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Capacity and voltage fading of Li₂MnO₃ is a major challenge for the application of this category of material, which is believed to be associated with the structural and chemical evolution of the materials. This paper reports the detailed structural and chemical evolutions of Li₂MnO₃ cathode captured by using aberration corrected scanning/transmission electron microscopy (S/TEM) after certain numbers of charge–discharge cycling of the batteries. It is found that structural degradation occurs from the very first cycle and is spatially initiated from the surface of the particle and propagates toward the inner bulk as the cyclic number increases, featuring the formation of the surface phase transformation layer and gradual thickening of this layer. The structure degradation is found to follow a sequential phase transformation: monoclinic C2/m → tetragonal I4₁ → cubic spinel, which is consistently supported by the decreasing …