Polyimide-PEO copolymers (“PEO” signifies polyethylene oxide) that have branched rod-coil molecular structures and that can be cured into film form at room temperature have been invented for use as gel electrolytes for lithium-ion electric- power cells. These copolymers offer an alternative to previously patented branched rod-coil polyimides that have been considered for use as polymer electrolytes and that must be cured at a temperature of 200 °C. In order to obtain sufficient conductivity for lithium ions in practical applications at and below room temperature, it is necessary to imbibe such a polymer with a suitable carbonate solvent or ionic liquid, but the high-temperature cure makes it impossible to incorporate and retain such a liquid within the polymer molecular framework. By eliminating the high-temperature cure, the present invention makes it possible to incorporate the required liquid.
The curing of a polyimide- PEO copolymer according to the invention results in formation of a gel network that is capable of conducting lithium ions. The PEO molecular segments provide the lithium-ion conductivity, while the imide segments and branching provide dimensional stability. The network can hold as much as four times its own weight of liquid while maintaining a high degree of dimensional stability. The liquid can aid significantly in the conduction of lithium ions, and in some circumstances, can increase cell cycle life. Electrolytes have been prepared that contain no volatile components, have a potential stability window of >4.5 V, and have exhibited stable Galvanic cycling between lithium metal electrodes in a coin cell for over 1,000 hours at 60 °C and 0.25 mA/cm2 current density.
The copolymer is synthesized in the following process (see figure):
- A PEO oligomer that is terminated with primary aliphatic amines on both ends is reacted with a dianhydride to make a polyamide-acid pre-polymer. The reaction takes place in a solvent, and the stoichiometry of the oligomer and the dianhydride is adjusted so that the resulting prepolymer is a linear polymer capped with amines on both ends. The solvent must be carefully chosen to solubilize the prepolymer, have a boiling temperature preferably between 150 and 200 °C, and to be inert to lithium metal and other cell ingredients.
- The polyamide-acid prepolymer is imidized in solution. The water generated in the imidization reaction is removed by azeotropic distillation.
- The appropriate additives (e.g., a lithium salt and a carbonate solvent) are dissolved in the polymer solution. A trifunctional molecule that reacts with the amine end caps at ambient temperatures to form a gel is then added to the solution. The gelation time and the properties of the resulting film can be adjusted by changing the length of the polymer chains. The properties of the film can also be adjusted through choice of the dianhydride, the length of the starting PEO oligomer molecules, and partial replacement of the trifunctional molecule with a difunctional molecule.
- The film can be packaged once gelation has occurred. Because the reaction solvent is inert toward all cell ingredients, it is not necessary to remove this solvent. Optionally, because the reaction solvent boils at a temperature ≈100 C° lower than does a typical cyclic carbonate solvent, the reaction solvent can be preferentially evaporated before packaging.
This work was done by Mary Ann B. Meador of Glenn Research Center and Dean M. Tigelaar of Ohio Aerospace Institute.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18205-1.