Sodium ion conducting flame-retardant gel polymer electrolyte for sodium batteries and electric double layer capacitors (EDLCs)

We report a trimethyl phosphate (TMP) based sodium ion conducting flame-retardant gel polymer electrolyte for safer electrochemical applications. The physical investigations reveal superior amorphicity and thermal stability of electrolyte utilizing TMP solvent as compared to the conventionally used binary mixture of ethylene carbonate (EC) and propylene carbonate (PC). The TMP based electrolyte membrane displays better ionic conductivity (∼ 1.40 mS cm−1) as compared to the membrane with EC:PC solvent mixture (∼ 0.72 mS cm−1) at 30°C with higher electrochemical stability window of ∼ 4.5 V and superior Na+ transport characteristics. The TMP based electrolyte has been utilized for proto-type sodium battery and EDLC application. The proto-type Na battery displays an open circuit potential of ∼ 2.3 V and specific discharge capacity of ∼ 225 mA h g−1. The electric double layer capacitor (EDLC) fabricated using the TMP based electrolyte and activated carbon electrodes shows specific capacitance of ∼100 F g−1 and is stable up to 4000 charge–discharge cycles.