Facile complex-coprecipitation synthesis of mesoporous Fe₃O₄ nanocages and their high lithium storage capacity as anode material for lithium-ion batteries

Highlights

• MFONs are synthesized by a facile complex-coprecipitation method.

• MFONs with high surface area lead to excellent electrochemical performance.

• MFONs anode retains a capacity of 573 mAh g⁻¹ at 1 A g⁻¹ after 300 cycles.

Abstract

In this study, high-quality mesoporous Fe₃O₄ nanocages (MFONs) have been synthesized by a facile complex-coprecipitation method at 100 °C with addition of triethanolamine and ethylene glycol. The as-prepared Fe₃O₄ nanocages possess a mesoporous structure and highly uniform dispersion. When used as an anode material for rechargeable lithium-ion batteries, MFONs anode shows high specific capacities and excellent cycling performance at high and low current rates. At a current density of 200 mA g⁻¹, the discharge specific capacities are 876 mAh g⁻¹ at the 2nd cycle and 830 mAh g⁻¹ at the 100th cycle. Even at the high current density of 1000 mA g⁻¹, MFONs anode still retains a stable capacity of 573 mAh g⁻¹ after 300 cycles. This superior electrochemical performance is attributed to the unique mesoporous cage-like structure and high specific surface area (133 m² g⁻¹) of MFONs, which may offer large electrode/electrolyte contact area for the electron conduction and Li⁺ storage. Furthermore, the good mechanical flexibility of the mesoporous nanocages can readily buffer the massive volume expansion/shrinkage associated with the reversible electrode reaction. These results indicate that MFONs can be used as a promising high-performance anode material for lithium-ion batteries.

Introduction

In the past several decades, magnetic nanomaterials have attracted much interest in functional materials and colloid interface science [1], [2], [3], [4], [5], [6]. Among them, magnetite (Fe₃O₄) has been widely utilized in many fields, such as magnetic ferrofluids [7], magnetic response imaging and sensing agents [8], [9], [10], catalysis [11], [12], [13], electrical device [14], [15], data storage [16], biomedical drug loading and delivery [17], [18], [19], [20], and energy storage devices etc [21], [22], [23], [24], [25], [26]. Recently, more attention has been devoted to the possibility of Fe₃O₄ as anode materials for lithium-ion batteries (LIBs) because of their high theoretical capacity (928 mAh g⁻¹), nontoxicity, natural abundance, low cost, and half-metal conduction mechanism [27], [28]. However, the cycling stability of capacity for Fe₃O₄ anode is still unsatisfactory due to the aggregation of Fe₃O₄ particles and the large volume variation during the charge-discharge cycling process, which inherently accompanies the electrochemical conversion reaction as expressed by the following reaction [29].

Fe₃O₄ + 8Li⁺ + 8e⁻ ↔ 3Fe + 4Li₂O (1)

In order to circumvent the above intractable problems, two typical methods have been presented. One way is to synthesize the Fe₃O₄/carbon composite materials [30], [31], [32], [33], [34], [35], [36], [37]. It can not only alleviate the inner volume change during the cycling process but also enhance the electronic conductivity of the composites. Numerous investigations have been focused on the hybrid anodes composed of Fe₃O₄ and carbon at the nanoscale. A series of Fe₃O₄/carbon hybrids, such as Fe₃O₄ nanoparticles embedded in a porous carbon matrix or a mesoporous carbon foam [30], [31], [32], [33], Fe₃O₄ nanoparticles with carbon matrix support, carbon-coated Fe₃O₄ nanostructures [34], [35], two-dimensional (2D) graphene/Fe₃O₄ [36], and carbon nanosheets/Fe₃O₄ composites [37], have all been extensively reported. However, to achieve the high-quality Fe₃O₄/carbon hybrids, the synthesis is normally operated at high temperatures (∼500–750 °C) under specific atmospheres (N₂ or Ar). The other promising way is to construct the nanostructure materials with various morphologies, including nanoparticles, nanotubes, nanosheets, nanowires, and nanorods [38], [39], [40], [41], [42], [43], [44]. It is assumed that these nanostructures may supply the large surface area, the sufficient contacts between active electrode and electrolyte materials, and the short diffusion lengths for the transport of electrons and lithium ions. In addition, the strain associated with Li⁺ intercalation and the volume variation of active electrode materials are normally accommodated, further improving their electrochemical performance. Some recent publications proposed that the capacity retention of iron oxides can be improved by hollow or porous nanostructures [45], [46], [47], [48], [49]. We expect the production of high-quality mesoporous Fe₃O₄ nanostructures, which can be used as stable anode materials for lithium-ion batteries. Furthermore, the high performance in specific electrochemical applications requires that the composite possesses some unique features, such as suitable particles size, narrow size distribution, large surface area, uniform mesoporous microstructure. These would result in high lithium storage capacity. The design of high-quality mesoporous Fe₃O₄ nanomaterials should be highly promising in power technology.

Such nanostructure, one of the most popular and extensively used nanomaterials, is of fundamental importance in preventing the aggregation, realizing the interaction between nanoparticles and other composites, and affecting the properties of nanoparticles. It is well known that magnetic nanoparticles tend to agglomerate because of strong magnetic dipole-dipole attractions between particles combined with van der Waals force and high surface energy [50]. The coprecipitation technique is the most important and widely used route to prepare the Fe₃O₄ nanoparticles in the laboratory. However, for classical coprecipitation route, the size distribution of particles is difficult to be optimized because of the complexity of controlling the nucleation, unlimited growth of the nanoparticles after nucleation, and the rapid hydrolysis reactions of the iron precursors.

Herein, we report a facile complex-coprecipitation method at 100 °C for the preparation of uniform mesoporous Fe₃O₄ nanocages assembled with nanoparticles as primary building blocks. In this study, triethanolamine (TEA) is used as a ligand and surface stabilizer; ethylene glycol (EG) serves as a template agent, which is crucial to the formation and transformation of the mesopore interiors. This mesoporous structure has an intimate relationship with the reversible capacity, increasing electrode contact area, and promoting the lithium ion diffusion. As a result, single-phase MFONs exhibit the good rate performance compared with some previous Fe₃O₄ nanostructures, and show the excellent cycling performance at high rates, suggesting the promising potential as a durable high-rate anode material for LIBs.