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) as a filler has a non-monotonic effect on polymer crystallinity and total ionic conductivity in PEO-NaNO3 electrolytes. The dual Lewis acidity and basicity of hexagonal boron nitride (h-BN) allows 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 ATR-Fourier Transform IR (FTIR) 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 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. These competing effects reveal that h-BN modulates both polymer crystallinity and charge mobility in our CPEs. Density Functional Theory (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 future CPEs for Na-ion batteries.
