Note: Descriptions are shown in the official language in which they were submitted.
CA 02299102 2000-02-22
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04645.0614
' SULFITE ADDITIVES FOR NONAQUEOUS
ELECTROLYTE RECHARGEABLE CELLS
15
BACKGROUND OF INVENTION
The present invention generally relates to an alkali
metal electrochemical cell, and more particularly, to a
rechargeable alkali metal cell. Still more particularly, the
present invention relates to a lithium ion electrochemical
cell activated with an electrolyte havina an addit.i.vP mrnvir~P~1
to achieve high charge/discharge capacity, long cycle life and
to minimize the first cycle irreversible capacity. According
to the present invention, the preferred additive to the
activating electrolyte is a sulfite compound.
Alkali metal rechargeable cells typically comprise a
carbonaceous anode electrode and a lithiated cathode
electrode. Due to the high potential of the cathode material
(up to 4.3V vs. Li/Li+ for Lil_xCo02) and the low potential of
the carbonaceous anode material (O.O1V vs. Li/Li+ for graphite)
in a fully charged lithium ion cell, the choice of the
electrolyte solvent system is limited. Since carbonate
solvents have high oxidative stability toward typically used
~lithiated cathode materials and good kinetic stability toward
carbonaceous anode materials, they are generally used in
lithium ion cell electrolytes. To achieve optimum cell
performance (high rate capability and long cycle life),
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solvent systems containing a mixture of a cyclic carbonate
(high dielectric constant solvent) and a linear carbonate (low
viscosity solvent) are typically used in commercial secondary
cells. Cells with carbonate based electrolytes are known to
deliver more than 1000 charge/discharge cycles at room
temperature.
Applicant's U.S. patent 6,153,338 is directed to a
quaternary mixture of organic carbonate solvents in the
activating electrolyte for a lithium ion cell capable of
discharge at temperatures below -20°C and down to as low as
-40°C while exhibiting good cycling characteristics. The
quaternary solvent system includes ethylene carbonate (EC),
dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and
diethyl carbonate (DEC).
Lithium ion cell design generally involves a trade off in
one area for a necessary improvement in another, depending on
the targeted cell application. The achievement of a lithium-
ion cell capable of low temperature cycleability by use of the
above quaternary solvent electrolyte, in place of a typically
used binary solvent electrolyte (such as 1. OM
LiPF6/EC:DMC=30:70, v/v which freezes at -11°C), is obtained at
the expense of increased first cycle irreversible capacity
during the initial charging (approximately 65 mAh/g graphite
for 1. OM LiPF6/EC:DMC:EMC:DEC=45:22:24.8:8.2 vs. 35 mAh/g
graphite for l.OM LiPF6/EC:DMC=30:70). Due to the existence of
this first cycle irreversible capacity, lithium ion cells are
generally cathode limited. Since all of the lithium ions,
which shuttle between the anode and the cathode during
charging and discharging originally come from the lithiated
cathode, the larger the first cycle irreversible capacity, the
lower the cell capacity in subsequent cycles and the lower the
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cell efficiency. Thus, it is desirable to minimize or even
eliminate the first cycle irreversible capacity in lithium ion
cells while at the same time maintaining the low temperature
cycling capability of such cells.
According to the present invention, these objectives are
achieved by providing an organic sulfite in the quaternary
solvent electrolyte. Lithium ion cells activated with these
electrolytes exhibit lower first cycle irreversible capacities
relative to cells activated with the same quaternary solvent
electrolyte devoid of the sulfite additive. As a result,
cells including the sulfite additive present higher subsequent
cycling capacity than control cells. The cycleability of the
present invention cells at room temperature, as well as at low
temperatures, i.e., down to about -40~C, is as good as cells
activated with the quaternary electrolyte devoid of a sulfite
additive.
5uuiriHtiY UF~ ~rHE IIvVENTtUN
It is commonly known that when an electrical potential is
initially applied to lithium ion cells constructed with a
carbon anode in a discharged condition to charge the cell,
some permanent capacity loss occurs due to the anode surface
passivation film formation. This permanent capacity loss is
called first cycle irreversible capacity. The film formation
process, however, is highly dependent on the reactivity of the
electrolyte components at the cell charging potentials. The
electrochemical properties of the passivation film are also
dependent on the chemical composition of the surface film.
