A Layer-Structured Electrode Material Reformed by a PO₄-O₂ Hybrid Framework toward Enhanced Lithium Storage and Stability

The surface framework of LiCoO₂ is modified through a surface treatment called phosphidation, which suppressed the unwanted phase transition occurring above 4.2 V. The surface instability of LiCoO₂ toward organic electrolytes is simultaneously improved by changing its surface structure from an O₂-based framework to a PO₄-based framework that can protect against HF attack during cycling. Phosphidated LiCoO₂ is successfully synthesized that showed greater stability in its bulk and surface structures. The phosphidated form enables faster Li⁺ diffusion and prevents irreversible phase transitions, especially when charged above 4.2 V, and consequently demonstrates excellent cycling performance and rate capabilities. The improved kinetics and stability resulting from phosphidation make LiCoO₂ highly suitable as a high-voltage cathode material for use in lithium ion batteries.