U.S. patent application number 14/616541 was filed with the patent office on 2015-06-04 for methods and electrolytes for electrodeposition of smooth films.
This patent application is currently assigned to BATTELLE MEMORIAL INSTITUTE. The applicant listed for this patent is Battelle Memorial Institute. Invention is credited to Xilin Chen, Fei Ding, Gordon L. Graff, Yuyan Shao, Wu Xu, Jiguang Zhang.
Application Number | 20150152566 14/616541 |
Document ID | / |
Family ID | 48901939 |
Filed Date | 2015-06-04 |
United States Patent
Application |
20150152566 |
Kind Code |
A1 |
Zhang; Jiguang ; et
al. |
June 4, 2015 |
METHODS AND ELECTROLYTES FOR ELECTRODEPOSITION OF SMOOTH FILMS
Abstract
Electrodeposition involving an electrolyte having a
surface-smoothing additive can result in self-healing, instead of
self-amplification, of initial protuberant tips that give rise to
roughness and/or dendrite formation on the substrate and/or film
surface. For electrodeposition of a first conductive material (C1)
on a substrate from one or more reactants in an electrolyte
solution, the electrolyte solution is characterized by a
surface-smoothing additive containing cations of a second
conductive material (C2), wherein cations of C2 have an effective
electrochemical reduction potential in the solution lower than that
of the reactants.
Inventors: |
Zhang; Jiguang; (Richland,
WA) ; Xu; Wu; (Richland, WA) ; Graff; Gordon
L.; (West Richland, WA) ; Chen; Xilin;
(Richland, WA) ; Ding; Fei; (Tianjin, CN) ;
Shao; Yuyan; (Richland, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Battelle Memorial Institute |
Richland |
WA |
US |
|
|
Assignee: |
BATTELLE MEMORIAL INSTITUTE
Richland
WA
|
Family ID: |
48901939 |
Appl. No.: |
14/616541 |
Filed: |
February 6, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13495727 |
Jun 13, 2012 |
8980460 |
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14616541 |
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13367508 |
Feb 7, 2012 |
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13495727 |
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Current U.S.
Class: |
205/239 ;
205/238 |
Current CPC
Class: |
C25D 3/20 20130101; C25D
3/46 20130101; C25D 3/22 20130101; C25D 3/02 20130101; C25D 3/56
20130101; C25D 3/38 20130101; C25D 3/48 20130101; C09D 5/4484
20130101; C25D 3/12 20130101; C25D 3/42 20130101; C25D 3/50
20130101; C25D 13/22 20130101; C25D 3/30 20130101; C25D 5/00
20130101 |
International
Class: |
C25D 3/56 20060101
C25D003/56 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under
Contract DE-AC0576RLO1830 awarded by the U.S. Department of Energy.
The Government has certain rights in the invention.
Claims
1. An electrolyte solution for electrodeposition of a first
conductive material (C1) on a substrate from one or more reactants
in the electrolyte solution, the electrolyte solution characterized
by a soluble, surface-smoothing additive comprising cations of a
second conductive material (C2), wherein cations of C2 have an
effective electrochemical reduction potential in the solution lower
than that of the reactants.
2. The electrolyte solution of claim 1, wherein C1 is a metallic
material and the reactants comprise cations of C1.
3. The electrolyte solution of claim 2, wherein C1 comprises a
metal selected from the group consisting of Li, Na, Mg, Al, Sn, Ti,
Fe, Ni, Cu, Zn, Ag, Pt, Au, and combinations thereof.
4. The electrolyte solution of claim 1, wherein C1 comprises an
electronic conductive polymer and the reactants comprise monomers
of the polymer.
5. The electrolyte solution of claim 1, wherein the cations of C2
are metal cations.
6. The electrolyte solution of claim 5, wherein the cations of C2
comprise a metal selected from the group consisting of Cs, Rb, K,
Ba, Sr, Ca, Li, and combinations thereof.
7. The electrolyte solution of claim 1, wherein the cations of C2
have an activity in solution such that the effective
electrochemical reduction potential of cations of C2 is lower than
that of the reactants.
8. The electrolyte solution of claim 1, wherein the cations of C2
have a concentration in solution such that the effective
electrochemical reduction potential of cations of C2 is lower than
that of the reactants.
9. The electrolyte solution of claim 1, wherein the concentration
of C2 cations is less than, or equal to, 30% of that of the
reactants.
10. The electrolyte solution of claim 1, wherein the concentration
of C2 cations is less than, or equal to, 5% of that of the
reactants.
