Nanoconfined Grain Boundaries Increase Conductivity of Polycrystalline Molecular Crystals

TL;DR

This study demonstrates that nanoconfined grain boundaries in polycrystalline molecular crystals significantly enhance ionic conductivity by providing faster pathways for Li+ ion migration, as revealed through molecular dynamics simulations.

Contribution

It reveals that grain boundaries in molecular crystals act as nano-confined, disordered regions with higher charge carrier concentration, increasing overall ionic conductivity.

Findings

Grain boundaries exhibit at least an order of magnitude higher diffusivity than grains.

Ion motion is sub-diffusive in grains and well-diffusive in grain boundaries.

Grain boundaries serve as reservoirs of ions migrating to faster-conducting regions.

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.