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

Abstract

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.