U.S. patent application number 09/196882 was filed with the patent office on 2002-01-24 for electrolytes having improved low temperature performance.
Invention is credited to BARKER, JEREMY, GAO, FENG, STUX, ARNOLD.
Application Number | 20020009651 09/196882 |
Document ID | / |
Family ID | 22727141 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020009651 |
Kind Code |
A1 |
BARKER, JEREMY ; et
al. |
January 24, 2002 |
ELECTROLYTES HAVING IMPROVED LOW TEMPERATURE PERFORMANCE
Abstract
The present invention provides a novel electrolyte solvent which
is usable with a variety of carbonaceous and metal oxide electrode
active materials, providing improved performance over a broad
temperature range, and which is stabilized to maintain cell
capacity over a number of cycles.
Inventors: |
BARKER, JEREMY; (REDMOND,
WA) ; GAO, FENG; (HENDERSON, NV) ; STUX,
ARNOLD; (BALTIMORE, MD) |
Correspondence
Address: |
LINDA DESCHERE
YOUNG & BASILE
3001 WEST BIG BEAVER ROAD
SUITE 624
TROY
MI
48084
|
Family ID: |
22727141 |
Appl. No.: |
09/196882 |
Filed: |
November 20, 1998 |
Current U.S.
Class: |
429/331 ;
429/223; 429/224; 429/231.1; 429/231.3; 429/231.8; 429/247;
429/254; 429/332; 429/333 |
Current CPC
Class: |
H01M 50/411 20210101;
H01M 4/587 20130101; H01M 50/426 20210101; H01M 10/0569 20130101;
Y02E 60/10 20130101; H01M 10/0565 20130101 |
Class at
Publication: |
429/331 ;
429/332; 429/333; 429/231.8; 429/231.1; 429/223; 429/224;
429/231.3; 429/254; 429/247 |
International
Class: |
H01M 010/40; H01M
004/58; H01M 004/48; H01M 004/52; H01M 004/50; H01M 002/16 |
Claims
What is claimed is:
1. An electrochemical cell which comprises a first electrode, a
counter electrode which forms an electrochemical couple with said
first electrode, and an electrolyte; said first electrode
comprising particles of carbonaceous intercalation active material;
and said electrolyte comprising a solvent mixture and a solute;
said solvent mixture comprising ethylene carbonate (EC), propylene
carbonate (PC), and a compound represented by R'COOR", where R' and
R" are each independently selected from the group consisting of
ethyl and propyl.
2. The electrochemical cell according to claim 1 wherein said
solvent mixture consists essentially of said propylene carbonate
(PC), said ethylene carbonate (EC), said compound represented by
R'COOR", and optionally one or more other organic solvents; with
said compound represented by R'COOR" being present in an amount by
weight which is not greater than the combined amount of said EC,
said PC, and said one or more other solvents.
3. The electrochemical cell according to claim 2 wherein each said
one or more other solvents is selected from the group consisting of
carbonates; lactones; propionates; five member heterocyclic ring
compounds; and organic solvent compounds having a low alkyl (1-4
carbon) group connected through an oxygen to a carbon, and
comprising C--O--C bonds.
4. The electrochemical cell according to claim 2 wherein each said
one or more other organic solvents is selected from the group
consisting of methyl ethyl carbonate (MEC), diethyl carbonate
(DEC), dipropyl carbonate (DPC), dimethyl carbonate (DMC), butylene
carbonate (BC), dibutyl carbonate (DBC), vinylene carbonate (VC),
diethoxy ethane (DEE), and mixtures thereof.
5. The electrochemical cell according claim 1 wherein said solvent
mixture consists essentially of said ethylene carbonate (EC), said
propylene carbonate (PC), said compound represented by R'COOR", and
dimethyl carbonate (DMC).
6. The electrochemical cell according to claim having said
EC/DMC/R'COOR" in a weight ratio of about 25:40:35 and said PC
present in an amount less than said compound represented by
R'COOR".
7. A lithium ion electrochemical cell which comprises a first
electrode, a counter electrode which forms an electrochemical
couple with said first electrode, and an electrolyte; each of said
electrodes having an intercalation active material, with at least
one said active material being a carbonaceous active material; and
said electrolyte comprising a solvent mixture and a solute; said
solvent mixture consisting essentially of ethylene carbonate (EC),
a compound represented by R'COOR", where R' and R" are each
independently selected from the group consisting of ethyl and
propyl, and dimethyl carbonate (DMC).
8. The electrochemical cell according to claim 7 wherein said
active material of said first electrode is carbonaceous and
consists of graphite particles.
9. The electrochemical cell according to claim 7 wherein said
active material of said counter electrode is lithium transition
metal oxide compound.
10. The electrochemical cell according to claim 9 wherein said
lithium transition metal oxide compound is selected from the group
consisting of LiMn.sub.2O.sub.4, LiNiO.sub.2, LiCoO.sub.2, and
mixtures thereof.
11. The electrochemical cell according to claim 9 wherein said
lithium transition metal oxide is lithium manganese oxide.
12. The electrochemical cell according to claim 11 wherein said
lithium manganese oxide is represented by the nominal general
formula Li.sub.1+xMn.sub.2-xO.sub.4 (-0.2.ltoreq.x.ltoreq.0.2).
13. The electrochemical cell according to claim 7 wherein said
electrolyte further comprises a separator in the form of a solid
matrix forming a network with voids interpenetrated by said solvent
mixture and said solute.
14. The electrochemical cell according to claim 13 wherein said
matrix is selected from the group consisting of polymeric acrylate,
porous polypropylene, porous polyethylene, and glass fiber
material.
15. The electrochemical cell according to claim 7 wherein said
compound represented by R'COOR" is ethyl propionate (EP) and said
solvent mixture consists of said EC.backslash.DMC.backslash.EP in a
weight ratio of 25:40:35.
