Solvation Time Scales in Polymer Electrolytes for Lithium Batteries*

The term "solvation" generally applies to the immediate neighborhood of the working ion in dilute liquid electrolytes. In this limit, the neighborhood - approximately a sphere - is dominated by solvent molecules, whence the term solvation. The solvent that dissolves ions can either be a polymer or a low molecular weight solvent (liquid). In the case of polymers, the solvation shell comprises polymer segments that translate coherently with the working ion for a short while before they diffuse away to Brownian motion. The time scale for this coherent translation (solvation lifetime) was measured in a polymer electrolyte comprising poly(pentyl malonate) and a lithium salt by quasi elastic neutron scattering (QENS). We obtained a value of about 1 nanosecond. This measurement was enabled by a unique QENS signature of solvation lifetime, arising due to the presence of multiple chains in the solvation sphere. The experimental results are compared with molecular dynamics simulations without resorting to any adjustable parameters. Our measurements may be characterized as "ultraslow", compared to the solvation life time in aqueous electrolytes of 1 picosecond. We use our simulations to estimate the solvation lifetime of organic liquid electrolytes - whether this value is 1 picosecond or in excess of 100 picoseconds remains unresolved.