Investigation on ionic conductivity enhancement of potassium-substituted NASICON solid electrolyte for sodium Ion batteries

Abstract

As interest in environmental protection grows, efforts have been made to replace existing fossil fuels with renewable energy sources including lithium ion batteries which has been studied most actively. Due to the rapidly growing battery market, the price of lithium increased rapidly Considering the economical aspect, the sodium ion battery which is cheaper than the lithium ion battery is attracting attention as the next generation battery. In addition to the cost of batteries, safety is an important issue. Conventional organic liquid electrolytes used mainly in batteries are flammable. So, for safe operation of the battery, conversion from liquid electrolytes to non-flammble inorganic solid electrolytes is required. Na superionic conductor (NASICON) is one of the promising candidates with high conductivity and stability. In this study, we introduced novel solid electrolytes $Na_3Zr_2Si_2PO_{12}$ with NASICON structure, substituted with potassium. From the results of the structural analysis using Rietveld refinement, it was found that the size of the sodium channel tends to increase with increasing potassium concentration. Using the lattice parameter data, it was confirmed that the higher the potassium concentration, the larger the total volume. The ionic conductivity was measured using electrochemical impedance spectroscopy to see if the expansion of the sodium channel actually affected the electrical properties. When the concentration of potassium was 0.1 mol, the highest ionic conductivity of 7.734 x $10^{-4}$ S/cm was obtained. However, when the concentration of potassium was higher than that, the ion conductivity decreased again due to the decrease in the amount of sodium as a charge carrier.