Topological Design of Highly Conductive Weakly Solvating Electrolytes for Ultrastable Sodium Metal Batteries Operating at −60 °C and Below

Abstract

Weakly solvating electrolytes (WSE) can favor reversible Na batteries at −40 °C for some extreme applications because of the low desolvation energy. However, it is challenging to enable reversible Na batteries at lower temperatures. Herein, we uncover that the low ionic conductivity of WSE reduces reaction kinetics at −60 °C. Accordingly, a highly conductive weakly solvating electrolyte (HCWSE) is designed by introducing additives of strongly solvating solvents and the dilution of NaPF₆. The additive can dominate the solvation sheath, increase the dissociation of NaPF₆ and the fluidity of the electrolyte, and thus greatly improve the ionic conductivity. Furthermore, the binding energy between Na⁺ and solvents is proposed as a descriptor to determine the solvating power of solvents, based on which a series of ultralow-temperature HCWSEs have been topologically designed by facilely introducing strong-solvation ether additives into the weak-solvation solvents. As a demonstration, the HCWSE showcases the long cycling of Na||Na cell at −60 °C with an overpotential of 42 mV under 1 mA cm^–2 for 1200 h. The Na||NNFM (Na_0.75Ni_0.25Fe_0.25Mn_0.5O₂) cell exhibits a reversible capacity of 79.2 mAh g^–1 after 160 cycles. The cells also achieve impressive performances at −70 °C.