High‐Performance All‐Solid‐State Silicon–Sulfur Batteries Enabled by Li 4 (BH 4 ) 3 I Hydride Electrolyte

Silicon anodes hold great potential for next‐generation battery systems due to their significantly higher capacity compared with conventional graphite anodes. However, the low electrochemical potential of lithiated phases in lithium–silicon alloys drives spontaneous electrolyte reduction, resulting in interfacial instability. Herein, a high‐performance silicon composite anode assembled with a Li 4 (BH 4 ) 3 I hydride electrolyte is developed. Prelithiation compensates for initial irreversible lithium consumption, while the combination of the Li 22 Si 5 alloy and hydride electrolyte suppresses interfacial degradation, enabling an initial Coulombic efficiency of 99.6%. In silicon–sulfur full cells, the opposite volume changes of the Li 22 Si 5 anode and sulfur cathode dynamically compensate for stress evolution during cycling and maintain intimate interfacial contact and structural integrity, enabling this configuration to deliver an ultra‐high areal capacity of 23.88 mAh cm −2 and retain 63.2% of its capacity after 1000 cycles, without observable cracking or structural degradation. These results provide an effective interfacial and structural design strategy for high‐performance silicon‐based anodes in all‐solid‐state batteries.