Secondary-Phase-Induced Charge-Discharge Performance Enhancement of Co-Free High Entropy Spinel Oxide Electrodes for Li-Ion Batteries

High entropy oxide (HEO) has emerged as a new class of anode material for Li-ion batteries (LIBs) by offering infinite possibilities to tailor the charge-discharge properties. While the advantages of single-phase HEO anodes are realized, the effects of a secondary phase are overlooked. In this study, two kinds of Co-free HEOs are prepared, containing Cr, Mn, Fe, Ni, and Zn, for use as LIB anodes. One is a plain cubic-structure high entropy spinel oxide HESO (C) prepared using a solvothermal method. The other HESO (C+T) contains an extra secondary phase of tetragonal spinel oxide and is prepared using a hydrothermal method. It is demonstrated that the secondary tetragonal spinel phase introduces phase boundaries and defects/oxygen vacancies within HESO (C+T), which improve the redox kinetics and reversibility during electrode lithiation/delithiation. Density functional theory calculation is performed to assess the phase stability of cubic spinel, tetragonal spinel, and rock-salt structures, and validate the cycling stability of the electrodes upon charging-discharging. The secondary-phase-induced rate capability and cyclability enhancement of HEO electrodes are for the first time demonstrated. A HESO (C+T)||LiNi0.8Co0.1Mn0.1O2 full cell is assembled and evaluated, showing a promising gravimetric energy density of approximate to 610 Wh kg(-1) based on electrode-active materials.

Automated summary

High entropy oxide (HEO) is a new type of anode material for Li-ion batteries that allows for customized charge-discharge properties. However, the impact of a secondary phase has been overlooked. In this study, two types of Co-free HEOs were prepared, one with a plain cubic structure and the other with an additional tetragonal spinel oxide phase. It was found that the secondary phase improved the redox kinetics and reversibility of the electrodes. The cycling stability of the electrodes was also validated using density functional theory calculations. This study demonstrates for the first time the enhanced rate capability and cyclability of HEO electrodes induced by a secondary phase.