Nanoconfined Grain Boundaries Increase the Conductivity of Polycrystalline Molecular Crystals

Abstract

Soft-solid molecular crystals consist of crystalline grains and fluid grain boundaries (GBs) that enhance the grain binding and transport of Li+ ions between the grains. The total ionic conductivity consists of ion migration in both the grains and GBs. To unravel these contributions in adiponitrile (Adpn):LiPF6 molecular crystals, the GB volume fraction was varied by changing the size of the crystals and the Adpn:LiPF6 molar ratio. Molecular dynamics (MD) simulations indicate that ion motion was subdiffusive in the grains and “well-diffusive” in the GBs, with GBs characterized as disordered nanoconfined regions of higher charge carrier concentration (∼1 M) than in saturated Adpn:LiPF6 solutions (0.04 M), and Li+ ions predominantly solvated by cyano groups with few contact ion pairs. The diffusivity in the GBs is at least an order of magnitude higher than that in the crystalline grains. The emergent picture is the grains as a reservoir of ions that migrate to faster-conducting GBs.