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Abstract

The surface reconstruction from the layered to rocksalt-type phase represents a primary deterioration pathway of layered-oxide cathodes in lithium-ion batteries, involving irreversible oxygen loss and transition metal migration. This degradation mechanism has primarily been attributed to the oxidative instability of highly delithiated cathodes at high voltages (>4.3 V vs Li/Li+). However, the battery degradation also occurs under seemingly stable voltage ranges, the origin of which remains unclear. Herein, a hidden mechanism to induce surface reconstruction and oxygen loss is proposed, termed the “quasi-conversion reaction”, which is revealed to occur during electrochemical reduction (discharge) processes just below 3.0 V (vs Li/Li+). Combined experiments and first-principles calculations unveil that the oxygens at the surface can be extracted from the cathode lattice by forming lithium oxides and oxygen vacancies, at significantly higher potentials than conventional conversion reaction, due to the instability of surface oxygens coordinated with fewer cations than in the bulk. The chemical incompatibility between lithium oxides and commercial carbonate-based electrolytes results in electrolyte decomposition, forming an organic-rich blocking layer and gaseous byproducts, which further increases the cell impedance. This study emphasizes the necessity of a thorough understanding of surface instability upon reduction to develop long-lasting batteries.