Phosphate-based gel polymer electrolyte enabling remarkably long cycling stable sodium storage in a wide-operating-temperature

Gel polymer electrolytes are regarded as one of the efficient strategies to substitute traditional liquid electrolytes for achieving safer high energy density sodium ion batteries. However, operating under wide temperature range especially at low temperature under zero is still a great challenge for most gel polymer electrolytes and becomes a hindrance to sodium ion batteries further development. Herein, a multifunctional phosphate gel copolymer electrolyte with larger crosslinking degree and encapsulation over a broader operation temperature range (−20 to 70 °C) is designed and synthesized via in situ free radical copolymerization. This gel polymer electrolyte displays a wide electrochemical stability window up to 5.1 V (vs. Na+/Na) and high ionic conductivity even at low temperature of − 20 °C (>1.0 × 10−3 S cm−1). Moreover, its outstanding interfacial stability is confirmed via alternating-current impedance and Na plating/stripping experiments. The morphology and X-ray photoelectron spectroscopy data of separators after long cycling suggest the gel electrolyte has ability to inhibit the decomposition of liquid electrolyte and suppress the growth of sodium dendrite, hence enhance the cycling stability of battery efficiently at wide temperature range. As a result, the assembled gel Na||Na3V2(PO4)3 battery delivers outstanding ultralong cycling stability with superior discharge capacity retention over wide temperature range, including at room temperature (0.002% capacity decay per cycle for 3000 cycles), 60 °C (0.005% fade per cycle for 1500 cycles) and −10 °C (0.002% fade per cycle for 1200 cycles).