One-Pot Synthesis of High-Performance Tin Chalcogenides/C Anodes for Li-Ion Batteries.

Tin chalcogenides are considered as promising anode materials for lithium-ion batteries (LIBs) due to their high theoretical lithium-storage capacity. Herein, we have successfully synthesized the composites of tin chalcogenides and graphite, that is, SnS/C, SnSe/C, and SnS0.5Se0.5/C, via a simple one-pot solid-state method. During the electrochemical test, they exhibit excellent lithium-storage ability and cyclic performance as the anode electrodes of LIBs due to the introduction of carbon. In particular, (i) SnS/C displayed a high specific capacity of 875 mAh g-1 at 0.2 A g-1 over 200 cycles; (ii) SnSe/C presents 734 mAh g-1 at 0.2 A g-1 after 100 cycles, and it delivers 690 mAh g-1 at 1.0 A g-1 over 300 cycles; and (iii) the SnS0.5Se0.5/C composite electrode delivers a specific capacity of 643 mAh g-1 at 0.5 A g-1 over 150 cycles. Furthermore, another series of tin-based composites have also been successfully fabricated (i.e., Sn/C, SnS2/C, SnSe2/C, and SnTe/C), showing the general applicability of the synthetic route applied here. Our synthetic approach demonstrates a promising route for the large-scale production of high-performance tin chalcogenides/C anode materials for LIBs and other battery systems (e.g., Na-ion and K-ion batteries).

Introduction

Due to the shortage of fossil energy and the environmental pollution caused by fossil energy, electric vehicles powered by lithium-ion batteries (LIBs) are more and more popular in the auto market.[1−3] However, facing the competition of internal combustion engines, there is an urgent demand for the improvement of energy densities of LIBs. Nowadays, the widely used anode material of LIBs is graphite, owing to its low cost and high chemical stability. However, the theoretical specific capacity of graphite is relatively low, only 372 mAh g–1.[4−7] Thus, massive research works have been done to develop alternative anodes with enhanced specific capacities.[8−10] Tin-based compounds such as tin metal,[11,12] tin oxides,[13,14] tin chalcogenides,[15,16] and tin phosphides[17,18] have been considered as a promising family for alternative anodes owing to their high theoretical specific capacity, cost-effectiveness, and eco-friendliness. However, these materials have huge volume change problems during the charging/discharging cycling, causing poor stability. Over the last decade, a lot of efforts have been dedicated to solve these problems. These efforts can be generally categorized into two types: (i) decreasing the particle size into nanoscale to protect the structural integrity and (ii) combining tin-based compounds with conductive materials (e.g., graphite) to alleviate volume expansion and particle aggregation.[19−23] Based on these efforts, a large number of literature about carbon-modified tin or tin chalcogenides, for example, Sn/C,[24,25] SnS/C,[26,27] and SnSe/C,[28,29] had been reported and they exhibited improved electrochemical properties for Li-ion batteries.

Up to now, the reported pathways for the synthesis of tin chalcogenides mainly include the solvothermal method,[26,28] hydrothermal method,[30] solution method,[31] coprecipitation method,[32] and chemical vapor transport.[33] However, these synthetic approaches have shown several drawbacks at present, such as complicated reaction conditions and relatively poor repeatability. Herein, we developed a simple one-pot route by employing tetraphenyltin as the tin source and successfully synthesized Sn(S, Se)/C composites without any complex equipment, harsh reagents, and reaction conditions. Elemental analysis revealed that the carbon contents of the as-synthesized SnS/C, SnSe/C, and SnS0.5Se0.5/C were 32.8, 31.6, and 36.8 wt %, respectively. During the electrochemical test, the as-synthesized Sn(S, Se)/C composites exhibited excellent electrochemical properties, which were comparable to the best properties of Sn(S, Se) anode materials reported in the literature. Furthermore, we extended the one-pot route to synthesize other series of tin-based composites, demonstrating the general applicability for the synthesis of high-performance tin-based anode materials.

