Anion Adsorption at the Inner‐Helmholtz Plane Directs Cathode Electrolyte Interphase Formation

Constructing a stable cathode‐electrolyte‐interphase (CEI) on cathode surface constitutes the foundation of realizing high‐voltag­e Li‐ion batteries, yet its formation, a highly heterogeneous process involving irreversible reactions between electrolyte components and cathode materials, remains poorly understood. Herein, combining multiple in‐situ/operando interfacial characterization techniques, we establish the correlation between interfacial structure and interphasial chemistry, and reveal the key role played by adsorptive behavior of various electrolyte components in the inner‐Helmholtz plane during CEI formation. Quartz crystal microbalance equipped with dissipation modification detects that difluorooxalatoborate (DFOB‐) anion preferentially adsorbed on LiCoO2 tends to expel carbonate solvents from the adsorption layer, thus suppressing their electrochemical decomposition at high voltages and leading to a more compact CEI derived from anions with limited contribution from organic ingredients. Consequently, the CoO2 lattice structure protected by the dense CEI remains intact despite near‐complete delithiation, thereby ensuring excellent cycling stability for 4.7 V operation of LiCoO2 cathode. The atomistic‐level insight into the key factors that govern CEI formation provides directive knowledge that accelerates electrolyte design for high‐voltage batteries.