The formation of a surface film is unavoidable for alkali
,.metal systems, and in particular, lithium metal anodes, and
lithium intercalated carbon anodes due to the relatively low
potential and high reactivity of lithium toward organic
electrolytes. The ideal surface film, known as the solid-
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electrolyte interphase (SEI), should be electrically
insulating and ionically conducting. While most alkali metal,
and in particular, lithium electrochemical systems meet the
first requirement, the second requirement is difficult to
achieve. The resistance of these films is not negligible, and
as a result, impedance builds up inside the cell due to this
surface layer formation which induces unacceptable
polarization during the charge and discharge of the lithium
ion cell. On the other hand, if the SEI film is electrically
conductive, the electrolyte decomposition reaction on the
anode surface does not stop due to the low potential of the
lithiated carbon electrode.
Hence, the composition of the electrolyte has a
significant influence on the discharge efficiency of alkali
metal systems, and particularly the permanent capacity loss in
secondary cells. For example, when 1.OM LiPF6/EC:DMC=30:70 is
used to activate a secondary cell, the first cycle
irreversible capacity is approximately 35 mAh/g of graphite.
However, under the same cycling conditions, the first cycle
irreversible capacity is found to be approximately 65 mAh/g of
graphite when 1. OM LiPF6/EC:DMC:EMC:DEC=45:22:24.8:8.2 is used
as the electrolyte. Further, lithium ion cells activated with
the binary solvent electrolyte of ethylene carbonate and
dimethyl carbonate cannot be cycled at temperatures less than
about -11°C. The quaternary solvent electrolyte of the
previously referenced U.S. patent 6,153,338, which enables
lithium ion cells to cycle at much lower temperatures, is a
compromise in terms of providing a wider temperature
application with acceptable cycling efficiencies. It would be
,.highly desirable to retain the benefits of a lithium ion cell
capable of operating at temperatures down to as low as about
-40~C while minimizing the first cycle irreversible capacity.
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According to the present invention, this objective is
achieved by adding a sulfite additive in the above described
quaternary solvent electrolytes. In addition, this
invention may be generalized to other nonaqueous organic
electrolyte systems, such as binary solvent and ternary
solvent systems, as well as the electrolyte systems
containing solvents other than mixtures of linear or cyclic
carbonates. For example, linear or cyclic ethers or esters
may also be included as electrolyte components. Although
the exact reason for the observed improvement is not clear,
it is hypothesized that the sulfite additive competes with
the existing electrolyte components to react on the carbon
anode surface during initial lithiation to form a beneficial
SEI film. The thusly formed SEI film is electrically more
insulating than the film formed without the sulfite additive
and, as a consequence, the lithiated carbon electrode is
better protected from reactions with other electrolyte
components. Therefore, lower first cycle irreversible
capacity is obtained.
In a preferred embodiment, the invention comprises an
electrochemical cell which comprises:
a) a negative electrode which intercalates with an
alkali metal;
b) a positive electrode comprising an electrode
active material which intercalates with the alkali metal;
c) a nonaqueous electrolyte activating the negative
and the positive electrodes; and
d) a sulfite additive of the formula RlOS (=0) (ORa)
provided in the electrolyte
wherein:
R1 is a group which has at least one unsaturated
hydrocarbon containing a C (sp' or sp3) -C (sp')
bond unit having the C(sp3) carbon directly
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connected to the -OSOz- functional group; and
R~ is selected from the group consisting of a
hydrogen atom and an organic group containing
1 to 12 carbon atoms;
wherein when R' is a hydrogen then R" is an unsaturated
hydrocarbon having 3-12 carbon atoms; and
wherein when Ra is an organic group containing 1 to 12
carbon atoms then Rl has the structure (R3) (R') (R5) C- wherein:
i) R3 is selected from the group consisting of an
aromatic group, an unsaturated organic group
and an unsaturated inorganic group; and
ii) R' and RS are independently selected from the
group consisting of an aromatic group, an
unsaturated organic group, an unsaturated
inorganic group and a saturated organic group
containing from 1 to 11 carbon atoms.
These and other objects of the present invention will
become increasingly more apparent to those skilled in the
art by reference to the following description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A secondary electrochemical cell constructed according
to the present invention includes an anode active material
selected from Groups IA, IIA, or IIIB of the Periodic Table
of Elements, including the alkali metals lithium, sodium,
potassium, etc. The preferred anode active material
comprises lithium.
