Boron-doping improving the long-term stability of methylated amorphous silicon electrodes for lithium-ion batteries

Thin-film electrodes offer a well-controlled geometry in order to comprehensively assess the behavior of active materials for Li-ion batteries. This approach has been used for studying silicon-based amorphous materials, which are of interest for being used as negative electrodes. Methylated amorphous silicon electrodes exhibit enhanced performance over pure amorphous silicon upon extended electrochemical cycling, but at the expense of a significant increase in the material resistivity. Boron doping increases the material conductivity by orders of magnitude, which alleviates limitations in terms of ohmic drop across the material during the first cycles, and allows for the practical use of methyl content higher than 10 % in the electrode material. This way, boron-doped methylated amorphous silicon electrodes exhibit enhanced rate performance and thin (100 nm) boron-doped 20 % methylated amorphous silicon electrodes exhibit high capacity retention over thousands of lithiation/delithiation cycles. Boron doping also allows for using thicker electrodes (up to a thickness of 1 µm), albeit at the expense of a decreased rate performance. Raman spectroscopy indicates that methylation increases the disorder in the material at intermediate range, and that boron doping increases disorder both at short and intermediate range. These structural changes are in line with a lowering of the material rigidity induced by methylation and boron doping, as measured by indentation tests, which at least in part might account for the enhanced long-term stability of boron-doped methylated amorphous silicon.