Short communicationLow-temperature preparation of mesoporous TiO2 honeycomb-like structure on TiO2 nanotube arrays as binder-free anodes for lithium-ion batteries
Graphical abstract
Introduction
Lithium-ion battery (LIB) is one of rechargeable batteries with promising applications in electronic products, such as smartwatch, toys, laptops and so on. Recently, more and more electric vehicles (EVs) and hybrid electric vehicles (HEVs) powered by LIBs emerge rapidly in the LIB markets. However, the power density and cycling stability of LIBs still remained to be improved [[1], [2], [3]]. Meanwhile, the most commercial LIBs with graphite anode exhibit the low specific capacity and poor lithiation rate capability [4]. Hence, to design and fabricate high-performance anodes for LIBs are still highly demanded.
Titanium-based compounds such as lithium titanate (Li4Ti5O12) and titanium dioxide (TiO2) are promising anode materials for LIBs. During the discharge/charge process, volume changes of Li4Ti5O12 (0.2%) and TiO2 (<4%) are both smaller than graphite (10%) [5,6]. But the theoretical specific capacity of Li4Ti5O12 is only 175 mAh g−1, that limits its further application. Compared to Li4Ti5O12, TiO2 has high theoretical specific capacity (336 mAh g−1), low cost and easy for preparation [7,8]. However, the poor electric conductivity and sluggish Li-ion diffusion hinder the practical application of TiO2 in LIBs, too.
For far, many methods have been used to improve the electrochemical performance of TiO2 anode. Amongst, designing nanostructured TiO2 is considered as a common approach to improve its Li-ion transport kinetics [9]. TiO2 with many different morphologies were prepared and their electric conductivities and Li-ion diffusion properties were studied. It has been demonstrated that, in particularly, one-dimensional (1D) TiO2 such as nanotubes, nanorods, nanowires could well avoid agglomeration of nanoparticles and exhibit excellent electrochemical performance as anodes [[10], [11], [12]]. Furthermore, the 1D TiO2 nanowire arrays (NWAs) or nanotube arrays (NTAs) on Ti foils can not only supply favorable electron and ion transport channels, but also avoid using the binder agent during the anode preparation. In a previous study, our group developed a simple hydrothermal method to prepare hierarchical TiO2-x on Ti foils, that showed manifest pseudocapacitive property [13]. Besides, the template method is also commonly used to prepare mesoporous TiO2 [14,15]. For instance, Zhao et al. [16] prepared single-layered TiO2 mesopores coated SiO2 (SiO2@SL-m TiO2) using Pluronic triblock copolymer F127 as a template, showed excellent electrochemical performance for sodium-ion storage. However, the template method usually needs to remove the template by etching or high-temperature treatment. Imai et al. prepared mesoporous anatase TiO2 through a template-free method on various substrates using TiF4 solution as precursor at 40–70 °C [[17], [18], [19]]. Cheng and Jiao et al. [20] synthesized mesoporous succulents-like TiO2/graphene aerogel in TiF4 solution at 60 °C. As an anode material for LIBs, this composite showed high reversible capacities of 663.2 and 215.5 mA h−1 at current densities of 0.1 and 5 A g−1, respectively.
In this work, mesoporous TiO2 honeycomb-like structure (HS) was generated in situ on the top of TiO2 nanotube arrays (NTAs) in TiF4 solution at 60 °C. It can be seen after treating with TiF4 for 4 h, the thickness and diameter of the as-generated TiO2 HS are about 59 nm and 121–153 nm, respectively. Meanwhile, the homogeneous mesopores (~3 nm) were formed also on the nanosheets of TiO2 HS. As binder-free anode of LIBs, TiO2 NTAs/HS exhibited a high discharge specific capacity (422 mAh g−1, 0.1 A g−1) even at a high current density of 2 A g−1 (96 mAh g−1) and good cycling stability. The outstanding electrochemical performance of TiO2 NTAs/HS should be contributed from the synergistic effects of NTAs morphology and the mesoporous structure of TiO2, supplying enriched electron transmission and Li-ion diffusion channels. Hence, TiO2 NTAs/HS should be a promising anode material for LIBs.
Section snippets
Experimental
As illustrated in Fig. 1, TiO2 NTAs were fabricated on Ti foils by a two-step anodic oxidation. Briefly, before the anodic oxidation process, the Ti foils (thickness ~ 0.25 mm, purity > 99%, Hebei Runhe Metal Products Co. Ltd., China) were cut into pieces of 2 × 3 cm2 and treated with a polishing solution including 1 g NH4F and 30 mL HNO3 in 30 mL H2O for 5 min, then washed with deionized water and ethanol for three times, respectively. Two pieces of Ti foils were used separately as anode and
Results and discussion
As shown in Fig. 2a-b, the TiO2 NTAs are oriented perpendicularly on the surface of Ti foil. The length and diameter of TiO2 nanotubes are about 17 μm and 65–136 nm, respectively. After dipping in the TiF4 solution for certain time, a thin TiO2 HS can be observed on the top of TiO2 NTAs (Fig. 2c-h). With the time in TiF4 solution was increased from 2 h to 6 h, the thickness of TiO2 HS is increased from 36 nm to 95 nm, and the diameter also enlarged. When treated 4 h, the structure of TiO2 HS is
Conclusions
In summary, the TiO2 HS was directly prepared on TiO2 NTAs with TiF4 solution as precursor at a low-temperature. When reacted in TiF4 solution for 4 h, the TiO2 HS was relatively neat with mesopores (~ 3 nm), which was beneficial to facilitate Li-ion and electron transportations. As a binder-free anode, the TiO2-4 achieves a superior discharge specific capacity (422 mAh g−1, 0.1 A g−1), excellent rate capability (96 mAh g−1, 2 A g−1) and long cycle stability (1000 cycles, 40 mAh g−1, 1 A g−1).
CRediT authorship contribution statement
Jinghao Huo: Conceptualization, Methodology, Writing - review & editing, Funding acquisition. Yujia Xue: Software, Investigation, Data curation, Writing - original draft. Yi Liu: Formal analysis, Visualization, Funding acquisition.Shouwu Guo: Validation, Resources, Supervision, Project administration.
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.
Acknowledgement
The Natural Science Foundation of Shaanxi University of Science and Technology (2016BJ-49) and the Natural Science Foundation of China (61704047) supported this work.
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