Effect of various ratios of poly(3-hexylthiophene) with polyvinyl alcohol gel-polymer electrolytes in flexible sodium-ion batteries using Samanea saman tree-leaf-derived carbon quantum dots decorated with SnO₂and NaVO₃

Gel-polymer electrolytes with high thermal stability and mechanical properties were considered suitable in rechargeable batteries so as to overcome the problems encountered in liquid electrolytes. In a previous study, flexible sodium-ion batteries (FSIBs) were fabricated using electrodes made of carbon quantum dots (CQDs) decorated with SnO2 and NaVO3 utilizing a polyvinyl alcohol (PVA)/P3HT gel-polymer electrolyte at a ratio of 1:0.05. In addition, various separators such as indium-doped tin oxide/polyoxyethylene tridecyl ether (ITO/PTE), rice paper (RP), silicone with three big holes (SIL BH), silicone with many small holes (SIL SH) and cellulose paper (CP) were tested in flexible Na-ion batteries. The SIL SH delivered a high specific discharge capacity of 4246 mAh g-1 in the initial cycle at 2 V and maintained a value of 71 mAh g-1 in the 50th cycle. With the aim of improving the cyclic ability of FSIBs, different weight ratios of PVA/P3HT (1:0.025, 1:0.05, 1:0.1, 1:0.15, 1:0.2) were tried in this work using the electrodes CQDs@SnO2 and CQDs@NaVO3. The above ratios were referred to as B25, B50, B100, B150 and B200, respectively. SIL SH was used as a separator. Cyclic voltammetry studies indicated that B150 had a high specific capacitance of 13 062 F g-1. B25 and B100 exhibited high discharge capacities (171 mAh g-1) and (151 mAh g-1) compared to that of other ratios B50 (75 mAh g-1), B150 (88 mAh g-1) and B200 (54 mAh g-1) in the 50th cycle at 2.0 V. This study reveals the scope of developing FSIBs of high capacity and cyclability at different voltages using carbonaceous electrodes and gel-polymer electrolytes with different ratios of PVA/P3HT. The focus of the present study is to bring out the optimum ratio of PVA/P3HT for maximizing the cyclic ability of FSIBs.