U.S. patent application number 15/977651 was filed with the patent office on 2018-09-13 for solid state electrolyte and electrode compositions.
The applicant listed for this patent is Wildcat Discovery Technologies, Inc.. Invention is credited to Marissa Caldwell, Deidre Strand.
Application Number | 20180261877 15/977651 |
Document ID | / |
Family ID | 53482911 |
Filed Date | 2018-09-13 |
United States Patent
Application |
20180261877 |
Kind Code |
A1 |
Strand; Deidre ; et
al. |
September 13, 2018 |
SOLID STATE ELECTROLYTE AND ELECTRODE COMPOSITIONS
Abstract
A lithium ion battery having an anode, a solid electrolyte, and
a cathode. The cathode includes an electrode active material, a
first lithium salt, and a polymer material. The solid electrolyte
can include a second lithium salt. The solid electrolyte can
include a ceramic material, a lithium salt, and a polymer
material.
Inventors: |
Strand; Deidre; (San Diego,
CA) ; Caldwell; Marissa; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wildcat Discovery Technologies, Inc. |
San Diego |
CA |
US |
|
|
Family ID: |
53482911 |
Appl. No.: |
15/977651 |
Filed: |
May 11, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14584841 |
Dec 29, 2014 |
9985313 |
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15977651 |
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61923135 |
Jan 2, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/056 20130101;
H01M 2300/0088 20130101; H01M 10/0525 20130101; H01M 2300/0068
20130101; Y02E 60/10 20130101; H01M 2300/0082 20130101 |
International
Class: |
H01M 10/056 20060101
H01M010/056; H01M 10/0525 20060101 H01M010/0525 |
Claims
1. A lithium ion battery, comprising: an anode; a cathode
comprising an electrode active material, a first lithium salt, and
an ion conducting polymer material; and a solid electrolyte
comprising a second lithium salt.
2. The battery of claim 1, wherein the first lithium salt and the
polymer material are arranged together in domains within the
cathode, the electrode active material is arranged in domains
within the cathode, interfaces are found where the domains of the
first lithium salt and the ion conducting polymer material and the
domains of the electrode active material meet, and the first
lithium salt is found preferentially at the interfaces.
3. The battery of claim 1, wherein the ion conducting polymer
material comprises poly(ethylene oxide).
4. The battery of claim 1, wherein the ion conducting polymer
material comprises poly(ethylene glycol).
5. The battery of claim 1, wherein the first lithium salt is
selected from the group consisting of lithium triflate
(LiCF.sub.3SO.sub.3), lithium tetrafluoroborate (LiBF.sub.4),
lithium hexafluorophosphate (LiPF.sub.6), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium bromide (LiBr), lithium
chlorate (LiClO.sub.3), lithium nitrate (LiNO.sub.3), lithium
bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2), lithium
difluoro(oxalato)borate (LiC.sub.2O.sub.4BF.sub.2), lithium
metaborate (Li.sub.2B.sub.4O.sub.7), lithium
bis(trifluoromethanesulfonyl)imide
(CF.sub.3SO.sub.2NLiSO.sub.2CF.sub.3), and combinations
thereof.
6. The battery of claim 1, wherein the first lithium salt comprises
lithium bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2) or lithium
bis(trifluoromethanesulfonyl)imide
(CF.sub.3SO.sub.2NLiSO.sub.2CF.sub.3).
7. The battery of claim 1, wherein the second lithium salt is
selected from the group consisting of lithium triflate
(LiCF.sub.3SO.sub.3), lithium tetrafluoroborate (LiBF.sub.4),
lithium hexafluorophosphate (LiPF.sub.6), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium bromide (LiBr), lithium
chlorate (LiClO.sub.3), lithium nitrate (LiNO.sub.3), lithium
bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2), lithium
difluoro(oxalato)borate (LiC.sub.2O.sub.4BF.sub.2), lithium
metaborate (Li.sub.2B.sub.4O.sub.7), lithium
bis(trifluoromethanesulfonyl)imide
(CF.sub.3SO.sub.2NLiSO.sub.2CF.sub.3), and combinations
thereof.
8. The battery of claim 6, wherein the second lithium salt
comprises lithium triflate (LiCF.sub.3SO.sub.3).
9. The battery of claim 1, wherein the first lithium salt migrates
into the solid electrolyte.
10. A lithium ion battery, comprising: an anode; a solid
electrolyte comprising an ion conducting ceramic material, a first
lithium salt, and an ion conducting polymer material; and a
cathode.
11. The battery of claim 10, wherein the cathode comprises a second
lithium salt.
12. The battery of claim 10, wherein the first lithium salt and the
ion conducting polymer material are arranged together in domains
within the solid electrolyte, the ion conducting ceramic material
is arranged in domains within the solid electrolyte, interfaces are
found where the domains of the first lithium salt and the ion
conducting polymer material and the domains of the ion conducting
ceramic material meet, and the first lithium salt is found
preferentially at the interfaces.
13. The battery of claim 10, wherein the ion conducting polymer
material comprises poly(ethylene oxide).
14. The battery of claim 10, wherein the ion conducting polymer
material comprises poly(ethylene glycol).
