Proposal of Na Conductive New Solid Electrolyte and Their Physicochemical Properties and Na-S Battery Performance

Abstract

Introduction In general, sodium-sulfur (Na-S) battery operates by redox reaction of sodium and sulfur during charge and discharge, and this battery system contains β-Al2O3 as electrolyte, Na metal as negative electrode and S as positive electrode, respectively. It has been applied as large-scale storage system owing to their high energy density and long-life. In contrast, this system has serious problems with fire risk owing to their chemical safety of Na metal and S at high temperature (ca. 573K) as liquid state. To solve these problems, we propose low-temperature operated Na-S battery using thermally stable organic gel electrolyte with polyether-based host polymer and sulfolane (SL)-based electrolyte solution. Herein, thermal stability and ionic conductivity of proposed Na conductive gel polymer electrolyte and charge-discharge property of Na-S cell were evaluated. Experimental Liquid electrolytes were prepared by mixing SL and NaTFSA using shading bottle in Ar-filled glove box. The molar ratio was changed to xSL:1NaTFSA (x=2,3,4,5 and 6, respectively). These liquid electrolytes and polyether-based macromonomer (P(EO)/(PO), polymer) were mixed with changing weight ratio as followed formula; y liquid electrolyte: (10-y)polymer (y=7 and 8). After Added DMPA as photo-initiator, solid gel polymer electrolytes were fabricated by photo-polymerization by UV irradiation. Prepared samples were expressed as (xSL)yEl+(10-y)Po. Thermophysical properties of fabricated gel polymer electrolytes were evaluated by thermogravimetric analysis (TG, Thermo plus EVO2, Rigaku) and differential scanning calorimeter analysis (DSC, Thermo plus EVO2, Rigaku). In addition, ionic conductivities were measured by electrochemical impedance method. The reversibility of Na-S batteries was evaluated by charge and discharge measurements at 333 K. Results and discussion The prepared electrolytes showed high flexibility at room temperature. Therefore, P(EO/PO) polymer matrix might be playing a role as framework phase to SL-based liquid electrolytes. Figure 1 shows the temperature dependence of the ionic conductivity of the gel electrolyte for each composition. The Ionic conductivity increases with composition of SL owing to their viscosity decreasing of SL electrolytes. Also, it was notable that high ionic conductivity observed in low SL composition at low temperature. Figure 2 shows charge and discharge profiles of prepared Na-S cells at 333 K. Sufficient charge and discharge operations at relatively low temperature (333 K) and exhibited a high capacity of approximately 350 mAh/g. Figure 1