ARTICLE

J. Cent. South Univ. (2019) 26: 1530-1539

DOI: https://doi.org/10.1007/s11771-019-4109-4

Lithium storage performance of {010}-faceted and [111]-faceted anatase TiO₂ nanocrystals

DU De-jian(杜德健)¹, DU Yi-en(杜意恩)^{1, 2}, YUE Wen-bo(岳文博)¹, YANG Xiao-jing(杨晓晶)¹

1. Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry,Beijing Normal University, Beijing 100875, China;

2. School of Chemistry & Chemical Engineering, Jinzhong University, Jinzhong 030619, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: As a popular anode material for lithium-ion batteries, anatase TiO₂ nanoparticles with exposed {001} facets usually exhibit exceptional lithium storage performance owing to more accessible sites and fast migration of lithium ions along the good crystalline channels. However, there are few researches on the lithium storage capability of TiO₂ nanocrystals with other high-energy facets owing to lack of effective synthesis method for controlling crystal facets. Herein, anatase TiO₂ nanocrystals with exposed {010}- and [111]-facets are successfully prepared by using the delaminated tetratitanate nanoribbons as precursors. The electrochemical properties of these TiO₂ nanocrystals with high-energy surfaces and the comparison with commercial TiO₂ nanoparticles (P25) are studied. It is found that the cycle and rate performance of TiO₂ nanocrystals is highly improved by reducing the particle size of nanocrystals. Moreover, TiO₂ nanocrystals with exposed {010}- and [111]-facets exhibit better lithium storage capacities in comparison with P25 without a specific facet though P25 has smaller particle size than these TiO₂ nanocrystals, indicating that the exposed facets of TiO₂ nanocrystals have an important impact on their lithium storage capacity. Therefore, the synthesis design of high-performance TiO₂ materials applied in the next-generation secondary batteries should both consider the particle size and the exposed facets of nanocrystals.

Key words: titanium dioxide; nanocrystal; exposed facet; lithium-ion battery

Cite this article as: DU De-jian, DU Yi-en, YUE Wen-bo, YANG Xiao-jing. Lithium storage performance of {010}-faceted and [111]-faceted anatase TiO₂ nanocrystals [J]. Journal of Central South University, 2019, 26(6): 1530-1539. DOI: https://doi.org/10.1007/s11771-019-4109-4.

1 Introduction

As a popular n-type semiconductor electrode material, titanium dioxide (TiO₂) has been potentially applied in solar cells, sensors, sodium- ion batteries and lithium-ion batteries (LIBs) owing to its low cost, robust capability, good safety and environmental friendliness [1-3]. Moreover, the volume change of TiO₂ upon Li-ion intercalation and deintercalation is below 4%, which favors the safety and cycling stability of LIBs [4]. The insertion reaction of Li⁺ into TiO₂ is expressed as follows: TiO₂ + xLi⁺ + xe^- → Li_xTiO₂ (x≈0.5), and this reaction is kinetically controlled by the solid-state diffusion of Li⁺ [5]. Therefore, the crystal phase, particle size, morphology and surface structure of TiO₂ can significantly affect its electrochemical performance. For instance, the small particle size and high specific surface area of TiO₂ nanocrystals offer increased interfacial contact area with electrolyte and shorten the diffusion length for lithium ions, thereby highly enhancing the lithium storage capacity of TiO₂ [6-8]. Moreover, some recent reports indicate that the exposed planes of TiO₂ may also play an important part in lithium storage in nanostructured TiO₂ [9-13]. To date, anatase TiO₂ with exposed {001} facets has been widely studied due to its higher surface energy (0.90 J/m²) in comparison with {101} and {010} facets (0.44 and 0.53 J/m²) [14]. Compared to anatase TiO₂ with exposed {101} facets, anatase TiO₂ with exposed {001} facets exhibits superior lithium storage capability because of more accessible sites available and fast diffusion of lithium ions along the good crystalline channels (i.e., [001] direction) [9-13]. Moreover, a high Coulombic efficiency and remarkable capacity retention are also achieved for anatase TiO₂ with exposed {001} facets. However, there are few researches on the electrochemical properties of TiO₂ nanoparticles with other high-energy facets such as {010} facets so far [15].

