Effects of Secondary Phase on Electrochemical Performance of Co-Free High Entropy Spinel Oxide Anodes for Lithium-Ion Batteries

Abstract

High entropy oxide (HEO) anode materials have recently gained significant attention in lithium-ion batteries (LIBs) due to their exceptional cycling stability, high specific capacity, and unique adjustable properties. The exclusive feature of HEO allows the use of endless element combinations to create new electrode materials with exciting and unexplored properties. In the context of Li-ion battery anode materials, unlike conventional thinking, where a single-phase high entropy oxide (HEO) is considered sufficient and necessary for delivering good electrochemical performance, we report here that this is not a necessary condition. This study demonstrates the advantages of a secondary phase in HEO. Two types of Co-free high entropy spinel oxide (HESO) are synthesized using solvothermal and hydrothermal methods, resulting in single-phase cubic HESO (HESO (C)) and binary-phase (cubic + tetragonal) HESO (HESO (C+T)), respectively. The secondary tetragonal spinel phase introduces phase boundaries and defects/oxygen vacancies in HESO (C+T), which can improve the redox kinetics and reversibility during electrode lithiation/delithiation. The minor tetragonal phase in the HESO (C+T) anode sustains after cycling, largely reducing the morphology deterioration due to improved reversibility, thus contributing to enhanced cyclability. 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 HEO electrodes upon lithiation/delithiation. This is the first time that the effects of a secondary phase in HEO on the electrochemical properties have been reported. A HESO (C+T)||LiNi0.8Co0.1Mn0.1O2 full cell delivers an energy density of ~610 Wh kg−1, demonstrating a great potential of the HESO (C+T) for use in LIBs. The results provide useful guidelines for designing new high-performance anode materials for LIBs.