Insight into the cation migration and surface structural evolution of spinel LiNi0.5Mn1.5O4 cathode material for lithium-ion batteries

Abstract

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is considered to be a promising cathode material for electric vehicles (EV) and hybrid electric vehicles (HEVs). Nevertheless, the structural transformation under a deep discharge voltage is still ambiguity. Herein, we report detailed studies of structural change of LNMO cathode under a wide voltage range of 5.0–1.5 V cycling by using synchrotron-based X-ray absorption spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The fundamental findings demonstrate that LNMO would undergo a complex phase transformation from cubic to tetragonal during deep discharge to a voltage lower than 3 V. It gives rise to a further reduction of Mn4+ to Mn3+ and this serious structural distortion leads to the formation of Li2Mn2O4-type tetragonal phase with space group of I41/amd, which is also confirmed by XANES simulations and density functional theory (DFT) calculations. Meanwhile, migration of Ni ions toward the surface from bulk lattice, leading to the rock-salt NiO phase formation with a space group of Fm-3 m. Finally, these observed evolutions on the surface are associated to the fast capacity fading for LNMO under such a wide voltage range. This work provides insight into understanding the capacity degradation mechanism of spinel cathodes and would inspire us to further design high-performance cathode materials for advanced lithium-ion batteries.