Enabling high performance all-solid-state lithium metal batteries using solid polymer electrolytes plasticized with ionic liquid

Abstract

Ionic conductivity needs to be improved significantly for solid polymer electrolytes to be considered competitive alternatives to organic liquid electrolytes for battery technology. The strategy employed here is to promote polymer microstructures that facilitate ion transport by developing an amorphous rather than crystalline polymer matrix. To this end the transport, thermal, and electrochemical properties of solid polymer electrolytes incorporating ionic liquid (ILSPEs) are investigated. ILSPEs are fabricated into homogenous films using a hot-pressing procedure and are composed of a blend of poly (ethylene oxide) (PEO), triethylsulfonium bis(trifluoromethylsulfonyl)imide ionic liquid (S2TFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt. The goal of this work is to establish composition-property relationships that enable the ambient temperature operation of ILSPEs in a lithium metal battery. Optimized ILSPE compositions are able to achieve a high total ionic conductivity and Li+ transference number, 0.96 mS/cm at 22 °C and 0.31 at 60 °C respectively, that meet commercial benchmarks. The ILSPEs also show resistance to oxidation, with passivation up to 4.5 V (vs. Li+), and long-term stability with lithium metal, which enable good rate performance during room temperature cycling with a lithium metal anode and lithium iron phosphate cathode. Initial tests with higher potential lithium cobalt oxide cathode demonstrate promising performance.