Experimental section

Materials

Tetraphenyltin (Sn(C6H5)4, 97% purity), sublimate sulfur (99.95% purity), selenium powder (99.9% purity), tellurium powder (99.9% purity), dibenzyl disulfide (98% purity), and dibenzyl diselenide (95% purity) were purchased from Aladdin Company. No further purification of reagents was done.

Synthetic Methods

For the typical synthesis of SnS/C and SnSe/C composites, powders of 0.2 g of tetraphenyltin and 0.015 g of sublimate sulfur or 0.037 g of selenium powder were mixed together and sealed in a vacuumized quartz tube. Then, the quartz tube was annealed at 873 K for 10 h. The SnS0.5Se0.5/C composite was prepared from a stoichiometric mixture of tetraphenyltin, sublimate sulfur, and selenium powder. The mixture was then sealed in a vacuumized quartz tube and annealed at 873 K for 10 h. Scheme schematically illustrates the synthetic process for Sn(S,Se)/C composites. Then, the as-prepared product was collected, washed several times with DI water and absolute alcohol, and dried in a vacuum oven at 60 °C.

Scheme 1

Schematic Illustration of Synthetic Routes for Sn(S,Se)/C Composites

Structural Characterization

The purity of the samples was examined by powder XRD on a Philips X’Pert diffractometer with a Cu Kα radiation (λ = 1.5418 Å). Field emission scanning electron microscopy (SEM, JEOLJSM-6700F) was used for the morphological studies of the prepared samples. X-ray photoelectron spectral studies were performed on Thermo ESCALAB 250. All spectra were calibrated by the C 1s peak resulting from the graphite. The stoichiometric ratios of tin chalcogenides were derived from the XPS data. TGA measurements were performed in an air environment from room temperature (RT) to 800 °C with a heating rate of 10 °C min–1. Raman spectroscopy was performed on a JY LABRAM-HR confocal laser micro-Raman spectrometer utilizing Ar+ laser excitation (λ = 514.5 nm). The carbon content was examined by elemental analysis (EA, Elemental vario EL cube, Thermal Conductivity Detector) under an oxygen atmosphere. Energy-dispersive spectrometer (EDS) images were taken at an acceleration voltage of 200 kV (JEOL-2010 HRTEM).

Electrochemical Characterization

An argon-filled glovebox (O2, H2O < 1 ppm) enclosing a counter electrode (lithium foil), a separator (Celgard 2400), and 1.0 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) in the volume ratio of 1:1 as the electrolyte was used to keep assembled coin-type 2016 cells. For the preparation of the working electrode, active material, polyvinylidene fluoride (PVDF), and carbon black at a weight ratio of 6:2:2 were mixed in N-methyl-2-pyrrolidone. The found mixture was ball-milled (10 h) to mix uniformly, and the obtained slurry was spread on the substrate (Cu foil) and dried (80 °C) in a vacuum oven (10 h). Cyclic voltammetric (CV) experiments were conducted at a scan rate of 0.1 mV s–1 (electrochemical workstation, CHI660E). Galvanostatic measurements were examined using a LAND-CT2001A instrument in the potential range of 0.01–3 V (vs Li/Li+) at a selected current density and constant RT. Here, the specific capacities and currents were calculated based on the total mass of the anode materials.

Results and Discussion

The phases of the as-prepared samples are examined by powder XRD, and the results are displayed in Figures a and 2a: orthorhombic SnS/C (JCPDS card no. 39-0354, Pmcn, a = 3.98 Å, b = 4.33 Å, c = 11.18 Å) and orthorhombic SnSe/C (JCPDS card no. 65-3811, Pnma, a = 11.59 Å, b = 4.21 Å, c = 4.41 Å), respectively. The morphologies and microstructures of the as-synthesized products are characterized by SEM, TEM, high-resolution TEM (HRTEM), high-angle annular dark-field STEM (HAADF-STEM), and EDX mapping. Figures b and S1 are the SEM images of the as-prepared SnS/C sample, revealing the uneven aggregation of particles. For the identification of the microstructure of the SnS/C sample, TEM characterization was conducted with the results shown in Figure c–h. In Figure c, the TEM image demonstrates the combination of SnS nanoparticles and carbon layers and the irregular morphology of the SnS nanoparticles marked with green circles. Figure d shows the HRTEM image of the SnS/C sample, and the selected lattice fringe is about 0.289 nm wide, which corresponds to the d-spacing of the (101) plane of SnS. The HAADF-STEM image in Figure e and the corresponding EDX images in Figure f–h further confirmed the distribution of SnS nanoparticles on carbon layers and the compositional elements (i.e., Sn, S, and C) of the SnS/C sample. The EDX (Figure S4) indicates that this composite is composed of C, Sn, and S elements and the signal of Cu arises from the TEM grid.

