Journal of Nanomaterials
Volume 2019 (2019), Article ID 9082132, 10 pages
https://doi.org/10.1155/2019/9082132

Research Article

Carbon-Coated SnO₂/Ti₃C₂ Composites with Enhanced Lithium Storage Performance

Zijing Wang, Fen Wang, Kaiyu Liu, Jianfeng Zhu, and Abdul Waras

School of Material Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an 710021, China

Correspondence should be addressed to Fen Wang; wangf@sust.edu.cn

Received 11 April 2019; Revised 14 June 2019; Accepted 31 August 2019; Published 7 October 2019

Guest Editor: Fei Ke

Copyright © 2019 Zijing Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Tin-based anode materials including oxides, composites oxides, and tin-based alloys are identified as promising candidates for energy storage attributed to the highest theoretical specific capacity. We introduce Ti₃C₂-MXene as structural skeletons and amorphous carbon as conductive networks for tin oxide in this work. Herein, carbon-coated kernel-like SnO₂ coupling with two-dimensional (2D) layered structure Ti₃C₂-MXene (C@SnO₂/Ti₃C₂) composites were prepared by a hydrothermal reaction and a further calcination process. The fabricated C@SnO₂/Ti₃C₂ nanocomposites exhibit smaller charge transfer resistance, larger Li⁺ diffusion coefficient, and better cycling stability than SnO₂/Ti₃C₂ and pure Ti₃C₂. Most of all, C@SnO₂/Ti₃C₂ nanocomposites display excellent initial capacity of 1531.5 mAh g⁻¹ at current density of 100 mA g⁻¹ and show outstanding rate performance of 540 mAh g⁻¹even after 200 cycles. In our work, we will provide a new research idea for the composite materials of metal oxides and two-dimensional layered materials in the field of electrode materials for batteries.

1. Introduction

Lithium-ion batteries (LIBs) are widely used because of their excellent specific capacity, superior cycle performance, and good safety performance. In recent years, it has become more and more widely used in mobile phones, electric vehicles, and laptops as well [1–4]. However, the current cycle stability and specific capacity of LIBs have not been able to meet the needs of the people, so it is necessary to develop LIBs with better capacity and higher cycle performance. As a new 2D transition metal carbide, MXene has a better electrochemical performance than any other carbon material in the electrode material of LIBs [5–19].

Ti₃C₂T (, OH, and F) is the most popular MXene. The 2D structure Ti₃C₂T can be obtained by corroding the Al layer in the Ti₃AlC₂ with a ceramic structure by HF. Furthermore, Ti₃C₂-MXenes have the following excellent properties, such as lower Li⁺ diffusion barrier and advanced Li storage capacity, electronic conductivity, and low operating voltage, combined with good surface hydrophilicity and excellent chemical stability and structural stability [12]. According to Sun et al.’s research, the capacity of Ti₃C₂ reaches 123.6 mAh g⁻¹ at 1 C rate and a coulombic efficiency of 47% [15]. Due to these advantages and disadvantages of Ti₃C₂, it is necessary to chemically modify Ti₃C₂ with high surface area to meet the capacity requirements of the battery.

Throughout the development of lithium-ion batteries, tin-based materials are one of the most commonly used anode materials for LIBs because of their nontoxicity and excellent theoretical capacity (782 mAh g⁻¹) [20–23]. Zhu et al. used a hydrothermal method to combine SnO₂ and graphene to obtain SnO₂/G nanomaterials. SnO₂/G shows excellent electrochemical performance of 860 mAh g⁻¹ after 50 cycles at 200 mA g⁻¹ [24]. In Wu et al.’s work, a SnO₂/graphene nanocomposite was proposed as an anode material for LIBs by a facile method, which shows a good specific capacity of 540 mAh g⁻¹ after 90 cycles [25]. According to the previous work, SnO₂-based materials have specific capacities but their capacity decays fast and the electrochemical stability needs to be improved. Therefore, it needs to be chemically modified with B having excellent chemical stability to improve its electrochemical performance.

