U.S. patent application number 09/222699 was filed with the patent office on 2001-09-06 for a rechargeable lithium electrochemical cell usable at low temperature.
Invention is credited to BARUSSEAU, SYLVIE, BIENSAN, PHILIPPE, HERREYRE, SYLVIE, PERTON, FRANCOISE.
Application Number | 20010019800 09/222699 |
Document ID | / |
Family ID | 9533803 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019800 |
Kind Code |
A1 |
HERREYRE, SYLVIE ; et
al. |
September 6, 2001 |
A RECHARGEABLE LITHIUM ELECTROCHEMICAL CELL USABLE AT LOW
TEMPERATURE
Abstract
The present invention provides a rechargeable lithium
electrochemical cell comprising an electrolyte containing a lithium
salt dissolved in a non-aqueous solvent, at least one positive
electrode, and at least one negative electrode of the paste type
containing an electrochemically active material which is a carbon
compound suitable for inserting lithium ions and a binder, the cell
being characterized in that said solvent contains at least one
saturated cyclic carbonate and at least one linear ester of a
saturated aliphatic monocarboxylic acid, and in that said binder is
a polymer having no fluorine.
Inventors: |
HERREYRE, SYLVIE; (EPINAY
SUR ORGE, FR) ; BIENSAN, PHILIPPE; (EPINAY SUR ORGE,
FR) ; PERTON, FRANCOISE; (BERUGES, FR) ;
BARUSSEAU, SYLVIE; (BRETIGNY SUR ORGE, FR) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS
2100 PENNSYLVANIA AVENUE, N.W.
WASHINGTON
DC
200373213
|
Family ID: |
9533803 |
Appl. No.: |
09/222699 |
Filed: |
December 29, 1998 |
Current U.S.
Class: |
429/332 ;
429/217; 429/231.4; 429/231.8 |
Current CPC
Class: |
H01M 4/621 20130101;
Y02T 10/70 20130101; H01M 4/622 20130101; H01M 10/0525 20130101;
H01M 10/0569 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
429/332 ;
429/231.8; 429/231.4; 429/217 |
International
Class: |
H01M 004/58; H01M
004/62; H01M 010/40 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 1998 |
FR |
98 15576 |
Claims
1. A rechargeable lithium electrochemical cell comprising an
electrolyte containing a lithium salt dissolved in a non-aqueous
solvent, at least one positive electrode, and at least one negative
electrode of the paste type containing an electrochemically active
material which is a carbon compound suitable for inserting lithium
ions and a binder, the cell being characterized in that said
solvent contains at least one saturated cyclic carbonate and at
least one linear ester of a saturated aliphatic monocarboxylic
acid, and in that said binder is a polymer having no fluorine.
2. A cell according to claim 1 or 2, in which the volume proportion
of said saturated cyclic carbonate lies in the range 5% to 60% of
said solvent, and the volume proportion of said linear ester lies
in the range 20% to 85% of said solvent.
3. A cell according to claim 1 or 2, in which the volume proportion
of said linear ester is not less than 50% of said solvent.
4. A cell according to any preceding claim, in which said saturated
cyclic carbonate is selected from propylene carbonate, ethylene
carbonate, butylene carbonate, and mixtures thereof.
5. A cell according to claim 4, in which said saturated cyclic
carbonate is ethylene carbonate.
6. A cell according to claim 4, in which said saturated cyclic
carbonate is propylene carbonate.
7. A cell according to claim 4, in which said saturated cyclic
carbonate is a mixture of ethylene carbonate and of propylene
carbonate.
8. A cell according to any preceding claim, in which said linear
ester is selected from acetate, a butyrate, a propionate, and a
mixture thereof.
9. A cell according to claim 8, in which said linear ester is ethyl
acetate.
10. A cell according to claim 8, in which said linear ester is
methyl butyrate.
11. A cell according to any preceding claim, in which said solvent
further comprises a saturated linear carbonate.
12. A cell according to claim 11, in which said saturated linear
carbonate is selected from dimethyl carbonate, diethyl carbonate,
ethyl methyl carbonate, propyl methyl carbonate, and mixtures
thereof.
13. A cell according to claim 11 or 12, in which said saturated
linear carbonate is dimethyl carbonate.
