Synergistic Regulation of Interfacial Potential and Anionic Covalency for High‐Voltage Cobalt‐Free All‐Solid‐State Batteries

High‐voltage cobalt‐free all‐solid‐state lithium batteries (ASSLBs) represent a promising pathway toward high‐energy‐density and sustainable energy storage. However, their practical viability is fundamentally hindered by a coupled interfacial failure mechanism involving kinetic bottlenecks at the space‐charge layer (SCL) and the electrochemical instability of interfacial lattice oxygen. Here, we propose a synergistic regulation to decouple these constraints in 5 V‐class LiNi 0.5 Mn 1.5 O 4 (LNMO) ASSLBs. We reveal that the large lithium (Li) chemical potential mismatch at the LNMO/electrolyte interface drives a Li‐deficient SCL, while the high voltage triggers interfacial oxygen release, causing severe interfacial structural degradation. To address this, a stable interface was constructed where interfacial potential and anion covalency are regulated synergistically. Specifically, a high‐dielectric BaTiO 3 (BTO) coating layer was introduced to regulate interfacial potential and suppress SCL formation, while sulfate‐derived S─O covalent bonds stabilized the interfacial lattice oxygen. Consequently, the BTO‐S‐LNMO ASSLB achieves a notable increase in reversible capacity from 52 to 116 mAh g −1 at 0.1 C and enables high‐rate capacity up to 3 C and exhibits long‐term durability at 1 C. This work establishes a paradigm of coupling dielectric regulation and anion‐chemistry stabilization to unlock the potential of high‐voltage LNMO ASSLBs.