U.S. patent application number 09/032660 was filed with the patent office on 2001-09-20 for novel electrochemically stable plasticizer.
Invention is credited to BARKER, JEREMY, LIU, PEIKANG, MITCHELL, PORTER, SWOYER, JEFFREY.
Application Number | 20010023039 09/032660 |
Document ID | / |
Family ID | 21866117 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010023039 |
Kind Code |
A1 |
LIU, PEIKANG ; et
al. |
September 20, 2001 |
NOVEL ELECTROCHEMICALLY STABLE PLASTICIZER
Abstract
A laminate structure or precursor paste thereof characterized by
being formed from a composition comprising a polymeric material and
a plasticizer. The plasticizer being at least one compound
represented by the following general formula 1 where R is a low
alkyl selected from the group consisting of methyl, ethyl, butyl,
pentyl and hexyl.
Inventors: |
LIU, PEIKANG; (HENDERSON,
NV) ; MITCHELL, PORTER; (LAS VEGAS, NV) ;
SWOYER, JEFFREY; (HENDERSON, NV) ; BARKER,
JEREMY; (HENDERSON, NV) |
Correspondence
Address: |
EMILY M ROGERS
ASSOCIATE GENERAL COUNSEL
VALENCE TECHNOLOGY INC
301 CONESTOGA WAY
HENDERSON
NV
89015
|
Family ID: |
21866117 |
Appl. No.: |
09/032660 |
Filed: |
February 27, 1998 |
Current U.S.
Class: |
429/217 ;
204/296; 29/623.1; 429/223; 429/224; 429/231.3; 429/254 |
Current CPC
Class: |
H01M 10/058 20130101;
H01M 10/0565 20130101; H01M 50/414 20210101; H01M 50/411 20210101;
H01M 10/052 20130101; H01M 50/46 20210101; H01M 10/0567 20130101;
H01M 4/1393 20130101; Y10T 29/49108 20150115; Y02E 60/10 20130101;
H01M 4/1391 20130101; H01M 4/131 20130101; H01M 6/188 20130101;
H01M 6/168 20130101; Y02P 70/50 20151101; H01M 4/62 20130101; H01M
4/133 20130101 |
Class at
Publication: |
429/217 ;
429/254; 29/623.1; 204/296; 429/223; 429/224; 429/231.3 |
International
Class: |
H01M 004/62; H01M
002/16; H01M 004/50; C25C 007/04; H01M 004/52 |
Claims
1. An electrode composition or precursor paste thereof
characterized by being formed from a composition initially
comprising an active material, a polymeric material, and a
plasticizer; and optionally from which composition at least a
portion of said plasticizer has been removed, after polymerization
of said polymeric material; said plasticizer being at least one
compound represented by the following general formula 4where R is a
low alkyl selected from the group consisting of methyl, ethyl,
butyl, pentyl and hexyl.
2. The electrode according to claim 1 wherein said plasticizer is
further characterized by electrochemical stability up to about 4.5
volts.
3. The electrode according to claim 1 wherein said polymeric
material is a copolymer of VdF (vinylidene) and HFP
(hexafluoropropylene).
4. A separator membrane, useful for preparing an electrolytic cell,
said membrane characterized by being formed from a composition
initially comprising a polymeric material and a plasticizer, and
optionally from which composition at least a portion of said
plasticizer has been removed, said plasticizer being at least one
compound represented by the following general formula 5where R is a
low alkyl selected from the group consisting of methyl, ethyl,
butyl, pentyl and hexyl.
5. The separator according to claim 4 wherein said plasticizer is
further characterized by electrochemical stability up to about 4.5
volts.
6. The separator according to claim 4 wherein said polymeric
material is a copolymer of VdF (vinylidene) and HFP
(hexafluoropropylene).
7. A rechargeable battery structure comprising a positive electrode
element, a negative electrode element, and a separator element
disposed therebetween characterized in that: at least one of said
elements comprises a flexible, self-supporting, polymeric matrix
film composition; each said element is bonded to contiguous
elements at its respective interfaces to form a unitary flexible
laminate structure; and at least one of said element films
comprises a composition initially comprising a polymeric material
and a plasticizer, and optionally from which composition at least a
portion of said plasticizer has been removed, said plasticizer
being at least one compound represented by the following general
formula 6 where R is a low alkyl having up to six carbon atoms.
8. The battery according to claim 7 where R is a low alkyl selected
from methyl, ethyl, butyl, pentyl and hexyl.
9. The battery according to claim 7 wherein said plasticizer is
further characterized by electrochemical stability up to about 4.5
volts.
10. The battery according to claim 7 wherein said polymeric
material is a copolymer of VdF (vinylidene) and HFP
(hexafluoropropylene).
11. A method of making a rechargeable battery structure which
structure comprises, in sequence, a positive electrode element, a
separator element, and a negative electrode element, characterized
in that each of said electrode and separator elements comprises a
flexible, polymeric matrix composition; said method comprising
forming a mixture comprising a casting solvent, a polymeric
material and a plasticizer, said plasticizer for at least one of
said elements being a compound represented by the following general
formula 7where R is a low alkyl having up to six carbon atoms;
casting the mixture and removing said casting solvent to form a
self-supporting film of said flexible, polymeric matrix
composition; bonding each said element to contiguous elements at
its respective interface to form a unitary flexible laminate
structure; and then removing at least a portion of said plasticizer
from said laminate structure.
12. The method according to claim 11 where R is a low alkyl
selected from methyl, ethyl, butyl, pentyl and hexyl.
13. The method according to claim 11 wherein said plasticizer is
further characterized by electrochemical stability up to about 4.5
volts.
14. The method according to claim 11 wherein said polymeric
material is a copolymer of VdF (vinylidene) and HFP
(hexafluoropropylene).
15. The method according to claim 11 wherein said plasticizer for
each of said elements is represented by said general formula.
16. An electrochemical cell which comprises a first electrode; a
counter electrode which forms an electrochemical couple with said
first electrode; and an electrolyte; said electrolyte comprising a
solute, and a solvent mixture; said solute consisting essentially
of a salt of a metal; and at least one of said electrodes
comprising: an active material; a polymeric material; a plasticizer
for said polymeric material wherein said plasticizer is at least
one compound selected from the group consisting of compounds
represented by the following general formula 8where R is a low
alkyl selected from methyl, ethyl, butyl, pentyl and hexyl.
17. The cell according to claim 16 wherein said plasticizer is
further characterized by electrochemical stability up to about 4.5
volts.
18. The cell according to claim 16 wherein said plasticizer is
further characterized by disassociatingly solubilizing said metal
salt.
