Sodium-ion batteries are promising for energy storage applications because of the natural abundance and low cost of sodium resources. However, safety hazards caused by flammable electrolytes have been one of the major obstacles to practical application. Adopting non-flammable all-phosphate electrolytes can improve safety but their reductive instability and inability to form stable solid electrolyte interphase films prevent them from being used with carbon-based anodes. Now, writing in Journal of the American Chemical Society, Cao, Fang, and colleagues demonstrate anion-cation interaction modulation by cosolvent addition allows flame-retardant low-concentration electrolytes to be used in sodium-ion batteries1.

High-concentration electrolytes are compatible with sodium-ion batteries as they form ion-solvent-coordinated structures that promote the reductive decomposition of anions to produce stable and dense solid electrolyte interphase films. However, they are not practical due to their high cost and high viscosity. The researchers previously found that the competition between cation-solvent and cation-anion interactions plays an important role in regulating the final ion-solvent-coordinated structures2. “In an electrolyte system with single phosphate solvents, the hard carbon anode cannot work unless the salt-to-solvent molar ratio is elevated to 1:2 to induce an ion-solvent-coordinated structure. Therefore, we focused here on making flame-retardant electrolytes with these all-phosphate solvents compatible at low-salt concentrations by enhancing the anion-cation interactions,” says Fang.

The team found that the salt-to-solvent molar ratio could be diluted to 1:5 by adding tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a cosolvent (Fig. 1a). “We investigated the effect of TFEP content on the solvation structure and found that the ion-solvent-coordinated structure could form which was compatible with the hard carbon anode for reversible Na+ storage,” explains Fang. “The addition of TFEP greatly enhances the cation-anion interaction but hardly affects the cation-solvent interaction, which allows the anion to emerge from the competition for coordination with the solvent and induces the ion-solvent-coordinated structures at a low salt concentration.”

Fig. 1: Enhancing anion-cation interactions to improve compatibility for sodium-ion batteries.
figure 1

a Solvation structures and interactions between the cation, anion, trimethyl phosphate (TMP) and tris(2,2,2-trifluoroethyl) phosphate (TFEP). b Photo of the fully charged pouch cell after being cut and put under a flame. Adapted from ref. 1 (https://doi.org/10.1021/jacs.4c01395). Used with permission of American Chemical Society, from ref. 1.

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The researchers used a combination of nuclear magnetic resonance, Raman spectroscopy, and density function theory simulations to probe the interactions of the different components in the electrolyte and define the stable solvation structure. The resultant sodium-ion battery showed a stable capacity retention of 84.5% after 2000 charge/discharge cycles and could be operated in temperatures from −20 to 60 °C. The fully charged battery could also maintain a steady voltage output even after cutting and put under a flame (Fig. 1b), demonstrating the safety benefits of using their non-flammable electrolytes.

The team is interested in continuing to design flame-retardant electrolytes. “We have solved the compatibility issue of non-flammable electrolytes with carbon-based anodes but compared with commercial electrolytes, the ionic conductivity is still low,” concludes Fang. “Our next goal is to design electrolytes with high ionic conductivity and good electrochemical compatibility that are fully flame retardant to realize a new generation of sodium-ion batteries with high-rate capacity and safety performance.”