Figure 1

(a) Powder XRD pattern of the SnS/C sample. (b) SEM, (c) TEM, and (d) HRTEM images of the SnS/C sample. (e–h) HAADF-STEM and corresponding EDX images of the SnS/C sample.

Figure 2

(a) Powder XRD pattern of the SnSe/C sample. (b) SEM, (c) TEM, and (d) HRTEM images of the SnSe/C sample. (e–h) HAADF-STEM and the corresponding EDX images of the SnSe/C sample.

From the SEM images in Figures b and S2, it could be found that the SnSe/C sample shows an unfixed lumpy morphology similar to that of the SnS/C sample. The TEM images in Figures c and S3 reveal that the diameter of the SnSe nanoparticles is around a few to tens of nanometers and the SnSe nanoparticles disperse over the carbon layer without severe agglomeration. The lattice fringe in the HRTEM image (Figure d) of the SnSe/C sample is about 0.304 nm wide, which matches the d-spacing of the (011) plane of SnSe. Similar to the SnS/C sample, the HAADF-STEM image in Figure e and the corresponding EDX images in Figure f–h further confirmed the distribution of SnSe nanoparticles on carbon layers and the compositional elements (i.e., Sn, Se, and C) of the SnSe/C sample. The EDX spectra in Figure S5 shows that the SnSe/C composite is composed of C, Sn, and Se elements and note that the trace oxygen signal may be caused by slight surface oxidation.

The images in Figure reveal the XPS characterization results of the SnS/C and SnSe/C samples. In Figure a, the two peaks at around 487.4 and 495.9 eV correspond to Sn 3d5/2 and 3d3/2, respectively.[34,35] The S 2p3/2 at 162.4 eV and S 2p1/2 at 163.9 eV shown in Figure b are ascribed to the binding energies of SnS, which suggests that no elemental sulfur (165.1 eV in binding energy) remains in the composite.[36,37] The C 1s peak at 284.7 eV corresponding to the C–C bonding in carbon is displayed in Figure S6.[38] The peaks at 487.6 and 496.1 eV in Figure c can be related to Sn 3d5/2 and 3d3/2 of SnSe.[34,35] As shown in Figure d, the peaks at 55.5 and 56.6 eV denote the spin–orbit splitting of 3d5/2 and 3d3/2 of Se.[39] The peak at 284.8 eV corresponds to the C 1s peak (Figure S7).[38]

Figure 3

XPS for (a) Sn 3d and (b) S 2p levels of the SnS/C sample. XPS for (c) Sn 3d and (d) Se 3d levels of the SnSe/C sample.

For electrochemical tests, CV was first utilized to understand the dynamic process of the SnS/C sample for lithiation/delithiation. Figure a reveals the CV curves of the first five cycles of the SnS/C sample. From the negative scanning curve of the first cycle, two reduction peaks located at 1.24 V could be detected, which are in connection with the irreversible decomposition of SnS into Sn and Li2S (1).[30] The reduction peak at 0.75 V is related to the reactions between the electrode materials and the electrolyte, which cause the formation of the SEI membrane.[9,10] The reduction peak at 0.2 V represents the Li-ion alloying with Sn (2).[31,40]

Figure 4

(a) CV curves of the SnS/C sample. (b) Galvanostatic charging–discharging curves of the SnS/C sample at 0.2 A g–1. (c) Rate capability of the SnS/C sample from 0.2 to 2.0 A g–1. (d) Cycling performance and Coulombic efficiency (CE) of the SnS/C sample at 0.2 A g–1.

