There has been enormous interest in energy storage devices in recent years, with the emergence of “water-in-salt” based electrolytes coming to the fore in the context of supercapacitors, and lithium-ion batteries. In fact, the “water-in-salt” electrolyte exhibits a wide voltage window; unfortunately, it shows a low intrinsic wettability on the carbon surface, exhibiting hydrophobic properties on the typical separator. In this work, we introduced carboxylate-modified cellulose for utilization as a novel hydrophilic separator in energy storage devices, especially “water-in-salt” electrolytes. The cellulose was modified through the esterification with succinic anhydride followed by microfluidization to obtain cellulose succinate nanofiber (SCNF). The SCNF increases negative surface charges, enhancing the strong electrostatic repulsion between fibrils and facilitating ion transportation. We successfully fabricated the SCNF separators with a high porosity (60%) and the carboxylate content of SCNF (∼3.9 mmol g−1), giving a hydrophilic surface. The applicability of the separator was demonstrated in “water-in-salt” electrolyte, which shows relatively hydrophobic on the commercial separator. Interestingly, the prepared SCNF separator can improve capacitive properties by reducing the cell resistance. It is found that the capacitance can be enhanced up to 2 to 7 times when compared with the supercapacitor fabricated using a commercial separator. This work could strongly advance the research and development of SCNF separators, and “water-in-salt” based electrolytes in the context of energy storage applications.