The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na MnO nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na MnO nanowall arrays can be extended to 0-1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g . The extended potential window for the Na MnO electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon-coated Fe O nanorod arrays are synthesized as the anode and can stably work in a negative potential window of -1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na MnO nanowall arrays as the cathode and carbon-coated Fe O nanorod arrays as the anode. In particular, the 2.6 V Na MnO //Fe O @C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg as well as excellent rate capability and cycle performance, outperforming previously reported MnO -based supercapacitors. This work provides new opportunities for developing high-voltage aqueous asymmetric supercapacitors with further increased energy density.

A surface-modified Co O ultrathin nanosheet (denoted as PCO) is reported via controllable phosphate ion functionalization for pseudocapacitors. An energy density of 71.6 W h kg (at 1500 W kg ) is achieved by the PCO-based pseudocapacitor. The unique porous nanosheet morphology, high surface reactivity, and fast electrode kinetics of PCO are found to be responsible for the enhanced pseudocapacitive performance.

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The insertion/deinsertion mechanism enables plenty of charge-storage sites in the bulk phase to be accessible to intercalated ions, giving rise to at least one more order of magnitude higher energy density than the adsorption/desorption mechanism. However, the sluggish ion diffusion in the bulk phase leads to several orders of magnitude slower charge-transport kinetics. An ideal energy-storage device should possess high power density and large energy density simultaneously. Herein, surface-modified Fe O quantum dots anchored on graphene nanosheets are developed and exhibit greatly enhanced pseudocapacitance via fast dual-ion-involved redox reactions with both large specific capacity and fast charge/discharge capability. By using an aqueous Na SO electrolyte, the oxygen-vacancy-tuned Fe O surface greatly enhances the absorption of SO anions that majorly increase the surface pseudocapacitance. Significantly, the Fe O -based electrode delivers a high specific capacity of 749 C g at 5 mV s and retains 290 C g at an ultrahigh scan rate of 3.2 V s . With a novel dual-electrolyte design, a 2 V Fe O /Na SO //MnO /Na SO asymmetric supercapacitor is constructed, delivering a high energy density of 75 W h kg at a power density of 3125 W kg .

Fiber‐shaped rechargeable batteries hold promise as the next‐generation energy storage devices for wearable electronics. However, their application is severely hindered by the difficulty in fabrication of robust fiber‐like electrodes with promising electrochemical performance. Herein, yolk–shell NiS2 nanoparticles embedded in porous carbon fibers (NiS2⊂PCF) are successfully fabricated and developed as high‐performance fiber electrodes for sodium storage. Benefiting from the robust embedded structure, 3D porous and conductive carbon network, and yolk–shell NiS2 nanoparticles, the as‐prepared NiS2⊂PCF fiber electrode achieves a high reversible capacity of about 679 mA h g−1 at 0.1 C, outstanding rate capability (245 mA h g−1 at 10 C), and ultrastable cycle performance with 76% capacity retention over 5000 cycles at 5 C. Notably, a flexible fiber‐shaped sodium battery is assembled, and high reversible capacity is kept at different bending states. This work offers a new electrode‐design paradigm toward novel carbon fiber electrodes embedded with transition metal oxides/sulfides/phosphides for application in flexible energy storage devices.