11. The electrolyte solution of claim 1, wherein the
surface-smoothing additive comprises an anion selected from the
group consisting of PF.sub.6.sup.-, AsF.sub.6.sup.-,
BF.sub.4.sup.-, N(SO.sub.2CF.sub.3).sub.2.sup.-,
N(SO.sub.2F).sub.2.sup.-, CF.sub.3SO.sub.3.sup.-, ClO.sub.4.sup.-,
I.sup.-, Cl.sup.-, OH.sup.-, NO.sub.3.sup.-, SO.sub.4.sup.2- and
combinations thereof.
12. The electrolyte solution of claim 1, wherein the substrate is
an electrode.
13. The electrolyte solution of claim 12, wherein the electrode
comprises lithium.
14. The electrolyte solution of claim 12, wherein the electrode
comprises carbon.
15. The electrolyte solution of claim 12, wherein the electrode is
an electrode in an energy storage device.
16. The electrolyte solution of claim 1, wherein the cations of C2
are not chemically or electrochemically reactive with respect to C1
or the reactants.
17. The electrolyte solution of claim 1, wherein the cations of C2
have a standard electrical reduction potential that is greater than
the effective electrochemical reduction potential of the
reactants.
18. The electrolyte solution of claim 2, wherein C1 comprises
lithium.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 13/495,727, filed Jun. 13, 2012, which is a continuation in
part of U.S. patent application Ser. No. 13/367,508, filed Feb. 7,
2012, now abandoned, each of which is incorporated in its entirety
herein by reference.
BACKGROUND
[0003] Electrodeposition is widely used to coat a functional
material having a desired property onto a surface that otherwise
lacks that property. During electrodeposition, electrically charged
reactants in an electrolyte solution diffuse, or are moved by an
electric field, to cover the surface of an electrode. For example,
the electrical current can reduce reactant cations to yield a
deposit on an anode. Or, anions of reactants in the electrolyte
solution can diffuse, or be moved by the electric field, to cover
the surface of a cathode, where the reactant anions are oxidized to
form a deposit on the electrode.
[0004] Electrodeposition has been successfully utilized in the
fields of abrasion and wear resistance, corrosion protection,
lubricity, aesthetic qualities, etc. It also occurs in the
operation of certain energy storage devices. For example, in the
charge process of a metal battery or metal-ion battery, metal ions
in the electrolyte move from the cathode and are deposited on the
anode. Some organic compounds with unsaturated carbon-carbon double
or triple bonds are used as additives in non-aqueous electrolytes
and are electrochemically reduced and deposited at the anode
surface or oxidized and deposited at the cathode surface to form
solid electrolyte interphase layers as protection films on both
anode and cathode of lithium batteries. Some other organic
compounds with conjugated bonds in the molecules are
electrochemically oxidized and deposited at the cathode surface to
form electric conductive polymers as organic cathode materials for
energy storage devices.
[0005] In most instances, the ideal is a smooth electrodeposited
coating. For example, a smoothly plated film can enhance the
lifetime of a film used for decoration, wear resistance, corrosion
protection, and lubrication. A smoothly plated film is also
required for energy storage devices, especially for secondary
devices. Rough films and/or dendrites generated on electrode
surfaces during the charge/discharge processes of these energy
storage devices can lead to the dangerous situations,
short-circuits, reduced capacities, and/or shortened lifetimes.
[0006] Roughness and/or dendrites can be caused by several reasons,
including the uneven distribution of electric current density
across the surface of the electrodeposition substrate (e.g., anode)
and the uneven reactivity of electrodeposited material and/or
substrate to electrolyte solvents, reactants, and salts. These
effects can be compounded in the particular case of repeated
charging-discharging cycles in energy storage devices. Therefore, a
need for improved electrolytes and methods for electrodeposition
are needed to enhance the smoothness of the resultant film.
SUMMARY
[0007] This document describes methods and electrolytes for
electrodeposition that result in self-healing, instead of
self-amplification, of initial protuberant tips, which are
unavoidable during electrodeposition and which give rise to
roughness and/or dendrite formation. For electrodeposition of a
first conductive material (C1) on a substrate from one or more
reactants in an electrolyte solution, embodiments of the
electrolyte solution described herein are characterized by a
soluble, surface-smoothing additive comprising cations of a second
conductive material (C2), wherein cations of C2 have an effective
electrochemical reduction potential (ERP) in the solution lower
than that of the reactants.
[0008] As used herein, cations, in the context of C1, C2, and/or
reactants, refer to atoms or molecules having a net positive
electrical charge. In but one example, the total number of
electrons in the atom or molecule can be less than the total number
of protons, giving the atom or molecule a net positive electrical
charge. The cations are not necessarily cations of metals, but can
also be non-metallic cations. At least one example of a
non-metallic cation is ammonium. Cations are not limited to the +1
oxidation state in any particular instance. In some descriptions
herein, a cation can be generally represented as X.sup.+, which
refers generally to any oxidation state, not just +1.