In our research, we have adopted a novel method to enhance the cycle stability and specific capacity of multilayer Ti₃C₂T (, OH) particles by loading SnO₂ nanoparticles (NPs) between Ti₃C₂T layers followed by coating amorphous carbon on its surface. In previous reports, there were many methods for preparing the nanocomposite, such as Wang et al.’s preparation of the SnO₂-Ti₃C₂ nanocomposite by a hydrothermal method and using it as the anode material of the lithium-ion battery [26]. Chen and Lou used the calcination method to prepare the SnO₂ nanorods to test their electrochemical performance [27]; Zheng et al. prepared SnO₂/Ti₃C₂ nanocomposites by microwave irradiation and tested their electrochemical properties [28]. Compared with the above methods, our method has the advantages of low temperature, simple and easy operation, and low requirements on equipment; in addition, it can make SnO₂ load all the more consistently on the layers of Ti₃C₂T, improving the consistency of the SnO₂, and, in the meantime, coat the amorphous carbon all the more effectively in the calcination stage, which is an increasingly productive and advantageous approach to synthesize C@SnO₂/Ti₃C₂ nanocomposites. Amorphous carbon connects isolated MXene particles, and it remarkably improves the electric transportation and decreases their contact resistance by coating these voids/gaps, which makes an inordinate contribution to the electrochemical properties of the as-prepared C@SnO₂/Ti₃C₂ composite.

2. Materials and Methods

2.1. Synthesis Procedure

Ti₃C₂-MXene was synthesized by a solid phase calcination method as reported previously [26]. All chemicals are purchased directly and do not require further processing. C@SnO₂/Ti₃C₂ nanocomposites were synthesized by a hydrothermal reaction and synthesized further by the calcination process. To obtain the Ti₃C₂ solution, mix 100 mg of as-prepared Ti₃C₂ and 100 mL of ultrapure water (UPW) and then sonicate for 2 h. Then, 3.0 g polyvinyl alcohol (PVA) was added to 100 mL UPW and stirred for 30 minutes to obtain PVA solution. After this, add 12.5 g SnCl₄·5H₂O to 60 mL UPW and then add 1 mL of HCl (36.0~38.0 wt.%), mix the solution with appropriate stirring, add Ti₃C₂ solution and PVA solution to it, and then adjust the pH value to 9~10 by injecting NH₃·H₂O (25.0~38.0 wt.%). Then, the mixed solution was quickly stirred in a water bath at 85°C for 1 h and then dried at 100°C for 24 hours. Finally, under argon protection, the target product is obtained by sintering 500°C in a tube furnace and marking it as C@SnO₂/Ti₃C₂. As a comparative sample, SnO₂/Ti₃C₂ is prepared in the same manner as above except that there is no addition of PVA.

Figure 1 demonstrates the preparation process of C@SnO₂/Ti₃C₂, when solutions of polyvinyl acetate (PVA) and SnCl₄·5H₂O are thoroughly mixed. The Sn⁴⁺ produced by the dissolved SnCl₄·5H₂O can be combined with -OH in the uniformly distributed PVA chain, and Sn⁴⁺ can be uniformly distributed in the solution, so PVA is an excellent surfactant, and the Ti₃C₂T (, F) is added. It allows the PVA chains to be oriented on the surface of the Ti₃C₂T layers by the mutual attraction between the functional groups. After calcination process, we can obtain C@SnO₂/Ti₃C₂ with different structure and morphology.

Figure 1: Synthesis schematic of C@SnO₂/Ti₃C₂ nanocomposites.

2.2. Material Characterization

The characterization of the morphology and structure for the as-prepared C@SnO₂/Ti₃C₂ nanocomposites will be carried out in the following instruments. Morphological characterization is carried out by field-emission scanning electron microscopy (FE-SEM, S4800) and transmission electron microscopy (TEM JEM2100F) equipped with an energy dispersive X-ray analyzer (EDX). Beyond that, the composition and structure of C@SnO₂/Ti₃C₂ nanocomposites are characterized in X-ray diffraction (XRD, D/MAX-2500).

