Pushing the Limits: Maximizing Energy Density in Silicon Sulfide Solid‐State Batteries

Abstract For the first time, we demonstrate a silicon solid‐state battery (SSB) architecture that achieves >400 Wh kg −1 , approaching the theoretical limit for silicon‐based SSBs. This configuration features a 99.9 wt% micro‐Si, a thin sulfide solid electrolyte (SSE), and a high‐loading NMC811. Key to these results is strategically selecting and evaluating the processing techniques, whether wet or dry, for the negative electrode, positive electrode and thin sheet‐type SSE. Excessive lithium incorporation into the silicon host, beyond the Li 3.75 +Si phase to form a LiSi composite, is essential to match the high capacity of the positive electrode. This SSB achieves over 1000 cycles for a 2 mAh cm −2 with ≈80% capacity retention and 94% capacity retention for 3 mAh cm −2 over 500 cycles at 25 °C. Post analysis identifies the primary capacity decay mechanisms as oxidation at the NMC/SSE interface and structural disruptions within NMC. Meanwhile, the Si electrode maintains a robust solid‐electrolyte interphase layer, minimizing capacity decay. This study highlights the necessity for improved NMC coatings, lattice oxygen stabilization, and a durable positive electrode‐electrolyte interface to improve the long‐term stability of SSBs. Strategies leading to a single‐layer pouch cell SSB exceeding 400 Wh kg −1 are developed.