In order to improve the electrochemical performances of lithium-sulfur batteries, it is crucial to understand profoundly their working mechanism and the limitation factors. This communication presents synchrotron-based in situ XRD studies of structural modifications occurring inside the cell upon cycling, since the active material changes constantly its form between solid and liquid phases.

A recent transmission electron microscopy (TEM) method using precession electron diffraction (PED) was used to obtain LiFePO 4 and FePO 4 phase mapping at the nanometer-scale level on a large number of particles of sizes between 50 and 300 nm in a partially charged cathode. Despite the similarity of the two phases (the difference of lattice parameters is <5%), the method gives clear results that have been confirmed using high-resolution transmission electron microscopy (HRTEM) and energy-filtered transmission electron microscopy/electron energy loss spectroscopy (EFTEM/EELS) experiments. The PED maps show that the particles are either fully lithiated or fully delithiated and, therefore, bring a strong support to the domino-cascade model at the nanoscale level (scale of a particle). A coreÀshell model or spinodal decomposition at mesoscale (scale of agglomerates of particles) is possible. Size effects on the transformation are also discussed.

Li 2 S 2 species, sometimes considered as a solid product, [ 18,19 ] has never been experimentally detected by XRD, thus questioning its real existence as a solid. Sulfur reduction (complete [ 13,16 ] or incomplete [ 10,20 ] and recrystallization at the end of charge [ 11,12,16 ] was also an unresolved question. Until today, there have been few reports released on the study of Li/S cells, where in situ XRD was applied. [7][8][9]21 ] In the majority of these works, the formation of a solid sulfur at the end of charge was confi rmed. The appearance of crystalline Li 2 S was not evidenced by Nelson et al., [ 9 ] while other groups [ 8,21 ] reported on its formation at different states of discharge along the lower plateau. Both the reports offered only qualitative interpretation of the results. By applying in situ and operando XRD, information about the structural changes of sulfur positive electrode versus exchanged capacity upon cycling could be obtained. [7][8][9] This enables a deeper insight into the complex mechanism of the Li/S cell, which from the point of view of further improvements, is essential.Here we report, for the fi rst time, a quantitative evaluation of Li 2 S formation and consumption upon cycling and propose a mechanism of solid product(s) formation. Monitoring of further cycles, as well as C-rate infl uence (C/20 and C/8), is also taken into consideration. These results were obtained from a pouch cell using nonwoven carbon sheet as a current collector and relatively high sulfur loading, along with a controlled electrolyte amount. The effect of the electrolyte excess and electrode morphology on the solid phases formation was not the scope of this paper, therefore not investigated any further. Nevertheless, these important parameters may be studied in the future.The obtained electrochemistry displays an expected voltage profi le of a typical Li/S cell, where discharge and charge capacities of 980 and 976 mAh g −1 , respectively, were obtained. The corresponding XRD patterns evolution is shown on Figure S1a (Supporting Information). As the discharge proceeds, the peak intensities of the elemental orthorhombic α-sulfur (PDF-2; No. 00-008-0247) linearly decrease as a function of capacity ( Figure S2, Supporting Information). They vanish at the end of the fi rst discharge plateau, proving a complete reduction of sulfur. This linear evolution can be associated with a one-step electrochemical process, based on the reduction reaction of S 8 to S 8 2− . [ 22 ] The practical capacity of this fi rst region (so-called "solid/soluble"; 175 mAh g −1 ) is slightly lower than the theoretically expected one (209 mAh g −1 ), possibly because of a self-discharge associated with partial S 8 dissolution [ 23 ] (further details are provided in the Supporting Information). In the region Lithium/sulfur batteries are considered as a promising candidate for a next-generation energy storage system. This technology is based on the electrochemical reaction between elemental sulfur and metallic lithium (16Li + S 8