The effect of particle surface facets on the kinetic properties of LiMn1.5Ni0.5O4 cathode materials

Abstract

Large LiMn_1.5Ni_0.5O₄ single crystals in plate shape with (112) surface facets and octahedral shape with (111) surface facets were obtained by molten-salt synthesis. The presence of transition-metal ordering in both samples was independently confirmed by SAED, FTIR, NMR, and electrochemical studies, demonstrating the excellent capability of each technique in distinguishing the ordered and disordered phases. The apparent chemical diffusion minima during Li extraction and insertion were correlated with the occurrence of the first-order phase transition, implying that phase boundary movement limits Li transport in the spinel cathodes. Despite a more ordered structure, nearly ten times less Mn³⁺ content, and increased two-phase boundary movement during delithiation and relithiation, the octahedral crystals exhibited superior rate capability and a larger chemical diffusion coefficient, suggesting the kinetic preeminence of (111) surface facets over (112). The dominating effect of particle morphology and the importance of morphology design in achieving optimal performance of the LiMn_1.5Ni_0.5O₄ spinel are clearly demonstrated for the first time.