*3.3. Microstructure after Cycling*

To interpret the capacity fading observed in Figure 4, XRD and electron microscopy were performed on the anodes after the 200 cycles were completed. The XRD characterization of the SnS/C anodes after 200 cycles is shown in Figure 5, indicating that the Sn phase, in addition to SnS, was present, after cycling. Using the Panalytical X' Pert Plus software, the grain size of the SnS alloy was estimated to be ~10 nm, which is the same as that of prior cycling, suggesting that crystal size was not affected by lithiation.

Figure 5 illustrates that pure Sn metal formed during the lithiation and delithiation process. This can occur because initial lithiation gives rise to the irreversible conversion reaction SnS + 2Li+ 2e−→Sn + Li2S, by which Sn is dispersed in a Li2S matrix. Further lithiation forms Li–Sn alloys, which is a reversible process during the charge/discharge cycle. The lack of the Li2S phase in the XRD of Figure 5 may be explained by the following three possibilities: (i) Li2S is soluble in the electrolyte [14], (ii) it may be amorphous, or (iii) the crystal size is too small to be detected by XRD.

**Figure 5.** XRD of the SnS/C composites after 200 cycles.

Although XRD has not been performed on cycled SnS anodes, these observations are consistent with those of XRD studies of SnS2 anodes [15], which reported the formation of metallic Sn during lithiation, as both conversion and alloying mechanisms occurred during the lithiation of SnS2. Furthermore, our conclusions are in accordance with detailed XRD studies for the first Li-ion insertion (but not deinsertion) in Sb2S3 electrodes [16], which showed that Sb2S3 underwent both a conversion reaction (forming a pure Sb phase) and an alloying reaction; however, neither Li–Sb nor Li–S was observed in XRD, which was attributed to them either being amorphous or having a very small crystal size.

As no other studies have examined the phases present upon complete Li deinsertion of SnS electrodes, high resolution TEM (HRTEM) was conducted on the cycled materials, as shown in Figure 6.

Comparing Figure 6 with the initial microstructure depicted in Figure 2 reveals that after cycling, the flower-like structure of the SnS particles was "destroyed". The initial SnS nanofibers decomposed into nanoparticles that were 2–10 nm. Similarly, as the XRD indicated, the SAED patterns in Figure 6 documented the existence of pure Sn and SnS phases; however, no C rings were observed, even though the area considered contained a lighter phase region. This indicated that the light contrast region surrounding the nanoparticles (in Figure 6) was not the initial carbon particle substrate, but solid electrolyte interface (SEI) or electrolyte residue. This is verified by the representative EDS spectrum in Figure 6 that illustrates that the light shaded area of Figure 6c a contained high contents of F, O, C, and P. This C would be from the binder and carbon additive of the electrode.

The TEM analysis thus illustrates that, during the Li insertion and deinsertion process, the SnS detached from the C substrate and dissolved into the electrolyte, getting dispersed within the inactive SEI layer. This is consistent with the continuous capacity fade, as the SnS that detached from the carbon could not store Li ions. Therefore, even though the addition of SnS seemed to benefit the performance of the AG and MCMB (as the initial capacity was high), it was actually a drawback, as with continuous cycling it dissolved in the electrolyte and resulted in a lower capacity than carbon materials.

To examine the extent by which this inactive SnS/Sn SEI layer formed, FESEM was performed on the cycled electrodes, as this method could depict a wider area than TEM. For all samples, the same microstructure was observed. Representative images are shown in Figure 7 for the 10SnS/AG and in Figure 8 for the 20SnS/MCMB. It can be seen that the initial flower-like structure was disrupted and large solid particles were present on the carbon surfaces. To better understand the composition throughout the anode, EDS mapping (Figures 7b–g and 8b–g) was also performed. It is of particular interest to see that the micron size particles in Figures 7a and 8a were not comprised of SnS, but had a

high C and F content, indicating that it was part of the SEI. These results are in accordance with the TEM images, since the EDS mapping illustrated that, during Li insertion and deinsertion, the flower-like SnS particles reduced into nanoparticles covering the anode surface, as Figures7g–f and 8g–f illistrate.

**Figure 6.** High resolution TEM (HRTEM) images with SAED and EDS results of the SnS/C composite anodes after 200 cycles. (**a**) 10SnS/AG, (**b**) 10SnS/MCMB, and (**c**) the results of EDS on 10SnS/AG.

**Figure 7.** FESEM images of (**a**) 10SnS/AG, (**b**–**g**) EDS mapping of the area in (**a**). The element being mapped is indicated in each image.

**Figure 8.** FESEM images of (**a**) 20SnS/AG, (**b**–**g**) EDS mapping of the area in (**a**). The element being mapped is indicated in each image.

Table 2 summarizes the EDS semiquantitative element analysis inside the micron particle of Figure 7a and outside the adjacent matrix, which again supports the conclusion that the SnS was distributed throughout the anode surface.


**Table 2.** EDS analysis after cycling of the area inside the particles and outside the adjacent matrix.

The high concentrations of F, O, C, and P (as seen in the EDS of Figures 7 and 8), together with the observed free-standing SnS particles (as seen in the TEM images of Figure 6), suggest that the electrode surface was coated by an electrochemically inactive layer, which was comprised of Sn and SnS nanoparticles distributed throughout the SEI layer. Such an electrochemically inactive coating increases internal resistance, and the Sn trapped within could not host Li ions, explaining the capacity and voltage loss seen in Figure 4.

The anodes that contained higher contents of Sn and S (20SnS/AG and 20SnS/MCMB) would have a higher Sn and S content in the inactive layer, which would result in a higher internal resistance and greater capacity loss (as seen in Figure 4), as compared to that in the lower content SnS samples. Therefore, even though the addition of SnS on the C resulted in

an initial increase in the capacity, the Sn and S dispersed into the electrolyte over continuous cycling, becoming part of the SEI layer and increasing the internal resistance of the cell. This conjecture was tested by comparing the impedance spectroscopy before and after 20 cycles for the 10SnS/AG and 20SnS/AG electrodes. In Figure 9 it can be seen that the impedance (i.e., interface/internal resistance) was higher for the 20SnS/AG sample, which supports the concept that SnS becomes chemically inactive, and the greater its content, the more negative its effects on electrochemical performance (as seen in Figure 4).

**Figure 9.** Electron impedance spectroscopy (EIS) diagrams for (**a**) 10SnS/AG and (**b**) 20SnS/AG, before and after 20 cycles.

Hence, the better performance noted for the 10SnS/AG materials was due to the lower Sn and S content that could dissolve into the solid electrolyte interface layer and increase internal resistance. These results show that the conversion reaction and alloying that SnS undergoes during cycling result in the dispersion of Sn particles within the electrolyte and SEI layer. Therefore, even though the capacity is initially higher, in the long term, the addition of SnS nanoflowers on graphite increases the internal resistance and results in a capacity that is even lower than that of the carbon substrate. In order to produce efficient SnS/C nanocomposites, the SnS must be fully isolated from the electrolyte using a protective coating.