15. The material of claim 10, wherein the ion conducting ceramic
material comprises a garnet material.
16. The material of claim 10, wherein the ion conducting ceramic
material comprises a cubic garnet phase, a sulfide glass, a lithium
ion conducting glass ceramic, or a phosphate ceramic material.
17. The battery of claim 10, wherein the first lithium salt is
selected from the group consisting of lithium triflate
(LiCF.sub.3SO.sub.3), lithium tetrafluoroborate (LiBF.sub.4),
lithium hexafluorophosphate (LiPF.sub.6), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium bromide (LiBr), lithium
chlorate (LiClO.sub.3), lithium nitrate (LiNO.sub.3), lithium
bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2), lithium
difluoro(oxalato)borate (LiC.sub.2O.sub.4BF.sub.2), lithium
metaborate (Li.sub.2B.sub.4O.sub.7), lithium
bis(trifluoromethanesulfonyl)imide
(CF.sub.3SO.sub.2NLiSO.sub.2CF.sub.3), and combinations
thereof.
18. The battery of claim 10, wherein the first lithium salt
comprises lithium bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2) or
lithium bis(trifluoromethanesulfonyl)imide
(CF.sub.3SO.sub.2NLiSO.sub.2CF.sub.3).
19. The battery of claim 11, wherein the second lithium salt is
selected from the group consisting of lithium triflate
(LiCF.sub.3SO.sub.3), lithium tetrafluoroborate (LiBF.sub.4),
lithium hexafluorophosphate (LiPF.sub.6), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium bromide (LiBr), lithium
chlorate (LiClO.sub.3), lithium nitrate (LiNO.sub.3), lithium
bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2), lithium
difluoro(oxalato)borate (LiC.sub.2O.sub.4BF.sub.2), lithium
metaborate (Li.sub.2B.sub.4O.sub.7), lithium
bis(trifluoromethanesulfonyl)imide
(CF.sub.3SO.sub.2NLiSO.sub.2CF.sub.3), and combinations
thereof.
20. The battery of claim 18, wherein the second lithium salt
comprises lithium triflate (LiCF.sub.3SO.sub.3).
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Non-Provisional
application Ser. No. 14/584,841 filed Dec. 29, 2014 entitled "Solid
State Electrolyte and Electrode Compositions", which claims
priority to and the benefit of U.S. Provisional Application No.
61/923,135 filed Jan. 2, 2014 entitled "Solid State Electrolyte and
Electrode Compositions". Each of these applications are
incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention is in the field of battery technology
and, more particularly, in the area of solid polymeric materials
and composites for use in electrodes and electrolytes in
electrochemical cells.
[0003] Conventional lithium ion batteries include a positive
electrode (or cathode as used herein), a negative electrode (or
anode as used herein), an electrolyte, and, frequently, a
separator. The electrolyte typically includes a liquid component
that facilitates lithium ion transport and, in particular, enables
ion penetration into the electrode materials.
[0004] In contrast, so-called solid-state lithium ion batteries do
not include liquid in their principal battery components.
Solid-state batteries can have certain advantages over liquid
electrolyte batteries, such as improvements in safety because the
liquids used in liquid electrolytes are often volatile organic
solvents. Solid-state batteries offer a wider range of packaging
configurations because a liquid-tight seal is not necessary as it
is with liquid electrolytes.
[0005] Further, solid state batteries can use lithium metal as the
anode, thereby dramatically increasing the energy density of the
battery as compared to the carbon-based anodes typically used in
liquid electrolyte lithium ion batteries. With repeated cycling,
lithium metal can form dendrites, which can penetrate a
conventional porous separator and result in electrical shorting and
runaway thermal reactions. This risk is mitigated through the use
of a solid nonporous polymer electrolyte.
[0006] The electrolyte material in a solid-state lithium ion
battery can be a polymer. In particular, poly(ethylene oxide)
("PEO") can be used in forming solid polymer electrolytes. PEO has
the ability to conduct lithium ions as positive lithium ions are
solubilized and/or complexed by the ethylene oxide groups on the
polymer chain. Solid electrolytes formed from PEO can have
crystalline and amorphous regions, and it is believed that lithium
ions move preferentially through the amorphous portion of the PEO
material. In general, ionic conductivities on the order of
1.times.10.sup.-6 S/cm to 1.times.10.sup.-5 S/cm at room
temperature can be obtained with variations on PEO based
electrolyte formulations. The electrolyte is typically formulated
by adding a lithium ion salt to the PEO in advance of building the
battery, which is a formulation process similar to liquid
electrolytes.
[0007] However, solid-state batteries have not achieved widespread
adoption because of practical limitations. For example, while
polymeric solid-state electrolyte materials like PEO are capable of
conducting lithium ions, their ionic conductivities are inadequate
for practical power performance. Successful solid-state batteries
require thin film structures, which reduce energy density, and thus
have limited utility.
[0008] Further, solid-state batteries tend to have a substantial
amount or degrees of interfaces among the different solid
components of the battery. The presence of such interfaces can
limit lithium ion transport and impede battery performance.
Interfaces can occur (i) between the domains of active material in
the electrode and the polymeric binder, (ii) between the cathode
and the solid electrolyte, and (iii) between the solid electrolyte
and the anode structure. Poor lithium ion transport across these
interfaces results in high impedance in batteries and a low
capacity on charge or discharge.