14. A cell according to any one of claims 11 to 13, in which the
volume proportion of said linear carbonate is no more than 40% of
said solvent.
15. A cell according to any preceding claim, in which said solvent
further comprises an unsaturated cyclic carbonate.
16. A cell according to claim 15, in which said unsaturated cyclic
carbonate is selected from vinylene carbonate, propylidene
carbonate, ethylene ethylidene carbonate, ethylene isopropylidene
carbonate.
17. A cell according to claim 15 or 16, in which said unsaturated
linear carbonate is vinylene carbonate.
18. A cell according to any one of claims 15 to 17, in which the
volume proportion of said unsaturated cyclic carbonate is no more
than 60% of said solvent.
19. A cell according to any preceding claim, in which said binder
contains an elastomer.
20. A cell according to claim 19, in which said elastomer is
selected from acrylonitrile and butadiene copolymer and styrene and
butadiene copolymer.
21. A cell according to claim 19 or 20, in which the proportion by
weight of said elastomer lies in the range 30% to 70% of said
binder.
22. A cell according to any preceding claim, in which said binder
contains a cellulose compound.
23. A cell according to claim 22, in which said cellulose compound
is a carboxymethyl cellulose of mean molecular weight greater than
about 200,000.
24. A cell according to claim 22 or 23, in which the proportion by
weight of said cellulose compound lies in the range 30% to 70% of
said binder.
25. A cell according to any preceding claim, in which said binder
is made up of a mixture of an elastomer and a cellulose
compound.
26. A cell according to claim 25, in which said binder is made up
of a mixture of an acrylonitrile and butadiene copolymer and
carboxymethyl cellulose having a mean molecular weight greater than
about 200,000.
27. A cell according to claim 25, in which said binder is made up
of a mixture of styrene and butadiene copolymer and carboxymethyl
cellulose having a mean molecular weight greater than about
200,000.
28. A cell according to any one of claims 25 to 27, in which the
proportion by weight of said elastomer lies in the range 30% to 70%
of said binder and the proportion by weight of said cellulose
compound lies in the range 30% to 70% of said binder.
29. A cell according to claim 28, in which the proportion by weight
of said elastomer lies in the range 50% to 70% of said binder and
the proportion by weight of said cellulose compound lies in the
range 30% to 50% of said binder.
30. A cell according to any one of claims 1 to 21, in which said
binder contains an acrylic polymer.
31. A cell according to claim 30, in which said polymer is an
acrylic acid homopolymer.
32. A cell according to claim 29 or 30, in which the proportion by
weight of said acrylic polymer lies in the range 20% to 60% of said
binder.
33. A cell according to any one of claims 1 to 21, in which said
binder is made up of a mixture of an elastomer and of an acrylic
polymer.
34. A cell according to claim 33, in which said binder is made up
of a mixture of an acrylonitrile and butadiene copolymer and an
acrylic acid homopolymer.
35. A cell according to claim 33, in which said binder is made up
of a mixture of styrene and butadiene copolymer and an acrylic acid
homopolymer.
36. A cell according to any one of claims 33 to 35, in which the
proportion by weight of said elastomer lies in the range 40% to 80%
of said binder and the proportion by weight of said acrylic polymer
lies in the range 20% to 60% of said binder.
37. The use of a cell according to any preceding claim, at
temperatures less than or equal to -20.degree. C.
Description
[0001] The present invention relates to a rechargeable lithium
electrochemical cell that is usable at low temperature.
[0002] A lithium electrochemical cell possesses an electrochemical
stack including a positive electrode comprising electrochemically
active material capable of inserting lithium into its structure
(generally an oxide of a transition metal, usually lithiated), and
a negative electrode that supplies the lithium ions. The electrodes
are placed on either side of a separator membrane that is generally
made of polyolefin. The electrochemical stack is impregnated in a
non-aqueous electrolyte that is solid or liquid. The electrolyte
contains a lithium salt dissolved in a mixture of organic
solvents.
[0003] The low temperature operation of such cells has frequently
been investigated. Attention has been given mainly to the
composition of the electrolyte.
[0004] According to document JP-08 195 221, low temperature
(-20.degree. C.) conductivity is increased when the solvent is made
up of ethylene carbonate, propylene carbonate, an acetic ester, and
at least one compound selected from diethyl or dimethyl carbonate,
and dimethoxyethane. The volume fraction of the acetic ester is not
more than 50%.
