Research Article  Open Access
Zijing Wang, Fen Wang, Kaiyu Liu, Jianfeng Zhu, Abdul Waras, "CarbonCoated SnO_{2}/Ti_{3}C_{2} Composites with Enhanced Lithium Storage Performance", Journal of Nanomaterials, vol. 2019, Article ID 9082132, 10 pages, 2019. https://doi.org/10.1155/2019/9082132
CarbonCoated SnO_{2}/Ti_{3}C_{2} Composites with Enhanced Lithium Storage Performance
Abstract
Tinbased anode materials including oxides, composites oxides, and tinbased alloys are identified as promising candidates for energy storage attributed to the highest theoretical specific capacity. We introduce Ti_{3}C_{2}MXene as structural skeletons and amorphous carbon as conductive networks for tin oxide in this work. Herein, carboncoated kernellike SnO_{2} coupling with twodimensional (2D) layered structure Ti_{3}C_{2}MXene (C@SnO_{2}/Ti_{3}C_{2}) composites were prepared by a hydrothermal reaction and a further calcination process. The fabricated C@SnO_{2}/Ti_{3}C_{2} nanocomposites exhibit smaller charge transfer resistance, larger Li^{+} diffusion coefficient, and better cycling stability than SnO_{2}/Ti_{3}C_{2} and pure Ti_{3}C_{2}. Most of all, C@SnO_{2}/Ti_{3}C_{2} 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. In our work, we will provide a new research idea for the composite materials of metal oxides and twodimensional layered materials in the field of electrode materials for batteries.
1. Introduction
Lithiumion 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].
Ti_{3}C_{2}T (, OH, and F) is the most popular MXene. The 2D structure Ti_{3}C_{2}T can be obtained by corroding the Al layer in the Ti_{3}AlC_{2} with a ceramic structure by HF. Furthermore, Ti_{3}C_{2}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 Ti_{3}C_{2} reaches 123.6 mAh g^{−1} at 1 C rate and a coulombic efficiency of 47% [15]. Due to these advantages and disadvantages of Ti_{3}C_{2}, it is necessary to chemically modify Ti_{3}C_{2} with high surface area to meet the capacity requirements of the battery.
Throughout the development of lithiumion batteries, tinbased 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 SnO_{2} and graphene to obtain SnO_{2}/G nanomaterials. SnO_{2}/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 SnO_{2}/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, SnO_{2}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 Ti_{3}C_{2}T (, OH) particles by loading SnO_{2} nanoparticles (NPs) between Ti_{3}C_{2}T 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 SnO_{2}Ti_{3}C_{2} nanocomposite by a hydrothermal method and using it as the anode material of the lithiumion battery [26]. Chen and Lou used the calcination method to prepare the SnO_{2} nanorods to test their electrochemical performance [27]; Zheng et al. prepared SnO_{2}/Ti_{3}C_{2} 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 SnO_{2} load all the more consistently on the layers of Ti_{3}C_{2}T, improving the consistency of the SnO_{2}, 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@SnO_{2}/Ti_{3}C_{2} 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 asprepared C@SnO_{2}/Ti_{3}C_{2} composite.
2. Materials and Methods
2.1. Synthesis Procedure
Ti_{3}C_{2}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@SnO_{2}/Ti_{3}C_{2} nanocomposites were synthesized by a hydrothermal reaction and synthesized further by the calcination process. To obtain the Ti_{3}C_{2} solution, mix 100 mg of asprepared Ti_{3}C_{2} 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 SnCl_{4}·5H_{2}O to 60 mL UPW and then add 1 mL of HCl (36.0~38.0 wt.%), mix the solution with appropriate stirring, add Ti_{3}C_{2} solution and PVA solution to it, and then adjust the pH value to 9~10 by injecting NH_{3}·H_{2}O (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@SnO_{2}/Ti_{3}C_{2}. As a comparative sample, SnO_{2}/Ti_{3}C_{2} is prepared in the same manner as above except that there is no addition of PVA.
Figure 1 demonstrates the preparation process of C@SnO_{2}/Ti_{3}C_{2}, when solutions of polyvinyl acetate (PVA) and SnCl_{4}·5H_{2}O are thoroughly mixed. The Sn^{4+} produced by the dissolved SnCl_{4}·5H_{2}O can be combined with OH in the uniformly distributed PVA chain, and Sn^{4+} can be uniformly distributed in the solution, so PVA is an excellent surfactant, and the Ti_{3}C_{2}T (, F) is added. It allows the PVA chains to be oriented on the surface of the Ti_{3}C_{2}T layers by the mutual attraction between the functional groups. After calcination process, we can obtain C@SnO_{2}/Ti_{3}C_{2} with different structure and morphology.
2.2. Material Characterization
The characterization of the morphology and structure for the asprepared C@SnO_{2}/Ti_{3}C_{2} nanocomposites will be carried out in the following instruments. Morphological characterization is carried out by fieldemission scanning electron microscopy (FESEM, S4800) and transmission electron microscopy (TEM JEM2100F) equipped with an energy dispersive Xray analyzer (EDX). Beyond that, the composition and structure of C@SnO_{2}/Ti_{3}C_{2} nanocomposites are characterized in Xray diffraction (XRD, D/MAX2500).
