Roles of Slab-Gliding-Induced Surface Nano-Steps in High-Voltage Instability of LiCoO₂

Rapid capacity decay caused by lattice oxygen (lattice-O) loss under high-voltage operation remains a critical challenge for layered LiCoO₂ (LCO) cathodes. Recently, various surface modification strategies have been explored to suppress lattice-O loss, yet the underlying mechanisms remain controversial. Herein, we identify slab-gliding-induced surface nanosteps on LCO and elucidate their role in driving lattice-O loss. These nanosteps (10–20 nm per step) are induced by local Co–O slab gliding during phase transitions upon deep delithiation, thereby exposing numerous active lattice O^n– (0 < n < 2) sites on the (003) and (104) planes, where oxygen vacancies (O_Vs) can form with significantly reduced formation energies. Consequently, this accelerates lattice-O loss and promotes the formation of a surface Li⁺-blocking layer, ultimately causing a rapid capacity decay. We further demonstrate that even if a rock-salt (RS) phase forms electrochemically in situ (e-RS) on the LCO surface, it fails to suppress the lattice-O loss due to the emergence of surface nanosteps. In contrast, a prefabricated RS layer (p-RS) with enhanced mechanical robustness effectively inhibits the formation of such nanosteps, thereby intrinsically suppressing lattice-O loss during cycling. This work identifies slab-gliding-induced surface nanosteps as a key structural trigger for lattice-O loss and demonstrates that prefabricated RS coatings offer an effective route to stabilize high-voltage LCO cathodes.