Solvation-Driven Interphase Engineering and Mechanical Failure Pathways in Large-Scale Anode-Free Lithium Metal Batteries

Abstract

Anode-free lithium–metal batteries promise ultrahigh energy density but remain limited by unstable interfaces and safety concerns. Here, we demonstrate that fluorinated-ether solvation chemistry directly dictates interphase formation, thermal behavior, and performance in large-format Cu‖NMC90 cells. We reveal that introducing 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) into a localized high-concentration electrolyte restructures the Li⁺ solvation shell into a previously unreported PF₆⁻-dominated coordination environment in practical cylindrical cells. This anion-rich solvation promotes preferential PF₆⁻ reduction and yields a dense LiF-rich solid-electrolyte interphase, as verified by nuclear magnetic resonance, X-ray photoelectron spectroscopy, and differential electrochemical mass spectrometry. The optimized electrolyte enables non-flammable behavior and high efficiency in 18650-format anode-free cells, achieving 278–308 Wh kg⁻¹ at the cell level (380–402 Wh kg⁻¹ at the jelly-roll level), nearly 100% Coulombic efficiency, and compliance with UN38.3 impact-safety standards. Importantly, we identify lithium-plating-induced mechanical expansion—not interfacial instability—as the dominant failure pathway, establishing a direct link between molecular solvation structure and macroscopic structural integrity. These findings define solvation-driven interphase engineering as a practical route toward safe, high-energy anode-free lithium–metal batteries.