[0009] In another example, the reactants might not technically be
cations but are positively charged species such as conductive
monomers/polymers. During the electrodeposition of a metal cation,
the cation gets the electron at the anode and is reduced to metal.
When forming a conductive polymer via electrodeposition, it is the
conjugated monomer, which can be neutral but with double or triple
bonds, that gets the electrons. The conjugated monomer re-arranges
the double or triple bonds among the same molecular structure and
forms new bonds among different molecules. The formed polymer is
either neutral or positively charged when protons are incorporated
onto the polymer moiety.
[0010] In one embodiment, C1 is a metallic material and the
reactants comprise cations of C1. Examples of suitable metallic
materials include, but are not limited to, elemental metals or
alloys containing Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Al, Ga,
In, Tl, Ge, Sn, Pb, As, Sb, Bi, Se, Te, Bi, Po, Sc, Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Pt,
Au, and/or Hg. Preferably, C1 is an elemental metal material
comprising Li, Zn, Na, Mg, Al, Sn, Ti, Fe, Ni, Cu, Zn, Ag, Pt, or
Au.
[0011] Alternatively, C1 can comprise an electronic conductive
polymer. In such instances, the reactants can comprise monomers of
the polymer. The monomers can be conjugated monomers that are
reduced at the anode during deposition. Examples of polymers can
include, but are not limited to, polyanaline, polypyrrole,
polythiophene, poly(3,4-ethylenedioxythiophene). Monomers of these
polymers can include, but are not limited to, analine, pyrrole,
thiophen, 3,4-ethylenedioxythiophene, respectively.
[0012] In another embodiment, the cations of C2 are metal cations.
Examples of metals for cations of C2 include, but are not limited
to, Li, Cs, Rb, K, Ba, La, Sr, Ca, Ra, Zr, Te, B, Bi, Ta, Ga, Eu,
S, Se, Nb, Na, Mg, Cu, Al, Fe, Zn, Ni, Ti, Sn, Sb, Mn, V, Ta, Cr,
Au, Ge, Co, As, Ag, Mo, Si, W, Ru, I, Fc, Br, Re, Bi, Pt, and/or
Pd. In preferred embodiments, cations of C2 are cations of Cs, Rb,
K, Ba, Sr, Ca, Li.
[0013] A cation of C2 might have a standard reduction potential
that is greater than that of the reactants. In such instances, some
embodiments of the electrolytes have an activity of C2 cations such
that the effective ERP of the C2 cations is lower than that of the
reactants (C1). Because activity is directly proportional to the
concentration and activity coefficient, which depend on the
mobility and solvation of the cation in the given electrolyte, a
lower activity can be a result of low concentration, low activity
coefficient of the cations, or both since the activity is the
product of the activity coefficient and concentration. The
relationship between effective ERP and activity is described in
part by the Nernst equation and is explained in further detail
elsewhere herein. In a particular embodiment, the concentration of
C2 cations is less than, or equal to, 30% of that of the reactants.
In another, the concentration of C2 cations is less than, or equal
to, 10% of that of the reactants. In yet another, the concentration
of C2 cations is less than, or equal to, 5% of that of the
reactants
[0014] The surface-smoothing additive can comprise an anion that
includes, but is not limited to, PF.sub.6.sup.-, AsF.sub.6.sup.-,
BF.sub.4.sup.-, N(SO.sub.2CF.sub.3).sub.2.sup.-,
N(SO.sub.2F).sub.2.sup.-, CF.sub.3SO.sub.3.sup.-, ClO.sub.4.sup.-,
I.sup.-, Cl.sup.-, OH.sup.-, NO.sub.3.sup.-, SO.sub.4.sup.2-, and
combinations thereof. Preferably, the anion comprises
PF.sub.6.sup.-.
[0015] In one embodiment, the substrate is an electrode. For
example, the substrate on which electrodeposition occurs can be an
electrode in an energy storage device. In particular instances, the
electrode can comprise lithium, carbon, magnesium, and/or sodium.
As used herein, electrode is not restricted to a complete structure
having both an active material and a current collector. For
example, an electrode can initially encompass a current collector
on which active material is eventually deposited to form an anode.
Alternatively, an electrode can start out as an active material
pasted on a current collector. After initial cycling, the active
material can be driven into the current collector to yield what is
traditionally referred to as an electrode.
[0016] Preferably, the cations of C2 are not chemically or
electrochemically reactive with respect to C1 or the reactants.
Accordingly, the surface-smoothing additive is not necessarily
consumed during electrodeposition.
