Charging behavior of ZnMn2O4 and LiMn2O4 in a zinc- and lithium-ion battery: an ab initio study

Zinc-ion batteries (ZIB) employing aqueous electrolytes have emerged as viable successors to the widely used lithium-ion batteries (LIBs), attributed to their cost-effectiveness, environmental friendliness, and intrinsic safety features. Despite these advantages, the performance of ZIBs is significantly hindered by the scarcity of suitable cathode materials, positioning manganese zinc oxide (ZnMn2O4) as a potential solution. In this study, we describe the ZnMn2O4 (ZMO) compound focusing on its properties variations during Zn extraction and potential battery applications. For the sake of comparison, we also analyze the same properties of the LiMn2O4 in its tetragonal phase (TLMO), for the first time, motivated by a recent discovery that the substitution of Zn ions by Li in ZMO forms isostructural TLMO compound at room temperature. The study was conducted within the density functional theory (DFT) framework, where the structural, electronic, magnetic, electrochemical, and spectroscopic properties of ZMO and TLMO are investigated under various conditions. Although both systems crystallize in tetragonal structures, they demonstrate distinct electronic and magnetic properties due todifferent oxidation states of the Mn. The TLMO exhibits a narrower band gap compared to ZMO, indicating enhanced electrical conductivity. In addition, TLMO presented a lower diffusion energy barrier than ZMO, indicating better ionic conductivity. To evaluate the potential application of these materials in battery technologies, we further explored their volume changes during charging/discharging cycles, simulating Zn or Li ions extraction. TLMO underwent a significant volume contraction of 5.8% upon complete Li removal, while ZMO experienced a more pronounced contraction of 12.5% with full Zn removal. By adjusting ion extraction levels, it is possible to reduce these contractions, thereby approaching more viable battery applications. Furthermore, spectroscopy results provide insights into the electronic transitions and validate the computational findings, consolidating our understanding of the intrinsic properties of ZMO and TLMO.