19. The cell according to claim 16 wherein said plasticizer
constitutes a portion of said solvent mixture.
20. The cell according to claim 19 wherein said solvent mixture
comprises, besides said plasticizer, at least one other solvent
selected from the group consisting of ethylene carbonate (EC),
dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl
carbonate (DPC), dibutyl carbonate (DBC), diethoxy ethane (DEE),
ethyl methyl carbonate (EMC), butylene carbonate (BC), vinylene
carbonate (VC), propylene carbonate (PC), and mixtures thereof.
21. The cell according to claim 16 wherein said first electrode
active material is selected from the group consisting of lithium
manganese oxide, lithium nickel oxide and lithium cobalt oxide; and
said first electrode polymeric material is a copolymer of VdF
(vinylidene) and HFP (hexafluoropropylene).
22. The cell according to claim 16 wherein said counter-electrode
active material is selected from the group consisting of
non-graphitic amorphous coke, graphitic carbon, graphites, and
mixtures thereof; and said counter-electrode polymeric material is
a copolymer of VdF (vinylidene) and HFP (hexafluoropropylene).
23. The cell according to claim 16 which further comprises a
separator, disposed between said first electrode and said
counter-electrode, and wherein said plasticizer and said
solubilized salt are distributed within said separator; said
separator is in the form of a solid matrix forming a network with
voids interpenetrated by said plasticizer and salt; and said matrix
is selected from the group consisting of polymeric acrylate, porous
polypropylene, porous polyethylene, and glass fiber material.
Description
FIELD OF THE INVENTION
[0001] This invention relates to electrochemical cells and
batteries, and more particularly, to lithium ion cells and
batteries.
BACKGROUND OF THE INVENTION
[0002] Lithium batteries are prepared from one or more lithium
electrochemical cells. Such cells have included an anode (negative
electrode), a cathode (positive electrode), and an electrolyte
interposed between electrically insulated, spaced apart positive
and negative electrodes. The electrolyte typically comprises a salt
of lithium dissolved in one or more solvents, typically nonaqueous
(aprotic) organic solvents. By convention, during discharge of the
cell, the negative electrode of the cell is defined as the anode.
During use of the cell, lithium ions (Li+) are transferred to the
negative electrode on charging. During discharge, lithium ions
(Li+) are transferred from the negative electrode (anode) to the
positive electrode (cathode). Upon subsequent charge and discharge,
the lithium ions (Li+) are transported between the electrodes.
Cells having metallic lithium anode and metal chalcogenide cathode
are charged in an initial condition. During discharge, lithium ions
from the metallic anode pass through the liquid electrolyte to the
electrochemically active material of the cathode whereupon
electrical energy is released. During charging, the flow of lithium
ions is reversed and they are transferred from the positive
electrode active material through the ion conducting electrolyte
and then back to the lithium negative electrode.
[0003] The lithium metal anode has been replaced with a carbon
anode, that is, a carbonaceous material, such as non-graphitic
amorphous coke, graphitic carbon, or graphites, which are
intercalation compounds. This presents a relatively advantageous
and safer approach to rechargeable lithium as it replaces lithium
metal with a material capable of reversibly intercalating lithium
ions, thereby providing the so-called "rocking chair" battery in
which lithium ions "rock" between the intercalation electrodes
during the charging/discharging/recharging cycles. Such lithium
metal free cells may thus be viewed as comprising two lithium ion
intercalating (absorbing) electrode "sponges" separated by a
lithium ion conducting electrolyte usually comprising a lithium
salt dissolved in nonaqueous solvent or a mixture of such solvents.
Numerous such electrolytes, salts, and solvents are known in the
art.
[0004] In the manufacturing of a battery or a cell utilizing a
lithium-containing electrode, there is an attempt to eliminate as
many undesirable impurities and unstable precursor components as
possible. Such undesirable impurities and precursors adversely
affect cell performance.
[0005] In a lithium battery or cell, it is important to eliminate
as many impurities and some precursor components which may affect
cell performance. Such impurities and precursor components cause
side reactions and are subject to breakdown because they are not
electrochemically stable. Loss of performance due to impurities and
breakdown of precursor compounds causing undesired side reactions
has led to the formation of cell components and assembly of the
cell under very controlled conditions. Performance problems have
also led to the removal and extraction of as many impurities and
precursor components as possible in order to minimize problems.
However, extraction techniques for removing such undesired
compounds are very time-consuming and very costly. Therefore, what
is needed is an understanding of the mechanism causing undesired
loss of performance and the resolution of same, which avoids the
need for costly and time-consuming process steps; and a new method
for forming battery components which avoids the need for costly
extraction and purification steps.
SUMMARY OF THE INVENTION
[0006] The present invention provides a novel composition from
which electrochemical cell component films are fabricated which
avoids undesired electrochemical breakdown of cell components; and
which avoids the need for complex purification steps to reduce or
substantially eliminate precursor components subject to
electrochemical breakdown.
[0007] The components of the cell are formed from a specifically
selected class of new plasticizers which are resistant to
decomposition by electrochemical breakdown. The new class of
plasticizers are characterized by electrochemical stability at
least up to about 4.5 volts.
[0008] In addition to their electrochemical stability, the
plasticizers of the invention have properties similar to those
desired in an electrolyte solvent.
[0009] The plasticizers of the invention are generally
characterized as dibasic esters based on adipates. They have the
general formula as shown in Table I, where "R" represents a low
alkyl selected from methyl, ethyl, propyl, butyl, pentyl and hexyl.
Accordingly, "R" represents a low alkyl, having up to six carbon
atoms. The plasticizers of the invention are further characterized
by electrochemical stability up to about 4.5 volts, and by
disassociatingly solubilizing the metal salt of the electrolyte.
The plasticizers of the invention have characteristics consistent
with desired electrolyte solvents, and they may be used as all or
part of the solvent mixture. However, it is preferred to remove at
least a portion of the plasticizer after casting the film.
[0010] The characteristics of the plasticizer include the ability
to disassociatingly solubilize the metal salt used for ion
transport in an electrochemical cell. Advantageously, the
plasticizer need not be extracted completely from precursor
components, the electrode and/or electrolyte, before final assembly
of the cell. This is because the plasticizer and the solubilized
salt become distributed within the separator of the completed cell
where the plasticizer along with other components of the solvent
mixture are dispersed for ion transport. Preferably, the solvent
mixture comprises, besides the plasticizer, at least one of those
solvents selected from the group consisting of ethylene carbonate
(EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl
carbonate (DPC), dibutyl carbonate (DBC), diethoxy ethane (DEE),
ethyl methyl carbonate (EMC), butylene carbonate (BC), vinylene
carbonate (VC), propylene carbonate (PC), and mixtures thereof.
