Topotactic phase transformation of lithiated spinel to layered LiMn0.5Ni0.5O2: the interaction of 3-D and 2-D Li-ion diffusion

Abstract

This study investigates the structural evolution of LiMn_0.5Ni_0.5O₂ cathode materials for Li-ion batteries as a function of synthesis temperature and its effect on electrochemical performance. It is demonstrated that, as the synthesis temperature increases from 400 to 900 °C, a gradual topotactic transformation occurs between a lithiated spinel structure, denoted herein as “lithium-excess spinel” LxS-LiMn_0.5Ni_0.5O₂ (or LxS-LMNO), and the well-known layered LiMn_0.5Ni_0.5O₂ structure prepared at high temperature, HT-LiMn_0.5Ni_0.5O₂ (HT-LMNO). The electrochemical capacity of the LiMn_0.5Ni_0.5O₂ electrodes follows a parabolic trend with increasing synthesis temperature, which is attributed primarily to the gradual transformation of 3-dimensional (3-D) to 2-dimensional (2-D) diffusion pathways for the Li ions. When synthesized at 400 °C, LxS-LiMn_0.5Ni_0.5O₂ electrodes perform well, benefitting from the 3-D network of channels within the LxS structure. By contrast, when prepared at 500–700 °C, LiMn_0.5Ni_0.5O₂ electrodes operate poorly, which is attributed to the formation of locally disordered structural arrangements that impede Li-ion diffusion. Such an increase in local disorder in the mid-temperature synthesis range is attributed to the structural frustration between the lithium-excess spinal and layered end-members. The transformation from the locally disordered to more ordered layered components between 700 °C and 900 °C enhances electrochemical performance. The study opens new avenues for designing next-generation Mn-rich cathode materials by fine-tuning the synthesis conditions as well as the composition and structure of LxS-LMNO electrodes.