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

Authors: Zijing Wang; Fen Wang; Kaiyu Liu; Jianfeng Zhu; Abdul Waras
Journal: Journal of Nanomaterials
 (2019)

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## 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.


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## Body

## 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].Ti3C2Tx (T=O, OH, and F) is the most popular MXene. The 2D structure Ti3C2Tx 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 Ti3C2Tx (T=F, OH) particles by loading SnO2 nanoparticles (NPs) between Ti3C2Tx 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 Ti3C2Tx, 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.Figure1 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 Ti3C2Tx (T=OH, F) is added. It allows the PVA chains to be oriented on the surface of the Ti3C2Tx 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.


## 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.Figure1 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 Ti3C2Tx (T=OH, F) is added. It allows the PVA chains to be oriented on the surface of the Ti3C2Tx 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,
(1)2dsinθ=nλ.Figure 2
(a) XRD patterns of the three different samples SnO2, Ti3C2, and C@SnO2/Ti3C2; (b) partial enlargement of XRD patterns in (a).
(a)
(b)It can be calculated that when the (002) plane is at 9.28°,d1=9.5228nm is obtained; when the (002) plane is offset by 0.43° to 8.85°, d2=9.9844nm is obtained, so it can be concluded that the layer spacing is increased that Δd=0.4616nm. 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 Ti3C2Tx, which are about nm in size; in addition to that, it can be clearly seen that the surface of the Ti3C2Tx 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 Ti3C2Tx 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.Figure 3
TEM images of Ti3C2Tx (a) and single-deck Ti3C2Tx 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.
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(f)Figure4 demonstrates the SEM pictures of samples. As shown in Figure 4(a), the SEM image of Ti3C2Tx which is the product obtained by treating Ti3AlC2 with HF solution environment [5]. As can be seen from Figure 4(a), Ti3C2Tx 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 Ti3C2Tx. 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.Figure 4
SEM images of Ti3C2Tx (a), SnO2-Ti3C2 nanocomposites (b, c), and C@SnO2/Ti3C2 nanoparticles.
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(b)
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(d)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 Ti3C2Tx and amorphous carbon. The mass of Ti3C2Tx 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.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.
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(b)
(c)
(d)
(e)
(f)


### 3.2. Performance of C@SnO2-Ti3C2 as Anodes

The electrochemical performance was assessed by utilizing the nanocomposites (Ti3C2Tx, 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 Ti3C2Tx after HF corrosion has functional groups, for example, hydroxyl groups and fluorine groups. During the lithiation process, lithium ions enter the Ti3C2Tx 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 LixSn and the Li ions from MXene sheets [9, 31]. The peak at 0.21 V indicates that Li ions enter the interlayer of Ti3C2Tx 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.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 Ti3C2Tx, SnO2-Ti3C2, and C@SnO2/Ti3C2; (c) charge/discharge profiles of Ti3C2Tx, SnO2-Ti3C2, and C@SnO2/Ti3C2 at 100 mA g−1; (d) cycling performance of Ti3C2Tx, 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.
(a)
(b)
(c)
(d)
(e)
(f)EIS of Ti3C2Tx, SnO2-Ti3C2, and C@SnO2/Ti3C2 are shown in Figure 6(b). Nyquist plots of Ti3C2Tx, 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 Ti3C2Tx, 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 Ti3C2Tx (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 Ti3C2Tx layers, which expands the Li storage capacity significantly.As shown in Figure6(d), the first discharge capacities for the pure Ti3C2Tx 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 Ti3C2Tx 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.Table1 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
Ti3C2Tx [15]
260 mA g−1
123.6 mAh g−1
69 mAh g−1/100
Ti2CTx [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−1/200However, 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.


## 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,
(1)2dsinθ=nλ.Figure 2
(a) XRD patterns of the three different samples SnO2, Ti3C2, and C@SnO2/Ti3C2; (b) partial enlargement of XRD patterns in (a).
(a)
(b)It can be calculated that when the (002) plane is at 9.28°,d1=9.5228nm is obtained; when the (002) plane is offset by 0.43° to 8.85°, d2=9.9844nm is obtained, so it can be concluded that the layer spacing is increased that Δd=0.4616nm. 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 Ti3C2Tx, which are about nm in size; in addition to that, it can be clearly seen that the surface of the Ti3C2Tx 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 Ti3C2Tx 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.Figure 3
TEM images of Ti3C2Tx (a) and single-deck Ti3C2Tx 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.
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(f)Figure4 demonstrates the SEM pictures of samples. As shown in Figure 4(a), the SEM image of Ti3C2Tx which is the product obtained by treating Ti3AlC2 with HF solution environment [5]. As can be seen from Figure 4(a), Ti3C2Tx 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 Ti3C2Tx. 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.Figure 4
SEM images of Ti3C2Tx (a), SnO2-Ti3C2 nanocomposites (b, c), and C@SnO2/Ti3C2 nanoparticles.
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(b)
(c)
(d)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 Ti3C2Tx and amorphous carbon. The mass of Ti3C2Tx 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.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.
(a)
(b)
(c)
(d)
(e)
(f)


## 3.2. Performance of C@SnO2-Ti3C2 as Anodes

The electrochemical performance was assessed by utilizing the nanocomposites (Ti3C2Tx, 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 Ti3C2Tx after HF corrosion has functional groups, for example, hydroxyl groups and fluorine groups. During the lithiation process, lithium ions enter the Ti3C2Tx 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 LixSn and the Li ions from MXene sheets [9, 31]. The peak at 0.21 V indicates that Li ions enter the interlayer of Ti3C2Tx 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.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 Ti3C2Tx, SnO2-Ti3C2, and C@SnO2/Ti3C2; (c) charge/discharge profiles of Ti3C2Tx, SnO2-Ti3C2, and C@SnO2/Ti3C2 at 100 mA g−1; (d) cycling performance of Ti3C2Tx, 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.
(a)
(b)
(c)
(d)
(e)
(f)EIS of Ti3C2Tx, SnO2-Ti3C2, and C@SnO2/Ti3C2 are shown in Figure 6(b). Nyquist plots of Ti3C2Tx, 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 Ti3C2Tx, 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 Ti3C2Tx (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 Ti3C2Tx layers, which expands the Li storage capacity significantly.As shown in Figure6(d), the first discharge capacities for the pure Ti3C2Tx 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 Ti3C2Tx 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.Table1 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
Ti3C2Tx [15]
260 mA g−1
123.6 mAh g−1
69 mAh g−1/100
Ti2CTx [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−1/200However, 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 Ti3C2Tx 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 Ti3C2Tx. 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.



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*Source: 9082132-2019-10-07.xml*