[0005] A rechargeable lithium electrochemical cell including an
anode of metallic lithium or of lithium alloy, and a cathode whose
active material is an electrically-conductive organic polymer, is
described in document FR-2 641 130. The electrolyte is a lithium
salt dissolved in a solvent, which salt is a combination of a
cyclic carbonate and a non-cyclic carbonate. That cell retains
sufficient discharge capacity at a temperature of less than
0.degree. C.
[0006] Document U.S. Pat. No. 4,056,663 describes a rechargeable
electrochemical cell having a metallic lithium anode and a metal
oxide cathode, in which the electrolyte comprises a solvent that
does not contain ether. The solvent is a mixture of carbonates or a
mixture of at least one carbonate and at least one ester.
[0007] According to document U.S. Pat. No. 4,983,476, improved low
temperature performance is obtained by replacing the metallic
lithium of the anode with a transition metal sulfide. The
electrolyte comprises an aprotic organic solvent such as methyl
formate, propylene carbonate, dimethyl carbonate, diethyl
carbonate, dimethoxyethane, tetrahydrofuran, and mixtures
thereof.
[0008] The most recent rechargeable lithium electrochemical cells
possess a negative electrode of the paste type having a conductive
support that acts as a current collector on which there is placed a
paste containing a binder and an electrochemically active material
which is a material that is capable of inserting lithium into its
structure. The greater safety of such cells makes them suitable for
a wider range of applications. For use in particular in an electric
vehicle or in radiocommunications, such cells must be capable of
operating at low temperature, and in particular at below
-20.degree. C.
[0009] To this end, document EP-0 482 287 suggests using a
rechargeable lithium electrochemical cell possessing a compressed
anode containing carbon, a cathode comprising a lithiated oxide,
and an electrolyte including a lithium salt dissolved in an organic
solvent comprising a cyclic ester and a linear ester.
[0010] A rechargeable electrochemical cell having a carbon anode as
described in document JP-09 022 738 contains an electrolyte whose
solvent comprises a cyclic carbonate, a linear carbonate, and up to
44% by volume ethyl acetate. The binder of the electrodes is
polyvinylidene fluoride, and the active material of the cathode is
a lithium cobalt oxide. That cell has improved performance at low
temperature. The document mentions a drop in performance at low
temperature when methyl propionate or ethyl propionate is used as
the solvent.
[0011] The discharged capacity of a rechargeable lithium
electrochemical cell in accordance with document EP-0-531 617 can
be increased. The cell comprises a carbon anode and a lithiated
oxide cathode. The electrolyte solvent is a mixture of a cyclic
carbonate, a linear carbonate, and a compound of formula RCOOR1,
where R is an ethyl radical and R1 is an alkyl group having 1 to 3
carbon atoms. The respective volume fractions thereof are
preferably 20% to 50%, 10% to 70%, and 10% to 70%.
[0012] Document EP-0 614 240 describes a rechargeable lithium
electro-chemical cell having a carbon anode and a metal oxide
cathode, with improved discharge at a high discharge rate,
particularly at low temperature. The cell contains an electrolyte
comprising a lithium salt and a mixture of aprotic solvents made up
by volume of 10% to 20% ethylene carbonate, 5% to 40% propylene
carbonate, and 50% to 85% dimethyl carbonate.
[0013] To improve high-rate discharge at low temperature, document
EP-0 766 332 proposes an electrochemical cell comprising paste
electrodes in which the binder is polyvinylidene fluoride (PVDF).
It has an anode comprising an electrochemically active material
based on carbon, and a cobalt oxide cathode. The solvent of the
electrolyte is, by volume, made up of 50% to 60% of a mixture of
cyclic carbonate and of cyclic ester, such as .gamma.-butyrolactone
or .gamma.-valerolactone, 20% to 40% of a linear carbonate, and 10%
to 25% of a linear ester.
[0014] Self-discharge during storage at low temperature is
decreased in a rechargeable lithium electrochemical cell having a
carbon anode, and a lithiated oxide cathode, in accordance with
EP-0 548 449. The electrolyte solvent is a mixture of three
components which are an aliphatic carboxylate, a cyclic carbonate,
and a linear carbonate. The respective volume proportions thereof
are preferably 10% to 80%, 20% to 50%, and not more than 70%.