[0017] The electrolyte also comprises a solvent. Examples of
solvents can include, but are not limited to, water or a
non-aqueous polar organic substance that dissolves the solutes at
room temperature. Blends of more than one solvent can be used. When
water or a protic organic substance is used as the solvent, C1 is
not a metal that reacts with water or the protic organic substance.
Generally, organic solvents can include, but are not limited to,
alcohols, ethers, aldehydes, ketones, carbonates, carboxylates,
lactones, phosphates, nitriles, sulfones, amides, five or six
member heterocyclic ring compounds, and organic compounds having at
least one C.sub.1-C.sub.4 group connected through an oxygen atom to
a carbon. Lactones may be methylated, ethylated and/or propylated.
Other organic solvents can include methanol, ethanol, acetone,
sulfolane, dimethyl sulfone, ethyl methyl sulfone, ethylene
carbonate, propylene carbonate, dimethyl carbonate, diethyl
carbonate, ethyl methyl carbonate, methyl propyl carbonate,
tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane,
1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane,
1,2-dibutoxyethane, acetonitrile, dimethylformamide, methyl
formate, ethyl formate, propyl formate, butyl formate, methyl
acetate, ethyl acetate, propyl acetate, butyl acetate, methyl
propionate, ethyl propionate, propyl propionate, butyl propionate,
methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate,
gamma-butyrolactone, 2-methyl-gamma-butyrolactone,
3-methyl-gamma-butyrolactone, 4-methyl-gamma-butyrolactone,
delta-valerolactone, trimethyl phosphate, triethyl phosphate,
tris(2,2,2-trifluoroethyl)phosphate, tripropyl phosphate,
triisopropyl phosphate, tributyl phosphate, trihexyl phosphate,
triphenyl phosphate, and combinations thereof. Still other
non-aqueous solvents can be used so long as they are capable of
dissolving the solute salts.
[0018] Methods for improving surface smoothness during
electrodeposition of C1 on a substrate surface can comprise
providing an electrolyte solution comprising reactants from which
C1 is deposited and a soluble, surface-smoothing additive
comprising cations of a second conductive material (C2) and
applying an electrical potential thereby reducing the reactants and
forming C1 on the substrate surface. The cations of C2 have an
effective electrochemical reduction potential in the solution lower
than that of the reactants. In preferred embodiments, the methods
further comprise accumulating cations of C2 at protrusions on the
substrate surface, thereby forming an electrostatically shielded
region near each protrusion. The electrostatically shielded region
can temporarily repel reactants, thus reducing the local effective
current density and slowing deposition at the protrusion while
enhancing deposition in regions away from the protrusions. In this
way, the growth and/or amplification of the protrusions are
suppressed and the surface heals to yield a relatively smoother
surface.
[0019] In one embodiment, the method is applied to
electrodeposition of lithium on a substrate surface. Lithium is an
effective example because Li.sup.+ ions have the lowest standard
ERP among metals (at a concentration of 1 mol/L, a temperature of
298.15 K (25.degree. C.), and a partial pressure of 101.325 kPa
(absolute) (1 atm, 1.01325 bar) for each gaseous reagent). C2
cations, which have standard EPR values that are greater than
lithium cations can have activity-dependent effective ERP values
that are lower than those of the lithium cations.
[0020] According to such embodiments, the method comprises
providing an electrolyte solution comprising lithium cations and a
soluble, surface-smoothing additive comprising cations of a second
conductive material (C2) selected from the group consisting of
cesium, rubidium, potassium, strontium, barium, calcium, and
combinations thereof. The cations of C2 have a concentration and
activity coefficient in solution such that the effective
electrochemical reduction potential of the cations of C2 is lower
than that of the lithium cations. The method further comprises
applying an electrical potential, thereby reducing the lithium
cations and forming lithium on the substrate surface. The method
further comprises accumulating cations of C2 at protrusions on the
substrate surface, thereby forming an electrostatically shielded
region near each protrusion and temporarily repelling the lithium
cations from the electrostatically shielded regions. In some
instances, the electrostatically shielded region has a higher
impedance to retard the further deposition of lithium cations.
[0021] In particular embodiments, the concentration of C2 cations
is less than, or equal to 30% of that of the lithium cations. In
others, the C2 cation concentration is less than, or equal to, 5%
of that of the lithium cations. Preferably, the surface-smoothing
additive comprises an anion comprising PF.sub.6.sup.- anion. The
substrate can be a battery anode that comprises lithium or that
comprises carbon.
[0022] The purpose of the foregoing abstract is to enable the
United States Patent and Trademark Office and the public generally,
especially the scientists, engineers, and practitioners in the art
who are not familiar with patent or legal terms or phraseology, to
determine quickly from a cursory inspection the nature and essence
of the technical disclosure of the application. The abstract is
neither intended to define the invention of the application, which
is measured by the claims, nor is it intended to be limiting as to
the scope of the invention in any way.
