Amorphous carbon coated TiO2 nanocrystals embedded in a carbonaceous matrix derived from polyvinylpyrrolidone decomposition for improved Li-storage performance
Graphical abstract
Introduction
Nanocrystals-based electrodes in high-performance lithium-ion batteries (LIBs) have attracted considerable interests due to the advantages of fast rate charge–discharge capabilities, long cycle life and high-energy density [1], [2], [3], [4]. However, the contact resistance of disconnected nanoparticles blocks the paths for the electronic transfer in the electrode materials, and consequently, reduces the power density. Moreover, the exposed interfaces between the electrodes and electrolyte arising from the nanocrystalline size also lead to the more undesired side reactions and the poor cycling performance. For example, it had been demonstrated that commercial grade, nanosized TiO2 (Degussa P25) exhibited a relatively poor Li-storage performance despite its relatively high specific surface area (∼50 m2 g−1), uniform nanoparticle and narrow size distribution [5], [6], [7], [8]. Therefore, nanocrystal-based LIBs have attracted less attention due to the above-mentioned reason. How to improve the performance of nanocrystal-based LIBs has become one key scientific topic we need to solve.
Recently, TiO2 has been most widely studied as a promising anode material for high-performance LIBs due to its high stability, wide availability of synthetic methods and cost-effective productivity [6], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. However, TiO2 material has an intrinsic poor electrical conductivity, and thus leading to poor electrochemical performance at high rate charge–discharge process. During the past few years, numerous research efforts have been carried out to improve the electrochemical performance of TiO2 by emplolying hydrothermal routes to obtain various complex hierarchical structures [5], [6], [9], [12], [13], [14], [21], [22]. These complex hierarchical structures have motivated chemists to design and fabricate a number of novel electrode materials, which have become one of the focuses of research and attention in LIBs. We have also recently reported the synthesis of hierarchical TiO2 structures with relatively high electrochemical properties, such as mesoporous hollow microspheres [21], [23], microtube arrays [22], anatase/rutile nanocomposite microspheres [6]. Unlike the most reported TiO2 hierarchical structures, a recent study by Bruce and co-workers had opened new and promising avenues for the utilization of nanocrystal electrode materials in LIBs [15]. Their work had clearly demonstrated the potential advantage and application of nanocrystal electrodes compared with traditional ones.
Recently, to tackle the poor conductivity of electrode materials, two common strategies have been used to solve this issue. For TiO2 anode materials, one is to enhance the electrical conductivity by coating with conductive materials such as carbon [24], [25], graphene [26], [27], conducting polymers [28], [29] or metal [10], [30]. The other is to dope N [22] or to introduce Ti3+ [31], modifying its crystal structure, to overcome its intrinsic poor conductivity drawbacks. Among most reported approaches, the hydrothermal method due to its simplicity is the most widely employed in the surface carbon coating technologies, which had been demonstrated that the as-obtained carbon layer can significantly improve the electronic conductivity of electrode materials and resulted in enhanced rate performance [24], [25], [32], [33]. In addition, it has been reported that the mesocrystals can be in situ carbon coated by introducing polyvinylpyrrolidone (PVP) [34]. With this as motivation, the synthesis of highly crystalline TiO2 nanocrystals fully coated with a conductive carbon layer would be an effective means by introducing PVP.
The control of the carbon content is a very important factor for evaluating the electrochemical properties since the carbon itself may involve the electrochemical reaction during charge–discharge process. A typical example is that 6 nm anatase nanoparticles with a high carbon content of up to 45 wt% reported by Baudrin and co-workers exhibited superior electrochemical properties [35]. However, important issues, for example, how much carbon content in the anode materials and how much nanocrystals size is appropriate for LIBs, have not been well answered, or even revealed. Although many previous studies were involved in the synthesis of carbon coated anode materials, the relationship between the electrochemical performance and carbon content is still not sufficiently clear. Therefore, understanding and exploring their correlations is necessary and critical for developing new electrode materials, which will have a positive effect on the nanocrystal electrode materials for their application in the high-performance LIBs in the future.
In this work, we specifically study TiO2 nanocrystals in an effort to understand how the nanocrystal size and carbon content influence the Li-ion diffusion kinetics and charge transfer kinetics via electrochemical characterizations. Herein, PVP was used as a carbon precursor of the conductive matrix to prepare TiO2–C nanocomposites by a one-step hydrothermal method. Moreover, an amorphous carbon layer was further covered on the surface of the TiO2 nanocrystals via in situ decomposition of PVP method at appropriate heat treatment temperature. The formation of the continuous conductive network could be more effective in improving the high-rate capability and cycle stability of the anodes. Furthermore, the effects of calcination temperature on the carbon content and nanocrystal size of TiO2–C are revealed. As an anode material for LIBs, the material achieved excellent rate capability, high charge–discharge capacities, good cycling performance.
Section snippets
Materials synthesis
Titanium butoxide and PVP were used as reagents without further purification. In a typical experiment, 1 mL of Titanium butoxide was slowly added to an aqueous solution of PVP (2 g of PVP dissolved in 60 mL of ethanol) in a flask under vigorously stirring. After being continuously stirred for 30 min in the flask, the solution was then transferred to a Teflon-lined stainless steel autoclave of 100 mL and kept at 180 °C for 14 h. The autoclave was cooled to room temperature, the as-prepared white
Results and discussion
Scheme 1 shows the fabrication process of the carbon-TiO2 nanocrystals in a continuous carbonaceous matrix and details in the experimental section. First, the aggregation of TiO2 units of hydrolysis of titanium butoxide occurred in ethanol to form small TiO2 nuclei. Here, the ethanol was utilized to control the titanium butoxide hydrolysis rate [36]. With an increasing reaction time, the TiO2 nanocrystals were then formed. In addition, we considered that PVP was widely applied as a surfactant
Conclusions
In summary, amorphous carbon coated TiO2 nanocrystals embedded in a carbonaceous matrix had been successfully prepared by a hydrothermal process in the presence of PVP, which acted as a ligand to control the nanocrystal growth, as well as carbon source. The unique conformation of this nanosized TiO2 was combined with its conductive carbon layer, which effectively enhanced its electrochemical performance. The highest charge–discharge capacity was achieved in the T-400 electrode with below 20 nm
Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 41272064), Department of Education, Guangxi Zhuang Autonomous Region of China (Nos. 200103YB061 and 201010LX188) and the fund from Guangxi Scientific Experiment Center of Mining, Metallurgy and Environment (No. KH2012YB004).
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