U.S. patent application number 15/829912 was filed with the patent office on 2018-07-05 for electrolyte additives for electrochemical devices.
This patent application is currently assigned to Natron Energy, Inc.. The applicant listed for this patent is Natron Energy, Inc.. Invention is credited to Shahrokh Motallebi, Colin Deane Wessells.
Application Number | 20180191033 15/829912 |
Document ID | / |
Family ID | 62709081 |
Filed Date | 2018-07-05 |
United States Patent
Application |
20180191033 |
Kind Code |
A1 |
Wessells; Colin Deane ; et
al. |
July 5, 2018 |
ELECTROLYTE ADDITIVES FOR ELECTROCHEMICAL DEVICES
Abstract
A system and method for stabilizing electrodes against
dissolution and/or hydrolysis including use of cosolvents in liquid
electrolyte batteries for three purposes: the extension of the
calendar and cycle life time of electrodes that are partially
soluble in liquid electrolytes, the purpose of limiting the rate of
electrolysis of water into hydrogen and oxygen as a side reaction
during battery operation, and for the purpose of cost
reduction.
Inventors: |
Wessells; Colin Deane; (Palo
Alto, CA) ; Motallebi; Shahrokh; (Los Gatos,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Natron Energy, Inc. |
Santa Clara |
CA |
US |
|
|
Assignee: |
Natron Energy, Inc.
Santa Clara
CA
|
Family ID: |
62709081 |
Appl. No.: |
15/829912 |
Filed: |
December 2, 2017 |
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15829912 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/485 20130101;
H01M 10/36 20130101; H01M 2300/0025 20130101; Y02B 90/10 20130101;
H01M 4/628 20130101; H01M 10/056 20130101; H01M 2250/10 20130101;
H01M 4/9008 20130101; H01M 10/0567 20130101; H01M 2250/20 20130101;
H01M 4/36 20130101; H01M 10/08 20130101; H01M 2300/0037 20130101;
Y02E 60/10 20130101; H01M 10/4235 20130101; H01M 2220/20 20130101;
H01M 4/60 20130101; H01M 2300/002 20130101; H01M 2300/0002
20130101; H01M 2300/0028 20130101; H01M 10/0569 20130101; H01M
8/188 20130101; H01M 10/0568 20130101; H01M 2300/004 20130101; H01M
4/505 20130101; H01M 4/58 20130101; H01M 2220/10 20130101; Y02E
60/50 20130101; H01M 2004/027 20130101; Y02T 90/40 20130101; H01M
10/44 20130101; H01M 2300/0091 20130101; H01M 10/345 20130101 |
International
Class: |
H01M 10/0569 20060101
H01M010/0569; H01M 10/34 20060101 H01M010/34; H01M 4/60 20060101
H01M004/60; H01M 4/90 20060101 H01M004/90; H01M 8/18 20060101
H01M008/18; H01M 10/08 20060101 H01M010/08; H01M 4/36 20060101
H01M004/36; H01M 4/505 20060101 H01M004/505; H01M 10/0568 20060101
H01M010/0568 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under ARPA-E
Award No. DE-AR000300 With Alveo Energy, Inc., awarded by DOE. The
government has certain rights in the invention.
Claims
1. A rechargeable electrochemical device, comprising: a first
electrode; a second electrode; an electrolyte coupled with said
electrodes; and a first additive in communication with said
electrolyte; wherein a first particular one electrode of said
electrodes includes a first variable potential material; and
wherein said first additive participates in a first predetermined
side-reaction with a first single one of said electrodes degrading
a charging efficiency of said first single one of said electrodes
for a duration of said first predetermined side-reaction.
2. The rechargeable electrochemical device of claim 1 wherein said
duration is preconfigured for a reduction in a relative
state-of-charge imbalance between said electrodes after charging of
said electrodes.
3. The rechargeable electrochemical device of claim 1 wherein said
first particular one electrode includes a first transition metal
cyanide coordination compound (TMCCC).
4. The rechargeable electrochemical device of claim 1 further
comprising a second additive in communication with said electrolyte
wherein a second particular one electrode of said electrodes,
different from said first particular one electrode of said
electrodes, includes a second variable potential material; and
wherein said second additive participates in a second predetermined
side-reaction with a second single one of said electrodes.
5. The rechargeable electrochemical device of claim 4 wherein said
second particular one electrode includes a second transition metal
cyanide coordination compound (TMCCC).
6. The rechargeable electrochemical device of claim 3 wherein said
TMCCC material includes a composition having the general chemical
formula A.sub.xM.sub.y[R(CN).sub.6-jL.sub.j].sub.z.nH.sub.2O,
wherein: A includes one or more cations; M includes one or more
metal cations; R includes one or more transition metal cations; and
L is a ligand substituted in the place of a CN.sup.- ligand; where
0.ltoreq.x.ltoreq.2; 0<y.ltoreq.4; 0<z.ltoreq.1;
0.ltoreq.j<6; and 0.ltoreq.n.ltoreq.5.
7. The rechargeable electrochemical device of claim 3 wherein said
electrolyte includes a total electrolyte volume V including a first
quantity of water comprising a first fraction V1 of said total
electrolyte volume V and including a second quantity of one or more
organic cosolvents together comprising a second fraction V2 of said
total electrolyte volume V, wherein V1>0.02, and wherein said
electrolyte consists essentially of a single phase.
8. The rechargeable electrochemical device of claim 7 wherein
V2>V1.
9. The rechargeable electrochemical device of claim 8 wherein said
one or more organic solvents includes a solvent containing a
cyanide group.
10. The rechargeable electrochemical device of claim 9 wherein said
second quantity V2 includes acetonitrile.
11. The rechargeable electrochemical device of claim 8 wherein said
one or more organic cosolvents includes a solvent containing a
sulfone group.
12. The rechargeable electrochemical device of claim 11 wherein
said sulfone group includes sulfolane.
13. The rechargeable electrochemical device of claim 8 wherein a
concentration of said additive is included within a range of 10 to
10,000 parts per million.
14. The rechargeable electrochemical device of claim 8 wherein said
additive includes one or more organic molecules.
15. The rechargeable electrochemical device of claim 14 wherein
said one or more organic molecules are configured to participate in
a reversible electrochemical redox reaction at one or more of said
electrodes during an application of charging energy to said
electrodes.
16. The rechargeable electrochemical device of claim 15 wherein
said additive includes a quinone group.
17. The rechargeable electrochemical device of claim 14 wherein
said one or more organic molecules are configured to participate in
an irreversible electrochemical redox reaction at one or more of
said electrodes during an application of charging energy to said
electrodes.
18. The rechargeable electrochemical device of claim 17 wherein
said irreversible electrochemical redox reaction results in a
polymerization of said one or more organic molecules.
19. The rechargeable electrochemical device of claim 18 wherein
said one or more organic molecules include a pyrrole group.
20. The rechargeable electrochemical device of claim 8 wherein said
additive includes a transition metal salt.
21. The rechargeable electrochemical device of claim 20 wherein
said salt is configured to participate in a reversible
electrochemical redox reaction at one or more of said electrodes
during an application of charging energy to said electrodes.
22. The rechargeable electrochemical device of claim 20 wherein
said salt is configured to participate in an irreversible reaction
at one or more of said electrodes during an application of charging
energy to said electrodes.
23. The rechargeable electrochemical device of claim 22 wherein
said salt includes a transition metal cation.
24. The rechargeable electrochemical device of claim 22 wherein
said salt includes a transition metal polyanion.
25. The rechargeable electrochemical device of claim 8 wherein said
additive includes an organometallic molecule.
26. The rechargeable electrochemical device of claim 25 wherein
said organometallic molecule is configured to participate in a
reversible electrochemical redox reaction at one or more of said
electrodes.
27. The rechargeable electrochemical device of claim 25 wherein
said organometallic molecule includes a metallocene.
28. The rechargeable electrochemical device of claim 25 wherein
said organometallic molecule is configured to participate in an
irreversible reaction at one or more of said electrodes during an
application of charging energy at said electrodes.
29. The rechargeable electrochemical device of claim 8 wherein said
additive includes a surfactant.
30. A method for reducing a relative state-of-charge imbalance of a
set of electrodes of a rechargeable electrochemical device during a
recharging process, the set of electrodes coupled to an electrolyte
and wherein at least one electrode of the set of electrodes
includes a first variable potential material, comprising: a)
performing the recharging process for a recharging duration which
charges the electrodes at different relative rates to tend to
produce a relative state-of-charge imbalance for the set of
electrodes; and b) reducing said relative state-of-charge imbalance
by interfering with a charging of at least one electrode of the set
of electrodes.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 15/442,634 filed 25 Feb. 2017; this
application is a continuation-in-part of U.S. patent application
Ser. No. 13/892,982 filed 13 May 2013 which claims benefit of U.S.
Patent Application No. 61/722,049 filed 2 Nov. 2012; and this
application is a continuation-in-part of U.S. patent application
Ser. No. 15/062,171 filed 6 Mar. 2016 which is a continuation of
U.S. patent application Ser. No. 14/231,571 (now U.S. Pat. No.
9,287,589) filed 31 Mar. 2014 which claims benefit of U.S. Patent
Application No. 61/810,684 filed 10 Apr. 2013, the contents of
which are all hereby expressly incorporated by reference thereto in
their entireties for all purposes.
FIELD OF THE INVENTION
[0003] The present invention relates generally to electrochemical
devices, and more specifically, but not exclusively, to balancing
electrode potential of electrodes in an electrochemical device in
which at least one electrode includes a material in which potential
may vary by state of charge.
[0004] The present invention also relates generally to rechargeable
energy accumulators, and more specifically, but not exclusively, to
stabilization of electrodes used with aqueous electrolytes and even
more particularly to stabilization of electrodes used with aqueous
electrolytes as part of an electrochemical cell.
BACKGROUND OF THE INVENTION
[0005] The subject matter discussed in the background section
should not be assumed to be prior art merely as a result of its
mention in the background section. Similarly, a problem mentioned
in the background section or associated with the subject matter of
the background section should not be assumed to have been
previously recognized in the prior art. The subject matter in the
background section merely represents different approaches, which in
and of themselves may also be inventions.
[0006] A wide variety of battery technologies have been developed
for portable and stationary applications, including lead acid,
lithium-ion, nickel/metal hydride, sodium sulfur, and flow
batteries, among others. Not one of these technologies is commonly
used for applications related to the stabilization and reliability
of the electric grid due to exorbitantly high cost, poor cycle and
calendar lifetime, and low energy efficiency during rapid cycling.
However, the development of lower cost, longer lived batteries is
likely needed for the grid to remain reliable in spite of the
ever-increasing deployment of extremely volatile solar and wind
power.
[0007] Existing battery electrode materials cannot survive for
enough deep discharge cycles for the batteries containing them to
be worth their price for most applications related to the electric
grid. Similarly, the batteries found in electric and hybrid
electric vehicles are long lived only in the case of careful
partial discharge cycling that results in heavy, large, expensive
battery systems. The performance of most existing battery electrode
materials during fast cycling is limited by poor kinetics for ion
transport or by complicated, multi-phase operational
mechanisms.
[0008] The use of Prussian Blue analogues (PBAs), which are a
subset of a more general class of transition metal cyanide
coordination compounds (TMCCCs) of the general chemical formula
A.sub.xP.sub.y[R(CN).sub.6].sub.z.nH.sub.2O (A=alkali cation, P and
R=transition metal cations, 0.ltoreq.x.ltoreq.2,
0.ltoreq.y.ltoreq.4, 0.ltoreq.z.ltoreq.1, 0.ltoreq.n), has been
previously demonstrated as electrodes in aqueous electrolyte
batteries. TMCCC electrodes have longer deep discharge cycle life
and higher rate capability than other intercalation mechanism
electrodes, and they enjoy their highest performance in aqueous
electrolytes. TMCCC cathodes rely on the electrochemical activity
of iron in Fe(CN).sub.6 complexes at high potentials. TMCCC anodes,
on the other hand, contain electrochemically active,
carbon-coordinated manganese or chromium.
[0009] The development of a symmetric battery in which both the
anode and the cathode are each a TMCCC is desirable because TMCCCs
have longer cycle life and can operate at higher charge/discharge
rates than other electrode systems. If one TMCCC electrode were to
be paired with a different kind of electrode, it is likely that the
full battery would not last as long, or provide the same high-rate
abilities as a symmetric cell containing a TMCCC anode and a TMCCC
cathode.
[0010] TMCCC cathodes are well understood, and the operation of a
TMCCC cathode for over 40,000 deep discharge cycles has been
previously demonstrated. These cathodes typically operate at about
0.9 to 1.1 V vs. the standard hydrogen electrode (SHE). One
challenge for the development of practical batteries using TMCCC
cathodes is their trace solubility in aqueous electrolytes. Their
partial dissolution into the battery electrolyte can result in a
decrease in battery charge capacity due to mass loss from the
electrodes and a decrease in efficiency due to side reactions with
the cathode's dissolution products.
[0011] In some embodiments, an order of production and assembly of
components of an electrochemical device may affect performance
metrics of the completed electrochemical device. For example, in
some instances of a cosolvent electrochemical device, it may be
better to add a chemical species to an electrolyte of the
electrochemical device before adding the electrolyte to the rest of
the electrochemical device.
[0012] The development of a TMCCC anode has proven much more
challenging than that of TMCCC cathodes because these materials
typically have reaction potentials either near 0 V or below -0.5 V
vs. SHE, but not in the range between -0.5 V and 0 V that is most
desirable in aqueous electrolytes, and because they operate only in
a narrow pH range without rapid hydrolysis to manganese dioxide
phases. As the useful electrochemical stability window of aqueous
electrolytes at approximately neutral pH (pH=5-8) extends from
about -0.4 V to 1 V vs. SHE, an anode reaction potential of 0 V
results in a cell voltage lower than the maximum that is possible
without decomposition of water. But, in the case of an anode
reaction potential below -0.5 V vs. SHE, the charge efficiency of
the anode can be poor due to rapid hydrolysis of water to hydrogen
gas. Finally, if the Mn(CN).sub.6 groups in the TMCCC anode
hydrolyze, the capacity of the electrode is rapidly lost.
[0013] For purposes of this application, electrode materials may be
divided into two classes: 1) electrode potential is constant with
respect to state of charge; and 2) electrode potential varies with
respect to state of charge.
[0014] For an electrochemical cell using electrodes of the first
class, there is no concern about unbalanced potentials on the
electrode are balanced across a range of charge of the cell.
[0015] However, for an electrochemical cell using one or more
electrodes of the second class, there is a possibility that there
could be unbalanced potentials on the electrode, particularly in an
event that the cell is not at maximum charge. Unbalanced potentials
reduce an energy density of the cell.
[0016] Commonly used materials for electrodes, such as lithium and
graphite, are materials of the first class. There are materials of
the second class that offer some improvements over these more
conventional materials. However electrochemical cells made with the
materials of the second class may have a degraded performance in
other areas, including the possibility of the unbalanced
potentials.
[0017] There could be advantages to addressing the possible
degradation when using electrode materials of the second class,
such as improving energy density while gaining the desired
advantages of the alternative electrode materials or for slowing
and/or preventing dissolution of electrodes into an operating
electrolyte to extend a calendar life of the electrodes.
BRIEF SUMMARY OF THE INVENTION
[0018] Disclosed is a system and method for addressing the possible
degradation when using electrode materials of the second class,
such as improving energy density while gaining the desired
advantages of the alternative electrode materials and/or slowing
and/or preventing dissolution of electrodes into an operating
electrolyte to extend a calendar life of the electrodes. The
following summary of the invention is provided to facilitate an
understanding of some of technical features related to use of
cosolvent electrolytes for more efficient and durable batteries,
and is not intended to be a full description of the present
invention and/or stabilization of TMCCC/PBA battery electrodes. A
full appreciation of the various aspects of the invention can be
gained by taking the entire specification, claims, drawings, and
abstract as a whole. The present invention is applicable to other
electrode types in addition to TMCCC cathodes and/or anodes, to
other electrochemical devices in addition to full, partial, and/or
hybrid battery systems including a liquid electrolyte, and to other
cell chemistries, materials, and analogues.
[0019] Some examples in this patent application concern the use of
solvents and cosolvents in liquid electrolyte batteries for
multiple purposes: the extension of the calendar and cycle life
time of electrodes that are partially soluble in liquid
electrolytes, the purpose of limiting the rate of electrolysis of
water into hydrogen and oxygen as a side reaction during battery
operation, and for the purpose of cost reduction. Cosolvents are
when two liquids are combined into a single solution, as in the
case of water and ethanol in wine, which may also contain dissolved
compounds such as salts. Herein is demonstrated a utility of these
cosolvent electrolytes using the model system of an aqueous sodium
ion electrolyte battery containing TMCCC electrodes, but the
benefits of cosolvents to the performance of liquid electrolyte
batteries apply generally to other electrode and battery systems as
well. One cost benefit occurs because an organic cosolvent as
disclosed herein allows one to have a higher voltage before water
is quickly split into hydrogen and oxygen. When the organic
cosolvent is relatively inexpensive, and the electrodes are the
same materials (as in some embodiments disclosed herein when the
anode has two different reaction potentials), then the organic
cosolvent lets the electrochemical device have a higher voltage for
about the same materials cost. Energy is equal to the product of
the charge and the voltage, so a higher voltage electrochemical
cell that gets more energy from the same materials will therefore
have a lower cost/energy.
[0020] Embodiments of the present invention broadly includes a
general concept of the use of cosolvents in liquid electrolyte
batteries, particularly, but not exclusively, in several areas,
including: first, the concept of using cosolvents to protect TMCCC
electrodes from dissolution and/or hydrolysis, and second, the
ability to use a hexacyanomanganate-based TMCCC anode with a
reaction potential so low that it can only be used when reduction
of water to hydrogen gas is suppressed (as is the case, for
example, when a cosolvent is used as herein described).
[0021] Included herein is description of a novel method for the
stabilization of TMCCC electrodes against dissolution and
hydrolysis, while simultaneously suppressing hydrogen generation at
the anode: for example an addition of a cosolvent to an aqueous
electrolyte. A cosolvent electrolyte is one in which multiple
liquid solvents are combined to form a single liquid phase, in
which the electrolyte salt and any additional additives are then
dissolved. The presence of a cosolvent can drastically change the
solubility and stability of materials including both TMCCCs and
electrolyte salts. The proper choice of cosolvent slows or prevents
the dissolution and/or hydrolysis of TMCCC electrodes, and it
allows for the high-efficiency operation of TMCCC anodes with
reaction potentials below -0.5 V vs. SHE. The final result is an
electrochemical device that operates at voltages of nearly double
those that can be achieved in simple aqueous electrolytes, with
longer electrode cycle and calendar lives.
[0022] Some embodiments of the present invention may include an
electrochemical device including at least a pair of electrodes in
chemical communication with one or more electrolytes, one, some, or
all of the electrodes may each include a variable potential
material, each such electrode including the same or different
variable potential material, and one or more additives to the
electrochemical device that each participates in a limited
side-reaction with one or more electrodes having variable potential
material. In response to charging the electrochemical device, each
limited side-reaction degrades charging of the related electrode(s)
for a limited duration. Those electrodes that do not participate in
one of the limited side-reactions may begin charging immediately at
full coulombic efficiency. Each electrode that is participating in
a limited side-reaction charges more slowly due to degraded
coulombic efficiency, for the duration of each applicable limited
side-reaction. As each limited side-reaction completes, the
associated electrode may then begin charging at full coulombic
efficiency. Proper configuration and coordination of appropriate
limited side-reactions allow different electrodes to be adjustably
charged to different potentials from the same charging source.
[0023] A battery (cell) that comprises an electrolyte and two
electrodes (an anode and a cathode), one or both of which is a
TMCCC material of the general chemical formula
A.sub.xP.sub.y[R(CN).sub.6-jL.sub.j].sub.z.nH.sub.2O, where: A is a
monovalent cation such as Na.sup.+, K.sup.+, Li.sup.+, or
NH.sub.4.sup.+, or a divalent cation such as Mg.sup.2+ or
Ca.sup.2+; P is a transition metal cation such as Ti.sup.3+,
Ti.sup.4+, V.sup.2+, V.sup.3+, Cr.sup.2+, Cr.sup.3+, Mn.sup.+,
Mn.sup.2+, Mn.sup.3+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+,
Ni.sup.2+, Cu.sup.+, Cu.sup.2+, or Zn.sup.2+, or another metal
cation such as Al.sup.3+, Sn.sup.2+, In.sup.3+, or Pb.sup.2+; R is
a transition metal cation such as V.sup.2+, V.sup.3+, Cr.sup.2+,
Cr.sup.3+, Mn.sup.2+, Mn.sup.3+, Fe.sup.2+, Fe.sup.3+, CO.sup.2+,
Co.sup.3+, Ru.sup.2+, Ru.sup.3+, Os.sup.2+, Os.sup.3+, Ir.sup.2+,
Ir.sup.3+, Pt.sup.2+, or Pt.sup.3+; L is a ligand that may be
substituted in the place of a CN.sup.- ligand, including CO
(carbonyl), NO (nitrosyl), or Cl.sup.-; 0.ltoreq.x.ltoreq.2;
0<y.ltoreq.4; 0<z.ltoreq.1; 0.ltoreq.j.ltoreq.6; and
0.ltoreq.n.ltoreq.5; and where the electrolyte contains water, one
or more organic cosolvents, and one or more salts, where: the
electrolyte is a single phase.
