Thermal-Induced Reversible Spinel-to-Layered Phase Transition in LiCoO₂ Cathodes

LiCoO₂ (LCO) materials always undergo an irreversible phase transition from the original layered phase (LP) to the electrochemically inert spinel phase (SP) during cycling that severely degrades their performance in lithium-ion batteries. Consequently, the relithiation of SP phase in degraded LCO, or in other words, the reverse SP-to-LP phase transition, is of urgent importance for the community. Herein, we investigate the feasibility and mechanism of thermal-induced relithiation and SP-to-LP transition in LCO. It is revealed that this phase transition occurs at a mild temperature region (∼300 °C). Moreover, the results from combined in situ/ex situ characterization suggest that the SP-to-LP phase transition follows a "nucleation-oscillation-aggregation" mechanism, which showcases a strong anisotropic characteristic while propagating from the surface toward the bulk of particles. These findings provide profound insights into the SP-to-LP phase transition and lay a solid scientific foundation for developing advanced synthesis and regeneration strategies in layered oxide cathodes.