Description
FIELD OF THE INVENTION
[0001] This invention relates to electrolytes which function as a
source of alkali metal ions for providing ionic mobility and
conductivity. The invention more particularly relates to
electrolytic cells where such electrolytes function as an ionically
conductive path between electrodes.
BACKGROUND OF THE INVENTION
[0002] Electrolytes are an essential member of an electrolytic cell
or battery. In one arrangement, a battery or cell comprises an
intermediate separator element containing an electrolyte solution
through which lithium ions from a source electrode material move
between cell electrodes during the charge/discharge cycles of the
cell. The invention is particularly useful for making such cells in
which the ion source electrode is a lithium compound or other
material capable of intercalating lithium ions, and where an
electrode separator membrane comprises a polymeric matrix made
tonically conductive by the incorporation of an organic solution of
a dissociable lithium salt which provides ionic mobility.
Early Lithium Metal Cells
[0003] Early rechargeable lithium cells utilized lithium metal
electrodes as the ion source in conjunction with positive
electrodes comprising compounds capable of intercalating the
lithium ions within their structure during discharge of the cell.
Such cells relied, for the most part, on separator structures or
membranes which physically contained a measure of fluid
electrolyte, usually in the form of a solution of a lithium
compound, and which also provided a means for preventing
destructive contact between the electrodes of the cell. Sheets or
membranes ranging from glass fiber, filter paper or cloth to
microporous polyolefin film or nonwoven organic or inorganic fabric
have been saturated with solutions of an inorganic lithium
compound, such as LiClO.sub.4, LIPF.sub.6, or LiBF.sub.4, in an
organic solvent to form such electrolyte/separator elements. The
fluid electrolyte bridge thus established between the electrodes
has effectively provided the necessary Li+ ion mobility at
conductivities in the range of about 10.sup.-3 S/cm.
Ion, Rocking Chair Cells and Polymer Cells
[0004] Lithium metal anodes cause dendrite formation during
charging cycles which eventually leads to internal cell
short-circuiting. Some success has been achieved in combatting this
problem through the use of lithium-ion cells in which both
electrodes comprise intercalation materials, such as lithiated
metal oxide and carbon (U.S. Pat. No. 5,196,279), thereby
eliminating the lithium metal which promotes the deleterious
dendrite growth. Another approach to controlling the dendrite
problem has been the use of continuous films or bodies of polymeric
materials which provide little or no continuous free path of low
viscosity fluid in which the lithium dendrite may propagate. These
materials may comprise polymers, e.g., poly (alkylene oxide), which
are enhanced in ionic conductivity by the incorporation of a salt,
typically a lithium salt such as LiClO.sub.4, LiPF.sub.6, or the
like. A range of practical ionic conductivity, i.e., over about
10.sup.-5 to 10.sup.-3 S/cm, was only attainable with these polymer
compositions at well above room temperature, however. (U.S. Pat.
Nos. 5,009,970 and 5,041,346.)
"Solid" and "Liquid" Batteries of the Prior Art
[0005] More specifically, electrolytic cells containing an anode, a
cathode, and a solid, solvent-containing electrolyte incorporating
an inorganic ion salt were referred to as "solid batteries". (U.S.
Pat. No. 5,411,820). These cells offer a number of advantages over
electrolytic cells containing a liquid electrolyte (i.e., "liquid
batteries") including improved safety factors. Despite their
advantages, the manufacture of these solid batteries requires
careful process control to minimize the formation of impurities.
Solid batteries employ a solid electrolyte matrix interposed
between a cathode and an anode. The inorganic matrix may be
non-polymeric [e.g., .beta.-alumina, silver oxide, lithium iodide,
etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers]
whereas the organic matrix is typically polymeric. Suitable organic
polymeric matrices are well known in the art and are typically
organic polymers obtained by polymerization of a suitable organic
monomer as described, for example, in U.S. Pat. No. 4,908,283.
[0006] Examples of solvents known in the art are propylene
carbonate, ethylene carbonate, .gamma.-butyrolactone,
tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme,
dimethylsulfoxide, dioxolane, sulfolane, diethoxyethane, and the
like. These are examples of aprotic, polar solvents.
[0007] More recently, a highly favored electrolyte/separator film
is prepared from a copolymer of vinylidene fluoride and
hexafluoropropylene. Methods for making such films for cell
electrodes and electrolyte/separator layers are described in U.S.
Pat. Nos. 5,418,091; 5,460,904; and 5,456,000 assigned to Bell
Communications Research, each of which is incorporated herein by
reference in its entirety. A flexible polymeric film useful as an
interelectrode separator or electrolyte member in electrolytic
devices, such as rechargeable batteries, comprises a copolymer of
vinylidene fluoride with 2 to 25% hexafluoropropylene. The film may
be cast or formed as a self-supporting layer retaining about 20% to
70% of a high-boiling solvent or solvent mixture comprising such
solvents as ethylene carbonate or propylene carbonate. The film may
be used in such form or after leaching of the retained solvent with
a film-inert low-boiling solvent to provide a separator member into
which a solution of electrolytic salt is subsequently imbibed to
displace retained solvent or replace solvent previously leached
from the polymeric matrix.
Electrolyte Performance
[0008] Regardless of which technique is used in preparing an
electrolyte/separator, problems occur including operability of the
electrolyte in a relatively narrow temperature range; loss of
effectiveness of the electrolyte; and electrolyte degradation.
There is presently no effective means to maintain the
serviceability of the electrolyte over a broad temperature range,
particularly low temperature.
[0009] In view of the above, it can be seen that it is desirable to
have an improved electrolyte which is operable over a relatively
broad temperature range, including low temperature, and which
maintains cell capacity in a variety of electrolyte/separator
configurations, including those described above as exemplary.