[0023] Various advantages and novel features of the present
invention are described herein and will become further readily
apparent to those skilled in this art from the following detailed
description. In the preceding and following descriptions, the
various embodiments, including the preferred embodiments, have been
shown and described. Included herein is a description of the best
mode contemplated for carrying out the invention. As will be
realized, the invention is capable of modification in various
respects without departing from the invention. Accordingly, the
drawings and description of the preferred embodiments set forth
hereafter are to be regarded as illustrative in nature, and not as
restrictive.
DESCRIPTION OF DRAWINGS
[0024] Embodiments of the invention are described below with
reference to the following accompanying drawings.
[0025] FIGS. 1A-1F are illustrations depicting an embodiment of
electrodeposition using an electrolyte having a surface-smoothing
additive.
[0026] FIGS. 2A-2D include SEM micrographs of Li films deposited in
an electrolyte with or without a surface-smoothing additive
according to embodiments of the present invention; (a) No additive;
(b) 0.05 M RbPF.sub.6; (c) 0.05 M CsPF.sub.6; (d) 0.15 M
KPF.sub.6.
[0027] FIGS. 3A-3B include SEM micrographs of pre-formed dendritic
Li film deposited in a control electrolyte for 1 hour and the same
film after another 14 hours' Li deposition in the electrolyte with
additive (0.05M CsPF.sub.6), respectively.
[0028] FIGS. 4A-4F include SEM micrographs of Li electrodes after
repeated deposition/stripping cycles in the control electrolytes
(a, b, and c) and with Cs.sup.+-salt additive (d, e and f).
[0029] FIGS. 5A-5B include SEM micrographs of Li electrodes after
100 cycles in coin cells of Li|Li.sub.4Ti.sub.5O.sub.12 containing
electrolytes without (a) and with (b) 0.05 M Cs.sup.+ additive.
[0030] FIGS. 6A-6F include optical and SEM micrographs of hard
carbon electrodes after charging to 300% of the regular capacity in
the control electrolyte (a, c, e) and in an electrolyte with 0.05 M
CsPF.sub.6 additive added in the control electrolyte (b, d, f).
DETAILED DESCRIPTION
[0031] The following description includes the preferred best mode
of one embodiment of the present invention. It will be clear from
this description of the invention that the invention is not limited
to these illustrated embodiments but that the invention also
includes a variety of modifications and embodiments thereto.
Therefore the present description should be seen as illustrative
and not limiting. While the invention is susceptible of various
modifications and alternative constructions, it should be
understood, that there is no intention to limit the invention to
the specific form disclosed, but, on the contrary, the invention is
to cover all modifications, alternative constructions, and
equivalents falling within the spirit and scope of the invention as
defined in the claims.
[0032] FIGS. 1-6 show a variety of embodiments and aspects of the
present invention. Referring first to FIG. 1, a series of
illustrations depict an embodiment of electrodeposition using an
electrolyte 104 having a surface-smoothing additive. The additive
comprises cations of C2 102, which have an effective ERP lower than
that of the reactants 103. FIG. 1 illustrates how an
electrostatically shielded region 106 can develop resulting in the
self-healing of the unavoidable occurrence of surface protrusions
105 that would normally form. During the initial stage of
deposition, both the reactants and the cations of C2 are adsorbed
on the substrate surface 100 (FIG. 1A) under an applied voltage
(E.sub.a) 101 slightly less than the reduction potential of the
reactant (E.sub.r) but larger than the additive reduction potential
(E.sub.C2/C2.sub.+), that is,
E.sub.r>E.sub.a>E.sub.C2/C2.sub.+). Reactants will be
deposited to form C1 on the substrate and will unavoidably form
some protuberance tips due to various fluctuations in the system
(FIG. 1B). A sharp edge or protrusion on the electrode exhibits a
stronger electrical field, which will attract more positively
charged cations (including both C1 and C2). Therefore, more cations
of C1 will be preferentially deposited around the tips rather than
on other smooth regions. In conventional electrodeposition,
amplification of this behavior will form the surface roughness
and/or dendrites. However, according to embodiments of the present
invention, the adsorbed additive cations (C2.sup.+) have an
effective ERP lower than E.sub.a (FIG. 1C) and will not be
deposited (i.e., electrochemically or chemically consumed, reacted,
and/or permanently bound) on the tip. Instead, they will be
temporarily electrostatically attracted to and accumulated in the
vicinity of the tip to form an electrostatic shield (FIG. 1D). This
positively charged shield will repel incoming reactants (e.g.,
like-charged species) at the protruded region and force them to be
deposited in non-protrusion regions. The net effect is that
reactants will be preferentially deposited in the smoother regions
of the substrate (FIG. 1E) resulting in a smoother overall
deposition surface (FIG. 1F). This process is persistently
occurring and/or repeating during electrodeposition. The
self-healing mechanism described herein resulting from embodiments
of the present invention appears to disrupt the conventional
roughness and/or dendrite amplification mechanism and leads to the
deposition of a smooth film of C1 on the substrate.