Since the plasticizer is not a preferred solvent, it preferably
constitutes a relatively small portion of the solvent mixture. The
plasticizer is preferably present in an amount less than the amount
by weight of any other one of the solvents included in the mixture.
Advantageously, the plasticizer is miscible with the aforesaid
common solvents. Other characteristics of the dibasic esters of the
invention based on adipate include, based on the exemplary dimethyl
adipate (DMA), a boiling point of 109-110.degree. C.; a melting
point of about 8.degree. C.; vapor pressure of about 0.2 mm;
specific gravity of about 1.063; and purity on the order of 98-99%.
The plasticizer in appearance is a colorless liquid, dialkyl
adipate.
[0011] Although the plasticizer of the invention may remain as a
part of the cell component (electrode and/or separator) after its
fabrication, it is preferred to remove at least a portion of the
plasticizer. In any event, the solubilizing plasticizer of the
invention, forming a part of the solvent mixture, is present in an
amount not greater than the amount by weight of any other one of
the organic solvent components. The preferred plasticizers are
dimethyl adipate (DMA) and diethyl adipate (DEA). The
characteristics of dimethyl adipate (DMA) as outlined above are
shown in Table II. The preferred dimethyl adipate is shown as an
entry in chemical structural formula in Table I.
[0012] The electrochemical cell of the invention comprises a first
electrode; a counter-electrode which forms an electrochemical
couple with the first electrode; and an electrolyte. The
electrolyte comprises the solute in solvent mixture. The solute is
essentially a salt of the metal. In the case of a lithium ion
battery, this is a lithium salt, such as LiPF.sub.6. According to
the invention, at least one of the electrodes comprises an active
material; a polymeric material functioning as a binder; and a
plasticizer for the polymeric material, where the plasticizer is at
least one compound selected from the group of dibasic esters
derived from adipate, according to the invention. Preferably, in
the case of a metal oxide electrode, the electrode composition
further comprises a conductive diluent such as graphite. The
preferred polymeric binder material is preferably a co-polymer of
polyvinylene difluoride (PVDF) and hexafluoropropylene (HFP). In
another aspect, the electrolyte/separator film is formed from the
co-polymer and plasticizer.
[0013] The plasticizers of the invention solve the difficult
processing problems associated with removal of conventional
plasticizers after formation of cell components and before their
assembly into a cell. The plasticizer of the invention may be used
to formulate any of the polymeric components of the cell, positive
electrode, negative electrode, and electrolyte/separator.
Plasticizers of the invention comprising adipate derivatives,
esters, are highly desirable due to their stability. Plasticizers
of the invention are stable under atmospheric conditions on
exposure to oxygen, humidity, and importantly, are
electrochemically stable. This is in contrast to plasticizers
conventionally used to form cell components. Such conventional
plasticizers must be removed prior to assembly of the cell as they
are not electrochemically stable. An additional advantage is that
the plasticizer of the invention has characteristics consistent
with properties desired for a solvent and functions as a part of
the solvent mixture when included in an electrochemical cell.
Therefore, advantageously, the plasticizers of the invention become
part of the electrode formulation performed characteristic function
as a plasticizer during formation of cell components from precursor
compounds, and then they remain as a part of the cell component
when the cell is assembled.
[0014] Objects, features, and advantages of the invention include
an improved electrochemical cell or battery having improved
charging and discharging characteristics; which maintains its
integrity over a prolonged life cycle as compared to presently used
batteries and cells. Another object is to provide electrode
mixtures comprising constituents which are stable when cycled in an
electrochemical cell, and which demonstrates high performance, and
which does not readily decompose during cell operation. It is also
an object of the present invention to provide cells which can be
manufactured more economically and conveniently, and to provide
cells with electrode and electrolyte components which are
compatible with one another, avoiding problems with undesired
reactivity, breakdown, and degradation of cell performance.
[0015] 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
[0016] FIG. 1 shows the performance of two cells prepared with a
negative electrode (anode) of carbonaceous material designated as
BG-35 cycled against a lithium metal electrode. The electrolyte is
EC/DMC (ethylene carbonate/dimethyl carbonate) in a ratio by weight
of 2:1; one molar LiPF6 electrolyte; and including 5 percent by
weight dimethyl adipate. FIG. 1 shows a voltage/capacity plot for
BG-35 graphite carbon electrode cycled with the lithium metal
counterelectrode, using constant current cycling at .+-.0.2
milliamps per square centimeter, between 0.01 and 2.0 volts, using
42 milligrams of the BG-35 active material. Here, the electrolyte
constitutes a mixture of 95% EC/DMC, LiPF.sub.6 and 5% DMA, more
specifically: 5% DMA (or 5 gram)+95% (or 95 gram) of 1M EC/DMC,
LiPF.sub.6 electrolyte.
[0017] FIG. 2 is a voltage/capacity plot similar to that described
for FIG. 1. The graphite is SFG-15/MCMB 2528 in a 50:50 weight
ratio. FIG. 2 is a voltage/capacity plot for the SFG-15/MCNB
graphite carbon electrode cycled with a lithium metal
counterelectrode using constant current cycling at .+-.0.2
milliamps per square centimeter, between 0.01 and 2.0 volts, using
36 mgs of the graphite active material. The electrolyte is one
molar LiPF6 in a solution of EC/DMC. The weight ratio of solvent is
2:1 of EC/DMC. In the formulation of FIG. 2, the DMA plasticizer
was essentially completely removed by methanol extraction.
[0018] FIG. 3 shows voltage/capacity plot for an electrode
formulation prepared similar to FIG. 2, except that the plasticizer
was essentially completely extracted by vacuum extraction.
[0019] FIG. 4 is a two-part graph, with FIG. 4A showing coulombic
efficiency versus cycles, and FIG. 4B showing discharge capacity
versus cycles. The cells have BG-35 negative electrode (anode) and
LMO (nominally LiMn.sub.2O.sub.4) positive electrode (cathode); the
electrolyte is EC/DMC/DMA:64.2% EC/30.8% DMC/5.0% DMA by weight,
with 1M LiPF.sub.6 salt.
[0020] FIG. 5 is a two-part graph, with FIG. 5A showing coulombic
efficiency versus cycles, and FIG. 5B showing discharge capacity
versus cycles. The cells have the same BG-35/LMO electrodes and
salt as FIG. 4, but the solvent weight ratio is 53.3% EC/26.7%
DMC/20.0% DMA.