[0009] Research on solid-state electrolyte materials tends to focus
primarily on the composition of the materials used to form the
electrolyte to increase ion conductivity. However, less attention
has been paid to solving the problem of increased impedance due to
conductivity losses at interfaces or addressing the transport of
ions through the electrode structures.
[0010] For example, U.S. Patent Publication 2013/0026409 discloses
a composite solid electrolyte with a glass or glass-ceramic
inclusion and an ionically conductive polymer. However, this solid
electrolyte requires a redox active additive. As another example,
U.S. Pat. No. 5,599,355 discloses a method of forming a composite
solid electrolyte with a polymer, salt, and an inorganic particle
(such as alumina). The particles are reinforcing filler for solid
electrolyte and do not transport lithium. As yet another example,
U.S. Pat. No. 5,599,355 discloses a composite solid state
electrolyte containing a triflate salt, PEO, and a lightweight
oxide filler material. Again, the oxide filler is not a lithium ion
conductor or intercalation compound.
[0011] More generally, ionically conductive polymers like PEO have
been disclosed with the use of a lithium salt as the source of
lithium ions in the solid electrolyte. For example, Teran et al.,
Solid State Ionics (2011) 18-21; Sumathipala et al., Ionics (2007)
13: 281-286; Abouimrane et al., JECS 154(11) A1031-A1034 (2007);
Wang et al., JECS, 149(8) A967-A972 (2002); and Egashira et al.,
Electrochimica Acta 52 (2006) 1082-1086 each disclose different
solid electrolyte formulations with PEO and a lithium salt as the
source for lithium ions. Still further the last two references
(Wang et al. and Egashira et al.) each disclose inorganic
nanoparticles that are believed to improve the ionic conductivity
of the PEO film by preventing/disrupting polymer crystallinity.
However, none of these formulations address the limitations of
solid electrolytes and provide the performance improvements seen in
the embodiments disclosed below.
BRIEF SUMMARY OF THE INVENTION
[0012] Embodiments of the present invention provide comparatively
high capacity and low impedance in solid-state batteries, that is,
batteries in which the electrodes and electrolyte are formed from
solid materials and are substantially free of liquid
components.
[0013] Embodiments of the present invention provide cathode
materials and composites formed from certain lithium salts, for
example lithium bis(oxalato) borate or lithium
bis(trifluoromethanesulfonyl)imide, used in combination with
poly(ethylene oxide), to improve capacity in a solid-state
battery.
[0014] Embodiments of the present invention provide electrolyte
materials formed from certain lithium salts, for example lithium
bis(oxalato) borate or lithium bis(trifluoromethanesulfonyl)imide,
used in combination with PEO, to decrease impedance in a
solid-state battery.
[0015] Embodiments of the present invention provide electrolyte
materials formed from certain lithium salts, for example lithium
bis(oxalato) borate or lithium bis(trifluoromethanesulfonyl)imide,
used in combination with PEO, to decrease the impedance of
polymer/ceramic composite solid-state electrolytes.
[0016] Embodiments of the present invention include a lithium ion
battery having an anode, a solid electrolyte, and a cathode. The
cathode comprises an electrode active material, a first lithium
salt, and a polymer material. The solid electrolyte can include a
second lithium salt.
[0017] Embodiments of the present invention include a lithium ion
battery having an anode, a solid electrolyte, and a cathode. The
solid electrolyte comprises a ceramic material, a first lithium
salt, and a polymer material. The solid electrolyte can include a
second lithium salt.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0018] FIGS. 1A, 1B, and 1C illustrate schematic representations of
cathode, electrolyte, and anode configurations according to certain
embodiments of the invention.
[0019] FIG. 2 illustrates improved capacity at a discharge rate of
C/100 from cells containing composite cathodes films formed from a
polymer/lithium salt formulation and a polymer/lithium salt solid
electrolyte according to certain embodiments of the invention.
[0020] FIG. 3 illustrates improved capacity at a discharge rate of
C/1000 from cells containing composite cathodes films formed from a
polymer/lithium salt formulation and a polymer/lithium salt solid
electrolyte according to certain embodiments of the invention.
[0021] FIG. 4 illustrates improved capacity in cells containing
pressed composite cathodes films formed from a polymer/lithium salt
formulation and a polymer/lithium salt solid electrolyte according
to certain embodiments of the invention.
[0022] FIG. 5 illustrates the measured ionic conductivity of
various polymer/lithium salt films according to certain embodiments
of the invention.
[0023] FIGS. 6A and 6B illustrate measurement of the impedance of
films containing polymer/lithium salt formulations according to
certain embodiments of the invention.
[0024] FIGS. 7A and 7B illustrate measurement of the time
dependence of electrical impedance of films containing
polymer/lithium salt formulations according to certain embodiments
of the invention.
[0025] FIG. 8 illustrates measurement of the ionic conductivity of
films containing ceramic/polymer/lithium salt formulations
according to certain embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0026] 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. Each term is
further explained and exemplified throughout the description,
figures, and examples. Any interpretation of the terms in this
description should take into account the full description, figures,
and examples presented herein.
