Compressing Carbon Nanocages by Capillarity for Optimizing Porous Structures toward Ultrahigh‐Volumetric‐Performance Supercapacitors

High volumetric energy density combined with high power density is highly desired for electrical double‐layer capacitors. Usually the volumetric performance is improved by compressing carbon material to increase density but at the much expense of power density due to the deviation of the compressed porous structure from the ideal one. Herein the authors report an efficient approach to increase the density and optimize the porous structure by collapsing the carbon nanocages via capillarity. Three samples with decreasing sizes of meso‐ and macropores provide us an ideal model system to demonstrate the correlation of volumetric performance with porous structure. The results indicate that reducing the surplus macropores and, more importantly, the surplus mesopores is an efficient strategy to enhance the volumetric energy density while keeping the high power density. The optimized sample achieves a record‐high stack volumetric energy density of 73 Wh L−1 in ionic liquid with superb power density and cycling stability. Collapsed carbon nanocages are obtained by making use of capillarity, featuring high density, large specific surface area, optimized pore distribution, and high conductivity. As supercapacitor electrode materials, such unique characteristics ensure sufficient space and efficient channels for charge storage and transport, leading to top‐level volumetric performances in both aqueous and ionic liquid electrolytes, especially the state‐of‐the‐art stack volumetric energy density with high stability in ionic liquid.