Revealing the roles of the solid-electrolyte interphase in designing stable, fast- charging, low- temperature Li- ion batteries

L Tao and HR Zhang and SR Shah and XX Yang and JW Lai and YJ Guo and JA Russell and DW Xia and JK Min and WB Huang and CG Shi and ZH Liang and DY Yu and S Hwang and H Xiong and LA Madsen and KJ Zhao and FF Shi and F Lin, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 122, e2420398122 (2025).

DOI: 10.1073/pnas.2420398122

Designing the solid-electrolyte interphase (SEI) is critical for stable, fast-charging, low-temperature Li-ion batteries. Fostering a "fluorinated interphase," SEI enriched with LiF, has become a popular design strategy. Although LiF possesses low Li-ion conductivity, many studies have reported favorable battery performance with fluorinated SEIs. Such a contradiction suggests that optimizing SEI must extend beyond chemical composition design to consider spatial distributions of different chemical species. In this work, we demonstrate that the impact of a fluorinated SEI on battery performance should be evaluated on a case-by-case basis. Sufficiently passivating the anode surface without impeding Li-ion transport is key. We reveal that a fluorinated SEI containing excessive and dense LiF severely impedes Li-ion transport. In contrast, a fluorinated SEI with well-dispersed LiF (i.e., small LiF aggregates well mixed with other SEI components) is advantageous, presumably due to the enhanced Li-ion transport across heterointerfaces between LiF and other SEI components. An electrolyte, 1 M LiPF6 in 2-methyl tetrahydrofuran (2MeTHF), yields a fluorinated SEI with dispersed LiF. This electrolyte allows anodes of graphite, mu Si/graphite composite, and pure Si to all deliver a stable Coulombic efficiency of 99.9% and excellent rate capability at low temperatures. Pouch cells containing layered cathodes also demonstrate impressive cycling stability over 1,000 cycles and exceptional rate capability down to -20 degrees C. Through experiments and theoretical modeling, we have identified a balanced SEI-based approach that achieves stable, fast- charging, low-temperature Li-ion batteries.

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