In secondary electrochemical systems, the anode
electrode comprises a material capable of intercalating and
de-intercalating the alkali metal, and preferably lithium.
A carbonaceous anode comprising any of the various forms of
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carbon (e. g., coke, graphite, acetylene black, carbon black,
glassy carbon, etc.) which are capable of reversibly retaining
the lithium species, is preferred. Graphite is particularly
preferred due to its relatively high lithium-retention
capacity. Regardless of the form of the carbon, fibers of the
carbonaceous material are particularly advantageous because
the fibers have excellent mechanical properties which permit
them to be fabricated into rigid electrodes that are capable
of withstanding degradation during repeated charge/discharge
cycling. Moreover, the high surface area of carbon fibers
allows for rapid charge/discharge rates. A preferred
carbonaceous material for the anode of a secondary
electrochemical cell is described in applicant's U.S.
Patent No. 5,443,928 to Takeuchi et al.
A typical secondary cell anode is fabricated by mixing
about 90 to 97 weight percent graphite with about 3 to 10
weight percent of a binder material which is preferably a
fluoro-resin powder such as polytetrafluoroethylene (PTFE),
polyvinylidene fluoride (PVDF),
Polyethylenetetrafluoroethylene (ETFE), polyamides and
polyimides, and mixtures thereof. This electrode active
admixture is provided on a current collector such as of a
nickel, stainless steel, or copper foil or screen by
casting, pressing, rolling or otherwise contracting the
active admixture thereto.
The anode component further has an extended tab or lead
of the same material as the anode current collector, i.e.,
preferably nickel, integrally formed therewith such as by
welding and contacted by a weld to a cell case of conductive
metal in a case-negative electrical configuration.
Alternatively, the carbonaceous anode may be formed in some
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other geometry, such as a bobbin shape, cylinder or pellet to
allow an alternate low surface cell design.
The cathode of a secondary cell preferably comprises a
lithiated material that is stable in air and readily handled.
Examples of such air-stable lithiated cathode materials
include oxides, sulfides, selenides, and tellurides of such
metals as vanadium, titanium, chromium, copper, molybdenum,
niobium, iron, nickel, cobalt and manganese. The. more
preferred oxides include LiNiOz, LiMnz04, LiCoOz, LiCoo,9zSno.oBOz
and LiCol_xNiXOz .
Before fabrication into an electrode for incorporation
into an electrochemical cell, the lithiated active material is
preferably mixed with a conductive additive. Suitable
conductive additives include acetylene black, carbon black
and/or graphite. Metals such as nickel, aluminum, titanium
and stainless steel in powder form are also useful as
conductive diluents when mixed with the above listed active
rna~etials. The electrode further comprises a fluoro-resin
binder, preferably in a powder form, such as PTFE, PVDF, ETFE,
polyamides and polyimides, and mixtures thereof.
To discharge such secondary cells, the lithium ion
comprising the cathode is intercalated into the carbonaceous
anode by applying an externally generated electrical potential
to recharge the cell. The applied recharging electrical
potential serves to draw the alkali metal ions from the
cathode material, through the electrolyte and into the
carbonaceous anode to saturate the carbon comprising the
anode. The resulting LiXC6 electrode can have an x ranging
between 0.1 and 1Ø The cell is then provided with an
"electrical potential and is discharged in a normal manner.
An alternate secondary cell construction comprises
intercalating the carbonaceous material with the active alkali
material before the anode is incorporated into the cell. In
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this case, the cathode body can be solid and comprise, but not
be limited to, such materials as manganese dioxide, silver
vanadium oxide, copper silver vanadium oxide, titanium
disulfide, copper oxide, copper sulfide, iron sulfide, iron
disulfide and fluorinated carbon. However, this approach is
compromised by the problems associated with handling lithiated
carbon outside of the cell. Lithiated carbon tends to react
when contacted by air.
The secondary cell of the present invention includes a
separator to provide physical segregation between the anode
and cathode active electrodes. The separator is of an
electrically insulative material to prevent an internal
electrical short circuit between the electrodes, and the
separator material also is chemically unreactive with the
anode and cathode active materials and both chemically
unreactive with and insoluble in the electrolyte. In
addition, the separator material has a degree of porosity
suz=icient to allow slow therethrough of the electrolyte
during the electrochemical reaction of the cell. The form of
the separator typically is a sheet which is placed between the
anode and cathode electrodes. Such is the case when the anode
is folded in a serpentine-like structure with a plurality of
cathode plates disposed intermediate the anode folds and
received in a cell casing or when the electrode combination is
rolled or otherwise formed into a cylindrical "jellyroll"
configuration.
