Dispersion of SnO₂ nanocrystals on TiO₂(B) nanowires as anode material for lithium ion battery applications

Abstract

TiO₂(B)@SnO₂ core–shell hybrid nanowires have been synthesized by a facile hydrothermal process and subsequent liquid phase reaction. Hybrid nanowire electrodes exhibit excellent reversible lithium storage capacity rate capability and good cyclability, mainly due to the particular architecture of the composite, which features an open continuous channel along its axis, facilitating lithium ion diffusion, and provides effective mechanical support for the TiO₂(B) core, alleviating the stress produced during discharge–charge cycling and also preventing the pulverization of the Sn nanoparticles. Owing to its superior electrochemical performance, this composite could be a promising potential anode material for lithium ion batteries.

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Introduction

SnO₂ nanomaterials have received considerable attention for their myriad of applications in gas sensors,^1–7 photocatalysis⁸ and lithium ion batteries,^9–11 owing to their attractive electronic, optical, and electrochemical properties. To date, various one-dimensional SnO₂ nanostructures, in such forms as nanorods/arrays,^12–14 nanobelts,^15,16nanowires,^2,3,17 and nanotubes,^18–19 have been prepared by many synthesis methods, including thermal evaporation,^5,20chemical vapor deposition,¹ the hydrothermal method,¹² the template method,^3,21etc. As one of the most promising anode materials for lithium ion batteries, one-dimensional (1D) SnO₂ nanomaterials have attracted particular attention because of their high capacity compared with that of graphite (372 mA h g⁻¹).^22,23 Nevertheless, practical implementation of 1D SnO₂ nanomaterials in lithium-ion batteries is greatly frustrated by the large initial irreversible capacity induced by Li₂O formation and the abrupt capacity fading caused by volume variation (up to 258%²⁴).

Using core–shell composite architectures may be one of most effective strategies to improve the Li⁺ storage capacity and cyclability of one-dimensional SnO₂-based nanomaterials because of the “buffering” and “mechanical support” functions^25,26 of the core materials during charge–discharge cycling. Presently, various core–shell 1D SnO₂-based composites, such as carbon/SnO₂,²⁷MgO/SnO₂,²⁸TiO₂/SnO₂,²⁹etc., have been prepared and have exhibited excellent optoelectronic performance, which enables the potential application of those materials in lithium ion batteries. However, up to now, only a few core–shell SnO₂ based composites besides carbon/SnO₂ have been explored for anode material applications in lithium ion batteries, probably because of their complicated chemical and physical synthesis processes. Thus, there are still many challenges in synthesizing one-dimensional TiO₂@SnO₂ hybrid nanomaterials by facile synthesis routes. TiO₂(B) is a metastable monoclinic modification of titanium dioxide. It has been demonstrated that the kinetics of lithium storage in TiO₂(B) are governed by a pseudo-capacitive Faradaic process, which is not limited by solid-state diffusion of Li ions.³⁰TiO₂(B) has an open structure with freely accessible channels for Li ion transport perpendicular to the (010) face, which allows easy Li⁺ transport.

Herein, we report TiO₂(B)@SnO₂ hybrid nanowires, each consisting of a TiO₂(B) nanowire core covered with a SnO₂ nanocrystal shell, which are simply prepared by a hydrothermal reaction between NaOH and TiO₂ powder, followed by a liquid phase reaction process in SnCl₂·2H₂O and urea solution, and finally by dehydrating and drying in an oven. These TiO₂(B)@SnO₂ hybrid nanowires exhibit excellent lithium storage capacity and good cyclability, revealing their potential for application in lithium ion batteries.

