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