Exploring the Frontiers of Polymer Electrolytes for Battery Applications : From Surface to Bulk

Abstract: Lithium-ion batteries have dominated the market since their inception in 1991 due to their unparalleled energy and power densities, but are now faced with new challenges. Growing demand for battery materials for energy intense applications and large-scale interim energy storage have emphasized the need for safe and sustainable battery electrolytes. In this context, non-flammable solid polymer electrolytes (SPEs) are a promising alternative to address the shortcomings of conventional liquid electrolytes. Despite its significance, little research has thus far been devoted to understanding the electrochemical stability of SPEs under the harsh conditions exerted by next-generation electrode materials.In this thesis, the stability and ramifications of interfaces in polycarbonate- and polyester-based SPEs have been investigated. The polycarbonate exhibited severe degradation upon contact with lithium compared to its ester counterpart. Volatile species stemming from polycarbonate and salt decomposition were observed independent of irreversible current response, thus also highlighting the limitations of voltammetry techniques to determine the electrochemical stability. Two novel techniques were thus devised to evaluate electrochemical stability of SPEs under more realistic conditions. Characterization of the electrode−polyester interface revealed formation of highly resistive interfacial layers composed of polymer, salt and impurity derivatives. The emergence of a detrimental resistance emanating from the polymer−polymer interface was also observed, thus identifying a crucial hurdle for double-layer SPEs as a strategy to extend the stability window.The application of polycarbonate/polyester-based polymer electrolytes for sodium-ion batteries was also studied. Sodium is far more abundant than lithium, and thereby an excellent chemistry platform to develop new sustainable battery materials. The polycarbonate exhibited an exceptional ability to dissolve large quantities of sodium salt without compromising the mechanical stability. Spectroscopic and thermal measurements revealed the emergence of an alternative ionic transport mechanism at concentrations within the polymer-in-salt regime, which was decoupled from the segmental motion of the polymer chains. By incorporating flexible polyester moieties in polycarbonates, an SPE with better transport properties compared to its individual subunits, and polyether counterparts, was obtained. Optimal salt concentration in this copolymer was dependent on the degree of crystallinity, determined by the portion of polyester. Finally, the practical application of these polymer electrolytes was demonstrated in solid-state sodium-ion batteries.