Journal of Energy Storage 46 (2022) 103899 2352-152X/© 2021 Elsevier Ltd. All rights reserved. Research Papers Sodium ion conducting fame-retardant gel polymer electrolyte for sodium batteries and electric double layer capacitors (EDLCs) Deepak Kumar a, b , Nitish Yadav c , Kuldeep Mishra d, * , Raza Shahid e , Tasnim Arif f , D. K. Kanchan g a Electronics and Mechanical Engineering School, Vadodara, Gujarat 390008, India b Gujarat Technological University, Ahmedabad, Gujarat 382424, India c Department of Physics, Indian Institute of Technology Delhi, Delhi 110016, India d Department of Physics and Materials Science, Jaypee University, Anoopshahr, Uttar Pradesh 203390, India e Department of Physics, Jamia Millia Islamia, New Delhi 110025, India f Department of Mechanical Engineering, Jaypee University of Engineering and Technology, Guna, Madhya Pradesh 473226, India g Department of Physics, ITM (SLS) Baroda University, Paldi, Vadodara, Gujarat 391510, India A R T I C L E INFO Keywords: Flame-retardant Gel polymer electrolyte Ionic conductivity Proto-type sodium battery EDLC ABSTRACT We report a trimethyl phosphate (TMP) based sodium ion conducting fame-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 elec- trolyte 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 specifc 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 specifc capacitance of ~100 F g 1 and is stable up to 4000 charge–discharge cycles. 1. Introduction Sodium-based energy storage systems, such as sodium ion batteries (SIBs) and sodium ion capacitors (SICs), have received considerable attention in last two decades as alternatives of lithium counterpart [1–7]. Being the fourth most abundant metal and similar cell chemistry as lithium in the commercially available energy storage devices, sodium-based energy storage systems appear most suitable for large scale energy storage systems. The abundance of natural reserves and relatively low cost of sodium salts, the suitable redox potential of sodium element and good electrochemical performance are important aspects which have attracted the researchers worldwide. Tremendous research has been done on electrolyte and electrode materials for sodium-based energy storage systems. Liquid electrolytes carrying salts dissolved in carbonate-based sol- vents, such as ethylene carbonate (EC), propylene carbonate (PC), diethylene carbonate (DEC) and dimethyl carbonate (DMC), have largely been proposed as electrolytes for sodium-based batteries [2,3,8]. However, due to high volatility and fammability, the presence of such solvents leaves risk of leakage and safety issues such as fre and explo- sion of device during the long time and large-scale usage. In this regard, a safer electrolyte system is much required to realize a high perfor- mance, low cost and fame retarded electrochemical energy storage device. Recently, quasi-solid state gel polymer electrolytes (GPEs) have been largely proposed as suitable electrolytes for energy storage devices [2–5, 9–11]. These electrolytes carry higher content of liquid electrolytes immobilized in polymer hosts such as polyvinylidene fuoride (PVdF), poly(vinylidene fuoride-hexafuoropropylene) (PVdF-HFP), Poly (methyl methacrylate) (PMMA) and Polyacrylonitrile (PAN). Due to high content of liquid electrolyte, the GPEs display excellent ionic conductivity and favourable electrochemical properties with fexibility and shape-versatility provided by the polymeric host. However, the risk of thermal runaway and burning of the device during the cell operation * Corresponding author. E-mail address: kuldeep.mishra@mail.jaypeeu.ac.in (K. Mishra). Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est https://doi.org/10.1016/j.est.2021.103899 Received 29 September 2021; Received in revised form 7 December 2021; Accepted 21 December 2021