[0027] The singular terms "a," "an," and "the" include the plural
unless the context clearly dictates otherwise. Thus, for example,
reference to an object can include multiple objects unless the
context clearly dictates otherwise.
[0028] 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.
[0029] The term "about" refers to the range of values approximately
near the given value in order to account for typical tolerance
levels, measurement precision, or other variability of the
embodiments described herein.
[0030] A "C-rate" refers to either (depending on context) the
discharge current as a fraction or multiple relative to a "1 C"
current value under which a battery (in a substantially fully
charged state) would substantially fully discharge in one hour, or
the charge current as a fraction or multiple relative to a "1 C"
current value under which the battery (in a substantially fully
discharged state) would substantially fully charge in one hour.
[0031] Ranges presented herein are inclusive of their endpoints.
Thus, for example, the range 1 to 3 includes the values 1 and 3 as
well as the intermediate values.
[0032] Solid-state batteries can be formed using polymeric
materials with ion conducting properties. The polymeric materials
can be used in the solid electrolyte. The polymer should have
suitable mechanical properties and thermal stability, in addition
to the desired level of ionic conductivity, and specifically
lithium ion conductivity. As with other applications using
polymeric materials, the properties of the solid structure can be
influenced by (i) the choice of polymer, (ii) the molecular weight
of the polymer, (iii) the polydispersity of the polymer, (iv) the
processing conditions, and (v) the presence of additives.
[0033] Poly(ethylene oxide) ("PEO") is a suitable polymer for use
in lithium ion solid-state batteries. PEO is a commodity polymer
available in a variety of molecular weights. PEO can range from
very short oligomers of about 300 g/mol (or 300 Da) to very high
molecular weights of 10,000,000 g/mol (or 10,000 kDa). At molecular
weights of 20 kDa and below, PEO is typically referred to as
poly(ethylene glycol) or PEG. PEO has been used as a separator in
conventional liquid electrolyte systems and, as described above, as
a component in a thin film solid electrolyte.
[0034] PEO processed into a structure can have both crystalline and
amorphous domains. Ionic conductivity happens more readily in the
amorphous domains and, therefore, processing conditions that
decrease crystalline domain size and/or the overall amount of
crystallinity are preferred. Some research has used carbonate
solvents, such as ethylene carbonate, dimethyl carbonate, or
diethyl carbonate, as plasticizers to improve ionic transport and
reduce interfacial impedance. However, this involves the addition
of a volatile, flammable liquid to the battery and negates much of
the safety benefits brought by a solid-state electrolyte. In PEO
systems, PEG can be added to achieve the desired processing
properties, such as a preferred solution viscosity, film modulus,
or film glass transition temperature.
[0035] While PEO is discussed herein as a preferred polymeric
material, it is understood that other polymers with equivalent
chemical, electrochemical, mechanical, and/or thermal properties
can be used in place of or in addition to PEO and/or PEO/PEG
mixtures. Further, copolymers that include PEO, PEG, or PEO-like
polymers in at least one segment of the copolymer can be suitable
for certain embodiments described herein. Thus, the embodiments
described herein that refer to PEO or PEO/PEG are understood to
encompass other such polymeric and co-polymeric materials.
[0036] According to some aspects discussed herein, certain lithium
salts added to polymeric materials improve the performance of
solid-state batteries. Specifically, a lithium salt concentration
in a PEO such that the ether oxygen (EO) to lithium ion ratio is
about 3.1 (that is, [EO]:[Li.sup.+]=3:1) results in maximum ionic
conductivity in the PEO films. In embodiments disclosed herein, the
[EO]:[Li.sup.+] ratio varies from about 2:1 to about 4:1, but is
preferably about 3:1 to achieve the desired conductivity.
Mechanical properties of the lithium salt/polymer composites are
controlled by the molecular weight of the PEO, the ratio of
PEO/PEG, and the process used to make the film (e.g., the type and
nature of the solvent used for casting).
[0037] Suitable lithium salts include, but are not limited to,
lithium triflate (LiCF.sub.3SO.sub.3), lithium tetrafluoroborate
(LiBF.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium bromide (LiBr), lithium
chlorate (LiClO.sub.3), lithium nitrate (LiNO.sub.3), lithium
bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2) (also referred to
herein as "LiBOB"), lithium difluoro(oxalato)borate
(LiC.sub.2O.sub.4BF.sub.2), lithium metaborate
(Li.sub.2B.sub.4O.sub.7), lithium
bis(trifluoromethanesulfonyl)imide
(CF.sub.3SO.sub.2NLiSO.sub.2CF.sub.3) (also referred to herein as
"LiTFSI"), and combinations thereof. In preferred embodiments, the
lithium salt is lithium triflate, LiBOB, LiTFSI, or combinations
thereof.
[0038] As discussed above, additives can be used to favorably
influence the properties of the final polymer structure. The
addition of lithium salts to PEO can result in favorable thermal
properties for the resulting mixture of salt and polymer.