2.3. Electrochemical Measurements
The assemblage of lithiumion batteries is necessary to test the electrochemical performance of asprepared C@SnO_{2}/Ti_{3}C_{2} 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 Nmethyl l2pyrrolidinone (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@SnO_{2}Ti_{3}C_{2}
XRD patterns of asprepared SnO_{2}, Ti_{3}C_{2}, and C@SnO_{2}/Ti_{3}C_{2} nanocomposites appear in Figure 2. As shown in Figure 2(a), the Ti_{3}C_{2}, SnO_{2} 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 Ti_{3}C_{2} [16]. The diffraction peaks of C@SnO_{2}/Ti_{3}C_{2} nanocomposites were indexed to the (110), (101), (200), and (211) planes of tetragonal SnO_{2} (JCPDS No. 411445). 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 crosssection steady changes and the betweenplanar separating increments. According to the Bragg equation,
(a)
(b)
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 Ti_{3}C_{2} and SnO_{2} have been effectively prepared in C@SnO_{2}/Ti_{3}C_{2} nanocomposites and there is no impurity.
The morphologies of the extraordinary 2D structure of Ti_{3}C_{2}, singledeck Ti_{3}C_{2}, SnO_{2}Ti_{3}C_{2}, and C@SnO_{2}/Ti_{3}C_{2} nanocomposites are seen by TEM, as shown in Figure 3. The morphologies of Ti_{3}C_{2}, singledeck Ti_{3}C_{2}, and SnO_{2}Ti_{3}C_{2} appear in Figures 3(a), 3(b), and 3(d) to compare with the C@SnO_{2}/Ti_{3}C_{2} composites. As can be seen from Figure 3(f), countless kernellike SnO_{2} NPs are scattered on the surface of layered Ti_{3}C_{2}T, which are about nm in size; in addition to that, it can be clearly seen that the surface of the Ti_{3}C_{2}T 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 Ti_{3}C_{2}T and it ensures that calcination improves the conjugated level of PVA, which can increase conductivity of C@SnO_{2}/Ti_{3}C_{2} composites. Moreover, due to calcination in argon gas, the SnO_{2} NPs in C@SnO_{2}/Ti_{3}C_{2} 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 SnO_{2} NPs, individually.
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(f)
Figure 4 demonstrates the SEM pictures of samples. As shown in Figure 4(a), the SEM image of Ti_{3}C_{2}T which is the product obtained by treating Ti_{3}AlC_{2} with HF solution environment [5]. As can be seen from Figure 4(a), Ti_{3}C_{2}T is an exceptionally uniform and remarkable 2Dlayered structure. As observed from the SEM image of SnO_{2}Ti_{3}C_{2} (Figure 4(b)), SnO_{2} nanoparticle with a particle size of around 30 nm (Figure 4(c)) is consistently upheld on the surface of Ti_{3}C_{2}T. Figure 4(d) is the SEM image of C@SnO_{2}/Ti_{3}C_{2} composites. It very well may be seen from the image that the surface of the C@SnO_{2}/Ti_{3}C_{2} composite is wrapped by a layer of gauzelike amorphous carbon and the clearly visible small protrusion is SnO_{2} NPs.
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Elemental mapping images of C@SnO_{2}/Ti_{3}C_{2} composites are shown in Figure 5. The inset in Figure 5(b) is the EDS quantitative analysis of C@SnO_{2}/Ti_{3}C_{2} 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 Ti_{3}C_{2}T and amorphous carbon. The mass of Ti_{3}C_{2}T 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@SnO_{2}/Ti_{3}C_{2} composites.
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3.2. Performance of C@SnO_{2}Ti_{3}C_{2} as Anodes
The electrochemical performance was assessed by utilizing the nanocomposites (Ti_{3}C_{2}T, SnO_{2}Ti_{3}C_{2}, and C@SnO_{2}/Ti_{3}C_{2}) as working electrodes and lithium foil as the counter electrode in halfcell 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@SnO_{2}/Ti_{3}C_{2} 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@SnO_{2}/Ti_{3}C_{2} 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 Li_{2}O while SnO_{2} 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 Ti_{3}C_{2}T after HF corrosion has functional groups, for example, hydroxyl groups and fluorine groups. During the lithiation process, lithium ions enter the Ti_{3}C_{2}T 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 Ti_{3}C_{2}T to increase the capacity of lithium. C@SnO_{2}/Ti_{3}C_{2} shows a couple of excellent redox peaks, suggesting C@SnO_{2}/Ti_{3}C_{2} has outstanding reversible performance during charging and discharging.