Illustrative separator materials include fabrics woven
from fluoropolymeric fibers of polyethylenetetrafluoroethylene
and polyethylenechlorotrifluoroethylene used either alone or
laminated with a fluoropolymeric microporous film. Other
suitable separator materials include non-woven glass,
polypropylene, polyethylene, glass fiber materials, ceramics,
a polytetraflouroethylene membrane commercially available
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under the designation ZITEX (Chemplast Inc.), a polypropylene
membrane commercially available under the designation CELGARD
(Celanese Plastic Company, Inc.) arid a membrane commercially
available under the designation DEXIGLAS (C. H. Dexter, Div.,
Dexter Corp.).
The choice of an electrolyte solvent system for
activating an alkali metal electrochemical cell, and
particularly a fully charged lithium ion cell is very limited
due to the high potential of the cathode material (up to 4.3V
vs. Li/Li+ for Lil_xCo02) and the low potential of the anode
material (O.O1V vs. Li/Li+ for graphite). According to the
present invention, suitable nonaqueous electrolytes are
comprised of an inorganic salt dissolved in a nonaqueous
solvent and more preferably an alkali metal salt dissolved in
a quaternary mixture of organic carbonate solvents comprising
dialkyl (non-cyclic) carbonates selected from dimethyl
carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate
(UYC:), ethylmethyl carbonate (EMC), methylpropyl carbonate
(MPC) and ethylpropyl carbonate (EPC), and mixtures thereof,
and at least one cyclic carbonate selected from propylene
carbonate (PC), ethylene carbonate (EC), butylene carbonate
(BC) and vinylene carbonate (VC), and mixtures thereof.
Organic carbonates are generally used in the electrolyte
solvent system for such battery chemistries because they
exhibit high oxidative stability toward cathode materials and
good kinetic stability toward anode materials.
Preferred electrolytes according to the present invention
comprise solvent mixtures of EC:DMC:EMC:DEC. Most preferred
volume percent ranges for the various carbonate solvents
,include EC in the range of about loo to about 500 DMC in the
range of about 5°s to about 75%; EMC in the range of about 50
to about 50°s; and DEC in the range of about 3% to about 45%.
Electrolytes containing this quaternary carbonate mixture
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exhibit freezing points below -50oC, and lithium ion cells
activated with such mixtures have very good cycling behavior
at room temperature as well as very good discharge and
charge/discharge cycling behavior at temperatures below -20oC.
Known lithium salts that are useful as a vehicle for
transport of alkali metal ions from the anode to the cathode,
and back again include LiPF6, LiBF9, LiAsF6, LiSbF6, LiC104,
LiA1C14, LiGaCl4, LiC (SOZCF3) 3, LiN03, LiN (SOzCF3) Z, LiSCN,
Li03SCFZCF3, LiC6F5S03, Li02CCF3, LiS03F, LiB (C6H5) 4 and LiCF3S03,
and mixtures thereof. Suitable salt concentrations typically
range between about 0.8 to 1.5 molar.
In accordance with the present invention, at least one
organic sulfite additive is provided in the electrolyte. The
sulfite additive preferably has the general formula
R10S (=O) (ORZ) , wherein R1 and RZ are the same or different and
they can both be a hydrogen atom or an organic group
containing 1 to 12 carbon atoms, and wherein at least R1 has
Lhe structure (k') (R") (R')C- if R'#H with at least R3 being an
aromatic substituent or an unsaturated organic or inorganic
group and wherein if any of the remaining groups of R9and RS
is a saturated organic group, the saturated organic group
contains 1 to 11 carbon atoms.
The greatest effect is found when diallyl sulfite, methyl
benzyl sulfite, ethyl benzyl sulfite, propyl benzyl sulfite,
butyl benzyl sulfite, pentyl benzyl sulfite, methyl allyl
sulfite, ethyl allyl sulfite, propyl allyl sulfite, butyl
allyl sulfite, pentyl allyl sulfite, mono-methyl sulfite,
mono-ethyl sulfite, mono-butyl sulfite, mono-propyl sulfite,
mono-pentyl sulfite, mono-allyl sulfite, mono-benzyl sulfite
,.and dibenzyl sulfite, and mixtures thereof are used as
additives in the electrolyte.
