Infrared <i>Operando</i> Study of the Solid-Electrolyte Interphase of Amorphous-Si Electrodes for Li-Ion Batteries: Effects of Methylation and Boron Doping

In lithium-ion batteries, the interest in delivering a high voltage leads to the use of electrodes working outside the electrochemical stability domain of electrolytes. This is especially the case for negative electrodes. Under such conditions, the stable operation of the battery requires the buildup of a passivation layer at the electrode surface, permeable to Li ions but blocking electrolyte decomposition. The stability of this passivation layer, the so-called solid-electrolyte interphase (SEI), is instrumental for sustained battery operation. In view of enhancing the specific battery capacity, silicon-based negative electrodes are appealing but suffer from an unstable SEI. Operando infrared spectroscopy has been used for analyzing the SEI formed on methylated amorphous silicon thin-film electrodes along lithiation/delithiation cycles. It provides a quantitative measurement of the SEI thickness and, with the exception of fluorinated compounds, an assessment of its chemical composition (organic carbonate, lithium carbonate, and polycarbonate content). The methyl content of the material was varied from 0 to 10%, and the influence of boron doping was also assessed. For undoped electrodes, increasing the methyl content enhances stability during electrochemical cycling but does not reduce the SEI growth rate, at least not within the first 30 cycles. The polycarbonate growth is also insensitive to the methyl content, contrary to lithium carbonate, which grows at a lower rate upon increasing methyl content. Remarkably, the combination of boron doping and high methyl content significantly lowers the SEI growth rate, opening perspectives for electrode passivation. Moreover, the polycarbonate component grows during the first cycles and then remains approximately constant, at least for about 100 cycles. The lithium carbonate component exhibits a low growth rate at a high methyl content, accounting for the slow residual growth rate of the SEI. An electrochemical mechanism is proposed to explain the effect of doping.