Room-Temperature Sodium–Sulfur Batteries with Liquid-Phase Sodium Polysulfide Catholytes and Binder-Free Multiwall Carbon Nanotube Fabric Electrodes

Charge/discharge of a room-temperature sodium–sulfur (Na–S) battery involves redox processes of a series of long-chain soluble sodium polysulfides (Na₂S_n, 4 ≤ n ≤ 8). By taking advantage of this, a room-temperature Na–S battery is developed with dissolved sodium polysulfide catholyte and a free-standing, binder-free multiwall carbon nanotube (MWCNT) fabric electrode. Use of liquid-phase sodium polysulfide as a cathode not only provides a facile dispersion and homogeneous distribution of the sulfur active material into the conductive matrix but also supplies a unique approach to mechanistically understand the ambient-temperature Na–S battery system. With the intermediate products (polysulfides) as the starting cathode, the electrochemical characteristics of the Na–S battery in the lower-voltage-plateau region can be readily studied without the impact from the transformation process of elemental sulfur into long-chain sodium polysulfides. The nanostructured, free-standing MWCNT fabric electrode in this battery system acts as a high-surface current collector. In comparison with the traditional solid sulfur–carbon composite cathode, the sodium polysulfide/MWCNT fabric cathode provides higher active-material utilization and capacity retention during cycling. Electrochemical studies reveal that the transition of the low-ordered sodium polysulfides (or disulfide) is mostly responsible for the capacity fade during cycling. Operation of the cells with the sulfur/dissolved sodium polysulfide redox couple provides a stable output capacity/energy.