Nanoconfined Grain Boundaries Increase Conductivity of Polycrystalline Molecular Crystals

Abstract

Soft-solid molecular crystals consist of crystalline grains and fluid grain boundaries (GB) that enhance 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 dynamic (MD) simulations indicate that ion motion was sub-diffusive in the grains and well-diffusive in the GBs, with GBs characterized as disordered nano-confined regions of higher charge carrier concentration (1M) than in saturated Adpn-LiPF6 solutions (0.04M), 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 in the crystalline grains. The emergent picture is the grains as a reservoir of ions that migrate to the faster-conducting GBs.