A supercapacitor, a safe and durable electrical energy storage device with fast charge–discharge capability, will achieve more widespread use if the specific energy can be improved. However, current understanding of pore characteristic effects on gravimetric capacitance has limited the development of electrode materials. We derive a model of ion packing in cylindrical nanopores, and it quantitatively reveals the significant impact of pore geometric characteristics on the gravimetric capacitance in neat ionic liquid, which is confirmed experimentally using a series of sponge-like carbons (carbon nanosponge). With the favourable ion packing proposed by the model, the electrode using the carbon nanosponge as an active material delivered double-layer capacitances of 290 F g−1 (20 °C) and 387 F g−1 (60 °C) with an operating cell voltage of 4 V. This study also provides systematical strategies for rational design of various carbon materials and ionic liquids by optimized ion packing for ultrahigh gravimetric capacitance.