[0015] An object of the present invention is to provide a
rechargeable lithium electrochemical cell having a carbon anode in
which performance during low temperature operation is better than
that of known cells.
[0016] The present invention provides a rechargeable lithium
electrochemical cell comprising an electrolyte containing a lithium
salt dissolved in a non-aqueous solvent, at least one positive
electrode, and at least one negative electrode of the paste type
containing an electrochemically active material which is a carbon
compound suitable for inserting lithium ions and a binder. The
invention is characterized in that said solvent contains at least
one saturated cyclic carbonate and at least one linear ester of a
saturated aliphatic monocarboxylic acid, and in that said binder is
a polymer having no fluorine.
[0017] The terms "linear ester of a saturated aliphatic
monocarboxylic acid" and "saturated aliphatic carboxylate" are used
to mean a compound of formula RC--O--OR' in which R is H or an
alkyl group, and R' is an alkyl group such as CH.sub.3 (methyl),
CH.sub.3--CH.sub.2 (ethyl), etc. . . . Said linear ester of a
saturated aliphatic monocarboxylic acid is, for example, a formiate
if R is H, an acetate if R is CH.sub.3, a propionate if R is
CH.sub.3--CH.sub.2, a butyrate if R is CH.sub.3-- (CH.sub.2).sub.2,
a valeriate if R is CH.sub.3--(CH.sub.2).sub.3, etc. . . .
[0018] In the solvent, the volume proportion of said saturated
cyclic carbonate lies in the range 5% to 60% of said solvent, and
the volume proportion of said linear ester lies in the range 20% to
85% of said solvent, the proportion of said linear ester is
preferably not less than 50% of said solvent.
[0019] Said saturated cyclic carbonate is selected from propylene
carbonate, ethylene carbonate, butylene carbonate, and mixtures
thereof.
[0020] In a first variant, said saturated cyclic carbonate is
ethylene carbonate.
[0021] In a second variant, said saturated cyclic carbonate is
propylene carbonate.
[0022] In a third variant, said saturated cyclic carbonate is a
mixture of ethylene carbonate and of propylene carbonate.
[0023] Said linear ester is selected from an acetate, a butyrate, a
propionate, and mixtures thereof. By way of example, it is possible
to select an ethyl acetate, a methyl acetate, a propyl acetate, an
ethyl butyrate, a methyl butyrate, a propyle butyrate, an ethyl
propionate, a methyl propionate, a propyl propionate.
[0024] In a first variant, said linear ester is ethyl acetate.
[0025] In a second variant, said linear ester is methyl
butyrate.
[0026] In another implementation of the invention, said solvent
further comprises a saturated linear carbonate.
[0027] Said saturated linear carbonate is selected from dimethyl
carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl
carbonate, and mixtures thereof. Said saturated linear carbonate is
preferably dimethyl carbonate.
[0028] The volume proportion of said linear carbonate is not more
than 40% of said solvent. When the solvent contains any, the volume
proportion of said linear carbonate preferably lies in the range 5%
to 40% of said solvent.
[0029] In another embodiment of the invention, said solvent further
comprises an unsaturated cyclic carbonate.
[0030] Said unsaturated cyclic carbonate is selected from vinylene
carbonate and derivatives thereof, in particular propylidene
carbonate, ethylidene ethylene carbonate, isopropylidene ethylene
carbonate. Said unsaturated linear carbonate is preferably vinylene
carbonate.
[0031] The term "derivatives of vinylene carbonate" is used to
cover compounds possessing at least one unsaturated bond connected
to a carbon atom of the cycle, for example propylidene carbonate,
ethylidene ethylene carbonate (or 4-ethylidene 1-3 dioxolane 2
one), or isopropylidene ethylene carbonate (or 4-isopropylidene 1-3
dioxolane 2 one).
[0032] The volume proportion of said unsaturated cyclic carbonate
is no more than 60% of said solvent. When the solvent contains any,
the volume proportion of said unsaturated cyclic carbonate
preferably lies in the range 0.5% to 10% of said solvent.
[0033] In a first embodiment of the invention, said binder contains
an elastomer.
