Intrinsic weaknesses of Co-free Ni-Mn layered cathodes for electric vehicles

Co-free Li[NixMn1-x]O-2 (NM) cathodes have received considerable attention owing to their cost competitiveness and cycling stability. However, Ni-rich, Co-free NM cathodes have not been extensively studied and the reasons for their poor performance at low temperatures remain unclear. Herein, the fundamental properties and electrochemical performances of Co-free Li[NixMn1-x]O-2 and conventional Li[NixMn1-x-y]O-2 (NCM) cathodes (x = 0.8 and 0.9) at various temperatures are compared. Although the cycling stability and rate capabilities of Co-free NM cathodes are comparable with those of NCM cathodes at elevated operating temperatures, their performances are significantly inferior to those of NCM cathodes at low temperatures. The absence of Co in layered oxide cathodes not only increases diffusion-hindering cation disorder in their Li layers but also reduces the tolerance of their ionic diffusivities to low temperatures, leading to poor rate capabilities at high C-rates and low temperatures. To improve the practical viability of Co-free layered oxide cathodes, it is necessary to resolve their serious weakness of poor low-temperature performance.