[0039] For example, Table 1 provides the results of Differential
Scanning Calorimetry testing of various salt and polymer
combinations. The molecular weight (MW) of the PEO is provided in
the first column in kiloDaltons. The identity of the lithium salt
additive is provided in the second column. Note that the first row
is a polymer formulation without any added lithium salt. The ratio
of PEO:PEG:Salt (weight %) is provided in the third column. The
melt onset temperature (T.sub.m onset), and peak melt temperature
(T.sub.m peak) are provided in the final two columns. Table 1
demonstrates that the ratio of PEO:PEG was about 4.26 and was the
same for all salt loading levels. Also, the 4.26 PEO:PEG ratio was
maintained for the different molecular weights of PEO used in this
testing. Thus, the thermal differences among the tested
formulations can be attributed to the presence of the salt.
TABLE-US-00001 TABLE 1 Thermal Data for PEO/Salt Combinations
Formulation T.sub.m T.sub.m PEO MW PEO:PEG:Salt (onset) (peak)
(kDa) Salt (weight %) (.degree. C.) (.degree. C.) 7,000 None
81:19:0 59 67 7,000 LiBOB 72.9:17.1:10 53 64 7,000 LiBOB
64.8:15.2:20 34 54 8,000 LiBOB 64.8:15.2:20 32 55 7,000 Li Triflate
64.8:15.2:20 57 65 8,000 Li Triflate 64.8:15.2:20 58 66
[0040] The addition of Li triflate to the PEO/PEG mixture did not
have a significant effect on the thermal properties of the PEO/PEG
as compared to the control. That is, the values for the onset of
the melting temperature and the peak melting temperature remained
similar to the unloaded PEO/PEG regardless of the molecular weight
of the PEO.
[0041] However, the addition of LiBOB salt to the PEO/PEG mixture
resulted in a decrease in the onset of the melting temperature and
a decrease in the peak melting temperature of the original polymer.
Specifically, a 10% LiBOB loading level decreased the onset of the
melting temperature and the peak melting temperature. A 20% LiBOB
loading level further decreased the onset of the melting
temperature and the peak melting temperature of the mixture. At a
20% LiBOB loading level and a slightly higher molecular weight PEO,
the decrease versus control in the onset of the melting temperature
and the peak melting temperature was maintained.
[0042] The decrease in the onset of the melting temperature and the
peak melting temperature is consistent with the LiBOB acting as a
solid plasticizer for the PEO/PEG mixture. Therefore, if the ionic
conductivity in the polymer is increased due to the plasticization,
no volatile or liquid plasticizer was actually required. As
described in more detail below, the benefit of LiBOB in improving
the cell capacity is observed for all the molecular weights of PEO
tested.
[0043] Using the formulations of polymer and salt generally
described above, electrolyte structures and electrode structures
can be formed for lithium ion batteries. In certain aspects, solid
electrolytes are formed from a polymer and a lithium salt. The
inclusion of a lithium salt, such as those disclosed herein and
their equivalents, can improve the performance of solid-state
batteries by the mechanism disclosed herein and other equivalent
mechanisms. For example, the inclusion of LiBOB or LiTFSI in a
PEO/PEG mixture can increase the conductivity of lithium ions
through a PEO/PEG structure and can reduce the interfacial
impedance between the electrolyte structure and the electrode
structure.
[0044] FIG. 1A depicts a schematic representation of a solid-state
battery. The cathode 10 includes domains of active material 10a and
domains of conductive carbon 10b. A binder may also be present in
the cathode 10 but is not pictured. The active material can be any
active material or materials useful in a lithium ion battery,
including the active materials in lithium metal oxides or layered
oxides (e.g., Li(NiMnCo)O.sub.2), lithium rich layered oxide
compounds, lithium metal oxide spinel materials (e.g.,
LiMn.sub.2O.sub.4, LiNi.sub.0.5Mn.sub.1.5O.sub.4), olivines (e.g.,
LiFePO.sub.4, etc.). Active materials can also include compounds
such as silver vanadium oxide (SVO), metal fluorides (e.g.,
CuF.sub.2, FeF.sub.3), and carbon fluoride (CF.sub.x). More
generally, the active materials for cathodes can include
phosphates, fluorophosphates, fluorosulphates, silicates, spinels,
and composite layered oxides.
[0045] In some embodiments, polymer/lithium salt materials and
composites described herein are used in the formation of anodes.
Appropriate active materials for use in such anodes include, but
are not limited to, graphitic and non-graphitic carbons, silicon
and silicon alloys, lithium tin oxide, other metal alloys, and
combinations thereof.
[0046] In FIG. 1A, the solid electrolyte structure 20 is formed
from any of the polymer/lithium salt formulations disclosed herein.
The solid electrolyte structure 20 is depicted as a uniform and
monolithic structure, but other configurations are possible. The
anode 30 is depicted in FIG. 1A and can be a lithium metal anode,
for example. The solid polymer electrolyte can include dispersions
of nanoparticles 30a, which may be incorporated to improve ionic
conductivity or mechanical properties.
[0047] The loading of the lithium salt in the polymeric material of
solid electrolyte structure 20 provides improved lithium ion
conduction as compared to an unloaded polymeric material. Further,
lithium ion transport across the electrolyte/anode interface 25 can
be enhanced by the lithium salt loaded solid electrolyte structure
20. The presence of the solid electrolyte structure 20 according to
embodiments disclosed herein reduces the impedance in the battery
and improves the battery capacity.
