Controlling Ionic Transport through the PEO-LiTFSI/LLZTO Interface

A highly resistive Polyethylene oxide-LiTFSI(PEO-LiTFSI)/ Lithium Lanthanum Zirconium Oxide (LLZO) interface, with a resistivity of 95 kOhms.cm2 (30°C) is believed to limit the total conductivity of ceramic-polymer composite electrolyte (CPE). To achieve higher ionic conductivity, the interfacial impedance (Rinterface) must be reduced to <~100 Ohms.cm2 to enable cell impedances comparable to Li-ion technology. The goal of this study was to investigate the origin of this high Rinterface. It was hypothesized that LLZO surface impurities and abrupt changes in Li-ion concentration between the PEO-LiTFSI/LLZO electrolytes contribute to the high impedance. By removing surface impurities through heat-treatment, the Rinterface was reduced to 180 Ohms.cm2 at 30°C. Optimization of Li-salt concentration in PEO to 15:1, resulted in reduction of Rinterface from 1.6 kOhms.cm2 to 421 Ohms.cm2. By understanding the underpinning mechanisms that govern the ceramic-polymer interface impedance, we believe it is possible to develop high conductivity CPE that are easy to fabricate and integrate into solid-state batteries.