(Invited) Instability and Inhomogeneity in the Lithiation of Methylated Amorphous Silicon Layers
Authors: Feng Yue , Ngo Thuy-Doan-Trang , Panagopoulou Marianthi , Cheriet Abdelhak , Koo Bon-Min , Charrier Anne , Henry de Villeneuve Catherine , Rosso Michel , Ozanam Francois 
Published: December 2020

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 practical use. We have shown that amorphous methylated silicon a-Si1−
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 spectroscopy and optical microscopy provide a thorough understanding of the mechanisms involved during these cycles. The lithiation of a-Si1−
x
(CH3)
x
:H, with various methyl contents (x = 0 - 0.12), was investigated using operando ATR-FTIR [2]. The first lithiation proceeds via a two-phase mechanism [3], 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 reveal that, 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 zones 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 appearance of these lithiation spots appears to be related to an electrostatic instability associated with the resistive character of the a-Si1−
x
(CH3)
x
:H layer. This process is reminiscent of that responsible for the formation of macropores on p-Si, but at a much larger length scale [5]. Experimental observations unravel the existence of defects determining the locations at which lithiation spots are formed. A simple model has been worked out for the lateral growth of the lithiation spots, which is in semi-quantitative agreement with the combined electrochemical and microscopy measurements. Complementary Raman spectroscopy experiments give some insight into the structural and electrochemical changes during the first lithiation/delithiation cycle.
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Figure 1