[0021] FIG. 6 is a two-part graph, with FIG. 6A showing coulombic
efficiency versus cycles, and FIG. 6B showing discharge capacity
versus cycles. The cells have the same BG-35/LMO electrodes and
salt as FIG. 4, but the solvent weight ratio is 60% EC/30% DMC/10%
DMA.
[0022] FIG. 7 shows cycling performance of respective 5%, 10% and
20% DMA cells taken from FIGS. 4, 5 and 6.
[0023] FIG. 8 is a two-part graph, with FIG. 8A showing coulombic
efficiency versus cycles, and FIG. 8B showing discharge capacity
versus cycles. The cells are BG-35/LMO, EC/DMC 1M LiPF.sub.6, with
DMA as separator plasticizer.
[0024] FIG. 9 is an illustration of a cross-section of a thin
battery or cell embodying the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] The invention provides, for the first time, a key cell
component which is stabilized against decomposition during cyclic
operation of an electrochemical cell. The components of the cell
are formed from a specifically selected class of new plasticizers
which are resistant to decomposition by electrochemical breakdown.
Such decomposition and resultant formation of byproducts, including
gaseous byproducts, are problems encountered with conventional
plasticizers used today. Advantageously, the plasticizer selected
for use in the present invention performs a dual function as both a
plasticizer and electrolyte solvent. Such dual function compound
has heretofore not been suggested. Before further describing the
invention, it is useful to understand problems associated with
present electrode and electrolyte formulations using conventional
plasticizers.
[0026] Conventional plasticizers, such as DBP (dibutyl phthalate)
are included in the precursor formulation from which electrode and
separator elements are formed. Other common plasticizers include
dimethylthalate, diethylthalate, trisbutoxyethyl phosphate, and
trimethyl trimellitate. The DBP (dibutyl phthalate) is particularly
preferred for use in combination with polymeric materials such as
VdF (vinylidene fluoride) and HFP (hexafluoropropylene), PVC, PAN
and the like.
[0027] Referring to U.S. Pat. Nos. 5,418,091; 5,456,000; 5,460,904;
and 5,540,741; it can be seen that such plasticizers are
essentially completely extracted immediately after formation of the
cell component, and before assembly of the completed cell. It is
necessary to essentially completely remove the plasticizer, DBP and
the like, because they are not electrochemically stable and will
decompose and interfere with cell performance. Each of the four
aforesaid patents is incorporated herein by reference in its
entirety, describing negative electrode, positive electrode, and
electrolyte formulations with removal of plasticizer before making
a cell. The present invention obviates the need for costly and
time-consuming removal of plasticizer.
[0028] In view of the difficulties mentioned above, very elaborate
extraction techniques are used to remove the plasticizer after it
has imparted the necessary properties to the precursor cell
components. The plasticizer is removed either by solvent
extraction, where it is transferred to a liquid solvent phase from
which it may be readily recovered, or by vacuum extraction. Those
skilled in the art will understand that solvent extraction and
vacuum extraction are energy intensive, complex, require series of
steps, good process control, and are very costly.
[0029] The present invention defines a new approach to solving the
problem. By the present invention, a new class of plasticizers are
selected which are electrochemically stable and have properties
similar to those desired in an electrolyte solvent. Such novel
plasticizers may remain in the cell component after fabrication
where they function as part of the solvent mixture. The
plasticizers of the invention are generally characterized as
dibasic esters based on adipates. They have the general formula as
shown in Table I, where "R" represents a low alkyl selected from
methyl, ethyl, butyl, pentyl, and hexyl. Accordingly, "R"
represents a low alkyl, having up to six carbon atoms. The
plasticizers of the invention are further characterized by
electrochemical stability up to about 4.5 volts, and by
disassociatingly solubilizing the metal salt of the electrolyte.
The plasticizers of the invention have characteristics consistent
with desired electrolyte solvents, and they may constitute a
portion of the solvent mixture.
[0030] The preferred characteristics of exemplary plasticizers of
the invention are given in Table II. It is preferred that the
solvent mixture of the electrolyte comprise an organic solvent
selected from the group consisting of ethylene carbonate (EC),
dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl
carbonate (DPC), dibutyl carbonate (DBC), diethoxy ethane (DEE),
ethyl methyl carbonate (EMC), butylene carbonate (BC), vinylene
carbonate (VC), propylene carbonate (PC), and mixtures thereof.
Refer to Table III for solvent characteristics. The plasticizer is
miscible with the other solvents and forms a part of the solvent
mixture.
[0031] Although the plasticizer of the invention may remain as a
part of the cell component after its fabrication, it is preferable
to remove at least a portion of it. In any event, the solubilizing
plasticizer of the invention, forming a part of the solvent
mixture, is present in an amount not greater than the amount by
weight of any other one of the organic solvent components. In an
exemplary mixture, the solvent comprised EC/DMC in a weight
proportion of 2:1, and also included the DMA plasticizer of the
invention (dimethyl adipate). The DMA was present in an amount of 5
percent by weight of the solvent mixture, and the EC/DMC/LiPF.sub.6
(1M) constituted 95 percent by weight.
[0032] Preferred plasticizers are dimethyl adipate (DMA) and
diethyl adipate (DEA). Dimethyl adipate is available from the
Dupont Chemical Company and is available under the trade name
"DBE-6", dibasic ester (dimethyl adipate), 99 percent purity. A 99
percent purity DMA is available from Aldrich Chemical Company,
Inc., of Milwaukee, Wis. Physical characteristics of the dimethyl
adipate available from both Dupont and Aldrich are given in Table
II.
[0033] According to Aldrich and Dupont, the dimethyl adipate is
synonymous with dimethyl hexanedionate, hexanedionic acid, dimethyl
ester (9CI) and methyl adipate. Another commercially available
formulation is DBE-4 product trade name, which represents a mixture
of DMA and DEA.
[0034] It should be noted that the melting point of DMA is lower
than the conventionally used DBP, therefore lamination of cell
electrode and separator parts would need to be lowered. Such
lamination presently occurs in a range of about 110 to 115.degree.
C., and such lamination preferably occurs with DMA at around
100.degree. C.
[0035] Graphite and lithium metal oxide electrode active materials
were used to prepare electrode formulations along with the novel
plasticizer of the invention, and then tested in electrochemical
cells. Test cells were also prepared having a polymeric separator
formed with the plasticizer of the invention. Selected results are
as recorded in FIGS. 1-8. A typical cell configuration will be
described with reference to FIG. 9.
