Surface-Engineered Boron Nitride Electrolytes for High Performance Solid-State Supercapacitors

Abstract

In the modern era of smart wearable electronics and compact transportation systems, there is an urge for the development of solid, lightweight, flexible and efficient energy storage devices. For realizing high performance energy devices, novel electrolyte materials with high energy density at enhanced power are demanding. The recent developments in the field of two-dimensional (2D) materials, including graphene, 2D hexagonal boron nitride (2D-hBN) and transition metal dichalcogenides, have exhibited promise for a wide range of applications. In particular, 2D-hBN, also known as “white graphene” is an isomorph of graphene with similar layered structure in a hexagonal lattice, which is uniquely featured by its exotic opto-electrical properties together with mechanical robustness, thermal stability, and chemical inertness. Due to its remarkable electrochemical properties, it is considered as a promising candidate that can be integrated with other 2D materials, for the next generation electrochemical energy storage and conversion applications including fuel cells, batteries and supercapacitors Moreover, h-BN possess electrically insulating behavior at wide range of humidity and temperatures, which makes it more versatile to be manipulated and used as an electrolyte membrane in electrochemical energy systems. In order to realise hBN nanosheets (BNNS) potential as an ion-conductive solid electrolyte in advanced energy devices, scalable production techniques involving simple and efficient green synthesis routes are crucially required. Liquid phase exfoliation and ball milling are versatile techniques for the production of large-scale, defect-free, few-layered 2D materials. Further, steps of chemical functionalization are involved to produce surface-engineered BNNS solid-state electrolyte membranes with high ionic conductivity and durability, for electrochemical energy storage technologies. In this project, the solvent-assisted exfoliation method has been employed to produce few-layered BNNS. In this method a polar solvent with similar surface energy to that of BNNS (e.g. organic solvents such as DMF, or acids) is used to cleave BNNS through ultrasonication energy to produce few-layered, stable dispersions of BNNS. Then, high surface area of BNNS was engineered with ion-conductive groups enabling the transfer of ions (e.g., H+). The augmentation of BNNSs significantly improved their ion-conductivity and electrochemical stability properties through covalent and non-covalent bonding with the hexagonal BN network. As a result, significant enhancement in both of in-plane and through-plane ion-conductivity was observed at different operating conditions (e.g., 0.34 S/cm at 25 °C and 50% relative humidity). The surface-engineered BNNS materials acted as high-performance solid electrolyte membranes in solid-state supercapacitors. In addition to electrochemical measurements, microscopic, spectroscopic and structural characterization tests were applied to investigate the chemical and surface structures, surface functionalization mechanism, the ion-conductivity mechanism, and physical properties of the developed BNNS.