Octahedral and truncated high-voltage spinel cathodes: the role of morphology and surface planes in electrochemical properties†
High-voltage spinel cathodes LiMn1.5Ni0.5O4 are promising candidates for large-scale energy-storage applications such as electric vehicles. However, the widespread adoption of this high-voltage spinel cathode is hampered by severe capacity fade, particularly at elevated temperatures, resulting from aggressive formation of a thick solid-electrolyte interphase (SEI) layer through side reactions with the electrolyte at the high operating voltage, cationic ordering between Mn4+ and Ni2+ ions in the crystal lattice, and formation of a rock salt LixNi1−xO impurity phase. While these issues have been explored, the wide variation in physical and electrochemical properties with different synthesis methods is not fully understood. In this investigation, we present how the synthesis conditions of the co-precipitation method influence the microstructure and morphology through nucleation and growth of crystals in solution. The samples were prepared by two similar wet-chemical routes and were characterized by microscopy and electrochemical methods to determine the role of microstructure and morphology in the electrochemical performance. Various factors such as the degree of cation ordering between Mn4+ and Ni2+, Mn3+ content, Ni–Mn ratio in the sample, change in lattice parameter with the state of charge, and surface crystal planes were examined to develop a better understanding of the factors influencing the electrochemical performance. It is found that the surface crystal planes, the arrangement of lithium ions near the surface, and the lithium diffusion mechanism have a dominant effect on the capacity retention and rate performance.