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Abstract Organic mixed ionic-electronic conductors (OMIECs) are a class of materials that can transport ionic and electronic charge carriers simultaneously. They have shown broad applications in soft robotics, electrochemical transistors, and bio-electronics. The structural response of OMIECs to the mixed conduction populates from molecular conformation to devices, presenting challenges in understanding their mechanical behavior and constitutive descriptions. Furthermore, OMIECs feature strong multiphysics interactions among mechanics, electrostatics, charge conduction, mass transport, and microstructural evolution. In this review, we summarize recent progress in mechanistic understanding of OMIECs and highlight dynamics and heterogeneity underlying each element of mechanics. We introduce strain activation and breathing, mechanical properties, and degradation of OMIECs upon electrochemical doping and dedoping. Drawing on the state-of-the-art experimental and simulation insights, we highlight the critical role of multiscale dynamics in governing the functionality of OMIECs. We discuss the current understanding and limitation of constitutive relations and present computational frameworks that integrate multiphysics. We synthesize mechanics-driven strategies—spanning strain modulation, material stretchability, and interfacial stability—from molecular design to macroscopic structural engineering. We conclude with our perspective on the outstanding questions and key challenges for continued research. This review aims to organize the fundamental mechanical principles of OMIECs, offering a multidisciplinary framework for researchers to identify, analyze, and address mechanical challenges in mixed conducting polymers and their applications. 

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.