Preparation of redox polymer cathodes for thin film rechargeable batteries

The present invention relates to the manufacture of thin film solid state electrochemical devices using composite cathodes comprising a redox polymer capable of undergoing oxidation and reduction, a polymer solid electrolyte and conducting carbon. The polymeric cathode material is formed as a composite of radiation crosslinked polymer electrolytes and radiation crosslinked redox polymers based on polysiloxane backbones with attached organosulfur side groups capable of forming sulfur-sulfur bonds during electrochemical oxidation.

BACKGROUND OF THE INVENTION 
The present invention relates to the manufacture of thin film solid state 
electrochemical devices using solvent-free polymerization of polymer solid 
electrolytes and redox polymer cathode materials. 
Solvent-free polymer electrolytes have generated significant interest in 
recent years, primarily due to the potential for producing thin film 
rechargeable lithium batteries with high power capability and energy 
density. They have been extensively described in the patent literature, 
e.g. U.S. Pat. Nos. 4,303,748 to Armand, et al.; 4,589,197 to North; 
4,547,440 to Hooper, et al.; 4,906,718 to Gornowicz, et al. and 4,228,226 
to Christiansen. The cells are construed from an alkali metal foil anode, 
an ionically conducting polymer solid electrolyte containing an alkali 
metal foil anode, an ionically conducting polymer solid electrolyte 
containing an alkali metal salt, and a cathode consisting of a composite 
of a powdered insertion material, such as TiS.sub.2, the polymer 
electrolyte and an electron conductor, such as finely dispersed carbon 
black. Like liquid electrolytes and solvent-swollen polyelectrolytes used 
in ion-exchange resins, solvent-free polymer electrolytes possess ion 
transport properties. Both cation transport and anion transport in these 
solid polymer electrolytes have been substantiated and are well documented 
in the prior art. 
In applications of solid polymer electrolytes to secondary solid state 
batteries, it would be preferable to have no anion migration, the result 
of which is less polarization and higher power output. Anion mobility 
produces a negative effect on the energy efficiency of the battery because 
it results in local concentration gradients which result in deleterious 
polarization of the cell, lowering the output current. 
Attempts have been made to immobilize the anion on the polymer chain in 
order to achieve specific cationic conductivity. Several approaches have 
included the synthesis of cationic single-ionic conductors based on 
carboxylate or sulfonate salts. (Tsuchida et al, Macromolecules 
21,96(1988)). These reported electrolytes are limited in their application 
due to low conductivity. Presumably, the low conductivity is due to the 
extensive ion pairing in these salts. High conductivity polymer solid 
electrolytes with specific cation conductivity have been synthesized by 
Skotheim et al (U.S. Pat. No. 4,882,243 (1989)) where the immobilized 
anionic moieties are based on sterically hindered phenol compounds. 
Sterically hindered phenol substituted polysiloxanes have demonstrated 
specific cation conductivity 100-1000 times higher than what has been 
previously achieved with covalently attached carboxylate or sulfonate 
salts. 
Polymer solid electrolytes are generally cast from a common organic solvent 
for the polymer and the alkali metal salts, such as methanol or 
acetonitrile. Disposing of the organic solvent poses an environmental 
hazard and adds a considerable manufacturing cost. It would be preferable 
to synthesize the polymer electrolyte using a solvent-free polymerization 
method where the polymerization is performed under actinic irradiation. 
Actinic irradiation is defined as ultraviolet, gamma ray or electron beam 
irradiation. 
M.-T. Lee et al., U.S. Pat. No. 4.830,939, describes a method for forming 
an interpenetrating polymeric network for use in solid state 
electrochemical cells, consisting of a liquid electrolyte trapped in a 
crosslinked polymer matrix. The two phase polymer electrolyte system is 
formed by subjecting a mixture consisting of a liquid monomeric or 
prepolymeric radiation polymerizable compound, a radiation inert ionically 
conducting compound, such as propylene carbonate (PC), and an alkali metal 
salt, such as lithium trifluoromethane sulfonate, to actinic radiation to 
thereby crosslink the radiation polymerizable material and form a solid 
matrix. 
Electrochemically, the composite electrolyte material described by Lee et 
al. behaves essentially as a liquid electrolyte, with the well known 
degradation problems associated with liquid electrolytes. The long term 
stability of liquid electrolyte based electrochemical cells is limited by 
corrosion at the electrode/electrolyte interface, leading tto the build up 
of passivating layers on the lithium electrode. In addition, with liquid 
electrolytes, co-insertion of the solvent and the alkali metal cation, 
e.g. lithium, into the cathode material results in degradation of the 
cathode material due to swelling and de-swelling upon discharging and 
charging of the battery. It would be preferable to use high conductivity 
polymer electrolytes containing no liquid components. 
The cathode materials used in manufacturing of thin flim lithium or sodium 
batteries have generally been intercalation compounds such as TiS.sub.2 
and V.sub.6 O.sub.13. The cells have had limited rate capability and low 
utilization of cathode capacity. The cathode is formed as a composite 
consisting of powdered intercalation material and the polymer electrolyte 
with finely dispersed carbon black as electrical conductor. The rate 
limiting factor is generally the diffusion of cation in the insertion host 
material. Recently, M. Liu et al (Proc. Electrochem. Soc. Meeting, Miami, 
Fla., Sep. 1989) have describe a new class of redox polymer based cathode 
materials with substantially improved rate capability for lithium or 
sodium secondary batteries. The materials are based on polymerization and 
depolymerization via sulfur-sulfur bonds during the charging and 
discharging of the battery. Electrochemical cells were made with lithium 
foil anode, an electrolyte consisting of polyethylene oxide (PEO) with a 
lithium salt, such as LiClO.sub.4 or LiSO.sub.3 CF.sub.3, and a cathode 
consising of a homogeneous mixture of PEO and a redox polymer, with added 
carbon black for electrical conductivity. The cells demonstrated 
considerably higher rate capabilities than comparable cells made with 
TiS.sub.2 cathodes. 
One drawback with the system described by Liu et al is the reliance on 
polymerization and depolymerization of the redox cathode. When 
depolymerized, the monomers could disperse into the polymer electrolyte 
over time, severly limiting the lifetime of the cell. A second problem 
arises from the polymer electrolyte and the redox polymer cathode having 
different polymeric backbones. The miscibility of different polymers is a 
well known problem. With different polymeric systems, phase segregation 
normally occurs. Basing the electrolyte and the electrode materials on the 
same polymeric backbone and employing high conducting single-ion 
conducting polymer electrolytes would be expected to result in improved 
long term stability and higher capacity. 
SUMMARY OF THE INVENTION 
A principal object of the present invention is to provide a method for 
forming a redox polymer composite cathode for use in solid state 
electrochemical cells. 
Another object of the invention is to provide high conductivity polymer 
electrolytes with exclusive cation conduction. 
Another object of the invention is to provide redox polymer cathode 
materials with redox properties based on the breaking and reforming of 
sulfur-sulfur bonds with the disulfide redox moieties covalently attached 
to polysiloxane backbones. 
Another object of the invention is to provide a method for forming a 
composite redox polymer cathode using actinic irradiation of monomeric and 
prepolymeric systems. 
Another object of the invention is to provide a class of redox polymer 
cathode materials based on polysiloxane backbones to be miscible with the 
polysiloxane based polymer electrolytes. The reduction and oxidation is 
based on the breaking and forming of sulfur-sulfur bonds or groups 
covalently attached to the polysiloxane backbone. Consequently, no 
depolymerization occurs. Without de-polymerization, long term stability 
should be enhanced. 
In accordance with the present invention, the electrolyte is formed by 
radiation crosslinking branched polysiloxanes with ethylene oxide and 
anionic side groups and radiation polymerizable moieties. The polysiloxane 
backbone provides high degree of local segmental mobility to assist in the 
ion motion. The highest conductivities measured for a polymer solid 
electrolyte has been with branched polysiloxanes. Crosslinking is 
necessary to provide the requisite mechanical stability to the polymer 
electrolyte and for the polymer to function as separator. 
The radiation polymerizable electrolyte composition may be coated together 
with an ionizable alkali metal salt, onto the anode or the cathode prior 
to radiation crosslinking. Exposure to radiation either during or 
subsequent to deposition produced a branched polysiloxane network with 
ethylene oxide side groups for cation complexation. 
In another embodiment of the invention, anionic groups are attached to the 
polysiloxane backbone together with ethylene oxide moieties to produce an 
exclusively cation conducting polymer electrolyte. The anionic groups can 
be sterically hindered phenol compounds, as described in U.S. Pat. No. 
4,882,243. 
The composite cathode is formed by preparing a homoseneous mixture of a 
branched polysiloxane redox polymer cathode material, a branched 
polysiloxane electrolyte material which may contain covalently attached 
anoinic moieties, both containing radiation polymerizable moieties, and an 
ionizable alkali metal salt, and curing the mixture by exposing it to 
actinic radiation. This produces a polysiloxane network which is a 
homogeneous mixture of the polymer electrolyte and the polymer redox 
cathode. 
The polymer electrolyte can be coated on an alkali metal foil or evaporated 
alkali metal film or a lithium-carbon composite film followed by 
deposition of the polymer composite cathode. 
Alternatively, the polymer cathode can be coated first on a current 
collector, such as nickel or aluminum foil or highly conducting polymer, 
followed by deposition of the polymer electrolyte and subsequent 
evaporation of the alkali metal, such as lithium, onto the cured polymer 
electrolyte, or coating of a lithium-carbon composite film onto the cured 
polymer electrolyte. These processes can be formed in a continuous 
deposition system with full automation of the deposition process.