[0024] A rechargeable electrochemical cell, includes a positive
electrode; a negative electrode; and an electrolyte having a total
electrolyte volume V including a first quantity of water comprising
a first fraction V1 of the total electrolyte volume V and including
a second quantity of one or more organic cosolvents together
comprising a second fraction V2 of the total electrolyte volume V;
wherein V1/V>0.02; wherein V2>V1; wherein a particular one
electrode of the electrodes includes a transition metal cyanide
coordination compound (TMCCC) material; and wherein the electrolyte
is a single phase.
[0025] A rechargeable electrochemical cell, includes a positive
electrode; a negative electrode; and an electrolyte having a total
electrolyte weight W including a first quantity of water comprising
a first fraction W1 of the total electrolyte weight W and including
a second quantity of one or more organic cosolvents together
comprising a second fraction W2 of the total electrolyte weight W;
wherein W1/W>0.02; wherein W2>W1; wherein a particular one
electrode of the electrodes includes a transition metal cyanide
coordination compound (TMCCC) material; and wherein the electrolyte
is a single phase.
[0026] A method for operating a rechargeable electrochemical cell
having a negative electrode disposed in a single phase liquid
electrolyte of a total electrolyte quantity Q including at least a
total quantity Q1 of water wherein Q1/Q is approximately 0.02 or
greater and wherein an electrolysis of the total quantity Q1 of
water below a first potential V1 initiates a production of hydrogen
gas at a first rate R1, including a) exchanging ions between the
negative electrode and the liquid electrolyte at an electrode
potential VE, VE<V1; and b) producing hydrogen gas at a second
rate R2 less than R1 responsive to the electrode potential VE;
wherein an electrolysis of the total electrolyte quantity Q a
second quantity of one or more organic cosolvents together
comprising a second fraction Q2 of the total electrolyte quantity Q
below a second potential V2 initiates the production of hydrogen
gas at the first rate R1, V2<V1; and wherein VE>V2.
[0027] A rechargeable electrochemical device, includes a first
electrode; a second electrode; an electrolyte coupled with the
electrodes; and a first additive in communication with the
electrolyte; wherein a first particular one electrode of the
electrodes includes a first variable potential material; and
wherein the first additive participates in a first predetermined
side-reaction with a first single one of the electrodes degrading a
charging efficiency of the first single one of the electrodes for a
duration of the first predetermined side-reaction.
[0028] A method for reducing a relative state-of-charge imbalance
of a set of electrodes of a rechargeable electrochemical device
during a recharging process, the set of electrodes coupled to an
electrolyte and wherein at least one electrode of the set of
electrodes includes a first variable potential material, including
a) performing the recharging process for a recharging duration
which charges the electrodes at different relative rates to tend to
produce a relative state-of-charge imbalance for the set of
electrodes; and b) reducing the relative state-of-charge imbalance
by interfering with a charging of at least one electrode of the set
of electrodes. In an embodiment, the reducing step b) may include
b1) communicating an additive to the electrolyte to induce a
predetermined side-reaction with the at least one electrode
including the first variable potential material; and b2) degrading
a charging efficiency of the at least one electrode for a duration
of the predetermined side-reaction.
[0029] A battery (cell) including: an electrolyte (which may be
aqueous or quasi-aqueous) and two electrodes (an anode and a
cathode), one or both of which is a TMCCC material of the general
chemical formula
A.sub.xP.sub.y[R(CN).sub.6-jL.sub.j].sub.z.nH.sub.2O, where: A is a
monovalent cation such as Na.sup.+, K.sup.+, Li.sup.+, or
NH.sub.4.sup.+, or a divalent cation such as Mg.sup.2+ or
Ca.sup.2+; P is a transition metal cation such as V.sup.2+,
V.sup.3+, Cr.sup.2+, Cr.sup.3+, Mn.sup.+, Mn.sup.2+, Mn.sup.3+,
Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+, Ni.sup.2+, Cu.sup.+,
Cu.sup.2+, or Zn.sup.2+, or another metal cation such as Al.sup.3+,
Sn.sup.2+, In.sup.3+, or Pb.sup.2+; R is a transition metal cation
such as V.sup.2+, V.sup.3+, Cr.sup.2+, Cr.sup.3+, Mn.sup.+,
Mn.sup.2+, Mn.sup.3+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+,
Ru.sup.2+, Ru.sup.3+, Os.sup.2+, Os.sup.3+, Ir.sup.2+, Ir.sup.3+,
Pt.sup.2+, or Pt.sup.3+; L is a ligand that may be substituted in
the place of a CN.sup.- ligand, including CO (carbonyl), NO
(nitrosyl), or Cl.sup.-; 0.ltoreq.j.ltoreq.6; 0.ltoreq.x.ltoreq.2;
0<y.ltoreq.4; 0<z.ltoreq.1; and 0.ltoreq.n.ltoreq.5.
[0030] A battery including an electrolyte in contact with two
electrodes, in which a conformal coating of a TMCCC of the general
chemical formula described herein on the surface of one or more of
the electrodes prevents dissolution of that electrode into the
electrolyte.
[0031] A battery including an electrolyte in contact with two
electrodes, in which a conformal coating of a TMCCC of the general
chemical formula described herein on the surface of the individual
particles of the electrochemically active material within the
electrode prevents dissolution of that material into the
electrolyte.
[0032] A battery including an electrolyte in contact with two
electrodes, in which a conformal coating of a mixed conducting
polymer such as polypyrrole on the surface of one or more of the
electrodes prevents dissolution of that electrode into the
electrolyte.
[0033] A battery including an electrolyte in contact with two
electrodes, in which a conformal coating of a mixed conducting
polymer such as polypyrrole on the surface of the individual
particles of the electrochemically active material within the
electrode prevents dissolution of that material into the
electrolyte.
[0034] An electrochemical apparatus including an operating aqueous
electrolyte including a quantity of water, a plurality of ions, and
an electrolyte additive distributed in the quantity of water; and a
first electrode disposed in the operating aqueous electrolyte, the
first electrode including a first TMCCC material having a general
chemical formula
A.sub.xP.sub.y[R(CN).sub.6-jL.sub.j].sub.z.nH.sub.2O, where: A is a
cation, P is a metal cation, R is a transition metal cation, and L
is a ligand substitutable in the place of a CN.sup.- ligand, and
0.ltoreq.j.ltoreq.6, 0.ltoreq.x.ltoreq.2, 0<y.ltoreq.4,
0<z.ltoreq.1, and 0.ltoreq.n.ltoreq.5, wherein the first TMCCC
material has a first specific chemical formula conforming to the
general chemical formula including a first particular cation
P.sub.1 and a first particular cation R.sub.1, wherein the first
electrode has a first rate of electrochemical capacity loss when
disposed in the operating aqueous electrolyte, and wherein the
first TMCCC material has a second rate of electrochemical capacity
loss when disposed in a second aqueous electrolyte consisting of
water and the plurality of ions without the electrolyte additive;
wherein the first rate of electrochemical capacity loss is less
than the second rate of electrochemical capacity loss.
[0035] A method for manufacturing an electrochemical apparatus
including a first electrode having a first TMCCC material with a
general chemical formula
A.sub.xP.sub.y[R(CN).sub.6-jL.sub.j].sub.z.nH.sub.2O, where: A is a
cation, P is a metal cation, R is a transition metal cation, and L
is a ligand substitutable in the place of a CN.sup.- ligand, and
0.ltoreq.j.ltoreq.6, 0.ltoreq.x.ltoreq.2, 0<y.ltoreq.4,
0<z.ltoreq.1, and 0.ltoreq.n.ltoreq.5, wherein the first TMCCC
material has a first specific chemical formula conforming to the
general chemical formula including a first particular cation
P.sub.1 and a first particular cation R.sub.1, and wherein the
first TMCCC material has a rate of electrochemical capacity loss
when disposed in an aqueous electrolyte including a plurality of
ions, the method including (a) disposing the first electrode in the
aqueous electrolyte; and (b) decreasing the rate of electrochemical
capacity loss by distributing an electrolyte additive into the
aqueous electrolyte.
[0036] Any of the embodiments described herein may be used alone or
together with one another in any combination. Inventions
encompassed within this specification may also include embodiments
that are only partially mentioned or alluded to or are not
mentioned or alluded to at all in this brief summary or in the
abstract. Although various embodiments of the invention may have
been motivated by various deficiencies with the prior art, which
may be discussed or alluded to in one or more places in the
specification, the embodiments of the invention do not necessarily
address any of these deficiencies. In other words, different
embodiments of the invention may address different deficiencies
that may be discussed in the specification. Some embodiments may
only partially address some deficiencies or just one deficiency
that may be discussed in the specification, and some embodiments
may not address any of these deficiencies.
[0037] Other features, benefits, and advantages of the present
invention will be apparent upon a review of the present disclosure,
including the specification, drawings, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The accompanying figures, in which like reference numerals
refer to identical or functionally-similar elements throughout the
separate views and which are incorporated in and form a part of the
specification, further illustrate the present invention and,
together with the detailed description of the invention, serve to
explain the principles of the present invention.
[0039] FIG. 1 illustrates a schematic of batteries using the higher
and lower anode reactions for CuHCF and MnHCMn;
[0040] FIG. 2 illustrates a unit cell of the TMCCC crystal
structure;
[0041] FIG. 3 illustrates a cyclic voltammogram of MnHCMn in
cosolvents;
[0042] FIG. 4 illustrates a cyclic voltammogram of MnHCMn in
cosolvents;
[0043] FIG. 5 illustrates a cyclic voltammogram of MnHCMn in
cosolvents;
[0044] FIG. 6 illustrates a cyclic voltammogram of MnHCMn in
cosolvents;
[0045] FIG. 7 illustrates a cyclic voltammogram and integrated
current of MnHCMn in 90% MeCN;
[0046] FIG. 8 illustrates a cyclic voltammogram of CuHCF in
cosolvents;
[0047] FIG. 9 illustrates a cyclic voltammogram of MnHCMn in 90% or
100% MeCN;
[0048] FIG. 10 illustrates a cycle life of MnHCMn in half
cells;
[0049] FIG. 11 illustrates a set of potential profiles of MnHCMn in
half cells;
[0050] FIG. 12 illustrates a cycle life of CuHCF in half cells;
[0051] FIG. 13 illustrates a set of GCPL vs. time profiles of
MnHCMn vs. CuHCF in the full cell;
[0052] FIG. 14 illustrates a full cell voltage profile;
[0053] FIG. 15 illustrates a full cell voltage profile of the cell
illustrated in FIG. 13;
[0054] FIG. 16 illustrates a representative secondary
electrochemical cell schematic having one or more TMCCC electrodes
disposed in contact with a cosolvent electrolyte as described
herein; and
[0055] FIG. 17-FIG. 30 illustrate seven pairs of charts
corresponding to Example A3-Example A9, each pair of charts
including an electrodes potential chart and a full cell voltage
chart;
[0056] FIG. 17-FIG. 18 illustrate a first pair of charts for
Example A3 comparing a control (no additive) to a
Cu(NO.sub.3).sub.2 additive;
[0057] FIG. 17 illustrates an electrode potentials chart for
Example A3; and
[0058] FIG. 18 illustrates a cell voltage chart for Example A3;
and
[0059] FIG. 19-FIG. 20 illustrate a second pair of charts for
Example A4 comparing a control (no additive) to a Benzoquinone
additive;
[0060] FIG. 19 illustrates an electrode potentials chart for
Example A4; and
[0061] FIG. 20 illustrates a cell voltage chart for Example A4;
and
[0062] FIG. 21-FIG. 22 illustrate a third pair of charts for
Example A5 comparing a control (no additive) to a Hydroquinone
additive;
[0063] FIG. 21 illustrates an electrode potentials chart for
Example A5; and
[0064] FIG. 22 illustrates a cell voltage chart for Example A5;
and
[0065] FIG. 23-FIG. 24 illustrate a fourth pair of charts for
Example A6 comparing a control (no additive) to a Ferrocene
additive;
[0066] FIG. 23 illustrates an electrode potentials chart for
Example A6; and
[0067] FIG. 24 illustrates a cell voltage chart for Example A6;
and
[0068] FIG. 25-FIG. 26 illustrate a fifth pair of charts for
Example A7 comparing a control (no additive) to a
Cu(NO.sub.3).sub.2 additive;
[0069] FIG. 25 illustrates an electrode potentials chart for
Example A7; and
[0070] FIG. 26 illustrates a cell voltage chart for Example A7;
and
[0071] FIG. 27-FIG. 28 illustrate a sixth pair of charts for
Example A8 comparing a control (no additive) to an Oxalic acid
additive;
[0072] FIG. 27 illustrates an electrode potentials chart for
Example A8; and
[0073] FIG. 28 illustrates a cell voltage chart for Example A8;
and
[0074] FIG. 29-FIG. 30 illustrate a seventh pair of charts for
Example A9 comparing a control (no additive) to a Pyrrole
additive;
[0075] FIG. 29 illustrates an electrode potentials chart for
Example A9;
[0076] FIG. 30 illustrates a cell voltage chart for Example A9;
and
[0077] FIG. 31 illustrates a set of charts for charging and
discharging under a set of different cases;
[0078] FIG. 32 illustrates a unit cell of the Prussian Blue crystal
structure;
[0079] FIG. 33 illustrates an X-ray diffraction spectrum of
CuHCF;
[0080] FIG. 34 illustrates a micrograph of CuHCF;
[0081] FIG. 35 illustrates X-ray diffraction spectra of MnHCMn;
[0082] FIG. 36 illustrates a micrograph of MnHCMn;
[0083] FIG. 37 illustrates baseline/control electrochemical cycling
of CuHCF;
[0084] FIG. 38 illustrates a UV-visible spectrum of CuHCF in water
and 1 M KNO.sub.3 pH=2;
[0085] FIG. 39 illustrates an ultraviolet-visible absorbance
spectrum of CuHCF in water and 10 mM Cu.sup.2+;
[0086] FIG. 40 illustrates the cycle life of CuHCF in 1 M KNO.sub.3
pH=2 with and without CU.sup.2+ added;
[0087] FIG. 41 illustrates galvanostatic cycling of
CuHCF/Cu.sup.2+/Cumetal in 2 sub-figures, including FIG. 41a and
FIG. 41b;
[0088] FIG. 41a illustrates potential profiles of the copper
hexacyanoferrate cathode and the copper anode, and the full cell
voltage, during galvanostatic cycling at a 1C rate in 1 M
KNO.sub.3; and
[0089] FIG. 41b illustrates the same data, plotted as a function of
the specific capacity of the copper hexacyanoferrate cathode;
[0090] FIG. 42 illustrates cyclic voltammetry of CuHCF and
PB/BG;
[0091] FIG. 43 illustrates capacity retention of PB/CuHCF and
CuHCF;
[0092] FIG. 44 illustrates capacity retention of CuHCF w/K.sup.+ in
PB dep solution;
[0093] FIG. 45 illustrates potential profiles of CuHCF and Prussian
Blue-coated CuHCF electrodes;
[0094] FIG. 46 illustrates morphologies of bare and Prussian
Blue-coated CuHCF electrodes in two sub-figures, including FIG. 46a
and FIG. 46b;
[0095] FIG. 46a illustrates scanning electron microscopy of a
freshly deposited slurry electrode of copper hexacyanoferrate
(80%), carbon black (10%), and polyvinylidene difluoride (10%) on a
carbon cloth substrate; and;
[0096] FIG. 46b illustrates the same sample, after electrochemical
reduction, followed by 40 minutes of exposure to a 2 mM aqueous
solution of Fe(CN).sub.3 and K.sub.3Fe(CN).sub.6;
[0097] FIG. 47 illustrates cycle life of CuHCF with PB coating on
the particles;
[0098] FIG. 48 illustrates potential profiles of CuHCF with PB
coating on the particles in two sub-figures, including FIG. 48a and
FIG. 48b;
[0099] FIG. 48a illustrates the potential profiles of electrodes
containing untreated copper hexacyanoferrate, and copper
hexacyanoferrate nanoparticles coated with Prussian Blue, during
galvanostatic cycling at a 1C rate in 1 M KNO.sub.3 (pH=2); and
[0100] FIG. 48b illustrates Galvanostatic cycling of an electrode
containing Prussian-Blue coated copper hexacyanoferrate
nanoparticles over a wider potential range;
[0101] FIG. 49 illustrates cycle life of CuHCF with PPy coating on
the particles; and
[0102] FIG. 50 illustrates potential profiles of CuHCF with PPy
coating on the particles.
DETAILED DESCRIPTION OF THE INVENTION
[0103] Embodiments of the present invention provide a system and
method for addressing the possible degradation when using electrode
materials of the second class, such as improving energy density
while gaining the desired advantages of the alternative electrode
materials. The following description is presented to enable one of
ordinary skill in the art to make and use the invention and is
provided in the context of a patent application and its
requirements.
[0104] Various modifications to the preferred embodiment and the
generic principles and features described herein will be readily
apparent to those skilled in the art. Thus, the present invention
is not intended to be limited to the embodiment shown but is to be
accorded the widest scope consistent with the principles and
features described herein.
Definitions
[0105] The following definitions apply to some of the aspects
described with respect to some embodiments of the invention. These
definitions may likewise be expanded upon herein.
[0106] As used herein, the singular terms "a," "an," and "the"
include plural referents unless the context clearly dictates
otherwise. Thus, for example, reference to an object can include
multiple objects unless the context clearly dictates otherwise.
[0107] Also, as used in the description herein and throughout the
claims that follow, the meaning of "in" includes "in" and "on"
unless the context clearly dictates otherwise.
[0108] As used herein, the term "set" refers to a collection of one
or more objects. Thus, for example, a set of objects can include a
single object or multiple objects. Objects of a set also can be
referred to as members of the set. Objects of a set can be the same
or different. In some instances, objects of a set can share one or
more common properties.
[0109] As used herein, the term "adjacent" refers to being near or
adjoining. Adjacent objects can be spaced apart from one another or
can be in actual or direct contact with one another. In some
instances, adjacent objects can be coupled to one another or can be
formed integrally with one another.
[0110] As used herein, the terms "couple," "coupled," and
"coupling" refer to an operational connection or linking. Coupled
objects can be directly connected to one another or can be
indirectly connected to one another, such as via an intermediary
set of objects.
[0111] As used herein, the terms "substantially" and "substantial"
refer to a considerable degree or extent. When used in conjunction
with an event or circumstance, the terms can refer to instances in
which the event or circumstance occurs precisely as well as
instances in which the event or circumstance occurs to a close
approximation, such as accounting for typical tolerance levels or
variability of the embodiments described herein.
[0112] As used herein, the terms "optional" and "optionally" mean
that the subsequently described event or circumstance may or may
not occur and that the description includes instances where the
event or circumstance occurs and instances in which it does
not.
[0113] As used herein, the term "size" refers to a characteristic
dimension of an object. Thus, for example, a size of an object that
is spherical can refer to a diameter of the object. In the case of
an object that is non-spherical, a size of the non-spherical object
can refer to a diameter of a corresponding spherical object, where
the corresponding spherical object exhibits or has a particular set
of derivable or measurable properties that are substantially the
same as those of the non-spherical object. Thus, for example, a
size of a non-spherical object can refer to a diameter of a
corresponding spherical object that exhibits light scattering or
other properties that are substantially the same as those of the
non-spherical object. Alternatively, or in conjunction, a size of a
non-spherical object can refer to an average of various orthogonal
dimensions of the object. Thus, for example, a size of an object
that is a spheroidal can refer to an average of a major axis and a
minor axis of the object. When referring to a set of objects as
having a particular size, it is contemplated that the objects can
have a distribution of sizes around the particular size. Thus, as
used herein, a size of a set of objects can refer to a typical size
of a distribution of sizes, such as an average size, a median size,
or a peak size.
[0114] As used herein, the term "electrolyte" means an
ion-conducting, but electronically insulating medium into which the
electrodes of an electrochemical cell are disposed. A liquid
electrolyte contains one or more liquid solvents and one or more
salts that readily disassociate when dissolved in these solvents.
Liquid electrolytes may also contain additives that enhance a
performance characteristic of the electrochemical cell into which
the electrolyte is disposed.