SUMMARY OF THE INVENTION
[0010] The present invention provides a novel electrolyte solvent
which is usable with a variety of carbonaceous and metal oxide
electrode active materials, providing improved performance over a
broad temperature range, and which is stabilized to maintain cell
capacity over a number of cycles. The electrolyte includes a
specifically selected class of solvents, and solvent combinations
using such new solvents. The new solvents, when used as
co-solvents, enhance the operable temperature range of the solvent
mixture. The solvents of the invention are esters, generally
characterized with lower melting points and higher boiling points
compared to the range observed for commonly used solvents, such as
dimethyl carbonate or diethyl carbonate. The novel, ester solvents
of the invention have further lower melting points and higher
boiling points than conventional solvents. The solvents are useful
as both high and low temperature solvents but are particularly
useful for low temperature applications such as start, light,
ignition (SLI). The compounds usable as solvents according to the
invention are compounds represented by the general formula R' COOR"
(alkyl aliphatic ester) where R' and R" are each independently
selected from the group consisting of ethyl and propyl.
[0011] In one embodiment, the ester represented by the general
formula is included in a solvent mixture which also comprises
ethylene carbonate (EC) and propylene carbonate (PC). In one
embodiment, the combined amount of the EC and PC is greater, on a
weight basis, than the amount of the ester of the formula stated
above.
[0012] In another embodiment, the solvent mixture further comprises
one or more other organic solvents along with the ester, and with
the EC and/or PC ester mixture. When such other additional organic
solvent or solvents is included in the mixture, it is preferred
that such solvent be selected from the group of carbonates;
lactones; propionates; five member hetercyclic ring compounds; and
organic solvent compounds having a low alkyl (1-4 carbon) group
connected through an oxygen to a carbon, and comprising C/O/C
bonds.
[0013] One preferred solvent mixture comprises EC, or EC and PC;
DMC; and the ester compound R' COOR" of the invention. A preferred
combination is EC/DMC/R' COOR" or EC/DMC/EP/EC in weight ratios as
follows: EC/DMC/EP at about 25:40:35 and at about 3:5:2 and
EC/DMC/EP/PC at about 58:29:12:1.
[0014] Advantageously, the solvent ester of the invention is usable
with a variety of cell electrode active materials including
lithium, transition metal oxide compounds such as
LiMn.sub.2O.sub.4, LiNiO.sub.2, LiCoO.sub.2, LiNiVO.sub.4, and
LiCoNiO.sub.2. It is most preferred that the electrode active
material be lithium manganese oxide represented by the nominal
general formula. Li.sub.1+xMn.sub.2-xO.sub.4
(-0.2.ltoreq.x.ltoreq.0.2)
[0015] Advantageously, the ester solvent of the invention is usable
with graphite active material consisting of particles which have an
interlayer distance spacing of 002 planes as determined by X-ray
diffraction of 0.33 to 0.34 nanometers; a crystallite size in the
direction of C-axis (L.sub.c) being greater than about 20
nanometers and less than about 2000 nanometers; and at least 90% by
weight of the graphite particles having a size less than about 60
microns. It is most preferred that the graphite particles have a
BET surface area greater than about 0.3 meters square per gram and
up to about 35 meters square per gram.
[0016] In the case where one or more additional organic solvents is
used in a solvent mixture along with the ester, the added solvents
are preferably organic solvents having a boiling point of about
80.degree. C. to about 300.degree. C. and are capable of forming a
solute with lithium salts. Preferably the added solvents are also
characterized by being aprotic, polar solvents. Preferred
additional organic solvents are ethylene carbonate (EC), dimethyl
carbonate (DMC), propylene carbonate (PC). The relative amounts of
the added solvents and the ester compound may vary so long as the
ester of the invention is present. One particularly useful
combination is a solvent mixture comprising EC/PC/DMC/R' COOR",
where R' and R" are each independently selected from the group
consisting of ethyl and propyl. Except for the present invention,
there is not known to be the use of solvent combinations comprising
EC/PC/R' COOR".
[0017] Advantageously, the solvent of the present invention
exhibits good performance even with carbonaceous electrode active
materials and with transition metal active electrode materials.
These materials are known to show poor performance when used with
more conventional organic solvents.
[0018] Objects, features and advantages of the invention include an
improved electrochemical cell or battery having good charging and
discharging characteristics; a large discharge capacity; good
integrity over a long life cycle; and operability over a large
temperature range and particularly relatively low temperature; and
which is stable with respect to carbonaceous and graphitic
electrode active material and stable with respect to metal oxide
electrode material.
[0019] These and other objects, features, and advantages will
become apparent from the following description of the preferred
embodiments, claims, and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a graph showing the results of pulse discharge
testing of a cell having 1480 milligrams lithium manganese oxide
(LMO) cathode active material, 493 milligrams BG-35 carbon graphite
anode active material, and electrolyte comprising one molar
LiPF.sub.6 in a solution of EC.backslash.DMC.backslash.EP in a
solvent volume ratio of 1:2:2, corresponding to a weight ratio of
about 25:40:35. The test was conducted at 20 milliamps per square
centimeter for temperatures ranging from +20.degree. C. to
-30.degree. C.
[0021] FIG. 2 is a graph showing the results of pulse discharge
testing of a cell having lithium manganese oxide cathode, BG-35
carbon graphite anode, and electrolyte comprising one molar
LiPF.sub.6 in a solution of EC.backslash.EMC.backslash.EP in a
solvent volume ratio of 1:2:2, corresponding to a weight ratio of
about 25:40:35. The test was conducted at 20 milliamps per square
centimeter for temperatures ranging from +20.degree. C. to
-20.degree. C.
[0022] FIG. 3 is a graph showing the results of pulse discharge
testing of a cell having lithium manganese oxide cathode, BG-35
carbon graphite anode, and electrolyte comprising one molar
LiPF.sub.6 in a solution of EC.backslash.EMC.backslash.EP in a
solvent weight ratio of 3:5:2. The test was conducted at 20
milliamps per square centimeter for temperatures ranging from
+20.degree. C. to -20.degree. C.
