Dual‐Functional Li ₂ B ₄ O ₇ Coating on Carbon Fibers for Enhanced Li ⁺ Transport and Stability in Sulfide All‐Solid‐State Batteries

Abstract Interfacial degradation between carbon additives and sulfide‐based solid electrolytes is a critical bottleneck hindering the development of all‐solid‐state batteries (ASSBs). Here, a dual‐functional lithium tetraborate (Li ₂ B ₄ O ₇ ) coating is introduced on carbon fibers to resolve this challenge. The conformal Li ₂ B ₄ O ₇ layer acts as a physical barrier to suppress parasitic reactions while its unique dielectric properties simultaneously facilitate Li ⁺ transport at the interface. Comprehensive spectroscopic and electrochemical analyses confirm that this Li ₂ B ₄ O ₇ ‐based interfacial engineering strategy effectively mitigates electrolyte decomposition and enhances Li ⁺ kinetics. Consequently, the Li ₂ B ₄ O ₇ ‐coated conductive additive enables outstanding electrochemical performance. In pellet‐type cells, the cathode retains 94.7% of its initial capacity after 400 cycles at 0.5 C. Furthermore, its practical viability is demonstrated in high‐loading (6 mAh cm ⁻² ), dry‐processed electrode cells, which deliver an impressive capacity of 139.4 mAh g ⁻¹ at a demanding 2 C rate. This work presents a scalable and highly effective approach to stabilizing the challenging carbon‐sulfide interface, paving the way for durable, high‐energy‐density ASSBs.