[0033] The additive cation (C2.sup.+) exhibits an effective ERP,
E.sub.Red, less than that of the cations (C1.sup.+) of the
reactants. In some instances, the standard ERP of the C2 cation
will be less than that of the reactants. Surface-smoothing
additives comprising such C2 species can be utilized with
appropriate reactants with few limitations on concentration and
activity coefficient. However, in some instances, the C2 cation
will have a standard ERP that is greater than that of the
reactants. The concentration and activity coefficient of the C2
cations can be controlled such that the effective ERP of the C2
cations is lower than that of the reactant cations. For example, if
the reactant is a Li.sup.+ ion, which has the lowest standard ERP
among metals, then the concentration and activity coefficient of C2
cations can be controlled such that the effective ERP is lower than
that of the lithium cations.
[0034] According to the Nernst equation:
E Red = E Red .phi. - RT F ln .alpha. Red .alpha. Ox ( 1 )
##EQU00001##
where R is the universal gas constant (=8.314 472 J K.sup.-1
mol.sup.-1), T is the absolute temperature (assume T=25.degree. C.
in this work), a is the activity for the relevant species
(.alpha..sub.Red is for the reductant and .alpha..sub.Ox is for the
oxidant). .alpha..sub.x=.gamma..sub.xc.sub.x, where .gamma..sub.x
and c.sub.x are the activity coefficient and the concentration of
species x. F is the Faraday constant (9.648 533 99.times.10.sup.4 C
mol.sup.-1), z is the number of moles of electrons transferred.
Although Li.sup.+ ion has the lowest standard reduction potential,
E.sub.Red(Li.sup.+), among all the metals when measured at a
standard conditions (1 mol/L), a cation (M.sup.+) may have an
effective reduction potential lower than those of lithium ion
(Li.sup.+) if M.sup.+ has an activity .alpha..sub.x much lower than
that of Li.sup.+. In the case of low concentration when the
activity coefficient is unity, .alpha. can be simplified as
concentration c.sub.x, then Eq. (1) can be simplified as:
E Red = E Red .phi. - 0.0516 V log 10 1 c Ox ( 2 ) ##EQU00002##
[0035] Table 1 shows several the reduction potentials for several
cations (vs. standard hydrogen electrode (SHE)) at various
concentrations assuming that their activity coefficients,
.gamma..sub.x, equal one. When the concentration of Cs.sup.+,
Rb.sup.+, and K.sup.+ is 0.01 M in an electrolyte, their effective
ERPs are -3.144 V, -3.098 V and -3.049 V, respectively, which are
less than those of Li.sup.+ at 1 M concentration (-3.040 V). As a
result, in a mixed electrolyte where the additive (Cs.sup.+,
Rb.sup.+, and K.sup.+) concentration is much less than Li.sup.+
concentration, these additives will not be deposited at the lithium
deposition potential. In addition to a low concentration c.sub.x, a
very low activity coefficient .gamma..sub.x (which is strongly
affected by the solvation and mobility of the cations in the given
solvent and lithium salt) may also reduce the activity of cations
and lead to an effective reduction potential lower than that of the
lithium ion (Li.sup.+) as discussed below.
TABLE-US-00001 TABLE 1 The effective reduction potential of
selected cations vs. SHE Li.sup.+ Cs.sup.+ Rb.sup.+ K.sup.+ Stand
reduction -3.040 V -3.026 V -2.980 V -2.931 V potential (1M)
Effective reduction -- -3.103 V -3.06 V -3.01 V potential at 0.05M*
Effective reduction -- -3.144 V -3.098 V -3.049 V potential at
0.01M* *Assume the activity coefficient .gamma..sub.x of species x
equals 1.
Surface Smoothing Exhibited in Electrodeposition of Lithium
[0036] Embodiments of the present invention are illustrated well in
the electrodeposition of lithium, since lithium ions have the
lowest standard ERP among metals. However, the present invention is
not limited to lithium but is defined by the claims.
[0037] The effect of several C2 cations has been examined for use
in surface-smoothing additives in the electrodeposition of lithium.
