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

Abstract 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)||LiNi 0.8 Co 0.1 Mn 0.1 O 2 full cell is assembled and evaluated, showing a promising gravimetric energy density of ≈610 Wh kg −1 based on electrode‐active materials.