Preparation of coal-based graphene quantum dots/α-Fe₂O₃ nanocomposites and their lithium-ion storage properties

Abstract

Nano-Fe₂O₃ particles on a nickel substrate have been obtained by electrodeposition technique adjusting the ratio of electrolyte solvent (DMF and water), and then it was used as the working electrode to obtain the C-GQDs/α-Fe₂O₃ composite material via second-step electrodeposition with the coal-based graphene quantum dots (C-GQDs) solution which had been prepared from Taixi anthracite powder as the electrolyte. The lithium-ion storage performance of C-GQDs/α-Fe₂O₃ composites as the anode in the lithium-ion battery was studied, and the results show that the composites exhibited excellent cyclability and rate capability. When the current density was 1 A/g, the specific capacitance of C-GQDs/α-Fe₂O₃ composites was up to 1582.5 mAh/g, and it could maintain 1320 mAh/g after 110 cycles. The specific capacitance was 1091 mAh/g at a high current density (5 A/g).

Introduction

The advantages of lithium-ion rechargeable batteries include high energy density, high operating voltage, long cycle life, absence of memory effect, and safe use. In recent years, lithium-ion rechargeable batteries have been widely used in portable appliances, electric vehicles, medical equipment, and other products [1], [2]. Graphite is currently used as an anode material in commercial lithium ion batteries [3]. However, due to its low theoretical capacity (372 mAh/g) and safety issues, graphite cannot meet the growing demand for lithium-ion batteries in high-power equipment applications. Therefore, finding an environmentally friendly and safe anode material with a high theoretical capacity to replace graphite is an urgent issue.

The use of Fe₂O₃ as an anode material for lithium ion batteries is popular because it is environmentally friendly, abundant, and low cost. It has a high theoretical capacity (1007 mAh/g) [4], [5], [6]. For example, Shakir et al. [7] designed a porous Fe₂O₃ aerogel structure encapsulated in three-dimensional graphene, and its reversible specific capacitance could reach 1129 mAh/g after 130 cycles at a current density of 0.2 A/g. Lee [8] reported a RGO/α-Fe₂O₃ hollow nanorod composite material—the composite material was synthesized through a one-step microwave-assisted hydrothermal method, and the unique porous structure showed excellent rate capability and cyclability. However, the Fe₂O₃ anode material exhibits poor electrical conductivity, and the changes of the volumetric expansion during the charge and discharge process will break the crystal structure. The changes during the insertion and extraction of lithium ions result in poor contact between the active material and the current collector. Consequently, the cycle life of the electrode is affected, leading to capacity loss and decreased electrochemical performance [9]. Researchers have attempted to solve the problem by constructing a graphene quantum dots conductive network and a unique nano-skeleton.

Taixi anthracite is a high-carbon mineral that is common in nature and has low ash, low phosphorus, low sulfur, high degree of graphitization, and good thermal stability [10], [11], [12], [13]. Coal-based graphene quantum dots (C-GQDs) were prepared via an acid oxidation method from Taixi anthracite powder, and two-step electrodeposition was used to prepare a self-supporting and binder-free C-GQDs/α-Fe₂O₃ composite electrode through a simple preparation process. This composite electrode could solve issues of hindered ion transfer, decreased electric conductivity, and apparent capacity loss due to the addition of a binder during the preparation of conventional electrodes. In addition, this new high-efficiency anode material for lithium-ion batteries was fabricated using a unique spatial structure of nano-Fe₂O₃ as well as an electrically conductive network possessing the “bridge effect” of C-GQDs. And this approach expands a way of high valued-added using of anthracite.