Experimental

Synthesis of TiO₂(B)@SnO₂ hybrid nanowires

TiO₂(B)@SnO₂ hybrid nanowires were fabricated via a facile hydrothermal process and subsequent liquid phase reaction. Typically, the precursors, titanate nanowires, were synthesized by adding 4.5 g TiO₂-anatase (99.8%, Aldrich) to a 22 ml 15 M aqueous solution of NaOH. After stirring for 1 h, the resulting suspension was transferred to a 30 ml Teflon-lined autoclave and heated at 150 °C for 72 h. The product was acid washed, which involved stirring the sample in 0.1 M HCl solution for 1 h, twice, resulting in the parent H₂Ti₃O₇ nanowires.^31–34 The material was then filtered, repeatedly washed with distilled water until pH ≈ 7, and dried at 80 °C for 20 h. Subsequently, the as-prepared materials were heated at 400 °C for 4 h in air. Then, 0.3 g as-prepared powder was dispersed into 20 ml de-ionized (DI) water to obtain a white suspension by ultrasonication and subsequent stirring. The suspension was subjected to a 60 °C water bath with continuous stirring. Then, 0.162 ml 37% hydrochloric acid was dropped into the suspension. After this process, based on many preliminary experiments, 0.67 g SnCl₂·2H₂O and 0.33 g urea were added in sequence. In the process above, the urea functions as the precipitant, owing to the slow hydrothermal reaction at this temperature. This thermostatic reaction lasted about 20 min with continuous stirring. The final product was obtained by washing with DI water, filtering, and drying.

Materials characterization

The morphology of the TiO₂(B)@SnO₂ nanowires was evaluated using a JEOL 7500FA field emission scanning electron microscope (FE-SEM, JEOL, Tokyo, Japan) and a JEOL 2011F transmission electron microscope (TEM, JEOL, Tokyo, Japan). The crystal structures of the TiO₂(B)@SnO₂ nanowires were obtained using X-ray diffraction (XRD) data (MMA, GBC, Australia). Energy dispersive X-ray spectroscopy (EDX, JEOL 7500FA) was used to confirm the concentrations of TiO₂ and SnO₂. X-Ray photoelectron spectroscopy (XPS) experiments were carried out on a VG Scientific ESCALAB 220IXL instrument using aluminum Kα X-ray radiation during XPS analysis.

Electrochemical characterization

Electrochemical properties were measured on electrodes prepared by compressing a mixture of as-prepared TiO₂(B)@SnO₂ hybrid nanowires, carbon black (Super P, MMM, Belgium), and poly(vinyl difluoride) (PVDF) binder in a weight ratio of 70 : 15 : 15 and pasting the mixture on copper foil. Pure lithium metal foil was used for the counter and reference electrodes. The electrolyte was LiPF₆ (1 M) in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 v/v; MERCK KgaA, Germany). Coin cells were assembled in a high-purity argon-filled glove box (Mbraun, Unilab, Germany). A LAND-CT2001A instrument was used with a charge-discharge current density of 30 mA g⁻¹ to measure the electrochemical capacity of the electrodes at room temperature by the galvanostatic method. Rate capability tests of the electrodes were then carried out systematically. The cut-off potentials for charge and discharge were set at 3.0 and 0.01 V versusLi⁺/Li, respectively. To explore the contribution of the TiO₂ in the composite to the capacity of the electrodes, the cycling performance of the electrodes was also evaluated with cut-off potentials of 3.0 and 1.0 V versusLi⁺/Li, respectively. Cyclic voltammetry (CV) was performed on a PARSTAT@2273 electrochemical workstation.

