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Chapter 5: In situ and operando XRD (a) (b) Figure 5-3. XRD patterns recorded at the initial state and at the end of 1st and 2nd charges (a). A zoomed view (b) shows slight changes in the Bragg peaks positions, clearly indicating the formation of another crystalline form, attributed to the monoclinic beta-S8. 5.3.2.a) β-Sulfur: physical characteristics According to Templeton et al.20, monoclinic sulfur was determined for the first time in 1965 by Sands. In 1976, Templeton et al.20 provided more complete report concerning β-S8 structure and its other physical and chemical properties. Monoclinic β-sulfur is known as another allotropic form of sulfur, usually stable at normal pressure, at the temperatures from 95°C to the melting point of sulfur (115°C)20,26. When molten sulfur is cooled, it may (but not always does) solidify as a monoclinic sulfur form. This is actually in agreement with our previous experiments, when preparing binder-free melted electrodes, and traces of monoclinic sulfur were detected (section 3.4.9). Below 95°C, orthorhombic α-sulfur is the most thermodynamically stable form, which is classically used as elemental sulfur powder for the electrodes preparation. Table 5-3 presents the crystalline structures of both sulfur allotropes. Alpha-sulfur (α-S8) phase contains 16 molecules of S8 rings in a face-centered unit cell, and is considered as orientationally ordered form. On the contrary, β-S8 is rather known as orientationally disordered structure21,22, and is composed of six molecules of S8 rings per unit cell. According to Pastorino and Gamba22, β-S8 crystals can be grown more easily than α-S8 ones from the melt phase, due to its disordered structure. It has been also reported that metastable pure crystals of β-S8 could be preserved for weeks at RT20,26. 164PDF Image | Accumulateur Lithium Soufre
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