Charge-Discharge Properties of HC/NaCrO2 Full-Cells Using Na[FSA]-[C3C1pyrr][FSA] Ionic Liquid Electrolytes

Abstract

Electrical energy storage (EES) devices are essential for new energy supply systems based on renewable energy resources such as solar and wind power. Although lithium-ion battery (LIB) is one of the candidates of EES devices, the limited amount of lithium and cobalt resources may lead to the instability in supply and the increase of LIB’s price. Thus, sodium-ion battery gathers increasing attention as an alternative EES device, especially for application as a large-scaled battery [1-3]. Our group has focused on the alkali metal amide-based ionic liquids as electrolytes for sodium-ion battery, and has confirmed that several electrode materials work properly in these electrolytes. For example, NaCrO2 positive electrode shows good charge-discharge characteristics in Na[TFSA]-Cs[TFSA] and Na[FSA]-K[FSA] systems at 423 K and 363 K, respectively [4,5]. With an aim to further lower the operation temperature, we developed Na[FSA]-[C3C1pyrr][FSA] (x(Na[FSA]) = 0.1 or 0.2) ionic liquid electrolyte, and reported that NaCrO2 positive electrode and hard carbon (HC) negative electrode exhibits stable cycling properties at temperatures between 298-363 K [6-9]. Moreover, we also applied an electrolyte with higher Na+ ion concentration (x(Na[FSA]) = 0.3-0.5), and obtained higher reversible capacities for NaCrO2 positive electrode [10].In the present study, we constructed the HC/NaCrO2 full-cell, and evaluated its cycling and rate properties. In particular, the effect of the Na+ ion concentration in the Na[FSA]-[C3C1pyrr][FSA] electrolyte and the operation temperature are discussed. Acknowledgments This study was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST), MEXT program "Elements Strategy Initiative to Form Core Research Center" (since 2012), MEXT; Ministry of Education Culture, Sports, Science and Technology, Japan, and JSPS Research Fellowships for Young Scientists (No. 15J02336).