The cations all have standard ERP values, E.sub.Red.sup..phi., that
are close to that of Li.sup.+ ions. The electrolyte comprised 1 M
LiPF.sub.6 in propylene carbonate. Electrolyte solutions with
surface-smoothing additives comprising 0.05 M RbPF.sub.6, 0.5 M
CsPF.sub.6, or 0.15 M KPF.sub.6 were compared to a control
electrolyte with no additives. CsPF.sub.6, RbPF.sub.6, and
Sr(PF.sub.6).sub.2 were synthesized by mixing stoichiometric amount
of AgPF.sub.6 and the iodide salts of Cs, Rb, or Sr in a PC
solution inside a glove box filled with purified argon where the
oxygen and moisture content was less than 1 ppm. The formed AgI was
filtered out from the solution using 0.45 .mu.m syringe filters.
The electrolyte preparation and lithium deposition were conducted
inside the glove box as well. Lithium films were deposited on
copper (Cu) foil substrates (10 mm.times.10 mm) in different
electrolyte solutions at the desired current densities using a
SOLARTRON.RTM. electrochemical Interface. After deposition, the
electrode was washed with DMC to remove the residual electrolyte
solvent and salt before the analyses.
[0038] Referring to the scanning electron microscope (SEM)
micrograph in FIG. 2A, when using the control electrolyte, the
electrodeposited film exhibited conventional roughness and dendrite
growth. The lithium film deposited in the electrolyte with 0.05 M
Rb.sup.+ as the C2 cation exhibits a very fine surface morphology
without dendrite formation as shown in FIG. 2B. Similarly, for the
lithium films deposited with 0.05 M Cs.sup.+ additive, a dramatic
change of the lithium morphology with no dendrite formation (see
FIG. 2C) was obtained compared with the control experiment.
Surprisingly, although E.sub.Red(K.sup.+) at 0.15 M is
theoretically .about.0.06 V higher than that of Li.sup.+ assuming
both K.sup.+ and Li.sup.+ have an activity coefficient of 1, K
metal did not deposit at the lithium deposition potential, and a
lithium film with a mirror-like morphology was obtained using
K.sup.+ as in the additive (FIG. 2D). This experimental finding
suggests that the activity coefficient .gamma..sub.x for K.sup.+
ion's in this electrolyte is much less than those of Li.sup.+
leading to an actual E.sub.Red(K.sup.+) lower than
E.sub.Red(Li.sup.+).
[0039] Generally, the concentration of the surface-smoothing
additive is preferably high enough that protrusions can be
effectively electrostatically shielded considering the effective
ERP, the number of available C2 cations, and the mobility of the C2
cations. For example, in one embodiment, wherein the C2 cation
comprises K.sup.+, the reactant comprises Li.sup.+ and C1 comprises
lithium metal, the concentration of K.sup.+ is greater than
0.05M.
[0040] Referring to FIG. 3A, a dendritic lithium film was
intentionally deposited on a copper substrate in a control
electrolyte for 1 hour. The substrate and film was then transferred
into an electrolyte comprising a surface-smoothing additive, 0.05 M
CsPF.sub.6 in 1 M LiPF.sub.6/PC, to continue deposition for another
14 hours. Unlike the dendritic and mossy film deposited in the
control electrolyte, the micrograph in FIG. 3B shows that a smooth
lithium film was obtained after additional electrodeposition using
embodiments of the present invention. The roughness, pits, and
valleys shown in FIG. 3A have been filled by dense lithium
deposits. The original needle-like dendritic whiskers have been
converted to much smaller spherical particles which will also be
buried if more lithium is deposited.
[0041] FIG. 4 includes SEM micrographs comparing the morphologies
of the lithium electrodes after repeated deposition/stripping
cycles (2.sup.nd, 3.sup.rd, and 10.sup.th cycle) in cells using the
control electrolyte (see FIGS. 4A, 4B, and 4C) and using
electrolyte with a surface-smoothing additive comprising 0.05M
Cs.sup.+ (see FIGS. 4D, 4E, and 4F). The large lithium dendrites
and dark lithium particles are clearly observed on the lithium
films deposited in the control electrolyte. In contrast, the
morphologies of the lithium films deposited in the
Cs.sup.+-containing electrolyte still retain their dendrite free
morphologies after repeated cycles. In all the films deposited with
the additives, lithium films exhibit small spherical particles and
smoother surfaces. This is in strong contrast with the needle-like
dendrites grown in the control electrolyte.
[0042] Electrolytes and methods described herein were also applied
in rechargeable lithium metal batteries. Coin cells with
Li|Li.sub.4Ti.sub.5O.sub.12 electrodes were assembled using the
control electrolyte. Similar cells were also assembled with
electrolytes containing a surface smoothing additive comprising
0.05 M Cs.sup.+. FIG. 5 contains SEM micrographs showing the
morphologies of the lithium metal anodes after 100 charge/discharge
cycles. Referring to FIG. 5A, the lithium electrode in the cell
with no additive exhibits clear surface roughness and formation of
dendrites. However, as shown in FIG. 5B, no dendritic lithium was
observed on the lithium electrode in the cell with the
surface-smoothing additive, even after 100 cycles.