[0023] FIG. 4 is a graph showing the results of pulse discharge
testing of a cell having lithium manganese oxide cathode, BG-35
carbon graphite anode, and electrolyte comprising one molar
LiPF.sub.6 in a solution of EC.backslash.DMC.backslash.EP in a
solvent weight ratio of 3:5:2. The test was conducted at 20
milliamps per square centimeter for temperatures ranging from
+20.degree. C. to -20.degree. C.
[0024] FIG. 5 shows start, light, ignition (SLI) test data for the
cell described as per FIG. 1 and where the conditions of the test
are 25 milliamps per square centimeter at -18.degree. C.
[0025] FIG. 6 shows start, light ignition (SLI) test data for the
same cell described with respect to FIG. 1, and undergoing testing
at 18.5 milliamps per square centimeter and a temperature of
-29.degree. C.
[0026] FIG. 7 is a two part graph. FIG. 7A shows the testing of a
cell having a lithium manganese oxide cathode and an anode
electrode active material designated as KX44 which is graphitic
carbon fibers. The cell has an electrolyte which comprises one
molar LiPF.sub.6 in a solution of
EC.backslash.DMC.backslash.EC.backslash.PC in a weight ratio of
58:29:12:1. The cell charge and discharge are at +/-2 milliamp
hours per centimeters square, between 3.0 and 4.2 volts for one to
ten cycles. The negative electrode contained 667 milligrams of the
KX44 active material and the positive electrode contained 2000
milligrams of the lithium manganese oxide active material. The
surface area of each of the electrodes was 48 square centimeters.
FIG. 7A is coulombic efficiency and FIG. 7B is discharge capacity,
each versus cycles.
[0027] FIG. 8 is a graph showing the results of testing a
comparative cell with a solvent which contains only EC/DMC, without
any R'COOR" of the invention. In this graph, the electrolyte was
one molar LiPF.sub.6 and 2:1 EC/DMC solvent. This electrolyte was
used in a cell having lithium manganese oxide (LMO) positive
electrode and a BG/35 negative electrode. The active mass of the
positive electrode was 1480 milligrams and the active mass of the
negative electrode was 493 milligrams. In FIG. 8 the comparative
cell was cycled at 10 milliamp hours per square centimeter and at
temperatures of 20.degree. C. down to -10.degree. C. The cycling
data is not shown for -10.degree. C. because the cell failed.
[0028] FIG. 9 is a diagrammic representation of a typical laminated
lithium-ion battery cell structure which is prepared with the
electrolyte salt of the present invention.
[0029] FIG. 10 is a diagrammic representation of a multicell
battery cell structure which is prepared with the electrolyte salt
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] In the present state of the art, the use of graphite as a
negative electrode material presents a problem when used with a
propylene carbonate electrolyte solvent. Cells containing graphite
and propylene carbonate and other similar electrolytes suffer from
very poor reversible capability during delithiation
(deintercalation). In addition, electrolyte decomposition occurs
and significant gas is released, posing a safety risk. The
electrolyte decomposition is thought to be because graphite has
many active sites in its structure.
[0031] It has been found that if a graphite negative electrode is
used in an electrolyte containing propylene carbonate as the
solvent, the solvent is apparently absorbed into the active sites
of the graphite negative electrode and readily generates gas
through decomposition. As a result, the decomposition of the
solvent prevents lithium ion as an active material from
intercalating into the graphite on charging the battery and causes
an increase in polarization; consequently, the battery capacity is
decreased. In other words, it is thought that graphite is catalytic
and causes breakdown of propylene carbonate. Such decomposition of
the propylene carbonate results in the evolution of the gas,
probably propylene.
[0032] In view of the difficulties mentioned above, propylene
carbonate is usable only with non-graphitic anodes and is not
usable with crystalline, ordered planar structure graphitic anodes.
It has recently been suggested to use dimethyl carbonate (DMC) in
combination with ethylene carbonate (EC) for any type of
carbonaceous anode. See for example, U.S. Pat. Nos. 5,352,548 and
5,192,629, each of which is incorporated by reference in its
entirety. However, such electrolyte is undesirable since the DMC
readily evaporates leaving behind the EC which quickly solidifies,
rendering the cell useless. In U.S. Pat. No. 5,474,862, Okuno et al
restrict the use of ethylene carbonate to graphite active material.
In Okuno '862, PC is not recommended for graphite. Ethylene
carbonate and propylene carbonate are never used together according
to Okuno. Okuno states that PC may only be used in combination with
amorphous carbon or with metallic lithium.
[0033] In one embodiment, the electrolyte of the invention
comprises a solvent mixture containing the novel solvent esters of
the invention and useable with EC and PC together. Such solvent
esters have lower melting points and higher boiling points compared
to the range observed for commonly used solvents, such as dimethyl
carbonate (DMC). The DMC does not have a high boiling point and is
not suitable for high temperature operation. It is not suitable for
low temperature operation due to its high melting point. The esters
of the invention have further lower melting point and higher
boiling point useful as both high and low temperature solvents.
Therefore, the advantages of temperature spread between the melting
point and the boiling point is achieved by the esters of the
invention by employing a preferred compound of the formula R' COOR"
for R' and R" are each independently selected from the group
consisting of ethyl and propyl. The preferred solvents are ethyl
propionate, ethyl butyrate, propyl propionate and propyl butyrate.
The physical characteristics of these solvents are shown in Table
I.
[0034] The ester compound of the invention is preferably used in a
solvent mixture. Such solvent mixture preferably includes one or
more other organic solvents. Such solvent mixture preferably
includes one or more other organic solvents having a boiling point
of about 80.degree. C. to about 300.degree. C. and where such other
solvent is capable of forming a solute with lithium salts.
