Unravelling the Role of AlxOy Coatings in LiNiO2 Cathodes via Operando and Surface-Sensitive X-ray Techniques

LiNiO2 (LNO) cathodes are regarded as promising materials for high-energy-density applications owing to their high practical capacity and competitive cost. However, over the course of cycling, they experience rapid capacity decay when cycled above 4.2 V (vs Li/Li+) through bulk and surface mediated instabilities. Surface engineering via ultrathin aluminium oxide (AlxOy) coatings has the potential to provide an effective approach to inhibit the surface degradation pathways in LNO, but their long-term benefits remain unclear. Further, it is not well understood whether AlxOy coatings can provide any surface-to-bulk stabilisation effects. Here, we employ an ultrathin and conformal film of AlxOy on polycrystalline LNO powder via a scalable powder atomic layer deposition (ALD) method. Electrochemical performance was evaluated in high-mass-loading pouch cells (LNO vs. graphite) in a voltage window of 3.0–4.1 V using a constant-current constant-voltage (CCCV) protocol. Cells containing the ALD-coated LNO cathodes demonstrated significantly improved capacity retention (91.2%) over 100 cycles at a C/3 rate. In-house operando X-ray diffraction data on pouch cells demonstrated that after aging, ALD-coated LNO showed less kinetically hindered delithiation as compared to the uncoated LNO, confirming a surface-to-bulk effect. Postmortem surface-sensitive spectroscopy showed both the HF scavenging role of the AlxOy coatings and suppression of dense surface reconstructions in coated LNO. These findings highlight the efficacy of our scalable surface engineering approach in mitigating interfacial degradation, reducing impedance growth and improving bulk delithiation kinetics, which enhances the long-term electrochemical performance of these ultra-Ni-rich cathode materials.