Ion Transport Dynamics in Poly(ethylene oxide) and Poly(pentyl malonate) Electrolytes for Lithium Batteries

The rate at which batteries can be charged is limited by the transport of the working ion through the electrolyte. In polymer electrolytes, the selective transport of Li+ ions is dependent on the solvation cage surrounding the Li+ ion. In this work, we measure cation and anion self-diffusion coefficients using pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR) in electrolytes containing poly(ethylene oxide) (PEO) or poly(pentyl malonate) (PPM) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We utilize molecular dynamics simulations to explore the connection between self-diffusion and the solvation cage structures of the Li+ ions. Decomposition of solvation cage dynamics allows for identification of two Li+ hopping mechanisms: interchain and intrachain hopping. We find that interchain hops are extremely rare in PEO/LiTFSI (i.e., back-and-forth intrachain hops are dominant). In contrast, inter- and intrachain hops in PPM/LiTFSI occur at similar rates. Interchain hopping is necessary for selective Li+ transport between battery electrodes and greater interchain hopping efficiency in PPM/LiTFSI results in improved Li+ transport.