Synthesis of γ-Phase Electrolytic Manganese Dioxide for Use in Aqueous Zinc Batteries

Interest in manganese dioxide for use in electrochemical energy storage systems has increased in recent years because it is environmentally friendly, low cost, and enables safer operating conditions. γ-phase electrolytic manganese dioxide (EMD) is of particular interest due to its larger 1x2 tunneling structure that enables greater ion transport and higher specific capacity. MnO _₂ can be synthesized from a redox reaction of KMnO _₂ and MnCl _₂ and the product can be annealed at 200°C for 32 hours for a phase transformation into γ-phase that is verified with XRD. EMD slurry is used to laminate carbon paper heat treated to 350°C for 8 hours to produce the cathode electrode material assembled into coin cells. Rate studies of EMD using CMC binder showed a specific capacity of ~120 mAhg ^ˉ¹ at 0.5C rate, approximately two times greater when compared to EMD with PVDF binder of 60 mAhg ^ˉ¹ at 0.5C rate. Cycling repeatability was similar between the two binders used with little to no deviation between charge/discharge cycles. Contact angle measurements of electrodes containing CMC produced a highly hydrophilic surface compared to electrodes with PVDF and is most likely responsible for the observed increase in capacity. EMD has the potential for use in electrochemical energy storage systems once an appropriate dopant used to stabilize the γ-phase can be determined.