α-methyl-γ-butyrolactone as novel solvent for low fluorinated and high-performance Sodium-Ion Batteries

This study introduces an innovative electrolyte for sodium-ion batteries (SIBs), which consists of sodium difluoro(oxalato)borate (NaDFOB) salt, dissolved in the α-methyl-γ-butyrolactone (α-Me-GBL) solvent. α-Me-GBL, is a positional isomer of γ-Valerolactone (GVL) and has properties similar to those of GVL. The electrolyte exhibited a wide electrochemical stability window, strong inhibition of anodic dissolution and excellent electrochemical stability (up to 4.3 V vs. Na + /Na). We showed that its use allows the realization of lab scale SIBs able to retain 97% of their initial capacity after 300 galvanostatic cycles at 0.5C. This outstanding stability is significantly higher than that observed for 1M NaDFOB in GVL. Post-mortem characterizations indicated the formation of thin interphase layers on both anode and cathode surfaces which are maintaining a homogenous elemental distribution across the entire electrode area over the cycling process. These results show that electrolytes containing different positional isomers can have different impact on the electrochemical performance of SIBs. Furthermore, since α-Me-GBL can be synthesized from bio-sources, they are offering novel indication towards the realization of advanced sustainable solvents suitable for SIBs.