[0048] In certain aspects, cathodes for solid-state batteries are
formed from an active material, a polymer, and a lithium salt
combination. The combination of lithium salts and PEO (or PEO/PEG)
can be incorporated in the cathode structure. The advantages of
improving ion transport and decreasing interfacial impedance are
also important within cathodes.
[0049] One of the benefits of liquid-containing batteries is that a
liquid electrolyte can penetrate the porous space of a cathode and
provide ion conduction paths. It is this benefit that perpetuates
the use of volatile liquid organics despite their safety issues. A
solid-state battery does not have liquid to facilitate ion
transport from within the cathode material.
[0050] Advantageously, embodiments of the polymer/lithium salt
formulations disclosed herein can be used as additives during the
formation of cathodes. FIG. 1B depicts a schematic representation
of a solid-state battery which has an anode 30, a solid electrolyte
structure 20, and a electrolyte/anode interface 25 similar to that
depicted in FIG. 1A. FIG. 1B depicts a cathode structure 10 that
includes domains of active material 10a and domains of conductive
carbon 10b. The cathode structure 10 further includes domains of a
polymer/lithium salt formulation 15. While FIG. 1B depicts the
components of cathode 10 as discrete domains, it is understood that
processing steps (such as heating, dissolution, and/or pressure)
can be used to create more intimate mixing of the cathode
components. During such mixing, interfaces will be formed among the
domains of the cathode. Advantageously, the polymer/lithium salt
formulations can reduce the interfacial impedance in the cathode.
Further, while the domains are depicted with a particle-type
configuration, the domains can be in other configurations,
including for example interpenetrating layers and/or films.
[0051] Further, FIG. 1B depicts domains of a polymer/lithium salt
formulation 15 and the solid electrolyte structure 20 with a
different pattern. This pattern difference indicates that the
actual formulation of the polymer/salt for use in the cathode may
be different from the formulation for use in the solid electrolyte.
For example, parameters including (i) the molecular weight of the
PEO, (ii) the ratio of PEO to PEG, (iii) the loading of salt; (iv)
the choice of salt or salts; and (v) the polymer/salt formulation
processing conditions can be varied and/or optimized for use in the
cathode as compared to the parameters selected for use in the
electrolyte.
[0052] In some aspects, solid state batteries according to
embodiments herein can have no lithium salt additive loaded in the
solid electrolyte if the cathode material includes polymer/lithium
salt formulations. The additive lithium salt within the cathode can
migrate out into the solid electrolyte over time and provide
benefits such as increased capacity and decreased impedance to the
overall battery. Still further, the salt in the solid electrolyte
can be different from the salt in the polymer/salt formulation in
the cathode. For example, lithium triflate can be used in the
electrolyte (with a polymer such as PEO or PEO/PEG) and LiBOB or
LiTFSI can be used in the cathode (with a polymer such as PEO or
PEO/PEG).
[0053] Embodiments of the polymer/lithium salt formulations
disclosed herein can be used as additives during the formation of
composite solid electrolytes. FIG. 1C depicts a schematic
representation of a solid-state battery which has an anode 30 and a
cathode 10 similar to that depicted in FIG. 1B. In FIG. 1C, the
electrolyte is formed from a composite of domains of a
polymer/lithium salt formulation 22 and domains of a lithium ion
conducting ceramic 28. In some cases, the lithium ion conducting
material can be a garnet material such as a cubic garnet phase
Li.sub.6.5La.sub.3Zr.sub.1.5Ta.sub.0.5O.sub.12(LLZTO), sulfides
such as Li.sub.10SnP.sub.2S.sub.12 (LSPS) and
P.sub.2S.sub.5--Li.sub.2S glass, lithium ion conducting glass
ceramics (LIC-GC) such as
Li.sub.1+x+yAl.sub.xTi.sub.2-xSi.sub.yP.sub.3-yO.sub.12, phosphates
such as Li.sub.1.3Ti.sub.1.7Al.sub.0.3(PO.sub.4).sub.3 (LTAP) or
Li.sub.2PO.sub.2N (LiPON), or combinations thereof.
[0054] A general formula for garnet materials, which can be
abbreviated as (LLMO), is:
Li.sub.3+xLa.sub.3-yA.sub.yM.sub.2O.sub.12 (1)
where M can be a variety of different elements, including but not
limited to, titanium (Ti), zirconium (Zr), niobium (Nb), tantalum
(Ta), antimony (Sb), bismuth (Bi) and combinations thereof, and A
can also be a variety of different elements, including but not
limited to, barium (Ba). Generally, x<=4 and y<=1. It is
intended that the garnet materials useful for embodiments disclosed
herein include those presently known to be useful and those
contemplated in future uses to be useful in lithium ion
batteries.
[0055] As with the other solid electrolyte formulations disclosed
herein, the polymer/lithium salt formulation can reduce interfacial
impedance in systems like those schematically depicted in FIG.
1C.
