Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 2 ~ 86099
AQUEOUS RECFAT~ART~ BATTERY
FIELD OF THE lNv~llON
The invention pertains to aqueous rechargeable bat-
teries and particularly to aqueous rechargeable batteries
which employ insertion compounds as the electrode
materials.
R~CR~ROUND OF THE INVENTION
In recent years, there have been significant advances
in the battery art such that gravimetric (energy per mass)
and volumetric (energy per volume) energy densities in
commercial rechargeable batteries have been substantially
increased. Improvements to nickel-metal hydride (NiMH)
batteries have led to reported energy densities of approxi-
mately 90 Wh/kg and over 300 Wh/l. Improvements to
lithium ion batteries have resulted in reported energy
densities of about 135 Wh/kg and 380 Wh/l for commercial
products according to Fujifilm Celltec Co. With such
advantageous characteristics, batteries using these
advanced electrochemical systems are generally preferred
for powering consumer electronics devices and their market
share is increasing. Such electrochemical systems also
appear very attractive for large battery applications such
as motive power for electric vehicles.
The requirements for large batteries can differ in
many ways from those for small consumer electronics bat-
teries however. For instance, gravimetric energy densityseems more important than volumetric energy density for
electric vehicles while the reverse is true for consumer
electronics devices. Further, safety concerns become an
even bigger issue for larger batteries. Also, the cost per
Wh must generally be markedly less for large batteries.
While they have attractive energy density characteristics,
NiMH batteries are relatively expensive compared to other
conventional systems (eg. Pb acid or NiCd) and do not
perform well at elevated temperature (eg. above 45~C).
~ - 2 - 2~6099
Lithium ion batteries also have very attractive energy
density characteristics but they are markedly more expens-
ive than NiMH batteries. Conventional lithium ion bat-
teries also employ flammable non-aqueous electrolytes and
thermally unstable lithium salts and thus fundamentally
pose a more substantial safety hazard than do aqueous
batteries. Indeed at the 13th International Seminar on
Primary and Secondary Battery Technology and Applications,
March 4-7, 1996, Boca Raton, Fl, USA (as reported in the
ITE Battery Newsletter No.2 Mar.-Apr. 1996), Mr. Jun
Sasahura of Toshiba suggested that safety requirements
already limit present Li-ion battery capacity to about 55
of its true or intrinsic energy density.
In Canadian patent application Serial No. 2,114,902,
Wainwright, filed February 3, 1994, available for public
inspection, August 3, 1995, aqueous rechargeable battery
systems are disclosed which operate much like conventional
lithium ion batteries except that aqueous electrolytes are
employed. That is, two different insertion compounds are
employed respectively as the cathode and anode electrodes
and an alkali metal (eg. lithium) or alkaline earth metal
species is 'rocked' during charge and discharge of the
battery (ie. species insertion takes place at one electrode
with simultaneous species extraction taking place at the
other electrode). The use of an aqueous electrolyte makes
such rechargeable battery systems fundamentally safer than
their non-aqueous counterparts. Also, the typical aqueous
electrolyte and typical aqueous battery construction are
markedly less expensive than their non-aqueous counter-
parts (The latter results from the typical non-aqueous
rechargeable battery needing much thinner electrode con-
structions than its aqueous counterpart to compensate for
the lower ionic conductivity of non-aqueous electrolytes.)
The downside of employing an aqueous electrolyte in a
system otherwise similar to a lithium ion battery is that
the stability range of an aqueous electrolyte to oxida-
tion/reduction is much less than many non-aqueous electro-
- 3 - 2 ~ 86 099
lytes. Consequently, the operating voltage of an aqueous
battery will need to be lower and more restricted than many
non-aqueous batteries. This places more constraints on
possible electrode material candidates and re~ults in a
reduction in energy output per unit species inserted/
extracted from the electrodes. The embodiments disclosed
in Canadian patent application Serial No. 2,114,902 include
combinations that have practical projected energ~ densities
which are competitive with Pb acid batteries (eg. about 35
Wh/kg). However, higher energy densities must be achieved
for applications such as electric vehicles. Also, only
limited results for the capacity loss versus cycle number
are obtained for the disclosed embodiments.