[0115] As used herein, the term "battery" means a rechargeable
electrochemical device that converts stored chemical energy into
electrical energy, including voltaic cells that may each include
two half-cells joined together by one or more conductive liquid
electrolytes.
[0116] As used herein, in the context of a cosolvent solution and a
majority or primary solvent of such cosolvent solution, the term
"majority" or "primary" means, for a two solvent cosolvent
solution, a solvent having 50% or greater volume of the total
solvent volume (% vol./vol.), or 50% or greater weight of the total
solvent weight (% weight/weight). For a cosolvent solution having
three or more solvents, the majority/primary solvent is the solvent
present in the greatest quantity (by volume or weight) as compared
to the quantities of any of the other solvents of the cosolvent
solution. These determinations are preferably made before
accounting for any salt or additive to the cosolvent solution. A
"minority" or "secondary" solvent in a cosolvent solution is any
other solvent other than the majority/primary solvent. For purposes
of this present invention when considering cosolvent solutions,
water is never a majority solvent and may be a minority/secondary
solvent. Water is purposefully present as minority solvent in
greater quantity than would be incidental or present as a
contaminant having 2% or greater volume of the total solvent volume
(% vol./vol.), or 2% or greater weight of the total solvent weight
(% weight/weight). An aqueous electrolyte includes water as a
majority solvent when in a cosolvent electrolyte and in some
instances water may be the only solvent present in a single-solvent
electrolyte. Cosolvent water, with water as a significant (e.g.,
about 2% or greater) solvent but not a majority solvent, may
produce an electrolyte that is sometimes referred to as
quasi-aqueous to indicate that water is present in more than trace
amounts but is not the majority solvent for two or more
cosolvents.
[0117] As used herein, the term "variable potential material" means
a material, that when used as an electrode in an electrochemical
device, experiences a variable potential as a function of state of
charge. Transition metal cyanide coordination compound (TMCCC)
materials are an example of a variable potential material. Other
examples include: transition metal oxides including but not limited
to lithium cobalt oxide, lithium nickel oxide, lithium manganese
oxide, lithium nickel manganese cobalt oxide, lithium nickel cobalt
aluminum oxide, manganese dioxide, sodium manganese oxide, sodium
cobalt oxide, and tungsten trioxide; sulfur, lithium sulfide;
carbons including but not limited to graphite, mesoporous carbons,
and activated carbons including charcoal; silicon including
nanostructured silicon; polymers including but not limited to
polypyrrole, polythiophene, polyanilene, and
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; and
combinations of one or more of the above.
[0118] As used herein, the term "additive" in the context of a
compound, substance, material, mixture, blend, composition, mix,
amalgamation, or other addition or assembly relative to an
electrochemical device including an electrolyte in chemical
communication to a variable potential material that is capable of
undergoing an electrochemical redox reaction with at least one
electrode of the electrochemical device. One or more additives may
be used in the electrochemical device. In some cases, this
electrochemical redox reaction may be irreversible, resulting in
consumption of the additive. In other cases, that reaction may be
reversible resulting in non-consumption of the additive, or
conversion of the additive through intermediate reactions of the
additive, allowing the additive to be recycled and reused, such as
through chemical recycling. In some embodiments, the additive may
be added to the electrolyte before the electrolyte is added to the
cell. In some embodiments, the additive may be added to the
electrodes before they are added to the cell. In some embodiments,
the additive may be added to the slurry or paste used to produce
the electrodes.
[0119] Electrode Materials
[0120] Some disclosed embodiments of the invention relate to
battery electrode materials in which dimensional changes in a host
crystal structure during charging and discharging are small,
thereby affording long cycle life and other desirable properties.
Such dimensional changes can otherwise result in mechanical
deformation and energy loss, as evidenced by hysteresis in battery
charge/discharge curves.
[0121] Some embodiments relate to a class of transition metal
cyanide coordination compound (TMCCC) electrode materials having
stiff open framework structures into which hydrated cations can be
reversibly and rapidly intercalated from aqueous (e.g., majority
water-based) electrolytes or other types of electrolytes. In
particular, TMCCC materials having the Prussian Blue-type crystal
structure afford advantages including greater durability and faster
kinetics when compared to other intercalation and displacement
electrode materials. A general formula for the TMCCC class of
materials is given by:
A.sub.xP.sub.y[R(CN).sub.6-jL.sub.j].sub.z.nH.sub.2O, where:
A is a monovalent cation such as Na.sup.+, K.sup.+, Li.sup.+, or
NH.sub.4.sup.+, or a divalent cation such as Mg.sup.2+ or
Ca.sup.2+; P is a transition metal cation such as Ti.sup.3+,
Ti.sup.4+, V.sup.2+, V.sup.3+, Cr.sup.2+, Cr.sup.3+, Mn.sup.+,
Mn.sup.2+, Mn.sup.3+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+,
Ni.sup.2+, Cu.sup.+, Cu.sup.2+, or Zn.sup.2+, or another metal
cation such as Al.sup.3+, Sn.sup.2+, In.sup.3+, or Pb.sup.2+; R is
a transition metal cation such as V.sup.2+, V.sup.3+, Cr.sup.2+,
Cr.sup.3+, Mn.sup.+, Mn.sup.2+, Mn.sup.3+, Fe.sup.2+, Fe.sup.3+,
Co.sup.2+, Co.sup.3+, Ru.sup.2+, Ru.sup.3+, Os.sup.2+, Os.sup.3+,
Ir.sup.2+, Ir.sup.3+, Pt.sup.2+, or Pt.sup.3+; L is a ligand that
may be substituted in the place of a CN.sup.- ligand, including CO
(carbonyl), NO (nitrosyl), or Cl.sup.-; 0.ltoreq.x.ltoreq.2;
0<y.ltoreq.4; 0<z.ltoreq.1; 0.ltoreq.j.ltoreq.6; and
0.ltoreq.n.ltoreq.5.
[0122] Figures
[0123] FIG. 1 illustrates a schematic of batteries using the higher
and lower anode reactions for the MnHCMn anode and the reaction
potential of the CuHCF cathode. This schematic shows the
operational modes of a battery containing a TMCCC cathode and a
TMCCC anode used together in two different electrolytes; 1) an
aqueous electrolyte, and 2) a cosolvent electrolyte. In the aqueous
electrolyte, rapid hydrogen evolution occurs above the lower
operational potential of the anode, so only the upper operational
potential of the anode can be used. The result is a 0.9 V cell.
But, in the cosolvent electrolyte, hydrogen production is
suppressed, resulting in efficient use of the lower operational
potential of the anode and a full cell voltage of 1.7 V.
[0124] FIG. 2 illustrates a unit cell of the cubic Prussian Blue
crystal structure, one example of a TMCCC structure. Transition
metal cations are linked in a face-centered cubic framework by
cyanide bridging ligands. The large, interstitial A sites can
contain water or inserted alkali ions.
[0125] FIG. 3 illustrates a cyclic voltammogram of MnHCMn in
cosolvents. Cyclic voltammetry of the lower operational potential
of manganese hexacyanomanganate(II/I) is shown in aqueous 1 M
NaClO.sub.4 and 1 M NaClO.sub.4 containing various concentrations
of acetonitrile. The position and hysteresis between the current
peaks vary only slightly with acetonitrile concentration,
indicating that the reaction mechanism and performance is largely
independent of the cosolvent.
[0126] FIG. 4 illustrates a cyclic voltammogram of MnHCMn in
cosolvents. Cyclic voltammetry of the lower operational potential
of manganese hexacyanomanganate(II/I) is shown in aqueous 1 M
NaClO.sub.4 and 1 M NaClO.sub.4 containing 95% solvent volume
acetonitrile and 5% solvent volume water. Reversible cycling is
achieved even with only 5% water present. The background current at
-0.9 V is 1 mA in purely aqueous electrolyte, but only 0.1 mA in
the primarily organic cosolvent electrolytes, demonstrating
improved coulombic efficiency with an organic primary
cosolvent.
[0127] FIG. 5 illustrates a cyclic voltammogram of MnHCMn in
cosolvents. Cyclic voltammetry of the lower operational potential
of manganese hexacyanomanganate(II/I) is shown 1 M NaClO.sub.4
containing 5% solvent volume water, 47.5% solvent volume
acetonitrile, and 47.5% solvent volume of one of sulfolane,
propylene glycol monoethyl ether, hydroxypropionitrile, or
gamma-valerolactone. In all cases, cycling of MnHCMn is shown to be
reversible.
[0128] FIG. 6 illustrates a cyclic voltammogram of MnHCMn in
cosolvents. Cyclic voltammetry of the lower operational potential
of manganese hexacyanomanganate(II/I) is shown 1 M NaClO.sub.4
containing 5% solvent volume water, 47.5% solvent volume
acetonitrile, and 47.5% solvent volume of one of ethylene
carbonate, dimethyl carbonate, or 1,3-dioxolane, or containing 5%
solvent volume water, 10% solvent volume acetonitrile, and 85%
solvent volume propylene carbonate. In all cases, cycling of MnHCMn
is shown to be reversible.
[0129] FIG. 7 illustrates a cyclic voltammogram and integrated
current of MnHCMn in 1 M NaClO.sub.4 in 90% solvent volume
acetonitrile and 10% solvent volume water. Main Figure: cyclic
voltammetry of MnHCMn(II/I) in 1 M NaClO.sub.4, 90%/10%
MeCN/H.sub.2O shows an extremely reversible reaction centered at
-0.75 V vs. SHE. The open circuit potential of the material is
above the upper reaction [MnHCMN(III/II)] so during the first
reductive sweep two reactions are observed. The peak current of
.+-.1.2 A/g is the equivalent of a 20C galvanostatic cycling rate,
indicating extremely fast kinetics. Inset Figure: integration of
the current during each scan gives the specific charge and
discharge capacity of the electrode. About 57 mAh/g is observed, in
close agreement with the approximate theoretical specific capacity
of 60 mAh/g. A coulombic efficiency of well over 95% is achieved.
There is little capacity fading, in agreement with GCPL
measurements of MnHCMn(II/I) in the same electrolyte.
[0130] FIG. 8 illustrates a cyclic voltammogram of CuHCF in
cosolvents containing varying amounts of acetone. Cyclic
voltammetry is shown of the copper hexacyanoferrate cathode in
aqueous 1 M NaClO.sub.4 and in 1 M NaClO.sub.4 containing up to 90%
solvent volume acetone and as little as 10% solvent volume water.
There is little change in the potential of the reaction with
increasing amounts of the cosolvent. No clear trend is observed in
the small effects of the cosolvent on the reaction potential and
kinetics of the charge and discharge of the electrode.
[0131] FIG. 9 illustrates a cyclic voltammogram of MnHCMn in 90% or
100% MeCN. Cyclic voltammetry is shown of the lower reaction
manganese hexacyanomanganate(II/I) in 1 M NaClO.sub.4 containing
either 100% solvent volume acetonitrile or 90% solvent volume
acetonitrile and 10% solvent volume water. The electrode has very
poor kinetics and a poor current response in the 100% solvent
volume acetonitrile electrolyte. In contrast, the addition of 10%
water to the acetonitrile results in a reaction with faster
kinetics and a higher peak current.
[0132] FIG. 10 illustrates a cycle life of MnHCMn in half cells.
During cycling in 1 M NaClO.sub.4 containing 90% solvent volume
acetonitrile and 10% solvent volume water, MnHCMn(II/I) shows good
cycle life, losing only 5% of its initial discharge capacity after
15 cycles. In contrast, in aqueous 1 M NaClO.sub.4 with no
acetonitrile present, 25% of the initial discharge capacity is lost
after 15 cycles.
[0133] FIG. 11 illustrates a set of potential profiles of MnHCMn in
half cells. The potential profiles of MnHCMn(II/I) are shown during
cycling in two different electrolytes: aqueous 1 M NaClO.sub.4
containing no organic cosolvent, and 1 M NaClO.sub.4 containing 90%
solvent volume acetonitrile and 10% solvent volume water. In both
electrolytes, the MnHCMn reaction is centered at -0.95 V vs.
Ag/AgCl, or equivalently, -0.75 V vs. SHE. Though both samples were
cycled at the same 1C rate, the sample operated in the purely
aqueous electrolyte shows a much lower capacity of 40 mAh/g as
rapid hydrolysis upon its insertion into the cell consumed one
third of its capacity. In contrast, the MnHCMn electrode operated
in the electrolyte containing the organic primary cosolvent had a
specific discharge capacity of over 55 mAh/g, much closer to the
maximum theoretical value (see FIG. 10).
[0134] FIG. 12 illustrates a set of coulombic efficiencies of
MnHCMn in half cells operated by galvanostatic cycling between
-0.95 V and -0.5 V vs. SHE. The coulombic efficiency is defined as
the ratio for each cycle of the discharge capacity divided by the
charge capacity. In the cell containing an electrolyte of 1 M
NaClO.sub.4 and 100% solvent volume water, a coulombic efficiency
of less than 99% is observed. In three identical cells each
containing an electrolyte of 1.4 M NaClO.sub.4, 95% solvent volume
acetonitrile, and 5% solvent volume water, a coulombic efficiency
of over 99.5% is observed.
[0135] FIG. 13 illustrates a cycle life of CuHCF in half cells.
During cycling of CuHCF at a 1C rate in aqueous 1 M NaClO.sub.4
containing no organic cosolvents, 4% of the initial discharge
capacity is lost after 50 cycles. In contrast, during cycling of
CuHCF at a 1C rate in 1 M NaClO.sub.4 containing 90% solvent volume
acetonitrile and 10% solvent volume water, zero capacity loss is
observed after 300 cycles.
[0136] FIG. 14 illustrates a set of GCPL vs. time profiles of
MnHCMn vs. CuHCF in the full cell. The potential profiles of the
CuHCF cathode and MnHCMn(II/I) anode in a full cell, and the full
cell voltage profile are shown. The electrolyte was 1 M
NaClO.sub.4, 10% solvent volume H.sub.2O, 90% solvent volume MeCN,
and cycling was performed at a 1C rate with the anode operated as
the working electrode. An excess of CuHCF was used in this case to
avoid any oxygen generation at high potentials, so the potential
profile of the cathode is flatter than that of the anode.
[0137] FIG. 15 illustrates a full cell voltage profile. The full
cell voltage profile is of the cell shown in FIG. 13. The average
voltage of the cell is 1.7 V, nearly double the voltage achievable
if the MnHCMn(III/II) reaction is used. The result is a cell with
significantly higher energy and power.
[0138] FIG. 16 illustrates a representative secondary
electrochemical cell 1600 schematic having one or more TMCCC
electrodes disposed in contact with a cosolvent electrolyte as
described herein. Cell 1600 includes a negative electrode 1605, a
positive electrode 1610 and an electrolyte 1615 electrically
communicated to the electrodes.
[0139] Overview
[0140] A battery (or cell) comprises an anode, a cathode, and an
electrolyte that is in contact with both the anode and the cathode.
Both the cathode and the anode contain an electrochemically active
material that may undergo a change in valence state, accompanied by
the acceptance or release of cations and electrons. For example,
during discharge of a battery, electrons are extracted from the
anode to an external circuit, while cations are removed from the
anode into the electrolyte. Simultaneously, electrons from the
external circuit enter the cathode, as do cations from the
electrolyte. The difference in the electrochemical potentials of
the cathode and anode results in a full cell voltage. This voltage
difference allows energy to be extracted from the battery during
discharge, or stored in the battery during charge.
[0141] The battery may be rechargeable and include electrodes that
may be made of variable potential material. Further, the battery
may include one or more additives in chemical communication with
the electrolyte. The additive(s) participate in one or more limited
side-reactions with one or more of the electrodes. These limited
side-reactions degrade charging of its associated electrode(s) for
the duration of the side-reaction. This allows other electrodes to
begin charging immediately at full coulombic efficiency.
Consequently, in response to a charging source, the electrodes may
have unbalanced charges. However, with appropriate selection and
coordination of the limited side-reaction(s), the overall energy
density of the electrochemical device may be greater than the case
without the limited side-reactions.
[0142] For example, an electrochemical device may include a
rechargeable battery including two variable potential electrodes
having the same nominal charge capacities and linear variation in
their electrochemical potentials with their states of charge, the
first of which (cathode) undergoes a full charge or discharge
between 1.0 volts and 1.5 volts as measured with respect to an
arbitrary reference electrode, and the second of which (anode)
undergoes a full charge or discharge between -1.0 volts and -1.5
volts with respect to the same reference electrode, in an
electrolyte. Nominally, the cell may reach a full charge at 3.0
volts, with the cathode reaching a potential of 1.5 volts at full
charge and the anode reaching a potential of -1.5 volts at full
charge, with respect to the reference electrode. During a discharge
of this cell to 2.0 volts, the cathode may discharge fully to a
potential of 1.0 volts and the anode may discharge fully to a
potential of -1.0 volts with respect to the reference electrode.
However, in some cases there may be limitations to the cell, or
with the chemistry of the cell, among a variety of other different
reasons, that the cell cannot charge to 3.0 volts, but rather to a
lower voltage. An example of this limitation is an electrolyte that
is only electrochemically stable over a 2.8 volt range, less than a
3.0 volt range required to fully charge both electrodes. An
embodiment of the present invention may address these situations.
Various cases are described herein, for example, see the discussion
below with respect to FIG. 31.
[0143] The electrolyte in a battery allows ions to flow from one
electrode to the other, but that insulates the two electrodes from
one another electronically. Typically battery electrolytes include
aqueous acids and salts in lead acid and bases nickel/metal hydride
batteries, and organic liquids containing lithium salts in
lithium-ion batteries. The electrolyte may also contain additives
that stabilize the electrodes, prevent side chemical reactions, or
otherwise enhance battery performance and durability. The
electrolyte may also contain multiple liquid components, in which
case they are known as cosolvents. The liquid component making up
the majority of the electrolyte is typically known as the primary
solvent, while those making up the minority are known as minority
solvents.
[0144] Organic cosolvents have been used in battery electrolytes in
some types of batteries. For example, commercial lithium-ion
battery electrolytes contain a variety of organic cosolvents,
including ethylene carbonate, diethyl carbonate, propylene
carbonate, and others. Those battery electrodes never include water
as a minority solvent. Other aqueous electrolyte batteries such as
lead acid, nickel/metal hydride, and flow batteries typically do
not use cosolvent electrolytes. There is no precedent among
previously documented battery systems for cosolvent electrolytes
containing water as a minority component.
[0145] An electrolyte containing organic cosolvents in combination
with water as a minority cosolvent offers several advantages in
comparison to electrolytes that are either primarily aqueous or
that contain solely organic cosolvents. First, when water is
present as only a minority cosolvent, its decomposition into
hydrogen and oxygen is suppressed, and a larger practical
electrochemical stability window is achieved (FIGS. 1, 4). Second,
electrode materials and other battery materials that are
water-sensitive and may decompose by a hydrolysis mechanism are
more stable when water is only a minority component of the system.
Third, water has higher ionic conductivity than the organic
solvents typically used in battery electrodes, so its presence as a
minority cosolvent increases the electrolyte conductivity.
[0146] Cosolvent electrolytes are of interest for the stabilization
of TMCCC electrodes that have inherent solubility in aqueous
battery electrolytes. Copper hexacyanoferrate (CuHCF) is a TMCCC
recently demonstrated to be a high performance battery electrode.
In the open framework structure of CuHCF, iron is six-fold,
octahedrally coordinated to the carbon ends of the cyanide
branching ligands, while copper is octahedrally
nitrogen-coordinated (FIG. 3). Depending on the method of
synthesis, the A sites in CuHCF may contain potassium or another
alkali cation such as sodium or lithium, or another type of cation
such as ammonium. More generally, for a TMCCC of the general
chemical formula A.sub.xP.sub.y[R(CN).sub.6].sub.z.nH.sub.2O,
alkali cations A.sup.+ and water occupy the interstitial A Sites,
transition metal P cations are six-fold nitrogen coordinated, and
transition metal R cations are six-fold carbon coordinated.
[0147] In the work described here, the electrochemical cells
contained a TMCCC working electrode, a counter electrode, an
electrolyte in contact with both the anode and cathode, and an
Ag/AgCl reference electrode used to independently measure the
potentials of the anode and cathode during charge and discharge of
the cell. When the electrode of interest was a cathode material,
then the working electrode was the cathode, and the counter
electrode was the anode. When the electrode of interest was an
anode material, then the working electrode was the anode, and the
counter electrode was the cathode. In the case that the cell did
not contain both a TMCCC cathode and a TMCCC anode, a capacitive
activated charcoal counter electrode was used to complete the
circuit while allowing the study of a single TMCCC electrode.
