Preparation And Investigation Of Electrochemical Properties Of Novel Sodium Electrolytes Based On TMS

In recent years, large research efforts are devoted to develop large-scale electric energy storage applications for renewable energy and smart grid in order to fulfil the increasing energy demand. Compared with lithium-ion batteries, sodium-ion batteries has distinct advantage for resource availability and low cost. However, much research work have been focused on cathode and anode material for Na-ion batteries. Electrolyte has always played the role as the bridge of cathode and anode. Therefore, developing rechargeable, high-efficient, low-cost, safe and environment-friendly electrolyte is also important.In this paper, sulfolane solvent(TMS) was first chosen as the main solvent for SIB batteries. In order to improve the physicochemical properties and electrochemical properties of the electrolyte, we choose the ethyl isocyanate(EI), p-toluene isocyanate(PTI), p-toluene sulfonyl isocyanate(PTSI), six methylene diisocyanate(HDI) as additives. Meanwhile, the electrochemical behavior of the Na3V2(PO4)3/Na half-cell and SiC-Sb-C/Na half-cell with these electrolytes were systematically evaluated by the tests of cyclic voltammetry, alternating-current impedance and charge-discharge performance. Finally, we have investigated the effects of different salts with TMS: HDI.The results show that the ionic conductivity of 0.4M NaClO4+TMS with different additives at room temperature can reach up to 10-3 S.cm-1, which can meet the requirement of commercial applications. In addition, four additives can drop the glass transition temperature of pure TMS, HDI is most effective to make the Tg down to-1.76 °C. What’s more, TMS-based electrolyte has much better thermal stability than common carbonate electrolyte. The thermal decomposition temperature of 0.4 M NaClO4+TMS:HDI(95:5, w/w) is 140.09 °C. Furthermore, NVP/Na half cells achieve the best performance with the electrolyte above, which delivered the initial discharge capacity of 103.9 mAhg-1, and maintained a reversible capacity of 100.1 mAhg-1 after 50 cycles. In particular, the NVP/Na half cells has much better low temperature(0 °C) cycle performance with HDI, which has maintained 92.7 mAhg-1of initial discharge capacity of 100.3 m Ahg-1 after 50 cycles.In order to seek the best sodium salts for TMS:HDI(95:5, w/w) system. By performing the test of ion conductivity at room temperature with electrolyte of various ratios. 0.25 mol/kg, 0.4mol/kg, 0.3mol/kg, 0.2 mol/kg were the optimum concentration, corresponding to NaPF6, NaClO4, NaCF3SO3, NaTFSI respectively. The NVP/Na half cells with 0.25 M NaPF6+TMS:HDI(95:5, w/w) achieve the best cycle performance, which delivered the initial discharge capacity of 109.6 mAhg-1, and maintained a reversible capacity of 88.3 mAhg-1 after 500 cycles. Indeed, the electrolyte has satisfactory compatibility with SiC-Sb-C electrode, the SiC-Sb-C half-cell has an initial capacity of 810.3 mAh g-1 and maintain a reversible capacity of 619.0 mAh g-1 after 50 cycles.At last, the surface morphologies of cathodes after 500 cycles and anodes after 10 cycles were observed by SEM, and the NVP remain homogeneous porous structures, while a smooth and even film was observed on the surface of the SiC-Sb-C. Meanwhile, the result of XPS, FTIR and EDS assist in the formation of SEI film and speculated possible electrode/electrolyte interface reactions and composition of the SEI film.