Understanding the Impact of Multi-Chain Ion Coordination in Poly(ether-Acetal) Electrolytes

Performant solid polymer electrolytes for battery applications usually have a low glass transition temperature and good ion solvation. Recently, to understand the success of PEO for solid-sate battery applications and explore alternatives, we have studied a series of polyacetals along with PEO, both from an experimental and a computational standpoint. We observed that even though the mechanism of transport may be more optimal in polyacetals, the lower glass transition temperature of the PEO-salt electrolyte system still makes it the best option, in this class of polymers, for battery applications. In this work, we explored the free-energy landscape of PEO and P(EO-MO) at various compositions and temperatures using metadynamics simulations to gain deeper insights into the various factors that affect the glass transition temperatures in these systems. In particular, we study the competition between intra- and inter-chain coordination of the cation in these systems that we had hypothesized in our previous work was responsible for the differences in the glass transition temperature. We observe that in PEO, the single-chain binding motif is thermodynamically more stable than the multi-chain binding motif, unlike P(EO-MO), where the opposite is true. We also show that multi-chain coordination, and the associated higher glass transition temperature, in P(EO-MO) is due to a larger strain energy for single-chain coordination that originates in the introduced OCO linkages (relative to PEO's consistent OCCO linkages). Furthermore, the type of pathways to move from one transition state to another in the various systems do not change at higher concentrations though the relative probability of cation–anion coordinated states increases. Calculations at different temperatures to understand the entropic effect on the stability of these coordination environments reveal that as we increase the temperature, single-chain coordination becomes relatively more stable due to the entropic cost of multi-chain coordination, reducing the number of accessible states for the polymer. The various insights into the factors that affect glass transition temperature in these systems suggest design principles for polymer electrolyte systems with lower glass transition temperatures that need further research to compete with PEO at the same absolute battery working temperatures.