[0036] The electrochemical cell or battery which uses the novel
plasticizer of the invention will now be described, with reference
to FIG. 9. By convention, an electrochemical cell comprises a first
electrode, a counterelectrode, which reacts electrochemically with
the first electrode, and an electrolyte which is capable of
transferring ions between the electrodes. A battery refers to one
or more electrochemical cells. Referring to FIG. 4, an
electrochemical cell or battery 10 has a negative electrode side
12, a positive electrode side 14, and an electrolyte/separator 16
therebetween. The negative electrode is the anode during discharge,
and the positive electrode is the cathode during discharge. The
negative electrode side includes current collector 18, typically of
nickel, iron, stainless steel, and copper foil, and negative
electrode active material 20. The positive electrode side includes
current collector 22, typically of aluminum, nickel, and stainless
steel, and such foils may have a protective conducting coating
foil, and a positive electrode active material 24. The
electrolyte/separator 16 is typically a solid electrolyte, or
separator and liquid electrolyte. Solid electrolytes are typically
referred to as polymeric matrixes which contain an ionic conductive
medium. Liquid electrolytes typically comprise a solvent and an
alkali metal salt which form an ionically conducting liquid. In
this latter case, the separation between the anode and cathode is
maintained, for example, by a relatively inert layer of material
such as glass fiber. Essentially, any lithium ion containing
conducting electrolyte may be used, that is stable up to 4.5 volts
or more. Essentially any method may be used to maintain the
positive and negative electrodes spaced apart and electrically
insulated from one another in the cell. Accordingly, the essential
features of the cell are the positive electrode, a negative
electrode electrically insulated from the positive electrode, and
an ionically conducting medium between the positive and negative
electrodes. Examples of a suitable separator/electrolyte, solvents,
and salts are described in U.S. Pat. No. 4,830,939 showing a solid
matrix containing an ionically conducting liquid with an alkali
metal salt where the liquid is an aprotic polar solvent; and U.S.
Pat. Nos. 4,935,317; 4,990,413; 4,792,504; 5,037,712; 5,418,091;
5,456,000; 5,460,904; 5,463,179; and 5,482,795. Each of the above
patents is incorporated herein by reference in its entirety.
Protective bagging material 40 covers the cell and prevents
infiltration of air and moisture.
[0037] Electrodes of the invention are made by mixing a binder,
plasticizer, the active material, and carbon powder (particles of
carbon). The binder desirably is a polymer. The plasticizer is
compatible with the polymer. A paste containing the binder,
plasticizer, active material and carbon is coated onto a current
collector. The positive electrode comprises a preferred lithium
manganese oxide active material of the invention. For the positive
electrode, the content is typically as follows: 60 to 80 percent by
weight active material; 2 to 8 carbon black, as the electric
conductive diluent; and 5 to 15 percent binder, preferably chosen
to enhance ionic conductivity; and 10 to 25 weight percent
plasticizer. Stated ranges are not critical. The amount of active
material may vary. These materials are mixed and blended together
with a casting solvent. Acetone is a suitable solvent. The mixture
is then coated onto a glass plate to achieve the desired thickness
for the final electrode. The negative electrode of the invention
preferably comprises about 55 to 75 percent by weight of graphite
active material, and more preferably, 60 to 70 percent by weight,
with the balance constituted by the binder and preferred
plasticizer. Preferably, the negative electrode is prepared from a
slurry, which is coated onto a glass plate using conventional
casting techniques as described with respect to the positive
electrode.
[0038] The electrolyte used to form a completed cell comprises an
organic solvent or solvent mixture with preferred solvents as shown
in Table III. The solvent also comprises the plasticizer of the
invention in an amount up to about 35 weight percent. The solvent
contains typically a one molar solution of a lithium metal salt,
such as LiPF6. The positive and negative electrodes are maintained
in a separated, spaced-apart condition using a fiberglass layer or
separator of an equivalent design. In an alternative embodiment,
the separator between the electrodes is also formed from a polymer
formulation using the plasticizer of the invention.
[0039] The electrochemical cell which utilizes the novel
plasticizer 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. For test purposes,
test cells were fabricated using lithium metal electrodes. When
forming cells for use as batteries, it is preferred to use an
intercalation metal oxide positive electrode and a graphitic carbon
negative electrode.
[0040] Various methods for fabricating electrochemical cells and
batteries and for forming electrode components are further
described immediately below. The invention is not, however, limited
by any particular fabrication method as the novelty lies in the
unique electrolyte. Accordingly, additional methods for preparing
electrochemical cells and batteries may be selected and are
described in the art, for example, in U.S. Pat. Nos. 5,435,054
(Tonder & Shackle); 5,300,373 (Shackle); 5,262,253 (Golovin);
4,668,595; 4,830,939 (Lee & Shackle); and particularly U.S.
Pat. Nos. 5,418,091; 5,456,000; 5,460,904, and 5,540,741 assigned
to Bell Communication Research. Each of the above patents is
incorporated herein by reference in its entirety.
EXAMPLE I
[0041] 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 Corporation,
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 IV. The binder was a copolymer
of polyvinylidene difluoride (PVDF) and hexafluoropropylene (HFP)
in a molar 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. The
plasticizer was dimethyl adipate. An electronic grade casting
solvent was used. The slurry was cast onto glass and a
free-standing electrode was formed as the casting solvent
evaporated. The slurry composition for the negative electrode was
as follows:
1 Component Wet Weight % Dry Weight % Graphite 23.4 56.0 Super P
0.9 2.2 Binder 6.8 16.4 Plasticizer 10.5 25.4 Solvent 58.4 -- Total
100.0 100.0
[0042] The counter-electrode was lithium metal. A glass fiber
separator was used between the electrodes to prevent them from
electrically shorting. An electrochemical cell of the first
electrode, separator, and counter-electrode was formed.
