With their high theoretical energy density (~2600 Wh.kg^-1), lithium/sulfur (Li/S) batteries are highly promising, but these systems are still poorly understood due to the complex mechanisms/equilibria involved. Replacing S₈ by Li₂S as the active material allows the use of safer negative electrodes, like silicon, instead of lithium metal. S₈ and Li₂S have different conductivity and solubility properties, resulting in a profoundly changed activation process during the first cycle. Particularly, during the first charge a high polarization and a lack of reproducibility between tests are observed [1] (Figure 1a). Differences observed between raw Li₂S material (micron-sized) and that electrochemically produced in a battery (nano-sized) may indicate that the electrochemical process depends on the particle size [2].

Then the major focus of the presented work is to deepen the understanding of the Li₂S material charge mechanism, and more precisely to characterize the effect of the initial Li₂S particle size both on the mechanism and the electrode preparation process. To do so, Li₂S nanoparticles were synthetized according to two ways: a liquid path synthesis [3] and a dissolution in ethanol, allowing Li₂S nanoparticles/carbon composites to be made [4]. Preliminary chemical and electrochemical tests show that starting with Li₂S nanoparticles could effectively suppress the high initial polarization (Figure 1b) but also influence the electrode slurry preparation. Indeed, it has been shown that classical formulation process - a slurry composed of Polyvinylidone Fluoride polymer dissolved in N-methyle-2-pyrrolidone - cannot be used with Li₂S nanoparticles. This reveals a complete different Li₂S material behavior regarding polymers and organic solvents when going at the nanometric scale. Then the coupling between two operando characterizations such as X-Ray Diffraction (XRD) and X-Ray Absorption and Emission Spectroscopy (XAS/XES) have been carried out in order to interpret the poorly understood first charge. The results allow to explain the electrochemical behaviors and particularly the polarization differences observed during the first charge between micrometric and nanometric Li₂S-based electrodes.

References

[1] S. Waluś et al., Electrochim. Acta, 180, 2015, 178–186.

[2] S. Waluś et al., Adv. Energy Mater., 2015.

[3] G. J.A, K. Wong, and S. Jick, J.C.S Chem. Comm, 1978, 6–7.

[4] F. Wu, H. Kim, A. Magasinski, J. T. Lee, H.-T. Lin, and G. Yushin, Adv. Energy Mater., 4, 2014.