Development of novel liquid and gel polymer electrolytes for room temperature sodium-sulfur batteries

Abstract

The ever-growing energy consumption requires promising next-generation advanced energy storage systems. Secondary batteries are an indispensable part of the energy storage market, with lithium-ion batteries (LIB) holding more than 85% of the market share. The booming electric vehicle (EV) market demands low-cost and long-range batteries. Uncertainty in the price of lithium raw materials, capital costs in grid energy storage systems, rapid material degradation, and environmental concerns are driving the increasing interest in non-lithium-based batteries. Sodium-based batteries are considered to be an alternative for LIBs due to their stable, low price, and high natural abundance of Na in the Earth’s crust (Li - 20 mg kg-1; Na - 28400 mg kg-1). However, intercalation chemistry limits the storage capacity and energy density of Na batteries. Therefore, low-cost, and highly abundant sulfur with a theoretical capacity of 1675 mAh g -1 has been studied as a conversion-type cathode material to enhance the energy density of the battery. High-temperature sodium-sulfur batteries (HT-SSB) operate at 300-350 °C, where both sodium and sulfur exist in a molten state, which leads to serious safety issues, including explosions. Multiple attempts have been made to reduce the operating temperature of the battery and develop room-temperature sodium-sulfur batteries (RT-SSB). The right choice of electrolyte plays a vital role in achieving high capacity, stable cycling, and safety of the battery. The most important criteria for an ideal electrolyte are high dissociation of the salt, high ionic conductivity, low activation energy, and chemical stability against all electrodes. The choice of an electrolyte salt for RT-SSBs is analogous to the salts used in LIBs. However, these salts have several disadvantages, such as moisture sensitivity, reduced electrochemical window, poor resistance against oxidation, corrosiveness to the current collector, explosiveness, and toxicity. Therefore, in this work, to achieve high capacity and stable cycling, three types of electrolytes were synthesized and characterized both chemically and electrochemically.