Understanding Transition-Metal Dissolution Behavior in LiNi0.5Mn1.5O4 High-Voltage Spinel for Lithium Ion Batteries

The high-voltage LiNi0.5Mn1.5O4 (LNMO) spinel is a promising candidate for a positive electrode in lithium ion batteries, but LNMO/graphite full-cells display severe capacity fading issues due to Mn dissolution. In this study, the dissolution behaviors of Mn and Ni were examined systematically under various conditions such as state of charge (SOC), temperature, storage time, and crystal structure of LNMO. In addition, surfaces of calendar- or cycle-aged LNMO and graphite electrodes were analyzed by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), or time-of-flight secondary ion mass spectrometry (TOF-SIMS). The chemical composition of aged electrolyte was determined by gas chromatography (GC) analysis after storage of LNMO electrodes under different conditions. It was found that the amounts of dissolved Mn and Ni and diethyl ether, a decomposition product of diethyl carbonate (DEC) in electrolyte, increased with SOC, temperature, and storage time. The decomposition of electrolyte can be explained, in part, by the self-discharge behavior of LNMO, which promotes electrolyte oxidation. Additional HF is believed to be generated during the formation of diethyl ether (via dehydration reaction from EtOH, another decomposition product of DEC), which accelerates Mn and Ni dissolution from LNMO. In addition, various reaction products that form as a result of Mn and Ni dissolution, such as LiF, MnF2, NiF2, and polymerized organic species, were found on the surface of LNMO electrodes, which will increase battery-cell impedance.