Localized "water-in-salt" (LWIS) electrolytes are promising candidates for the next generation of high-voltage aqueous electrolytes with low viscosity/salt beyond high-salt electrolytes. An effective yet high-function diluent mainly determines the properties of LWIS electrolytes, being a key issue. Herein, the donor number of solvents is identified to serve as a descriptor of interaction intensity between solvents and salts to screen the organic diluents having few impacts on the solvation microenvironment and intrinsic properties of the original high-salt electrolyte, further leading to the construction of a novel low-viscosity electrolyte with a low dosage of the LiNO3 salt and well-kept intrinsic Li+-NO3--H2O clusters. Nonsolvating diluents, especially acetonitrile (AN) that has never been reported previously, are presented with the capability of constructing a LWIS electrolyte with nonflammability, electrode-philic features, lower viscosity, decreased salt dosage, and a greatly enhanced ion diffusion coefficient by about 280 times. This strongly relies on a huge difference of about 5000 times in coordination and solubility between AN and H2O toward LiNO3 (0.05 vs 25 mol kgsolvent-1) and the moderate interaction between AN and H2O. Multi-spectroscopic techniques and molecular dynamics simulations uncover the solvation chemistry at the microscopic level and the interplay among cations, anions, and H2O without/with AN. The identified unique diluting and nonsolvating effects of AN reveal well-maintained cation-anion-H2O clusters and enhanced intermolecular hydrogen bonding between AN and H2O, further reinforcing the H2O stability and expanding the voltage window up to 3.28 V. This is a breakthrough that is far beyond high-viscosity/salt electrolytes for high-voltage and high-rate aqueous supercapacitors.