DETAILED DESCRIPTION OF THE INVENTION 
The polymer electrolyte network of the present invention is formed from 
branched polysiloxanes capable of crosslinking to form a network when 
exposed to actinic radiation. The branched polysiloxanes contain radiation 
polymerizable groups, such as polethylenically unsaturated moieties. More 
specifically, the polysiloxanes are selected from the formula: 
##STR1## 
wherein: 
##STR2## 
k=0-100, 1=0-100, m is at least 2, n=0-100, k+1+m+n is equivalent to a 
viscosity of up to 1.0 Pa.s, x=3=30, y=2=12, z=2-12 and the molar ratio of 
CH.sub.2 CH.sub.2 O units to alkali metal salt is 5-40. In general, 
R.sub.3 is an anionic moiety capable of being covalently attached to a 
polysiloxane backbone. 
Ionizable alkali metal salts useful in this invention include lithium and 
sodium salts where the anions may be selected from the group consisting of 
ClO.sub.4-, SO.sub.3, BF.sub.4-, CF.sub.3 COO--, PF.sub.6 --, N(SO.sub.2 
CF.sub.3).sub.2 -- and SCN--, 
The composite cathode materials are made from a mixture of carbon powder, 
polysiloxane electrolytes and polysiloxane redox polymers selected from 
the formula: 
##STR3## 
wherein: 
##STR4## 
k, l, m, n, x, y, z are defined as above and p=1-3, q=1-10 and r=1-30. 
Upon electrochemical reduction, intra- or inter-polymeric sulfur-sulfur 
bonds are formed. The bonds are broken upon electrochemical oxidation, 
with alkali metal cations inserted as counterions for electrical 
neutrality. The result is a high concentration of alkali metal cations 
inserted into the composite cathode.