Surface modifications of boron nitride nanosheets for poly(vinylidene fluoride) based film capacitors: advantages of edge-hydroxylation

Boron nitride nanosheets (BNNSs) are ideal candidates to endow polymers with high breakdown strength (E-b) and energy storage density (U-e). Surface modifications, including non-covalent or covalently bonding, are adopted to improve their dispersion in polymer matrix, but, usually, they suffer from inferior dielectric properties. The key is to improve their compatibility within polymer without sacrificing excellent dielectric properties, which remains to be a great challenge. In this study, three methods were designed to modify BNNSs within poly(vinylidene fluoride) (PVDF) for a comparative study. The first one involves a non-covalent polydopamine (PDA) coating (PDA-BNNSs); the crystal lattice was well-preserved, but additional PDA was introduced. The second method involves basal hydroxylation via nitric acid (HNO3) oxidation (BOH-BNNSs) with hydroxyl groups (-OH) mainly located at the basal plane, thus, seriously damaging the in-plane crystal. The third one is edge-hydroxylation via ball-milling (EOH-BNNSs) with -OH groups being regulated at the boundary; thus, integrated basal crystal is largely maintained. It was very interesting to find that the dispersion, interfacial interaction and orientation of BNNSs in PVDF matrix are more or less similar for all three surface-modification methods, but the enhancement of E-b is very different. PDA-BNNS and BOH-BNNS filled PVDF nanocomposites exhibited a limited enhancement of E-b, while a greatly improved E-b (approximate to 146.6%) was observed for EOH-BNNS filled PVDF nanocomposites in comparison with that of PVDF/BNNSs nanocomposites. Therefore, edge-hydroxylation could endow PVDF/EOH-BNNS nanocomposites with impressive E-b (585 kV mm(-1)) and U-e (16.23 J cm(-3)). Because the low E-b of PDA and the decreased band gap energy (E-g) of BOH-BNNSs (2.73 eV) are both disadvantages for achievement of high E-b. Inversely, well-preserved basal crystal lattices could be obtained if -OH groups are regulated at the boundary, which makes EOH-BNNSs not only display better dispersion and interfacial interaction but also retain higher E-g (5.76 eV). This study demonstrates the importance of maintaining an integrated crystal lattice of BNNSs when exposed to surface modification for the first time. Our study provides guidance for surface-modification of BNNSs and preparation of polymer-based capacitors with excellent performance.