[0034] Preferably, said elastomer is selected from a copolymer of
acrylonitrile and of butadiene, and a copolymer of styrene and of
butadiene.
[0035] The proportion by weight of said elastomer lies in the range
30% to 70% of said binder.
[0036] In a second embodiment of the invention, said binder
contains a cellulose compound.
[0037] Preferably, said cellulose compound is a carboxymethyl
cellulose having a mean molecular weight greater than about
200,000.
[0038] The proportion by weight of said cellulose compound lies in
the range 30% to 70% of said binder.
[0039] In a third embodiment of the invention, said binder is made
up of a mixture of an elastomer and of a cellulose compound.
[0040] In a first variant, said binder is made up of a mixture of a
copolymer of acrylonitrile and of butadiene, and of carboxymethyl
cellulose having a mean molecular weight of greater than about
200,000.
[0041] In a second variant, said binder is made up of a mixture of
a copolymer of styrene and of butadiene and carboxymethyl cellulose
having a mean molecular weight greater than about 200,000.
[0042] In the binder, the proportion by weight of said elastomer
lies in the range 30% to 70% of said binder and the proportion by
weight of said cellulose compound lies in the range 30% to 70% of
said binder.
[0043] The proportion by weight of said elastomer preferably lies
in the range 50% to 70% of said binder and the proportion by weight
of said cellulose compound preferably lies in the range 30% to 50%
of said binder.
[0044] In a fourth embodiment of the invention, said binder
contains an acrylic polymer.
[0045] Said polymer is preferably a homopolymer of acrylic
acid.
[0046] The proportion by weight of said acrylic polymer lies in the
range 20% to 60% of said binder.
[0047] In a fifth embodiment of the invention, said binder is made
up of a mixture of an elastomer and of an acrylic polymer.
[0048] In a first variant, said binder is made up of a mixture of a
copolymer of acrylonitrile and of butadiene, and a homopolymer of
acrylic acid.
[0049] In a second variant, said binder is made up of a mixture of
a copolymer of styrene and of butadiene, and of a homopolymer of
acrylic acid.
[0050] In the binder, the proportion by weight of said elastomer
lies in the range 40% to 80% of said binder and the proportion by
weight of said acrylic polymer lies in the range 20% to 60% of said
binder.
[0051] The cell of the invention has a paste negative electrode
comprising a conductive support and an active layer containing the
active material and the binder.
[0052] The conductive support can be a two-dimensional support,
such as a solid or perforated foil, an expanded metal, a grid, or a
cloth, or it can be a three-dimensional support such as a felt or a
foam having fibers that are metallic, metal-plated, or made of
carbon.
[0053] The active material is a material suitable for inserting
lithium ions at low potential (i.e. not exceeding 1.5 V). The
material is preferably selected from carbon in crystal form, such
as graphite powder or fibers, graphitizable carbon compounds of low
crystal content, such as coke, non-graphitizable carbon compounds
of low crystal content, such as vitreous carbon and carbon black,
and mixtures thereof.
[0054] The cell of the invention has a positive electrode whose
active material is a material suitable for inserting lithium ions
at high potential (i.e. not less than 2.5 V). This material is
preferably selected from a lithiated oxide of a transition metal,
such as nickel, cobalt, manganese, vanadium, and iron, a sulfide, a
sulfate, and mixtures thereof.
[0055] The cell of the invention contains a liquid or solid
electrolyte containing a lithium salt. The lithium salt is
preferably selected from lithium perchlorate LiClO.sub.4, lithium
hexafluoroarsenate LiAsF.sub.6, lithium hexafluorophoshate
LiPF.sub.6, lithium tetrafluoroborate LiBF.sub.4, lithium
trifluoromethanesulfonate LiCF.sub.3SO.sub.3, lithium
trifluoromethanesulfonimide LiN(CF.sub.3SO.sub.2).sub.2 (LiTFSI),
or lithium trifluoromethanesulfonemethide
LiC(CF.sub.3SO.sub.2).sub.3 (LiTFSM).
[0056] The present invention also provides the use of a cell of the
invention at very low temperatures, i.e. temperatures less than or
equal to -20.degree. C.
[0057] Other characteristics and advantages of the present
invention appear from the following examples, naturally given as
non-limiting illustrations.
