Tailoring electrolyte solvation structure to enhance rate capability, cycle life, and safety in Prussian-blue-based sodium-ion battery

Prussian-blue (PB) cathodes paired with hard carbon (HC) anodes are promising for low-cost sodium-ion batteries (SIBs), but practical deployment is limited by rapid degradation at high charge/discharge rates and safety issues arising from electrolyte-driven interfacial reactions. Here, we develop a localized high-concentration electrolyte (LHCE) based on NaFSI in diglyme with a non-solvating fluorinated diluent (TTE) and benchmark against a diluted ether electrolyte (DE), a high-concentration ether electrolyte (HCE). HC||PB full cells with LHCE deliver outstanding high-rate durability, sustaining 80% capacity for ~1200 cycles at 2 C and strongly outperforming HCE and DE. Raman, Small-angle X-ray scattering (SAXS), and ab initio molecular dynamics (AIMD) reveal that LHCE increases anion involvement in the Na⁺ primary solvation sheath (higher contact ion pairs and aggregates fraction), which shifts interphase formation toward anion-derived products. Post-mortem analyses show that LHCE forms thinner, more inorganic FSI-derived SEI/CEI on both electrodes, suppressing parasitic reactions, mitigating PB degradation and Fe migration, and reducing polarization growth under high-rate operation. In multilayer pouch cells, LHCE retains 82% capacity after 500 cycles with stable Coulombic efficiency (~99.3%), generates negligible gas (0.26 mL/Ah), and improves thermal safety by delaying exothermic onset in accelerating rate calorimetry relative to conventional carbonated-based electrolyte (CBE). Overall, solvation-structure engineering via LHCE provides a practical pathway to simultaneously enhance rate capability, cycle life, and safety in PB-based SIBs. Localized high-concentration solvation enriches anion-involved Na⁺ coordination (CIP/AGG) to regulate interfacial chemistry. This solvation structure drives uniform, inorganic FSI-derived SEI/CEI formation on hard carbon and Prussian Blue, suppressing parasitic reactions and Fe cross-talk. PB||HC cells achieve high-rate long-life cycling with minimal gas evolution and improved thermal safety. • LHCE-based electrolyte boosts PB||HC high-rate capability and cycle life. • Anion-involved Na⁺ solvation (CIP/AGG-rich) was analyzed by Raman/SAXS/AIMD. • Inorganic-dominated SEI and CEI suppress Fe cross-talk and polarization growth. • Full cell with LHCE provide 80% retention ~ 1200 cycles at 2 C and 915 cycles at 1 C. • Pouch cell performs 82% after 500 cycles, 0.26 mL/Ah gas, high thermal stability.