Evolution of Spinel LiMn₂O₄ Single Crystal Morphology Induced by the Li₂MnO₃ Phase during Sintering

The most severe problems for adoption of LiMn2O4 (LMO) as a low-cost and sustainable cathode in lithium-ion batteries are manganese dissolution and structural degradation, especially at an elevated temperature. Developing large single crystals (SCs) for LMO could be a feasible solution since it significantly reduces electrode/electrolyte interfaces where degradation can occur, while exceptionally high ionic diffusivity of its spinel structure could guarantee decent kinetics. In this work, we discovered a unique correlation between morphology and synthesis conditions, especially oxygen partial pressure in a successful development of defect-free faceted LMO SCs. Further experimental and theoretical studies identified that crystal growth of spinel LMO can be dramatically promoted by the Li2MnO3 impurity, which is spontaneously generated at low oxygen partial pressure during high temperature synthesis. Meanwhile, electrochemical performances were found to be controlled by both impurity and crystallite size. We believe that with more understanding of synthesis parameters, LMO single crystals could achieve optimal electrochemical performance.