[0148] Electrochemical characterization of electrodes was performed
using cyclic voltammetry (CV) and galvanostatic cycling with
potential limitation (GCPL). During the CV technique, the potential
of the working electrode is swept at a constant rate between high
and low cutoff potentials, and the resulting current into or out of
the electrode is measured. During the GCPL technique a constant
current is applied to the cell until the working electrode reaches
a maximum or minimum potential; upon reaching this potential
extreme, the sign of the current is reversed.
[0149] Researchers have used TMCCCs as battery electrodes in cells
containing aqueous and organic electrolytes For example, the
reversible reduction of Prussian Blue to Everitt's Salt has allowed
its use as an anode in aqueous cells. However, the electrochemical
potential of Prussian Blue is relatively high, so using it as an
anode with a TMCCC cathode results in a low full cell voltage of
0.5-0.7 V vs. SHE. Such low voltages make these cells impractical,
as many cells in series would be required to achieve the high
voltages needed for many applications.
[0150] Chromium hexacyanochromate (CrHCCr) has also been used as an
anode in full cells that also contained Prussian Blue cathodes, and
an aqueous/Nafion electrolyte. The performance of these cells was
limited by the low potential and poor coulombic efficiency of
CrHCCr in aqueous electrolytes and the use of acidic electrolytes
in which CrHCCr hydrolyzes.
[0151] TMCCC anodes containing electrochemically active
hexacyanomanganate groups have also been recently demonstrated.
Examples include manganese hexacyanomanganate (MnHCMn), and zinc
hexacyanomanganate (ZnHCMn). In hexacyanomanganate-based TMCCC
anodes, the hexacyanomanganate groups undergo two electrochemical
reactions. First, Mn.sup.III(CN).sub.6 can be reversibly reduced to
Mn.sup.II(CN).sub.6 at potentials near or above 0 V vs. SHE.
Second, Mn.sup.II(CN).sub.6 can be reduced to Mn.sup.I(CN).sub.6 at
lower potentials, typically below -0.4 V vs. SHE. In general, the
lower reaction cannot be efficiently used in aqueous electrolytes
due to the simultaneous generation of hydrogen gas at such low
potentials. One exception is chromium hexacyanomanganate (CrHCMn),
which has a lower reaction potential of about -0.35 V, but
high-purity CrHCMn is extraordinarily difficult to synthesize due
to its affinity to form other phases such as mixed cyanides and
oxides of chromium. In no prior art has the lower reaction of any
hexacyanomanganate-based TMCCC been used with high coulombic
efficiency in aqueous electrolytes.
[0152] Though the use of a basic electrolyte would result in a
lower potential for the onset of H.sub.2 generation, TMCCCs rapidly
decompose at high pH except in the presence of an excess of free
cyanide anions, which are a severe safety hazard. Mildly acidic or
neutral electrolytes are needed for them to be stable. Thus, only
the upper reaction of MnHCMn can be used without deleterious
H.sub.2 production. As the upper stability limit of these aqueous
electrolytes is near 1 V, MnHCMn can be paired with a cathode such
as CuHCF to produce a battery with an average full cell voltage of
about 0.9-1 V.
[0153] TMCCCs have also been used as cathodes, but not as anodes,
in organic electrolyte batteries. Most commonly, they have been
used as cathodes in place of the standard LiCoO.sub.2 cathode found
in high-voltage organic electrolyte Li-ion cells. A number of
studies have demonstrated TMCCCs containing electrochemically
active iron and/or manganese as cathodes in these high voltage
cells.
[0154] TMCCCs have not been previously used as battery electrodes
in cosolvent electrolytes in which water is a minority cosolvent.
In recently published patent application, we described the
opportunity to do so for the specialized case of water acting as
the primary cosolvent. However, a practical cosolvent had not yet
been identified, and the cosolvent electrolytes described in that
document decompose into multiple phases under some circumstances,
making them impractical for use in an actual battery. In addition,
in that previous work, the idea of a cosolvent was described and
claimed in the context of an organic liquid additive to a water,
with water as the primary solvent of the electrolyte. Herein we
describe for the first time the principles for selecting cosolvents
and electrolyte salts to combine with water to produce stable,
single phase aqueous cosolvent electrolytes in which TMCCC
electrodes operate with high efficiency, fast kinetics, and long
lifetime. In addition, we demonstrate for the first time the
operation of TMCCC electrodes in aqueous cosolvent electrolytes in
which water is a minority solvent of the electrolyte, and an
organic solvent is the primary solvent.
[0155] U.S. Patent Application No. 61/722,049 filed 2 Nov. 2012
includes a discussion of various electrolyte additives to aqueous
electrolytes, as well as coatings on the electrodes of
electrochemical cells, that can improve a rate of capacity loss.
U.S. Patent Application No. 61/760,402 filed 4 Feb. 2013 includes a
discussion of a practical TMCCC anode. Both of these patent
applications are hereby expressly incorporated in their entireties
by reference thereto for all purposes.
[0156] Herein we discuss and demonstrate for the first time the use
of a practical aqueous cosolvent electrolyte for batteries
containing a TMCCC anode and a TMCCC cathode. The use of an organic
liquid as the primary solvent, with water as a minority solvent has
no significant effects on the kinetics or reaction potentials of
either TMCCC anodes or TMCCC cathodes as compared to the
performance of those electrolytes in aqueous electrolytes
containing no organic solvents. In addition, the cosolvent
stabilizes TMCCCs against dissolution and hydrolysis, resulting in
greater electrode stability and longer cycle and calendar life.
[0157] Our previous demonstration of the operation of TMCCC
cathodes in cosolvent electrolytes did not demonstrate the use of
an organic solvent as the majority electrolyte, and it considered
only the effect of an organic minority cosolvent on the performance
of TMCCC cathodes without showing the reduction to practice of a
full cell containing a cosolvent electrolyte. Furthermore, it did
not address the extreme sensitivity of hexacyanomanganate-based
TMCCC anodes to electrolyte composition. For these reasons, among
others, the work described here is novel and independent.
[0158] The addition of an organic cosolvent as the majority
component to the battery electrolyte is especially important for
the performance and lifetime of TMCCC anodes. Whereas without any
cosolvents, the upper reaction of the MnHCMn anode must be used in
aqueous electrolytes, here we show that the addition of a cosolvent
to the electrolyte suppresses electrolysis of water to hydrogen
gas. In a full cell also containing a CuHCF cathode, the result in
an increase in average discharge voltage from about 0.9 V to about
1.7 V (FIG. 1). Nearly doubling the cell voltage has extraordinary
ramifications for the performance and cost of the battery. Energy
scales proportionally with voltage, while power scales with the
square of the voltage. Thus, nearly doubling the voltage while
using the same electrode materials results in about twice the
energy and nearly four times the power, at about the same materials
cost. Without the presence of a cosolvent that limits the rate of
hydrogen production at the anode, cells with a TMCCC anode and
cathode cannot achieve high efficiency at voltages above about 1.3
V. Thus, the addition of the cosolvent increases the maximum
practical voltage, energy, and power of the cell.
[0159] Prior study of TMCCCs in organic electrolytes did include
the use of organic cosolvent electrolytes in some cases. However,
in anhydrous conditions, the kinetics of TMCCC electrodes are
vastly reduced, making these electrodes impractical for high power
applications. In this work, we demonstrate for the first time the
use of aqueous cosolvent electrolytes containing non-negligible
amounts of water. That water must be present for the TMCCC
electrodes to be rapidly charged or discharged.
[0160] As a first example, acetonitrile (also known as methyl
cyanide, or MeCN) is chosen as a cosolvent to be used in
electrolytes for batteries containing TMCCC electrodes. MeCN is
fully miscible with water and is electrochemically stable over a
much wider potential range than water itself. High purity,
anhydrous MeCN is used in commercial ultracapacitors. Here,
reagent-grade MeCN was used, as low voltage cells are less
sensitive to electrolyte impurities that may result in parasitic
side reactions at extreme potentials.
[0161] The choice of MeCN provides an additional benefit for the
specific case of a battery containing TMCCC electrodes. In a
cosolvent electrolyte containing MeCN as the primary solvent, the
solvation shells of the TMCCC electrode particles will primarily be
cyanide groups in which nitrogen faces the particle. This completes
the six-fold nitrogen coordination of P-site cations in the
particle at the surface or adjacent to hexacyanometalate vacancies.
The result is improved material stability via suppression of
dissolution via the formation of a hydration shell.
[0162] Other examples of organic solvents include ethylene
carbonate, propylene carbonate, and dimethyl carbonate; sulfolane;
1,3 dioxolane; propylene glycol monoethyl ether;
hydroxypropionitrile; diethylene glycol; gamma-valerolactone;
acetone; ethylene glycol and glycerol. Organic cosolvents must be
polar to allow them to form miscible single phase solutions with
water and a salt, but they may be either protic or aprotic.
[0163] It is desirable when using hexacyanomanganate-based TMCCC
anodes to use water as only a minority cosolvent, and organic
liquids as the primary cosolvents. The manganese-carbon bond in
hexacyanomanganate is labile and cyanide can be replaced by water
and/or hydroxide. The choice of a larger, less polar organic
species as the primary solvent results in weaker bonding to Mn and
steric hindrance, both of which protect the hexacyanomananate group
from suffering ligand exchange leading to its decomposition.
[0164] Proper selection of the electrolyte cosolvents, salts, and
any additional additives will result in a single-phase system in
which all of the components are miscible and do not phase
segregate. Phase segregation in a battery electrolyte is
undesirable because ion transport will occur primarily in the phase
containing the higher salt concentration, while the other, less
conductive phase or phases will impede the transport of ions. It is
not enough to simply choose liquids that are miscible, as the
addition of a salt can lead to decomposition of the electrolyte
into multiple phases: for example, one that is mostly water, that
has a high salt concentration, and that contains a small amount of
the organic solvent, and a second phase that is mostly organic
solvent, and contains little water or salt leads to poor
performance when there is phase segregation, a problem addressed by
proper selection of electrolyte cosolvents.
[0165] A very limited number of common electrolyte salts that are
highly water soluble are also appreciably soluble in organic
solvents. This is because most organic solvents have dielectric
constants much lower than that of water. In other words, organic
solvents are typically not as polar as water, so the formation of a
solvation shell during the dissolution of an ionic salt is not
energetically favorable. For example, potassium nitrate, which has
a saturation of 3.6 M in water at room temperature, is only
sparingly soluble in most organic solvents.
[0166] Here, to demonstrate a reduction to practice of the
operation of TMCCC electrodes in cosolvent electrolytes containing
an organic primary solvent, an embodiment may use sodium
perchlorate hydrate as the electrolyte salt in cosolvent
electrolytes of water and MeCN. The choice of NaClO.sub.4.H.sub.2O
is based on its ability to dissolve in high concentrations (greater
than 1 M) over the entire range of cosolvent ratios from 100%/0%
water/MeCN to 0%/100% water/MeCN without forming biphasic
systems.
[0167] The ternary phase diagrams describing the solubility of
salts such as NaClO.sub.4 in cosolvents such as water/MeCN are
tabulated. The general need for high salt concentration and a
monophasic electrolyte can be used to select other combinations of
salts and cosolvents from these data.
[0168] Other cosolvents besides acetonitrile that can be used with
water in electrolytes for use in batteries containing TMCCC anodes
include, but are not limited to, methanol, ethanol, isopropanol,
ethylene glycol, propylene glycol, glycerine, tetrahydrofuran,
dimethylformamide, and other small, polar linear and cyclic
alcohols, polyols, ethers, and amines. However, while many of these
solvents are fully miscible with pure water, they are not miscible
in the presence of concentrated salt. For example, more than a few
percent isopropyl alcohol will phase-segregate from concentrated
aqueous salts of sodium, which this will not occur if acetonitrile
is used in the place of isopropyl alcohol. A proper selection of
the cosolvents and the salt will result in a single-phase
solution.
[0169] CuHCF was synthesized as reported previously. An aqueous
solution of Cu(NO.sub.3).sub.2, and a second aqueous solution of
K.sub.3Fe(CN).sub.6 were added to water by simultaneous, dropwise
addition while stirring. The final concentrations of the precursors
were 40 mM Cu(NO.sub.3).sub.2 and 20 mM K.sub.3Fe(CN).sub.6. A
solid, brown precipitate formed immediately. It was filtered or
centrifuged, washed, and dried. In a prior study, CuHCF synthesized
by this method was found to have the composition
K.sub.0.7Cu[Fe(CN).sub.6].sub.0.7.2.8H.sub.2O. The CuHCF was found
to have the cubic Prussian Blue open framework crystal structure
using X-ray diffraction (XRD). The CuHCF was composed of
nanoparticles about 50 nm in size, as verified by scanning electron
microscope (SEM).
[0170] MnHCMn was produced state by adding a 10 mL aqueous solution
containing 0.0092 mmol KCN to a 10 mL aqueous solution containing
0.004 mmol MnCl.sub.2.4H.sub.2O under constant stirring in the dark
in a nitrogen atmosphere. After stirring the solution for 20
minutes, the resulting dark green precipitate was centrifuged,
washed with methanol, and dried at room temperature in a nitrogen
atmosphere. Analysis of this material using X-ray diffraction
showed that it had the monoclinic crystal structure characteristic
of MnHCMn(II) synthesized by a similar method Composition analysis
using inductively coupled plasma optical emission spectrometry
(ICP-OES) revealed that this material was
K.sub.0.4Mn[Mn(CN).sub.6].sub.0.6.nH.sub.2O (0<n<4).
[0171] Aqueous cosolvent electrolytes were prepared from
reagent-grade NaClO.sub.4.H.sub.2O, de-ionized water, and reagent
grade MeCN. All electrolytes were pH-neutral, but not buffered. The
salt was dissolved in a concentration of 1 M in cosolvents with
solvent volume ratios of 100%/0%, 90%/10%, 50%/50%, 10%/90%, and
0%/100% water/MeCN.
[0172] Electrodes containing the freshly synthesized TMCCCs were
prepared as reported previously. The electrochemically active
material, carbon black, and polyvinylidene difluoride (PVDF) binder
were ground by hand until homogeneous, and then stirred in
1-methyl-2-pyrrolidinone (NMP) solvent for several hours. This
slurry was deposited on an electronically conductive carbon cloth
substrate using a doctor blade or spatula. Other substrates
including foils and meshes of stainless steel and aluminum can also
be used. These electrodes were dried in vacuum at 60.degree. C. For
practical batteries, the binder is preferably selected such that it
is stable against dissolution or excessive swelling in the
cosolvent electrolyte, but is still fully wetted by the cosolvent.
Methods for determining binder/electrolyte compatibilities such as
Hansen Solubility Parameter analysis are well known.
[0173] Activated charcoal counter electrodes were prepared by
grinding the charcoal with PVDF before stirring in NMP for several
hours, followed by deposition and drying on conductive substrates
following the same procedure as in the case of electrodes
containing a TMCCC.
[0174] Electrochemical Characterization
[0175] Half-cell measurements were performed on TMCCC electrodes in
cosolvent electrolytes. The cell contained the working electrode,
an Ag/AgCl reference electrode, an activated charcoal counter
electrode, and the deaerated electrolyte. Cyclic voltammetry was
performed on the working electrode.
Example 1
[0176] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by cyclic voltammetry.
The reaction potentials of the reactions of MnHCMn with 1 M
Na.sup.+ were found to be about -0.76 V and 0.04 V vs. SHE. The
potential of the lower reaction of MnHCMn varied only slightly with
the addition of MeCN to the electrolyte, from 0% MeCN to 95% MeCN
(FIG. 3-4). The magnitude and sign of the small shift in reaction
potential showed no trend with MeCN concentration (FIG. 3).).
Furthermore, MnHCMn was found to cycle reversibly in 95% MeCN at
with Na.sup.+ salt concentrations of both 1 M and 1.4 M.
Example 2
[0177] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by cyclic voltammetry.
MnHCMn was found to cycle reversibly in cosolvent electrolytes
containing water as a minority cosolvent comprising 5% of the total
solvent volume, and with equal quantities of MeCN and a second
organic cosolvent comprising the remaining 95% of the total solvent
volume (FIG. 5-6). These second organic cosolvents were one of:
sulfolane, propylene glycol monoethyl ether, hydroxypropionitrile,
gamma-valerolactone, ethylene carbonate, dimethyl carbonate, and
1,3-dioxolane. In these example electrolytes, the solvent volume of
MeCN is as little as 10%, with another primary organic cosolvent
such as propylene carbonate comprising 85% solvent volume. These
electrolyte compositions of matter demonstrate the use of multiple
organic cosolvents in combination with water as a minority
cosolvent.
Example 3
[0178] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by cyclic voltammetry.
Over 55 mAh/g of specific discharge capacity was achieved for the
lower reaction of MnHCMn in a cosolvent electrolyte of 1 M
NaClO.sub.4 in 90% MeCN and 10% water (FIG. 7). This is comparable
to the 50-60 mAh/g capacities typically achieved for the upper
reaction of MnHCMn at 0.05 V in aqueous electrolytes. With no loss
in specific capacity of the anode, but a gain in full cell voltage
of about 0.8 V, full cells that operate by using the lower reaction
of MnHCMn will have nearly double the energy of those that operate
by using the upper reaction of MnHCMn, with the same electrode
materials (and associated costs). This makes the use of the lower
reaction, and therefore, the use of a cosolvent electrolyte,
critically important to the economics and viability of the
battery.
Example 4
[0179] A CuHCF electrode was disposed in a half cell in the
configuration described above and operated by cyclic voltammetry.
The half cells contained electrolytes of 1 M NaClO.sub.4 and
quantities of water and acetone up to 90% acetone. During cyclic
voltammetry the reaction potential of CuHCF with 1 M Na.sup.+ was
observed to be centered at 0.84 V vs. SHE, which is consistent with
the previously observed value (FIG. 8). The reaction potential and
peak current hysteresis of CuHCF during CV varied only slightly, 1
M NaClO.sub.4 cosolvents containing increasing amounts of acetone
up to 90% of the total solvent volume.
Example 5
[0180] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by cyclic voltammetry.
In this example, the half cell contained an electrolyte of pure
MeCN and no water, and 1 M NaClO4. A much lower peak current of
MnHCMn was observed in MeCN electrolyte without water added as a
minority cosolvent (FIG. 9). In contrast, the CV curves shown in
FIG. 3, FIG. 4, and FIG. 8 show that there is little change in the
voltage difference between the peak currents in oxidation and
reduction. This qualitatively indicates that the kinetics of the
reaction of both MnHCMn and CuHCF with Na.sup.+ do not change in
the presence of MeCN, up to the case of a 95% MeCN primary solvent.
This example demonstrates that a minimum amount of water must be
present in the cosolvent electrolyte to allow reversible electrode
cycling that yields useful discharge capacity.
Example 6
[0181] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by cyclic voltammetry.
In this example, the half cell contained an electrolyte of either
pure water with no organic cosolvents and 1 M NaClO.sub.4, or of
95% solvent volume basis MeCN, with 5% solvent volume basis water
and 1 M or 1.4 M NaClO.sub.4 (FIG. 4). The background current
observed at -0.9 V vs. S.H.E. was approximately 1 mA in the aqueous
electrolyte containing no organic cosolvents. In the 95% volume
basis MeCN electrolytes, the background current at -0.9 V vs.
S.H.E. was less than 0.1 mA. Background current during a cyclic
voltammetry scan indicates a side reaction such as the
decomposition of water that harms coulombic efficiency. This
example demonstrates that the addition of a majority organic
cosolvent results in an improvement in the coulombic efficiency of
the MnHCMn anode.
Example 7
[0182] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by galvanostatic cycling
at a 1C rate between -0.9 V and -0.6 V vs. S.H.E. In aqueous 1 M
NaClO.sub.4 containing no organic cosolvents, the MnHCMn electrode
lost 25% of its initial specific discharge capacity after 15 cycles
(FIG. 10). However, in a cosolvent electrolyte of 1 M NaClO.sub.4
containing 90% solvent volume MeCN and 10% solvent volume water as
a minority cosolvent, less than 5% capacity loss was observed after
15 cycles. This demonstrates that the use of an organic cosolvent
as the majority cosolvent solvent and water as a minority cosolvent
significantly increases the cycle lifetime of the MnHCMn anode.
Example 8
[0183] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by galvanostatic cycling
at a 1C rate between -0.9 V and -0.5 V vs. S.H.E. In aqueous 1 M
NaClO.sub.4 containing no organic cosolvents, the MnHCMn electrode
had an initial discharge capacity of about 40 mAh/g. In 1 M NaClO4
containing 90% solvent volume MeCN and 10% solvent volume water as
a minority cosolvent, a specific discharge capacity of about 55
mAh/g was achieved. This demonstrates that the use of the organic
primary cosolvent prevents the decomposition of MnHCMn that can
result in significant, immediate capacity loss.
