Sol-gel synthesis of mesoporous Co₃O₄ octahedra toward high-performance anodes for lithium-ion batteries

Highlights

• Mesoporous Co₃O₄ octahedra have been prepared with simple sol-gel method.

• Porous structure buffers the volume change and shortens the diffusion path of Li⁺.

• Anisotropy octahedral morphology guarantees structural stability.

• It exhibits high capacity, superior capacity retention and excellent rate capability.

Abstract

Unique mesoporous Co₃O₄ octahedra are prepared with the sol-gel method as negative electrode material for lithium-ion batteries. Such an interesting nanostructure that has inherited the combined advantages of anisotropy morphological structure stability and porous feature, which can buffer the volume changes during the electrochemical cycles and shorten the diffusion path of Li⁺/electron transport, exhibits remarkable electrochemical performance, including high reversible capacity (1195 mAh g⁻¹ at rate of 200 mA g⁻¹), good capacity retention (capacity fading of 0.4% per cycle upon 60 cycles), and excellent rate capability (681 mAh g⁻¹ at high rate of 2 A g⁻¹).

Introduction

The need for high-performance rechargeable batteries in electronic devices and electrical/hybrid vehicles has led to dramatic development of lithium-ion batteries (LIBs). Graphite, the widely used commercial anode material, can not fulfill the increasing demand due to its low theoretical capacity (372 mAh g⁻¹). Intensive research efforts have been devoted to developing new high-performance (high capacity, high power and high rate capability) anode materials for the next generation LIBs [1]. Among all the materials applicable for LIBs anodes, transition metal oxides have always been considered as the more promising candidate to replace the commercial graphite anode due to their high theoretical capacity (∼500-1000 mAh g⁻¹) [2], [3], [4], [5], [6], [7], [8], [9]. For instance, as a promising anode material of LIBs, Co₃O₄ has recently been subjected to extensive research, owing to its low cost, high theoretical capacity (890 mAh g⁻¹, calculated based on the conversion mechanism: Co₃O₄ +8Li⁺ + 8e⁻ = 3Co + 4Li₂O), good chemical/thermal stability, and environmental benignity [2]. Unfortunately, there remain major challenges for its practical use in LIBs, because Co₃O₄ electrode usually suffers from bad capacity retention upon cycling and poor rate capability. The electrode failure is mostly attributed to its intrinsic drawback of low ionic and electronic conductivity, which results in slow charge/discharge rate; and the significant volume change during repeated Li uptake and removal reactions leading to electrode pulverization. One generally accepted method to overcome these obstacles is to design composites of Co₃O₄ mixed with carbon materials or conductive substrates [5], [10], [11], [12], [13]. However, the additional conductive carbon or substrates will sacrifice the capacity of Co₃O₄, and lead to the emergence of undesirable interfaces and defects, causing lower electron transfer velocities and electrolyte diffusion efficiency. In addition, it is accepted that the electrochemical performance of Co₃O₄ strongly depends on its particle sizes, shapes and morphological structures. Indeed, it is indicated in many reports that the electrochemical performance of Co₃O₄ anode, including capacity and stability, will evidently be improved when the materials possess either small size, or appropriate pore-size distribution and special morphology, especially the combination of the above structural characteristics. In this regard, more and more efforts have been made to synthesize novel and diverse tailored nanostructured Co₃O₄ materials with excellent electrochemical performance [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. For instance, Wang et al. prepared a series of multishelled Co₃O₄ hollow spheres assembled with nanosheets, which exhibited excellent cycle performance and enhanced lithium storage capacity [18]. Du et al. synthesized porous Co₃O₄ nanotubes with good electrochemical properties [19]. Wang et al. prepared multishelled hollow Co₃O₄ microspheres as high-performance anode materials in LIBs [20]. All these reports have revealed that the hollow/porous structures can undoubtedly improve the electrochemical performance of Co₃O₄ materials. According to other literatures, the specific anisotropy morphologies such as nanocube [21], [22] and octahedral [23], [24] will also effectively improve the cyclability and reversible capacity of Co₃O₄ anode. But the well-defined porous Co₃O₄ with anisotropy structures for LIBs anodes have rarely been reported. So there is an emergency to develop a method for the synthesis of novel high-performance Co₃O₄ anode materials with porous anisotropy structures that possess the combined advantages of the above structural characteristics.

In the past few years, Pluromic type triblock copolymer (HO(CH₂CH₂O)₁₀₆(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₁₀₆H) (F127) has been proved to be an efficient capping agent and template to synthesize porous metal oxide anode materials [27], [28], [29]. However, to the best of our knowledge, the anisotropy metal oxides with porous structure have not been reported previously. Herein, for the first time, we demonstrate a soft-chemical sol-gel route for the synthesis of nanosized porous Co₃O₄ octahedra with F127 as soft-template. The as-obtained unique architecture intrinsically possesses stable octahedral structure and porous features, which can alleviate the pulverization, buffer the drastic volume changes, and shorten the diffusion path of Li⁺/electron transport, thus leading to improved electrochemical performance.