Recently, Feng’s group and our group cooperatively reported a novel synthesis of {010}- faceted and [111]-faceted anatase nanoparticles by using exfoliated tetratitanate nanoribbons as precursors [16, 17]. The particle sizes and the exposed facets of TiO₂ nanocrystals are easily controlled by changing pH values of solutions. Furthermore, it is interesting to find that the photocatalytic activities of these TiO₂ catalysts increase in the order of surface without a specific facet (commercial P25)<[111]-faceted surface< {010}-faceted surface, indicating a serious effect of the exposed facets on the photocatalytic properties of anatase TiO₂ [16]. It is reasonable that TiO₂ nanocrystals with various exposed facets would also exhibit different electrochemical performance. In this work, {010}-faceted and [111]-faceted anatase TiO₂ nanoparticles with various morphologies are prepared for studying the relationship between the exposed facets and the lithium storage property of TiO₂ nanocrystals. The results suggest that the crystalline phase, particle size, morphology and exposed facets are highly depended on the pH values of solutions. As a result, nanorod-shaped anatase TiO₂ nanocrystals with exposed [111]-facets are formed at pH 3, while rhombic-/cuboid-shaped and spindle-shaped anatase TiO₂ nanocrystals with expose {010}-facets are obtained at pH 5 and 7, respectively. To conveniently represent these TiO₂ nanocrystals, the samples are denoted as TiO₂-T, where T is the pH values (3, 5 and 7). The electrochemical tests indicate that TiO₂-3 exhibits better cycle and rate performance than TiO₂-5 and TiO₂-7 due to its small particle size. The large contact area between TiO₂-3 and electrolyte is favorable to the diffusion and intercalation of lithium ions. However, although P25 has smaller particle size than TiO₂-T, these anatase TiO₂ nanocrystals with exposed {010}- and [111]-facets still exhibit better lithium storage capacity than P25 without a specific facet. It is inferred that lithium ions are more facile to be attached to the high-energy facets of TiO₂ nanocrystals, resulting in the electrochemical performance improvement of TiO₂-T. Consequently, the electrochemical property of anatase TiO₂ can be remarkably increased by controlling not only the particle size and morphology, but also the exposed facets of TiO₂.

2 Experimental

2.1 Sample preparation

2.1.1 Tetratitanate nanoribbon

Potassium tetratitanate fibers (K₂Ti₄O₉) with layered structure were firstly immersed in 1 L of HCl solution (1 mol/L) with stirring for three days to obtain a protonated H₂Ti₄O₉·nH₂O (HTO) [16]. The acid exchange was repeated more than three times. The product was then collected by centrifugation and dried by vacuum freezing. The TMA⁺-intercalated tetratitanate compound (TMA-HTO) was prepared by immersing the HTO sample (5.7 g) in tetramethylammonium hydroxide (TMAOH) solution (57 mL, 12.5%) and then calcination at 100 °C for 24 h. The as-synthesized TMA-HTO was dispersed in 570 mL of distilled water with stirring for 24 h. A colloidal suspension (exfoliated HTO) was collected after removing the large particles.

2.1.2 Anatase with various facets

The pH value of the colloidal solution was tuned to 3, 5 and 7 respectively by adding a HCl or TMAOH solution. The solutions (60 mL) with different pH were then transferred into a 100 mL Teflon-lined autoclave and heated at 170 °C for 24 h. After the autoclave was cooled down, TiO₂ nanocrystals were obtained after centrifugation and freeze-drying.

2.2 Sample characterization

Crystalline characterizations of samples were performed by X-ray diffraction (XRD) on a Phillips X’pert Pro MPD diffractometer. The morphologies and structures of samples were observed by scanning electron microscope (SEM) on a JEOL JSM-6700F with a voltage of 5 kV, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) on a JEOL JEM-2011 electron microscope with a voltage of 200 kV.

2.3 Electrochemical measurements

The electrochemical properties of samples were measured by using a LR2032-type coin cell. The negative electrode was Li metal and the positive electrode was a mixed slurry on a copper foil, including active materials (80%), super P carbon black (10%) and polyvinylidene difluoride (PVDF, 10%), and N-methyl-2-pyrrolidine (NMP) was used as solvent. 2.5 mg/cm² mass loading is necessary for active materials. The electrolyte was prepared by dissolving LiPF₆ (1 mol/L) in a mixed solution (EC:DEC:DMC with a 1:1:1 volume ratio). Coin cell was assembled in a glove box filled with Ar. Cyclic voltammetry and electrochemical impedance spectroscopy were tested on a Gamry Interface 1000 electrochemical station in the voltage range of 0.01-3.0 V (versus Li/Li⁺) with a scan speed of 0.5 mV/s or with a frequency range of 100 kHz to 10 mHz, respectively. LAND test system was used to measure the galvanostatic charge–discharge performance in the voltage range of 0.01-3.0 V under a constant current of 0.05–1 A/g.

