Graphene microsheets from natural microcrystalline graphite minerals: scalable synthesis and unusual energy storage

Abstract

Mass production of graphene from graphite at a low cost is essential for its practical application since there is huge storage of natural graphite minerals on earth. However, extracting graphite from the minerals usually involves a complex and polluted purification process. Here, natural microcrystalline graphite minerals were directly used to produce high-quality graphene microsheets at a high yield of >70% through a scalable electrochemical & mechanical exfoliation approach. The graphene microsheets present the features of small sheet sizes of 0.2–0.6 μm² and <5 atomic layers, few defects and high purity. The graphene microsheets can be highly dispersible in various solvents (the absorption coefficient of graphene microsheets dispersed in isopropanol is around 11.00 cm⁻¹) and printable/paintable to make conductive films with a low sheet resistance of ∼10 ohm sq⁻¹. The graphene products were used for energy-storage electrodes for a supercapacitor and a lithium ion battery. The supercapacitor reaches a high-rate areal performance of 77 mF cm⁻² area capacity at a high charge/discharge rate of 20 mA cm⁻². Notably, graphene anode batteries have a high coulombic efficiency of 99.2% and a high reversible specific capacity of 390 mA h g⁻¹ (after 220 cycles) at 40 mA g⁻¹ and of 200 mA h g⁻¹ at 595 mA g⁻¹ for a fast charge/discharge time of 17 min. This investigation demonstrates that graphene microsheets can be directly prepared from natural graphite minerals at high yield and low cost and potentially used for high-rate energy storage.