Hexagonal Boron Nitride Modulates Crystallinity and Charge Mobility in Poly(ethylene oxide)–NaNO3 Electrolytes

Composite solid polymer electrolytes (CSPEs) for solid-state sodium (Na) batteries are attractive due to their high modulus, good mechanical properties, and overall safety relative to liquid electrolytes. Important CSPE properties, such as crystallinity and ionic conductivity, are closely tied to the physicochemical characteristics of the filler material. In this work, we investigate how 2D hexagonal boron nitride (2D h-BN) flakes influence polymer crystallinity and ionic conductivity in poly(ethylene oxide) (PEO)-based CSPEs for Na-ion conduction using NaNO3 as a model salt. Using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and electrochemical impedance spectroscopy (EIS), we find that polymer crystallinity increases in the presence of the h-BN flakes, whereas the total ionic conductivity decreases relative to h-BN-free samples. Quantum mechanical DFT calculations reveal the ability of h-BN to bind with both ions of the dissociated salt, more strongly so with the Na+ cation, which has hitherto not been reported in the context of Na-based polymer electrolytes. The combined experimental and computational efforts in this work provide key physical insights into the importance of filler geometry and chemical characteristics (i.e., Lewis acidity and Lewis basicity) in the design of CSPEs for Na-ion conduction.