Example 9
[0184] A MnHCMn electrode was disposed in a half cell in the
configuration described above and operated by galvanostatic cycling
at a 1C rate between -0.95 V and -0.5 V vs. S.H.E. In aqueous 1 M
NaClO.sub.4 containing no organic cosolvents, the MnHCMn electrode
had coulombic efficiency of less than 99% (FIG. 12). In 1.4 M
NaClO.sub.4 containing 95% solvent volume MeCN and 5% solvent
volume water as a minority cosolvent, a coulombic efficiency of
over 99.5% was achieved in three identical cells.
Example 10
[0185] A CuHCF electrode was disposed in a half cell in the
configuration described above and operated by galvanostatic cycling
at a 1C rate. CuHCF loses 4% of its initial capacity after 50
cycles at a 1C rate in aqueous 1 M NaClO.sub.4 (FIG. 13). In
contrast, CuHCF is completely stable and shows zero capacity loss
after 300 cycles when operated in an electrolyte of 1 M NaClO.sub.4
containing 90% solvent volume MeCN as the primary cosolvent and 10%
solvent volume water.
Example 11
[0186] In this example, MnHCMn and CuHCF electrodes were disposed
as anode and cathode, respectively, in a full cell also containing
a reference electrode as described above. The electrolyte was 1 M
NaClO.sub.4 in 90% solvent volume MeCN and 10% solvent volume
water. These full cells were operated such that the anode was
controlled by the reference electrode as the working electrode. The
cathode was oversized such that the capacity of the anode limited
the capacity of the full cell. The MnHCMn anode was
galvanostatically cycled at 1C as the working electrode between
-0.9 V and -0.5 V vs. SHE. Highly reversible cycling of the full
cell is achieved in this primarily organic cosolvent electrolyte
(FIG. 14). Negligible capacity loss of either the CuHCF cathode or
the MnHCMn anode was observed for 30 cycles, as shown by the
consistent duration of each cycle shown in FIG. 13. This full cell
operates at an average voltage of 1.7 V, nearly double that of the
0.9 V cell achievable if the upper reaction of MnHCMn is used (FIG.
1, FIG. 7, and FIG. 15). As the electrode materials in these two
cells are identical, and only their mode of operation is changed,
the higher voltage cell offers nearly twice the energy at the same
materials cost. On a basis of the masses and densities of two TMCCC
electrodes, a 1.7 V cell will have a specific energy of 50 Wh/kg
and an energy density of 90 Wh/L.
[0187] FIG. 16 illustrates a representative secondary
electrochemical cell 1600 schematic having one or more TMCCC
electrodes disposed in contact with a cosolvent electrolyte as
described herein. Cell 1600 includes a negative electrode 1605, a
positive electrode 1610 and an electrolyte 1615 electrically
communicated to the electrodes. One or both of negative electrode
1605 and positive electrode 1610 include TMCCC as an
electrochemically active material. A negative current collector
1620 including an electrically conductive material conducts
electrons between negative electrode 1605 and a first cell terminal
(not shown). A positive current collector 1625 including an
electrically conductive material conducts electrons between
positive electrode 1610 and a second cell terminal (not shown).
These current collectors permit cell 1600 to provide electrical
current to an external circuit or to receive electrical
current/energy from an external circuit during recharging. In an
actual implementation, all components of cell 1600 are
appropriately enclosed, such as within a protective housing with
current collectors externally accessible. There are many different
options for the format and arrangement of the components across a
wide range of actual implementations, including aggregation of
multiple cells into a battery among other uses and
applications.
[0188] Electrolyte 1615, depending upon implementation, includes a
set of conditions that affect production of hydrogen and oxygen gas
responsive to the operating voltages of the electrodes. In general,
at a first electrode voltage V1 relative to a reference electrode,
initiation of more than an incidental quantity of hydrogen gas will
begin to be produced at a particular rate R1 that is consequential
for the particular application. Pure water, under comparable
conditions, begins the production of hydrogen gas at rate R1 using
a second electrode voltage V2 that is greater than V1 (as shown in
FIG. 1, this voltage is less negative). Cell 1600 may be operated
at an electrode voltage less than V2 but greater than V1 to achieve
a greater cell voltage between the electrodes while producing
hydrogen gas at second rate R2 less than R1.
[0189] Additives
[0190] Electrolytes may contain not only solvents and salts, but
additives as well. These additives may be included for many
reasons, including but not limited to: to improve the calendar or
cycle life of one or both of the electrodes, to change the
coulombic efficiency of one or both of the electrodes, to change
the rate of electrochemical oxidation or reduction of one or both
of the electrodes, to prevent chemical or electrochemical
decomposition of one or more other electrolyte components, to
prevent chemical dissolution or other chemical reactions of one or
both of the electrodes with the electrolyte or its components, to
change the ionic conductivity, viscosity, or transference numbers
of the electrolyte, to change the surface tension of the
electrolyte, to change the wetting of at least one of the separator
and electrodes, to change the flammability or volatility of the
electrolyte, and to decrease the corrosion or degradation of at
least one cell component including the separator, electrodes,
current collectors, tabs, terminals, and packaging.
[0191] Electrolyte additives may be effective in concentrations as
low as nanomolar, or at somewhat higher concentrations such as
micromolar, or millimolar, up to concentrations of about 100 mM.
Any particular electrolyte additive might be most effective at a
particular concentration. Multiple additives might be added to the
same electrolyte in different concentrations. The most effective
concentrations of additives and combinations of additives might
vary with the salts and cosolvents also present in the
electrolyte.
[0192] Some electrolyte additives may be chemically active and
undergo a reaction with another chemical species in the cell, such
as with an electrolyte solvent, salt, or another additive, or such
as an electrode component such as an electrochemically active
electrode material, a binder, a conductive additive, another
electrode additive, or a current collector.
[0193] Electrolyte additives may be electrochemically active and
undergo electrochemical reactions in the cell. These
electrochemical reactions may occur on, in, or near one or both
electrodes. These reactions may be reversible or irreversible. They
may result in the production of a new species that is soluble in
the electrolyte, or in the production of an insoluble product on an
electrode surface or elsewhere in the cell, or in the production of
a gas. Additives may undergo an electrochemical reaction in which
one or more other species of the electrolyte or at least one
electrode are also reactants. These electrochemical reactions may
occur within the normal operating electrochemical potential ranges
of at least one of the electrodes, or they may occur outside of it
during overdischarge or overcharge of the cell. They may also occur
when the electrolyte is initially added to the cell and before the
cell is charged or discharged for the first time. Finally, they may
occur if the temperature of the cell exceeds its normal range, or
if foreign substances such as air enter the cell.
[0194] Chemical or electrochemical reactions of an electrolyte
additive may alter at least one of the composition, morphology,
crystal structure, chemical or electrochemical activity, or other
properties of at least one of the surface or bulk of one or more of
the electrodes, separator, current collectors, or other cell
components.
[0195] The additive or the chemical or electrochemical reaction
products of the additive may change one or more of the chemical or
physical properties of at least one other cell component such as
the solubility, reactivity towards other species in the cell,
thermal stability, thermal conductivity, ionic or electronic
conductivity, transference numbers of the electrolyte salt species,
viscosity, or other properties.
[0196] The chemical composition of an electrolyte additive is
limited only by its ability to be dissolved, suspended, dispersed,
or otherwise distributed in at least one region of the
electrolyte.
[0197] An electrolyte additive may include an inorganic component
such as a metal cation, including but not limited to alkali metal
cations (lithium, sodium, potassium, rubidium, etc.), alkaline
earth cations (beryllium, magnesium, calcium, etc.), aluminum,
transition metal cations (scandium, titanium, chromium, vanadium,
iron, manganese, nickel, cobalt, copper, zinc, zirconium, heavy
metal cations (cerium, lead, bismuth, etc.), or intermetallics
(gallium, tin, antimony, etc.). These metal cations may have a
valence state of one or more of 1+, 2+, 3+, 4+, or 5+.
[0198] An electrolyte additive may include an inorganic component
such as an anion, including but not limited to halogens (fluorine,
chlorine, bromine, iodine, etc.), polyatomic anions containing
oxygen including but not limited to sulfate, nitrate, perchlorate,
carbonate, phosphate, and borate, polyatomic anions containing
fluorine including but not limited to tetrafluoroborate,
hexafluorophosphate, hexafluoroarsenate, or other inorganic anions.
These anions may have a valence state of one or more of 1-, 2-, 3-,
or a more negative valence state.
[0199] An electrolyte additive may include a cation containing
nitrogen, including but not limited to ammonium, mono substituted
ammoniums, disubstituted ammoniums, trisubstituted ammoniums, or
tetrasubstituted ammonium. The substituted groups may include
linear, branched, or ringed hydrocarbon groups, or groups
containing alcohols, ketones, carboxylates, esters, nitriles, or
other functional groups. For example, an electrolyte additive might
include ethyl ammonium, or tert-butyl ammonium, or ethyl phenyl
ammonium, or tetrabutyl ammonium.
[0200] An electrolyte additive may include a polyatomic anion such
as a carboxylate (formate, acetate, oxalate, etc.), an alkoxide
(ethoxide, isopropoxide, butoxide, etc.), a thiol (decanethiol,
etc.), an amine (ethylenediamine, ethylenediaminetetraacetate)
[0201] An electrolyte additive may include a neutral organic
species such as a quinone (benzoquinone, hydroquinone, etc.), a
sulfone (dimethyl sulfone, ethyl methyl sulfone, etc.), a carbonate
(propyl carbonate, pentyl carbonate, vinyl carbonate, etc.), ethers
including crown ethers, ethylene glycol, polyethyleneglycol,
polypropyleneglycol
[0202] An electrolyte additive may include an organometallic
species such as an organometallic anion including but not limited
to hexacyanoferrate, pentacyanonitrosylferrate, hexacyanocobaltate,
hexacyanochromate, or a neutral organometallic species including
but not limited to ferrocene.
[0203] An electrolyte additive may include a surfactant such as a
cationic surfactant including but not limited to octenidine
dihydrochloride, cetrimonium bromide, cetylpyridinium chloride,
benzalkonium chloride, benzethonium chloride, distearyl dimethyl
ammonium chloride, dioctadecyldimethylammonium bromide, an anionic
surfactant including but not limited ammonium lauryl sulfate,
sodium dodecyl sulfate, sodium myreth sulfate, dioctyl
sulfosuccinate, perfluorooctane sulfonate, perfluorobutanesulfonic
acid, sodium dodecylbenzenesulfonate, or a zwitterionic surfactant
including but not limited to sultaines
(3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate) and
cocamidopropyl hydroxysultaine, betaines such as
cocamidopropylbetaines and phosphatidylethanolamine,
phosphatidylchloride and sphingomyelins
[0204] An electrolyte additive may include a reducing agent such as
an inorganic reducing agent including but not limited to sodium
thiosulfate, sodium dithionite, sodium hydrosulfine, sodium
borohydride, sodium cyanoborohydride, sodium triacetoxyborohydride,
potassium borohydride, nickel borohydride, potassium
tetrahydroborate, chromium (low valent), indium (low valent),
titanium (low valent), iron, phosphorous acid, hydrogen, hydrazine
and strontium or an organic reducing agent including but not
limited to formic acid, acetic acid, oxalic acid, malic acid,
citric acid, 3-mercaptopropionic acid, triphenylphosphite,
triphenylphosphine, trimethyl phosphine, triethylphosphine,
tributylstannane, tetramethyldisiloxane, sodium
hydroxymethanesulfinate, polymethylhydrosiloxane and
formaldehayde.
[0205] An electrolyte additive may also include an oxidizing agent
such as an inorganic oxidizing agent including but not limited to
iron(III), oxygen, oxone, ozone, osmium tetroxide, manganese (IV)
oxide, iodine, hydrogen peroxide, chromium trioxide, chlorine,
bleach, ammonium peroxydisulfate, ammonium cerium (IV) nitrate,
sodium ferricyanide, potassium permanganate, potassium
peroxydisulfate, selium oxide, sodium bromate, sodium nitrite,
sulfur, or an organic oxidizing agent including but not limited to
quinone, benzoquinone, benzaldehyde, benzyl peroxide,
N-Bromosuccinimide, ter-butyl hydroperoxide, tert-butyl nitrite,
tert-butyl hypochlorite, dimethylsulfoxide, peracetic acid,
pyridine N-oxide. FIG. 17-FIG. 30 illustrate seven pairs of charts
corresponding to Example A3-Example A9, each pair of charts
including an electrodes potential chart and a full cell voltage
chart.
Example A1 (Control with No Additives)
[0206] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. For all additive examples (A1-A9), the
first TMCCC material found in the first electrode had an
approximate composition of Na1.7Fe0.3Mn0.7[Fe(CN)6], and the second
TMCCC material found in the second electrode had an approximate
composition of Na2Mn2(CN)6. The electrolyte contained 1 M sodium
perchlorate in a solution of 75% sulfolane, 20% acetonitrile, and
5% water. The second electrode contained a larger quantity of
electrochemically active material than did the first electrode. By
applying constant positive or negative currents to the first
electrode, the cell was repeatedly charged to a high voltage, and
then discharged to a low voltage. Because the second electrode
contained an excess of TMCCC material, it was not fully charged by
the time the first electrode reached its full charge state. Then,
during discharge of the cell, the second electrode was
over-discharged because it had never reached its full charge state
during the preceding charging of the cell.
Example A2 (Control with No Additives)
[0207] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate in a solution of 75% sulfolane, 20% acetonitrile, and
5% water. The second electrode contained a smaller quantity of
electrochemically active material than did the first electrode. By
applying constant positive or negative currents to the first
electrode, the cell was repeatedly charged to a high voltage, and
then discharged to a low voltage. Because the first electrode
contained an excess of TMCCC material, it was not fully charged by
the time the second electrode reached its full charge state.
Example A3 (Cu(NO.sub.3).sub.2 Additive)
[0208] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate and a Cu(NO.sub.3).sub.2 additive in a 4:1 molar ratio
with respect to the second TMCCC material, in a solution of 75%
sulfolane, 20% acetonitrile, and 5% water. The second electrode
contained a smaller quantity of electrochemically active material
than did the first electrode. By applying constant positive or
negative currents to the first electrode, the cell was repeatedly
charged to a high voltage, and then discharged to a low voltage.
Reductive electroplating of the dissolved Cu2+ electrolyte additive
to form metallic copper on the surface of the second electrode
increased the electrochemical charge capacity of that electrode,
which allowed the first electrode to be charged to a higher
potential than as seen in Example A2. As a result, the voltage of
the cell was increased, thereby increasing the energy of the cell.
During subsequent charge-discharge cycling, the copper on the
surface of the second electrode catalyzed the decomposition of
water in the electrolyte, further increasing the effective charge
capacity of the second electrode. This allowed the first electrode
to be charged to higher voltages during successive charge-discharge
cycles, resulting in further increases to the cell energy as
charge-discharge cycling continued. FIG. 17-FIG. 18 illustrate a
first pair of charts for Example A3 comparing a control (no
additive) to a Cu(NO.sub.3).sub.2 additive, FIG. 17 illustrates an
electrode potentials chart for Example A3, and FIG. 18 illustrates
a cell voltage chart for Example A3.
Example A4 (Benzoquinone Additive)
[0209] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate and a benzoquinone additive in a 4:1 molar ratio with
respect to the second TMCCC material, in a solution of 75%
sulfolane, 20% acetonitrile, and 5% water. The second electrode
contained a smaller quantity of electrochemically active material
than did the first electrode. By applying constant positive or
negative currents to the first electrode, the cell was repeatedly
charged to a high voltage, and then discharged to a low voltage.
Reduction of the dissolved benzoquinone electrolyte additive by the
second electrode to form hydroquinone increased the electrochemical
charge capacity of that electrode, which allowed the first
electrode to be charged to a higher potential than as seen in
Example A2. As a result, the voltage of the cell was increased,
thereby increasing the energy of the cell. During subsequent
charge-discharge cycling, the second electrode continued to reduce
the remaining benzoquinone, further increasing the effective charge
capacity of the second electrode. This allowed the first electrode
to be charged to higher voltages during successive charge-discharge
cycles, resulting in further increases to the cell energy as
charge-discharge cycling continued. FIG. 19-FIG. 20 illustrate a
second pair of charts for Example A4 comparing a control (no
additive) to a Benzoquinone additive, FIG. 19 illustrates an
electrode potentials chart for Example A4, and FIG. 20 illustrates
a cell voltage chart for Example A4.
Example A5 (Hydroquinone Additive)
[0210] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate and a hydroquinone additive in a 4:1 molar ratio with
respect to the first TMCCC material, in a solution of 75%
sulfolane, 20% acetonitrile, and 5% water. The second electrode
contained a larger quantity of electrochemically active material
than did the first electrode. By applying constant positive or
negative currents to the first electrode, the cell was repeatedly
charged to a high voltage, and then discharged to a low voltage.
Oxidation of the dissolved hydroquinone electrolyte additive by the
first electrode to form benzoquinone increased the electrochemical
charge capacity of that electrode, which allowed the second
electrode to be charged to a lower potential than as seen in
Example A1. As a result, the voltage of the cell was increased,
thereby increasing the energy of the cell. During subsequent
charge-discharge cycling, the first electrode continued to oxidize
the remaining hydroquinone, further increasing the effective charge
capacity of the first electrode. This allowed the second electrode
to be charged to lower voltages during successive charge-discharge
cycles, resulting in further increases to the cell energy as
charge-discharge cycling continued. FIG. 21-FIG. 22 illustrate a
third pair of charts for Example A5 comparing a control (no
additive) to a Hydroquinone additive, FIG. 21 illustrates an
electrode potentials chart for Example A5, and FIG. 22 illustrates
a cell voltage chart for Example A5.
Example A6 (Ferrocene Additive)
[0211] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate and a ferrocene additive in a 4:1 molar ratio with
respect to the first TMCCC material, in a solution of 75%
sulfolane, 20% acetonitrile, and 5% water. The second electrode
contained a larger quantity of electrochemically active material
than did the first electrode. By applying constant positive or
negative currents to the first electrode, the cell was repeatedly
charged to a high voltage, and then discharged to a low voltage.
Oxidation of the dissolved ferrocene electrolyte additive by the
first electrode to form ferrocenium increased the electrochemical
charge capacity of that electrode, which allowed the second
electrode to be charged to a lower potential than as seen in
Example A1. As a result, the voltage of the cell was increased,
thereby increasing the energy of the cell. During subsequent
charge-discharge cycling, the first electrode continued to oxidize
the remaining ferrocene, further increasing the effective charge
capacity of the first electrode. This allowed the second electrode
to be charged to lower voltages during successive charge-discharge
cycles, resulting in further increases to the cell energy as
charge-discharge cycling continued. FIG. 23-FIG. 24 illustrate a
fourth pair of charts for Example A6 comparing a control (no
additive) to a Ferrocene additive, FIG. 23 illustrates an electrode
potentials chart for Example A6, and FIG. 24 illustrates a cell
voltage chart for Example A6.
Example A7 (Cu(NO.sub.3).sub.2 Additive)
[0212] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate and a Cu(NO.sub.3).sub.2 additive in a 1:1 molar ratio
with respect to the second TMCCC material, in a solution of 75%
sulfolane, 20% acetonitrile, and 5% water. The second electrode
contained a smaller quantity of electrochemically active material
than did the first electrode. By applying constant positive or
negative currents to the first electrode, the cell was repeatedly
charged to a high voltage, and then discharged to a low voltage.
Reductive electroplating of the dissolved Cu2+ electrolyte additive
to form metallic copper on the surface of the second electrode
increased the electrochemical charge capacity of that electrode,
which allowed the first electrode to be charged to a higher
potential than as seen in Example A2. As a result, the voltage of
the cell was increased, thereby increasing the energy of the cell.
During subsequent charge-discharge cycling, the copper on the
surface of the second electrode catalyzed the decomposition of
water in the electrolyte, further increasing the effective charge
capacity of the second electrode. This allowed the first electrode
to be charged to higher voltages during successive charge-discharge
cycles, resulting in further increases to the cell energy as
charge-discharge cycling continued. FIG. 25-FIG. 26 illustrate a
fifth pair of charts for Example A7 comparing a control (no
additive) to a Cu(NO.sub.3).sub.2 additive, FIG. 25 illustrates an
electrode potentials chart for Example A7, and FIG. 26 illustrates
a cell voltage chart for Example A7.
