Stability enhancing ionic liquid hybrid electrolyte for NVP@C cathode based sodium batteries

• Published in: Sustainable energy & fuels 2018-01, Vol.2 (3), p.566-576
• Main Authors: Manohar, C. V., Raj K, Anish, Kar, Mega, Forsyth, Maria, MacFarlane, Douglas R., Mitra, Sagar
• Format: Article
• Language: English
• Subjects: Batteries; Cathodes; Composite materials; Discharge capacity; Electrochemical analysis; Electrochemistry; Electrode materials; Electrolytes; Electron microscopy; Ionic liquids; Ions; Nonaqueous electrolytes; Photoelectron spectroscopy; Sodium; Sodium channels (voltage-gated)
• ISSN: 2398-4902; 2398-4902
• DOI: 10.1039/C7SE00537G

Cost-efficient, high-voltage, stable sodium-based cathodes are needed to develop commercial-scale sodium batteries. In this work, a Na 3 V 2 (PO 4 ) 3 /carbon (NVP@C) composite sodium cathode material is synthesized by a novel, facile, two-step, solid state method. This material delivered a discharge capacity of 115 mA h g −1 at 0.5C rate with a conventional organic electrolyte. Improvements in stable cycling were found when NVP@C was paired with a “hybrid” electrolyte comprising a [50 : 50] v/v mixture of 1 M sodium bis(fluorosulfonyl)amide (NaFSI) in an organic electrolyte and an ionic liquid, N -methyl- N -propyl pyrrolidinium bis(trifluoromethanesulfonyl)amide (C 3 mpyrTFSI). Sodium batteries based on the NVP@C cathode retained 95% of their initial capacity after 100 cycles at 0.5C rate. We show that the hybrid electrolyte enhanced the electrochemical performance of the NVP@C cathode material by forming a stable SEI (solid-electrolyte interphase) layer on the surface. Electron microscopy and X-ray photoelectron spectroscopy were used to study the SEI layers on electrodes that had been subjected to 100 cycles with hybrid or conventional organic electrolytes. The hybrid electrolyte produced a less resistive, highly Na + ion permeable SEI layer, explaining its superior sodium battery performance, compared to that found with the conventional organic electrolyte.