Research ArticleTi–C bonds reinforced TiO2@C nanocomposite Na-ion battery electrodes by fluidized-bed plasma-enhanced chemical vapor deposition
Graphical abstract
Introduction
Carbon coatings have been extensively used in surface modification of nanoparticles used as active materials for energy storage [[1], [2], [3], [4], [5]]. Since a considerable number of energy storage materials have low electrical conductivity, a carbon coating layer is one of the effective ways to address the issue. In addition, carbon coating is often used to improve the structural stability, chemical stability, and electrochemical reversibility of active nanomaterials [[6], [7], [8], [9], [10], [11], [12], [13]].
A variety of methods, such as sol-gel, spray pyrolysis, ball-milling, co-precipitation, and chemical vapor deposition (CVD), have been developed for diverse carbon coatings on active materials for energy storage. Carbon coatings achieved by incorporating organic precursor coating (e.g., sucrose, glucose, polyaniline, and so on) followed by pyrolysis is a widely used method [14]. However, uneven carbon coatings often arise because of the incompatibility between some carbon precursors and nanoparticles. Alternatively, some advanced techniques, such as in-situ polymer-functionalized nanoparticles, have been developed [15,16]. Despite the much improved electrochemical performance and cycling stability, cheaper, less complex and higher-precision processes are highly desired. One of the key challenges is the sufficient bonding strength between the carbon layer and the coated particles, which is rarely achieved because the formation of interfacial carbon bonds is energy demanding and difficult to form at low, industry-relevant temperatures.
Another major drawback of high-temperature processing is that active materials often undergo undesired phase transitions at high temperature, which degrades their electrochemical performance. This happens for anatase titanium dioxide (TiO2) which is one of the most promising energy materials for negative electrodes of sodium-ion batteries (SIBs), with the theoretical specific capacity of 335 mA h g−1, environmental friendliness, Earth abundance, and excellent cycling stability [[17], [18], [19]]. Above 600 °C, the anatase phase is easily transformed into the rutile phase with lower capacity of 270 mA h g−1 [20]. Therefore, it is crucial to accomplish uniform carbon coatings at lower process temperatures.
Fluidized bed is a long-established and mature industrial powder batch processing technology. The heat and mass transfer efficiencies are very high around the fluidized particles, leading to efficient and uniform powder processing [[21], [22], [23]]. The combination of fluidized bed and CVD techniques has been used for thin film deposition on various powder materials [9,[24], [25], [26]]. Further modification of this approach has led to a scalable synthesis of carbon nanotubes (CNTs) based on the floating catalysis method, thus opening opportunities for commercial CNT manufacturing [27]. The fluidized bed coupled with plasma can perform particle surface treatment more efficiently at reduced temperature due to the energetic free electrons and free radicals [14,[28], [29], [30], [31], [32]]. Recently, we have reported a uniform graphene coating on NaTi2(PO4)3 nanoparticles by a fluidized bed plasma-enhanced CVD (FB-PECVD) [33].
On the other hand, stimulated by the energetic plasmas, new phases that are beneficial to energy storage can be produced. This could be useful for the design of energy storage materials. We have recently demonstrated the effective transformation of SnO2/nitrogen-doped graphene (NG) and Fe2O3/NG to SnO2@Sn/NG core-shell structure and Fe2O3/Fe3O4/NG mesoporous structure, respectively, in a plasma-enhanced fluidized bed reactor. The newly generated metallic Sn and the conductive Fe3O4 both promoted the electrochemical performances of the composite materials [34,35].
Here we hypothesize that the TiO2/TiC hybrid phase can be formed by plasma processing of TiO2 nanoparticles in the presence of carbon precursor. Since the reported conductivity of TiC can be as high as 3 × 107 S/cm [36], far beyond that of some multilayer graphenes (2700 S/cm [37]), the TiO2/TiC hybrid phase can be expected to perform much better than the TiO2 pure phase in electrochemical energy storage devices.
Here, we report on a one-step carbon coating on commercially available anatase TiO2 nanoparticles (TiO2@C) using the FB-PECVD. Thanks to the efficient mass and heat transfer in the fluidized bed, the carbon layer is uniformly and rapidly grown on the TiO2 particles. The use of plasmas also enables carbon coating at significantly lower temperatures, thereby ensuring no phase transition to rutile TiO2. Importantly, plasma exposure induces the formation of Ti–C bonds at the TiO2/C interface, which makes the structure of TiO2@C more stable and also enhances the electronic coupling between TiO2 and carbon. As a negative material for SIBs, the TiO2@C shows superior sodium storage performances: specific capacity of 290.2 mA h g−1 at 50 mA g−1 and 101.2% capacity retention (119.8 mA h g−1) over 300 cycles at 4000 mA g−1. This carbon coating method can be extended to different types of active materials, while the generation of hybrid phases will enrich the design strategies of advanced energy storage materials.
Section snippets
Material synthesis
The fabrication of TiO2@C is schematically illustrated in Fig. 1. Anatase TiO2 nano powder (1 g, Aladdin, purity > 99.8%, 25 nm) was loaded into a quartz fluidized bed. After pumping down to 10 Pa, a mix of methane (10 standard cubic centimeter per minute, sccm) and argon (10 sccm) was flown upward in the reactor. The power of the magnetron was ramped up to 800 W to ignite the plasma. Then, the pressure was adjusted to 100 Pa which ensured powder fluidization and plasma operation
Low-temperature production of TiO2@C
Compared with the fixed bed, the most prominent advantage of fluidized bed reactors is the floating movement of particles, which ensures homogenous mixing of gas and solid particles, as well as the efficient heat and mass transfer (Fig. 1a). All nanoparticles are exposed in the gas of free radicals, ions, and electrons in the FB-PECVD reactor (Fig. 1b), which is essential for uniform carbon coating. By the activation of plasma, the coating temperature is substantially reduced. The temperature
Conclusion
In summary, TiO2@C composite energy storage material with uniform carbon coating was successfully obtained by a versatile single-step FB-PECVD method. The TiO2@C electrodes exhibit a Na-storage capacity up to 290.2 mAh g−1 at 50 mA g−1. Excellent long-term cycling stability with capacity retention of 101.2% (119.8 mA h g−1) was also obtained after 300 cycles at 4000 mA g−1. The excellent electrochemical performance can be attributed to the uniform carbon coating with superior defects as ion
CRediT authorship contribution statement
Shuyue Yao: Investigation, Writing - original draft. Yujie Ma: Validation, Writing - original draft. Tianhao Xu: Methodology, Investigation. Zhongyue Wang: Validation, Resources. Peng Lv: Validation, Resources. Jiajin Zheng: Validation, Resources. Chen Ma: Visualization. Kehan Yu: Conceptualization, Methodology, Writing - review & editing, Funding acquisition. Wei Wei: Supervision, Project administration. Kostya (Ken) Ostrikov: Conceptualization, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is financially supported by the Program of Distinguished Professor of Jiangsu Province. We thank Chunyuan Song at Nanjing University of Posts and Telecommunications for technical support of Raman spectroscopy. K.O. thanks the Australian Research Council (ARC) and QUT Centre for Materials Science for partial support.
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