3 Results and discussion

3.1 Materials characterization

Anatase TiO₂ nanocrystals are prepared from as-synthesized HTO nanoribbons under hydrothermal treatment (see details in Section 2). According to our previous research, the crystalline phase (anatase single phase) is highly depended on the temperature (>150 °C) and pH (3-13) [16]. Herein, TiO₂ nanocrystals are prepared at 170 °C in solutions with pH of 3, 5 and 7, respectively. The XRD patterns of these TiO₂ nanocrystals are shown in Figure 1(a). All characteristic peaks in the XRD patterns are indexed into the tetragonal structure of anatase TiO₂. Moreover, it is found that the morphology of the nanocrystals is also influenced by the pH values. The SEM image of TiO₂-3 (Figure 1(b)) reveals that nanorod-like particles with an average size of about 60 nm in length and about 40 nm in width are formed at pH 3. At pH 5, two morphologies of TiO₂-5, cuboid and rhombic shapes, can be found in the SEM image (Figure 1(c)). The size of mainly cuboid particles is about 70 nm in length and about 55 nm in width. The morphology of TiO₂-7 changes to spindle shape when pH increases to 7 (Figure 1(d)), and the particle size increases to about 160 nm in length and about 55 nm in width. The particle size of these TiO₂ nanocrystals was analyzed by using Nano Measurer software. The histogram of particle length based on statistical results (Figures 2(a)-(c)) shows that the average size of TiO₂ nanocrystals is 58.8 nm for TiO₂-3, 69.8 nm for TiO₂-5 and 144.1 nm for TiO₂-7. This means that the crystalline size increases with the increase of pH values under the same condition. Our previous study indicates that the cracks in the precursor [Ti₄O₉]^2- nanoribbons appear along the [010]-direction of the HTO below pH 3 and along [001]-direction above pH 7, leading to the different morphologies and particle sizes of anatase TiO₂ nanocrystals [16].

The morphologies and exposed facets of anatase TiO₂ nanocrystals were further studied by TEM. TEM images also confirm the formation of TiO₂ nanocrystals with a nanorod shape (Figure 3(a)), a cuboid or rhombic shape (Figure 3(c)) and a spindle shape (Figure 3(e)) respectively, which is in accord with the SEM observation. The nanocrystal in the HRTEM image of TiO₂-3 (Figure 3(b)) exhibits lattice spacings of 0.352 and 0.352 nm, which are assigned to the (101) and (011) planes of TiO₂ with an angle of ca. 82.1°. Therefore, the dominant exposed facets of TiO₂-3 are perpendicular to [111] crystal zone axis, and denoted as [111]-facets. In contrast, the lattice spacings of 0.351 and 0.473 nm in the HRTEM image of TiO₂-5 (Figure 3(d)) correspond to the (101) and (002) planes of TiO₂ with an angle of ca. 68.3°. Therefore, the dominant exposed facets of TiO₂-5 are the {010} facets and vertical to the [010] direction. The lattice spacings of 0.352 and 0.352 nm in the HRTEM image of TiO₂-7(Figure 3(f)) correspond to the (101) and planes of TiO₂, and the angle between the two facets is 136.6°, also indicating that the {010} facets are the dominant exposed facets of TiO₂-7. In a word, nanorod-shaped TiO₂ nanoparticles with exposed [111]- facets were formed at pH 3, whereas rhombic-/cuboid- and spindle-shaped TiO₂ nanoparticles with exposed {010}-facets were formed at pH 5, and 7, respectively.

Figure 1 XRD patterns of TiO₂-T (T=3, 5, 7) (a) and corresponding SEM images of TiO₂-3 (b), TiO₂-5 (c) and TiO₂-7 (d)

Figure 2 Particle size distribution of TiO₂-3 (a), TiO₂-5 (b), TiO₂-7 (c) and P25 (d)

Figure 3 TEM and HRTEM images of TiO₂-3 (a, b), TiO₂-5 (c, d) and TiO₂-7 (e, f) (Insets of (a, c, e) are structural models of TiO₂-3, TiO₂-5 and TiO₂-7, respectively)

