Thermochemical and electrochemical investigation of high capacity SnSx- based anode materials for lithium-ion batteries

In recent years tin sulfides, particularly SnS and SnS₂, have attracted significant attention in the energy storage research community. Their high theoretical specific capacities (1111 mAh g^-1 for SnS and 1209 mAh g^-1 for SnS₂) make them promising candidates for anode-active materials for Li-ion batteries. Additionally, they possess the advantages of being cost-effective and environmentally friendly. During the lithiation process, Sn-S-based materials undergo a conversion reaction with Li, forming Li₂S and Sn. Further lithiation triggers the alloying reaction between Sn and Li, forming various Sn-Li intermetallic phases. Notably, these newly formed Sn-Li phases have significantly larger unit cell volumes than their parent phases, causing the active material particles to crack on repeated alloying and dealloying of tin. Ultimately, the continuous cracking usually leads to pulverisation of the anode material and reduced contact between the particles. These mechanisms contribute to cell degradation and capacity fade at higher cycle numbers.
This thesis focuses on assessing and understanding the structure-processing-property relations in Sn-S materials and Sn-S-based electrodes. Such investigations can yield measurable enhancements in the electrochemical performance of this materials class.
Both precipitation and hydrothermal methods were used to synthesise SnS and SnS₂ materials. In all cases, SnCl_x·yH₂O (x= 2; y= 2 for SnS and x= 4; y= 5 for SnS₂) was used as a Sn source and thioacetamide as a S source. The physicochemical properties of the as-synthesised materials were examined by powder X-ray diffraction (PXRD), and structural parameters were extracted using Rietveld analysis of the measured patterns. Furthermore, the sample morphologies were characterised by scanning electron microscopy (SEM), and the crystallinity of the samples was further investigated by transmission electron microscopy (TEM). The presence of surface impurities, which may be formed during material synthesis, was examined by X-ray photoemission spectroscopy (XPS). Furthermore, the as-prepared materials were subjected to drop solution calorimetry to determine the enthalpy of formation.
The electrochemical cycling performance of the as-prepared materials was investigated in half-cell configurations ( Sn-S working electrodes vs Li/Li+ counter electrodes). In this way, comparisons were drawn between materials with the same chemical composition but different particle sizes and morphology and materials with different chemical compositions and very similar morphology.
In addition to the above experiments, in-situ dilatometry was performed to quantify the changes in the thickness of SnS and SnS₂ electrodes during cycling. Ex-situ XRD and ex-situ SEM were utilised to investigate the role of crystal chemistry and particle morphology on electrochemical performance post-mortem. The results showed that the electrodes made of partially amorphous SnS₂, which was synthesised via the precipitation, expands the least on alloying. Additionally, the cycling data show that partially amorphous SnS₂ performs better than SnS and well-crystallised SnS₂. Two effects could be attributed to those results. Firstly, it is hypothesized that thicker in-situ Li₂S layers are formed via the conversion mechanism on particles with lower specific surface areas. These thicker layers can better restrain the volume changes of Sn during cycling. Secondly, due to the intercalation step prior to the conversion reaction in SnS₂, it is postulated that Li₂S and Sn are homogeneously distributed on the atomic scale after the conversion reaction, which further helps minimise the volume expansion of Sn during cycling.