Nickel-based materials: Toward practical application of the aqueous hybrid supercapacitors

Aqueous hybrid supercapacitors (AHSCs) consisting of a high-capacity battery-type electrode and a high-rate capacitive-type electrode can be promising candidates for next-generation supercapacitors due to their excellent energy density and high safety. Nonetheless, battery-type electrodes store charges through the bulk-phase faradic reaction, which is sluggish compared to capacitive-type electrodes and render volume expansion of the materials during cycling, resulting in degraded rate capability and short cycling lifespans (rarely above 3000 cycles). Nickel-based materials are the typical battery-type electrode materials in aqueous KOH electrolytes that store energy through the reversible redox between different valence states of Ni ions, and have been extensively studied in recent years because of their high theoretical capacities, sufficient natural abundance, and low costs. The achievements and obstacles of nickel-based materials are essential for developing nickel-based materials and other battery-type electrodes in AHSCs. Considering this, the critical progress and developing trends of nickelbased materials applied in AHSCs are thoroughly summarized. The current opinions on the energy storage mechanism of a general electrode and the specific reaction mechanism of nickel-based materials in AHSCs are concluded. Also, recent advancements in facile synthesis, design strategies, and electrochemical performances are discussed. Finally, the developing trends, crucial challenges, and possible solutions regarding nickel-based materials and AHSCs are presented for commercialization.