Investigation of the Effect of Extra Lithium Addition and Postannealing on the Electrochemical Performance of High-Voltage Spinel LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> Cathode Material

The LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> (LNMO) spinel is an attractive cathode candidate for next generation lithium-ion batteries as it offers high power and energy density. In this paper, the effects of extra amounts of lithium addition and postannealing process on the physicochemical and electrochemical properties of the spherical LNMO material were investigated. The experimental results show that the amount of lithium and the postannealing process have significant impacts on the Mn<sup>3+</sup> content, phase impurity (rock-salt phase) and phase structures (<i>Fd</i>3<i>m</i> and <i>P</i>4<sub>3</sub>32) of the spherical LNMO cathode materials, so as their electrochemical performance. In particular, the phase transition from <i>Fd</i>3<i>m</i> to <i>P</i>4<sub>3</sub>32 and the Mn<sup>3+</sup> content of the LNMO spinels were found to be adjusted by lithium additions and the postannealing process. With the presence of Mn<sup>3+</sup>, the absence of the impurity phase (rock-salt phase) and the cation ordering in the spinels, the electrochemical rate performance and capacity retention of the products could be significantly improved. In a half cell test, LNMO cathode material with 5% of lithium excess (based on theoretical formula calculation) displays a high specific discharge capacity of 123 mAh g<sup>–1</sup> at 2 C rate with excellent capacity retention of 84% after 500 cycles at 55 °C. All these findings show the important roles of the synergic effects of Mn<sup>3+</sup> content, phase impurity (rock-salt phase) and phase structures (<i>Fd3m</i> and <i>P</i>4<sub>3</sub>32) on the electrochemical performance improvement of LNMO-based cathode materials, which will guide the preparation of LNMO-based cathode material with excellent electrochemical performance.