Synergistic Benefit of Atomic to Nanoscale Disorder: Electrochemistry and Structural Evolution of Hybrid Layered-Spinel High Entropy Oxide Cathode Material

The high entropy design of cathode materials can be used to advance materials properties. Here the strategy is applied to Li- and Mn-rich oxides, where voltage fade has been an ongoing issue that has limited their practical implementation as lithium-ion battery cathode materials. Notably, the compositional complexities of high entropy oxides (HEOs) pose challenges to understanding the structure–function relationship. Bulk to atomic level characterization of two HEOs with similar nominal compositions, Li_xMn_0.6Ni_0.1Co_0.1Fe_0.1Al_0.1O_y, where x = 1.5 for HEO-L and x = 0.5 for HEO-H, was accomplished herein to probe structural and chemical changes and to understand how high entropy designs at the atomic level and (dis)ordered phase compositions at the nanoscale contribute to the observed electrochemistry. X-ray diffraction and electron microscopy were used to probe changes in the crystal lattice and microstructure. HEO-L was found to display a higher initial capacity but underwent continuous structural evolution from its layered phase to a spinel-like phase. Spectroscopic characterization revealed the formation of oxygen vacancies and reduction of the Mn, Ni, and Co transition metals. The depreciating voltage profile was attributed to metal reduction caused by oxygen loss after electrochemical cycling, resulting in the activation of a lower voltage redox couple. In contrast, HEO-H exhibited a small lattice expansion but maintained its layered-spinel hybrid structure after 100 cycles with minimal capacity and voltage fade. Limited oxygen loss and metal ion reduction were also observed. The stable electrochemistry was attributed to the structural stability of HEO-H, resulting in reversible phase transitions between the cubic spinel and tetragonal phase.