Example A8 (Oxalic Acid Additive)
[0213] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate and an oxalic acid additive in a 1:1 molar ratio with
respect to the second TMCCC material, in a solution of 75%
sulfolane, 20% acetonitrile, and 5% water. The second electrode
contained a smaller quantity of electrochemically active material
than did the first electrode. By applying constant positive or
negative currents to the first electrode, the cell was repeatedly
charged to a high voltage, and then discharged to a low voltage. An
electrochemical reaction of the dissolved oxalic acid electrolyte
additive with the second electrode increased the electrochemical
charge capacity of that electrode, which allowed the first
electrode to be charged to a higher potential than as seen in
Example A2. As a result, the voltage of the cell was increased,
thereby increasing the energy of the cell. During subsequent
charge-discharge cycling, the copper on the surface of the second
electrode catalyzed the decomposition of water in the electrolyte,
further increasing the effective charge capacity of the second
electrode. This allowed the first electrode to be charged to higher
voltages during successive charge-discharge cycles, resulting in
further increases to the cell energy as charge-discharge cycling
continued. FIG. 27-FIG. 28 illustrate a sixth pair of charts for
Example A8 comparing a control (no additive) to an Oxalic acid
additive, FIG. 27 illustrates an electrode potentials chart for
Example A8, and FIG. 28 illustrates a cell voltage chart for
Example A8.
Example A9 (Pyrrole Additive)
[0214] An electrochemical cell was prepared containing two
electrodes and an electrolyte. The first electrode contained a
first TMCCC material having a high electrochemical potential, and
the second electrode contained a second TMCCC material having a low
electrochemical potential. The electrolyte contained 1 M sodium
perchlorate and a pyrrole additive in a 0.4:1 molar ratio with
respect to the first TMCCC material, in a solution of 75%
sulfolane, 20% acetonitrile, and 5% water. The second electrode
contained a larger quantity of electrochemically active material
than did the first electrode. By applying constant positive or
negative currents to the first electrode, the cell was repeatedly
charged to a high voltage, and then discharged to a low voltage.
Oxidative polymerization of the pyrrole electrolyte additive at the
surface of the first electrode increased the electrochemical charge
capacity of that electrode, which allowed the first electrode to be
charged to a higher potential than as seen in Example A2. As a
result, the voltage of the cell was increased, thereby increasing
the energy of the cell. During subsequent charge-discharge cycling,
the copper on the surface of the second electrode catalyzed the
decomposition of water in the electrolyte, further increasing the
effective charge capacity of the second electrode. This allowed the
first electrode to be charged to higher voltages during successive
charge-discharge cycles, resulting in further increases to the cell
energy as charge-discharge cycling continued. FIG. 29-FIG. 30
illustrate a seventh pair of charts for Example A9 comparing a
control (no additive) to a Pyrrole additive, FIG. 29 illustrates an
electrode potentials chart for Example A9, and FIG. 30 illustrates
a cell voltage chart for Example A9.
[0215] As demonstrated in Examples A1-A9, more particularly in
Examples A3-A9, one or more side-reactions exist with an
additive-containing electrolyte and one or more electrodes coupled
to the electrolyte. These side-reactions may decrease coulombic
efficiency of charging an electrode (e.g., degrade it) and do not
appear in these examples to completely suspend charging for a
duration of the side-reaction. It may be the case that some
embodiments include an ability to suspend charging for some period,
such as a case where the side-reaction occurs with much greater
efficiency than the actual charging of the electrode. The
efficiency of the side-reaction may be increased or decreased as
desired by increasing or decreasing a concentration of the additive
in the electrolyte.
[0216] As noted herein, these side-reactions may be reversible or
irreversible. Further, these additives may part of an electrode or
electrode assembly. To be reversible, it may be necessary for the
additive to dissolve or disassociate into the electrolyte, diffuse
across the cell, and react with another electrode. In this fashion,
it may be possible in some embodiments to reset to its initial
redox state after consumption during the side-reaction.
[0217] These side-reactions may include chemical and
electrochemical reactions. The additive could undergo a chemical
reaction with one of the electrodes. For example, the additive
could bond to the surface of the electrode, and then act as a
catalyst for reactions with the electrolyte that decrease the
charging efficiency.
[0218] The additive could also, or in lieu of, undergo an
electrochemical reaction with one of the electrodes. For example,
an embodiment may add Cu.sup.2+ cations to the electrolyte, then
the additive may be reduced electrochemically on the surface of an
electrode to form copper metal: Cu.sup.2++2e-=Cu.
[0219] In the case of an additive that undergoes reversible
reactions, it may be more likely that these would be
electrochemical. For example, should an embodiment add a ferrocene
additive to the electrolyte, it could be stable at the anode, but
it may oxidize at the cathode to ferrocinium. That ferrocinium
would be stable at the cathode, but it would reduce back to
ferrocene at the anode. Then, that newly reduced ferrocene could
diffuse back to the cathode and be reoxidized to ferrocinium. This
cyclic oxidation and reduction of ferrocene at the two electrodes
may then continue for the entire duration of operation of the
cell.
[0220] In some secondary storage systems, there may be a desire to
improve coulombic efficiencies by suppressing undesired
side-reactions. In contrast, the embodiments described herein
intentionally add further inefficiencies in a manner that is
beneficial to the operation of the system. For example, one
discovered benefit is that in some embodiments it may be easier to
balance inefficiencies of two electrodes when one or more of the
inefficiencies are larger.
[0221] Regarding a duration of these side-reactions, as long as
there is a quantity of unconsumed additive present during charging,
the charging efficiency is degraded. In the cases of a reversible
side-reaction having sufficient additive to last an entire period
of charging, then each charging and discharging cycle may replenish
the additive and allow consistent repeatable charging degradation
for the entire operational life of the cell. Even in an embodiment
that includes a very large quantity of an additive that is consumed
by an irreversible reaction, it might take the entire first charge,
or even a number of successive charge/discharge cycles to consume
the additive fully. The additive may be periodically replenished
based upon the quantity, rate of consumption, and operation of a
device. In some designs for electrochemical devices, there may be a
port through which more electrolyte is added to the cell. An
embodiment of the present invention could implement or take
advantage of such a feature by "topping off" the cell with more
additive by adding an additive, an additive precursor, and/or an
electrolyte having such additive or additive precursor into the
device through the port.
[0222] While the mechanism for degrading charging efficiency
described herein has focused primarily on chemical and
electrochemical side-reactions, some embodiments may employ
something else in addition, or as an alternative. That something
else may include, for example, a case in which one electrode might
burn off charge by reacting with the electrolyte itself at very
high temperature. Or, it might undergo a spontaneous phase change
at very high temperature. Thus, heating might be a substitute for
an electrolyte additive. Other substitutes may also exist in some
embodiments to replace or supplement an additive.
[0223] FIG. 31 illustrates a set of charts for charging and
discharging under a set of different cases. In FIG. 31, there is a
representation of Potential (V) vs. Arbitrary Reference Electrode
as a function of time (T) and a representation of Full Cell Voltage
(V) as a function time (T) for four different cases: case 1, case
2, case 3, and case 4. Each case includes an electrode charge phase
(for a cathode and an anode) followed by an electrode discharge
phase. The cathode charge phase includes an initial state-of-charge
(A) charging to a maximum state-of-charge (B) with the cathode
discharge phase including the SOC B discharging to a final
state-of-charge (C). The anode charge phase includes an initial
state-of-charge (D) charging to a maximum negative state-of-charge
(E) with the anode discharge phase including the SOC E discharging
to a final state-of-charge (F). The full cell voltage illustrates a
full cell voltage at a beginning of the charge phase (G),
completion of the charging and beginning of discharging (H), and a
completion of the electrode discharge phase (I).
[0224] Case 1 represents an ideal case where the cell is able to
fully charge and discharge. In case 1, an electrolyte (or other
mechanism) does not limit cell charging. Both electrodes can fully
charge and discharge between +1.0 V and +1.5 V and -1.0 and -1.0 V
and -1.5 V, resulting in a full cell voltage that ranges between
2.0 V and 3.0 V with 100% capacity utilization. Case 2 represents a
non-ideal case with both electrodes starting at the same initial
state-of-charge and experiencing equal charge efficiencies. For
example, an electrolyte stability range is 2.8 V (not 3.0 V as in
case 1). When both electrodes start as the same SOC, then they
reach maximum SOCs of 80% and the cell experiences 80% capacity
utilization. 80% capacity utilization is optimum for this
electrolyte with this set of conditions. Case 3 represents a
non-ideal case with the electrodes starting at different SOCs and
experiencing equal charge efficiencies. The electrolyte stability
range is 2.8 V and the cathode starts at a higher SOC (40%) than
the anode (0%). With this set of conditions, the anode reaches a
maximum SOC of 60% when the cell is fully charged which reduces the
capacity utilization to 60%, which is less than optimum for this
cell (for example, compare to case 2). Case 4 represents a
modification to case 3 by inclusion of an additive to the electrode
that decreases a charging efficiency of the cathode by 50%. The
boundary and initial conditions of case 4 are the same as case 3
except for the selective modification to the cathode charging
efficiency (e.g., such as by use of a side-reaction as described
herein). In this case 4, the cathode charges from 40% to 80% SOC
while the anode charges from 0% to 80%, producing an optimum
capacity utilization for the cell by the use of the additive. Table
I below summarizes the specific case states-of-charge (SOC) and
cell utilization:
TABLE-US-00001 TABLE I SOC/Utilization by Case No. Capacity CASE A
(%) B (%) C (%) D (%) E (%) F (%) G (%) H (%) I (%) Util (%) 1 0
100 0 0 100 100 0 100 0 100 2 0 80 0 0 80 0 0 80 0 80 3 40 100 40 0
60 0 0 60 0 60 4 40 80 0 0 80 0 0 80 0 80
[0225] In case 1, the cell may reach a full charge at 3.0 volts,
with the cathode reaching a potential of 1.5 volts at full charge
and the anode reaching a potential of -1.5 volts at full charge,
with respect to the reference electrode. During a discharge of this
cell to 2.0 volts, the cathode may discharge fully to a potential
of 1.0 volts and the anode may discharge fully to a potential of
-1.0 volts with respect to the reference electrode.
[0226] In case 2 with a cell reaching only 2.8 volts, when both
electrodes have the same initial state of charge and equal charge
efficiencies, then the cathode may charge to 80% state of charge at
1.4 volts with respect to the reference electrode and the anode to
80% state of charge at -1.4 volts with respect to the reference
electrode. During a subsequent discharge, the cell may discharge
80% of its theoretical capacity before either of the two electrodes
reaches a state of charge of zero.
[0227] However, there instead may be an unbalanced charging that
limits the discharge capacity of the cell (case 3). In case 3, the
cathode may have an initial state of charge of 40% at 1.2 volts
with respect to the reference electrode, while the anode may have
an initial state of charge of zero at -1.0 volts with respect to
the reference electrode. In this case, the full cell may reach its
maximum voltage of 2.8 volts with the cathode fully charging to
100% state of charge 1.5 volts with respect to the reference
electrode, and the anode partially charging to 60% at -1.3 volts
with respect to the reference electrode (a relative state-of-charge
imbalance of 40% between the electrodes). During a subsequent
discharge, the cell may discharge only 60% of its theoretical
capacity before the anode reaches a state of charge of zero. In
case 3, the discharge capacity of the cell is not optimized for the
boundary condition of an electrolyte having a 2.8 volt stability
window because the anode has been charged to a lower state of
charge than the cathode when the cell voltage reaches 2.8 volts, so
there is less discharge capacity available.
[0228] The maximum discharge capacity is achieved only when the
states of charge of the cathode and anode are equal at the maximum
cell voltage. This may be achieved by use of a limited
side-reaction in which the coulombic efficiency of at least one
electrode is degraded. For example, consider case 4 in which the
cathode has an initial state of charge of 40% at 1.2 volts with
respect to the reference electrode, the anode has an initial state
of charge of zero, at -1.0 volts with respect to the reference
electrode, and in which an additive to the electrolyte degrades the
coulombic efficiency of the charging of the cathode to 50% of the
coulombic efficiency of the charging of the anode. In this case 4,
during charging of the cell the state of charge of the cathode may
increase from 40% to 80% while the state of charge of the anode may
increase from zero to 80%, at which point the cell reaches a
maximum voltage of 2.8 volts. During a subsequent discharge, the
cell may discharge 80% of its theoretical capacity before either
electrode reaches a state of charge of zero. In this case 4, the
addition of an electrolyte additive to degrade the coulombic
efficiency of charging of the cathode allowed the electrodes to
reach a balanced state of charge when the cell reached its maximum
voltage, which resulted in optimization of the discharge capacity
of the cell.
[0229] Some embodiments relate to a class of electrode materials
having stiff open framework structures into which hydrated cations
can be reversibly and rapidly intercalated from aqueous (e.g.,
water-based) electrolytes or other types of electrolytes. In
particular, open framework structures with the Prussian Blue-type
crystal structure afford advantages including greater durability
and faster kinetics when compared to other intercalation and
displacement electrode materials. A general formula for a TMCCC/PBA
class of materials is given by:
A.sub.xP.sub.y[R(CN).sub.6-jL.sub.j].sub.z.nH.sub.2O, where:
A is a monovalent cation such as Na.sup.+, K.sup.+, Li.sup.+, or
NH.sub.4.sup.+, or a divalent cation such as Mg.sup.2+ or
Ca.sup.2+; P is a transition metal cation such as Ti.sup.3+,
Ti.sup.4+, V.sup.2+, V.sup.3+, Cr.sup.2+, Cr.sup.3+, Mn.sup.+,
Mn.sup.2+, Mn.sup.3+, Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+,
Ni.sup.2+, Cu.sup.+, Cu.sup.2+, or Zn.sup.2+, or another metal
cation such as Al.sup.3+, Sn.sup.2+, In.sup.3+, or Pb.sup.2+; R is
a transition metal cation such as V.sup.2+, V.sup.3+, Cr.sup.2+,
Cr.sup.3+, Mn.sup.+, Mn.sup.2+, Mn.sup.3+, Fe.sup.2+, Fe.sup.3+,
Co.sup.2+, Co.sup.3+, Ru.sup.2+, Ru.sup.3+, Os.sup.2+, Os.sup.3+,
Ir.sup.2+, Ir.sup.3+, Pt.sup.2+, or Pt.sup.3+; L is a ligand that
may be substituted in the place of a CN.sup.- ligand, including CO
(carbonyl), NO (nitrosyl), or Cr; 0.ltoreq.x.ltoreq.2;
0<y.ltoreq.4; 0<z.ltoreq.1; 0.ltoreq.j.ltoreq.6; and
0.ltoreq.n.ltoreq.5; wherein j is typically zero but may have a
non-integer value when the PBA material includes a mixture of
multiple types of R(CN).sub.6-jL.sub.j groups, for example half
R(CN).sub.6 and half R(CN).sub.5L.sub.1, then the average in the
whole material is a non-integer j.
[0230] FIG. 32 illustrates a unit cell of the Prussian Blue crystal
structure. The unit cell of copper hexacyanoferrate, a TMCCC. In
this material and all other TMCCCs, transition metal cations are
linked in a face centered cubic framework by cyanide bridging
ligands. In this case, iron is six-fold carbon coordinated, while
copper is six-fold nitrogen coordinated. Each unit cell contains
eight smaller cubic subcells, at the center of which is a large
interstitice designated as the "A Site". The A Sites contain
zeolitic water and mobile alkali cations such as Na.sup.+ or
K.sup.+ or NH.sub.4.sup.+. During the electrochemical cycling of a
TMCCC, alkali cations are inserted or removed from the A Sites.
[0231] FIG. 33 illustrates an X-ray diffraction spectrum of CuHCF.
The fully indexed powder X-ray diffraction spectra of copper
hexacyanoferrate and Prussian Blue. Copper hexacyanoferrate has the
well-known face-centered cubic open framework structure of Prussian
Blue.
[0232] FIG. 34 illustrates a micrograph of CuHCF. Scanning electron
microscopy of copper hexacyanoferrate shows that the material is
composed of agglomerations of 20-50 nm grains. These agglomerations
can be as large as several microns.
[0233] FIG. 35 illustrates X-ray diffraction spectra of MnHCMn. The
powder X-ray diffraction spectrum of freshly synthesized, fully
reduced manganese(II) hexacyanomanganate(II), and of the same
material after partial oxidation. In the latter case, a
symmetry-breaking distortion in the framework structure is
eliminated during oxidation, forming the more common face-centered
cubic phase (as in FIG. 32 and FIG. 33).
[0234] FIG. 36 illustrates a micrograph of MnHCMn. Scanning
electron microscopy of manganese hexacyanomanganate, as synthesized
by a simple, one-step synthesis method.
[0235] FIG. 37 illustrates baseline/control electrochemical cycling
of CuHCF. The fractional capacity retention of copper
hexacyanoferrate during galvanostatic cycling at a 1C rate between
0.8 and 1.05 V vs. standard hydrogen electrode (SHE) in 1 M
KNO.sub.3 (pH=2) with a Ag/AgCl reference electrode and an
activated charcoal counter electrode. These two cells represent a
consistently observed loss of 7%/50 cycles under these
conditions.
[0236] FIG. 38 illustrates a UV-visible spectrum of CuHCF in water
and 1 M KNO.sub.3 pH=2. Ultraviolet-visible absorbance spectroscopy
of aqueous solutions that had contained 1 mg CuHCF electrode per 1
g of solution for 24 hours. The presence of concentrated K+
drastically reduces the soluble ferricyanide signal (peak at 420
nm).
[0237] FIG. 39 illustrates an ultraviolet-visible absorbance
spectrum of CuHCF in water and 10 mM Cu.sup.2+. The
ultraviolet-visible absorbance spectra of solutions that had
contained 1 mg CuHCF electrode per 1 g of solution for 24 hours.
The addition of dilute (10 mM) copper nitrate results in a
near-total elimination of the absorbance peak due to soluble
ferricyanide. This demonstrates that P.sup.m+ electrolyte additives
slow or prevent the dissolution of APR(CN).sub.6 TMCCCs.
[0238] FIG. 40 illustrates the cycle life of CuHCF in 1 M KNO.sub.3
pH=2 with and without Cu.sup.2+ added. Galvanostatic cycling of
copper hexacyanoferrate against a metallic copper anode in 1 M
KNO.sub.3/0.1 M Cu(NO.sub.3).sub.2 (pH=2) at a 1C rate results in a
steep initial capacity loss, followed by stabilization of the
electrode, with zero capacity loss between cycle 20 and cycle 25.
In contrast, the rate capacity loss observed in a control cell
containing an activated charcoal anode and no Cu(NO.sub.3).sub.2 in
the electrolyte is constant. After 20 cycles, the rate of the
continuing capacity loss is greater in the control cell than in the
cell containing the Cu(NO.sub.3).sub.2 electrolyte additive and the
Cu metal anode.
[0239] FIG. 41 illustrates galvanostatic cycling of
CuHCF/Cu.sup.2+/Cumetal in 2 sub-figures. FIG. 41a): The potential
profiles of the copper hexacyanoferrate cathode and the copper
anode, and the full cell voltage, during galvanostatic cycling at a
1C rate in 1 M KNO.sub.3 (pH=2) with 0.1 M Cu(NO.sub.3).sub.3
added. FIG. 41b) The same data, plotted as a function of the
specific capacity of the copper hexacyanoferrate cathode. Cycling
is highly reversible.
[0240] FIG. 42 illustrates cyclic voltammetry of CuHCF and PB/BG.
Cyclic voltammetry (scan rate 1 mV/s) of copper hexacyanoferrate
and Prussian Blue electrodes in 1 M KNO.sub.3 (pH=2) electrolyte.
The reaction potential of copper hexacyanoferrate is centered at
0.95 V, while the oxidation of Prussian Blue to Berlin Green is
centered at nearly 1.2 V. This means that copper hexacyanoferrate
can be fully oxidized before appreciable oxidation of the Prussian
Blue occurs. In the case of a Prussian Blue coating on a copper
hexacyanoferrate electrode, the electrode can be charged and
discharged without oxidizing the coating.
[0241] FIG. 43 illustrates capacity retention of PB/CuHCF and
CuHCF. Copper hexacyanoferrate electrodes that have been coated by
a thin film of Prussian Blue have improved capacity retention in
comparison to uncoated electrodes.
[0242] FIG. 44 illustrates capacity retention of CuHCF w/K.sup.+ in
PB dep solution. The capacity retention of copper hexacyanoferrate
does not improve if the electrodes are exposed to a solution
containing both Prussian Blue precursors and a more concentrated
potassium salt.
[0243] FIG. 45 illustrates potential profiles of CuHCF and Prussian
Blue-coated CuHCF electrodes. The potential profiles of bare and
Prussian Blue-coated copper hexacyanoferrate electrodes. The
coating does not have an appreciable effect on the electrochemical
behavior of the electrode.
[0244] FIG. 46 illustrates morphologies of bare and Prussian
Blue-coated CuHCF electrodes in two sub-figures: FIG. 46a)
illustrates scanning electron microscopy of a freshly deposited
slurry electrode of copper hexacyanoferrate (80%), carbon black
(10%), and polyvinylidene difluoride (10%) on a carbon cloth
substrate and FIG. 46b) illustrates the same sample, after
electrochemical reduction, followed by 40 minutes of exposure to a
2 mM aqueous solution of Fe(CN).sub.3 and K.sub.3Fe(CN).sub.6. A
film of Prussian Blue has clearly precipitated on the surface of
the sample, as the grains at the surface are larger and form a more
continuous surface than seen in FIG. 46a).
