Revisit Solvation Chemistry in High-Concentration Ether-based Electrolytes to Stabilize Alloy-Type Anodes of Sodium Ion Batteries

Abstract

Alloy-based anodes are promising for sodium-ion batteries but suffer from large volume variation during cycling. Although ether-based electrolytes benefit many alloy-based anodes, they fail for Sb. Here, mechanical property and volume variation are proposed to explain the greater challenge with Sb (versus Bi and Sn). Then, a high-concentration electrolyte is developed for commercial Sb powders (μ-Sb) by using THF as the solvent for its good solubility to sodium salts, low molecular polarity and small molecule size. Theoretical simulations indicate that the increasing salt concentration makes the anions in the solvation sheath shift from monodentate to polydentate. This change results in weakened P-F bonds, reduced LUMO energy levels and increased Na-ion number within one cluster, promoting the formation of a NaF-rich solid electrolyte interphase on the surface. Such an interphase greatly enhances ion conductivity and mechanical resilience, improving the electrochemical performance of μ-Sb. This electrolyte is also compatible to Sn and Bi. These results advance our understandings of solvation structures and interface chemistry critical for high-performance sodium-ion batteries.