Solid polymer electrolyte compositions

Details Solid polymer electrolyte compositions Solid polymer electrolyte compositions

1998-08-06

2001-11-13

Brouillette, Gabrielle (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

C429S188000, C429S325000, C429S306000, C429S321000, C429S313000

Reexamination Certificate

active

06316149

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to preparing compositions useful as electrolytes for alkali metal polymer batteries and the electrochemical cells and batteries made with such electrolytes.
Alkali metal polymer batteries generally include a positive electrode, a negative electrode, and an electrolyte positioned between the electrodes. The electrolyte can act to separate the electrodes and, because it is ionically conductive, provides a path through which ions flow from one electrode to the other during charging and discharging of the cell. Electrolytes can be solid or liquid. Liquid electrolytes include aqueous and nonaqueous solutions and molten salts. Solid electrolytes include ionically conducting polymers. Electrolytes can include salts, and liquids or polymers or combinations thereof capable of solvating the salt.
Lithium polymer batteries that include a lithium metal negative electrode, a metal oxide positive electrode and a solid electrolyte offer the advantage of high energy storage capacity and rechargeability. To optimize the cycle number or cycle life of a battery or cell, it is desirable to minimize or eliminate the growth of dendrites. Dendrite growth can result from the nonuniform plating of lithium onto the negative electrode during the charging step. Dendrites can puncture the electrolyte and make contact with the opposite electrode, thus causing an internal short circuit in the cell, which can compromise cell performance and shorten its useful life.
SUMMARY OF THE INVENTION
In one aspect, the invention features an electrolyte composition that includes a solid, ionically conductive polymer, organically modified oxide particles that include organic groups covalently bonded to the oxide particle, and an alkali metal salt. The electrolyte composition is essentially free of lithiated zeolite. In preferred embodiments, the organically modified oxide particles are essentially free of organic groups capable of ionically interacting with the alkali metal salt. The organically modified oxide particles preferably are essentially free of organic groups that include hydrogen atoms capable of forming hydrogen bonds.
Useful oxide particles are selected from the group consisting of silica, alumina, titania, zirconia, and combinations thereof. In one preferred embodiment, the oxide particles include silica. In some embodiments, the organic groups are selected from the group consisting of alkyl, alkene, alkyne, aryl, aldehyde, ketone, ester tertiary amide, ethers, and halogenated derivatives thereof, siloxanes, or combinations thereof.
In other embodiments, the electrolyte includes at least about 1% by volume organically modified oxide particles.
In preferred embodiments, the alkali-metal salt includes a lithium salt selected from the group consisting of lithium hexafluorophosphate, lithium trifluoromethane sulfonate, lithium bis(trifluoromethylsulfonyl)imide, and combinations thereof.
In one embodiment, the solid polymer includes a polyalkylene oxide, e.g., polyethylene oxide, having a molecular weight greater than about 100,000. In other embodiments, the solid polymer includes a copolymer that includes the reaction product of monomers selected from the group consisting of ethylene oxide, propylene oxide, allyl glycidyl ether, and combinations thereof.
In some embodiments, the electrolyte may include a low-melting oxygenated organic compound selected from the group consisting of cyclic carbonate (e.g., ethylene carbonate), dialkyl carbonate, ether, polyether, and combinations thereof.
In one embodiment, the electrolyte is a pressure sensitive adhesive.
In another embodiment, when the above-described electrolyte composition is incorporated into a cell, the cell is characterized by a voltage variation of no greater than ±3 mV over a period of five charge-discharge cycles, where each half cycle consists of about 1 mAh/cm
2
lithium being transferred through the electrolyte at a cycling rate of 0.5 mA/cm
2
.
In other aspects, the invention provides a cell that includes a first electrode, a second electrode, and the above-described electrolyte composition. In one embodiment, the cell includes a first electrode, a second electrode, and the above-described electrolyte composition adhered to a major surface of at least one of the electrodes. The cell is characterized by a voltage variation of no greater than about ±3 mV over a period of five charge-discharge cycles, where each half cycle consists of about 1 mAh/cm
2
lithium being transferred through the electrolyte at a cycling rate of 0.5 mA/cm
2
.
In another aspect, the invention features a composite electrode that includes the above-described electrolyte composition.
Electrolyte compositions of the invention have adhesive properties (preferably, pressure sensitive adhesive properties) and are capable of being adhered to electrodes. When laminated to the electrode, the electrolyte maintains direct contact with the electrode, which facilitates ionic transfer between electrodes and uniform deposition of the electrode metal (e.g., lithium) on the surface of the electrode during charge-discharge cycling.
Electrochemical cells and batteries constructed from the electrolyte compositions of the invention exhibit uniform plating of electrode materials, impedance levels similar to the impedance of the solid, ionically conductive polymer, and the ability to cycle repeatedly at constant cell voltage levels. The electrolyte compositions also exhibit unexpectedly stable levels of impedance (i.e., the internal impedance of the cell does not increase significantly) during cell cycling and are capable of cycling for greater than 50 cycles in a lithium-lithium cell, at four hours/cycle, with a current density 0.5 mA/cm
2
, at a depth of discharge of 1 mAh/cm
2
, and at 80° C. In addition, the electrolyte compositions possess sufficient ionic conductivity at room-temperature such that batteries that include the electrolyte compositions can be discharged at low current densities at these temperatures.
The organically modified particles provide improved structural integrity to the electrolyte compositions, which allows batteries and cells to be constructed from thin (i.e., less than about 20 &mgr;m) electrolyte films. The strengthened electrolyte also is relatively less penetrable by dendrites compared to solid polymer electrolytes that do not include organically modified oxide particles. Consequently, electrochemical cells with long cycle life can be achieved. The stronger electrolyte films also exhibit improved web handling at higher speeds compared to electrolyte films constructed solely of the solid, ionically conductive polymer. In addition, the electrolyte compositions of the invention are less combustible than solid polymer electrolytes due to the incorporation of non-combustible inorganic fillers.
Composite anodes and composite cathodes that include the electrolyte composition also exhibit the above-described properties.
The material costs associated with primary and secondary batteries are lowered due to decreased use of the relatively more expensive conductive polymer that has been reinforced with the less expensive organically modified oxide particles. Processing steps previously required to strengthen the component layers (e.g., chemical crosslinking of polymers) can be eliminated, thereby lowering the costs associated with battery manufacture without reducing performance.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.


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