On the Localized Nature of the Structural Transformations of Li 2 MnO 3 Following Electrochemical Cycling

Although the Li‐excess layered‐oxide Li 2 MnO 3 has a high theoretical capacity, structural transformations within the oxide during electrochemical cycling lead to relatively low experimental capacities, hindering its use in practical applications. Here, aberration‐corrected scanning transmission electron microscopy/electron energy loss spectroscopy and high‐resolution X‐ray diffraction are used to characterize the oxide following electrochemical cycling. Microscopy reveals the coexistence of regions with local monoclinic, spinel, and rock‐salt symmetries, indicating localized and inhomogeneous structural evolutions. Crystal structure transformations are observed both at the particle surface and in the bulk. At the surface, these transformed regions resemble spinel Mn 3 O 4 or rock‐salt MnO, consistent with oxygen loss. In the bulk, the regions resemble defect spinels, such as the layered‐spinel Li x Mn 4/3 O 4 , which suggest a partial phase transformation consistent with oxygen retention. Both microscopy and diffraction data of the cycled sample indicate areas of pristine Li 2 MnO 3 ; the presence of such areas, in close proximity to Li x Mn 4/3 O 4 areas, suggests that the layered to spinel structure transformation is partially reversible. Spinel, disordered rock salt, and pristine areas are also observed in Li 2 MnO 3 samples intentionally damaged by electron beam irradiation. This observation indicates that the dynamic processes resulting in phase transformations can be studied for a variety of oxide systems by a judicious selection of irradiation conditions.