3.2 Electrochemical performance

The impact of the particle size and exposed facets on the electrochemical behaviors of anatase TiO₂ nanocrystals was studied by cyclic voltammetry and galvanostatic measurements. Figures 4(a), (c), (e) show the cyclic voltammograms (CVs) of TiO₂-3, TiO₂-5 and TiO₂-7 in the 1st, 2nd, 3rd and 6th cycles, respectively. There are two obvious cathodic and anodic peaks at about 1.5 and about 2.2 V, corresponding to the reversible transition from tetragonal anatase (I4₁/amd) to orthorhombic Li_0.5TiO₂ (Imma) [9, 12]. It is worth noting that the reduction and oxidation peaks still remain in the CV curves of TiO₂-3 after the first cycle, whereas these peaks become inapparent in the CV curves of TiO₂-5 and TiO₂-7 with the increase of cycle number. The reduction peak at about 0.5 V is associated with the formation of solid electrolyte interface (SEI) films on the surface of TiO₂ nanocrystals [18, 19]. This peak would disappear as the SEI films become stable. Figures 4(b), (d), (f) depicts voltage profiles of these TiO₂ nanocrystals for the 1st, 2nd, 10th and 50th cycles. Two voltage plateaus at 1.7 and 2.1 V are observed in both charge and discharge curves of TiO₂-3. On the contrary, the voltage plateaus are not distinct in the charge/discharge curves of TiO₂-5 and TiO₂-7, which is well-matched with the CV results. It is inferred that lithium ions are more facile to be attached to the high-energy facets of TiO₂ nanocrystals, which is beneficial to the Li-ion intercalation and conversion reactions. In addition, according to the electrochemical properties of TiO₂-5 and TiO₂-7 with the same facets, the Li-ion intercalation performance is also influenced by the particle size, or in other words, the specific surface area of TiO₂ nanocrystals.

Figure 4 CV curves (a, c, e) and voltage profiles (b, d, f): (a, b) TiO₂-3; (c, d) TiO₂-5; (e, f) TiO₂-7

The electrochemical properties of these TiO₂ nanocrystals are also measured, and TiO₂-3 shows the best cycle performance and rate capability among these TiO₂ nanocrystals (Figures 5(a), (b)). For instance, the reversible capacity of TiO₂-3 is maintained at about 160 mA h/g at 0.05 A/g over 100 cycles, which is comparable to the theoretical capacity of anatase TiO₂ (167.5 mA·h/g). On the contrary, TiO₂-5 and TiO₂-7 deliver the capacities of only about 90 and about 60 mA·h/g respectively over 100 cycles. The discharge capacity of TiO₂-3 is about 100 mA h/g as the current rate rises to 1 A/g, whereas the discharge capacities of TiO₂-5 and TiO₂-7 drop to about 60 and about 30 mA·h/g, respectively. The small particle size and large surface area of TiO₂-3 can enhance the electrolyte/ electrode contact area and shorten lithium diffusion pathways, thereby improving the electrochemical performance of TiO₂-3. It should be noted that the increased contact area also causes a larger SEI film formed on TiO₂-3, which leads to more irreversible capacity losses and serious capacity fading in the initial cycles. The Coulombic efficiencies of TiO₂-3, TiO₂-5 and TiO₂-7 are 59.3%, 39.8% and 41.5% in the first cycle respectively, and reach to above 98% after the 10th cycle. The low initial Coulombic efficiencies are attributed to the formation of SEI films on TiO₂ nanocrystals and the irreversible generation of lithium titanate. It is interesting to note that TiO₂-3 has an abnormal higher initial Coulombic efficiency than TiO₂-5 and TiO₂-7, implying that the growth behavior of SEI film is totally different on different faceted crystals [19]. Accordingly, in addition to the particle size, the exposed facet also plays a significant part in the electrochemical properties of TiO₂ nanocrystals.

The electron and Li⁺ transport in these TiO₂ nanocrystals were evaluated by using electrochemical impedance spectroscopy (EIS). Figure 5(c) shows the Nyquist plots consisting of a semicircle at the high frequency area, which is associated with the charge transfer resistance (R_ct), and a sloping straight line at the low frequency area, which is related to the diffusion performance of Li⁺ ions in electrodes (Z_w) [20, 21]. The R_ct values of samples were obtained after fitting curves with the equivalent circuit model (Figure 5(d)). The charge transfer resistance of TiO₂-3 is about 55.8 Ω, slightly lower than those of TiO₂-5 (about 78.6 Ω) and TiO₂-7 (about 87.1 Ω). The electron transfer relys on the magnitude of the band-gap, which is influenced by the particle size and defects of anatase [22]. More sub-band-gap states between the valence bands and conductions result in the lower charge-transfer resistance of TiO₂-3. Therefore, TiO₂-3 with lower resistance would display superior electrochemical performance as compared to other TiO₂ samples [23].