2.3. Electrochemical Measurements

The assemblage of lithium-ion batteries is necessary to test the electrochemical performance of as-prepared C@SnO₂/Ti₃C₂ nanocomposites. Right off the bat, the dynamic materials, poly(vinylidene) fluoride (PVDF), and acetylene black are completely mixed in an agate mortar at a weight proportion of 80 : 10 : 10, then N-methyl l-2-pyrrolidinone (NMP) is added dropwise, grinding is continued until the mixture was uniformly glue like, and the mixture was connected to the surface of the Cu foil by a coater, dried in a vacuum at 120°C for 12 h, and cut into small wafers. At the same time, lithium foil is used as the counter electrode, and the LIBs with the CR2032 coin type are assembled in the vacuum glove box. The electrochemical performance of the assembled battery is tested on an Ametek PARSTAT 4000 electrochemistry workstation, including cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS). The charge and discharge performance of the gathered battery is estimated on a Land CT2001A cycler.

3. Results and Discussion

3.1. Characterization of C@SnO₂-Ti₃C₂

XRD patterns of as-prepared SnO₂, Ti₃C₂, and C@SnO₂/Ti₃C₂ nanocomposites appear in Figure 2. As shown in Figure 2(a), the Ti₃C₂, SnO₂ have a standard XRD design as detailed in the literature [5, 20]. The peaks distinguished at (002), (006), (008), and (0010) were allotted as the diffraction peaks of Ti₃C₂ [16]. The diffraction peaks of C@SnO₂/Ti₃C₂ nanocomposites were indexed to the (110), (101), (200), and (211) planes of tetragonal SnO₂ (JCPDS No. 41-1445). In addition, there is a small peak at about 20° demonstrating the presence of the amorphous carbon. Figure 2(b) shows partial magnification of XRD designs for these examples; the peaks at 9.28° indexed to the (002) plane move 0.43° towards 8.85°; the result shows the cross-section steady changes and the between-planar separating increments. According to the Bragg equation,

(a)

(b)

Figure 2: (a) XRD patterns of the three different samples SnO₂, Ti₃C₂, and C@SnO₂/Ti₃C₂; (b) partial enlargement of XRD patterns in (a).

It can be calculated that when the (002) plane is at 9.28°, is obtained; when the (002) plane is offset by 0.43° to 8.85°, is obtained, so it can be concluded that the layer spacing is increased that . This outcome demonstrates that both Ti₃C₂ and SnO₂ have been effectively prepared in C@SnO₂/Ti₃C₂ nanocomposites and there is no impurity.

The morphologies of the extraordinary 2D structure of Ti₃C₂, single-deck Ti₃C₂, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂ nanocomposites are seen by TEM, as shown in Figure 3. The morphologies of Ti₃C₂, single-deck Ti₃C₂, and SnO₂-Ti₃C₂ appear in Figures 3(a), 3(b), and 3(d) to compare with the C@SnO₂/Ti₃C₂ composites. As can be seen from Figure 3(f), countless kernel-like SnO₂ NPs are scattered on the surface of layered Ti₃C₂T, which are about nm in size; in addition to that, it can be clearly seen that the surface of the Ti₃C₂T sheet is covered with a large amount of amorphous carbon like a tissue. At the same time, the individual components have been marked with arrows in Figure 3(f). Corrugated amorphous carbon emerged on the surface of Ti₃C₂T and it ensures that calcination improves the conjugated level of PVA, which can increase conductivity of C@SnO₂/Ti₃C₂ composites. Moreover, due to calcination in argon gas, the SnO₂ NPs in C@SnO₂/Ti₃C₂ with a uniform size of about 150 nm are clearly observed in Figure 3(f). From the HRTEM pictures in Figures 3(c) and 3(e), the interplanar distances of 0.331 nm and 0.262 nm may be distinguished as d (110) and d (101) of SnO₂ NPs, individually.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3: TEM images of Ti₃C₂T (a) and single-deck Ti₃C₂T nanoparticles (b); HRTEM image (c) and TEM image (d) of the SnO₂-Ti₃C₂ nanoparticles; HRTEM image (e) and TEM image (f) of the C@SnO₂/Ti₃C₂ nanoparticles.

