Small congeneric solvents for practical sodium metal batteries

Abstract

Recent advances in electrolyte engineering have boosted the cycling stability of rechargeable metal batteries by weakening cation solvation, leading to more cation-anion pairing/aggregation, but compromising their kinetic properties. Here, we report an electrolyte design with small congeneric solvents exhibiting up to ∼20× higher ionic conductivity than their larger counterparts, ∼0.4 V higher electrode potential, and balanced redox kinetics relative to diffusion—contributing to superior high-rate plating/stripping reversibility. Solvents with similar structures as N , N -dimethyltrifluoromethane sulfonamide (DMTMSA) were generated and screened using a molecular design and a three-dimensional Zernike model, respectively, revealing the essential contributions of solvent geometry, affinity, and reactivity to electrochemical performance. A model small solvent, N , N -dimethylsulfamoyl fluoride demonstrates extraordinarily stable cycling performance against a high-voltage NaNi0.33Mn0.33Fe0.33O2 cathode and a Na-metal anode. Spectroscopic analysis and molecular dynamics simulations reflect the corresponding changes in ion-dipole interaction and solvation structures. Moreover, the small congeneric solvent principle for electrolyte design enables the development of other practical alkali-metal batteries and electrolyte systems.