Lithium-ion battery represents a quite promising energy storage technology addressing a large range of applications, including automotive applications. In order to meet their requirements, it is however necessary to find higher capacity electrode materials. Considering its specific capacity of 3579 mAh.g^-1, silicon is one of the most promising materials to replace graphite as a negative electrode in this technology, opening the route to cheaper, though still safe, batteries. However Si-based electrodes suffer from poor cyclability due to the large volumetric expansion of Si particles upon cycling, as well as an unstable solid electrolyte interphase (SEI) related to a perpetual electrolyte degradation at the surface of the active material. In the end, this leads to Si particle pulverization (Si expansion) and loss of electrical contact within the electrode (Si expansion and SEI formation). The SEI dynamics of Si particles has been extensively studied with a large range of electrolytes in the literature [1-4]. However, all the studies describing the failure mechanisms of Si electrodes have been achieved in a half-cell setup so far. In such configuration, the Si electrode is cycled versus a lithium metal counter electrode: the supply of lithium is not limited.

In the present work, an in-depth characterization of the SEI forming within Si electrodes cycled in a full-cell setup, with LiNi_1/3Mn_1/3Co_1/3O₂ as the positive electrode, has been achieved by using a combination of advanced characterization tools. ⁷Li/¹⁹F MAS NMR and STEM-EELS lead respectively to a global description of the SEI at the electrode scale, and to a local description of the SEI at the particle scale, while XPS allows to characterize the nature of the SEI developing at the electrode surface and FIB-TOF-SIMS brings complementary information concerning Li distribution throughout the electrode depth. Full batteries were prepared in a three electrodes configuration Swagelok cells. The latter were dismantled at different cycle numbers (1^st, 10^th and 100^th at the end of lithiation and at the end of delithiation) in an argon glovebox before subsequent characterization. All electrochemical cycles were achieved in the electrolyte (1M LiPF₆ in EC-DEC carbonates mixture) at a limited capacity of Si electrodes (1200mAh.g^-1): this was necessary in order to keep the possibility of reaching a sufficiently high number of electrochemical cycles for the study.

In a first phase, it was observed that Si electrodes simply soaked in the electrolyte enhances its degradation (this being highly favored by increasing the temperature) without the formation of a proper SEI at the surface of the silicon particles. This leads to a thin and heterogeneous interface layer, which remains quite stable after the first degradation step. Upon cycling, heterogeneous thick patches of LiF and carbonates appear at the surface of the Si particles: the development of the inorganic part of the SEI mostly occurs during the early stages of cycling, while an incessant degradation of the organic solvents of the electrolyte occurs endlessly. After extended cycling, all the lithium from the positive electrode is consumed, either trapped in an intermediate part of the SEI or in the electrolyte. While the cell cannot function properly anymore, degradation of the organic electrolyte solvents goes on, leading to the formation of Li-free organic degradation products thickening the SEI. Such comprehensive work allows for a better study of the failure mechanisms of Si-based composite electrodes for Li-ion batteries cycled in a full-cell setup. This also emphasizes the importance of achieving multiprobe and multiscale analysis by using various advanced characterization tools for this kind of study: such approach allows to investigate various regions of an electrode at various scales, favouring a better understanding of the underlying mechanisms.

[1] M. N. Obrovac et al, J. Electrochem. Soc., 154 (2007) A103

[2] U. Kasavajjula et al, J. Power Sources, 163 (2007) 1003

[3] D. Mazouzi et al, Electrochem. Solid-State Lett., 12(11) (2009) A215

[4] W-J. Zhang, J. Power Sources, 196 (2011) 13