Nanoconfined Grain Boundaries Increase the Conductivity of Polycrystalline Molecular Crystals

Paul, Shujit Chandra; Goddard, William A., III; Zdilla, Michael J.; Prakash, Prabhat; Wunder, Stephanie L.

doi:10.1021/acsmaterialslett.5c01267

Published March 2, 2026 | Version Published

Journal Article Open

Nanoconfined Grain Boundaries Increase the Conductivity of Polycrystalline Molecular Crystals

1. Temple University
2. California Institute of Technology

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.

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Acknowledgement

This work is supported by National Science Foundation Grant (Award No. DMR 2138432), NSF MRI grant (CHE-2215854) and a Department of Energy Grant (Award No. DE-SC0023356). W.A.G. acknowledges support from the Hong Kong Quantum AI Lab, AIR@ InnoHK of the Hong Kong Government. W.A.G. thanks the U.S. National Science Foundation (CBET-2311117) for support.

Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Supplemental Material

Experimental details (crystal preparation, characterization, thermal and electrochemical measurements, XRD) and computational details (molecular models, simulations protocols), powdered XRD data, additional SEM images, thermogravimetric analysis data, ¹H NMR spectra, conductivity data for cooling cycles, table for activation energy barriers, MD model initial structures, and mean-squared displacement vs time plots from MD (PDF)

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