Re-examining the effect of ZnO on nanosized 5 V LiNi0.5Mn1.5O4 spinel: An effective procedure for enhancing its rate capability at room and high temperatures

Based on the known effectiveness of ZnO as a hypothetical coating, its ability to expand the rate capability of nanosized LiNi0.5Mn1.5O4 was examined. The additive was characterized by X-ray photoelectron spectroscopy (XPS) analysis. The intensity of the 2p peak for Zn in the XPS concentration depth profiles remained unchanged over a long sputtering period; therefore, rather than forming a coating layer over the spinel particles, as commonly described in the literature, ZnO seemingly deposits as uniformly dispersed nanoparticles in the bulk material. TEM images were consistent with this alternative model for explaining where the oxide is located. ZnO-treated spinel cycled at low rates (C/4) at room temperature was found to exhibit an increase in delivered capacity (more than a 10%) with good capacity retention. The increase was less marked at high cycling rates (8C). Cells cycled at 50 °C exhibited similarly improved properties; however, the increase in capacity at low cycling rates was somewhat lower. These changes are explained in the light of the results obtained by treating the spinel with HF at room temperature: ZnO clearly hindered dissolution of the spinel. In fact, ZnO partially protects the spinel from the attack of HF traces in LiPF6 based electrolyte during prolonged contact between the two. This protective effect, however, is insubstantial at high charge/discharge rates, where contact between the spinel and electrolyte is shorter.