Synthesis of amorphous nickel-cobalt-manganese hydroxides for supercapacitor-battery hybrid energy storage system

In this work, amorphous nickel-cobalt-manganese hydroxide (NiCoMn-OH) was hydrothermally synthesized using a mixed solvent strategy and used as positive electrode materials for supercapacitor-battery hybrid energy storage system. The experimental results show that the mixed solvent is indispensable to form the amorphous phase of NiCoMn-OH, which exhibits significantly improved electrochemical activity and rate capability in comparison with the crystalline counterpart because of more grain boundaries and ion diffusion channels in the former phase. A strong synergy between the transition metal ions in the amorphous NiCoMn-OH is found to significantly contribute to the electrochemical activity, rate capability and cycling stability. In addition to battery behavior, the amorphous NiCoMn-OH exhibits pseudocapacitive behavior, which contributes approximately 40% to the total energy storage capacity. The pseudocapacitive property significantly enhances the rate performance. The robust synthesis method described in this paper was also used to fabricate the NiCoMn-OH porous network on Ni foam, which shows a specific capacity close to its theoretical value, indicating a complete utilization of the electroactive material. Furthermore, a supercapacitor-battery hybrid cell fabricated with the amorphous NiCoMn-OH as the positive electrode and reduced graphene oxide (RGO) as the negative electrode exhibits both high-energy and high-power performances with a specific energy of 42.8 Wh kg(-1) at a specific power of 749Wkg(-1) or a specific energy of 19.9 Wh kg(-1) at a specific power of 20.9 kW kg(-1).