[0035] Even a small amount of the ester compound solvents of the
invention is helpful to the mixture, therefore, the lower limit is
greater than zero. A practical range by weight is up to 85% ester
compound solvent to other organic solvents in the mixture. The
ester is effective in electrolyte solutions comprising a solute
consisting essentially of a salt of lithium, and a solvent
consisting essentially of one or more apractic, polar solvent
compounds in combination with the ester.
[0036] Preferably, the aprotic polar solvent is selected from the
group consisting of carbonates, lactones, propionates, five member
ring compounds, and organic solvent compounds having a low alkyl
group (1-4 carbons) connected through an oxygen to a carbon and
comprising C--O--C bonds.
[0037] It is preferred that the aprotic, polar solvent is a
carbonate selected from the group consisting of propylene carbonate
(PC), ethylene carbonate (EC), methyl ethyl carbonate (MEC),
diethyl carbonate (DEC), dipropyl carbonate (DPC), dimethyl
carbonate (DMC), butylene carbonate (BC), dibutyl carbonate (DBC),
vinylene carbonate (VC), and mixtures thereof. (Table II). Note
that methyl ethyl carbonate (MEC) and ethyl methyl carbonate (EMC)
are used interchangeably. The physical characteristics of these
solvents are shown in Table II.
[0038] A battery or cell which utilizes the novel family of salts
of the invention will now be described. Note that the preferred
cell arrangement described here is illustrative and the invention
is not limited thereby. Experiments based on full and half cell
arrangements were conducted as per the following description.
[0039] Polymeric electrolytic cells comprise polymeric film
composition electrodes and separator membranes. In particular,
rechargeable lithium battery cells comprise an intermediate
separator element containing an electrolyte solution through which
lithium ions from a source electrode material move between cell
electrodes during the charge/discharge cycles of the cell. In such
cells an ion source electrode is a lithium compound or other
material capable of intercalating lithium ions. An electrode
separator membrane comprises a polymeric matrix made ionically
conductive by the incorporation of an organic solution of a
dissociable lithium salt which provides ionic mobility. Strong,
flexible polymeric electrolytic cell separator membrane materials
retain electrolyte lithium salt solutions and remain functional
over temperatures ranging well below room temperature. These
electrolyte membranes are used either in the usual manner as
separator elements with mechanically assembled battery cell
components, or in composite battery cells constructed of
successively coated layers of electrode and electrolyte
compositions.
[0040] Performance data for the preferred solvent mixtures of the
invention are shown in FIGS. 1-8, as a result of testing in actual
cells. Before further describing the invention, the construction of
a typical ion cell will now be described with reference to FIGS. 9
and 10.
[0041] A typical laminated battery cell structure 10 is depicted in
FIG. 9. It comprises a negative electrode side 12, a positive
electrode side 14, and an electrolyte/separator 16 therebetween.
Negative electrode side 12 includes current collector 18, and
positive electrode side 14 includes current collector 22. A copper
collector foil 18, preferably in the form of an open mesh grid,
upon which is laid a negative electrode membrane 20 comprising an
intercalation material such as carbon or graphite or low-voltage
lithium insertion compound, dispersed in a polymeric binder matrix.
An electrolyte separator film 16 membrane of plasticized copolymer
is positioned upon the electrode element and is covered with a
positive electrode membrane 24 comprising a composition of a finely
divided lithium intercalation compound in a polymeric binder
matrix. An aluminum collector foil or grid 22 completes the
assembly. Protective bagging material 40 covers the cell and
prevents infiltration of air and moisture.
[0042] In another embodiment, a multicell battery configuration as
per FIG. 10 is prepared with copper current collector 51, negative
electrode 53, electrolyte/separator 55, positive electrode 57, and
aluminum current collector 59. Tabs 52 and 58 of the current
collector elements form respective terminals for the battery
structure.
[0043] The relative weight proportions of the components of the
positive electrode are generally: 50-90% by weight active material;
5-30% carbon black as the electric conductive diluent; and 3-20%
binder chosen to hold all particulate materials in contact with one
another without degrading ionic conductivity. Stated ranges are not
critical, and the amount of active material in an electrode may
range from 25-85 weight percent. The negative electrode comprises
about 50-95% by weight of a preferred graphite, with the balance
constituted by the binder. A typical electrolyte separator film
comprises approximately two parts polymer for every one part of a
preferred fumed silica. Before removal of the plasticizer, the
separator film comprises about 20-70% by weight of the composition;
the balance constituted by the polymer and fumed silica in the
aforesaid relative weight proportion. The conductive solvent
comprises the solvent of the invention and suitable salts.
Desirable salts and solvent/salt ratios are described in U.S. Pat.
Nos. 5,712,059 and 5,418,091. One example is a mixture in a weight
ratio of about 90 parts or more of solvent to 10 parts or less of
salt. Therefore, the range of salt content may be very broad.
[0044] Those skilled in the art will understand that any number of
methods are used to form films from the casting solution using
conventional meter bar or doctor blade apparatus. It is usually
sufficient to air-dry the films at moderate temperature to yield
self-supporting films of copolymer composition. Lamination of
assembled cell structures is accomplished by conventional means by
pressing between metal plates at a temperature of about
120-160.degree. C. Subsequent to lamination, the battery cell
material may be stored either with the retained plasticizer or as a
dry sheet after extraction of the plasticizer with a selective
low-boiling point solvent. The plasticizer extraction solvent is
not critical, and methanol or ether are often used.
[0045] Separator membrane element 16 is generally polymeric and
prepared from a composition comprising a copolymer. A preferred
composition is the 75 to 92% vinylidene fluoride with 8 to 25%
hexafluoropropylene copolymer (available commercially from Atochem
North America as Kynar FLEX) and an organic solvent plasticizer.
