Cost attractive hydrogel electrolyte for low temperature aqueous sodium ion batteries

Low temperature tolerance of aqueous sodium ion batteries (ASIBs) represents a high challenge, eventhough ASIBs are attractive for large scale energy-storage application due to their high safety, low cost and environmental friendliness. For the first time, this work reports a Na 2 SO 4 -SiO 2 hydrogel type electrolyte operated at low temperature as -30℃ using fumed silica as the gel matrix and methyl alcohol as the anti-freezing additive. Due to the inter-molecular bonding between SiO 2 and Na 2 SO 4 , the precipitation of Na 2 SO 4 and the continue growth of the Na 2 SO 4 grain in supersaturated electrolyte is highly suppressed at low temperature. The Na 2 SO 4 -SiO 2 hydrogel electrolyte possesses a high ionic conductivity as 0.070 mS cm −1 at -30℃. An aqueous sodium ion cell with the as-proposed SiO 2 Na 2 SO 4 hydrogel electrolyte has been developed based on active carbon (AC) as the cathode and carbon coated NaTi 2 (PO 4 ) 3 (NTP@C) as the anode. At -30℃, the cell possesses a reversible capacity of 61.8 mAh g −1 (based on NTP@C) at a current density as 0.13 A g −1 (0.13 mA cm −2 ) with a high cycling stability. This work provides a new path for the development of aqueous sodium ion batteries for low temperature operation.