(Invited) The Crucial Role of Nano-Sized Porosity in the Lithiation and Stability of Methylated Amorphous Si Electrodes for Li-Ion Batteries

Abstract

Silicon electrodes are attracting for lithium-ion batteries but their use is impeded by their poor mechanical stability upon operation (huge volume variation takes place upon electrochemical Li insertion in and extraction from the material) and a poorly-controlled growth of surface layers resulting from the electrolyte decomposition products (the so-called solid-electrolyte interphase, SEI). A thin layer of methylated amorphous silicon a-Si(1-x)(CH3) x :H has a much better resistance to electrochemical lithiation/delithiation cycles than amorphous silicon a-Si:H [1]. Methylated amorphous silicon is a carbon-silicon alloy, deposited using PECVD, in which carbon is incorporated in the bulk material under the form of methyl groups CH3 only. We have studied the lithiation processes in the material for various methyl contents x ranging from 0 (a-Si:H) to 0.12. The lithiation process has been investigated using operando infrared spectroscopy in geometry of attenuated total reflection (ATR) where the active thin layer (30 nm) is directly deposited at the surface of the ATR prism. For all the investigated values of the methyl content x in the material, it was observed that the first lithiation proceeds according to a two-phase mechanism [2]: a (highl²y) lithiated phase with a well-defined Li content z (Li z Si phase) progressively invades the active-material film; if lithiation is performed in galvanostatic mode, this regime corresponds to the observation of a potential plateau. When lithiation is pursued beyond the potential plateau, a single-phase behavior is observed: the Li concentration in the active film becomes uniform and increases as long as the lithiation current is maintained. Delithiation and subsequent lithiation/delithiation cycles proceed homogeneously according to this single-phase behavior. When the electrode is thicker (200 nm), extensive electrochemical cycling results in the cracking of the film. The appearance of the crack network can be followed by operando optical microscopy. When the active film is deposited on a reflecting substrate such as stainless steel, the lithiation can also be followed using the same technique by monitoring of interference colors, since the lithiation results in a thickening of the film and in a change of its refractive index. The quantitative analysis of the two-phase regime of the first lithiation shows that the Li content z in the invading phase depends in a non-trivial way upon the methyl content x of the material: z first decreases upon increasing x, then for values of x larger than 0.05, z increases when increasing x. This behavior can be rationalized in the framework of a model describing the first lithiation of a-Si(1-x)(CH3) x :H. Noteworthy, the regime in which z increases upon increasing x can be ascribed to the existence of a methyl-induced porosity of the material at the nanometric scale. Moreover, the careful analysis of color patterns during lithiation demonstrates that the locations at which the lithiation occurs is not homogeneous at the very beginning of the second cycle: color change can be identified at the locations where future cracks will appear (at the end of the second delithiation of the electrode or later). Possible implications regarding the optimization of the material for its use as electrodes in Li-ion batteries will be discussed.