With the rapid development of portable electronic devices, electrified transportation, and renewable energy conversion, there has been an increasing and urgent demand for advanced energy storage devices [1], [2], [3]. Electrochemical double-layer capacitors (EDLCs), also known as supercapacitors, owing to their high power density (> 10 kW Kg−1), rapid charging/discharging rate, ultralong cycle lifetime and wide operating temperatures, have attracted tremendous attention. However, compared with widely used batteries, commercial supercapacitors suffer from a low energy density [4], [5], [6], [7]. Especially, the energy density at a high power density is too low to meet the requirement of practical usage [8].

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A high cell operating voltage provides both high energy density (E = 1/2 CV2) and power density (P = V2/4Rs) [7,9]. Thus, the current trend in the development of supercapacitor involves the switch from an aqueous electrolyte with low operating voltage of 1 V due to the thermodynamic decomposition of water, to an organic electrolyte which allows a much higher voltage window of about 2.5 V [9], [10], [11], [12], [13]. However, the much bigger size of organic ions arises new challenge to the pore size engineering of electrode materials: pore sizes above 0.4 nm can be active in electrochemical double layer charging in aqueous solution [14], while the optimal pore size in organic electrolytes is as large as 0.8 nm [15]. Moreover, if the carbon is exclusively microporous, the equivalent resistance and diffusion resistance will be rather high due to ions trapping effects [16]. Thus, the existence of a certain amount of mesopores (i.e., pores between 2 and 50 nm) is necessary to meet the requirement of fast ion diffusion [9,15,17,18].

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