[0056] The formulations disclosed above have been tested in various
configurations. The lithium salt/polymer combinations are useful as
components of cathode materials and solid-state electrolyte
materials. As will be apparent from the example below,
PEO/PEG/LiBOB and PEO/PEG/LiTFSI perform well in both the cathode
and the solid electrolyte in enabling high cell capacity. Of
course, other formulations can enable improvements in battery
performance as compared to unloaded polymeric materials.
[0057] The following examples describe specific aspects of some
embodiments of the invention to illustrate and provide a
description for those of ordinary skill in the art. The examples
should not be construed as limiting the invention, as the examples
merely provide specific methodology useful in understanding and
practicing some embodiments of the invention.
Examples
[0058] Preparation of Solid Electrolyte Films.
[0059] A solution of PEO, PEG, and the desired lithium salt or
salts is prepared by weighing the desired ratios of solids,
followed by addition of a solvent (such as acetonitrile). The
solution is stirred aggressively overnight in an argon filled glove
box (M-Braun, O.sub.2 and humidity content <0.1 ppm). A film is
cast from the slurry using a doctor blade onto a Teflon substrate,
and is then air-dried. The film is annealed at 100 degrees C. under
vacuum for 12 hours, and then cooled. A freestanding film can then
be peeled from the substrate, and cut or punched to the appropriate
size and shape. The punched films are dried at 60 degrees C. under
vacuum for about an hour.
[0060] Preparation of Cathode Films.
[0061] A stock solution containing PEO (molecular weight can be
chosen based on the desired properties of the finished structure),
conductive carbon black, and an additive lithium salt (such as
LiBOB) in desired ratios (such as those identified herein) is
prepared in a solvent (such as benzonitrile) by stirring overnight.
The PEO and LiBOB dissolve, while the carbon is suspended in the
polymer/salt solution. The stock solution is then added to an
Ag.sub.2V.sub.4O.sub.11 ("SVO") cathode powder in the desired
amount, and the resulting suspension is stirred overnight. The
resulting slurry is then coated onto a current collector, and then
dried at 60 degrees C. until substantially all of the solvent is
evaporated. Films are then pressed to a desired density. The
pressed cathode films are dried at 60 degrees C. under vacuum
overnight prior to cell assembly.
[0062] Battery Cell Assembly.
[0063] Battery cells were formed in a high purity argon filled
glove box (M-Braun, O.sub.2 and humidity content <0.1 ppm). The
silver-vanadium oxide ("SVO") cathode film described previously and
a lithium metal anode electrode were used. The SVO//Li pairing is
typical of a primary battery chemistry. Each battery cell includes
the composite cathode film prepared as described above, a solid
polymer electrolyte prepared as described above, and a lithium
metal anode film. No liquid electrolyte components were added to
the battery cell. Annealing of the stack of cathode/electrolyte
films was done at 110 degrees C. on a hot plate for 1 hour prior to
putting in the cell with lithium and crimping the cell together.
All assembly was done under argon.
Example 1
[0064] After cell assembly, the cells were held at open current
voltage for 12 hours at 37 degrees C. The cells were then held at
37 degrees C. and discharged at the desired C-rate to determine the
capacity. FIG. 2 shows a comparison at C/100 for various salts
formulated into the cathode slurry and FIG. 3 shows a comparison at
C/1000. The bands in each of FIGS. 2 and 3 represent the standard
deviation of the replicates of the control. All solid electrolytes
in the cells contained lithium triflate. Thus, FIGS. 2 and 3
compare the presence of a given lithium salt/polymer formulation in
the cathode material, with a single lithium salt/polymer
formulation as the electrolyte. The cathodes in FIGS. 2 and 3 were
unpressed, which is relevant because pressing cathodes prior to use
will improve the eventual performance of the battery. The additive
amounts are expressed as a percentage of the amount loaded into the
polymeric material.
[0065] FIG. 2 demonstrates that at a discharge rate of C/100, LiBOB
performs better than the other lithium salts and better than
cathodes with no lithium salt. FIG. 2 shows results from cathodes
containing LiBOB that yielded approximately 40-80 mAh/g (as
compared to the theoretical capacity of 270 mAh/g). In combination,
FIGS. 2 and 3 demonstrate that the capacity observed when LiBOB is
incorporated into the cathode is dependent upon the discharge rate.
FIG. 3 demonstrates that at a slower rate, C/1000, formulations
containing no salt can yield about 30 mAh/g of capacity. FIG. 3
shows results from cathodes containing LiBOB that reach
approximately 100 mAh/g.
Example 2
[0066] After pressing the films at greater than 1 ton/cm.sup.2, the
C/100 capacity reaches 220 mAh/g, which is 81% of theoretical
capacity, for composite cathodes that contain LiBOB, as depicted in
FIG. 4. That is, in FIG. 4, the cathode is formed with LiBOB/PEO
domains as described in the synthetic methods section above. The
solid electrolyte is formed to include PEO/lithium triflate. The
anode is based on lithium metal. At C/1000, 88% of the theoretical
capacity of this electrochemical cell arrangement is demonstrated
in testing. Similar results can be achieved when LiTFSI is used in
the composite cathode.
Example 3
[0067] Electrochemical impedance spectroscopy is used to determine
the ionic conductivity of PEO/lithium salt films. A film with known
thickness and area is placed between two polished stainless steel
("SS") disks, and an AC voltage (10 mV) is applied at varying
frequencies. The resulting amplitude change and phase shift in the
response is used to calculate ionic conductivity of the film. FIG.
