(506d) Thin Film Composite Separators for Energy Dense Lithium Batteries

Energy-dense, next generation solid state batteries offer a solution to the inherent safety concerns of conventional lithium ion systems that employ flammable liquid electrolytes. Essential to these batteries is the development of solid lithium ion conductors that possess high ionic conductivity, thermal and chemically stability, mechanical robustness, and nonflammability. Preparing flexible, robust, thin inorganic solid membranes amenable to high throughput, low-cost manufacturing is equally important. Aiming to satisfy these requirements, 50 nm of lithium phosphorous oxynitride (Lipon), a thin film solid electrolyte, was deposited onto microporous membranes, creating thin film composite (TFC) separator that are alternatives to conventional microporous separators. Lipon provides a dense, isotropic, chemically stable interface with Li metal, while the porous membrane accommodates a second electrolyte component for incorporation into conventional cathodes. The Lipon layer exhibits relatively low interfacial resistance with liquid electrolytes (2.75 W cm²), mitigates electrolyte solvent consumption, and inhibits chemical cross-over contamination from cathodes.

The functionality of these thin film composite separators was further enhanced by engineering the microporous membranes. A facile additive manufacturing approach for microporous membranes based on polymerization induced phase separation (PIPS) was developed. Difunctional 1,4-butanediol diacrylate (BDDA) was mixed with ethylene carbonate (EC) and then photopolymerized. During polymerization, the EC became insoluble as the BDDA cures into a crosslinked network and phase separates through spinodal decomposition. By controlling the polymerization kinetics and EC content, the pore sizes and porosities were tuned between 6.8 to 22nm and 15.4% to 38.5%, respectively. Lithium ion battery half cells consisting of LiNi_0.5Mn_0.3Co_0.2O₂ cathodes and pBDDA separators were shown to undergo reversible charge/discharge cycling with an average discharge capacity of 142 mAh/g and a capacity retention of 98.4% over 100 cycles - comparable to cells using state-of-the-art separators. Moreover, similar discharge capacities were achieved in rate performance tests due to the high ionic conductivity and electrolyte uptake of the film. The pBDDA separators were shown to be thermally stable to 374°C and exhibited negligible thermal shrinkage up to 150°C, unlike conventional polyolefin separators which shrink by more than 30% at the same temperature, compromising cell safety. Efforts to further engineer composite separators using the PIPS method will be discussed.