Development of Low Fluorinated, Sustainable, and Recyclable Electrolytes Based on γ‐Valerolactone for High‐Performance Sodium‐Ion Batteries

This study introduces a dual salt novel electrolyte for sodium-ion batteries (SIBs), consisting of sodium difluoro(oxalato)borate (NaDFOB) and sodium bis(fluorosulfonyl)imide (NaFSI) salts dissolved in the bio-based γ-valerolactone (GVL) solvent. Besides its renewable origin, the electrolyte exhibited strong inhibition of anodic dissolution and excellent electrochemical stability (up to 4.3 V vs. Na$^+$/Na). It delivered outstanding cycling stability, with ∼87 % capacity retention after 100 cycles in P2-Na$_{2/3}$Al$_{1/9}$Fe$_{1/9}$Mn$_{2/3}$Ni$_{1/9}$O$_2$ (P2-AFMNO) cathode half cells and ∼80 % retention after 200 cycles in lab- scale full cells with hard carbon anodes when cycled within a wide voltage window of 1.5–4.3 V. Post mortem X-ray photoelectron spectroscopy analysis helped gaining deeper understanding about the decomposition products formed on the interphases. A simple and sustainable water-based process is employed to successfully recover the GVL solvent. The recovery method enabled recover 85 % of GVL solvent from the recycling process. The feasibility of recycling is further demonstrated by reusing the recovered GVL-based electrolyte in full cells, which achieved performance comparable to that of the pristine GVL-based electrolyte and exhibited excellent long-term stability, retaining approximately 83 % of its capacity after 100 cycles.