Further work on the a~orementioned embodiments by M.
Zhang et al. (mentioned in Materials Technology, Vol. 11,
No. 1, Jan./Feb. 1996, p9-12) indicate that marked
improvements in the capacity loss versus cycle number can
be achieved by judicious choice of electrolyte for the
specific VO2(B) anode material employed. The VO2(B) anode
shows a rapid loss of lithium insertion capacity in an
electrolyte having pH = 11.3, but not in an electrolyte
having pH = 9.1. This is presumed to arise from a dissol-
ution of the electrode itself into the electrolyte. With
optimum electrolyte selection, excellent cycling character-
istics can be expected. However, the possible dissolutionof this specific anode or even any anode material in a
basic electrolyte further restricts the choices available
to the battery engineer designing an optimum sy~tem.
It is therefore desirable to identify other insertion
compounds for these aqueous electrochemical systems that
not only have absolute potentials for insertion which are
compatible with aqueous electrolytes, but which also have
greater capacities for insertion of an alkali metal or
alkaline earth metal species and which are also more stable
in basic solutions. As in non-aqueous electrochemistries,
lithium is particularly desirable for use as an inserted
species.
- 4 - 2t 8 609q
Conventional non-aqueous lithium ion battery cathodes
such as LiCoO2, LiNio2, and LiMn2O4 have absolute potentials
in a range compatible for use as a cathode in an aqueous
lithium ion battery. LiNio2 is attractive because it i8
characterized by a large reversible capacity for lithium
insertion. LiMn2O4, on the other hand, exhibits less
reversible capacity but advantageously exhibits a relative-
ly flat or constant voltage over this reversible range
(thereby resulting in a battery with almost constant
voltage during operation and consequently making it easier
to engineer electrolyte stability over the reversible
range) and the raw materials used in its preparation are
less expens1ve.
As reported in Army Research Lab report number ARL-TR-
422, Feb. 1995, E. Plichta et al., tested various conven-
tional lithium insertion oxides and/or sul~ides as both
cathode and anode electrode materials for aqueous lithium
ion batteries. In this article, no working combination was
prepared with practical energy densities. The spinel
compound LiMn2O4 was found suitable as a cathode material in
principle although it was found to decompose during over-
charge before oxygen evolution occurred in the specific
embodiments tested (eg. batteries with electrolyte of
pH~8.5). This situation might prevent the use of a conven-
tional oxygen-hydrogen recombination reaction for over-
charge protection. Accordingly, if such recombination
reactions are desired, an alternate material choice is
required or the electrolyte must be modified such that
oxygen evolution occurs before cathode decomposition.
Modifications might involve increasing the pH of the
electrolyte which lowers the potential at which oxygen
evolution can occur (as discussed in J. Electrochem. Soc.,
Vol. 142, No. 6, June 1995, W. Li et al.).
Conventional non-aqueous lithium ion battery anodes
and lithium metal alloy anodes are generally unsuitable for
use as an anode in an aqueous lithium ion battery because
their absolute potentials are close to that of lithium
~ _ 5 _ 2 1 8 6 ~99
metal and hence are outside a range compatible for aqueous
electrolytes (as demonstrated in Journal of Power Sources,
55 (1995), 41-46, R.L. Deutscher et al.). Some well known
insertion compound oxides (eg. the aforementioned VO2(B)) or
sulfides (eg.TiS2) have been suggested, but to date, no
materials have been identified which are compatible with
basic or very basic electrolytes over a large reversible
range of inserted lithium.
Lately, novel high capacity insertion compounds are
being discovered at a rapid rate. Carbons, amorphous tin
oxides, and polymers exhibiting reversible capacities for
lithium of order of 600, 800, and 500 Ah/g have recently
been discovered and are described in Canadian patent
application Serial No. 2,149,853, Xue, filed May 19, 1995,
Canadian patent application Serial No. 2,134,052, Idota et
al., filed October 21, 1994, published April 23, 1995, and
U.S. Patent No. 5,441,831, Okamoto et al., granted August
15, 1995, respectively. The first two of these are con-
sidered suitable for use as anode materials and the third
product is considered suitable for use as a cathode
material in otherwise conventional non-aqueous lithium ion
batteries.