Figure 5 Electrochemical properties of TiO₂-T:

3.3 Comparison with P25

Besides the particle size, the high-energy facets of TiO₂ nanocrystals may also influence their electrochemical behavior. P25 is a well-known commercial TiO₂ powder containing about 80% anatase phase and 20% rutile phase (Figure 6(a)) [24]. SEM and TEM images (Figures 6(b) and (c)) reveal that P25 are mostly irregular spherical nanocrystals without a specific facet on the surface. In a typical HRTEM image of P25 (Figure 6(d)), the lattice spacings of 0.351 and 0.241 nm are assigned to the (101) and (004) planes of TiO₂ with an angle of ca. 68.3°, indicating the exposure of {010} facets on the basal plane. Furthermore, the (101) facets are parallel to the lateral faces of P25, indicating that the exposed facets are {101} on the lateral surface. The above results show that the small number of cuboid nanocrystals mainly expose {010} facets on the basal plane and {101} facets on the lateral surface. The main particle size of P25 is only 26.7 nm, much smaller than those of as-synthesized TiO₂ nanocrystals with exposed {010}- and [111]-facets (Figure 2D).

In theory, P25 should exhibit superior electrochemical performance than these TiO₂ nanocrystals due to its smaller particle size. However, the cycle and rate performance of P25 is merely comparable to those of TiO₂-7 (Figure 7), implying that the exposed facet serves as an important factor for the lithium storage capacity of TiO₂ as well. As reported previously, lithium ions prefer to be attached to the {010} facets of anatase rather than the {001} and {101} facets, and it is convenient for Li-ion transport along the [100] and [010] directions [25]. Therefore, TiO₂-5 with exposed {010}-facets exhibit slightly better electrochemical performance than P25 though TiO₂-5 has larger particle size. The [111]-facets are more complicated and their effects on the lithium transport need to be deeply studied in future.

Figure 6 XRD pattern (a), SEM (b), TEM (c) and HRTEM images (d) of P25

Figure 7 Cycle performance of P25 at 0.05 A/g (a) and rate performance of P25 at 0.05–1 A/g (b)

4 Conclusions

Anatase TiO₂ nanocrystals with exposed {010}- and [111]-facets are successfully prepared by using exfoliated tetratitanate nanoribbons as precursors. The primary exposed facets of TiO₂ nanoparticles are controllable by adjusting the pH values of reaction solution. The cycle and rate performance of these TiO₂ nanocrystals are highly improved by reducing the particle size of nanocrystals. Moreover, these TiO₂ nanocrystals exhibit better electrochemical performance than P25 without a specific facet though P25 has smaller particle size. These results reveal that the electrochemical properties of anatase TiO₂ are affected not only by the crystal size and morphology, but also by the surface facets.

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(Edited by FANG Jing-hua)

中文导读

具有{010}和[111]晶面的锐钛矿型TiO₂纳米晶体的储锂性能

摘要：作为锂离子电池的负极材料，具有{001}晶面的锐钛矿型TiO₂纳米晶体通常展现出良好的储锂性能，这是由于该晶面具有更多的接触位点，同时锂离子沿着该晶格方向的扩散速度较快。然而，由于缺乏有效的控制TiO₂晶面的合成方法，对具有其它高能晶面TiO₂晶体的储锂性能的研究较少。本文以剥离的钛酸盐纳米带为前驱体，通过水热法合成了具有{010}和[111]晶面的锐钛矿型TiO₂纳米晶体。通过对该TiO₂纳米晶体的电化学性能进行研究，并与商业用的TiO₂纳米颗粒(P25)的性能进行比较，发现通过减小TiO₂纳米晶的粒径，可以大幅度提高TiO₂纳米晶体的循环和倍率性能。此外，尽管P25具有更小的粒径，相比没有特定晶面的P25，具有{010}和[111]晶面的TiO₂纳米晶体仍展现出更好的储锂能力。这表明TiO₂纳米晶体的暴露晶面对其储锂性能同样具有重要影响。因此，在设计合成具有高性能的TiO₂电极材料时，要同时考虑纳米晶体的粒径和暴露晶面等因素。

关键词：二氧化钛；纳米晶体；暴露晶面；锂离子电池

Foundation item: Projects(21573023, 51572031) supported by the National Natural Science Foundation of China; Project supported by the Grants-in-Acid for Doctor Research Funds; Project supported by “1331 Project” for Innovation Team Construction Plan Funds of Jinzhong University, China; Project supported by “1331 Project” for 2018 Key Innovation Team Construction Plan Funds of Shanxi Province, China

Received date: 2018-12-05; Accepted date: 2019-02-02

Corresponding authors: YUE Wen-bo, PhD, Professor; Tel: +86-10-58804229; E-mail: wbyue@bnu.edu.cn; ORCID: 0000-0003-3809- 6733; YANG Xiao-jing, PhD, Professor; Tel: +86-10-58805476; E-mail: yang.xiaojing@bnu.edu.cn; ORCID: 0000-0003-3620-3507