Understanding charge compensation in nickel-rich NMC cathodes is crucial for optimizing electrochemical performance. This study uses various spectroscopic techniques to investigate the correlation between transition metal oxidation states and Li depth profile in NMC811. It reveals different charge compensation mechanisms based on local Li content, aiding the development of high-energy Li-ion batteries with improved stability.

Understanding charge compensation mechanisms in nickel-rich lithium nickel–manganese–cobalt alloy oxide (NMC) cathodes is crucial for optimizing their electrochemical performance. This study employs complementary spectroscopic techniques with varying probing depths, including X-ray absorption near-edge structure spectroscopy, hard X-ray photoemission spectroscopy, X-ray photoelectron spectroscopy, and ion beam analysis, to investigate the correlation between transition metal oxidation states and Li depth profile in NMC811. This approach reveals different charge compensation mechanisms depending on the local Li content, which exhibits near-surface gradients. The results show that Ni oxidation is the main charge compensation mechanism for moderate delithiations at a Li stoichiometry x > 0.35. When the Li content drops below the threshold x = 0.3, which occurs near the surface for high state of charge (SOC), compensation by Ni/Li mixing prevails. Meanwhile, the Li-depleted surface (with x ≪ 0.3) shows evidence of transformation into electrochemically inactive Li-free cubic phases. The voltage window used to cycle the NMC811 prevents the bulk Li content from falling below x = 0.35, so the different charge compensation mechanisms coexist for high SOCs, allowing to compare between their kinetics. This study contributes to a comprehensive understanding of the electrochemical behavior of nickel-rich NMCs, aiding the development of high-energy Li-ion batteries with improved stability.