Revealing Mechanism of Li₃PO₄ Coating Suppressed Surface Oxygen Release for Commercial Ni-Rich Layered Cathodes

Surface modification through Li3PO4 (LPO) coating is a brilliant approach to inhibit oxygen release of Ni-rich layered Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) cathodes, particularly under highly delithiated states, which can help tackle the thermal runaway issue for industrial applications. However, the mechanism of LPO prohibiting the release of the oxygen from the surface of NCM811 is still unclear. Herein we systematically investigate the critical role of the LPO protective layer through combining atomic-scale microscopy and in/ex situ techniques. In particular, results from electrochemical impedance spectroscopy, galvanostatic intermittent titration technique test, focused ion beam-high resolution transmission electron microscopy, and in situ X-ray diffraction, unequivocally identifies that the LPO layer can inhibit the transformation from layered-like to rock-salt-like structure. Ex situ soft X-ray absorption spectroscopy and in situ Raman further reveal the improved intensity of metal–oxide bonds with the suppression of phase transition, thus inhibiting surface oxygen release. This work provides inspiration for better understanding of surface coating on stabilizing the Ni-rich cathodes.