Cathode-Electrolyte Interphase Engineering through Artificial Surface Coating in 4.55 V High-Voltage LiCoO2 Cathodes

Increasing the high cutoff potential of LiCoO₂ (LCO) has proven challenging, due to lattice structure deterioration and cathode electrolyte interphase (CEI) instability. This work reports an effective strategy for tuning the CEI and stabilizing the cathode surface structure; we used atomic layer deposition (ALD) to synthesize Al₂O₃-capped LCO (ACLCO). Direct cryogenic transmission electron microscopy (cryo-TEM) characterization of the CEI reveals that, at 4.55 V voltage, ACLCO suppresses unstable CEI growth and enhances the stability of the layered LCO lattice. More specifically, as the cycling process drives electrolyte corrosion, the Al₂O₃ surface coating partially transforms into a thermodynamically stable AlF₃ layer, which inhibits continuous side reactions in the high oxidation state, protecting the layered crystal lattice from transformation to rock-salt phases. Our comprehensive study describes a new technique for direct visualization of the CEI, demonstrates the efficacy of ACLCO, and reveals the mechanism by which it stabilizes the LCO cathode.