[0043] Surface-smoothing additives comprising higher valence
cations can also be used. Examples include, but are not limited to,
Sr.sup.2+, which have E.sub.Red.sup.+ values of -2.958 V (assuming
.gamma.=1) versus a standard hydrogen potential. The lower activity
of these cations can result in an effective ERP lower than that of
Li.sup.+ ions. The larger size and higher charge should be
accounted for in the non-aqueous electrolyte. Lithium films were
deposited using the control electrolyte along with electrolytes
comprising 0.05 M Sr(PF.sub.6).sub.2. Deposition from the
electrolyte comprising 0.05 M Sr.sup.2+ results in a lithium film
that is smooth, free of dendrites, and void of Sr in/on the anode.
This again indicates that the activity coefficient for Sr.sup.2+ in
these solutions is less than unity.
[0044] Using this approach, C2 cations of the surface-smoothing
additive are not reduced and deposited on the substrate. The C2
cations are not consumed because these cations exhibit an effective
reduction potential lower than that of the reactant. In contrast,
traditional electrodeposition can utilize additives having a
reduction potential higher than that of the reactants; therefore,
they will be reduced during the deposition process and "sacrificed
or consumed," for example, as part of an SEI film or as an alloy to
suppress dendrite growth. As a result, the additive concentration
in the electrolyte will decrease with increasing charge/discharge
cycles and the effect of the additives will quickly degrade. In
contrast, the C2 cations described herein will form a temporary
electrostatic shield or "cloud" around the dendritic tips that
retards further deposition of C1 in this region. This "cloud" will
form whenever a protrusion is initiated, but it will dissipate once
applied voltage is removed or the protrusion is eliminated.
Accordingly, in some embodiments, the applied electrical potential
is of a value that is less than, or equal to, the ERP of the
reactants and greater than the effective ERP of the cations of
C2.
[0045] Lithium films having an SEI layer on the surface and
deposited using electrolytes comprising 0.05 M Cs.sup.+, Rb.sup.+,
K.sup.+, or Sr.sup.2+ additives were analyzed by x-ray
photoelectron spectroscopy (XPS), Energy-dispersive X-ray
spectroscopy (EDX) dot mapping, and Inductively coupled plasma
atomic emission spectroscopy (ICP/AES) methods. XPS and EDX results
did not show Cs, Rb, K, and Sr elements in the SEI films within the
detectable limits of the analysis instruments. In addition, ICP-AES
analysis did not identify Cs, Rb, K, and Sr elements in the bulk of
deposited lithium film (including the SEI layer on the surface)
within detectable limits.
[0046] Dendrite formation is not only a critical issue in
rechargeable lithium metal batteries, but also an important issue
in high power lithium ion batteries because lithium metal dendrites
can grow at the anode surface when the lithium ions cannot move
quickly enough to intercalate into the anode, which can comprise
graphite or hard carbon, during rapid charging. In this case, the
lithium dendrites can lead to short circuits and thermal runaway of
the battery. Accordingly, a carbonaceous anode is described herein
to demonstrate suppression of lithium dendrite growth in a lithium
ion battery. FIG. 6 compares the optical (6A and 6D) and SEM images
(6B, 6C, 6E, and 6F) of lithium particles formed on the hard carbon
anode after it was charged to 300% of its theoretical capacity in a
control electrolyte (without additives) and in an electrolyte
having a surface smoothing additive comprising 0.05 M CsPF.sub.6. A
significant amount of lithium metal was deposited on the surface of
carbon electrode (see grey spots in FIG. 6A) for the sample
overcharged in the control electrolyte. FIGS. 6B and 6C show clear
dendritic growth on the electrode surface. In contrast, no lithium
metal deposition was observed on the surface of carbon electrode
(see FIG. 6D) for the sample overcharged in the electrolyte with
0.05M Cs.sup.+ additive (the white line on the bottom of the carbon
sample is due to an optical reflection). After removing a small
piece of carbon from the sample (see the circled area in FIG. 6D),
it was found that excess lithium was preferentially grown on the
bottom of the carbon electrode as shown in FIGS. 6E and 6F.
[0047] While a number of embodiments of the present invention have
been shown and described, it will be apparent to those skilled in
the art that many changes and modifications may be made without
departing from the invention in its broader aspects. The appended
claims, therefore, are intended to cover all such changes and
modifications as they fall within the true spirit and scope of the
invention.
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