Evaluation of Physicochemical Properties and Battery Performancesfor PEO/SL-Based Na Conductive Solid Electrolyte
Authors: Yokomaku Yuji , Hiraoka Koji , Inaba Kohei , Miyauchi Hibiki , Seki Shiro 
Published: July 2021

Introduction

In recent years, power generations utilized renewable energy are attracted attention, and power resource is variegated for the realization of sustainable society. Among them, the amount of power generation as solar power generation and wind power generation is unstable due to weather and environmental factors. In order to popularize such renewable energy, it is considered required to have a power storage system for stores the generated power electricity. As one of the promising candidates for them, there is the lithium-ion battery which is widely utilized such as smartphones and laptop computers. However, this battery has the problem such as cost and resource due to utilizing rare material of lithium and is desired to instead another element to lithium. Sodium resources are inexhaustible and distributed everywhere, it has similar chemical properties due to same monovalent alkali metal like lithium. One of the sodium-based batteries is the sodium-sulfur (Na-S) battery, which operates at high temperatures and is primarily used for peak-shaving and load-leveling applications. In general, sodium-sulfur (Na-S) batteries use sodium and sulfur as negative and positive electrode active materials, respectively, and has possible to produced low-cost due to few resource restrictions of the main content materials. The β-alumina ceramics are used as an electrolyte and acts as the Na+-conduction membrane layer and the separator between two liquid state electrodes. Due to operating at high temperature, it utilizes the advantage of quit high ionic conductivity of electrolyte at high temperature and improves the interfacial formation of electrolyte/electrode by ensuring active materials are molten. However, β-alumina is easy to damage by various thermal and mechanical stresses and there are risks of the direct reaction of molten sodium and sulfur, and leak out of the module. Therefore, it is desired to develop low temperature operating Na-S battery, we focused on solid polymer electrolytes (SPE) having high flexibility for the realization of safety low temperature operating Na-S battery. SPE is expected to improve safety by decrease operating temperature, but it has the problem of low ionic conductivity at room temperature to compare liquid electrolytes, therefore we focused on gel polymer electrolyte (GPE) by added sulfolane (SL) solvent which high thermal stability. This study proposed sodium conductive new solid polymer electrolytes and evaluated their physicochemical properties and Na-S battery performance.

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) and differential scanning calorimeter analysis (DSC). 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.

Result and discussion

Proposed GPE has high flexibility at room temperature. Remarkable weight changes were not observed below 373 K, and it was expected to apply as the low-temperature operated Na-S battery. Fig.1 shows DSC curves of GPE. The glass tradition temperature (T
g) increased with polymer composition, when the case of high composition SL, T
g exhibited almost the same value as T
g of the pure polymer without SL. It was suggested that the increase in free PEO chains owing most Na+ forms a solvate structure with SL. Fig.2 shows temperature dependence of ionic conductivities. Ionic conductivities increased with SL composition, it was suggested that improving ionic mobility due to decreasing microscopic viscosity according to adding SL. The highest ionic conductivity of 2.3 × 10-3 Scm-1 was observed when the highest electrolyte composition and SL composition were optimized. In this presentation, we will also report about electrochemical properties, such as transference number of Na+ and activation energy at Na/GPE interfaces, time dependences of interfacial stabilization at Na/GPE interface, and battery performance of [Na|GPE|SPAN] (Sodium-Sulfur) and [Na|GPE|NaCoO2] (Na-ion) cells.

Figure 1