Li3InCl6 as an Active Filler Improves Interfacial Stability of PEO-LiTFSI Composite Solid Polymer Electrolytes for All-Solid-State Lithium Metal Batteries

Composite solid polymer electrolytes (CSPEs) with inorganic fillers are of great interest for advanced all-solid-state lithium–metal batteries (ASSLMBs), enabling high energy density and enhanced safety. Despite increasing interest, the effects of different inorganic fillers on the interface between CSPEs and the lithium metal anode (LMA) remain not fully understood. Herein, we systematically study the contact behavior between the LMA and CSPEs containing PEO, LiTFSI, as well as oxide- (Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 ), sulfide- (Li 6 PS 5 Cl), and halide-based (Li 3 InCl 6 ) fillers by evaporating lithium metal stepwise onto CSPE thin films under ultrahigh vacuum conditions, followed by in-system X-ray photoelectron spectroscopy (XPS) analysis. XPS results demonstrate that halide-based fillers can facilitate the formation of electron-rich Li–O and Li–S species at the interfaces, acting as a passivation layer, which can prevent further decomposition of the CSPE and thus improve interfacial stability. In contrast, the incorporation of oxide- and sulfide-based fillers does not promote the interfacial reactions. Impedance spectroscopy as a function of resting time also shows that the incorporation of a halide-based filler reduces the CSPE/LMA interfacial impedance, which is beneficial for increasing the critical current density for lithium dendrite formation. The incorporation of active inorganic fillers to induce protective interphases offers a novel strategy for interfacial engineering of ASSLMBs.