PNNL

Abstract

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 it against a diluted ether electrolyte (DE), a high-concentration ether electrolyte (HCE), and a conventional carbonate electrolyte (CBE). HC||PB full cells with LHCE deliver outstanding high-rate durability, sustaining 80% capacity for >1000 cycles at 2C and strongly outperforming HCE and DE. Raman, SAXS, and AIMD reveal that LHCE increases anion involvement in the Na? primary solvation sheath (higher CIP/AGG fraction), which shifts interphase formation toward anion-derived products. Post-mortem analyses (XPS, HR-TEM, TOF-SIMS) show that LHCE forms thinner, more inorganic NaF/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 (0.076 mL), and improves thermal safety by delaying exothermic onset in accelerating rate calorimetry relative to CBE. Overall, solvation-structure engineering via LHCE provides a practical pathway to simultaneously enhance rate capability, cycle life, and safety in PB-based SIBs.