Anisotropic Ion Conductivity in Boron Nitride–containing PEO Electrolytes

Engineering filler orientation provides an attractive route to boosting directional ion conductivity in polymer electrolytes for batteries. In this work, we study composite polymer electrolytes (CPEs) comprising 2D planar boron nitride (BN) flakes dispersed within sodium bisfluorosulfonylimide–doped poly(ethylene oxide) (PEO) matrices near (~1.88 vol.%), and above (18.8 vol.%) the critical percolation threshold for the filler. BN is chosen due to its unique Lewis chemistry which enables better salt dissociation in the electrolytes, and salt concentration is fixed at a sodium : ether ratio (r) of 0.05. Hot–pressing is used to preferentially orient the filler within the polymer electrolyte matrix, confirmed using x–ray diffraction and electron microscopy. From impedance spectroscopy data corrected for both polymer crystallinity and filler volume fraction, we observe a 12x–15x enhancement in conductivity along the direction of the BN flake surfaces (in–plane conductivity) relative to through–plane conductivity across the film thickness. Our analysis indicates that this anisotropy originates primarily from improvements in the in–plane conductivity, pointing to the critical role of polymer–filler interfacial effects in governing ion transport. Our findings have implications in the better design and engineering of composite polymer electrolytes for emerging battery applications.