EXAMPLE 1
[0058] Non-aqueous electrolytes were prepared, constituted by
lithium hexafluorophosphate LiPF.sub.6 at a concentration of 1 M
dissolved in organic solvents that were mixtures of solvents
selected from ethylene carbonate EC, propylene carbonate PC,
dimethyl carbonate DMC, diethyl carbonate DEC, methyl and ethyl
carbonate EMC, ethyl acetate EA, methyl acetate MA, methyl butyrate
MB, ethyl propionate EP, and methyl propionate MP.
[0059] The viscosities of the electrolytes were measured as a
function of temperature.
[0060] The results are given in Table 1 below. Viscosity is given
in mm.sup.2/sec.
1 TABLE 1 Viscosity Electrolyte -40.degree. C. -20.degree. C.
.degree. C. +20.degree. C. EC/PC/DMC solid 4.0 2.4 20/20/60
EC/PC/EMC 35 11 5.3 3.2 20/20/60 EC/DMC/EMC 16 6.8 3.4 2.1 25/15/60
EC/EA 17 6.0 3.6 2.5 50/50 EC/PC/EA 12 5.4 3.2 2.1 15/25/60
EC/PC/MA 7.4 3.7 2.3 1.6 15/25/60 EC/PC/MP 9.6 4.6 2.8 1.9 15/20/65
EC/DMC/MP 7.4 3.8 2.3 1.6 15/35/50
[0061] At lower temperatures, the lowest viscosities were given by
electrolytes containing a methyl acetate or propionate (the
EC/PC/MA and EC/DMC/MP mixtures). As for the EC/PC/DMC mixture
which does not contain a linear ester of a monocarboxylic acid, it
was solid at that temperature.
[0062] Measurements of conductivity as a function of temperature
were performed for the above-prepared electrolytes. The results are
given in Table 2 below. Conductivity is expressed in mS/cm.
2 TABLE 2 Conductivity Electrolyte -40.degree. C. -30.degree. C.
-20.degree. C. -10.degree. C. 0.degree. C. +20.degree. C. EC/PC/DMC
solid solid 4.2 4.8 7.5 11 20/20/60 EC/PC/DEC solid 1.2 1.9 2.8 3.9
6.4 20/20/60 EC/PC/EMC 1 1.5 2.3 3.5 4.8 7.8 20/20/60 EC/DMC/EMC
1.4 2.0 3.2 4.0 5.3 7.6 25/15/60 EC/EA 1.1 3.2 5.3 7.1 9.1 13 50/50
EC/PC/EA 2.3 2.7 4.9 5.5 7.3 11.2 20/20/60 EC/DMC/EA 4 5.6 7.1 8.6
10.2 13.3 15/25/60 EC/PC/MA 3.2 5.3 7.4 9.6 12 16.6 20/20/60
EC/PC/MB 1.6 2.3 3.4 5.0 6.5 9.9 20/20/60 EC/PC/MB/VC 1.6 2.3 3.5
4.9 6.3 9.5 19/19/57/5 EC/PC/EP 2.0 2.5 3.6 4.8 6.5 9.6 20/20/60
EC/PC/MP 2.3 3.4 4.3 5.7 7.2 10.4 20/20/60 EC/DMC/MP 3.5 4.7 6.2
7.7 9.0 11.7 15/32/20
[0063] Only the following mixtures EC/PC/EA, EC/DMC/EA, EC/PC/MA,
EC/PC/EP, EC/PC/MP, and EC/DMC/MP still had conductivity greater
than 2 milliSiemens/cm at a temperature of -40.degree. C.
EXAMPLE 2
[0064] A rechargeable lithium electrochemical cell was prepared in
4/5A format, having a nickel positive electrode, a carbon negative
electrode, a separator, and a non-aqueous electrolyte.
[0065] The positive electrode was of the paste type on an aluminum
foil. The paste contained the electrochemically active material
which was a substituted lithium nickel oxide LiNiMO.sub.2 (where M
is at least one doping element), a binder which was polyvinylidene
fluoride (PVDF), and a carbon-based conductive material.
[0066] The negative electrode was of the paste type on a copper
foil. The paste contained 85% by weight of electrochemically active
material constituted by a mixture of graphitized carbon compounds
and/or graphites, and 15% by weight of a binder which was
polyvinylidene fluoride (PVDF).