[0043] The electrolyte used to form the completed final cell or
battery comprised a solution of 95 percent by weight EC/DMC and 5
percent by weight DMA, which remained after formation of the
electrode. The weight ratio of EC to DMC was 2:1. The solvent
included one molar LiPF.sub.6 salt. The two electrodes were
maintained in separated condition using a fiberglass layer. The
electrolyte solution interpenetrated the void spaces of the
fiberglass layer. The results of constant current cycling are shown
in FIG. 1. FIG. 1 shows a voltage/capacity plot of BG-35 graphite
cycled with a lithium metal electrode using constant current
cycling at .+-.0.2 milliamps per square centimeter, between 0.01
and 2.0 volts vs. Li/Li.sup.+. In FIG. 1, the results of cycling
two similar cells are shown. One cell is designated L0147900 (1479)
and the other is L0148000 (1480). The data for the 1479 cell is
given below, followed directly by the data for the 1480 cell stated
in parentheses. The cycling data was obtained using 42 (40)
milligrams of the BG-35 active material. The electrolyte is as
stated above. The test was conducted under ambient conditions. In
the first half-cycle, lithium is removed from metallic electrode
and intercalated into the graphite electrode. Once essentially full
intercalation at the graphite electrode was completed,
corresponding to about Li.sub.1C.sub.6, the voltage had dropped to
approximately 0.1 volts, representing about 400 (335) milliamp
hours per gram, corresponding to about 16.8 (13.4) milliamp hours,
based on the 42 (40) milligrams of active material. In the
second-half cycle, lithium is de-intercalated from the graphite and
returned to the metallic electrode, until the average voltage is
approximately 2 volts vs. Li/Li.sup.+. The deintercalation
corresponds to approximately 352 (295) milliamp hours per gram,
representing approximately 14.8 (11.8) milliamp hours, based on the
42 (40) milligrams of active material. This completes an initial
cycle. The percentage difference between the 16.8 (13.4) milliamp
hours per gram capacity "in" and the 14.8 (11.8) milliamp hours per
gram capacity "out", divided by the initial 16.8 (13.4) capacity
"in" corresponds to a surprisingly low 12 percent first cycle loss
for each of cells 1479 and 1480.
Example II
[0044] The flexibility of the plasticizer of the invention can be
further understood by reference to the following examples and the
results shown in FIGS. 2 and 3. For comparative purposes,
electrodes were prepared using DMA as plasticizer, as mentioned
above, but having DMA extracted after formation of the electrode.
Methanol was used as the extraction solvent. Graphite electrodes
were prepared as described in Example I and according to the weight
proportions shown therein, except that the graphite was a
combination of SFG-15 and MCMB 2528 in a 50:50 weight ratio. The
electrodes comprised 36 milligrams of active material. The area of
the electrodes used in FIGS. 2 and 3 are the same as that shown in
FIG. 1, namely 2.4 sq. centimeters. The aforesaid 36 milligrams of
graphite active material corresponds to a 56 percent loading. The
electrode slurry casting formulation comprised, on a weight basis:
25.4% DMA; 56% Graphite (50% SFG-15/50% MCMB 2528); 16.4% Kynar
2801 (PVDF:HFP); and 2.2% Super P (MMM Carbon) carbon black.
[0045] FIG. 2 shows a voltage/capacity plot of SFG-15 and MCMB,
2528,X 50:50 graphite cycled with a lithium metal electrode using
constant current cycling at .+-.0.2 milliamps per square
centimeter, between 0.01 and 2 volts vs. Li/Li.sup.+. In FIG. 2,
the results of cycling two similar cells are shown. One cell is
designated LO1475 (1475) and the other is LO1476 (1476). The data
for the 1475 cell is given below, followed directly by the data for
the 1476 cell stated in parentheses. The cycling data was obtained
using 36 milligrams of the active material. The electrolyte is one
molar LiPF.sub.6 in a solution of EC/DMC in a 2:1 weight ratio. In
this case, essentially all of the DMA was extracted by methanol.
Therefore, DMA did not form a detectable part of the solvent
solution. As in the case with respect to FIG. 1, in the first-half
cycle, lithium is removed from the metallic electrode and
intercalated into the graphite electrode. When essentially full
intercalation of the graphite electrode is complete, corresponding
to LiC.sub.6, the voltage has dropped to approximately 0.01 volts,
representing about 383 (386) milliamp hours per gram, corresponding
to about 13.8 (13.9) milliamp hours, based on 36 (36) milligrams of
active material. In the second half cycle, the lithium is
deintercalated from the graphite and returned to the metallic
electrode until the average voltage is approximately 2 volts vs.
Li/Li.sup.+. The deintercalation corresponds to approximately 341
(344) milliamp hours per gram, representing approximately 12.3
(12.4) milliamp hours based on 36 (36) milligrams of active
material. This completes an initial cycle. The percentage
difference between the 13.8 (13.9) milliamp hours per gram capacity
"in" and the 12.3 (12.4) milliamp hours per gram capacity "out",
divided by the initial 13.8 (13.9) capacity "in", corresponds to a
surprisingly low first cycle loss. As shown in FIG. 2, for the two
cells (1475 and 1476) tested; the first exhibited a first cycle
loss of 10.9 percent, and the second exhibited a first cycle loss
of 10.8 percent.
EXAMPLE III
[0046] A graphite electrode was fabricated in the same manner as
described for Example II, except that the DMA plasticizer was at
least partially removed by vacuum. FIG. 3 shows a voltage capacity
plot of the SFG-15/MCMB 2528 electrode cycled with a lithium metal
electrode, using constant current cycling at .+-.0.2 milliamps per
square centimeter, between 0.01 and 2.0 volts vs. Li/Li.sup.+,
using 35 milligrams of the graphite active material. The
electrolyte is one molar LiPF.sub.6 in a solution of EC/DMC in a
2:1 weight ratio. In this case, essentially all of the DMA was
extracted by vacuum. Therefore, DMA did not form a detectable part
of the solvent solution. No DMA in the electrolyte. In the first
half cycle, lithium is removed from the metallic electrode and
intercalated into the graphite electrode as described above. Then,
the lithium is deintercalated from the graphite and returned to the
metallic electrode, as described in the examples above. The
percentage difference between the 12.4 milliamp hours per gram
capacity "in" and the 10.9 milliamp hours per gram capacity "out",
divided by the initial 12.4 capacity "in", corresponds to a
surprisingly low 12 percent first cycle loss. It can be seen again
that DMA may successfully remain in the cell as a plasticizer
without extraction, or extraction may be done if desired. The
capacity of the cell is not affected by the DMA due to
electrochemical stability of the DMA and its suitability to form a
portion of the solvent mixture.
EXAMPLE IV
[0047] Positive electrodes were also fabricated by solvent casting
of the invention, casting a slurry of lithium manganese oxide,
conductive carbon, binder, plasticizer and solvent, as in a manner
similar to Example I. A preferred lithium manganese oxide (LMO)
cathode was formed, and the lithium manganese oxide was
LiMn.sub.2O.sub.4, supplied by Kerr-McGee (Soda Springs, Id.); and
the conductive carbon was Super P, available from MMM carbon. Kynar
Flex co-polymer, described above, was used as the binder, along
with the preferred plasticizer of the invention. Electronic grade
acetone was used as a casting solvent. The cathode 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 evaporated. An exemplary
cathode slurry composition is as follows:
2 Component Wet Weight % Dry Weight % LiMn.sub.2O.sub.4 28.9 65.0
Super P 2.5 5.5 Binder 4.5 10.0 Plasticizer 8.7 19.5 Solvent 55.4
-- Total 100.0 100.0
[0048] Positive electrodes for cells are easily prepared, as noted
above, using the preferred plasticizer. The plasticizer may be
removed, only partially removed, or remain in the cell in
accordance with the examples described above with respect to the
negative electrode.