SUMMARY OF THE lNV~NllON
The invention represents an improvement over those
embodiments disclosed in Canadian Patent application Serial
No. 2,114,902, Wainwright, filed February 3, 1994. The
improvement involves the use of a polymer for at least one
of the insertion compounds. Improved energy density
characteristics can be obtained by using certain polymers
as insertion compounds for the electrodes and more options
can become available for the selection of electrolyte salts
and pH.
Although polymers were not previously considered as
alternatives, polymer electrodes exhibiting large revers-
ible capacities for inserted alkali or alkaline earth
- 6 - 2186099
metals can be particularly useful as electrode materials in
aqueous rechargeable insertion compound batteries. Poly-
mers generally can be less prone to dissolution or decompo-
sition in basic aqueous electrolytes. The "Handbook of
Plastics and Elastomers", C.A.Harper, Editor-in-chief,
1975,McGraw-Hill, states "Generally speaking, inorganic
salt solutions, weak aqueous alkaline solutions ... do not
have an adverse effect upon plastics, resins, or
elastomers."
Carbon-sulfur polymers, such as those described in the
aforementioned U.S. Patent No. 5,441,831, can exhibit
relatively constant voltages over a wide insertion range
for alkali metals, especially lithium. Such carbon-sulfur
polymers are particularly attractive ~or use, not only as
cathode materials in non-aqueous batteries, but as anode
materials in aqueous lithium ion batteries. The anode
polymer can be poly(carbon disul~ide) having the formula
(CSx) nl wherein x is a number from about 1.2 to 2.33 and n
is a number greater than or equal to 2.
Although various cathode materials may be considered,
a preferred embodiment of the invention combines a
poly(carbon disulfide) anode with a lithium manganese oxide
spinel cathode, denoted Li~n2O4, wherein lithium can, in
principle, be reversibly inserted over a value of y ranging
from 0 to about 2.
The aqueous electrolyte can comprise one or more
lithium salts. To maintain stability of the electrolyte
against oxidation/reduction, a basic electrolyte is pre-
ferred (ie. pH >7). A very basic electrolyte may be
preferred for a poly(carbon disulfide) anode/ lithium
manganese oxide spinel cathode embodiment. LioH may be
employed to adjust pH and other non-hydroxide salts of
lithium may be used as a source of additional lithium ions
in the electrolyte solution.
- 7 - 2186099
BRIEF DESCRIPTION OF THE DRAWINGS
The provided Figure illustrates certain non-optimized
aspects of the invention, but should not be construed as
limiting in any way.
Figure 1 shows the overall voltage of the battery of
Example 1 versus capacity and also shows the voltages of
the individual electrodes therein versus Li/Li+.
EMBODIMENTS OF THE lNv~NlloN
A variety of actual constructions, sizes, configur-
ations, etc. are possible for the battery of the invention.
All share a fundamental construction which represents a
mixture of conventional aqueous and non-aqueous battery
constructions. The basic operation is similar to a non-
aqueous lithium ion battery in that the cathode and anode
electrodes comprise a first and second insertion compound
respectively in electrical contact with respective cathode
and anode current collectors. (Herein, insertion compounds
are broadly defined as host materials into which a species
can be inserted and extracted without irreversible effect
on the structure of the host). In physical contact with
both electrodes is an aqueous electrolyte comprising a
dissolved salt of the inserted species A of the battery.
During operation of the battery, ions of A migrate to and
from each electrode through the aqueous electrolyte.
Concurrently, electrons migrate to and ~rom each electrode
via an external circuit. (Note that some hydrogen inser-
tion can inherently be expected to occur to some limited
extent in both electrodes. Additionally therefore, some
limited 'rocking' of hydrogen may also occur between the
electrodes.)
As explained in the aforementioned Canadian patent
application Serial No. 2,114,902, the electrode materials
are preferably selected such that the largest operating
- 8 _ 2l 8 6099
voltage is obtained without decomposing the aqueous elec-
trolyte into H2 and ~2 by electrolysis. (Note that, as with
conventional aqueous batteries, practical batteries may be
constructed that operate beyond the fundamental thermody-
namic stability limits of the electrolyte. It is possiblein principle to operate at significant overvoltages before
significant gas evolution occurs.) Of course, the elec-
trodes themselves must also not decompose or dissolve.