In the positive scanning curve of the first cycle, the two broad oxidation peaks at 0.51 and 0.65 V are in relation with the delithiation process of LiSn. The oxidation peaks located at 1.87 V originated from the reversible conversion reaction of SnS.

The charging–discharging curves for the 1st, 2nd, 3rd, 10th, and 200th cycles at a current density of 0.2 A g–1 of the SnS/C sample are presented in Figure b. Consistent with the CV, an obvious platform around 1.3 V can be observed in the initial discharging process. Note that in the subsequent cycles, the platform around 1.3 V gradually weakens due to the irreversible decomposition of SnS. During the charging process, the platform located at around 0.55 V is ascribed to the extraction of Li ions from the discharged SnS/C sample. The charging–discharging curves in Figure c exhibit the excellent rate performance of the SnS/C sample. The specific capacities at the currents of 0.2, 0.5, 1.0, and 2.0 A g–1 are found to be 1076, 958, 837, and 661 mAh g–1, respectively, and after charging/discharging at high current densities, the specific capacity can recover to 886 mAh g–1 at 0.2 A g–1. Figure d displays the excellent cyclic performance of the SnS/C sample at 0.2 A g–1. In the first cycle, the specific capacities are found to be 3164 and 1693 mAh g–1 during the discharging/charging process, respectively, suggesting an initial Coulombic efficiency (CE) of 53.5% due to the irreversible decomposition of SnS. Then, the CE of the SnS/C sample increases to 98% and remains almost unchanged during the subsequent cycles. After cycling 200 times at 0.2 A g–1, the specific capacity of the SnS/C sample can still remain around 875 mAh g–1. Since the specific capacity of graphite is lower than that of SnS, the specific capacity of bare SnS should be higher than the value of the SnS/C sample obtained here. Considering the theoretical specific capacity (i.e., 372 mAh g–1) of graphite and the 32.8 wt % carbon content of the SnS/C sample measured here, the specific capacity of the SnS nanoparticles synthesized through the one-pot route could be roughly estimated to be about 1120 mAh g–1 at 0.2 A g–1. The SnS without carbon showed a rapid and continuous capacity attenuation during the electrochemistry cycling process and decreased to 163 mAh g–1 after 50th cycles.[33] In comparison to the pure SnS electrode, the SnS/C electrode presented here evidently displays a better Li-storage ability due to the introduction of carbon material.

The electrochemical properties of the SnSe/C sample were measured in the same way as that for the SnS/C sample. Figure b presents the 1st, 2nd, 3rd, 10th, and 100th charging–discharging curves of the SnSe/C sample at the current of 0.2 A g–1. It could be observed that the SnSe/C sample also shows a discharging platform observed at 1.4 V in the first charging/discharging cycle, and the platform disappears in the subsequent cycles, which is assigned to the irreversible degradation of SnSe into metallic tin and Li2Se:[28,29]

Figure 5

(a) CV curves of the SnSe/C sample. (b) Galvanostatic charging–discharging curves of the SnSe/C sample at 0.2 A g–1. (c) Rate capability of the SnSe/C sample from 0.2 to 2.0 A g–1. (d) Cycling performance and Coulombic efficiency (CE) of the SnSe/C sample at 1.0 A g–1. (e) Cycling performance and CE of the SnSe/C sample at 0.2 A g–1.

During the subsequent charging processes, the two platforms located at 0.5 and 1.8 V originate from the dealloying reaction of Sn with Li (Figure a, discussed later). The first CV curve of the SnSe/C sample in Figure a was also collected, in which the redox peaks are in accordance with the charging–discharging curves in Figure b. Obviously, the reduction peak around 0.25 V represents the Li alloying with Sn:[28,29]

