A novel monolithic carbon sponge is prepared by a ZnCl₂-assisted carbonization of commercial melamine foam (MF) at 800 °C in a N₂ atmosphere. The as-prepared nitrogen/oxygen co-doped carbon sponges (NOCSs) can be directly applied as monolithic electrodes for supercapacitors without using any additives. It is found that ZnCl₂ activation plays an important role in improving the supercapacitive behavior of NOCSs. Compared with the NOCS without ZnCl₂ activation treatment, the optimal NOCS-1/10, obtained with a ZnCl₂/MF weight ratio of 1 : 10, has the best capacitive performance due to its unique porous architecture, rich pseudocapacitance-active species, and good electron/ion transfer feature. It exhibits a high specific capacitance (242 F g⁻¹ at a current density of 0.5 A g⁻¹ and 325 F g⁻¹ at a scan rate of 10 mV s⁻¹) and good electrochemical cyclability (97% after 10 000 cycles at 5.0 A g⁻¹). When being further assembled into an all-solid-state supercapacitor (PVA/KOH electrolyte), it can deliver an energy density of 4.33 W h kg⁻¹ at a power density of 250 W kg⁻¹ and 3.13 W h kg⁻¹ at a power density of 3 kW kg⁻¹.

Rechargeable aqueous zinc-ion batteries have shown great potential for grid-scale applications owing to their high safety, low cost and sustainability. However, they are still limited by the lack of suitable cathode materials with high energy density and satisfactory cycling stability. Herein, a highly reversible zinc-ion battery system with (NH₄)₂V₆O₁₆·1.5H₂O nanobelts as the cathode materials and aqueous ZnSO₄ solution as the electrolyte has been developed. The (NH₄)₂V₆O₁₆·1.5H₂O nanobelts deliver a superior reversible specific capacity of 479 mA h g⁻¹ at 0.1 A g⁻¹ with a desirable energy density of 371.5 W h kg⁻¹ and exhibit an impressive cycling stability, i.e. 152 mA h g⁻¹ retained after 3000 cycles at 5 A g⁻¹. The high energy density and long-term cycling stability make (NH₄)₂V₆O₁₆·1.5H₂O a promising candidate for grid-scale energy storage. This work could be extended to choose and construct other ideal high-performance cathode materials for ZIBs and other multivalent metal ion storage systems.