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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 LiPF₆in 2-methyl tetrahydrofuran (2MeTHF), yields a fluorinated SEI with dispersed LiF. This electrolyte allows anodes of graphite, μSi/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 °C. Through experiments and theoretical modeling, we have identified a balanced SEI-based approach that achieves stable, fast-charging, low-temperature Li-ion batteries. 

Abstract Mechanical failure and its interference with electrochemistry are a roadblock in deploying high-capacity electrodes for Li-ion batteries. Computational prediction of the electrochemomechanical behavior of high-capacity composite electrodes is a significant challenge because of (i) complex interplay between mechanics and electrochemistry in the form of stress-regulated Li transport and interfacial charge transfer, (ii) thermodynamic solution non-ideality, (iii) nonlinear deformation kinematics and material inelasticity, and (iv) evolving material properties over the state of charge. We develop a computational framework that integrates the electrochemical response of batteries modulated by large deformation, mechanical stresses, and dynamic material properties. We use silicon as a model system and construct a microstructurally resolved porous composite electrode model. The model concerns the effect of large deformation of silicon on charge conduction and electrochemical response of the composite electrode, impact of mechanical stress on Li transport and interfacial charge transfer, and asymmetric charging/discharging kinetics. The study captures the rate-dependent, coupled electrochemomechanical behavior of high-capacity composite electrodes that agrees well with experimental results.