The preparation of azobenzene mesogenic thiol ligand with six methylene spacer and decyloxy tail C₁₀H₂₁O-Ph-N=N-Ph-C₆H₁₂SH was conducted according to our previously reported procedures.^[16] The raw material Sn (II) chloride dihydrate (98%) was provided by Alfa Aesar and used without purification. All of the other chemical reagents were purified with standard procedures.

The synthesis procedure of the azobenzene mesogenic tin thiolate is illustrated in Fig. 1. In a typical synthesis, 4-(6-mercaptohexyloxy)-4′-decyloxy-azobenzene (1.50 g, 3.19 mmol) and Sn (II) chloride dihydrate (360 mg, 1.59 mmol) were, respectively, dissolved in tetrahydrofuran (THF) by ultrasonication. Then, the Sn (II) chloride solution was added into the 4-(6-mercaptohexyloxy)-4′-decyloxy-azobenzene solution dropwise with slow magnetic stirring for 30 min at room temperature. The precipitate formed was collected after several washing-centrifugation cycles and dried at 45 °C under vacuum overnight to obtain azobenzene-containing tin thiolate as the precursor compound, and then the precursor was carbonized at 600 °C for 2 h under Ar atmosphere with a heating rate of 2 °C·min⁻¹ to achieve SnS@N/C composite.

Fig. 1.

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	Fig. 1. Preparation of the precursor azobenzene-containing tin thiolate.

The x-ray diffraction (Bruker D8 Advance diffractometer) and transmission electron microscopy (TEM, JEOL, JEM-2100) were employed for the structure and morphology investigation. Differential scanning calorimetry (DSC) analyses were carried out on a Mettler Toledo calorimeter associated with a cooling accessory with a 10 °C·min⁻¹ heating or cooling rate under N₂ gas flow. Small-angle x-ray scattering (SAXS) investigation was performed on a high-flux instrument (SAXSess mc², Anton Paar) with an imaging-plate (IP) spanning broad q ranging from 0.06 nm⁻¹ to 29 nm⁻¹, with q = (4π sinθ)/λ, Cu K_α radiation λ = 0.1542 nm, collecting at 40 kV/50 mA for 5 min. Typically, aluminum foil was used for the powder sample encapsulation and the collected x-ray analysis data were processed with the associated SAXSquant software 3.80. Polarized optical microscopy (POM) observation and photography were conducted on a PM6000 optical microscope associated with a Leitz-350 heating stage and a Nikon (D3100) digital camera. Raman spectra were recorded using a Renishaw 1000 spectrometer system with a 633-nm He–Ne laser operating at 2 mW with 10 s exposure time. The x-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 5000 Versa probe system, using monochromatic Al K_α radiation (1486.6 eV) operating at 25 W.

Electrochemical measurements were performed using 2032 type coin cells. The working electrode slurry consisted of active material (70 wt%), conducting carbon (20 wt%), and polyvinyldifluoride (PVDF, 10 wt%) in N-methylpyrrolidone (NMP) and was coated onto a copper foil current collector. The above slurry was dried at 110 °C in a vacuum oven and Li foil was taken as the counter electrode. Coin cells were assembled in an argon-filled glove-box with an electrolyte of 1M LiPF₆ in an ethylene carbonate and diethyl carbonate (EC/DMC, 1:1 volume) mixture, and Celgard 2500 separator was used. Electrochemical tests were conducted using an automatic galvanostatic charge–discharge unit, Land CT2001 system, with a potential range of 0–2.0 V versus Li/Li⁺ electrode at room temperature.

The thermal properties of precursor compound azobenzene-containing tin thiolate and its corresponding ligand mesogenic thiol were investigated by DSC for comparison, and the thermograms of the first cooling and second heating cycles are shown in Fig. 2(a). The azobenzene thiol exhibits a 30 K interval smectic (SmA) LC mesophase, as revealed by the typical focal conic POM texture (Fig. 3(b)). The azobenzene-containing tin thiolate shows a lamellar mesophase with focal conic mosaic POM texture (Fig. 3(d)) similar to the smectic phase of mesogenic thiol.

Fig. 2.

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	Fig. 2. (a) DSC thermograms of mesogenic azobenzene thiol and tin thiolate precursor during the 1st cooling and 2nd heating scans at a rate of 10 °C·min−1. (b) SAXS profiles of ligand thiol and the precursor tin thiolate at their LC state.

Fig. 3.

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	Fig. 3. Representative POM images of mesogenic azobenzene thiol at (a) 55 °C, Cr; (b) 92 °C, SmA; and precursor tin thiolate at (c) 90 °C, Cr; (d) 134 °C, lamellar phase.

