Weakly-coordinating non-fluorinated diluents for local high-concentration electrolytes in advanced lithium metal batteries

Abstract

Local high-concentration electrolytes (LHCEs) resolve the contradiction between ion transport efficiency and interface stability of traditional electrolytes in lithium metal batteries (LMBs) through the innovative design of macroscopically low viscosity and locally high concentration. Nevertheless, fluorinated diluents widely used in current LHCEs possess inherent defects, including high environmental toxicity, poor biodegradability, and high synthesis costs, which severely restrict the sustainable and large-scale application of LMBs. Recently, non-fluorinated diluents have attracted growing attention as viable alternatives, yet their roles remain insufficiently understood and often oversimplified as inert viscosity modifiers. In this perspective, we systematically classify non-fluorinated diluents into chlorinated and halogen-free systems, and further correlate their molecular structures with solvation behavior, interfacial chemistry, and electrochemical performance. By critically analyzing recent advances, we reveal that non-fluorinated diluents are not intrinsically benign or universally inert; instead, their effectiveness arises from a delicate balance between weak interactions, transport regulation, and interphase formation. Finally, we outline key challenges and future directions, including parameter-guided molecular design, multi-component electrolyte engineering, advanced in situ characterization, and sustainability-driven evaluation. This perspective aims to provide comprehensive theoretical support for the structural optimization of non-fluorinated diluents and the innovation of electrolyte formulations, thus promoting the technological progress and industrialization of LMBs.