Spontaneous Formation of a Li–Al Oxide Coating Layer on Ni-Rich Positive Electrode Materials and Improvement of Electrochemical Performance

Abstract Surface coating with a lithium-ion conductor is an effective strategy for improving the electrochemical performance of positive electrode materials for lithium-ion batteries. However, preparing a uniform coating by a simple, cost-effective, and solvent-free process remains challenging. Here, we show that simply mixing Ni-rich LiNi0.85Mn0.10Co0.05O2 (NMC) positive electrode material with Al2O3 nanoparticles (NPs) causes the residual alkali species on the NMC surface to spontaneously react with the Al2O3 NPs, forming a Li–Al oxide layer and thereby improving the electrochemical performance of Ni-rich NMC. Full-cell tests at 45 °C showed that 0.25 wt % Al2O3 NPs markedly improved capacity retention, particularly at high rates. pH titration and hard X-ray photoelectron spectroscopy revealed that residual LiOH reacted with Al2O3 NPs to form Li–Al oxides on the NMC surface before cycling. This indicates a spontaneous reaction between Al2O3 NPs and residual LiOH during dry mixing. Energy-dispersive X-ray spectroscopy mapping further revealed that a thin Al-rich layer a few nanometers thick was formed even on surface regions distant from the Al2O3 NPs, suggesting that the Al2O3 NPs reacted with LiOH distributed across the entire NMC surface, thereby forming a nanoscale coating layer. Postmortem scanning transmission electron microscopy coupled with electron energy-loss spectroscopy analysis confirmed a reduced NiO-like rock-salt layer after cycling, indicating that the self-formed Li–Al oxide layer suppresses surface degradation. This solvent-free coating approach offers a simple, cost-effective, and environmentally friendly route for improving the electrochemical performance of Ni-rich NMC.