5 shows the measured ionic conductivity of PEO/lithium salt films.
The concentration of the LiBOB is different by half than that of
the lithium triflate (10% by weight for LiBOB and 20% by weight for
lithium triflate), but both films are similar in ionic
conductivity. The incorporation of LiBOB or lithium triflate into
the PEO does not result in significant changes in ionic
conductivity of the film as measured in this test and depicted in
FIG. 5.
Example 4
[0068] Electrochemical impedance spectroscopy is used to determine
the ionic conductivity of PEO/lithium salt films placed between
lithium substrates instead of the stainless steel disks of Example
3. The impedance is determined by where the data cross the x-axis.
The impedance of films containing the lithium triflate is an order
of magnitude higher than those containing LiBOB, as depicted in
FIGS. 6A and 6B (where 6B is a magnified view of the data near the
origin of the plot depicted in FIG. 6A). This data demonstrates
that the interfacial impedance on lithium substrates is reduced for
PEO containing LiBOB. Thus, while FIG. 5 confirmed that there was
not a significant change in ionic conductivity when comparing LiBOB
and lithium triflate, FIG. 6 shows that polymer/LiBOB formulations
can improve the impedance performance of a cathode as compared to
polymer/lithium triflate formulations.
Example 5
[0069] Electrochemical impedance spectroscopy is used to determine
the time-dependence of the ionic conductivity of PEO/lithium salt
films placed between lithium substrates. FIGS. 7A and 7B
demonstrate that the interfacial impedance does not increase with
time, indicating that a passivation film does not build up on the
lithium metal due to reaction of the LiBOB with the lithium metal
anode. Rather, the interfacial impedance decreases with time when
LiBOB is used, in contrast to the lithium triflate control.
Electrolyte formulations containing LiBOB are stable on the
reductive lithium metal surface as measured by monitoring the
impedance.
Example 6
[0070] Electrochemical impedance spectroscopy is used to determine
the ionic conductivity of LLZTO/PEO/lithium salt films placed
between stainless steel disks. In composites with PEO, LLZTO, and
lithium salts, incorporation of LiBOB or LiTFSI result in a
surprising increase in ionic conductivity as shown in FIG. 8. LLZTO
is a ceramic inclusion that acts as an intercalation compound and
facilitates ionic conductivity. However, without the lithium salt
additives in the film, the LLZTO is only somewhat effective.
Surprisingly, the LiBOB and LiTFSI increased the effectiveness of
the ceramic inclusion.
[0071] Without being bound to any particular hypothesis or
mechanism of action, the addition of lithium salts (and LiBOB or
LiTFSI in particular) may increase the presence of amorphous
domains in the polymer. It is hypothesized that the softer
amorphous domains (as compared to the harder crystalline domains)
enable better lithium transport and also form better interfaces
than the original (and less amorphous) polymer material. These
better interfaces can be particularly effective in the presence of
ceramic inclusions that are added to facilitate ionic conductions,
such as garnet-type ceramic inclusions.
[0072] Certain embodiments disclosed herein demonstrate improved
power performance as compared to control and the ability to achieve
nearly theoretical capacity out of the cathode.
[0073] Certain embodiments disclosed herein demonstrate
significantly lower impedance at the solid electrolyte/lithium
metal anode interface and improved conductivity.
[0074] Certain embodiments disclosed herein facilitate lithium salt
migration from a comparatively thick cathode and into a thin
electrolyte, thereby increasing the lithium salt content in the
electrolyte from an initial amount of as low as 0% in the solid
electrolyte.
[0075] Certain embodiments disclosed herein demonstrate that the
combination of using LiBOB and/or LiTFSI as an additive in both the
cathode and electrolyte formulations enables high discharge
capacities in an all solid-state battery.
[0076] Various embodiments of solid electrolyte formulations
disclosed herein benefit from the discovery of the reduction of
interfacial impedance between a conducting polymer (such as PEO)
and a lithium-ion-intercalating ceramic inclusion (such as LLZTO).
As compared to prior art formulations, the present formulations use
ceramic inclusions that can transport and/or intercalate lithium
rather than as non-conductive filler materials.
[0077] Notably, the formulations disclosed herein can be used in
cathode formulations. Thus, the ceramic materials that intercalate
lithium and the lithium salts that reduce interfacial impedance can
be used with conducting polymers that act as binders when
formulating cathode materials.
[0078] While the invention has been described with reference to the
specific embodiments thereof, it should be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted without departing from the true spirit and scope
of the invention as defined by the appended claims. In addition,
many modifications may be made to adapt a particular situation,
material, composition of matter, method, or process to the
objective, spirit and scope of the invention. All such
modifications are intended to be within the scope of the claims
appended hereto. In particular, while the methods disclosed herein
have been described with reference to particular operations
performed in a particular order, it will be understood that these
operations may be combined, sub-divided, or re-ordered to form an
equivalent method without departing from the teachings of the
invention. Accordingly, unless specifically indicated herein, the
order and grouping of the operations are not limitations of the
invention.
* * * * *