(410i) How Does Hexagonal Boron Nitride Affect Ionic Conductivity in PEO/NaX Electrolytes?

Composite polymer electrolytes (CPEs) containing fillers like particles and flakes can lead to safer sodium–ion batteries (SIBs) by potentially replacing incumbent liquid electrolytes. CPEs are attractive due to their high modulus, ease of processing, and the absence of solvent which prevents undesirable side reactions at the electrode. Key ion transport properties like total ionic conductivity (IC) and cationic transference number (TN) are governed by polymer crystallinity (X_C), polymer glass transition temperature, filler geometry, and intercomponent interactions in the CPEs. Here, we investigate the effect of 2–dimensional hexagonal boron nitride (h–BN) on X_C and IC in polymer–salt complexes. h–BN is unique due to its dual Lewis chemistry (boron is electrophilic and nitrogen is nucleophilic) and its planarity which can influence polymer crystallization behavior. We investigate two sets of polymer–salt complexes based on poly(ethylene oxide) (PEO) polymer, and sodium nitrate (NaNO₃) or sodium bis(fluorosulfonylimide) NaFSI salt. Moderately–concentrated and highly–concentrated regimes are studied. We observe from x–ray diffraction and thermal analysis a non–monotonic variation of X_C with increasing h–BN loading. This phenomenon results from a competition between enhanced heterogeneous nucleation from the h–BN surfaces at low filler loading and polymer spherulitic confinement from smaller interparticle distances at high filler loading, both below the critical percolation threshold. Electrochemical impedance spectroscopy reveals the ability of h–BN to modulate IC, which depends upon both X_c and local solvate structure. Local solvate structure is elucidated using vibrational spectroscopy. In the melt, IC with and without h–BN remain invariant for both salt systems. Lastly, quantum mechanical calculations provide key molecular insight into the underlying intercomponent interactions. Based on these findings, we propose possible mechanisms by which h–BN affects ion transport in our CPEs. Our results underscore the importance of filler geometry, filler chemistry, and filler–polymer–salt interactions in the design of CPEs for beyond–lithium energy storage applications.