Li-Rich Li[Li_1/6Fe_1/6Ni_1/6Mn_1/2]O₂ (LFNMO) Cathodes: Atomic Scale Insight on the Mechanisms of Cycling Decay and of the Improvement due to Cobalt Phosphate Surface Modification

Lithium-rich Li[Li_1/6Fe_1/6Ni_1/6Mn_1/2]O₂ (0.4Li₂MnO₃-0.6LiFe_1/3Ni_1/3Mn_1/3O₂, LFNMO) is a new member of the xLi₂MnO₃·(1 − x)LiMO₂ family of high capacity–high voltage lithium-ion battery (LIB) cathodes. Unfortunately, it suffers from the severe degradation during cycling both in terms of reversible capacity and operating voltage. Here, the corresponding degradation occurring in LFNMO at an atomic scale has been documented for the first time, using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), as well as tracing the elemental crossover to the Li metal anode using X-ray photoelectron spectroscopy (XPS). It is also demonstrated that a cobalt phosphate surface treatment significantly boosts LFNMO cycling stability and rate capability. Due to cycling, the unmodified LFNMO undergoes extensive elemental dissolution (especially Mn) and O loss, forming Kirkendall-type voids. The associated structural degradation is from the as-synthesized R-3m layered structure to a disordered rock-salt phase. Prior to cycling, the cobalt phosphate coating is epitaxial, sharing the crystallography of the parent material. During cycling, a 2–3 nm thick disordered Co-rich rock-salt structure is formed as the outer shell, while the bulk material retains R-3m crystallography. These combined cathode–anode findings significantly advance the microstructural design principles for next-generation Li-rich cathode materials and coatings.