[0245] FIG. 47 illustrates cycle life of CuHCF with PB coating on
the particles. Fractional capacity retention of a standard copper
hexacyanoferrate electrode, and an electrode containing Prussian
Blue-coated copper hexacyanoferrate during galvanostatic cycling at
a 1C rate in 1 M KNO.sub.3 (pH=2). The use of a Prussian Blue
coating stabilizes the individual copper hexacyanoferrate particles
against dissolution.
[0246] FIG. 48 illustrates potential profiles of CuHCF with PB
coating on the particles in two sub-figures: FIG. 48a illustrates
the potential profiles of electrodes containing untreated copper
hexacyanoferrate, and copper hexacyanoferrate nanoparticles coated
with Prussian Blue, during galvanostatic cycling at a 1C rate in 1
M KNO.sub.3 (pH=2) and FIG. 48b illustrates Galvanostatic cycling
of an electrode containing Prussian-Blue coated copper
hexacyanoferrate nanoparticles over a wider potential range.
Prussian Blue is electrochemically active at 0.4 V vs. SHE. About
20% of the total capacity of the electrode occurs at low potential,
indicating a ratio of copper hexacyanoferrate to Prussian Blue of
about 4:1. This is in agreement with the 4:1 ratio of copper
hexacyanoferrate to Prussian Blue precursors added during the
coating treatment procedure.
[0247] FIG. 49 illustrates the cycle life of CuHCF with PPy coating
on the particles. An electrode containing polypyrrole-coated copper
hexacyanoferrate shows a completely stable capacity for at least 50
cycles at a 1C rate in 1 M KNO.sub.3 (pH=2). In comparison, a
control sample containing uncoated copper hexacyanoferrate loses
about 7% of its capacity after the same duration of cycling.
[0248] FIG. 50 illustrates potential profiles of CuHCF with PPy
coating on the particles. A large, irreversible charge capacity is
observed during the first charge of polypyrrole-coated copper
hexacyanoferrate. Cycling after the first charge is extremely
reversible. The reversible reaction centered at 0.95 V vs. SHE is
consistent with the one observed for uncoated copper
hexacyanoferrate, showing that the polypyrrole coating is inactive
in this potential range.
[0249] A battery (or cell) comprises an anode, a cathode, and an
electrolyte that is in contact with both the anode and the cathode.
Both the cathode and the anode contain electrochemically active
material that may undergo a change in valence state, accompanied by
the acceptance or release of cations and electrons. For example,
during discharge of a battery, electrons are extracted from the
anode to an external circuit, while cations are removed from the
anode into the electrolyte. Simultaneously, electrons from the
external circuit enter the cathode, as do cations from the
electrolyte. The difference in the electrochemical potentials of
the cathode and anode results in a full cell voltage. This voltage
difference allows energy to be extracted from the battery during
discharge, or stored in the battery during charge.
[0250] Prussian Blue is a well-known material phase of iron cyanide
hydrate of the chemical formula
K.sub.xFeIII[FeII(CN).sub.6].sub.z.nH.sub.2O (0.ltoreq.x,
0<z.ltoreq.1; n.apprxeq.4). This material has been produced
industrially for centuries for use as a pigment and dyestuff. It is
also a well-known electrochromic material, and has been studied for
use as a cathode in electrochromic displays. FIG. 32 illustrates
Prussian Blue as having a face-centered cubic crystal structure. In
this structure, cyanide bridging ligands link transition metal
cations in a spacious open framework. The structure contains large
interstitial sites commonly called the "A Sites." Each unit cell
contains eight A Sites, each of which may contain zeolitic water,
interstitial alkali cations, or both.
[0251] For example, copper hexacyanoferrate (CuHCF) is a TMCCC
recently demonstrated to be a high performance battery electrode.
In the open framework structure of CuHCF, iron is six-fold,
octahedrally coordinated to the carbon ends of the cyanide
branching ligands, while copper is octahedrally
nitrogen-coordinated as shown in FIG. 32. Depending on the method
of synthesis, the A sites in CuHCF may contain potassium or another
alkali cation such as sodium or lithium, or another type of cation
such as ammonium. More generally, for a TMCCC of the general
chemical formula A.sub.xP.sub.y[R(CN).sub.6].sub.z.nH.sub.2O,
alkali cations A.sup.+ and water occupy the interstitial A Sites,
transition metal P cations are six-fold nitrogen coordinated, and
transition metal R cations are six-fold carbon coordinated.
[0252] Herein the electrochemical cells used to test electrode
properties contained a TMCCC working electrode, a counter
electrode, an electrolyte in contact with both the anode and
cathode, and a Ag/AgCl reference electrode used to independently
measure the potentials of the anode and cathode during charge and
discharge of the cell. When the electrode of interest was a cathode
material, then the working electrode was the cathode, and the
counter electrode was the anode. When the electrode of interest was
an anode material, then the working electrode was the anode, and
the counter electrode was the anode. In the case that the cell did
not contain both a TMCCC cathode and a TMCCC anode, a capacitive
activated charcoal counter electrode was used to complete the
circuit while allowing the study of a single TMCCC electrode.
[0253] Several measurement and characterization techniques were
used to examine the materials and electrodes described here.
Physical characterization of TMCCC materials was performed using
X-ray diffraction (XRD) and scanning electron microscopy (SEM).
Electrochemical characterization of electrodes was performed using
galvanostatic cycling with potential limitation (GCPL). During the
GCPL technique a constant current is applied to the cell until the
working electrode reaches a maximum or minimum potential; upon
reaching this extreme potential, the sign of the current is
reversed.
[0254] In the application, sometimes a shorthand reference is made
to a "standard" method for materials synthesis. Those references
include this following discussion, sometimes the context includes a
modification or adjustment of a portion of this synthesis. CuHCF
was synthesized using existing techniques, such disclosed in
Wessells, C. D., et al. Copper hexacyanoferrate battery electrodes
with long cycle life and high power. Nature Comm., 2, 550 (2011).
An aqueous solution of Cu(NO.sub.3).sub.2, and a second aqueous
solution of K.sub.3Fe(CN).sub.6 were added to water by
simultaneous, dropwise addition while stirring. The final
concentrations of the precursors were 40 mM Cu(NO.sub.3).sub.2 and
20 mM K.sub.3Fe(CN).sub.6. A solid, brown precipitate formed
immediately. It was filtered or centrifuged, washed, and dried. In
a prior study, CuHCF synthesized by this method was found to have
the composition K.sub.0.7Cu[Fe(CN).sub.6].sub.0.7.2.8H.sub.2O. FIG.
33 illustrates that CuHCF was found to have the cubic Prussian Blue
open framework crystal structure using XRD. The CuHCF was composed
of nanoparticles about 50 nm in size, as verified by SEM as shown
in FIG. 34.
[0255] Manganese hexacyanomanganate (MnHCMn) was synthesized using
a single-step procedure such as disclosed in Her, J.-H., et al.
Anomalous Non-Prussian Blue Structures and Magnetic Ordering of
K.sub.2MnII[MnII(CN).sub.6] and Rb.sub.2MnII[MnII(CN).sub.6].
Inorg. Chem., 49, 1524 (2010). A 10 mL aqueous solution containing
0.5 g KCN was slowly added to a 10 mL aqueous solution containing
0.5 g of MnCl.sub.2 in a N.sub.2 atmosphere. A dark green
precipitate slowly formed. This precipitate was centrifuged,
washed, and dried with no exposure to air or oxygen. X-ray
diffraction of the freshly synthesized material revealed a
monoclinic structure indicative of a slight distortion to the
standard Prussian Blue open framework structure as shown in FIG.
35. After partial oxidation, the cubic phase was found to form.
This result indicates an approximate chemical formula
K.sub.2MnII[MnII(CN).sub.6].nH.sub.2O. SEM of FIG. 36 illustrates
that the MnHCMn was composed of 1-5 .mu.m agglomerations of
200-1000 nm particles.
[0256] Aqueous electrolytes were prepared from reagent-grade salts
such as KNO.sub.3 or NaClO.sub.4 and de-ionized water. These alkali
salt electrolytes are typically pH-neutral. For cases in which the
electrolytes were acidified, the pH was lowered using
HNO.sub.3.
[0257] Electrodes containing the freshly synthesized TMCCCs were
prepared using various techniques known in the art. The
electrochemically active material, carbon black, and polyvinylidene
difluoride (PVDF) binder were ground by hand until homogeneous, and
then stirred in 1-methyl-2-pyrrolidinone (NMP) solvent for several
hours. This slurry was deposited on electronically conductive
substrates such as aluminum foil or carbon cloth using a doctor
blade or spatula. These electrodes were dried in vacuum or a
N.sub.2 atmosphere at 60.degree. C.
[0258] Activated charcoal counter electrodes were prepared by
grinding the charcoal with PVDF before stirring in NMP for several
hours, followed by deposition and drying on conductive substrates
following the same procedure as in the case of electrodes
containing a TMCCC.
[0259] As a control test, CuHCF electrodes (5 mg CuHCF) were cycled
at a 1C rate (one hour charge or discharge) by GCPL between 0.8 and
1.05 V with respect to the standard hydrogen electrode (SHE) in a
cell that also contained a Ag/AgCl reference electrode, an
activated charcoal counter electrode, and 15 mL of aqueous 1 M
KNO.sub.3 (pH=2) electrolyte. FIG. 37 illustrates that about 7%
capacity loss is observed after 50 cycles.
Electrode Life Extension Method 1: P Electrolyte Additives
[0260] The dissolution of a TMCCC occurs by the following general
process: APR(CN).sub.6.fwdarw.A.sup.++P.sup.m++R(CN).sub.6.sup.n-
where A is an alkali cation, P and R are transition metal cations,
and n=-1(m+1). The dissolution process will continue until the
saturation limit of the dissolution products is reached. At this
chemical equilibrium, the thermodynamic driving force for further
dissolution is zero.
[0261] The thermodynamic driving force for a chemical process
occurring at constant temperature and pressure is the change in the
Gibbs Free Energy (.DELTA.G). It is related to the equilibrium
constant (Keq) of a reaction by the following expression:
.DELTA.G=-RTln(Keq), where R is the ideal gas constant and T is the
absolute temperature. The equilibrium constant for the dissolution
of a TMCCC is the product of the chemical activities of the
dissolution products, divided by the chemical activity of solid
TMCCC: Keq=(aAaPaR(CN).sub.6)/aAPR(CN).sub.6 where a.sub.i is the
chemical activity of the i.sup.th species. As .DELTA.G=0 and R and
T are nonzero constants, Keq=1, and therefore,
(aAaPaR(CN).sub.6)/aAPR(CN).sub.6=1 as well. In most conditions,
the chemical activity of a species can be approximated by the
concentration of that species, so
cAcPcR(CN).sub.6/cAPR(CN).sub.6=1, where c.sub.i is the
concentration of the i.sup.th species.
[0262] As cAcPcR(CN).sub.6/cAPR(CN).sub.6=1, the introduction of an
additional quantity of one or more species A.sup.+, P.sup.m+, or
R(CN).sub.6.sup.n- to the system must result in the precipitation
of APR(CN).sub.6 from dissolved A.sup.+, P.sup.m+, or
R(CN).sub.6.sup.n- until the equilibrium constant
Keq=cAcPcR(CN).sub.6/cAPR(CN).sub.6 is again equal to one. For
example, the dissolution of the CuHCF cathode is described by the
following expression:
KCuFe(CN).sub.6=K.sup.++Cu.sup.2++Fe(CN).sub.6.sup.3-, and the
corresponding equilibrium constant is
Keq=(cKcCucFe(CN).sub.6)/cCuHCF=1. Therefore, CuHCF will be less
soluble in a concentrated K.sup.+ electrolyte than in pure water,
as a higher cK must result in lower equilibrium cCu and
cFe(CN).sub.6. FIG. 38 illustrates confirmation of this result by
measurement of the dissolved ferricyanide concentration
(cFe(CN).sub.6) in either pure water or 1 M KNO.sub.3 (pH=2) by
ultraviolet-visible (UV-vis) absorption spectroscopy.
[0263] Following the same principle, the addition of either
P.sup.m+ or R(CN).sub.6.sup.n- to the electrolyte will also shift
the chemical equilibrium to favor less dissolution of the solid
APR(CN).sub.6 phase. In the case of the CuHCF cathode with a
Cu.sup.2+ electrolyte additive, this result has been confirmed by
both UV-vis spectroscopy and by electrochemical testing of
electrodes as illustrated in FIG. 39 and FIG. 40.
[0264] The same principles are valid for the case of the MnHCMn
anode. This material hydrolyzes rapidly in pure water or dilute
alkali salt solutions. However, it is much more stable, and
therefore capable of reversible electrochemical cycling, in
concentrated alkali salt solutions such as saturated sodium
perchlorate. Furthermore, enhanced stability is observed upon the
addition of Mn.sup.2+ to the electrolyte. A similar effect can also
be achieved by the addition of CN.sup.- anions to the electrolyte,
as their presence shifts the equilibrium towards MnHCMn, and away
from a hydrolyzed product and dissolved CN.sup.- anions. These
results for the stabilization of the MnHCMn anode, in combination
with those for the CuHCF cathode demonstrate that the general
concept of a P.sup.2+ electrolyte additive to enhance the stability
of a TMCCC applies to both anodes and cathodes.
Electrode Life Extension Method 2: Combination of P.sup.2+
Electrolyte Additive with P Metal Anode
[0265] In most previous studies of TMCCC battery electrodes,
another TMCCC or a capacitive carbon counter electrode was chosen.
In a 1983 patent, Itaya et al briefly describe the use of a TMCCC
cathode in combination with a metallic zinc anode in an aqueous
NH.sub.4Cl electrolyte. Metallic anodes operate by dissolution
during oxidation (discharge) and by electroplating of the metal
from cations in solution during reduction (charge).
[0266] The choice of a metallic anode P for use in an electrolyte
containing a P.sup.m+ additive and a TMCCC cathode of the general
formula APR(CN).sub.6 is advantageous for at least two reasons.
First, the presence of P.sup.m+ in the electrolyte stabilizes the
TMCCC cathode against dissolution. Second, the initial presence of
P.sup.m+ in the electrolyte allows the battery to start in a
discharged state. Without the addition of P.sup.m+ to the
electrolyte, no electrodeposition can occur at the anode.
[0267] FIG. 41 illustrates GCPL of the CuHCF cathode against a
metallic Cu anode in an electrolyte containing 100 mM
Cu(NO.sub.3).sub.2. Analogous systems include, but are not limited
to nickel hexacyanoferrate/Ni.sup.2+/Ni and zinc
hexacyanoferrate/Zn.sup.2+/Zn. Furthermore, the P.sup.m+/P anode
system need not match the transition metal cation found in the
TMCCC cathode. For example, the CuHCF cathode could be operated in
an electrolyte containing Zn.sup.2+ and a Zn metal anode.
Electrode Life Extension Method 3: Electroless Deposition of TMCCC
Coatings
[0268] A method for the stabilization of TMCCC electrodes against
dissolution is the use of a conformal coating that limits their
contact with water. However, for an electrode with a coating layer
to be useful, the coating must be conductive to alkali cations such
as Na.sup.+ and K.sup.+, or it will prevent the charge and
discharge of the TMCCC. Few materials systems are capable of rapid
Na.sup.+ or K.sup.+ conduction at room temperature.
[0269] Regular Prussian Blue is much less soluble than many of its
analogues. Also, reduced Prussian Blue analogues/TMCCCs have been
observed to be less soluble than oxidized ones. In addition,
electrochemical oxidation of mixed-valent KFeIIIFeII(CN).sub.6 to
Berlin Green (FeIIIFeIII(CN).sub.6) occurs at a higher potential
than the analogous oxidation of TMCCC cathodes such as CuHCF.
Therefore, if a CuHCF electrode is coated with a thin, conformal
film of reduced, insoluble Prussian Blue, the CuHCF electrode may
undergo electrochemical cycling as usual. If the Prussian Blue
coating is continuous and conformal, the CuHCF electrode will not
dissolve; however, the high ionic conductivity of Prussian Blue
allows the electrode to still operate at high rates.
[0270] Because the oxidation potential of Prussian Blue to Berlin
Green is higher than the oxidation potential of CuHCF, a film of
Prussian Blue can be easily deposited onto CuHCF by an electroless
reductive precipitation method.
[0271] The reduction of Berlin Green to Prussian Blue is analogous
to the reduction of the CuHCF cathode, as in each case,
carbon-coordinated iron in the framework crystal structure is
reduced from Fe.sup.3+ to Fe.sup.2+. Unlike Prussian Blue, fully
oxidized Berlin Green is sparingly soluble, so dilute solutions of
Fe.sup.3+ and Fe(CN).sub.6.sup.3- can be readily prepared.
Electroless deposition of Prussian Blue onto a low-potential
electrode (such as CuHCF) will occur from a dilute solution
containing Fe.sup.3+ and Fe(CN).sub.6.sup.3- if that electrode has
a potential below that of the reduction of Fe.sup.3+ to Fe.sup.2+
(V0=0.771 V vs. SHE). In the case of a CuHCF electrode, this occurs
by the following two-step mechanism:
K.sub.2CuFeII(CN).sub.6+xFe.sup.3+=K.sub.2-xCu[FeII(CN).sub.6].sub.1-x[F-
eIII(CN).sub.6].sub.x+x(Fe.sup.2++K.sup.+)
x(K.sup.++Fe.sup.2++Fe(CN).sub.6.sup.3-=KFeIIIFeII(CN).sub.6
[0272] It is reasonable to expect that the reduction of Fe.sup.3+
results in the formation of a thin film of Prussian Blue on the
surface of the CuHCF electrode, as there is widespread precedent
for the electrodeposition of Prussian Blue films by the same
mechanism: reduction of iron cations, and subsequent Prussian Blue
precipitation, from a dilute aqueous solution of Fe.sup.3+ and
Fe(CN).sub.6.sup.3+.
[0273] The oxidation of Prussian Blue to Berlin Green occurs at a
higher potential than the reaction potential of CuHCF (FIG. 42).
This means that CuHCF electrodes in electrical contact with
Prussian Blue can be cycled without oxidizing the Prussian Blue to
the more soluble Berlin Green. Therefore, a conformal coating of
insoluble Prussian Blue prevents the slow dissolution of CuHCF
electrodes.
[0274] This technique offers several advantages: electroless
deposition of Prussian Blue is fast and inexpensive; alkali ion
transport in Prussian Blue is extremely rapid; and the same
technique could be used to stabilize TMCCC anodes (in fact, it
could be used on any electrode family).
[0275] Methods: [0276] 1 cm.sup.2 slurry electrodes containing 5 mg
of CuHCF were prepared using the standard methods described herein.
[0277] An aqueous deposition solution of 2 mM Fe(NO.sub.3).sub.3
and 2 mM K.sub.3Fe(CN).sub.6 was prepared. [0278] As-synthesized
CuHCF has an open circuit potential near 1.05 V vs. SHE, too high
for electroless deposition of Prussian Blue from Fe.sup.3+ and
Fe(CN).sub.6.sup.3-. Thus, a preparative electrochemical reduction
to 0.7 V was performed by galvanostatic discharge at a 1C rate in 1
M KNO.sub.3 (pH=2). [0279] The discharged CuHCF electrodes were
washed, dried, and then placed in 10 mL of the deposition solution
for 30 minutes. They were then washed, dried, and inserted into
batteries for testing. [0280] Though the fresh electrodes were
black (due to the carbon in the slurry), after exposure to the
deposition, the electrodes appeared slightly bluish.
[0281] The CuHCF electrodes were cycled at 1C between 0.8 and 1.05
V against a Ag/AgCl reference electrode and an activated charcoal
counter electrode in 15 mL of 1 M KNO.sub.3 (pH=2).
[0282] Before exposure to the deposition solution, CuHCF electrodes
were discharged to 0.7 V vs. SHE. After deposition of the Prussian
Blue coating for 30 minutes, their open circuit potential was found
to be 0.85 V vs. SHE. From the previously reported galvanostatic
potential profile of CuHCF (FIG. 33) this corresponds to a charge
fraction of about 5% for the CuHCF, or 3 mAh/g based on its
specific capacity of 60 mAh/g. As each sample contained 5 mg CuHCF,
the total charge expended during Prussian Blue deposition was 15
.mu.Ah. From the 10.16 .ANG. lattice parameter of Prussian Blue and
the planar geometric area of the electrode, this total charge
corresponds to the deposition of a film with a thickness of 1.1
.mu.m. However, as the electrode is extremely rough with a larger
true surface area than its planar one, a true Prussian Blue coating
thickness of .about.500 nm is reasonable.
