Insight into the atomic structure of high-voltage spinel Lini_0.5mn_1.5o₄ cathode material in the first cycle

Application of high-voltage spinel LiNi_0.5Mn_1.5O₄ cathode material is the closest and the most realistic approach to meeting the midterm goal of lithium-ion batteries for electric vehicles (EVs) and plug-in hybrid electric vehicles (HEVs). However, this application has been hampered by long-standing issues, such as capacity degradation and poor first-cycle Coulombic efficiency of LiNi_0.5Mn_1.5O₄ cathode material. Although it is well-known that the structure of LiNi_0.5Mn_1.5O₄ into which Li ions are reversibly intercalated plays a critical role in the above issues, performance degradation related to structural changes, particularly in the first cycle, are not fully understood. Here, we report detailed investigations of local atomic-level and average structure of LiNi_0.5Mn_1.5O₄ during first cycle (3.5-4.9 V) at room temperature. We observed two types of local atomic-level migration of transition metals (TM) ions in the cathode of a well-prepared LiNi_0.5Mn_1.5O₄//Li half-cell during first charge via an aberration-corrected scanning transmission electron microscopy (STEM). Surface regions (2 nm) of the cycled LiNi_0.5Mn_1.5O₄ particles show migration of TM ions into tetrahedral Li sites to form a Mn₃O₄-like structure. However, subsurface regions of the cycled particles exhibit migration of TM ions into empty octahedral sites to form a rocksalt-like structure. The migration of these TM ions are closely related to dissolution of Ni/Mn ions and building-up of charge transfer impedance, which contribute significantly to the capacity degradation and the poor first-cycle Coulombic efficiency of spinel LiNi_0.5Mn_1.5O₄ cathode material. Accordingly, we provide suggestions of effective stabilization of LiNi_0.5Mn_1.5O₄ structure to obtain better electrochemical performance.