Ultrasmall ZnMn₂O₄ Cathodes for High-Energy and High-Power Aqueous Zinc-Ion Secondary Batteries

Current lithium-ion batteries are widely used for various energy storage devices. However, improvement of LIB performances such as high energy density, cost effective, and safety is urgently required, with increasing demand for lithium-ion batteries (LIBs) as energy storage devices. Rechargeable zinc-ion batteries, using aqueous electrolytes with mild acidity and zinc metal anodes, are promising for stationary power supply due to their environment-friendly, cost-effectiveness, and safety. For high energy density cathode materials, vanadium oxides and manganese oxides were widely developed. However, suitable cathode materials with high capacity and long cycling stability are still progressing. Spinel-type ZnMn₂O₄ is reported as highly cyclable cathode materials, though only one-electron reaction per Mn is available.^[1] Theoretically, two-elecctron reactions using Mn^2+/4+can be utilized in spinel cathode materials.^[2] Here, we apply the ultrasmall ZnMn₂O₄ spinel nanoparticle-graphene composite as cathode material to accelerate the fast Zn intercalation/deintercalation.<br/>The ultrasmall ZnMn₂O₄ spinel was prepared by alcohol reduction process.^[3] The Rietveld analysis of the X-ray diffraction (XRD) pattern suggests the obtained nanoparticles as tetragonal spinel. X-ray absorption spectroscopy (XAS) also shows the valence state of Mn as 3+. According to the Transmission Electron Microscope (TEM) image, primary particles of 5–10 nm were strongly aggregated into micron size secondary particles. The aggregation is effectively suppressed by dispersing graphene into the reaction solution. The ZnMn₂O₄-graphene composite cathode exhibits the reversible capacity of 450 mAh g^–1 at 100 mA g^–1, a high value because of a reversible two-electron reaction, while ZnMn₂O₄ nanoparticle shows only 300 mAh g^–1. Downsizing particles is an effective way to enhance its specific capacity.<br/><br/>[1] L. Chen <i>et al.</i>, <i>J. Power Sources</i> <b>425</b>, 162 (2019).<br/>[2] S. Okamoto <i>et al.</i>,<i> Adv. Sci.</i> <b>2</b>, 1500072 (2015).<br/>[3] H. Kobayashi <i>et al</i>.,<i> RSC Adv.</i> <b>9</b>, 36434 (2019).