Facile synthesis of CNTs@TiO₂ composites by solvothermal reaction for high-rate and long-life lithium-ion batteries

Highlights

• CNTs@TiO₂ composites were prepared using solvothermal reaction.

• Glycerol acts as reactant and confined agent during solvothermal process.

• CNTs@TiO₂ composites show good high-rate and cycling properties as anode materials.

• Rapid conductive network formed by CNTs and pseudocapacitance ensure good properties.

Abstract

Multi-walled carbon nanotubes (CNTs)@TiO₂ composites with different contents of CNTs were prepared by employing solvothermal process using ethanol-glycerol mixture as solvent and subsequent calcination. The role of glycerol during solvothermal process was discussed and the microstructures of the as-prepared CNTs@TiO₂ composites were characterized by scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. The results indicate glycerol is one of the solvothermal reactants and also play an important role for the coating of TiO₂ particles on the surface of CNTs because of its high viscosity. Moreover, with the increasing of the content of CNTs in composites, the surface areas of composites increase and TiO₂ layer becomes more uniform. Notably, CNTs@TiO₂ composites with 42 wt% CNTs have a specific surface area of 265.7 m² g⁻¹, and exhibit excellent high-rate performance and cyclic stability. Their specific capacities at 1C, 2C, 5C, 10C, 20C, 30C and 40C are 248, 228, 225, 212, 200, 194 and 191 mAh·g⁻¹, respectively. Even at 50C and 60C, their specific capacities are still as high as 187 and 184 mAh·g⁻¹, respectively. Moreover, 90.1% of the reversible capacity is retained after 1000 cycles at 10C. The excellent performance can be ascribed to the electronic and ionic rapid conductive network formed by CNTs in composites and the synergistic contribution from pseudocapacitance.

Introduction

Anatase titanium dioxide (TiO₂) has been intensively studied as one of the most prominent anode materials for lithium-ion batteries (LIBs), owing to its low cost, chemical and physical stability, environmental benignity and favorable theoretical capacity [[1], [2], [3]]. In addition, it has been demonstrated that the volume variation during charging-discharging processes is less than 4%, which affords anatase TiO₂ excellent structural stability and long cyclic life as anode material for LIBs [[4], [5], [6]]. Moreover, its high charging-discharging platform can essentially avoid the formation of lithium dendrites, thus improving the safety of LIBs [7,8]. However, poor intrinsic electronic conductivity (10⁻¹²-10⁻⁷ S cm⁻¹) and low lithium-ion diffusion coefficient (10⁻¹⁵-10⁻⁹ cm² s⁻¹) of anatase TiO₂ restrict its electrochemical performance, especially at high charging-discharging rate [[9], [10], [11]].

To improve the electrochemical performances of TiO₂ anode materials, multiple attempts have been made to tune their properties. One way is to synthesize nano-TiO₂ materials with various morphologies and hierarchical structures, which can not only shorten the lithium-ion diffusion distance but also provide large electrode-electrolyte contact interface [[12], [13], [14]]. Moreover, nanoscale particle size of electrode materials can bring about pseudocapacitive reaction associated with surface-controlled reactions which are not kinetically limited by solid-state diffusion [[15], [16], [17], [18]]. Ion doping has also been found to be an effective way to enhance the conductivity of TiO₂, by modifying the number of free charge carriers or changing slightly the lattice distance [19,20]. In addition, fabricating composites of TiO₂ and high conductive carbon materials (e.g., graphene [[21], [22], [23]] or carbon nanotubes (CNTs) [24,25]) is also an effective approach to enhance the energy-storage kinetics of TiO₂. Carbon materials in composites could suppress the agglomeration of TiO₂ particles, mitigate the mechanical stress caused by insertion and extraction of lithium ions, and compensate for the poor electronic and ionic conductivities of TiO₂.

Generally, CNTs with high conductivity, electrochemical stability and excellent mechanical property, have been widely used as matrix to fabricate CNTs@TiO₂ composites which exhibit improved electrochemical performances as anode materials of LIBs. It is well known that the synthetic strategy usually plays an important role in material structures which will determine their performances. Different methods (e.g., sol-gel method [26] and self-assembled method [27]) have been investigated to prepare CNTs@TiO₂ composites. Solvothermal process has also been proved to be an efficient strategy for synthesizing of oxides and their nanocomposites with controllable structures [[28], [29], [30]].

In this paper, CNTs@TiO₂ composites with anatase TiO₂ nanoparticles anchored on CNTs were fabricated by facile solvothermal process followed by calcination, as shown in Fig. 1. In solvothermal process, glycerol acts not only reactant which reacts with tetrabutyl titanate (TBT) to form the precursor of TiO₂ (i.e., titanium glycerolate), but also play an important role for the coating of TiO₂ particles on the surface of CNTs. The as-prepared CNTs@TiO₂ composites exhibit excellent high-rate capabilities and cyclic stabilities as anode materials for LIBs, which should be attributed to the rapid conductive network of ions and electrons formed by CNTs and the pseudocapacitive contribution.