Anion-participating solvation enables compositionally graded interphases for durable subzero lithium-ion batteries

Abstract

Low-temperature operation remains a critical bottleneck for lithium-ion batteries (LIBs), since the coupled limitations in Li+ transport, desolvation and interfacial stability severely undermine energy delivery and long-term cyclability. Here, rather than merely relying on low-freezing-point solvents to improve electrolyte fluidity, we design a lithium difluoro(oxalato)borate (LiDFOB)-based electrolyte combining fluoroethylene carbonate (FEC), dimethyl sulfite (DMS) and 1,1,2,2-Tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE) to simultaneously regulate the solvation structure and interfacial reactions under subzero conditions. Molecular dynamics simulations reveal that the LiDFOB + DMS/HFE/FEC electrolyte reconstructs the Li+ solvation sheath from solvent-dominated coordination in LiPF6 + ethylene carbonate/dimethyl carbonate to an anion-dominated, aggregate-rich structure. Sputtering-time-dependent X-ray photoelectron spectroscopy reveals a compositionally graded graphite solid electrolyte interphase in which LiF remains the predominant fluorine-containing species and the relative contribution of Li-O-containing species increases with continued sputtering, while total-ion-normalized time-of-flight secondary ion mass spectrometry provides complementary semiquantitative trends for selected interphase fragments. A compositionally differentiated hybrid cathode interphase containing borate-related components and reduced organic accumulation is also observed on LiCoO2. As a result, LiCoO2||Li cells using the LiDFOB-based electrolyte retain 93.77% of the initial discharge capacity after 500 cycles at 1.0 C and 25 °C. Even at -20 °C, they deliver 139.6 mAh g-1 and retain 132.8 mAh g-1 after 500 cycles at 0.2 C, corresponding to 95.1% retention. Meanwhile, LiCoO2||graphite full cells also exhibit markedly improved cycling stability at -20 °C, delivering 116.8 mAh g-1 after 400 cycles at 0.2 C. Rather than optimizing solvation structure or interphase chemistry in isolation, this work links anion-involved Li+ coordination to depth-dependent interfacial evolution and identifies solvation-to-interphase coupling as a practical design principle for extending the cycling life of low-temperature LIBs.