Figure 4 demonstrates the SEM pictures of samples. As shown in Figure 4(a), the SEM image of Ti₃C₂T which is the product obtained by treating Ti₃AlC₂ with HF solution environment [5]. As can be seen from Figure 4(a), Ti₃C₂T is an exceptionally uniform and remarkable 2D-layered structure. As observed from the SEM image of SnO₂-Ti₃C₂ (Figure 4(b)), SnO₂ nanoparticle with a particle size of around 30 nm (Figure 4(c)) is consistently upheld on the surface of Ti₃C₂T. Figure 4(d) is the SEM image of C@SnO₂/Ti₃C₂ composites. It very well may be seen from the image that the surface of the C@SnO₂/Ti₃C₂ composite is wrapped by a layer of gauze-like amorphous carbon and the clearly visible small protrusion is SnO₂ NPs.

(a)

(b)

(c)

(d)

Figure 4: SEM images of Ti₃C₂T (a), SnO₂-Ti₃C₂ nanocomposites (b, c), and C@SnO₂/Ti₃C₂ nanoparticles.

Elemental mapping images of C@SnO₂/Ti₃C₂ composites are shown in Figure 5. The inset in Figure 5(b) is the EDS quantitative analysis of C@SnO₂/Ti₃C₂ nanocomposites, which has the mass percentage and atomic percentage of each element. We know that the active material used in the negative electrode material of a LIB with the CR2032 coin type is about 3.76 mg, calculated from the mass percentage, and has a carbon content of about 0.7246 mg and a Ti content of about 1.9394 mg. Carbon is mainly supplied by Ti₃C₂T and amorphous carbon. The mass of Ti₃C₂T can be calculated according to the Ti content, and the remaining carbon is the mass of amorphous carbon. The calculated carbon content is about 0.3963 mg. The bright regions correspond to the elements tin, titanium, oxygen, and carbon. Sn, Ti, O, and C are distributed uniformly throughout the composite material, which further confirms the structure of the C@SnO₂/Ti₃C₂ composites.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5: Elemental mapping images of C@SnO₂/Ti₃C₂ nanoparticles: (a) TEM image; (b) electron dispersive spectroscopy (EDS) pattern of (c) tin, (d) titanium, (e) oxygen, and (f) carbon distribution in the selected area.

3.2. Performance of C@SnO₂-Ti₃C₂ as Anodes

The electrochemical performance was assessed by utilizing the nanocomposites (Ti₃C₂T, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂) as working electrodes and lithium foil as the counter electrode in half-cell batteries. The thickness of the electrode and the accurate mass of active material are 300 μm and 3.76 mg, respectively. Figure 6(a) presents the CV curves of C@SnO₂/Ti₃C₂ at a scan rate of 0.1 mV s⁻¹ in the voltage range of 0.01~3.00 V (vs. Li/Li⁺). As can been seen, the characteristic reduction peak was found nearly 0.69 V in the first lithiation process for the C@SnO₂/Ti₃C₂ electrode. The reduction peak of 0.69 V might be generated by the formation of a solid electrolyte interphase (SEI) layer on the surface of the active material and the formation of Li₂O while SnO₂ chemically reacts to form Sn. Nonetheless, it vanished in the following cycles, indicating that the irreversible reaction happened [9, 29]. It is important that the Ti₃C₂T after HF corrosion has functional groups, for example, hydroxyl groups and fluorine groups. During the lithiation process, lithium ions enter the Ti₃C₂T layer and interact with these functional groups, resulting in the irreversibility of the first cycle [15, 30]. The obvious peak near 0.02 V which compares to the lithiation of carbon rises in the active materials. In the first delithiation process, there are two distinct anodic peaks situated at 0.58 V and 1.25 V, ascribing to the dealloying process for LiSn and the Li ions from MXene sheets [9, 31]. The peak at 0.21 V indicates that Li ions enter the interlayer of Ti₃C₂T to increase the capacity of lithium. C@SnO₂/Ti₃C₂ shows a couple of excellent redox peaks, suggesting C@SnO₂/Ti₃C₂ has outstanding reversible performance during charging and discharging.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 6: (a) CV curves of C@SnO₂/Ti₃C₂ from 3.0 V to 0.01 V vs. Li/Li⁺ at a scan rate of 0.1 mV s⁻¹; (b) EIS for Ti₃C₂T, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂; (c) charge/discharge profiles of Ti₃C₂T, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂ at 100 mA g⁻¹; (d) cycling performance of Ti₃C₂T, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂ at 100 mA g⁻¹and the coulombic efficiency of C@SnO₂/Ti₃C₂; (e) charge/discharge profiles of C@SnO₂/Ti₃C₂ at 100 mA g⁻¹; (f) rate performance of the C@SnO₂/Ti₃C₂ nanocomposite electrode.

