Controlling homogeneity of the first lithiation in methylated amorphous silicon

Methylated amorphous silicon (a-Si_1-x(CH₃)_x:H) exhibits a longer life time as an anode material in Li-ion batteries as compared to pure amorphous silicon (a-Si:H). However, operando optical microscopy of thin-film electrodes shows that the first lithiation turns from spatially uniform to non-uniform by increasing the methyl content of the material. The non-uniform lithiation appears to result from an electrostatic instability related to the large resistivity of methylated amorphous silicon. Lithiation spots nucleate either instantaneously at the beginning of the lithiation through dielectric breakdown, or more progressively at morphological defects where the thickness of the layer is reduced. In both cases damage is generated by the high current density flowing through the locations at which lithiation spots nucleate. Boron doping of methylated amorphous silicon, which decreases the material resistivity, turns the lithiation of thin-film electrodes from inhomogeneous to homogeneous.