Such a copolymer composition is also preferred for the preparation
of the electrode membrane elements, since subsequent laminate
interface compatibility is ensured. The plasticizing solvent may be
one of the various organic compounds commonly used as casting
solvents, for example, carbonates. Higher-boiling plasticizer
compounds such as dibutyl phthalate, dimethyl phthalate, diethyl
phthalate, and tris butoxyethyl phosphate are particularly
suitable. Inorganic filler adjuncts, such as fumed alumina or
silanized fumed silica, may be used to enhance the physical
strength and melt viscosity of a separator membrane and, in some
compositions, to increase the subsequent level of electrolyte
solution absorption.
[0046] In the construction of a lithium-ion battery, a current
collector layer of aluminum foil or grid is overlaid with a
positive electrode film, or membrane, separately prepared as a
coated layer of a dispersion of intercalation electrode
composition. This is typically an intercalation compound such as
LiMn.sub.2O.sub.4 (LMO), LiCoO.sub.2, or LiNiO.sub.2, powder in a
copolymer matrix solution, which is dried to form the positive
electrode. An electrolyte/separator membrane is formed as a dried
coating of a composition comprising a solution containing VdF:HFP
copolymer and a plasticizer solvent is then overlaid on the
positive electrode film. A negative electrode membrane formed as a
dried coating of a powdered carbon or other negative electrode
material dispersion in a VdF:HFP copolymer matrix solution is
similarly overlaid on the separator membrane layer. A copper
current collector foil or grid is laid upon the negative electrode
layer to complete the cell assembly. Therefore, the VdF:HFP
copolymer composition is used as a binder in all of the major cell
components, positive electrode film, negative electrode film, and
electrolyte/separator membrane. The assembled components are then
heated under pressure to achieve heat-fusion bonding between the
plasticized copolymer matrix electrode and electrolyte components,
and to the collector grids, to thereby form an effective laminate
of cell elements. This produces an essentially unitary and flexible
battery cell structure.
[0047] Examples of forming cells containing metallic lithium anode,
intercalation electrodes, solid electrolytes and liquid
electrolytes can be found in U.S. Patent Nos. 4,668,595; 4,830,939;
4,935,317; 4,990,413; 4,792,504; 5,037,712; 5,262,253; 5,300,373;
5,435,054; 5,463,179; 5,399,447; 5,482,795 and 5,411,820; each of
which is incorporated herein by reference in its entirety. Note
that the older generation of cells contained organic polymeric and
inorganic electrolyte matrix materials, with the polymeric being
most preferred. The polyethylene oxide of U.S. Pat. No. 5,411,820
is an example. More modern examples are the VDF:HFP polymeric
matrix. Examples of casting, lamination and formation of cells
using VdF:HFP are as described in U.S. Pat. Nos. 5,418,091;
5,460,904; 5,456,000; and 5,540,741; assigned to Bell
Communications Research, each of which is incorporated herein by
reference in its entirety.
[0048] As described earlier, the electrochemical cell which
utilizes the novel solvent of the invention may be prepared in a
variety of ways. In one embodiment, the negative electrode may be
metallic lithium. In more desirable embodiments, the negative
electrode is an intercalation active material, such as, metal
oxides and graphite. When a metal oxide active material is used,
the components of the electrode are the metal oxide, electrically
conductive carbon, and binder, in proportions similar to that
described above for the positive electrode. In a preferred
embodiment, the negative electrode active material is graphite
particles. When forming cells for use as batteries, it is preferred
to use an intercalation metal oxide positive electrode and a
graphitic carbon negative electrode. Various methods for
fabricating electrochemical cells and batteries and for forming
electrode components are described herein. The invention is not,
however, limited by any particular fabrication method as the
novelty lies in the unique electrolyte.
Example I
[0049] A graphite electrode was fabricated by solvent casting a
slurry of BG-35 graphite, binder, plasticizer, and casting solvent.
The graphite, BG-35, was supplied by Superior Graphite, Chicago,
Ill. The BG series is a high purity graphite derived from a flaked
natural graphite purified by heat treatment process. The physical
features are given in Table III. The binder was a copolymer of
polyvinylidene difluoride (PVDF) and hexafluoropropylene (HFP) in a
wt. ratio of PVDF to HFP of 88:12. This binder is sold under the
designation of Kynar Flex 2801.RTM., showing it's a registered
trademark. Kynar Flex is available from Atochem Corporation. An
electronic grade solvent was used. The slurry was cast onto glass
and a free standing electrode was formed as the casting solvent
evaporated. The electrode composition was approximately as follows
on a dry weight % basis: 60% graphite; 16% binder; 21% plasticizer
and 2% conductive carbon.
[0050] An electrode cathode was also fabricated by solvent casting
a slurry of lithium manganese oxide (LMO), conductive carbon,
binder, plasticizer, and solvent. The lithium manganese oxide used
was LiMn.sub.2O.sub.4 supplied by Kerr-McGee (Soda Springs, Id.);
the conductive carbon used was Super P (MMM carbon), Kynar Flex
2801.RTM. was used as the binder along with a plasticizer, and
electronic grade acetone was used as the solvent. The slurry was
cast onto aluminum foil coated with polyacrylic acid/conductive
carbon mixture. The slurry was cast onto glass and a free standing
electrode was formed as the solvent was evaporated. The cathode
electrode composition was approximately as follows on a dry weight
% basis: 65% LiMn.sub.2O.sub.4; 5.5% graphite, 10% binder; and
19.5% plasticizer.
[0051] A rocking chair battery was prepared comprising a graphite
anode, an intercalation compound cathode, and a novel electrolyte
additive of the invention. The negative electrode comprising BG-35
was prepared as described above. The lithium manganese oxide
positive electrode was also prepared in accordance with the above
description. The active mass of the negative electrode was 493
milligrams and the active mass of the positive electrode was 1480
milligrams. A first electrolyte solution of 1 molar LiPF.sub.6 in a
solvent of EC/DMC/EP (1:2:2 by volume and about 25:40:35 by weight)
was prepared. The two electrode layers were arranged with an
electrolyte layer in between, and the layers were laminated
together using heat and pressure as per the Bell Comm. Res. patents
incorporated herein by reference earlier. The results of pulse
charge type testing of this cell are shown in FIG. 1.
