It is well known that silicon presents one of the most important anode materials for the improvement of lithium-ion cells in terms of energy density. Indeed, silicon’s high theoretical specific capacity to lithium (more than 3800 mAh/g at room temperature), environmental friendliness, low potential compared to lithium and material abundance turn silicon to a strong candidate for the replacement of carbon-based anodes [1,2]. However, one of the main drawbacks of silicon’s application to the lithium-ion technology is its poor electrochemical cycling stability over several galvanostatic cycles, mainly due to silicon’s huge volume change (around 300%) during lithiation and delithiation that leads to high internal mechanical stress and therefore to film fracturing and delamination [3]. Many alternatives have been proposed that alleviate this mechanical expansion issue either through the use of nanostructured silicon sometimes combined with carbon-based materials or with special coatings and binders with the current collector [1,4]. Even though many years ago in 1999 amorphous silicon (a-Si) was initially proposed by Bourdereau et al. [5], it is quite recently that it has been found that a-Si can be more resistive to fractures and pulverisation upon lithiation [6] than crystalline silicon (c-Si). It has been demonstrated by McDowell et al., that amorphous silicon compared to the crystalline can resist more efficiently the large volume modifications at least for spheres with 870 nm of diameter and this has been attributed to the different lithiation mechanism [7]. Recently, Farmakis et al. [8] have demonstrated micro-grain structured amorphous silicon anodes delivering specific capacity of more than 2000 mAh/g and 2.0 mAh/cm²for more than 50 cycles. As a goal to comprehend the performance of the silicon and the related lithium kinetics, this contribution attempts to study the lithiation mechanism at micro-grain structures amorphous silicon structures with the aid of impedance spectroscopy.

For this purpose, silicon/metal Lithium half cells were prepared with silicon being previously deposited by DC sputtering on rough copper foil and electrochemical characterization including impedance spectroscopy was conducted. After the application of a two semi-circle fitting model, the charge transfer and the Solid Electrolyte Interphase impedances are extracted. It is shown that the charge transfer resistance is highly dependent on the State Of Charge (SOC) and consequently on the charging rate. It is observed that the charge transfer resistance is highly increased (5-6 times) when silicon anode lithiation exceeds 80% of SOC and not. The results can be explained by considering that lithiation of amorphous silicon strongly depends on x at Li_xSi alloy throughout the silicon bulk material.

References

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