EIS of Ti₃C₂T, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂ are shown in Figure 6(b). Nyquist plots of Ti₃C₂T, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂ comprise a straight line at low frequencies and a semicircle at high frequencies. As can be seen from the figure, the semicircle of C@SnO₂/Ti₃C₂ is the smallest among all samples and the slope of the line of C@SnO₂/Ti₃C₂ is the largest. Therefore, it can be concluded that the minimum impedance of C@SnO₂/Ti₃C₂ means that it has excellent conductivity, which is credited to the uniform dispersion of SnO₂ NPs and the presence of large amounts of amorphous carbon. At the same time, the increase of conductivity is beneficial to the improvement of electrochemical performance of C@SnO₂/Ti₃C₂.

With the galvanostatic charge/discharge profiles of Ti₃C₂T, SnO₂-Ti₃C₂, and C@SnO₂/Ti₃C₂ anodes at a current density of 100 mA g⁻¹, all the samples are tested over the voltage range of 0.01~3.00 V as presented in Figure 6(c). The first-cycle discharge and charge capacities of as-prepared C@SnO₂/Ti₃C₂ anodes are 811.4 and 1531.5 mAh g⁻¹, respectively, which are just about 3 times as much as that of unadulterated Ti₃C₂T (580.5 mAh g⁻¹) and higher than SnO₂-Ti₃C₂ nanocomposites (1030.1 mAh g⁻¹). The initial capacity loss is about 53% for C@SnO₂/Ti₃C₂. The voltage plateau and slope of the discharge/charge profiles correspond to the CV curves recently reported in our work. The enormous capacity decay is due to the arrangement of the SEI layer and the functional groups, including fluorine and hydroxyls, on the surface of the active material. These outcomes affirm that an exceedingly conjugated carbonaceous polymer improves conductivity and the introduction of SnO₂ NPs amplifies the d-spacing of Ti₃C₂T layers, which expands the Li storage capacity significantly.

As shown in Figure 6(d), the first discharge capacities for the pure Ti₃C₂T and SnO₂/Ti₃C₂ nanocomposites are 581.1 mAh g⁻¹ and 1031.1 mAh g⁻¹, respectively. However, C@SnO₂/Ti₃C₂ demonstrates extraordinarily the first capacity of 1531.5 mAh g⁻¹ at 100 mA g⁻¹; the first charge and discharge capacity can stay at around 540 mAh g⁻¹ even after 200 cycles and the relevant coulombic efficiency of C@SnO₂/Ti₃C₂ remains at around 98% (Figure 6(d)). The outstanding electrochemical reversibility of C@SnO₂/Ti₃C₂ is attributed to the carbon coating layers. In addition to improving conductivity, the amorphous carbon can also alleviate the volume expansion of SnO₂ NPs. For a comparison, pure Ti₃C₂T begins to decay rapidly after the third charge and discharge cycles and remains at around 82.3 mAh g⁻¹ at 200 cycles. However, SnO₂-Ti₃C₂ shows lower reversible capacities than C@SnO₂/Ti₃C₂. In addition, from Figure 6(d) we can also see that the coulomb efficiency of C@SnO₂/Ti₃C₂ can still be maintained between 98% and 99% and with no significant attenuation after the tenth cycles. Moreover, C@SnO₂/Ti₃C₂ likewise has great rate capacity as appears in Figure 6(f). As the current density recovers from 1000 to 100, the capacity of C@SnO₂/Ti₃C₂ also recuperates to 454.3 mAh g⁻¹, indicating a good capacity reversibility of C@SnO₂/Ti₃C₂. It can be seen from Figure 7 that at the current density of 300 mA g⁻¹, the reversible specific capacity of C@SnO₂/Ti₃C₂ still remains at around 480 mAh g⁻¹ even after 200 cycles.