Example II
[0052] Another cell was prepared in accordance with the method of
Example I, except that the electrolyte was 1 molar LiPF.sub.6 in a
solvent of EC/EMC/EP (volume ratio of 1:2:2 and about 25:40:35 by
weight). The results of testing are shown in FIG. 2.
Example III
[0053] An additional cell was prepared in accordance with the
method of Example I, except that the electrolyte was 1 molar
LiPF.sub.6 in a solvent of EC/EMC/EP in a weight ratio of 3:5:2.
The results of testing are shown in FIG. 3.
[0054] Example IV
[0055] A cell was prepared in accordance with the methods of
Example I, except that the electrolyte solution comprised 1 molar
LiPF.sub.6 in a solvent of EC/DMC/EP in a weight ratio of 3:5:2.
The results of testing are shown in FIG. 4.
[0056] FIGS. 5 and 6 are for SLI pulse data at 25 mA/cm.sup.2 for
30 seconds at -18.degree. C. and 19 mA/cm.sup.2 for 30 seconds at
-29.degree. C. The cell tested was the same as described for
Example I, having BG35/LMO and 1 molar LiPF.sub.6 in electrolyte of
EC/DMC/EP at volume ratio of 1:2:2.
[0057] FIGS. 1-6 summarize the pulse data for the experimental
electrolytes of Examples I-IV. The pulse discharge tests are
conducted at selected temperatures for a period of time. The time
is selected to represent a desired operating condition, for
example, starting a car. The time limit normally is 30 seconds. The
voltage limit is above 1.8 volts; and the current changes with
temperature of test being conducted. The best performance combines
the aspects of high current discharge at low temperature with the
highest voltage for over 30 seconds. The initial pulse data were
collected at 20 mA/cm.sup.2 for 30 seconds (FIGS. 1-4). Two
iterations were then monitored for the EC/DMC/EP at 1:2:2 measured
at target current densities at -18.degree. C. and -29.degree. C. as
per FIGS. 5 and 6. The 1.8 volt cut-off is shown in FIGS. 5 and 6.
The best low temperature performance was demonstrated by the
EC/DMC/EP at 1:2:2 volume ratio. This electrolyte is suitable for
applications where improved high and low temperature performance
are both required.
Example V
[0058] Another cell was prepared in accordance with the methods of
Example I, except that the negative electrode active material was a
carbon material designated as KX44. This carbon electrode active
material is graphitic in nature and is in the form of graphitic
carbon fibers having a filament diameter in the range of 4-20
microns and typically on the order of 9 microns. These carbon
fibers are sold by PETOCA, Ltd. and are designated melblon fibers,
which are carbon fibers suitable for industrial use. These carbon
fibers have high strength due to their formation from petroleum
pitch (mesophase pitch) as the starting material. The physical
features of the KX44 carbon fibers are given in Table IV. The
electrolyte of this example was 1 molar LiPF.sub.6 salt in a
solvent mixture of EC/DMC/EP/PC at a weight ratio of
58:29:12:1.
[0059] FIG. 7 is a two-part graph. FIG. 7A shows the excellent
rechargeability and FIG. 7B shows the excellent cyclability and
capacity of the cell prepared in accordance with Example V. The
capacity was determined at constant current cycling for cycles 1 to
10 consistent with the test parameters described above. FIG. 7
shows long cycle life demonstrated by the relatively slow capacity
fade with cycle numbers. The recharge ratio data shows the absence
of any appreciable side reactions and decompositions over the
extended life cycling. This can be more particularly seen from FIG.
7A. The recharge ratio maintains its value exceptionally close to
1. The cell maintains close to 100 percent of its capacity over
extended cycling to 10 cycles. The combination of slow, minimal
capacity fade along with excellent recharge ratio demonstrates the
absence of any appreciable side reactions. The cell of FIG. 7
containing 1 M LiPF.sub.6 EC/DMC/EC/PC at weight ratio of
58:29:12:1 cycled well with low capacity fade and indicated good
compatibility of the solvent in the graphite/LMO system.
Comparative Example
[0060] For comparison purposes, an additional cell was prepared in
accordance with the methods of Example I, except that the solvent
only contained EC/DMC. The solvent was 1 molar LiPF.sub.6 in 2:1
EC/DMC. This electrolyte was also used in a cell having an LMO
lithium manganese oxide positive electrode and a BG-35 negative
counter-electrode. The active mass of the positive electrode was
1480 milligrams and the negative electrode was 493 milligrams.
[0061] FIG. 8 contains the results of cycling the comparative cell
at 10 mA/cm.sup.2. This cell did not perform as well as the
EC/DMC/EP solvent based cell of the earlier Examples. This cell
failed at -10.degree. C. At 0.degree. C., 10.degree. C. and
20.degree. C., this cell showed poor performance relative to the
earlier examples.
1TABLE I Ester (R'COOR") Solvent Properties EP EB PP PB Molecular
Weight (g/mol) 102.13 116.16 116.16 130.19 Boiling Point (.degree.
