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.