Multifunctional Lithium-Ion Battery Separators with Additional Crosslinkers

Electrochemical energy storage in the form of lithium-ion batteries is increasingly important as the world transitions towards renewable energy sources and an electrified consumer vehicle fleet. An essential component of lithium-ion batteries is the battery separator, a thin microporous membrane that physically separates the anode and cathode while providing for facile lithium ion transport. For optimal battery performance, separators must have large porosity for efficient transport while also being mechanically and thermally robust. In particular, the thermal stability of the separators is critical to lithium-ion batteries safety. Herein, polymerization-induced phase separation (PIPS) is explored as a fast, scalable, low-cost approach for fabricating battery separators. In PIPS, monomers are mixed with ethylene carbonate to prepare a homogenous precursor solution. Upon irradiation with UV light, the monomers polymerize, and the ethylene carbonate becomes insoluble in the polymer and phase separates into nanoscale domains. The EC domains form the separator pores, which are vital to the separator performance.

This project explores how the concentration of bisphenol A diacrylate (BPADA) comonomer in the separator formulation consisting of butanediol diacrylate, poly(ethylene glycol diacrylate), BPADA, and EC influences key separator properties. Photopolymerization cure kinetics were accelerated by adding BPADA. With 1% weight photo-initiator in the monomer solution, the cure was complete within 30 seconds – assessed from the residual unconverted vinyl functional groups measured using Fourier Transform Infrared Spectroscopy. Additionally, increasing the amount of BPADA yielded higher elastic moduli, with a 79% increase for 40% weight BPADA versus no BPADA, and strength of the separators, approximately doubling the strength for 20% weight BPADA versus no BPADA. The PIPS separators also had increased thermal stability relative to state-of-the-art Celgard separators, undergoing negligible thermal shrinkage under abuse conditions (30 minutes at 150˚C). Finally, the optimized compositions possessed porosities of 17%, enabling reversible cycling of NMC532/Lithium half-cells cycled at C/3 at 30˚C over 100 cycles, with 1M LiPF6 in 3:7 EC:DMC used as the electrolyte.