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

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 BaMO_{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.