Results and discussion

As shown in Fig. 1, X-ray diffraction (XRD) analysis of the as-synthesized parent titanate nanowires and TiO₂@SnO₂ hybrid nanowires reveals that the parent titanate nanowires are H₂Ti₃O₇^31–34 with monoclinic structure (JCPDS 47-0561) and undergo consecutive phase transitions to TiO₂(B) (JCPDS 74-1940), with the final stage consisting of TiO₂@SnO₂ hybrid nanowires, after hydrothermal processing and the subsequent liquid phase reaction. There are no sharp characteristic peaks in the X-ray diffraction pattern, possibly because of the poor crystallinity of the titanate just after the hydrothermal process, as well as the interference of a thick, nanocrystalline or amorphous SnO₂ particle layer on the surface of the hybrid nanowires. Furthermore, TiO₂(B) is the main phase of the TiO₂@SnO₂ hybrid nanowires, belonging to space groupC2/m (12) with lattice parameters of a = 12.1787 Å, b = 3.7412 Å, c = 6.5249 Å, and β = 107.054°, while H₂Ti₃O₇ is the main phase of the parent titanate nanowires. Additionally, no obvious characteristic peak of SnO₂ has been detected, which is due to the nanocrystalline or amorphous nature of the SnO₂ in the composite. The as-synthesized nanowires display a homogenous, one-dimensional morphology, as shown in the field-emission scanning electron microscope (FE-SEM) images (Fig. 2 and ESI,† Fig. S1). The H₂Ti₃O₇ nanowires have diameters of 30–120 nm and extend to a few micrometres in length (Fig. 2(a–b) and ESI,† Fig. S1(a,b)). In order to reduce the electrostatic charging during the FE-SEM imaging of the H₂Ti₃O₇ sample, gold nanoparticles were evaporated on it and can clearly be seen in Fig. 2(b) and Fig. S1(b).† After heat-treatment and the subsequent liquid phase reaction process, as shown in Fig. 2(c,d) and Fig. S1(c,d),† these TiO₂(B)@SnO₂ hybrid nanowires become slightly thicker than the H₂Ti₃O₇ precursor nanowires, mainly due to the encapsulation of the TiO₂ nanowires by a thick layer of porous SnO₂ nanoparticles (see Fig. 2(d)), which, to some extent, facilitate lithium ion storage and Li⁺ diffusion due to the high specific surface area and high porosity. More interestingly, some TiO₂(B)@SnO₂ hybrid nanowires have aggregated to form a three-dimensional (3D) microstructure. There are also many mesopores or gaps formed by these 3D microstructures in the electrode material, which favor electrolyte diffusion.

Fig. 1 X-Ray diffraction patterns of as-prepared H₂Ti₃O₇ nanowires and TiO₂(B)@SnO₂ nanowires.

Fig. 2 FE-SEM images of as-synthesized H₂Ti₃O₇ nanowires and TiO₂(B)@SnO₂ nanowires: (a) H₂Ti₃O₇ nanowires, (b) H₂Ti₃O₇ nanowires at high magnification and at higher magnification (inset), (c) TiO₂(B)@SnO₂ nanowires, (d) close-up of a few TiO₂(B)@SnO₂ nanowires with porous surface structure, with inset showing details. (Gold nanoparticles with grain sizes of ∼5–10 nm were evaporated on the surface of the H₂Ti₃O₇ nanowire samples to reduce electrostatic charging during SEM imaging.)

Transmission electron microscope (TEM) images combined with selected-area electron diffraction (SAED) patterns reveal the fine microstructure of the parent H₂Ti₃O₇ nanowires and the TiO₂(B)@SnO₂ hybrid nanowires, respectively (Fig. 3 and Fig. S2(a,b)†). Fig. 3(a) shows the TEM image and SAED pattern (inset) of a single H₂Ti₃O₇ nanowire. Each H₂Ti₃O₇ nanowire is actually a monocrystal with monoclinic structure extending along the [200]* direction (Fig. 3(a) and its inset), which is similar to the results of Peng's group.^35,36Fig. 3(b) shows a TEM image and SAED pattern (inset) of random H₂Ti₃O₇ nanowires. Due to the random orientation of these nanowires, the two most intensive diffraction rings were indexed as the (200) and (003) planes of H₂Ti₃O₇ (see Fig. 3(b) and its inset). As shown in Fig. 3(c), a TiO₂(B)@SnO₂ nanowire, inheriting the monoclinic structure characteristics of a H₂Ti₃O₇ nanowire, extends along the [010]_B* direction, according to the SAED pattern of a single hybrid nanowire (Fig. 3(c) inset). Similarly, due to the random orientation of the hybrid nanowires, the two most intensive diffraction rings were indexed as the (002) and (110) planes of TiO₂(B) (see Fig. 3(d) and its inset). Additionally, it can be observed that SnO₂ nanoparticles are decorated on the surface of the TiO₂(B) nanowire core (Fig. 3(c,d) and Fig. S2(a,b)†), according to the obvious fringes of the SnO₂ nanocrystals in the high resolution TEM (HRTEM) image (Fig. S2(b)†). However, it is difficult to distinguish the SnO₂ phase in the composite from the SAED patterns, which may be attributable to the nanocrystalline nature of the SnO₂ nanoparticles in the composite. Thus, both HRTEM and SAED investigations confirm the presence of single-crystalline TiO₂(B) and SnO₂ nanocrystals. This SnO₂ nanocrystal layer forms an effective core–shell 1D structure in conjunction with the TiO₂(B) nanowire itself. It has been reported that TiO₂(B) nanowires extending along the [010] direction^31–34 are characterized by continuous, freely accessible, parallel channels which will enhance Li⁺ diffusion. Therefore, in the hybrid nanowires, the TiO₂(B) core could provide continuous channels favorable to Li⁺ diffusion along the length axis of the nanowire, in addition to its mechanical support function for the ∼30% (wt%) of porous SnO₂ nanocrystals (Fig. S2(c)†). This supporting function of the TiO₂(B) core to some extent helps to alleviate the stress caused during lithium intercalation/de-intercalation.