As for the rate capabilities of the SnSe/C sample, the specific capacities at 0.2, 0.5, 1.0, 2.0, and 0.2 A g–1 estimated from Figure c are around 861, 789, 702, 593, and 738 mAh g–1, respectively. They only undergo a capacity loss of 31.1% with the current density increasing from 0.2 to 2.0 A g–1, suggesting a relatively good rate capability. Figure e displays the cyclic performance of the SnSe/C sample at the current density of 0.2 A g–1. After the first cycle, the SnSe/C sample shows a high specific capacity with the CE nearly 100%, indicating an excellent ability of energy storage. After 100 cycles at 0.2 A g–1, the specific capacity of the SnSe/C sample maintains around 734 mAh g–1. Moreover, the SnSe/C sample displays a specific capacity of nearly 690 mAh g–1 over 300 cycles at 1.0 A g–1, as shown in Figure d. In the same way, for estimating the specific capacity of the SnS nanoparticles, the specific capacity of the SnSe nanoparticles is estimated to be around 901 mAh g–1 at 0.2 A g–1 and 837 mAh g–1 at 1.0 A g–1. The SnSe without carbon maintained a specific capacity of only 271 mAh g–1 at 0.5 A g–1 over 100th cycles,[28] indicating that the carbon-modified SnSe/C electrode demonstrated in this work delivers a significantly enhanced cycling property than the pure SnSe electrode.

To obtain further insight into intrinsic electrochemical mechanisms of composite electrodes, electrochemical impedance spectroscopy (EIS) was used to study the conductivity of the SnS/C and SnSe/C samples before and after electrochemical cycling. Figure a displays the equivalent circuit model, where Rs is the internal resistance between the electrolyte and electrode and CPE and Rct correspond to the constant phase element and charge transfer resistance, respectively. The Nyquist plots of the two samples are displayed in Figure b,e. The semicircle at the medium-frequency region represents the internal resistances in the electrode.[41] We find that SnS/C and SnSe/C electrodes show observable impedance decrease after electrochemical processes compared to impedance values before cycling, demonstrating that two composites retain good electron transport during the cycled process, which may be ascribed to the carbon introduction.[42,43] On the other hand, the ex situ SEM was carried out to study the morphologies of the two samples after cycling. These two composites maintain similar irregular morphologies before and after cycling, which were considered to be advantageous to the cycling stability due to the good electron transport between the carbon and corresponding matrix, as displayed in Figures c,d,f,g.[44,45]

Figure 6

(a) Equivalent circuit model to fit the Nyquist plots. Rs is the contact resistance between the electrode and electrolyte; Rct is charge transfer resistance; and CPE is the constant phase element. (b) EIS of the SnS/C sample before cycling and after 200 cycles at 0.2 A g–1, (c) Low-magnified and (d) high-magnified SEM images of the SnS/C composite electrode after 200 cycles at 0.2 A g–1. (e) EIS of the SnSe/C composite electrode before cycling and after 100 cycles at 0.2 A g–1. (f) Low-magnified and (g) high-magnified SEM images of the SnSe/C composite electrode after 200 cycles at 0.2 A g–1.

As discussed above, SnS/C, SnSe/C, and SnS0.5Se0.5/C composites were successfully synthesized via a simple and cost-effective one-pot route and displayed superior electrochemical properties when utilized as anode materials for LIBs due to the hybridizations of carbon. It can be clearly observed from Table and Figure S11 that the as-synthesized SnS/C, SnSe/C, and SnS0.5Se0.5/C composites deliver outstanding lithium-storage capabilities exceeding or comparable to those of the Sn(S,Se)-based anode materials that have been reported in the literature, demonstrating the feasibility of the one-pot route developed in this work for the synthesis of high-performance Sn-based anode materials. The structure and electrochemical performances of other tin-based anode materials are presented in Figures S12–S15.

Table 1

Summary of Electrochemical Performance of Sn(S,Se)/C Anodes in This Work and That of the Previously Typical Literature