Polymer insertion compounds are generally more stable and
less prone to dissolution in basic electrolytes than are
typical inorganic insertion compounds. Thus, their use
would generally provide ~or greater options with regards to
electrolyte salts and pH selection in the aqueous electro-
lyte.
As explained in the aforementioned Canadian patent
application Serial No. 2,114,902, the selection of electro-
lyte salt/s and pH can be complex as many mutually depend-
ent factors must be considered. The use of electrode
materials that are stable in the presence of most salts and
over a wide pH range does however simplify this process.
The electrolyte preferably has high ionic conductivity and
has sufficient salt concentration to prevent electrolyte
depletion during operation of the battery. This implies
having a substantial cation concentration which may addi-
tionally help to bind the water to the dissociated saltions (thereby preventing reaction with inserted lithium to
some extent) and to prevent the water from decomposing into
H2 and ~2~ It may be desirable to use more than one dis-
solved salt in the electrolyte in order to meet all these
conditions simultaneously.
Preferred embodiments will combine the advantages of
both aqueous and non-aqueous battery constructions where
possible. As typical aqueous electrolytes have much higher
ionic conductivities than typical non-aqueous electrolytes,
the thicker electrode constructions of aqueous batteries
may be employed resulting in a simpler, less expensive
construction than those of non-aqueous batteries. However,
~ 21 86099
unlike Pb acid batteries for instance, the aqueous electro-
lyte in the battery of the invention does not substantially
participate in its basic electrochemical operation. Thus,
relatively high loadings o~ active electrode can be
expected in the battery. For instance, the active elec-
trode materials constitute about 50~ by weight in today's
typical small cylindrical non-aqueous lithium ion batteries
in commercial use, and these batteries employ relatively
large area, yet thin electrodes. Thicker electrode con-
structions in larger batteries are expected to allow ~or anincrease in achievable electrode loading since the relative
contributions of separator, current collectors, and con-
tainer may be reduced. Also, the relative weight o~ the
container has recently been reduced in certain commercial
non-aqueous lithium ion batteries by using aluminum instead
of steel. It is therefore not unreasonable to expect that
electrode loadings corresponding to 60~ by weight of the
total battery will be possible in aqueous battery construc-
tions.
Further, it can be preferred to provide for overcharge
protection via oxygen recombination reactions as ~ound in
many conventional aqueous systems. This involves engineer-
ing the battery such that after a full recharge, continued
charging or overcharging results in controlled, limited
oxygen evolution at the cathode without otherwise decompos-
ing the cathode. Batteries are usually assembled somewhat
electrolyte starved such that it is easier and hence faster
for the evolved oxygen to migrate back to the anode where
recombination can occur. Hydrogen evolution at the anode
is preferably avoided as much as is possible. Additives or
inhibitors may be used to increase the hydrogen over-
potential at the anode and hence suppress generation of
hydrogen gas. Batteries may also be slightly cathode
limited to avoid evolving hydrogen at the anode. (Other-
wise, the capacities of both electrodes would generally bebalanced in order to maximize overall battery capacity.)
The voltages at which both oxygen and hydrogen are evolved
~ 21 86099
-- 10
will of course atrongly depend on the electrolyte pH
selected.
In commercial Li ion batteries, it is conventional to
load the total amount o~ the inserted species A into the
~irst insertion compound prior to constructing the battery.
Nonetheless, it may be advantageous to load a portion of
the total amount of the inserted species A into either the
first or second insertion compounds, or both, during assem-
bly. In other circumstances, it may be desirable to add an
excess o~ a salt o~ A in order to electrochemically insert
additional species A into an electrode and hence into the
battery prior to completing the battery assembly. (The
electrochemical method for accomplishing this is the
subject of the invention of Canadian patent application
Serial No. 2,114,492, Dahn, filed Jan. 28, 1994.)
Hardware requirements (including current collectors
and container) for the batteries of the invention can also
be expected to share similarities to other aqueous systems.
