Unveiling the physiochemical aspects of the matrix in improving sulfur-loading for room-temperature sodium–sulfur batteries

Abstract

The sulfur cathode in Na/S batteries possesses a very high theoretical specific capacity of about 1675 mA h g⁻¹ and specific energy of 1230 W h kg⁻¹ (which is over five times that of the LiCoO₂ cathode in Li-ion batteries), besides high abundance and cost-effectiveness of the electrode materials. The sulfur cathode in Na/S batteries undergoes various electrochemical processes, where a series of soluble sodium polysulfides are formed during the discharge reaction, which adversely affects the operation of the cell. Furthermore, the viable application of RT-Na/S batteries is severely challenged by various obstacles, including their short-life and low-sulfur utilization, which become more serious when sulfur loading is increased to the practically acceptable level of over 5 mg cm⁻². Thus, there have been innovative efforts in recent years to manipulate the physiochemistry of the matrix to overcome these barriers toward the practical application of RT-Na/S batteries with an improved sulfur loading close to practical limits. The rational design of the matrix (i.e., physicochemical aspects) with a high-sulfur utilization and long lifespan are two crucial challenges that Na/S batteries are experiencing currently and require immediate attention to be addressed. This review highlights the recent progress on tuning the physiochemistry of the matrix through chemical and physical means to realize an improved sulfur-loading. Particularly, basic insight into the chemical binding, strategies for mesoscale assembly, unique architectures, and configurational innovation in the cathode are the specific focus. Finally, novel strategies to improve sulfur-loading are proposed to guide the future development of high-sulfur loading RT-Na/S batteries.