Constructing Ionically Conductive Surface Layers for High-Voltage Layered Cathodes via Their Metastable Framework

Increasing the upper cut-off voltage of O3-type layered LiCoO₂ cathodes is a promising strategy to enhance their specific energy density, attracting significant recent attention. However, this approach induces severe surface reconstruction and poor cycling performance as a result of oxygen loss. To overcome this limitation, we propose a novel synthesis strategy that employs a metastable O2-type LiCoO₂ framework combined with a minor Li-defective T2-type Li_1-x CoO₂ phase, which is transformed via thermal treatment into a stable O3-type LiCoO₂ capable of high-voltage operation up to 4.6 V. Surprisingly, this thermal treatment results in the formation of a thin, uniform spinel LiCo₂O₄ layer on the O3-type LiCoO₂ surface. This Li-ion conductive surface layer not only facilitates Li-ion transport but also inhibits structural collapse along the c -axis during high-voltage cycling. Furthermore, it effectively mitigates oxygen loss from the LiCoO₂ cathode during long-term cycling. Consequently, the modified O3-type LiCoO₂ cathode exhibits a high-capacity retention of 88% at 1 C over 200 cycles (3.0–4.6 V), substantially outperforming its unmodified counterpart (39%). This strategy of converting Li-poor metastable phases into a thermodynamically stable layered structure with a self-formed protective surface layer opens a new avenue for developing high-capacity, stable layered cathodes for advanced lithium-ion batteries.