Revealing Capacitive Slope Capacity of Open Pore Carbon for Ultrahigh-Rate Sodium-Ion Storage.

Abstract

Disordered carbon materials exhibit slope and plateau Na+ storage capacities. Compared with the well-investigated plateau capacities from the intercalation/filling mechanism, the "capacitive" slope storage remains relatively unidentified. Herein, the effects of open pore sizes and solid electrolyte interface (SEI) layers are investigated on slope capacity and thus categorize the "capacitive" behaviors into three distinct scenarios. Conventionally, the complete desolvation and pseudocapacitive Na+ slope capacities arise from the sieving of ethylene carbonate (EC)-SEI layers. Differently, an electric double-layer (EDL) capacitive adsorption of partially desolvated Na+ ions is revealed in large open pores of 0.5-2 nm in the diethylene glycol dimethyl ether (DGDE) electrolyte in a large potential window of 3-0.01 V vs Na+/Na. Remarkably, DGDE-SEI does not block open pores or sieve solvation shells, resulting in an exceptionally high initial coulombic efficiency of 92.4% and ultrahigh-rate capabilities. When the open pore size decreases to <0.5 nm (accessible to CO2 but inaccessible to Ar) or becomes closed pores in DGDE electrolyte, the narrow pores themselves sieve solvation shells and subsequent pseudocapacitive Na+ storage for slope capacity. The EDL capacitive slope capacity of open porous carbon highlights the ultrafast (dis)charging abilities and stable cycles, which are highly promising for high-power sodium-ion storage devices.