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EIS of Ti_{3}C_{2}T, SnO_{2}Ti_{3}C_{2}, and C@SnO_{2}/Ti_{3}C_{2} are shown in Figure 6(b). Nyquist plots of Ti_{3}C_{2}T, SnO_{2}Ti_{3}C_{2}, and C@SnO_{2}/Ti_{3}C_{2} comprise a straight line at low frequencies and a semicircle at high frequencies. As can be seen from the figure, the semicircle of C@SnO_{2}/Ti_{3}C_{2} is the smallest among all samples and the slope of the line of C@SnO_{2}/Ti_{3}C_{2} is the largest. Therefore, it can be concluded that the minimum impedance of C@SnO_{2}/Ti_{3}C_{2} means that it has excellent conductivity, which is credited to the uniform dispersion of SnO_{2} 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@SnO_{2}/Ti_{3}C_{2}.
With the galvanostatic charge/discharge profiles of Ti_{3}C_{2}T, SnO_{2}Ti_{3}C_{2}, and C@SnO_{2}/Ti_{3}C_{2} 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 firstcycle discharge and charge capacities of asprepared C@SnO_{2}/Ti_{3}C_{2} anodes are 811.4 and 1531.5 mAh g^{−1}, respectively, which are just about 3 times as much as that of unadulterated Ti_{3}C_{2}T (580.5 mAh g^{−1}) and higher than SnO_{2}Ti_{3}C_{2} nanocomposites (1030.1 mAh g^{−1}). The initial capacity loss is about 53% for C@SnO_{2}/Ti_{3}C_{2}. 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 SnO_{2} NPs amplifies the dspacing of Ti_{3}C_{2}T layers, which expands the Li storage capacity significantly.
As shown in Figure 6(d), the first discharge capacities for the pure Ti_{3}C_{2}T and SnO_{2}/Ti_{3}C_{2} nanocomposites are 581.1 mAh g^{−1} and 1031.1 mAh g^{−1}, respectively. However, C@SnO_{2}/Ti_{3}C_{2} 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@SnO_{2}/Ti_{3}C_{2} remains at around 98% (Figure 6(d)). The outstanding electrochemical reversibility of C@SnO_{2}/Ti_{3}C_{2} is attributed to the carbon coating layers. In addition to improving conductivity, the amorphous carbon can also alleviate the volume expansion of SnO_{2} NPs. For a comparison, pure Ti_{3}C_{2}T begins to decay rapidly after the third charge and discharge cycles and remains at around 82.3 mAh g^{−1} at 200 cycles. However, SnO_{2}Ti_{3}C_{2} shows lower reversible capacities than C@SnO_{2}/Ti_{3}C_{2}. In addition, from Figure 6(d) we can also see that the coulomb efficiency of C@SnO_{2}/Ti_{3}C_{2} can still be maintained between 98% and 99% and with no significant attenuation after the tenth cycles. Moreover, C@SnO_{2}/Ti_{3}C_{2} likewise has great rate capacity as appears in Figure 6(f). As the current density recovers from 1000 to 100, the capacity of C@SnO_{2}/Ti_{3}C_{2} also recuperates to 454.3 mAh g^{−1}, indicating a good capacity reversibility of C@SnO_{2}/Ti_{3}C_{2}. It can be seen from Figure 7 that at the current density of 300 mA g^{−1}, the reversible specific capacity of C@SnO_{2}/Ti_{3}C_{2} still remains at around 480 mAh g^{−1} even after 200 cycles.
Table 1 shows the electrochemical properties of different MXenebased nanomaterials as anode materials for LIBs. As can be seen from the table, the electrochemical performance of C@SnO_{2}/Ti_{3}C_{2} is significantly better than other MXenebased nanomaterials. Meanwhile, the electrochemical properties of the previously reported MXenebased 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].

However, in this paper, we rely on metal oxides (SnO_{2} NPs) with high theoretical capacity to increase the Li^{+} storage capacity of C@SnO_{2}/Ti_{3}C_{2}. At the same time, the presence of SnO_{2} NPs can increase the interlayer spacing of C@SnO_{2}/Ti_{3}C_{2}, thereby increasing the lithium storage of the C@SnO_{2}/Ti_{3}C_{2} capacity. In addition, SnO_{2} NPs improve the conductivity by the transportation of Sn^{4+}, 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@SnO_{2}/Ti_{3}C_{2} nanocomposites with amazing electrochemical performance as anode material for LIBs are synthesized via hydrothermal strategy pursued by a simple calcination. Due to Ti_{3}C_{2}T providing C@SnO_{2}/Ti_{3}C_{2} with a decent skeleton structure, amorphous carbon gives C@SnO_{2}/Ti_{3}C_{2} with fantastic electrical conductivity; C@SnO_{2}/Ti_{3}C_{2} 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 SnO_{2} NPs, high conductivity of amorphous carbon, and chemical stability of Ti_{3}C_{2}T. Another reason is that SnO_{2} NPs improve the conductivity by the transportation of Sn^{4+} and Li^{+} insertion/extraction into the anode. These outstanding properties show that amorphous carbon and metal oxide composites in 2Dlayered 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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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.