Ionic Transport Properties and Additive Effect of Lithium Polysulfides in Binary Conducting Poly(ethylene oxide)-Based Copolymer Electrolytes

Abstract

Despite the high impact lithium–sulfur (Li–S) batteries can bring in terms of specific energy and battery lifetime, their full advantage has not yet been realized due to inherent issues associated with this technology. The intermediate polysulfide products produced in the positive electrode during discharge dissolve and diffuse in the electrolyte, leading to capacity fading and low Coulombic efficiency. A promising solution to this issue is the use of a solid polymer electrolyte that combines the advantages of an ion-conducting poly(ethylene oxide) (PEO) phase and a mechanically reinforced phase, such as polystyrene (PS), that can suppress the nonuniform electrodeposition of Li onto Li metal. In this work, the possibility of using PS–PEO–PS triblock copolymer as an electrolyte or binder in a Li–S battery was investigated by characterizing the thermodynamical, morphological, and ionic transport properties of lithium polysulfides species (Li₂S_x, with x = 4 and 8). Thermodynamic results showed that the long-chain lithium polysulfide (Li₂S₈) is more soluble in the copolymers compared to the short-chain polysulfide (Li₂S₄). Meanwhile, the addition of Li₂S₄ and Li₂S₈ in the mesostructured block copolymer influences both the phase transition (lamellar or hexagonal) and the domain spacing in a fashion similar to the conventional LiTFSI salt. In terms of ionic transport, the mobility of the polysulfides (S₄^2– and S₈^2–) in the copolymers is reduced compared to the TFSI^– anion, and the cationic transference number remains in the range of 0.5 compared to 0.15 for LiTFSI. To move toward the application, the introduction of Li₂S₄ into the block copolymer electrolyte is also used as an additive in the presence of LiTFSI salt, resulting in a very low interfacial resistance with the Li metal electrode. The results of these investigations would guide the design of solid polymer electrolytes for application in Li–S batteries.