[0067] A microporous polyolefin separator was placed between the
electrodes to form an electrochemical stack. The stack was
spiral-wound and a spool was obtained which was inserted into a
metal can. The electrochemical stack was impregnated with one of
the above-described electrolytes.
EXAMPLE 3
[0068] A rechargeable lithium electrochemical cell analogous to the
cell of Example 2 was prepared except that it had a carbon negative
electrode of the type comprising paste on a copper foil. The paste
contained 96% by weight of electrochemically active material
constituted by a mixture of graphitized carbon compounds and/or of
graphites, and 4% by weight of a binder constituted by an
equal-weight mixture of a copolymer of acrylonitrile and of
butadiene (2% by weight of NBR) and of carboxymethyl cellulose (2%
by weight of CMC) having a mean molecular weight of not less than
200,000.
EXAMPLE 4
[0069] A rechargeable lithium electrochemical cell was prepared
analogous to the cell in Example 3, with the exception that the
binder of the negative electrode was an equal-weight mixture of a
copolymer of styrene and of butadiene (2% by weight of SBR) and of
carboxymethyl cellulose (2% by weight of CMC) having a mean
molecular weight of not less than 200,000.
[0070] The cells of Examples 2 to 4 were evaluated
electrochemically by means of the following tests.
[0071] Cycling test at ambient temperature (25.degree. C.) and at
high discharge rates:
[0072] A first cycle was performed at a normal rate in order to
determine the characteristics of the cells:
[0073] initial charging at a rate of 0.5 I.sub.c (where I.sub.c is
the current required for discharging the nominal capacity C.sub.n
of said cell in one hour) for a period of 3 hours; and
[0074] an initial discharge at 0.2 I.sub.c down to a voltage of 2.7
volts.
[0075] The initially discharged capacity C.sub.i was measured in
milliAmpere-hours per gram of positive active material during the
initial discharge at normal rates at25.degree. C.
[0076] Thereafter the following cycling was performed at a high
discharge rate:
[0077] charging at a rate of 0.5 I.sub.c for 3 hours; and
[0078] discharging at I.sub.c down to 2.7 V.
[0079] The capacities discharged during the discharges of cycles 50
and 200 were measured. These measurements make it possible to
determined the loss of capacity F.sub.50 and F.sub.200 in % per
cycle, respectively after 50 cycles and after 200 cycles.
[0080] Storage test:
[0081] An initial test was performed in order to measure the
initially discharged capacity C.sub.i in mAh/g of the positive
active material:
[0082] charging at 0.5 I.sub.c for 3 hours; and
[0083] discharging at 0.2 I.sub.c down to 2.7 V.
[0084] Then the following succession of operations was performed
four times over prior to taking measurements:
[0085] charging at a rate of I.sub.c for 3 hours;
[0086] storing for 8 days at 40.degree. 0C.; and
[0087] discharging at 0.2 I.sub.c down to 2.7 V.
[0088] The loss of capacity S expressed in % of C.sub.i was
measured during the discharge following the fourth period of
storage.
[0089] The results of the above tests are given in Table 3
below.
3 TABLE 3 Electrolyte Binder C.sub.i F.sub.50 F.sub.200 S EC/PC/DMC
PVDF 139 0.3 01.2 10.9 20/20/60 EO/EA PVDF 149 0.45 0.31 25 50/50
EC/PC/EA PVDF 150 1 0.36 30.8 15/25/60 EC/DMC/EA PVDF 137 15.4
15/25/60 EC/PC/DMC/VC NBR + 144 0.10 0.03 1 19/19/57/5 CMC
EC/DMC/EA/VC NBR + 144 0.09 0.03 1 14/24/57/5 CMC EC/DMC/EA/VC SBR
+ 145 0.10 14/24/57/5 CMC
[0090] When the binder of the negative electrode was PVDF, the
cells whose electrolyte solvent contained EA presented losses of
capacity in cycling at ambient temperature and in storage that were
greater than those for cells in which the electrolyte contained no
EA.
[0091] At ambient temperature, cells containing EA and having a
negative electrode binder made up of elastomer+CMC in accordance
with the invention presented initial capacity, and stability during
cycling and storage that were considerably better than those of
prior art cells in which the negative binder was PVDF.
