Aqueous Supercapacitor with Ultrahigh Voltage Window Beyond 2.0 Volt

Supercapacitors (SCs), a new type of green electrochemical energy storage device with high power density and long-term durability, show great potential to replace nonrenewable energy sources. Benefiting from both economic and environmental advantages, aqueous SCs represent a fabulous prospect in many industries. However, the relative low voltage window of aqueous SCs hinders their further development. In the recent years, especially within the last five years, the significant achievements in aqueous SCs with ultrahigh voltage window (>2.0 V) have been reported. Herein, the effects of theoretical mechanisms on voltage window are first introduced, which provide the fundamental guidance to enlarge the voltage window of aqueous SCs. Subsequently, the strategies for constructing the aqueous SCs over 2.0 V are comprehensively summarized and classified into the electrode modification by structural engineering, metal cations doping and constructing advanced composites, and the electrolyte optimization by preparing Water in Salt and novel mixed electrolyte. Finally, via the discussion of current progresses and drawbacks of these >2.0 V aqueous SCs, their future development directions are proposed.