(Digital Presentation) Influence of Li2s on the Volume Expansion of Sn Particles during Lithiation of Tin Sulfides

Abstract

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 mAhg-1 for SnS and 1209 mAhg-1 for SnS2) make this group of materials good candidates for anode active materials for Li-ion batteries. Moreover, their low cost and environmental benignity further contribute to the appeal. During lithiation, tin sulfides undergo a conversion reaction with Li in which Li2S and Sn are formed. Further lithiation triggers the alloying reaction between Sn and Li, resulting in the formation of various Sn-Li intermetallic phases. The newly formed Sn-Li phases have a significantly higher unit cell volume than their parent phases, causing cracking of particles on repeated alloying and dealloying. Ultimately, the continuous cracking of particles leads to pulverization of anode material, reduced contact between the particles and capacity fade.In our work, we explored the influence of Li2S formed during the conversion reaction on the stability of the solid electrolyte interphase (SEI) layer and the overall cycling performance of SnS and SnS2 materials. The precipitation reaction was used to synthesised both SnS and SnS2 materials. Furthermore, SnS2 was also synthesised via the widely used hydrothermal method. In all cases, SnClx ·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 physio-chemical 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 characterized 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 formed during material synthesis, was examined by X-ray photoemission spectroscopy (XPS).The cycling performance of SnS2 prepared via the precipitation reaction was compared to SnS2 prepared via hydrothermal method as well as to SnS prepared via precipitation reaction. In this way we were able to draw comparisons between materials with the same chemical composition but different particle size and morphology as well as materials with different chemical composition and very similar morphology.The length changes of SnS and SnS2 electrodes during cycling were investigated by the in-situ dilatometry technique, and several ex-situ methods such as ex-situ XRD and ex-situ SEM were utilized to investigate the role of crystal chemistry and particle morphology on electrochemical performance post-mortem. The results showed that on alloying, the electrode comprised of partially amorphous SnS2 synthesised via the precipitation expands the least. Additionally, the cycling data show that partially amorphous SnS2 electrochemically outperformed both SnS and highly crystalline SnS2. This may be attributed to two effects. Firstly, thicker Li2S layers are generally formed on particles with lower surface areas, thereby better restraining 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, which further helps to minimize the volume expansion during cycling.