EXAMPLE V
(5% DMA Solvent)
[0049] Graphite (BG-35) and LMO electrodes, prepared as described
above, were tested in a cell having an electrolyte composition
comprising DMA. The electrolyte used to form the completed final
cell, or battery, comprised of solution of 95% by weight EC/DMC and
5 weight % DMA. The electrolyte salt was one molar LiPF.sub.6. The
weight ratio of EC to DMC was 2:1. 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
Communication Research patents listed earlier.
[0050] FIG. 4 contains the results of testing of three cells with
cell designated 1261 showing data points with open squares, cell
1263 data in the form of a straight line, and cell 1264 showing
data designated with filled-in boxes. FIG. 4 is a two-part graph.
FIG. 4A shows the good rechargeability of the LMO/BG-35 graphite
cells. FIG. 4B shows the good cycling and capacity of the cells.
Charge and discharge are at .+-.2.0 amp hours per centimeter
square, between 3.0 and 4.2 volts for up to about 100 cycles. In
FIG. 4A, the coulombic efficiency versus cycle is very good. In
FIG. 4B, after 100 cycles, approximately 82-83% capacity is
maintained.
EXAMPLE VI
(10% DMA Solvent)
[0051] Cells were prepared as per Example V, except that the
solvent mixture contained a greater proportion by weight of
DMA.
[0052] Graphite (BG-35) and LMO electrodes, prepared as described
above, were tested in a cell having an electrolyte solution of 90%
by weight EC/DMC and 10 weight % DMA. The electrolyte salt was one
molar LiPF.sub.6. The weight ratio of EC to DMC was 2:1. The two
electrode layers were laminated with the electrolyte layer in
between as described above.
[0053] FIG. 5 contains the results of testing of four cells with
cell designated 1257 showing data points with open squares, cell
1259 data in the form of a straight line, cell 1260 showing data
designated with filled-in boxes, and cell 1258 data shown as
filled-in circles. FIG. 5 is a two-part graph. FIG. 5A shows the
excellent rechargeability of the LMO/BG-35 graphite cells. FIG. 5B
shows the excellent cycling and capacity of the cells. Charge and
discharge are under same conditions as Example V. In FIG. 5A, the
coulombic efficiency versus cycle is very good, and in FIG. 5B, it
can be seen that after 100 cycles, approximately 81-83% capacity is
maintained.
EXAMPLE VII
(20% DMA Solvent)
[0054] Graphite (BG-35) and LMO electrodes, prepared as described
above, were tested in a cell having an electrolyte solution of 80%
by weight EC/DMC and 20 weight % DMA. The electrolyte salt was one
molar LIPF.sub.6. The weight ratio of EC to DMC was 2:1. The two
electrode layers were laminated with the electrolyte layer in
between as described above.
[0055] FIG. 6 contains the results of testing of three cells with
cell designated 1266 showing data points with open squares, cell
1268 data in the form of a straight line, and cell 1269 showing
data designated with filled-in boxes. FIG. 6 is a two-part graph.
FIG. 6A shows the good rechargeability of the LMO/BG-35 graphite
cells. FIG. 6B shows the good cycling and capacity of the cells.
Charge and discharge are under the same conditions as Example V. In
FIG. 6A, the coulombic efficiency versus cycle is very good, and in
FIG. 6B, it can be seen that after 100 cycles, approximately 78-80%
capacity is maintained.
[0056] To further emphasize the good coulombic efficiency and
discharge capacity versus cycles, exemplary data from the 5% DMA
(Example V), 10% DMA (Example VI), and 20% DMA (Example VII) were
combined in single graph. This can be seen in FIG. 7.
EXAMPLE VIII
[0057] Several cells were prepared, similar to the aforesaid
examples of graphite and LMO electrodes, but also using a separator
formed with the DMA plasticizer of the invention. The six cells
were SFG-15/MCMB 2528, 50:50, 56% active material. In this case,
the DMA was totally removed after the cell was laminated and before
activation with the electrolyte. Therefore, the electrode and
separator preparation with the removal of the DMA was similar to
the processes described earlier with respect to FIGS. 2 and 3,
where the DMA plasticizer was totally removed. The results of
testing the six cells are shown in FIG. 8. Cell 2042 is shown by a
dashed line with open squares; cell 2043 is shown by a dashed line
with dots; cell 2044 data is designated by a gray line; data for
cell 2045 is shown by a dashed line with filled-in squares; data
for cell 2047 is shown by a solid line with open squares; and data
for cell 2049 is shown by a fixed solid line with large black
(filled-in) squares. The data of FIG. 8 clearly demonstrates that
DMA is a stable alternate plasticizer and provides performance for
electrodes and separators equivalent to that obtained with
conventional DBP plasticizer. FIG. 8A shows that coulombic
efficiency is maintained for as many as 400 cycles. FIG. 8B shows
that in the case of cell 2044 after 400 cycles, 83% of initial
capacity is maintained, at 2 milliamps per centimeter square life
cycling. The data obtained at one milliamp per centimeter square
life cycling for less than 250 cycles is also shown for comparative
purposes.
[0058] Reviewing the data in the various figures, it is clear that
DMA is acceptable for use as a plasticizer for separator polymeric
electrolyte layer, and that it is not necessary to remove it before
the activation step. The activation step indicates the step at
which electrolyte solvent and salt is added to the cell. Therefore,
it is possible to include DMA as a portion of the electrolyte
solvent salt mixture. It is also acceptable to use DMA as
plasticizer in each laminate layer of the cell, anode, cathode, and
separator, and it is not necessary to remove it, thus permitting it
to form a part of the cell solvent mixture. Under current
processing techniques, the extraction step currently practiced for
removing the plasticizer is useful for removing water. Therefore,
it is unlikely that all plasticizer will be permitted to remain in
the cell, since its extraction is coincident with water removal.
However, since the DMA is a stable plasticizer, one not need be
concerned with removing DMA down to a point of nearly undetectable
amounts, as is presently done in the case of DBP. DMA was included
as part of the electrolyte formulation to prove its electrochemical
stability in FIGS. 1, 4, 5, 6, and 7, and this stability was
clearly proven. It is suggested that the greatest amount of DMA
includable in the electrolyte system preferably up to about 20%. As
shown in FIGS. 4 through 7, BG-35/LMO cells with 5-20 weight
percent of DMA in the electrolyte all show reasonable first-cycle
loss. These first-cycle losses ranged from 14-19 percent, and
cycling performance was 78-82 percent, after about 100 cycles.
