The Effect of Boron Nitride Flakes on Sodium Ion Transport in PEO Electrolytes

Improving the total ionic conductivity (σ) of solid polymer electrolytes (SPEs) is critical to the development of solid-state sodium (Na) batteries. In this work, we investigate the effect of two-dimensional (2D), dual-Lewis hexagonal boron nitride (h-BN) filler on polymer structure and ion transport properties of P(EO)24:Na+ and P(EO)4:Na+ mixtures of poly(ethylene oxide) (PEO)-bis (fluorosulfonylimide) (NaFSI). Below the critical percolation concentration threshold for the h-BN flakes, X-ray diffraction and differential scanning calorimetry studies show that an increase in h-BN concentration initially induces an increase in PEO crystallinity followed by a decrease due to competing effects between heterogeneous nucleation of PEO lamellae and its spherulitic confinement, respectively. Raman spectroscopy reveals that h-BN improves NaFSI dissociation in the semi-dilute SPEs which is supported by density functional theory calculations. Our calculations suggest that PEO can almost fully dissociate a NaFSI molecule with a coordination number of 6. We propose an h-BN-"assisted" mechanism to explain this observation, wherein h-BN aids PEO in better matching the dissociation energy of the NaFSI salt by virtue of its dual-Lewis surface chemistry. A corresponding 4× increase in σ is observed for the P(EO)24:Na+ SPEs using electrochemical impedance spectroscopy. The P(EO)4:Na+ SPEs do not show this increase, which is likely due to a significantly different local solvation environment wherein contact ion pairs and aggregates (AGGs) dominate. Our findings highlight the role of filler chemistry in the design and development of composite solid polymer electrolytes for Na batteries.