Liquid–Solid Conversion Chemistry Enables Fast‐Charging Solid‐State Li–Se ₂ I ₂ Batteries

Solid‐state batteries (SSBs) are promising next‐generation energy storage devices due to their high energy densities and inherent safety. However, achieving fast charging in energy‐dense SSBs remains a significant challenge, primarily limited by poor solid–solid interfacial contact, high interfacial resistances, and sluggish redox kinetics. Here, we address this challenge by reporting a solid‐state Li–Se 2 I 2 battery that incorporates a low‐melting‐point (∼56°C) molecular liquid, Se 2 I 2 , to create a unique liquid–solid electrochemical interface. We elucidate the underlying six‐electron conversion mechanism and probe its redox dynamics. This engineered interface enables exceptional fast‐charging performance. The solid‐state Li–Se 2 I 2 battery delivers a high specific capacity of ∼536 mAh g −1 at 0.5C (70°C) and retains 115 mAh g −1 at an ultrahigh rate of 20C. Moreover, it exhibits a stable electrolyte resistance and an ultralow charge transfer resistance, leading to remarkable long‐term cycling stability, retaining 79% capacity retention over 1200 cycles at 10C, equivalent to a high current density of 5.974 mA cm −2 . This work on Se 2 I 2 electrochemistry establishes a novel pathway for developing fast‐charging and energy‐dense solid‐state batteries.