Deciphering Nanoscale Spatially Separated Effects in High‐Voltage Aqueous Electrolyte for High‐Rate Supercapacitors

ABSTRACT Localized “water‐in‐salt” (LWIS) electrolytes featuring low viscosity offer a promise for high‐voltage supercapacitors with rapid charge/discharge capability. However, the electrochemical performances of LWIS electrolytes rely heavily on the selection of the diluent. By screening 20 organic solvents, 1,3‐dioxolane (1,3‐DX) that featuring low viscosity and weak interactions with H ₂ O, is identified as an optimal spacer/diluent, which refines the electrolyte coordination structure through the formation of interconnected yet dynamic networks to dissipate dense crosslinked cation‐anion‐water clusters into loose isolated ones, triggering a spacial redistribution that alleviates ionic crowding while preserving the essential Na ⁺ −ClO ₄ ⁻ −H ₂ O coordination structure. This molecule‐scale design endows the resulting 8 mol kg ⁻¹ LWIS electrolyte with a high output voltage of 2.8 V in an asymmetric supercapacitor, low viscosity (7.33 mPa s), and a remarkable 1,622‐fold enhancement in ion diffusion coefficient at 25°C compared to the conventional 17 m WIS electrolyte. Accordingly, the electrolyte endows symmetric supercapacitors with exceptional charge‐discharge kinetics, reliable operation at −30°C, and robust cycling stability over 150 000 cycles. Our work presents a foundational guideline for diluent selection in advanced LWIS electrolytes, paving the way for aqueous supercapacitors that deliver concurrently high energy and power density.