Unlocking limited electric double-layer capacity via electrochemically-driven continuous partial desolvations in carbon nanopores

Electrochemical capacitors are primarily limited by the low electric double-layer storage capacity and narrow operating window that avoids the formation of solid electrolyte interface layers. Herein, we demonstrate that electric double-layer adsorption of solvated Na⁺ in carbon nanopores is achievable under a large offset potential of -2.95 V vs. potential of zero charge in diethylene-glycol-dimethyl-ether electrolyte, even accompanying with the as-formed solid electrolyte interface layers. The largely enlarged offset potential in situ drives the continued partial desolvations in carbon nanopores, which largely reduces the average solvation numbers from 2.1 to 0.6, leading to a high electric double-layer capacitance of 172 F g^-1, a high capacity of 508 C g^-1 and high initial coulombic efficiency of 92.2% at 0.1 A g^-1 (0.5 mA cm^-2), together with high-rate capability and long-term cycling stability. Thereby, such increased electric double-layer charge storage enables the redesign and assembly of sodium-ion capacitor pouch cells that display a high specific density of 40 Wh kg^-1 (on cell level) and 30,000 cycles at a fast (dis)charging rate of 51 C (20 mA cm^-2). Such sodium-ion capacitors are assembled without any pretreatments that are beneficial for scale-up fabrications in industry.