Impact of the silicon particle size on the pre-lithiation behavior of silicon/carbon composite materials for lithium ion batteries

High-capacity silicon-based negative electrode materials can significantly boost the energy density of lithium ion batteries. However, several disadvantages arise due to the severe volume changes during lithiation/de-lithiation, which particularly include the low initial Coulombic efficiency and related consumption of active lithium. Therefore, pre-lithiation is considered as a mandatory processing step when applying high amounts of silicon (e.g., >10 wt%). Due to its simplicity, pre-lithiation via direct contact to Li metal in presence of electrolyte is well studied. However, systematic studies of the pre-lithiation of silicon/carbon composite materials have only rarely been reported so far. Here, pre-lithiation by use of Li metal is thoroughly investigated to answer three major questions with regard to the effectiveness of the pre-lithiation behavior for silicon/carbon negative electrodes. Firstly, the pre-lithiated capacity is evaluated by electrochemical means to conclude whether it is of irreversible or reversible nature. Secondly, the homogeneity of pre-lithiation with regard to species of the binary composite material is investigated by electrochemical and analytical methods. Thirdly, the spatial distribution of the pre-lithiation is examined on macroscopic (electrode) and microscopic levels (particle) via glow discharge-mass spectrometry. All these investigations are made with regard to the silicon particle size, ranging from ≈180 nm to ≈6.3 μm.