Therefore, it appears that up to 20% DMA in the electrolyte
formulation is acceptable. In that regard, FIGS. 2 and 3 show
graphite half-cells, where the DMA plasticizer was totally removed
by methanol (FIG. 2) or vacuum extraction (FIG. 3). In comparing
FIGS. 1, 2, and 3, it can be seen that all three cells show
reasonable first-cycle loss (10.8-12.1%). FIG. 8 demonstrates using
DMA as the separator plasticizer, where DMA was totally removed
before the activation step, showing it to be a stable alternate
plasticizer for processing.
[0059] When reviewing the data of all of the FIGS. 1 through 8,
several conclusions are obtained. The first cycle loss, when using
DMA as a plasticizer, is relatively low. The first cycle loss, when
using DMA as a part of the electrolyte is also relatively low. This
demonstrates the electrochemical stability of DMA. The good
capacity retention and cyclability is demonstrated for the various
conditions, both half-cells and full cells, for FIGS. 1 through 8.
Therefore, it is possible to conclude that the DMA is a very good
alternate plasticizer for processing (FIGS. 2, 3, and 8) and it
also has the potential for not being removed before cell activation
with the electrolyte. That is, it shows great potential for saving
process time and cost by remaining in the cell as part of the
electrolyte solvent in an amount of up to about 20 wt percent DMA,
based on the formulation shown herein of 20 weight percent DMA; and
80% EC/DMC (2:1 ratio) with one molar LiPF.sub.6.
[0060] Additional physical features of the polymer binder,
plasticizer, active materials, and additives (such as fillers) will
now be described.
[0061] The plasticizer of the invention is not limited for use with
co-polymers of vinylidene fluoride and hexafluoropropylene. The
polymeric material for use with the plasticizers of the invention
may be selected from a broader class. More particularly, the
polymer may be selected from polymers and copolymers of vinyl
chloride, acrylonitrile, vinylidene fluoride, vinyl chloride and
vinylidene chloride, vinyl chloride and acrylonitrile, vinylidene
fluoride with hexafluoropropylene, vinylidene fluoride with
hexafluoropropylene and a member of the group consisting of vinyl
fluoride, tetrafluoroethylene, an trifluoroethylene. The preferred
polymer composition is a copolymer of VdF and HFP, more preferably,
the polymer composition is 75 to 92% vinylidene fluoride and 8 to
25% hexafluoropropylene. These copolymers are available
commercially from, for example, Atochem North America as Kynar
FLEX. This polymer composition is preferred for both the
preparation of the electrodes and the separator membrane.
[0062] Inorganic fillers, such as fumed alumina or silanized fumed
silica, may be used to enhance the physical strength and melt
viscosity of the solid-state components, namely, the electrodes and
separators, and to facilitate absorption of electrolyte solution in
the completed cell. The active materials for inclusion in the
positive electrode are not limited and may include any of a number
of conventionally used positive electrode active materials such as
LiMn.sub.2O.sub.4, LiCoO.sub.2, and LiNiO.sub.2. Active materials
for inclusion in the negative electrode include petroleum coke,
microbead carbon coke, synthetic graphite, natural graphite,
synthetic graphitized carbon fibers and whiskers, and metal oxides.
Those skilled in the art will understand that metal chalcogenides
may be used as positive and negative electrode active materials.
Completed cells are formed by laminating the electrodes and
separator membranes described above, under heat and pressure, to
form a unitary battery structure. The battery is activated by
including the electrolyte solution comprising the solvent and metal
salt.
3TABLE I 2 3 DMA (dimethyl adipate, CAS# 627-93-0) dibasic
ester
[0063]
4 TABLE II Dibasic Ester Dimethyl Adipate DMA Boiling Point
109.degree. C. to 110.degree. C. Melting Point 8.degree. C. Vapor
Pressure 0.2 mm (20.degree. C.) Specific Gravity 1.063 Appearance
Colorless Liquid Purity 98-99%
[0064]
5TABLE III Characteristics of Organic Solvents PC VC EC DMC DEC BC
MEC DPC Boiling Temperature (C.) 240 162 248 91.0 126 230 107
167-168 Melting Temperature (C.) -49 22 39-40 4.6 -43 -- -55 --
Density (g/cm.sup.3) 1.198 1.35 1.322 1.071 0.98 1.139 1.007 0.944
Solution Conductivity (S/cm) 2.1 .times. 10.sup.-9 -- <10.sup.-7
<10.sup.-7 <10.sup.-7 <10.sup.-7 6 .times. 10.sup.-9
<10.sup.-7 Viscosity (cp) at 25.degree. C. 2.5 -- 1.86 (at
40.degree. C.) 0.59 0.75 2.52 0.65 -- Dielectric Constant at
20.degree. C. 64.4 -- 89.6 (at 40.degree. C.) 3.12 2.82 -- -- --
Molecular Weight 102.0 86.047 88.1 90.08 118.13 116.12 104.10
146.19 H.sub.2O Content <10 ppm -- <10 ppm <10 ppm <10
ppm <10 ppm <10 ppm <10 ppm Electrolytic Conductivity 5.28
-- 6.97 11.00 5.00 <3.7 -- -- (mS/cm) 20.degree. C. 1M
LiAsF.sub.6 (1.9 mol) (1.5 mol)
[0065]
6 TABLE IV Carbon Material BG-35 SFG-15 MCMB-2528 Surface Area
(m.sup.2/g) (BET) 7 8.8 N/A Coherence Length L.sub.c (nm) >1000
>120 >1000 Density (g/cm.sup.3).sup.2 0.195 2.26 2.24
Particle Size.sup.1 <36 <16 37 Median Size d.sub.50 (.mu.m)
17 8.1 22.5 Interlayer Distance c/2 (nm) N/A 0.335 0.336
.sup.1Maximum size for at least 90% by weight of graphite
particles. .sup.2In xylene.
[0066] In summary, the invention solves the difficult processing
problems associated with removal of conventional plasticizers after
formation of cell components and before their assembly into a cell.
Plasticizers such as DBP have always been a problem, since DBP
readily decomposes when subjected to conditions of cyclic operation
in an electrochemical cell. Although DBP and similar compounds have
been popular as plasticizers, their deterioration due to
electrochemical instability is highly problematic. In contrast, the
plasticizer family of the invention, which comprises adipate
derivatives, esters, are highly desirable and have a wide voltage
operating range while avoiding decomposition in a cell.
[0067] 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.
[0068] The embodiments of the invention in which an exclusive
property or privilege is claimed, are defined in the following
claims.
* * * * *