Understanding the Multi‐Electron Conversion Mechanism and High Performance of NbCl 5 Cathodes in All‐Solid‐State Lithium Batteries

ABSTRACT Conversion‐type cathodes are promising candidates for high‐energy all‐solid‐state lithium batteries due to their high capacities enabled by multi‐electron redox chemistry. However, their practical application is severely hindered by large voltage hysteresis and rapid capacity decay arising from sluggish Li‐ion transport and structural degradation during conversion reactions. Herein, we develop a high‐valent NbCl 5 cathode with continuous electronic pathways, enabling highly reversible four‐electron conversion with ultralow discharge–charge overpotentials. The NbCl 5 cathode delivers a reversible capacity of 281 mAh g −1 at 0.1 C and retains 70% of its initial capacity at 1.0 C after 2000 cycles. Notably, it exhibits minimal voltage hysteresis across four electrochemical plateaus (0.08, 0.03, 0.03, and 0.01 V, respectively), achieving an energy efficiency of 98.9%, which surpasses reported conversion‐type and most layered oxide intercalation cathodes. We revealed that conversion‐induced interfaces between crystalline LiCl and low‐valent niobium chloride nanodomains facilitate Li‐ion migration within the cathode bulk and across the cathode/solid electrolyte interface. Meanwhile, the internal framework remains structurally stable during prolonged conversion reactions, as evidenced by only a slight increase in porosity from 2.48% to 3.55% after 200 cycles. This chemo‐mechanical synergy accelerates Li‐ion transport and maintains intimate interfacial contact, thereby enabling ultralow voltage hysteresis and durable cycling performance.