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The phase transformation from spinel LiMn2O4 to layered rock-salt NaxMnO2 via Na insertion-extraction cycles is crucial for a LiMn2O4 positive electrode in a Na-ion battery. To reveal the atomic-scale mechanism of the structural conversion, we applied advanced techniques of analytical electron microscopy to a Na-containing LiMn2O4 specimen, formed by lithiation of a thin MnO wafer containing Na impurity. Scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) analyses revealed that Na and Li are separately distributed in the two phases of the specimen, which are layered NaxMnO2 and spinel LiMn2O4 phases confirmed by annular bright field (ABF)-STEM observation. The large difference in the ionic radii of Na and Li is considered to be the reason for the clear phase separation without atomic-scale mixture. EDX analysis showed that the layered NaxMnO2 phase with P3 structure exhibits local variations in Na composition, with the maximum value of x = 0.6. High-resolution transmission electron microscopy (HRTEM) and ABF-STEM imaging clearly showed that an epitaxial LiMn2O4/NaxMnO2 interface is formed without any lattice dislocations, however, the interface has a low inclination angle due to the lattice mismatch, and local distortions are induced in the NaxMnO2 phase. The epitaxial lattice transition suggests smooth generation of a Na-inserted phase with a layered structure in a spinel LiMn2O4 crystal, which should contribute to the superiority of the LiMn2O4 electrode in a Na-ion battery

Transmission electron microscopy investigation of the LiMn2O4/NaxMnO2 interface as a model study of a Na-ion battery electrode