Hollow CuO nanoparticles in carbon microspheres prepared from cellulose-cuprammonium solution as anode materials for Li-ion batteries

Highlights

• Biomass derived hollow CuO@C microspheres were prepared by pyrolysis.

• The hollow structure of the microspheres formed following the Kirkendall effect.

• The hollow CuO@C microspheres exhibited excellent electrochemical performance.

Abstract

Biomass derived carbon with hollow metallic structures is considered as eco-friendly and sustainable materials, especially as electrode materials, absorbents and catalysts. Herein, hollow CuO nanoparticles (NPs)@carbon (C) microspheres were fabricated by directly using a cellulose-cuprammonium solution as the starting metallic precursor along with the carbonization and oxidation processes. The hollow CuO NPs were formed spontaneously during the oxidation process following a modified Kirkendall effect. The cellulose derived carbon assisted the formation of the hollow structure and prevented the aggregation of CuO NPs during the thermal treatment. As an integrated material, CuO@C microspheres were used as anodes for lithium-ion batteries. The anodes of CuO@C microspheres exhibited a good cycling stability with a discharge capacity of 630 mA h/g at a rate of 0.1 C (1 C = 1 A/g), which is close to the theoretical specific capacity of the anodes of the pure CuO (670 mA h/g), and a good rate performance with high discharge capacities of 361 mA h/g at a rate of 5 C, which is much higher than that of the bare CuO microspheres (57 mA h/g at 5C). The excellent electrochemical performance could be attributed to the unique carbon combined hollow CuO NPs. This work provided a new pathway for preparing biochar based metallic materials with the inexpensive precursor and high applicable performance.

Introduction

Cellulose, as the most abundant and chemical compatible material, has been widely used as an efficient matrix to combine with other materials [1], [2], [3]. Because of the porous structure, the cellulose matrix in the hybrid not only acts as substrate but also sufficiently stables the nanostructures within it. Carbon-based materials and metallic nanoparticles (NPs) are the most common materials embedded into cellulose [4], [5]. Various hybrid materials based on cellulose were fabricated [6], [7], [8], and have been used in the fields such as thermal conductive film [9], biosensor [10], catalyst [11], [12], transparent conductor [13], optical device [14] and electrodes [15]. Integrating cellulose hosted materials with different NPs may afford new designed materials with advanced set of functions and enlarge the application of cellulose.

Hollow materials are a bunch of materials which contain the hollow structure inside them. Proper holes give these materials large surface areas and considerable inner volume to adapt to the volume changes during electronic and catalytic processes [16], for cargo delivery [17], or to form channels enabling the free movement of substances within the materials [18]. Template method is the most common way to create well-shaped hollow materials. SiO₂ [19], bubbles [20], water soluble salts [21] and carbon-based polymers [22] have been used as efficient templates to form hollow structural materials. However, these processes are all involved the steps to remove the templates, which complicate the method and induce uncertainties. Nowadays, non-template method gradually became a significant approach to fabricate hollow materials [23]. Oswald ripening [24] and Kirkendall effect [25] are the well-known mechanism for the formation of hollow NPs. However, the non-template method also faces the shortages especially for the cost of precursor and low-yields. To further develop the application of hollow materials, finding a new approach to make integrated hollow materials, lower the cost, and increase the yields is still a priority and challenge.

Copper oxides, like other transitional metal oxides [26], [27], [28], [29] have shown remarkable potential as an energy conversion media [30], [31], [32] for lithium-ion batteries (LIBs), due to their abundance, environmental friendliness, chemical stability and high theoretical lithium storage capacity (670 mA h/g) [33], [34]. However, traditional CuO anodes suffer from a large initial irreversible capacity loss, poor cycling stability, and undesirable rate performance [35], [36]. To overcome these problems, basic strategies have been applied to relieve the volume variation and to improve the electrical contact of the anodes. Most methods require multiple steps, and the growth of the particles needs to be carefully controlled, which would limit the mass production of the anodes. Therefore, a facile method to produce CuO-based LIB anodes is necessary to fulfil these drawbacks.

Herein, we describe a new approach to fabricate carbon microspheres embedded with hollow CuO NPs. In order to simplified the process of making precursor and lower the cost, the copper-enriched cuprammonium-cellulose solution was chosen, as it naturally contains a lot of copper compounds and favourable for preparing copper NPs/cellulose materials [37]. Chemical crosslinked cuprammonium-cellulose microspheres were prepared and used as precursors to fabricate the hollow CuO NPs embedded carbon microspheres. The spontaneous formation of the hollow structure is associated with Kirkendall effect. Cellulose and its derived carbon played the key role in the formation of hollow structure. The obtained microspheres had two-level structures including micro-sized spheres and nano-sized hollow particles. When using as anodes of LIBs, the combination of carbon and the hollow CuO NPs provided higher electrochemical properties than the bare CuO microspheres. Above all, this is a facile and reliable method to create multiply hollow NPs hybridized carbon based materials.