Moreover, SAXS investigation was performed to probe into the ordered structure and phase behavior of the tin thiolate precursor and the corresponding mesogenic azobeneze thoil. Figure 2(b) shows the SAXS profiles of the ligand and precursor samples at their LC state. The precursor demonstrated a more organized lamellar structure compared with its mesogenic azobenzene thiol. For the tin thiolate precursor, upon cooling down to the mesophase state at 139 °C, in between the temperature range 145 °C and 133 °C consistent with the DSC measurement, three reflections existed in the small-angle range with q values of 1.23 nm⁻¹, 2.46 nm⁻¹, 3.67 nm⁻¹ in ratio of 1:2:3, indicating an ordered lamellar mesophase. Therefore, it is worth noting that the precursor metal thiolates well inherited the liquid crystallinity of the corresponding ligand mesogenic thiols with enhanced heat resistance in virtue of the introduction of metal ions such as tin in this case.^[15,16]

Figure 4(a) presents the XRD patterns of as-prepared SnS@N/C nanocomposite. All the diffraction peaks can be easily attributed to orthorhombic SnS crystal (JSPDS card No. 39-0354). No diffraction signals from precursor or other impurities were observed, suggesting complete conversion of the precursor into SnS nanocrystals with sharp peaks compared to the standard line-style diffractions of SnS single crystal. Raman spectrum of the composites as shown in Fig. 4(b) with two broad peaks at 1341 cm⁻¹ and 1583 cm⁻¹ respectively at an intensity ratio of 0.95 could be well assigned to typical D and G bands of amorphous carbon, manifested the amorphous character of the coated carbon layer.^[32] Generally, the conductive amorphous carbon matrix with porous structure not only favors the diffusion of lithium ions, but also provides a buffer layer to cushion the large volume change during the lithiation–delithiation, contributing to the significantly improved cycling performance and overall capacity.^[33]

Fig. 4.

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	Fig. 4. (a) XRD patterns of SnS@N/C nanocomposite and pure SnS crystal (JCPDS card No. 39-0354). (b) Raman spectra of the SnS@N/C nanocomposite. (c) The full XPS spectrum of as-prepared SnS@N/C nanocomposite with the inset showing the S 2p spectrum in magnified scale. (d) High-resolution XPS spectra of Sn 3d.

The surface composition of the SnS@N/C nanocomposite was investigated by x-ray photoelectron spectroscopy (XPS). Figure 4(c) shows the full XPS spectrum of the SnS@N/C nanocomposite with signals of all concerned elements Sn, S, C, N, and O, demonstrating that the formation of SnS nanocrystals coated with N-doped carbon SnS@N/C, with the content of N species in the nanocomposite was determined to be 7.04 wt% based on the elemental analysis. It should be mentioned that many researchers have revealed that nitrogen doping is an efficient way to boost the electrochemical performance of the carbon based materials, thanks not only to their enhancing the electric conductivity, but also offering more active sites for lithium ion insertion and extraction, thus benefiting for the improved cycling performance and rate capacity of the electrode.^[31] As shown in Fig. 4(d), the high resolution Sn 3d spectrum displays two characteristic peaks at 486.4 eV (Sn 3d_5/2) and 494.7 eV (Sn 3d_3/2), manifesting that some SnS nanoparticles close to the surface of the carbon material were oxidized into SnO₂, which explained the existence of element O in the full XPS spectra. Nevertheless, the occurrence of little SnO₂ might increase the general capacity of the material in view of the much higher theoretical capacity of SnO₂ (1494 mAh·g⁻¹) compared to that of SnS (782 mAh·g⁻¹).^[34]

Transmission electron microscopy (TEM) was used to investigate the morphology of the as-prepared SnS@N/C nanocomposite. As shown in Fig. 5(a), SnS nanoparticles (dark dots) with relatively uniform size were found to be homogenously inserted onto the carbon layer, possessing an average size of around 4 nm as measured from the high-revolution TEM (HRTEM) image shown in Fig. 5(b).

Fig. 5.

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	Fig. 5. (a) Transmission electron microscopy (TEM). (b) High-resolution TEM (HRTEM) images of SnS@N/C nanocomposite.

The electrochemical behavior of the nanocomposites was investigated through evaluation with the nanocomposite based materials as anodes of LIBs. The rate performance of the SnS@N/C composite anode was assessed with the cell cycled at various current densities from 100 mA·g⁻¹ to 1000 mA·g⁻¹. As shown in Fig. 6(a), the nanocomposite offered a reversible capacity of about 604.6 mAh·g⁻¹, 532.6 mAh·g⁻¹, 389.0 mAh·g⁻¹, and 300.4 mAh·g⁻¹ at the current density of 100 mA·g⁻¹, 200 mA·g⁻¹, 500 mA·g⁻¹, and 1000 mA·g⁻¹, respectively. The capacity could recover to 587.7 mAh·g⁻¹ when the current density returned to 100 mA·g⁻¹, which is close to the original level, suggesting that the morphology of the as-prepared SnS@N/C composite was well preserved during cycling. The good rate capacity of the SnS@N/C nanocomposite originated from the combination of nano-sized SnS particles, uniform distribution and N-doped carbon coating layer. Figure 6(b) shows the cycling performance of the SnS@N/C nanocomposite anode at a scan rate of 100 mA·g⁻¹. Upon cycling, the discharge and charge capability slightly decreased at the beginning for several cycles and then leveled off with a high columbic efficiency of up to 99%. After 90 cycles, the discharge and charge capacity still maintained 493.4 mAh·g⁻¹ and 477.9 mAh·g⁻¹, respectively. These results revealed that the SnS@N/C nanocomposite demonstrated quite good cycle stability.

Fig. 6.

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	Fig. 6. (a) Rate performance of SnS@N/C nanocomposite anode at various current rates. (b) Cycling performance of SnS@N/C nanocomposite anode at a rate of 100 mA·g−1.