[0283] The deposition of a Prussian Blue coating consistently
improved the capacity retention of the CuHCF electrode. The
fractional capacity retention of two control samples and four
samples with Prussian Blue coatings is shown in FIG. 43. The
improvement of CuHCF capacity retention is reproducibly achieved
using the Prussian Blue coating step. Improving the completeness of
the coverage of the conformal Prussian Blue coating through
optimization of the coating procedure will further improve the
magnitude and reliability of the stabilizing effect of the coating
layer.
[0284] In some publications in which thin films of Prussian Blue
are electrodeposited, a supporting electrolyte such as 0.1 M KCl or
K.sub.2SO.sub.4 is used. This aids electrodeposition, as the ionic
conductivity of the solution is much higher in the presence of a
more concentrated salt. To determine whether or not a supporting
electrolyte enhances the electroless deposition of Prussian Blue
coatings on CuHCF electrodes, the coating step was performed in the
same 2 mM Fe.sup.3+/Fe(CN).sub.6.sup.3- solution, with 0.1 M
KNO.sub.3 added. At the end of the coating step, the electrodes
were washed and dried. Their color remained black, and did not show
evidence of a bluish tint. As shown in FIG. 34, there is no
improvement in the capacity retention.
[0285] FIG. 44 illustrates that the presence of excess K.sup.+ in
the deposition solution prevents the rapid growth of the Prussian
Blue film on CuHCF. This result can be qualitatively explained be
the presence of K.sup.+ on the right side of the first step of the
deposition mechanism. The presence of excess K.sup.+ shifts the
equilibrium to the left side, so the CuHCF would have to be reduced
to a lower open circuit potential to reduce Fe.sup.3+ to Fe.sup.2+
in the presence of excess K.sup.+.
[0286] Finally, the effect of the Prussian Blue coating step on the
potential profile of the CuHCF electrode was examined. As shown in
FIG. 45, there is no discernible difference between the shapes of
potential profiles of samples treated with the deposition solution
and fresh control samples.
[0287] The morphologies of bare and Prussian Blue-coated CuHCF
electrodes were examined using SEM (FIG. 46). The fresh sample is
composed of easily distinguished individual nanoparticles. However,
the coated sample is composed of nanoparticles that are bound
together in a continuous coating layer. Exposure of the CuHCF
electrode to the deposition solution results in the formation of a
conformal thin film of Prussian Blue. This film is directly
responsible for improved electrode lifetime during battery
operation because it acts as a barrier to CuHCF dissolution.
Electrode Life Extension Method 4: Combination of TMCCC Coating
with P.sup.2+ Electrolyte Additives
[0288] The method for electrode stabilization described in
electroless deposition of TMCCC Coatings is now generalized. Other
analogues besides Prussian Blue itself may be used as a protective
coating against dissolution for another TMCCC, and a protective
coating of the formula APR(CN).sub.6 may be used in combination
with a P.sup.m+ electrolyte additives and a P metal anode.
[0289] When the reduction potential of the APR(CN).sub.6 coating is
higher than the oxidation potential of the TMCCC to be protected,
then the same electroless deposition procedure as in the case of a
Prussian Blue coating can be used. For example, nickel
hexacyanoferrate (NiHCF) has a lower reaction potential than zinc
hexacyanoferrate (ZnHCF). Electroless deposition of a conformal
film of ZnHCF onto a NiHCF electrode will occur spontaneously if
that electrode is placed in a solution containing Zn.sup.2+ and
Fe(CN).sub.6.sup.3-:
K.sub.2NiFeII(CN).sub.6+Zn.sup.2++FeIII(CN).sub.6.sup.3-.fwdarw.KNiFeIII-
(CN).sub.6+KZnFeII(CN).sub.6
[0290] Or, in the case of materials not containing excess potassium
in the A sites in their structures:
K.sub.2Ni.sub.3[FeII(CN).sub.6].sup.2+3Zn.sup.2++2FeIII(CN).sub.6.sup.3--
.fwdarw.Ni.sub.3[FeIII(CN).sub.6].sup.2+K.sub.2Zn.sup.3[FeII(CN).sub.6].su-
p.2
[0291] This reaction occurs spontaneously because ZnHCF and other
TMCCCs are less soluble when reduced than when oxidized. Therefore,
for this reaction to yield a conformal thin film on the electrode,
but not the additional spontaneous precipitation of oxidized ZnHCF
particles, the concentrations of the Zn.sup.2+ and
FeIII(CN).sub.6.sup.3- precursors must be greater than the
saturation limit of reduced ZnHCF, but lower than the saturation
limit of oxidized ZnHCF. Or, more generally, the spontaneous
precipitation of a A1+xPR(CN).sub.6 film with a high reduction
potential onto a TMCCC electrode with a lower reduction potential
will occur if the precursors P.sup.m+ and R(CN).sub.6.sup.n- are
present in concentrations greater than the saturation limit of
reduced A1+xPR(CN).sub.6, but lower than the saturation limit of
oxidized A.sub.xPR(CN).sub.6.
[0292] The use of a Prussian Blue coating can be used in
combination with a Fe electrolyte additive, but it is by the
reduction potential of aqueous Fe.sup.3+ to Fe.sup.2+ at 0.771 V.
To avoid reversibly oxidizing and reducing the Fe in the
electrolyte, the potentials of the cathode and anode must both
remain below 0.771 V. For TMCCC cathodes with reaction potentials
higher than this, a different coating must be chosen. For example,
a coating of ZnHCF can be combined with a Zn.sup.2+ electrolyte
additive because Zn.sup.2+ cannot be oxidized further in aqueous
solutions. Furthermore, as the Zn.sup.2+ electrolyte additive can
be paired with a metallic Zn anode, a general cell of the following
form can also be constructed: A TMCCC cathode of the general
formula APR(CN).sub.6, coated by another TMCCC of the general
chemical formula AP'R'(CN).sub.6, with an electrolyte additive
P'.sup.m+ and a metallic anode P'. Combinations include a ZnHCF
coating/Zn.sup.2+ electrolyte additive/Zn metal anode and a NiHCF
coating/Ni.sup.2+ electrolyte additive/Ni metal anode. Furthermore,
a ZnHCF cathode or ZnHCF-coated cathode could be paired with a
Zn.sup.2+ electrolyte additive and a galvanized steel anode, as the
zinc in the galvanized surface layer would provide an adequate
charge capacity.
Electrode Life Extension Method 5: Coating of Individual TMCCC
Particles with a TMCCC Shell
[0293] A protective coating of insoluble Prussian Blue or a TMCCC
can be applied not only to entire electrodes, but to the individual
particles that compose the electrode. In one prior case, unrelated
to the use of TMCCCs as battery electrodes, nanoparticles of a
TMCCC were coated with a conformal shell of another TMCCC. The
advantage of this method of electrode stabilization is that if
performed correctly, every particle of electrochemically active
material has a conformal shell that prevents its dissolution.
However, a larger total mass of protective layer is needed because
of the larger surface area.
[0294] Copper hexacyanoferrate was synthesized as described herein.
Sodium thiosulfate (Na.sub.2S.sub.2O.sub.3), a reducing agent, was
added by dropwise to the solution containing the CuHCF
nanoparticles 15 minutes after their initial precipitation. The
Na.sub.2S.sub.2O.sub.3 was added in a 0.8:1 molar ratio to the
potassium hexacyanoferrate precursor used to make the CuHCF. During
this process, the color of the solution changed from brown to
purple. The low oxidation potential of Na.sub.2S.sub.2O.sub.3
results in the reduction of the CuHCF nanoparticles. After this
chemical reduction step, the electrochemical potential of the CuHCF
was below 0.771 V, low enough to spontaneously reduce Fe.sup.3+ to
Fe.sup.2+.
[0295] The chemically reduced CuHCF was centrifuged and washed with
water to remove excess Cu.sup.2+ left over from its precipitation.
It was then redispersed in pure water by sonication. Finally, by
dropwise addition, a Prussian Blue deposition solution of 10 mM
Fe(NO.sub.3).sub.3 and 10 mM K.sub.3Fe(CN).sub.6 was added to the
solution of reduced CuHCF particles. This solution was slowly added
until the molar ratio of the Fe(NO.sub.3).sub.3 and
K.sub.3Fe(CN).sub.6 to the hexacyanoferrate in the CuHCF reached
1:4. The solution changed color from purple to dark blue,
indicating that the Fe.sup.3+ was reduced to Fe.sup.2+, and that
the Fe.sup.2+ then reacted with the Fe(CN).sub.6.sup.3- to form
Prussian Blue. This process is analogous to the reduction of iron
that occurs during the exposure of electrodes containing CuHCF to
the Prussian Blue deposition solution described herein. The rest of
the electrode preparation method was the same as described
above.
[0296] The reduction by thiosulfate is necessary only because the
CuHCF was synthesized in a fully oxidized state, and its potential
was too high to reduce the Fe.sup.3+ to Fe.sup.2+. In the case that
some other TMCCC is chosen to be the coating layer (for example, to
be paired with a P.sup.2+ electrolyte additive and a P metal
anode), then the chemical reduction step of the electrode
nanoparticles may not be necessary, as described in the discussion
of a combination of TMCCC coating with P.sup.2+ electrolyte
additives for the case of a NiHCF electrode and a ZnHCF
coating.
[0297] As shown in FIG. 47, an electrode containing CuHCF particles
coated with Prussian Blue lost less than 1% of its capacity after
50 galvanostatic cycles at a 1C rate. In comparison, an electrode
containing uncoated CuHCF particles lost about 7% of its capacity
after the same duration of cycling. This result conclusively
demonstrates that the coating of individual particles with Prussian
Blue results protects them from dissolution in the battery
electrolyte.
[0298] The performance of the CuHCF electrode is similar with and
without a conformal Prussian Blue coating of the individual
particles. As shown in FIG. 48, the potential profiles of
electrodes containing bare and coated particles are similar (FIG.
48a).
[0299] Prussian Blue can be electrochemically reduced near 0.4 V
vs. SHE. The Prussian Blue coating is electrochemically active at
low potential (FIG. 17b), confirming that the coating treatment
indeed resulted in the successful deposition of Prussian Blue
coating. The ratio of the observed capacities of CuHCF to Prussian
Blue is about 4:1 between 0.2 and 1.05 V vs. SHE. This is
consistent with the 4:1 molar ratio of CuHCF to Prussian Blue
precursors present during the coating procedure.
Electrode Life Extension Method 6: Coating of TMCCC Particles with
Polymer Coatings by Redox Deposition
[0300] Other coatings besides TMCCCs may be used to protect a TMCCC
battery electrode (or its constituent particles) from dissolution.
Such a coating material must be conductive to cations such as
Na.sup.+ or K.sup.+ so that the electrode can be charged and
discharged, and it should not be significantly soluble in aqueous
electrolytes. If it has non-negligible solubility in aqueous
electrolytes, then its dissolution products must be
electrochemically inactive in the potential window of the anode and
cathode of the battery.
[0301] A variety of mixed conducting polymers including
polypyrroles and polythiophenes are known to intercalate cations
such as Nat They are insoluble in aqueous electrolytes. Therefore,
a conformal polymer coating can protect a TMCCC electrode from
dissolution.
[0302] For example, CuHCF was synthesized by the standard method
described herein. Pyrrole was then added by slow, dropwise addition
to the solution in a 1:2 mass ratio with respect to the CuHCF
already present. The solution immediately turned black, as the
pyrrole was oxidized to polypyrrole upon contact with the CuHCF
nanoparticles. The rest of the electrode preparation method was the
same as the standard method.
[0303] The use of a polypyrrole coating stabilizes the CuHCF
against dissolution in the battery electrolyte. As shown in FIG.
49, the capacity of an electrode containing polypyrrole-coated
CuHCF nanoparticles is completely stable for 50 galvanostatic
cycles at a 1C rate in 1 M KNO.sub.3 (pH=2). In comparison, a
control electrode containing untreated CuHCF loses about 7% of its
capacity during cycling under the same conditions.
[0304] The initial charge of polypyrrole-coated CuHCF shows a
large, irreversible capacity. However, the electrode is completely
stable in charge and discharge after the first few cycles. Little
difference is observed between the first discharge and the charge
and discharge during the 20th cycle as illustrated in FIG. 50.
[0305] In some embodiments having a specific chemical formula for
the PBA material, e.g., copper hexacyanoferrate or manganese
hexacyanomangate, depositing a coating may include polymerization
of polythiophene.
Other Stabilization Methods
[0306] Below are described several additional methods for the
stabilization of TMCCCs against dissolution, and therefore, the
extension of the operational life of TMCCC electrodes.
[0307] Complexation with amines: a variety of amines have been
shown to form strong complexes with hexacyanoferrate, and
therefore, can coordinate strongly to the surface of a TMCCC
particle to form a protective layer. These include simple diamines
such as ethylene diamine, and larger aromatic amines such as Nile
Blue. Furthermore, oxides of cyclic amines such as pyridine-n-oxide
can be used to coat TMCCC particles. Addition of one or more of
these amines during the synthesis of the TMCCC, or as an
electrolyte additive, will result in a conformal surface coating
layer that stabilizes the TMCCC against dissolution.
[0308] Anions of insoluble P2.sup.+ salts: soluble or trace-soluble
alkali cation salts such as sodium fluoride, carbonate, or oxalate
can be added during the synthesis of a TMCCC, or as an electrolyte
additive. During the dissolution of a TMCCC, the Pm.sup.+ cation
hydrates and leaves the surface of the particle to enter the
aqueous solution. Transition metal salts of anions such as
fluoride, carbonate, oxalate, and others are insoluble, and
therefore, will react with the Pm+ at the surface of the TMCCC
particle to form an insoluble coating layer.
[0309] Thin films of insoluble transition metal sulfides such as
CdS, Cu.sub.2S, MnS, and ZnS are commonly fabricated for
semiconductor devices including photodiodes. Deposition of these
films from aqueous solution can be easily accomplished by reaction
of a transition metal cation Pm.sup.+ with a sulfide precursor such
as thiourea, thiosulfate, or sodium sulfide. Many transition metal
sulfides are good sodium ion conductors, so a metal sulfide coating
of a TMCCC will protect it against dissolution while still allowing
it to react electrochemically. These metal sulfides are unstable
against hydrolysis at high potentials, and are most fit for use on
TMCCC anodes with reaction potentials near or below SHE.
[0310] Similarly, small molecules containing thiol groups can
coordinate to the transition metal cations Pm+ on the surface of a
TMCCC particle. Examples include simple thiols such as decanethiol,
and more complicated molecules such as cysteine.
[0311] Additionally, extremely thin (5-10 nm) conformal layers of
metal oxides such as Al.sub.2O.sub.3, SiO.sub.2, and TiO.sub.2 can
be readily grown on the surfaces of TMCCC nanoparticles using a
sol-gel decomposition process from organometallic precursors. For
example, in the case of SiO.sub.2, the slow addition of dilute
tetraethyl orthosilicate (silicon tetraethoxide) to the aqueous
solution containing newly synthesized TMCCC particles will result
in the hydrolysis as polymerization of SiO.sub.2 nanoparticles,
which form a thin, continuous film on the surface of the TMCCC
particle. These oxides are completely insoluble in water, so they
provide a robust barrier to dissolution of the TMCCC. In addition,
as they are extremely thin, and in some cases (such as
Al.sub.2O.sub.3) have good Na+ conductivity, they do not strongly
limit the transport of alkali cations in and out of the TMCCC
during electrochemical cycling.
[0312] In the discussion regarding additives and coatings, there
are descriptions of situations in which an additive to the
electrolyte "sticks" bonds, or otherwise attaches to a surface of
an electrode material and thereby form a coating. Additives are
described as situations in which soluble chemical components of the
electrode are added to the electrolyte which does not result in a
coating but does reduce/prevent dissolution. There are also complex
situations in which a Prussian Blue coating is applied to an
electrode and then components of the coating are added to the
electrolyte to stabilize and resist dissolution which
slows/eliminates a rate of capacity loss. A coating of an electrode
may occur after an electrode is completely formed, or constituent
materials that will be used to form the electrode are coated before
the electrode is formed. The material(s) added to the electrolyte
that is/are used to form a coating (distinguished from additives as
a class of substances added to the electrolyte that directly
stabilize the electrode) are not referred to herein as additives.
These coating-forming material(s) bond to the surface of the
electrode to form the stabilizing coating.
[0313] In some situations, for example Pyridine-N-oxide and the
thiols and organic molecules that stick to a surface of an
electrode. These materials are classified herein as coatings and
not polymers as they do not bond together (polymerize) into big
polymer strands. A hybrid solution includes attachment of small
molecules onto a surface of the electrode, or electrode
constituents, and then polymerizing these small molecules all
together into a single polymer coating for the electrode.
[0314] When coating a PBA electrode with a conformal coating layer
of a PBA coating material, the disclosed embodiments preferably use
a different PBA for the coating than is used for the electrode. The
coating material is selected to be more stable than the electrode
material, and/or the coating material allows a use of A, P, or
R(CN).sub.6 electrolyte additives in way that is better (e.g., less
expensive, more stable, or the like). In these conformal coatings,
the "P" transition metal cations of the electrode and of the
coating may be the same or different cations, and when the same the
coating PBA has a different A and/or R(CN).sub.6 material.
[0315] In the case of additives, some embodiments provide for
electrodes having multiple P transition metal cations (e.g., P1 and
P2). The electrolyte may be presaturated with a P1 additive, a P2
additive, or both a P1 additive and a P2 additive. For more than 2
P transition metal cations in the electrode, all the different
permutations of one or more corresponding additive may be employed.
In some cases the electrolyte additive may be of a P transition
metal cation that is not present in an electrode of the system.
[0316] The system and methods above have been described in general
terms as an aid to understanding details of preferred embodiments
of the present invention. In the description herein, numerous
specific details are provided, such as examples of components
and/or methods, to provide a thorough understanding of embodiments
of the present invention. Some features and benefits of the present
invention are realized in such modes and are not required in every
case. One skilled in the relevant art will recognize, however, that
an embodiment of the invention can be practiced without one or more
of the specific details, or with other apparatus, systems,
assemblies, methods, components, materials, parts, and/or the like.
In other instances, well-known structures, materials, or operations
are not specifically shown or described in detail to avoid
obscuring aspects of embodiments of the present invention.
[0317] Reference throughout this specification to "one embodiment",
"an embodiment", or "a specific embodiment" means that a particular
feature, structure, or characteristic described in connection with
the embodiment is included in at least one embodiment of the
present invention and not necessarily in all embodiments. Thus,
respective appearances of the phrases "in one embodiment", "in an
embodiment", or "in a specific embodiment" in various places
throughout this specification are not necessarily referring to the
same embodiment. Furthermore, the particular features, structures,
or characteristics of any specific embodiment of the present
invention may be combined in any suitable manner with one or more
other embodiments. It is to be understood that other variations and
modifications of the embodiments of the present invention described
and illustrated herein are possible in light of the teachings
herein and are to be considered as part of the spirit and scope of
the present invention.
[0318] It will also be appreciated that one or more of the elements
depicted in the drawings/figures can also be implemented in a more
separated or integrated manner, or even removed or rendered as
inoperable in certain cases, as is useful in accordance with a
particular application.
[0319] Additionally, any signal arrows in the drawings/Figures
should be considered only as exemplary, and not limiting, unless
otherwise specifically noted. Furthermore, the term "or" as used
herein is generally intended to mean "and/or" unless otherwise
indicated. Combinations of components or steps will also be
considered as being noted, where terminology is foreseen as
rendering the ability to separate or combine is unclear.
[0320] The foregoing description of illustrated embodiments of the
present invention, including what is described in the Abstract, is
not intended to be exhaustive or to limit the invention to the
precise forms disclosed herein. While specific embodiments of, and
examples for, the invention are described herein for illustrative
purposes only, various equivalent modifications are possible within
the spirit and scope of the present invention, as those skilled in
the relevant art will recognize and appreciate. As indicated, these
modifications may be made to the present invention in light of the
foregoing description of illustrated embodiments of the present
invention and are to be included within the spirit and scope of the
present invention.
[0321] Thus, while the present invention has been described herein
with reference to particular embodiments thereof, a latitude of
modification, various changes and substitutions are intended in the
foregoing disclosures, and it will be appreciated that in some
instances some features of embodiments of the invention will be
employed without a corresponding use of other features without
departing from the scope and spirit of the invention as set forth.
Therefore, many modifications may be made to adapt a particular
situation or material to the essential scope and spirit of the
present invention. It is intended that the invention not be limited
to the particular terms used in following claims and/or to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
any and all embodiments and equivalents falling within the scope of
the appended claims. Thus, the scope of the invention is to be
determined solely by the appended claims.
* * * * *