In Situ Lithium Nitridosilicate Interlayer via Reactive Anolyte Design for All‐Solid‐State Silicon Anodes

ABSTRACT The solid‐electrolyte interphase in liquid‐electrolyte systems reaches a self‐passivating steady state by electrolyte design. However, the interphase in all‐solid‐state batteries (ASSBs) arises from mutual electrochemical decomposition between the anode and solid electrolyte, yielding unpredictable composition, poor ionic conductivity, and insufficient mechanical compliance. Instead of suppressing this interfacial reaction, we introduce a reactive anolyte design strategy in which vacancy‐rich β‐Li 3 N undergoes a controlled in situ interfacial reaction with Si, resulting in a purpose‐built lithium nitridosilicate interlayer directly on the surfaces of Si particles within the composite anode. This interlayer with the anolyte simultaneously establishes fast Li + transport pathways, suppresses electrochemical sintering, and accommodates large volumetric strain. A pre‐lithiation protocol further shifts the anode operation window into the kinetically favorable region and supplements the Li inventory. The resulting composite Si anode paired with a LiCoO 2 cathode delivers outstanding rate capability (19C, current density of 25.5 mA/cm 2 ), long cycling stability (over 5000 cycles), and high areal capacity (above 10 mAh/cm 2 ). Beyond the Li‐Si‐N system, the reactive anolyte principle could be extended to other alloy‐ and conversion‐type active materials, offering both a practical solution to the Si anode interface problem and a conceptual framework for rational interphase engineering in next‐generation ASSBs.