Li-ion technology is among the most commercially viable solutions for electrochemical storage, particularly for the electric vehicle (VE) market. The fading of such systems is often related to the degradation of the electrode active materials in various ageing conditions; it is thus mandatory to better understand the causes of these failures. It is well known that the nature and amount of the solid electrolyte interphase (SEI) which forms at the surface of the active material during cycling, can be one of the main issues preventing durability of the cells. For that purpose, our lab is actively working on the characterization of such degradation mechanisms, trying to understand some fundamental aspects of lithiation or lithiation loss mechanisms, nature and dynamics of the SEI etc.

Previous works allowed us to describe some fundamental aspects of silicon electrodes degradation in various configurations by using XPS, AES and ToF-SIMS combined with in situ FIB (known as FIB-ToF-SIMS) cross analysis [1-3]. This has recently been applied to the study of commercial graphite electrodes [4]. In this study, the electrodes have been cycled (vs NMC positive electrodes) at 5°C following 2 end-of-voltage windows strategies: 2.70V-4.20V (0%SOC-100%SOC), 3.42V-4.08V (10%SOC-90%SOC).The relationship between the end-of-voltage windows and the SOC limits were measured in the beginning of life at 25°C, and so these voltage ranges do not correspond strictly to specified SOC ranges for other temperatures, C-rates, or after ageing. It is observed that the former are fading a lot faster than the latter (cycled in the limited 3.42-4.08 V potential range). The relationship between the end-of-voltage windows and the SOC limits were measured in the beginning of life at 25°C, and so these voltage ranges do not correspond strictly to specified SOC ranges for other temperatures, C-rates, or after ageing. It is observed that the former are fading a lot faster than the latter (cycled in the limited 3.42-4.08 V potential range). ). A complete characterization study, involving also XRD and NMR, showed that the generation of SEI is much more important between 90 and 100% SOC, inducing lithium trapping in the graphite particles while also favoring some lithium plating. A scheme (Fig.1) has been proposed to explain the evolution of such electrodes in these particular cycling conditions.

These results show the high impact of cycling policy on the integrity of Li-ion cells. They also emphasize the ability of FIB-ToF-SIMS, used in combination with XPS (as used in [2] and [4] but applied in this study to graphite electrodes), to unveil the underlying ageing mechanisms of Li-ion negative electrodes.

[1] E. Radvanyi et al, Phys. Chem. Chem. Phys., 2014,16, 17142-17153

[2] A. Bordes et al, Chem. Mater., 2016, 28 (5), 1566-1573

[3] N. Dupré et al, Chem. Mater., 2016, 28 (8), 2557-2572

[4] B. Pilipili Matadi et al, J. Electrochem. Soc., 2017, 164 (12), A2374-A2389