Nucleation and Growth of Lithiation Patterns on Methylated Amorphous Silicon Layers Monitored By Operando Optical Microscopy

Abstract

Silicon is one of the best candidates as an anode material for Li-ion batteries, owing to its high capacity. However, the large variation in volume induced by lithiation leads to a degradation of the material incompatible with its use in practice. We have shown that amorphous methylated silicon a-Si 1-x (CH3) x :H has a better cyclability while keeping a comparable capacity [1]. This was attributed to a lower stiffness of the methylated material as compared to pure silicon. AFM nanoindentation experiments confirm this property.The first lithiation/delithiation cycles are of prime importance for the evolution of the material during subsequent cycling. Operando monitoring by attenuated-total-reflection Fourier-transform infrared (ATR-FTIR) spectroscopy and optical microscopy provide a thorough understanding of the mechanisms involved during these cycles. The lithiation of a-Si 1-x (CH3) x :H, with various methyl contents (x = 0 - 0.12), was investigated using operando ATR-FTIR [2]. As previously reported for pure crystalline and amorphous silicon [3], the first lithiation proceeds via a two-phase mechanism whatever the methyl content is. The lithium concentration z of the invading Li-rich phase depends on x, first decreasing for x < 0.05 and increasing above (Fig.1). This behavior was tentatively explained by two distinct effects: the softening of the material due to a methyl-induced lowering of its reticulation degree and its cohesion, and the presence of nanovoids at higher methyl content.Operando observations by optical microscopy were performed on thin layers during the first lithiation. Unlike pure silicon, the methylated silicon undergoes a first spatially inhomogeneous lithiation [4]: lithiated zones appear on the surface as colored spots (Fig.2), which grow circularly and progressively invade the surface. These spots have a thickness greater than that of the initial layer, which can be estimated from their color change. The morphology of the lithiation spots and their evolution are accurately determined by ex situ AFM. The resistive character of the layer is found to be responsible for this inhomogeneous behavior. A simple model has been worked out and is in semi-quantitative agreement with the combined electrochemical and microscopy measurements.Using Raman spectroscopy, we investigated the properties of fully lithiated, semi-lithiated and non-lithiated layers, in and out of the lithiation spots. Our results give some insight into the structural and electrochemical changes during the first cycle. They also show the low stability of the material under illumination and electrochemical conditions.