Amorphous carbon coated TiO₂ nanocrystals embedded in a carbonaceous matrix derived from polyvinylpyrrolidone decomposition for improved Li-storage performance

Highlights

• Calcination temperature and carbon content were found to affect electrochemical properties of LIBs.

• The relationship between the LIBs performance and the carbon content is revealed.

• The TiO₂/C composite prepared at 400 °C exhibited large capacity.

Abstract

A facile method has been developed for the fabrication of TiO₂-C nanocomposite anode material, where a thin amorphous carbon layer is coated on the surface of TiO₂ nanocrystals for lithium-ion batteries (LIBs) application. To study the structure–property correlations, the effects of different carbon content and nanocrystals size of nanocomposites on the electrochemical properties of LIBs are systematically studied by post-calcination at various temperatures. Based on detailed experimental results, it is demonstrated that amorphous carbon-coated TiO₂ nanocrystals calcinated at 400 °C show the best electrochemical performance as compared with its counterparts at 500 °C and 600 °C. At 400 °C, the enhanced electronic conductivity from the decomposition of polyvinylpyrrolidone (PVP) seems to be the main reason for the improved capacity of TiO₂ nanocrystals-based LIBs. This unique architecture of anodes materials provides many important features for high-performance LIBs, such as fast ion transport and relatively high electrical conductivity, thus leading to the outstanding electrochemical performance of the electrodes. Such an electrode yields 228 mA h g⁻¹ capacity (1 C = 170 mA g⁻¹) even after 100 cycles. This method is proven to be an effective technique for improving the electrochemical performance and stability of TiO₂ based anode electrodes, especially for nanocrystal electrodes application in LIBs.

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.