Synthesis And Electrochemical Performance Of Manganese Oxide Based Positive Electrodes For Rechargeable Aqueous Zinc Batteries

There is an increasing demand for environmentally friendly energy resources.The wind and solar systems provide good opportunities,but they supply energy intermittently and need to be coupled with energy storage systems.In comparison to the highly developed rechargeable lithium-ion batteries,aqueous zinc batteries possess better safety and lower cost.The zinc metal anode provides high theoretical capacity,low potential and good compatibility with water.Nevertheless,a suitable cathode is still under investigation.In this work,a few manganese oxide-based materials were synthesized and tested as the cathode materials for aqueous zinc batteries.Three crystal phases（α,β and δ）of MnO₂ were prepared by microwave-assisted hydrothermal/solvothermal synthesis.Theδ-MnO₂/PEG composite electrode was also obtained using partially exfoliated graphite（PEG）as the substrate.In addition,the Mn₃O₄ material was obtained at different temperatures and composited with reduced graphene oxide（rGO）.The crystal structures of the synthesized materials were characterized by X-ray diffraction（XRD）while the morphology was obtained by scanning electrode microscopy（SEM）.The manganese oxide-based materials were used as the cathode in zinc cells.The electrochemical performances were tested by cyclic voltammetry（CV）,galvanostatic charge discharge and electrochemical impedance spectroscopy.Among the three crystal structures,the β-MnO₂ shows the smallest charge transfer resistance and highest specific capacity.It delivers 182 mAh/g capacity at 100 mA/g during the first cycle,which increases to 248 mAh/g after 5 activation cycles.At the high current density of 1000 mA/g,97 mA/g capacity is still maintained.Using PEG as the substrate helps to improve the electrical conductivity of the electrode.The synthesis temperature of 160℃ is the most suitable for high performance Mn₃O₄ electrode.The performance is further improved by coupling with rGO,with the obtained Mn₃O₄/rGO electrode delivers 142 mAh/g capacity at the current density of 100 mA/g.The electrolyte composition also shows important effect on the electrochemical performance of manganese oxide electrodes.