Achieving Ultrahigh Volumetric Energy Storage by Compressing Nitrogen and Sulfur Dual‐Doped Carbon Nanocages via Capillarity

Abstract High volumetric performance is a challenging issue for carbon‐based electrical double‐layer capacitors (EDLCs). Herein, collapsed N,S dual‐doped carbon nanocages (cNS‐CNC) are constructed by simple capillary compression, which eliminates the surplus meso‐ and macropores, leading to a much increased density only at the slight expense of specific surface area. The N,S dual‐doping induces strong polarity of the carbon surface, and thus much improves the wettability and charge transfer. The synergism of the high density, large ion‐accessible surface area, and fast charge transfer leads to state‐of‐the‐art volumetric performance under the premise of high rate capability. At a current density of 50 A g⁻¹, the optimized cNS‐CNC delivers a high volumetric capacitance of 243 and 199 F cm⁻³ in KOH and EMIMBF₄ electrolyte, with high energy density of 7.9 and 93.4 Wh L⁻¹, respectively. A top‐level stack volumetric energy density of 75.3 Wh L⁻¹ (at power density of 0.7 kW L⁻¹) and a maximal stack volumetric power density of 112 kW L⁻¹ (at energy density of 18.8 Wh L⁻¹) are achieved in EMIMBF₄, comparable to the lead–acid battery in energy density but better in power density with 2–3 orders. This study demonstrates an efficient strategy to design carbon‐based materials for high‐volumetric‐performance EDLCs with wide practical applications.