Atomic insights into surface orientations and oxygen vacancies in the LiMn₂O₄ cathode for lithium storage

Surface anisotropy of spinel LiMn₂O₄ crystal is of significance for strengthening the lithium storage. In this work, we investigate the evolutions of surface orientations and oxygen vacancies in the LiMn₂O₄ cathode materials by the spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) technique. Firstly, the {111} lattice planes appear in the formed LiMn₂O₄ octahedra, and then the {110} and {001} crystal planes are observed in the subsequent truncated octahedral at a higher temperature or a longer annealing time. The LiMn₂O₄ crystals with dominated {111} and {110} planes and an appropriate amount of oxygen vacancies exhibits a superior cycling performance and rate performance, suggesting a synergistic effect of surface orientation and concentration of oxygen vacancies achieves an enhanced electrochemical performance. The introduction of oxygen vacancies can accelerate the diffusion of lithium-ions along {110} plane and strengthen the rate performance of LiMn₂O₄, as elucidated by the combination of Cs-STEM observation, X-ray photoelectronic spectra analysis and property measurements.