Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates in general to a lithium
electrochemical cell and in particular, to a lithium electro-
chemical cell including lithium as the anode, the lithium
intercalating compound L~ CoO~(0< x <l) as the cathode, and a
solution.o~ a lithium salt in a mixed organic solvent o~ methyl
~ormate (MF) and diethylcarbonate (DEC) as the electrolyte.
The ~ubJect matt~r of this application l~ rel~ted to
the 3ubiect matter o~ U.S. Patent No. 4,7B6,499 lssued on
Z2 November, 1988 ~or "Lithium Electrochemical Cell Includlng
AProtlc Solvent-~lalkyl Carbon~te SolYent Mixture" an~ a~ ned
to a common a~slgnee. In that patent, there ls de~cribed and
claimed a lithlum electrochemical cell lncluding lithium as the
anode, non-~toichiometric (NS)-VbO~3 a~ the cathode, and a
solution of a lithium salt in a mixed organic ~olvent of methyl
formate and diethylcarbonate as the electrolyte.
Another lithium intercalating compound, to wit, Li~CoO~
(0< x <l~ is particularly attractive for battery applications.
because o~ its inherently high energy content. However, the known
Li~CoO~ cathode material/solvent combinations are susceptible to
oxidation during charge and reduction during discharge that
results in losses in cell capacity and cycle~ e. In addition to
oxidation and reduction o~ the electrolyte, both the cathode and
anode are subject to reaction with the solvent and electrolyte.
This can result in poor lithium cyclability and structural
rearrangement o~ the active material which may limit
rechargeability.
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The general object of this invention is to provide an
improved lithium alectrochemical cell including Li~CoO~(0< x <1)
as the cathode active material. A more particular object of the
invention is to provide an intercalating solvent system ~or L~ CoO~
(0< x <1) that produces higher energy lithium cell~ also
charact~rized by increased resistance to solvent oxidation and
improved lithium cycling efficiencies.
It has now been ~ound that the a~orementioned objects can
be attained by employing a system including lithium as the anode,
LixCoOa(O< x ~1) as the cathode, and a solution of a lithium salt
in a mixed organic solvent Oe MF and DEC as the electrolyte.
The solution can be, ~or example, 1 to 2 mol dm s LiA~
in the mixed organic solvent. Though the use o~ LiAsF,~ as th~
electrolyte salt is pre~erred, other electrolyte salts can be used
such as the soluble salts o~`light metals, for example,
tetra~luoroborates, tetrachloroaluminates, perchlorates,
hexa~luorophosphates, and halides o~ lithium.
The mass percent o~ the DEC in the mixed organic solvent
can vary ~rom 10 to 100 mass percent. The instant invention
identi~ies and demonstrates that the addition o~ DLC to ester
containing electrolytes, such as LiAsF~ in MF, results in
signi~icant improvements in the electrolytes resistance to
electrochemical oxidation and improved lithium cycling
efficiencies. In addition, when these electrolytes containing the
DEC additives are used in Li/LixCoO~ electrochemical cells, there
is signiEicant improvements in the cell cycling behavior over
cells without the DEC additive.
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The drawing compares cycling results obtained ~or
additions o~ DEC and dimethylcarbonate (DMC) to LiAsF~ in MF
electrolyte in a Li/LixCoOl electrochemical cell.
The drawing shows the dramatic improvement in cycli~g
behavior ~or the electrolyte containing DEC as opposed to DMC.
The Li/Li~CoO~ cells are cycled between either 4.3Vto 3.5V or 4.3V
to 2.5V where the charging rate is 0.5 mAcm~, the discharge rate
is 2.0 mAcm~, and the temperature is 25~C. The LixCaO~ cathodes
include a mixture o~ 80 weight percent Li~CoO~, 10 weight percent
carbon diluent, and 10 weight percent Teflon binder. The cathode
mixture is roll pressed onto aluminum substrate5 and sintered in a
vacuum oven at 280~C ~or 1 hour. The cycling is per~ormed on
identically prepared cells consisting o~ ~lag electrodes sealed in
a glass pressure vessel where Celgard 2400 is used as separators
and a glass ~iber wick ~or drawing electrolyte in between the
electrode.
Interestingly, homologues o~ DEC such as DMC have been
utilized in lithium cells and are known to be su~iciently stable
towards lithium. However, although DMC and DEC show structural
similarities, they behave very di~erently in the presence o~
lithium, both chemically and electrochemically. DMC produces a
high cycling e~iciency o~ S0 percent as compared to DEC which is
O percent. This is due to the reactive nature o~ DEC with lithium
as opposed to the more stable DMC solvent. However, even though
DMC is more stable with lithium, its addition to the LiAsF~-MF
electrolyte does not result in the improved results observed with
DEC. ThereEore, where the success~ul use o~ DMC as a solvent in
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lithium cells may imply the possible use oE a similar solvent such
as DEC, this is not made obvious due to the lack o~ lithium
stability o~ the neat DEC electrolyte, thus precluding such
application~. Furthermore, one would not ~ind obvious tha
discovery that a mixture o~ the unstable solvent DEC with ano~her
solvent would produce an improved mixture suitably stable ~or use
in a lithium cell. It is only through its addition to other ester
electrolytes that the use o~ DEC in lithium cells is possible.
The use o~ DEC as a solvemt additive in electrolytes ~or
use in either primary, rechargeable, or reserve electrochemical
cells is not considered to be limited to the instance where
lithium is the anode. That is, other light metals or composites
may be applicable as the anode such as sodium, potassium and
aluminum, or any conductively doped polymeric material or similar
compound~ Moreover, the positive electrode or cathode, may be any
oxide, sul~ide or combinations o~ titanium, vanadium, chromium,
manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium,
molybdenum, ha~nium, tantalum, or tungsten or any conductively
doped polymeric material or similar compound.
We wish it to be un~derstood that we do not desire to be
limited to the exact details as described ~or obvious
modi~ications will occur to a person skilled in the art~
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