Fig. 3 (a) TEM image and SAED pattern (inset) of single H₂Ti₃O₇ nanowire, (b) low-magnification TEM image and SAED pattern (inset) of H₂Ti₃O₇ nanowires, (c) TEM image and SAED pattern (inset) of single TiO₂(B)@SnO₂ nanowire, (d) low-magnification TEM image and SAED pattern (inset) of H₂Ti₃O₇ nanowires. (In these TEM patterns, subscript ‘B’ represents the ‘TiO₂(B)’.)

X-Ray photoelectron spectroscopy (XPS) analysis of the TiO₂(B)/SnO₂ hybrid nanowires was performed from 0 to 1100 eV (Fig. 4). The Ti2p spectrum (Fig. 4(a)) for TiO₂(B)/SnO₂ comprises two symmetrical peaks with binding energies (BEs) of 458.89 eV and 464.61 eV, which are attributable to Ti2p3/2 and Ti2p1/2, respectively. The separation between these two peaks is 5.72 eV, slightly larger than the energy splitting reported for TiO₂.^37,38 This is possibly because of the encapsulation of TiO₂(B) in SnO₂ nanocrystals. Similarly, there are two symmetrical peaks with BEs at 487.20 eV and 495.46 eV in the Sn3d spectrum (Fig. 4(b)) of TiO₂(B)@SnO₂, which are derived from Sn3d5/2 and Sn3d3/2, respectively. The separation between them, equal to 8.26 eV, is in good agreement with the energy splitting reported for SnO₂.^39–41 As for the O1s spectrum shown in Fig. 4(c), a portion could come from TiO₂, as evidenced by the O1s BE peak at ∼529.90 eV,⁴² while the peak at 530.40 eV may be due to the SnO₂ in the composite. Additionally, there are two higher O1s BE peaks at 531.10 eV and 532.187 eV, which possibly originate from the OH⁻ radical, adsorbed oxygen, or adsorbed H₂O.^37,42 These results combined with the XRD, FE-SEM, TEM, and XPS results, confirm that the as-synthesized TiO₂(B)/SnO₂ hybrid nanowire composites have a TiO₂(B) crystal core inside a shell of porous SnO₂ nanocrystals. In addition, some pores or gaps are formed by bundling of the nanowires (Fig. S1†) in the sample, all of which would favor the enhancement of the electrochemical performance of the electrode, due to their contribution in terms of accommodating the electrolyte and favoring lithium ion diffusion during the charge–discharge process.

Fig. 4 XPS high-resolution spectra of the (a) Ti2p, (b) Sn3d, and (c) O1s regions for the as-prepared TiO₂(B)@SnO₂ nanowires.

