Unlocking Sulfur-Containing Polymers as Cathodes in Room-Temperature Rechargeable Sodium–Sulfur Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries have emerged as a promising energy storage technology capable of meeting the demands of future large-scale energy storage systems, owing to their high theoretical energy density (1274 Wh·kg⁻¹), abundant sodium resources, and low cost of raw materials. Nevertheless, the practical deployment of room-temperature sodium–sulfur batteries is hindered by several critical challenges, including the intrinsically low electrical conductivity of sulfur, the dissolution and shuttle effect of sodium polysulfide intermediates during electrochemical reactions, pronounced volume expansion, and irreversible redox processes. These issues collectively lead to poor cycling stability and low coulombic efficiency.
Sulfur-containing polymers are capable of forming stable covalent bonds with sulfur species, thereby enhancing electrochemical performance while maintaining high sulfur loading. This thesis investigates the application of sulfur-containing polymers as cathode materials for sodium–sulfur batteries to overcome the aforementioned challenges. On this basis, the potential applications of novel sulfur-containing polymer that tailored specifically to meet the requirements of Na-S batteries are explored, along with a detailed examination of their underlying electrochemical mechanisms. ... mehrFurthermore, functionalized separators are employed to further improve the capacity and stability of sulfur-containing polymer cathodes.
Sulfurized polyacrylonitrile (SPAN) was used against a Na metal anode to attempt to solve the shuttle effects. SPAN is a structurally stable and highly conductive covalent sulfur polymer. In this polymer, sulfur is embedded in a chain-like form into the carbon nitrogen framework, forming an amorphous and chemically bonded composite cathode material. This structure not only effectively avoids the formation of long-chain polysulfides, thereby suppressing shuttle effects, but also significantly enhances the overall electronic/ion conductivity of the electrode and improves interface reaction kinetics. In addition, SPAN materials exhibit low volume expansion and good structural reversibility during charge and discharge processes, providing a new approach for achieving high cycling stability. However, the decay mechanism of SPAN positive electrode in ether-based electrolyte RT Na-S batteries remains poorly understood. In this study, sulfurized polyacrylonitrile (SPAN) was applied as a cathode material in ether-based electrolyte, the structural evolution of the SPAN cathode during the first few cycles was explained and the variations in the structure of SPAN and reaction in SPAN//Na cells within different depths of discharge (DOD) was explored. The first part of the results (chapter 5.1) deals with the impact effect of deep discharge below 1.0V on the structure of SPAN, and its impact on the equilibrium of polysulfides in the electrolyte and on the growth of sodium dendrites at the negative electrode. On this basis, to further enhance the cycling stability and rate performance, carbon-coated functionalized separators are incorporated in the cell.
Additionally, a new type of sulfur rich polyamide (P1) is used as the positive electrode for sodium sulfur batteries. Rich sulfur polyamide has the dual advantages of high sulfur content and polymer skeleton. On the one hand, the covalently bonded sulfur atoms effectively increase the theoretical capacity and are expected to significantly reduce the shuttle effect caused by the dissolution of polysulfides. On the other hand, the amide groups in the polyamide backbone not only endow the material with excellent structural stability, but are also promising to enhance the synergistic effect of sodium ion and electron conduction. The second part of this work (chapter 5.2) explores the electrochemical performance and working mechanism of P1 in sodium-metal batteries and explains the irreversible electrochemical behavior of the P1. The nucleophilic reaction between polysulfides and active nitrogen in amide groups leads to irreversible bond type conversion from C-N to C-S/S-N/C(=S)-N, resulting in chemical fixation of sulfur and inability of sulfur chains to regenerate during charge process, leading to irreversible loss of active sulfur and low coulombic efficiency. It provides valuable insights in polymers containing highly active nitrogen sites (amide groups in P1) about working and failure mechanism in sodium-metal batteries. On this basis, MoS2 functional separator was used to improve capacity, stability and kinetics by chemical anchoring ability and catalytic activity of molybdenum metal sites.
Inspired by the uniform covalent distribution of sulfur atoms in sulfur-containing polymers that can improve electron conduction pathways and reaction reversibility, solvent method was used for synthesis sulfur-carbon composite material that attempted to combine sulfur and carbon as sulfur-carbon composite, which attempt to design sulfur-carbon composite with uniform sulfur distribution. This method can enhance the interface contact between sulfur and carbon to improve electron/ion transfer efficiency and further improve electrochemical reaction kinetics and cycling stability. Meanwhile, the conductive carbon matrix should provide a robust framework to enhance electronic conductivity and accommodate the volume changes during cycling. Moreover, MoS2 functional separator was applied to cell with CS cathode to improve capacity and stability by molybdenum metal sites, which can inhibit shuttle effects by chemical anchoring ability, and catalytic reaction between sulfur and sodium by catalytic activity. At the same time, the insight of the sodium storage behavior and electrochemical mechanism of the CS cathode is explored by cyclic voltammetry.
These discoveries not only provide new insights into the application of sulfur-containing polymers as positive electrodes for RT Na-S batteries, but also provide new insights into the electrochemical mechanism of these materials in metal batteries (such as Li-metal batteries and K-metal batteries). On this basis, these results provide new solutions to overcome the challenges of RT Na-S batteries.