Electrochemical energy storage is essential to electrified transportation and the utilization of clean, renewable energy. Energy-dense, next generation solid state batteries address the inherent safety concerns of conventional lithium ion systems by utilizing solid lithium ion conductors that are thermally and chemically stable, mechanically robust, and nonflammable. Yet, preparing flexible, robust, thin inorganic solid membranes for integration into conventional cell architectures is challenging. To address this issue, thin film composite separators have been developed where 50 nm of lithium phosphorous oxynitride (Lipon), a thin film solid electrolyte, is deposited onto conventional microporous polypropylene separators (Celgard 2500). 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 exhibited relatively low interfacial resistance with liquid electrolytes (2.75 W cm2), mitigated electrolyte solvent consumption, and inhibited chemical cross-over contamination from sulfur and LiMn2O4 cathodes. Further functionality can be engineered into these thin film composite separators by engineering the microporous membrane supports. 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 half-cells prepared with Celgard and pBDDA (PIPS) separators showed comparable cycling performance, but the pBDDA was much more thermally stable and mechanically robust, undergoing negligible thermal shrinkage up to 150°C (Celgard 2500 shrinks by more than 30% at the same temperature). Efforts to engineer composite separators with pBDDA will be discussed.

Thin film composite separators for energy dense lithium batteries

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ACS Fall 2022

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