The electrochemical performance of the TiO₂(B)/SnO₂ hybrid nanowire electrode was subsequently investigated as an anode material for lithium ion batteries. Fig. 5(a) shows cyclic voltammograms (CVs) of the TiO₂/SnO₂ hybrid nanowires from the first to the fifth cycle at a scan rate of 0.1 mV s⁻¹ in the voltage range 0.0–3.0 V. The curve of the initial cycle is different from later ones, especially with respect to the subsequent gradual disappearance of some high voltage CV peaks, possibly owing to the formation of an inactive solid–electrolyte interface (SEI) on the surface of the active material, which is inclined to intercept reversible Li ion pathways into/out of the TiO₂(B) core in the discharge–charge process.⁴³ In the first cycle, the cathodic/anodic peak pairs at 0.012 V, 0.175 V, 0.437 V, and 0.584 V are mainly related to the formation of various Li_xSn phases such as Li₂Sn₅, LiSn, Li₇Sn₃, Li₅Sn₂, Li₁₃Sn₅, Li₇Sn₂, and Li₂₂Sn₅ during the charge–discharge process,¹¹ while the small cathodic/anodic peak pair at around 0.968 V and 1.265 V is possibly derived from Li₂O formation and electrolyte decomposition,⁴⁴ which causes a large irreversible capacity in the first few cycles that gradually disappears in the subsequent cycles, as shown in Fig. 5(a). There remains two obvious cathodic/anodic peak pairs at 1.45 V and 1.583 V, and 1.51 V and 1.663 V in the first cycle, while the two cathodic peaks change to and stabilize at 1.483 V and 1.551 V, respectively, in the subsequent cycles, which is probably attributable to lithium ion insertion into/extraction out of TiO₂(B).^30,45 Another small cathodic/anodic peak pair at 1.716 V and 1.95 V corresponds to the reaction of Li⁺ with a trace amount of anatase⁴⁶ in the composite. After the first CV cycle, the electrode demonstrates quite reversible behavior in terms of the reaction, according to Fig. 5(a). It is apparent that the TiO₂ core provides a supporting function which alleviates the pulverization and drastic volume variation of the SnO₂. There may also be the formation of an electrochemically-active Sn/Sn-Ti-O bilayer⁴³ in the TiO₂@SnO₂ hybrid nanowire, which functions as a stable molecular membrane for Li-ion insertion and extraction processes, and will thus facilitate reversible Li-ion behavior.

Fig. 5 Electrochemical performance of TiO₂(B)@SnO₂ nanowire electrode cycled between 0.01 and 3.0 V vs.Li⁺/Li: (a) cyclic voltammograms of TiO₂(B)@SnO₂ nanowire electrode from the first cycle to the fifth cycle at a scan rate of 0.1 mV s⁻¹ in the voltage range 0.01–3.0 V. (b) Voltage profiles of TiO₂(B)@SnO₂ nanowire composite electrode at the current density of 30 mA g⁻¹ for selected cycles. (c) Capacity–cycle number curves and coulombic efficiency from the first cycle to the 50th cycle of TiO₂(B)@SnO₂ nanowires at the current density of 30 mA g⁻¹, compared with SnO₂ nanoparticle and SnO₂/C nanoparticle electrodes cycled in the same voltage range 0.01–3 V.⁴⁷ (d) Charge–discharge capacity of TiO₂@SnO₂ nanowire electrode as a function of the charge–discharge rate (30 mA g⁻¹, 100 mA g⁻¹, 200 mA g⁻¹, 500 mA g⁻¹, 1000 mA g⁻¹).

The discharge–charge cycling performance was evaluated between 3 V and 0.01 V (versusLi/Li⁺) at a constant current of approximately 30 mA g⁻¹ up to 50 cycles. Fig. 5(b) presents the voltage profiles of a TiO₂(B)@SnO₂ hybrid nanowire electrode at a current density of 30 mA g⁻¹. The first discharge and charge steps deliver a specific capacity of 1566.3 and 916.8 mA h g⁻¹, respectively, with an initial coulombic efficiency of 58.5%. The large initial capacity loss of the TiO₂(B)@SnO₂ composite electrode can be partly attributed to the formation of a thick SEI layer on the electrode,⁴⁴ the high specific surface area of the porous SnO₂ nanocrystals in the TiO₂(B)@SnO₂ composite, which consume a much greater quantity of irreversible Li⁺ to form the SEI layer as compared with common SnO₂ materials, and further, some Li⁺ pathway interception by the Li₂O layer formed on the TiO₂(B) core during discharge–charge cycling.⁴³Fig. 5(c) displays the capacity–cycle number curves from the first cycle to the 50th cycle of TiO₂(B)@SnO₂ nanowires at a current density of 30 mA g⁻¹. For comparison, the capacity–cycle number curves from the first cycle to the 30th cycle (0.01–3 V) of the SnO₂ nanoparticle and SnO₂/C nanoparticle electrodes prepared by the molten salt method in our laboratory⁴⁷ has also been shown in Fig. 5(c). Compared to the SnO₂ and SnO₂/C nanoparticle electrodes, the TiO₂(B)@SnO₂ composite electrode exhibits greatly improved cycling performance and a higher reversible specific capacity of approximately 463 mA h g⁻¹ up to 50 cycles, with high coulombic efficiency of nearly 100%. Fig. 5(d) shows the rate capability of the TiO₂(B)@SnO₂ nanowire electrode, which delivers a capacity of about 873 mA h g⁻¹ at a current density of 30 mA g⁻¹ after 5 cycles. This value decreases to 769 mA h g⁻¹ at 100 mA g⁻¹, 712 mA h g⁻¹ at 200 mA g⁻¹, 602 mA h g⁻¹ at 500 mA g⁻¹, 477 mA h g⁻¹ at 1000 mA g⁻¹, and finally returns to 837 mA h g⁻¹ at 30 mA g⁻¹ after 5 cycles, which shows the excellent rate capability of the TiO₂(B)@SnO₂ nanowire electrode.

