Thermal-Induced Reversible Spinel-to-Layered Phase Transition in LiCoO ₂ Cathodes

(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.