(266g) Thiol-Ene Polymer Electrolytes: Polymer and Electrochemical Properties

Thermomechanical and electrochemical properties of thiol-ene polymers were evaluated to explore the feasibility of crosslinked, non-crystalline, thiol-ene networks as highly conductive polymer electrolytes. Polymer electrolytes have the potential to mitigate stability and safety concerns accompanying incumbent liquid electrolyte systems in lithium-ion batteries. The thermal properties associated with low glass transition temperature (T_g) polymers are thought to improve contact with electrodes and ionic flow through the membranes compared to other liquid electrolyte alternatives such as solid electrolytes. Thus, thiol-ene polymers with sub-room temperature T_gs were explored as solid polymer electrolytes (SPEs).

Lithium salt was dissolved in resins consisting of thiol-containing compounds and various multi-functional alkene monomers: aliphatic monomers, aromatic monomers, and bio-based monomers derived from lignocellulosics. The resins were subsequently cured in radically-induced addition polymerizations to create flexible thio-ether networks with T_gs ranging from –52 °C to –20 °C. Monomer functionality, aromatic content, and thiol functionality were varied to explore the structure-property relationships of the SPEs. SPEs containing monophenolic compounds exhibited the lowest T_gs and neat polymer crosslink densities, the lowest being -48 °C and 0.21 mmol cm^-3, respectively, and the highest conductivities, the highest reaching 7.65x10^-4 S cm^-1. The flexibility of the low T_g networks assisted with total ionic transport through the polymer electrolytes, yielding higher conductivities. Though SPEs containing higher aromatic content displayed higher T_gs, higher neat polymer crosslink densities, and lower overall conductivities, they also displayed increased cationic transference numbers compared to the SPEs containing monophenolic compounds.

An additional area of improvement for many types of polymer electrolytes lies in mechanical stability. Employing aromatic compounds, such as lignin, is known to increase the mechanical and thermal stability of resulting polymer networks. Thus, the lignin-based thiol-ene polymers were explored for application as polymer electrolytes in this work as well. The room temperature conductivities of the polymers were determined and electrochemical evaluations as gel polymer and solid polymer electrolytes were performed.