The above described compounds are only intended to be
exemplary of those that are useful with the present invention,
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and are not to be construed as limiting. Those skilled in the
art will readily recognize sulfite compounds which come under
the purview of the general formula set forth above and which
will be useful as additives for the electrolyte to reduce
voltage delay according to the present invention.
While not intending to be bound by any particular
mechanism, it is believed that due to the presence of the
sulfite additive comprising at least one unsaturated
hydrocarbon containing a C (sp2 or sp3)-C (spa) bond unit having
the C(sp3) carbon directly connected to the -OSOZ- functional
group, the bond between oxygen and at least one of the group R1
and RZ is readily severed and the sulfite intermediate is able
to compete effectively with the other electrolyte solvents or
solutes to react with lithium and form a sulfite salt, i.e.,
lithium sulfite, or the lithium salt of a sulfite reduction
product on the surface of the anode. The resulting salt is
more conductive than lithium oxide which may form on the anode
in crie absence of the organic sulfite additive.
In fact, it is believed that the lithium sulfite or the
lithium salt of a sulfite reduction product on the surface of
the anode provides for the existence of charge delocalization
due to resonance equilibration at the anode surface. This
equilibration allows lithium ions to travel easily from one
molecule to the other via a lithium ion exchange mechanism.
As a result, beneficial ionic conductance is realized.
Accordingly, it is believed that the present organic sulfite
compounds compete more effectively with the other solvents in
the electrolyte to form an sonically more conductive and
electrically more insulative film than is provided by alkyl
sulfites and cyclic sulfites such as ethylene sulfite and
propylene sulfite, for example. As a consequence, the
chemical composition and perhaps the morphology of the
carbonaceous anode surface protective layer is believed to be
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changed with concomitant benefits to the cell's cycling
characteristics.
The assembly of the cell described herein is preferably
in the form of a wound element cell. That is, the fabricated
cathode, anode and separator are wound together in a
"jellyroll" type configuration or "wound element cell stack"
such that the anode is on the outside of the roll to make
electrical contact with the cell case in a case-negative
configuration. Using suitable top and bottom insulators, the
wound cell stack is inserted into a metallic case of a
suitable size dimension. The metallic case may comprise
materials such as stainless steel, mild steel, nickel-plated
mild steel, titanium or aluminum, but not limited thereto, so
long as the metallic material is compatible for use with
components of the cell.
The cell header comprises a metallic disc-shaped body
with a first hole to accommodate a glass-to-metal
seal/terminal pin feedthrough and a second hole for
electrolyte filling. The glass used is of a corrosion
resistant type having up to about 50°s by weight silicon such
TM TM TM
as CABAL 12, TA 23 or FUSITE 425 or FUSITE 435. The positive
terminal pin feedthrough preferably comprises titanium
although molybdenum, aluminum, nickel alloy, or stainless
steel can also be used. The cell header comprises elements
having compatibility with the other components of the
electrochemical cell and is resistant to corrosion. The
cathode lead is welded to the positive terminal pin in the
glass-to-metal seal and the header is welded to the case
containing the electrode stack. The cell is thereafter filled
with the electrolyte solution comprising at least one of the
sulfite additives described hereinabove and hermetically
sealed such as by close-welding a stainless steel ball over
the fill hole, but not limited thereto.
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The above assembly describes a case-negative cell, which
is the preferred construction of the exemplary cell of the
present invention. As is well known to those skilled in the
art, the exemplary electrochemical system of the present
invention can also be constructed in a case-positive
configuration.
The concentration limit for the sulfite additive is
preferably about O.OO1M to about 0.20M. The beneficial effect
of the sulfite additive will not be apparent if the additive
concentration is less than about O.OO1M. On the other hand,
if the additive concentration is greater than about 0.20M, the
beneficial effect of the additive will be canceled by the
detrimental effect of higher internal cell resistance due to
the thicker anode surface film formation and lower electrolyte
conductivity.
It is appreciated that various modifications to the
inventive concepts described herein may be apparent to those
vi vruimary sKill in trie art without departing from the spirit
and scope of the present invention as defined by the appended
claims.