The good electrochemical performance of the TiO₂(B)@SnO₂ composite electrode may be attributed to the following factors: first, as shown in Scheme S1 (ESI†), the core–shell structure of the TiO₂(B)@SnO₂ nanowire, consisting of a TiO₂(B) crystal core inside a shell of porous SnO₂ nanocrystals, is favorable to lithium ion diffusion during charge–discharge cycling, owing to the open continuous channel in the TiO₂(B) core along its axis. Second, the porous amorphous or nanocrystalline SnO₂ particles with their relatively higher specific surface area compared to micron-sized particles ensure electrode–electrolyte contact during discharge–charge cycling and make the greatest contribution to lithium storage capacity, owing to the lower capacity of the hybrid nanowire electrode when cycled beyond 50 cycles between 1.0 and 3.0 V vs.Li⁺/Li, as shown in Fig. S3,† which is mainly attributable to the TiO₂(B) in the hybrid nanowire. Additionally, the TiO₂(B) core in the composite, to some extent, acts as an effective mechanical support to alleviate the stress aroused during lithium intercalation–de-intercalation, which may play a critical role in the excellent lithium storage capacity and cyclability of the electrode. Finally, the mesopores formed by 3D microstructures (bundles of nanowires) also contribute to the improved electrochemical performance of the electrode, possibly because they facilitate the electrolyte or lithium ion diffusion during charge–discharge processes.

Conclusions

In summary, TiO₂(B)@SnO₂ hybrid nanowires consisting of a TiO₂(B) crystal core inside a shell of porous SnO₂ nanocrystals have been fabricated by a facile hydrothermal process and subsequent liquid phase reaction. This particular architecture of TiO₂(B)@SnO₂ hybrid nanowires has an open continuous channel along the nanowire axis, facilitating lithium ion diffusion, as well as providing effective mechanical support of the TiO₂(B) core, alleviating the stress caused during discharge–charge cycling, and further preventing the pulverization of the Sn nanoparticles. Therefore, this composite delivers a high reversible capacity of ∼463 mA h g⁻¹ after 50 cycles at a current density of 30 mA g⁻¹, with a high coulombic efficiency of nearly 100%. It also delivers a reversible discharge capacity as high as 477 mA h g⁻¹, when cycled at a current density of 1000 mA g⁻¹. Thus, this composite is a promising potential anode material for lithium-ion batteries, even though the composition and structure of these materials require further improvement by optimizing the fabrication processes of the composite.

Acknowledgements

Part of this work was funded by an Australian Research Council (ARC) Discovery Grant (DP1094261), the Postdoctoral Foundation Program of Fuzhou University (BSH-0601), the Natural Science Foundation Program of Fujian Province (2010J01332, A0510011), the Initial Foundation Program of the Ministry of Education for Returned Exchange Personnel (LXKQ201101), and the Talent Foundation Program of Fuzhou University. The authors also would like to thank Dr Tania Silver at the University of Wollongong for critical reading of the manuscript and Mr. Attard Darren for his great contribution.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c1ra00500f/

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