Consideration with regards to possible chemical and/or
electrochemical corrosion must be made in the choice of
this hardware, particularly if strongly basic electrolytes
are employed. As with some Pb acid batteries, it may be
desirable to adopt a design that allows ~or replenishment
of the electrolyte over time in order to compensate ~or
losses due to electrolysis.
A preferred embodiment of the invention is an aqueous
battery wherein lithium is the inserted species. For anode
materials, a class of carbon-sulfur polymer insertion
compounds is pre~erred as their voltage characteristics can
be ~airly constant over a wide insertion range ~or lithium,
and their voltages (typically about 2.5 V versus Li/Li+) are
at an absolute potential near that for hydrogen evolution
in the electrolyte. The polymer poly(carbon disul~ide)
described in the aforementioned U.S. Patent No. 5,441,831
is particularly preferred as an anode since it is charac-
terized by a very large reversible capacity for lithium
over a voltage range of from about 2.1 to 2.7 V versus
~ 2 1 86û99
Li/Li+. The structure of poly(carbon disulfide) is charac-
terized by repeating units having C-S bonds in the chain
and branches having C=S bonds. The following example
illustrates the possible capacity advantages that might be
5 achieved by employing poly(carbon disulfide) as an anode
material in an aqueous lithium ion battery.
EXAMPLE
The voltage and capacity characteristics i~or an
aqueous rechargeable battery are illustrated in Fig. 1 for
an electrochemical couple comprising a lithium manganese
oxide spinel cathode (denoted LiyMn204) and a poly(carbon
disulfide) anode. The lithium manganese oxide spinel
15 cathode is considered as cycling with a 115 mAh/g revers-
ible capacity at voltages versus Li/Li+ ranging from 3.8 to
4.2 V (see for instance, J. Electrochem. Soc., Vol. 143,
No.1, plO9, Fig. 9, sample A-1). The poly(carbon
disulfide) anode is considered as cycling with a 460 mAh/g
20 reversible capacity at voltages versus Li/Li+ ranging from
2.8 to 2.1 V (see Eor instance, a:Eorementioned U.S. Patent
No. 5,441,831). The battery i9 assumed to comprise 4 g
of spinel cathode material and 1 g of poly(carbon
disul:Eide) anode material and the total active electrode
25 weight (5 g) amounts to 60~ of the overall battery weight.
Figure 1 shows the approximate individual cathode and
anode voltages versus Li/Li+ (based on low rate discharge
data given in the cited references) as well as the expected
overall battery voltage during a discharge (given by the
30 difi~erence in cathode and anode voltages). Under the above
conditions, the battery delivers 460 mAh at an average
voltage of about 1.5V (ranging over about 1.0-2.1 V) and
therei~ore has a gravimetric energy density o~ about 83
Whr/kg, which is competitive with commercial nickel metal
35 hydride batteries. [Note that hysteresis between charge
and discharge voltage curves and/or operation at high rate
implies that either the charge voltage will have to be
~ - 12 - 2186099
somewhat higher than that shown in Fig. 1 or that the
achieved capacity will be somewhat lower.]
The aqueous electrolyte salt and pH are selected such
that hydrogen evolution does not occur. Ideally, the
electrolyte also allows for full recharge followed shortly
after by the onset of oxygen evolution on OC for
recombination purposes. From thermodynamic principles, a
fairly basic electrolyte seems preferred, and can be
obtained by using LiOH as a salt. Other Li salts (eg.
nitrate, chloride, etc.) may also be used to provide for
more cations if desired. Note that some concentration of
OH- near pH=10 may be required to stabilize Lil~nMn204 in
aqueous solution, but that excessive concentration of OH-
can result in the spontaneous reaction of ~ilnMn204 with Li+
and OH- to make LiMn204, oxygen, and water as described by
Kanoh et al. in J. Electrochem. Soc., Vol. 140, No.11,
p3162-66.
As will be apparent to those skilled in the art in the
light of the foregoing disclosure, many alterations and
modifications are possible in the practice of this inven-
tion without departing from the spirit or scope thereof.
Accordingly, the scope of the invention is to be construed
in accordance with the substance defined by the following
clalms.
