Hexagonal Boron Nitride Modulates Crystallinity and Charge Mobility in PEO–NaNO3 Electrolytes

Composite polymer electrolytes (CPEs) hold great promise for the development of safe and sustainable batteries. In this study, we find that 2D hexagonal boron nitride (h-BN) has a non-monotonic effect on polymer crystallinity and total ionic conductivity in PEO-NaNO3 electrolytes. The dual Lewis acidity and basicity of h-BN allow it to interact with dissociated salt ions and the polymer matrix itself. PEO crystallinity was quantified using differential scanning calorimetry (DSC) and X-ray diffraction (XRD), and complex formation between NaNO3, PEO, and h-BN was studied using IR spectroscopy. Total ionic conductivity was determined using electrochemical impedance spectroscopy (EIS) as a function of temperature. We find that h-BN has two competing effects on polymer crystallinity and charge mobility in CPEs: 1) nucleation-enhanced crystallization of PEO on h-BN surfaces at low h-BN loading, and 2) spherulitic confinement of PEO at higher h-BN weight loading. DFT calculations confirm strong attractive interactions between h-BN and both free ions (Na+ and NO3-), and we also find lesser attractive interactions between h-BN and PEO. These new findings for Na-polymer electrolytes support our experimental results. Our findings highlight the importance of filler geometry and chemical characteristics in designing CPEs for Na-ion batteries.