Solvation Sheath Exchange with Weak-Solvent Molecules Enables the Development of Sodium Dual-Ion Pouch Cells

This study introduces a new electrolyte formulation enabling low-potential intercalation of PF 6 – anions. The electrolyte consists of an optimized mixture of ethyl methyl carbonate (EMC, a primary solvating medium) and trimethyl phosphate (TMP, a weakly solvating medium). While solvation in either EMC or TMP alone is unstable, adding TMP to EMC modifies the solvation sheath and enhances the solvation dynamics. Theoretical calculations reveal that a favorable solvation environment can be achieved through a balanced exchange of EMC with TMP molecules, thereby improving the overall stability and electrochemical performance of the cell. Sodium dual-ion cells, comprising sodium metal and synthetic graphite as the anode and cathode, respectively, exhibit an impressive cycling stability and high Coulombic efficiency when paired with the optimized electrolyte system. We believe that the strategy of solvation-shell exchange using weakly solvating solvents to modulate solvation dynamics could pave the way for the design of next-generation electrolyte systems for high-performance sodium dual-ion batteries.