Lithiation Mechanism of Methylated Amorphous Silicon Unveiled by Operando ATR‐FTIR Spectroscopy

The lithiation mechanism of methylated amorphous silicon, a‐Si1−x(CH3)x:H, with various methyl contents (x = 0 ‐ 0.12) is investigated using operando attenuated total reflection Fourier transform infrared spectroscopy. As in hydrogenated amorphous silicon, a‐Si:H, the first lithiation proceeds via a two‐phase mechanism. The concentration of the invading Li‐rich phase nonmonotonously depends on the methyl content of the active material. This behavior is tentatively explained by two distinct effects: a softening of the material due to a methyl‐induced lowering of its reticulation degree and its cohesion, and the presence of nanovoids at high enough methyl content. The first lithiation of a‐Si1−x(CH3)x:H is biphasic. Methyl groups lower the material cohesion and at concentrations >5%, induce the formation of nanovoids. It yields a nontrivial dependence of the Li‐content of the invading phase as a function of the methyl content of the material.