Stabilizing Oxygen Redox in Polycrystalline Li‐Rich Layered Cathodes via Efficient Grain Boundary Passivation

ABSTRACT High‐voltage cathodes with hybrid anion and cation redox activities, such as Li‐rich and Mn‐rich oxides, offer a pathway to higher energy density in lithium‐ion batteries (LIBs). However, those cathodes suffer from oxygen loss from both the surface and grain boundaries, especially in traditional polycrystalline cathodes with secondary‐particle morphology. Interfacial passivation is needed, yet uniform, high‐quality coating at both primary‐ and secondary‐particle levels remains challenging. Here, using a tannic acid‐assisted process, we demonstrate an in situ formed Ba M O 3− δ ( M = Ni, Co, and Mn) perovskite phase at surfaces and grain boundaries, offering effective passivation to suppress structural and electrochemical degradation. Differential electrochemical mass spectrometry (DEMS), soft X‐ray spectroscopy (SXAS) and resonant inelastic X‐ray spectroscopy (RIXS) reveal that the passivated grain boundaries effectively mitigate oxygen release, promote the retention of oxidized oxygen within the grains, and improve oxygen‐redox reversibility. The stabilization is facilitated by strong M ─O bonding at primary‐particle surfaces, improving energy‐density stability and mitigating voltage decay. The improved electrochemical performance results from the reliable coating method, which constructs sequential interfacial bonding along grain boundaries. Using earth‐abundant elements only, this process may offer high‐quality passivation for a variety of oxide cathodes used in LIBs and beyond.