C.) 99 120 122 142 Melting Point (.degree. C.) -73 -93 -76 --
Density (g/cm.sup.3) 0.891 0.878 0.881 0.873 Solution Conductivity
(S/cm) <10.sup.-7 <10.sup.-7 <10.sup.-7 <10.sup.-7
Viscosity (cp at 25.degree. C.) -- -- -- -- Dielectric Constant --
-- -- -- Water Content (ppm) <30 <30 <30 <30
Electrolytic Conductivity 6.3 -- -- -- at 23.degree. C. 1 M
LiPF.sub.6 Note: 1) EP = ethyl propionate
C.sub.2H.sub.5COOC.sub.2H.s- ub.5 2) EB = ethyl Butyrate
C.sub.3H.sub.7COOC.sub.2H.sub.5 3) PP = propyl propionate
C.sub.2H.sub.5COOC.sub.3H.sub.7 4) PB = propyl butyrate
C.sub.3H.sub.7COOC.sub.3H.sub.7
[0062]
2TABLE II Characteristics of Organic Solvents PC VC EC DMC Boiling
Temperature (C.) 240 162 248 91.0 Melting Temperature (C.) -49 22
39-40 4.6 Density (g/cm.sup.3) 1.198 1.35 1.322 1.071 Solution
Conductivity 2.1 .times. -- <10.sup.-7 <10.sup.-7 (S/cm)
10.sup.-9 Viscosity (cp) at 25.degree. C. 2.5 -- 1.86 0.59 (at
40.degree. C.) Dielectric Constant 64.4 -- 89.6 3.12 at 20.degree.
C. (at 40.degree. C.) Molecular Weight 102.0 86.047 88.1 90.08
H.sub.2O Content <10 ppm -- <10 ppm <10 ppm Electrolytic
Conductivity 5.28 -- 6.97 11.00 (mS/cm) 20.degree. C. 1M
LiAsF.sub.6 (1.9 mol)
[0063]
3TABLE II Continued DEC BC MEC DPC Boiling Temperature (C.) 126 230
107 167-168 Melting Temperature (C.) -43 -- -55 -- Density
(g/cm.sup.3) 0.98 1.139 1.007 0.944 Solution Conductivity (S/cm)
<10.sup.-7 <10 - 7 6 .times. <10.sup.-7 10.sup.-9
Viscosity (cp) at 25.degree. C. 0.75 2.52 0.65 -- Dielectric
Constant at 20.degree. C. 2.82 -- -- -- Molecular Weight 118.13
116.12 104.10 146.19 H.sub.2O Content <10 <10 <10 <10
ppm ppm ppm ppm Electrolytic Conductivity 5.00 <3.7 -- --
(mS/cm) 20.degree. C. lM LiAsF.sub.6 (1.5 mol)
[0064]
4 TABLE III Carbon Material BG-35 Surface Area (m.sup.2/g) (BET) 7
Coherence Length L.sub.c (nm) >1000 Density (g/cm.sup.3).sup.2
0.195 Particle Size.sup.1 <36 Median Size d.sub.50 (.mu.m) 17
Interlayer Distance c/2 (nm) N/A .sup.1Maximum size for at least
90% by weight of graphite particles. .sup.2In xylene.
[0065]
5TABLE IV Properties of KX44 PROPERTY Diameter .mu.m 9.0 Fiber
Density g/cm.sup.3 2.23 Surface Area m.sup.2/g 1.22 Particle Size
10% D .mu.m 9.3 50% D .mu.m 16.2 90% D .mu.m 51.6 XRD d.sub.002 nm
0.3363 Lc.sub.002 nm 52 Ash Content wt % 0.01.dwnarw. Water
Adsorption wt % 0.01.dwnarw.
[0066]
6TABLE V Comparative Specifications of Carbon Materials Surface
Area Coherence Particle Median Interlayer Carbon (m.sup.2/g, Length
Lc Density Size Size d.sub.50 Distance Material BET) (nm)
(g/cm.sup.3) (.mu.m) (nm) C/2 (nm) KX 44 1.22 <100 2.24 <70
16 0.3363 BG 35 7 >1000 2.25 <36 17 n/a
[0067] In summary, the invention solves the problems associated
with conventional electrolytes. Solvents containing DMC have always
been a problem since DMC readily boils off. EC readily solidifies,
and it is necessary for the cell to achieve a temperature of
40.degree. C. to melt the EC and prevent it from solidifying. In
addition, mixtures of DMC/EC have been found to result in
decomposition evidenced by solution color change and/or by
formation of gas. In contrast, solvents of the invention provide
highly desirable wide temperature operating range while avoiding
decomposition of cell components. It is thought that the solvents
of the invention also help overcome problems associated with
reactive active materials and avoids the consequences of catalytic
reaction which catalyzes decomposition of electrolyte solvent.
Therefore, the solvents of the invention are an improvement over
conventional solvents.
[0068] Based on the performance above, the electrolytes of the
invention are considered to be usable with a variety of
carbonaceous active materials. This is demonstrated by the better
performance of EP/EC/DMC as compared to EP/EC/EMC. The latter is
not even operable at low temperatures with a carbonaceous graphite
electrode. The carbonaceous materials (carbons) usable with the
electrolyte of the invention range from highly structural to
amorphous and from powders to fibers. Such materials have well
documented physical properties. Some carbons are highly structured,
highly crystalline, highly graphitic, anisotropic graphites having
a nearly perfect layered structure and preferably formed as
synthetic graphites and heat treated up to about 3000.degree. C.
Examples are the SFG and the KS graphites as supplied by the
manufacturer Lonza G. & T., Limited (Sins, Switzerland). Some
carbons are graphitic carbons which have relatively very large
crystal size (L.sub.c greater than 2000) and are fully graphitized.
BG grades from Superior are purified natural graphite. Some carbons
are non-graphitic carbons. These are considered amorphous,
non-crystalline, disordered, and are generally petroleum cokes and
carbon blacks, as such, supplied by Lonza under the designation
FC-250 and Conoco (USA) under the designation XP and X-30.
[0069] While this invention has been described in terms of certain
embodiments thereof, it is not intended that it be limited to the
above description, but rather only to the extent set forth in the
following claims.
[0070] The embodiments of the invention in which an exclusive
property or privilege is claimed are defined in the following
claims.
* * * * *