Nano-Co₃O₄ anchored helical carbon nanofibers as an anode material for Li-ion batteries

Highlights

• A novel Co₃O₄/HCNFs anode composite was first introduced for lithium-ion batteries.

• A simple two-step approach was successfully used to prepare the Co₃O₄/HCNFs composite material.

• Co₃O₄/HCNFs composites make an obvious advantage as an anode material for lithium-ion batteries.

Abstract

In the current work, the Co₃O₄/helical carbon nanofibers (Co₃O₄/HCNFs) composite was successfully obtained by using a simple and inexpensive two-step method and applied as an anode material for lithium-ion batteries (LIBs). The results showed that Co₃O₄ nanoparticles (the nanoparticle size of 10–20 nm) were anchored on the surface of helical carbon nanofibers. The Co₃O₄/HCNFs anode displays an excellent cyclability of 853 mAh/g after 200 cycles under the current density of 200 mA/g (327.7 %, 183.7 %, and 172.0 % higher than the pure HCNFs, pure Co₃O₄, and Co₃O₄-HCNFs respectively). The superior electrochemistry capability of the Co₃O₄/HCNFs composites are mainly thankful to the specific 3D helical structure of HCNFs, which can not only accommodate the volume expansion of nano-Co₃O₄ during charge–discharge process, but also greatly enhance the conductivity of Co₃O₄. Therefore, this research provides a novel method to exploiting an excellent and stable anode material for enhanced LIBs.

Introduction

Li-ion batteries (LIBs) have been broadly applied in electric vehicles, portable devices and other energy storage systems, due to their inherent advantages of high stability and environmental friendliness, in the past decades [1], [2], [3], [4], [5]. Meanwhile, with the intensification of the fossil fuel consumption and the growth of environmental pollution, the demand for stable energy storage system like lithium-ion batteries is increasing. Nevertheless, the commercial graphite as the mainstream anode cannot fulfill the requirement of rapid exploitation of the high-energy–density storage system [6], [7], [8], [9]. Consequently, it is anxious to exploit novel anode materials with lower cost, better cycling stability, environmental friendliness, and higher energy density for LIBs.

In recent years, more and more researchers focused on anode materials [10], [11], [12]. Electrochemically active transition metal oxides as a class of anode materials, such as SnO₂ [13], ZnO [14], Fe₂O₃ [15], CoO [16], and Co₃O₄ [6] have been widely researched as electrode materials for LIBs. Note that the Co₃O₄ has a higher theoretical capacity of 890 mAh/g [17], [18], but it has low conductivity and huge volume change (about 200 %) during charging and discharging progress, which will lead to rapid pulverization and capacity decay of the LIBs [19]. Many researchers have investigated the modification of Co₃O₄ nanoparticle, and various Co₃O₄-based composites were prepared as anode materials for LIBs. Currently, new carbon materials can be used to modify Co₃O₄, including graphene (GO), carbon nanotubes (CNTs), carbon microspheres (CM), carbon fibers (CNFs), etc. Wang et al. [10] employed an easy thermal decomposition method to prepare Co₃O₄-NC/CNTs anode composites, which displays a discharge capacity of 441 mAh/g after 50 cycles at 100 mA/g. Ren et al [20] obtained a Co₃O₄/GO anode composite with a network of interpenetrating 3D porous nanostructure by using hydrothermal treatment, having an excellent electrochemistry performance. For these researches, however, the tedious process operation, expensive cost and poor practical applicability limit the development of Co₃O₄-based composites. In fact, it is difficult to find a facile and inexpensive method for synthesizing Co₃O₄-based anode material with stable and outstanding performance.

According to our latest research [21], [22], the helical carbon nanofibers (HCNFs) were obtained by low-temperature chemical vaper deposition (CVD), and this method is simple, reproducible and relatively low cost. In this research, Co₃O₄/HCNFs composite was obtained by employing a simple method and served as a higher performance anode material for LIBs. The results of electrochemical tests showed that the Co₃O₄/HCNFs composite reaches excellent cycling performance (853.0 mAh/g under 200 mA/g after 200th cycles) and a higher capacity retention ratio (77.1 %). Furthermore, an outstanding ratio performance of 567.3 mAh/g was obtained at 2000 mA/g. As a matter of fact, the Co₃O₄/HCNFs composite have a lot of inherent advantages as an anode, such as: (1) the characteristic helical structure of HCNFs can provide a well entangling 3D structure for anode materials. (2) the excellent electrical conductivity of HCNFs can significantly improve weaker conductive of Co₃O₄ nanoparticles. (3) a more stable network structure for the accommodation of volume expansion of Co₃O₄ nanoparticle was obtained, compared with other composites. In general, our research provides a facile way to develop an excellent anode material for enhanced LIBs, and the preparation process of Co₃O₄/HCNFs composite is shown in Fig. 1.