materials	cycled capacities	cycle number	reference
SnS/C composite	875 mAh g–1 at 0.2 A g–1	200	this work
SnSe/C composite	734 mAh g–1 at 0.2 A g–1	100	this work
	690 mAh g–1 at 1.0 A g–1	300
SnS0.5Se0.5/C composite	643 mAh g–1 at 0.5 A g–1	150	this work
SnS@rGO	791 mAh g–1 at 0.1 A g–1	100	(26)
SnS/C hybrid	466 mAh g–1 at 0.05 A g–1	50	(27)
SnS nanoflower	580 mAh g–1 at 0.05 A g–1	30	(31)
SnS/polypyrrole nanosheet	703 mAh g–1 at 1.0 A g–1	500	(32)
SnS nanosheet	560 mAh g–1 at 0.78 A g–1	50	(33)
SnS@polypyrrole-nanobelt/carbon nanotube	757 mAh g–1 at 1.0 A g–1	500	(46)
SnS nanocrystal	867 mAh g–1 at 0.1 A g–1	70	(47)
net-like SnS/C film	540 mAh g–1 at 0.04 A g–1	40	(48)
SnS/C nanofibers	648 mAh g–1 at 0.2 A g–1	500	(40)
	548 mAh g–1 at 0.5 A g–1	500
SnS/C nanosphere	936 mAh g–1 at 0.1 A g–1	50	(49)
	830 mAh g–1 at 0.5 A g–1	250
SnS/TiO2@C nanosphere	672 mAh g–1 at 0.5 A g–1	100	(50)
NG–SnS composite	790 mAh g–1 at 0.5 A g–1	900	(51)
SnS/C nanocomposite	696 mAh g–1 at 0.5 A g–1	200	(52)
	423 mAh g–1 at 2.0 A g–1	200
SnS/graphene nanosheet	840 mAh g–1 at 0.1 A g–1	150	(53)
YDSC-SnS@NSC	561 mAh g–1 at 1.0 A g–1	100	(54)
	344 mAh g–1 at 2.0 A g–1	600
SnS@C nanoparticle	484 mAh g–1 at 0.04 A g–1	40	(55)
Se-doped SnS/C nanofiber	700 mAh g–1 at 0.2 A g–1	100	(56)
SnS/C nanocomposite	607 mAh g–1 at 1.0 A g–1	200	(57)
SnS/NC nanocomposite	832 mAh g–1 at 0.1 A g–1	100	(58)
SnS/graphene	535 mAh g–1 at 0.05 A g–1	50	(59)
flower-likeC@SnS	594 mAh g–1 at 0.5 A g–1	100	(60)
SnS/spherical grapheme framework	800 mAh g–1 at 0.078 A g–1	100	(61)
	527 mAh g–1 at 0.78 A g–1	300
SnSe/C nanocomposite	633 mAh g–1 at 0.5 A g–1	100	(28)
SnSe@CNF composite	840 mAh g–1 at 0.2 A g–1	100	(29)
SnSe nanocrystal	510 mAh g–1 at 0.1 A g–1	70	(47)
SnSe–MWCNT hybrid	651 mAh g–1 at 0.05 A g–1	50	(62)
SnSe quantum dot	550 mAh g–1 at 0.5 A g–1	1500	(63)
SnSe/C nanofiber	405 mAh g–1 at 1.0 A g–1	500	(64)
SnS0.5Se0.5 nanocomposite	625 mAh g–1 at 0.5 A g–1	1000	(34)
SnS0.5Se0.5 nanoplate	681 mAh g–1 at 0.5 A g–1	200	(65)

Conclusions

Tin chalcogenide-based composites, that is, Sn(S, Se)/C, were successfully synthesized by a facile one-pot route. During the electrochemical test, they displayed excellent lithium-storage capabilities, revealing their potential as the anode materials for LIBs. In particular, the SnS/C composite delivers a specific capacity of 875 mAh g–1 after 200 cycles at a current density of 0.2 A g–1, the SnSe/C composite electrode delivers a specific capacity of 734 mAh g–1 over 100 cycles at 0.2 A g–1 and 690 mAh g–1 at 1.0 A g–1 after 300 cycles, and the SnS0.5Se0.5/C composite electrode delivers a capacity of 643 mAh g–1 at 0.5 A g–1 over 150 cycles, respectively. The excellent lithium-storage capabilities could be ascribed to the synergistic effect of the nanometer-scale size of the tin chalcogenide particles and the conductivity of carbon. In addition, other series of tin-based composites, that is, Sn/C, SnS2/C, SnSe2/C, and SnTe/C, were also synthesized in a similar way, demonstrating the general applicability of the one-pot route developed in this work.