Analysis of the Li Distribution in Si-Based Negative Electrodes for Lithium-Ion Batteries by Soft X‑ray Emission Spectroscopy

Understanding the distribution of various Li x Si phases is essential for producing lithium-ion batteries with Si-based electrodes. Herein, we developed a facile and rapid analysis method by utilizing soft X-ray emission spectroscopy to determine the Li distribution in the Si layer based on the relationship between the intensity ratio of peaks arising from the Li–Si and Si–Si bonds and x in Li x Si. We also investigated the effect of different electrolytes, phosphorus-doping into Si, and cycle number on the x distribution. The results demonstrated that the distribution of an amorphous Li-rich phase (x = 2.00–3.75) affected the Si expansion. Heterogeneous distribution of the phase caused local accumulation of high strain and a drastic increment in the Si thickness, which led to capacity decay. This work provides insights into the relationship between the Li distribution and Si thickness, which determines the cycling performance of Si-based electrodes.