Figure 7: Cycling performance of C@SnO₂/Ti₃C₂ nanocomposite at a current density of 300 mA g⁻¹.

Table 1 shows the electrochemical properties of different MXene-based nanomaterials as anode materials for LIBs. As can be seen from the table, the electrochemical performance of C@SnO₂/Ti₃C₂ is significantly better than other MXene-based nanomaterials. Meanwhile, the electrochemical properties of the previously reported MXene-based nanomaterials are inadmissible, especially in terms of capacity and cycle stability. In their work, storing Li⁺ relies on MXene having larger lattice parameters, while larger lattice parameters imply larger layer spacing, which is beneficial for Li⁺ storage [15].

Table 1: Comparison of the performances of the various MXene-based materials for LIBs.

Electrode material
Current density
Initial capacity
Steady capacity/cycle number

Ti₃C₂T [15]
260 mA g⁻¹
123.6 mAh g⁻¹
69 mAh g⁻¹/100

Ti₂CT [14]
260 mA g⁻¹
480 mAh g⁻¹
110 mAh g⁻¹/80

Ti₃C₂/CNF [32]
320 mA g⁻¹
848 mAh g⁻¹
320 mAh g⁻¹/80

C@SnO₂/Ti₃C₂
300 mA g⁻¹
1531.5 mAh g⁻¹
540 mAh g⁻¹/200

However, in this paper, we rely on metal oxides (SnO₂ NPs) with high theoretical capacity to increase the Li⁺ storage capacity of C@SnO₂/Ti₃C₂. At the same time, the presence of SnO₂ NPs can increase the interlayer spacing of C@SnO₂/Ti₃C₂, thereby increasing the lithium storage of the C@SnO₂/Ti₃C₂ capacity. In addition, SnO₂ NPs improve the conductivity by the transportation of Sn⁴⁺, which promotes the release and insertion speed of Li⁺ in the electrode material. In this way, the combination of metal oxide and MXene provides a promising research strategy for LIB anode materials.

4. Conclusions

In this work, C@SnO₂/Ti₃C₂ nanocomposites with amazing electrochemical performance as anode material for LIBs are synthesized via hydrothermal strategy pursued by a simple calcination. Due to Ti₃C₂T providing C@SnO₂/Ti₃C₂ with a decent skeleton structure, amorphous carbon gives C@SnO₂/Ti₃C₂ with fantastic electrical conductivity; C@SnO₂/Ti₃C₂ nanocomposites display excellent initial capacity of 1531.5 mAh g⁻¹ at current density of 100 mA g⁻¹ and show outstanding rate performance of 540 mAh g⁻¹ even after 200 cycles. The exceptional electrochemical performance credits the uniform dispersion of SnO₂ NPs, high conductivity of amorphous carbon, and chemical stability of Ti₃C₂T. Another reason is that SnO₂ NPs improve the conductivity by the transportation of Sn⁴⁺ and Li⁺ insertion/extraction into the anode. These outstanding properties show that amorphous carbon and metal oxide composites in 2D-layered material MXene have comprehensive application prospects in energy storage.

Data Availability

The data used to support the findings of this study are available from the corresponding authors upon request.

Conflicts of Interest

There is no conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51472153, 51572158) and the Graduate Innovation Fund of Shaanxi University of Science & Technology.

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