Conversion type reaction have revolutionized Li-ion batteries and beyond in term of performance as well as of fundamental aspects. To tackle the volume expansion issue occurring during lithiation/delithiation, composite materials made of electrochemically active/inactive elements have been successfully developed.^1,2 However, direct evidence of the electrochemical conversion reaction as well as the role of the inactive elements over long term cycling still remains to be elucidated.

To tackle this issue, we propose advanced surface Auger / µXPS / ToF-SIMS imaging to analyze the chemical and morphological evolution of clean electrodes cross-sections, prepared by ion-milling. These innovative approaches were used to study ternary conversion materials, MSnSb (M=Ti or Nb) for which unexpected electrochemical performance were observed at high temperature.^3,4 It provided a direct proof that the electrochemical conversion mechanism still occurred even after 400 cycles, which was unexpected. Importantly, it highlights the role of the inactive element (Ti or Nb) in the long term reversibility of the conversion reaction. Overall, a gradual shell to core expansion/break up of MSnSb particles is revealed during the continuous conversion reactions which leads to highly porous structures after 400 cycles. Finally, it will be showed that the developed approaches will benefit to the study of all-solid-state batteries for which buried interfaces have to be reached.

References

1. V. Aravindan, Y. S. Lee, and S. Madhavi, Adv. Energy Mater., 5, 1402225 (2015).
2. A. Mukhopadhyay and B. W. Sheldon, Prog. Mater. Sci., 63, 58 (2014).
3. L. Madec, G. Gachot, G. Coquil, H. Martinez, and L. Monconduit, J. Power Sources, 391, 51 (2018).
4. G. Coquil, M. T. Sougrati, S. Biscaglia, D. Aymé-Perrot, P. F. Girard, and L. Monconduit, Electrochim. Acta, 281, 619 (2018).