[0092] Cycling test at low temperature:
[0093] This test was performed as follows:
[0094] charging at a rate of 0.5 I.sub.c for 3 hours; and
[0095] discharging at 0.2 I.sub.c, 0.5 I.sub.c, I.sub.c or 2
I.sub.c down to a voltage of 2 volts.
[0096] The initially discharged capacity C.sub.i in mAh/g during
the first discharge was measured. Thereafter the discharged
capacities C.sub.-20.degree. C. and C.sub.-30.degree. C. were
measured respectively at -20.degree. C. and at -30.degree. C.,
being expressed in % of C.sub.i at ambient temperature, for the
various discharge rates.
[0097] The results are given in Table 4 below.
4 TABLE 4 C.sub.-20.degree.C. (% C.sub.i) C.sub.-30.degree.C. (%
C.sub.i) Electrolyte Binder C.sub.i 0.21.sub.C 0.51.sub.C I.sub.C
21.sub.C 0.21.sub.C 0.51.sub.C I.sub.C 21.sub.C EC/PC/DMC/VC PVDF
140 64 18 0 20 0 0 19/19/57/5 EC/PC/EA PVDF 142 42 24 2 29 2 0
15/25/60 EC/DMC/EA PVDF 145 49 28 10 57 18 0 15/25/60 EC/PC/DMC/VC
NBR/ 145 71 46 10 38 0 0 19/19/57/5 CMC EC/DMC/EA/VC NBR 144 75 77
62 57 71 47 14/24/57/5 CMC EC/DMC/EA/VC SBR 145 58 68 14/24/57/5
CMC
[0098] When the binder of the negative electrode was PVDF, the
cells whose electrolyte contained EA had a higher discharged
capacity at low temperature, particularly at a high discharge rate,
than did the cells which did not contain any EA. Under difficult
conditions (-30.degree. C., I.sub.c), the discharged capacity was
18% in the best of cases.
[0099] When the binder of the negative electrode was a mixture of
elastomer+CMC, it can be seen that the results were of the same
order of magnitude as before when the cell had an electrolyte which
did not contain acetate.
[0100] Surprisingly, when an electrode containing a negative binder
of elastomer+CMC is associated with an electrolyte containing ethyl
acetate in a cell of the invention, very much better results are
obtained. Under the most extreme conditions (-30.degree. C.,
2I.sub.c), nearly half the capacity of that cell was still
usable.
EXAMPLE 4
[0101] A rechargeable lithium electrochemical cell was prepared
analogous to the cell of Example 2 except that the
electrochemically active material of the positive electrode was a
lithium cobalt oxide LiCoO.sub.2.
[0102] The cell was evaluated electrochemically using the tests
described above.
[0103] For the cycling test at ambient temperature (25.degree. C.)
at a high discharge rate, and for the storage test, the results are
given in Table 5 below.
5 TABLE 5 Electrolyte Binder C.sub.i F.sub.50 F.sub.200 S
EC/DMC/DEC/VC NBR + 128 0.04 0.03 10 38/38/19/5 CMC EC/DMC/EA/VC
NBR + 129 0.05 0.05 10 14/24/57/5 CMC EC/PC/MB/VC NBR + 128 0.05
0.05 7 19/19/57/5 CMC
[0104] It can be seen that the cycling performance at ambient
temperature and the storage performance were of the same order for
all of the cells tested.
[0105] The results of the low temperature cycling test are given in
Table 6 below.
6 TABLE 6 C.sub.-20.degree.C. C.sub.-30.degree.C.
C.sub.-30.degree.C. Electrolyte Binder C.sub.i 0.21.sub.C
0.21.sub.C I.sub.C 0.21.sub.C I.sub.C EC/DMC/DEC/VC NBR + 129 86 0
0 0 0 38/38/19/5 CMC EC/DMC/EANC NBR + 129 100 94 99.7 76 0
14/24/57/5 CMC EC/PC/MB/VC NBR + 128 100 94 99 77 0 19/19/57/5
CMC
[0106] Cells with electrolytes containing EA or MB gave better
results from -20.degree. C., than cells whose electrolytes did not
contain any; at -40.degree. C. they still gave more than
three-fourths of their initial ambient temperature capacity.
* * * * *