Understanding the effects of surface reconstruction on the electrochemical cycling performance of the spinel LiNi0.5Mn1.5O4 cathode material at elevated temperatures

Abstract

Detailed investigation of the influence of surface modification using a typical oxide (TiO₂) on the electrochemical cycling performance of LiNi_0.5Mn_1.5O₄ at room temperature (25 °C) and elevated temperature (55 °C) is reported. This spinel cathode material is commonly surface-modified with various metal oxides to improve its electrochemical cycling performance in lithium ion batteries. However, the underlying mechanisms of such a treatment, with respect to the surface crystal structure and chemistry evolution, have remained unclear. Bare-LiNi_0.5Mn_1.5O₄ and TiO₂-modified LiNi_0.5Mn_1.5O₄ both show excellent cycling performance, i.e. almost no capacity retention and ∼99% coulombic efficiency for 150 cycles, at room temperature. However, at 55 °C the latter shows significantly better electrochemical cycling performance, with 93% capacity retention and ∼96% coulombic efficiency for 100 cycles, than the former with 70% capacity retention and ∼93% coulombic efficiency. Via advanced electron microscopy techniques, we observed that titanium ions migrated into the surface region of LiNi_0.5Mn_1.5O₄ during the surface modification process at high temperature and reconstructed the (1–3 nm) surface spinel structure into a rocksalt-like structure and the subsurface (several nanometers) into a pseudo-rocksalt-like structure. The reconstruction of the surface and subsurface of the LiNi_0.5Mn_1.5O₄ spinel cathode material mitigates not only the migration of Mn ions from the bulk into the electrolyte but also the formation of a solid state electrolyte interface, which plays a critical role in the improvement of electrochemical cycling performance at elevated temperatures.