Fundamental experiments on the measurement of Li diffusion in electrochemically lithiated silicon electrodes for lithium
The knowledge of Li diffusion in lithium silicide (LixSi) is nowadays of importance for the Li-ion battery (LIB) research and development community. About twenty times more charge (Li+) can be stored per Si atom (Li3.75Si) than per C atom (LiC6), while carbon is still the main storage material in commercial LIBs. Consequently, huge effort is made to replace carbon by silicon in LIBs. Unfortunately, the diffusion of high amounts of Li+ into silicon during LIB operation is accompanied by a huge volume expansion of up to 400 % leading to a pulverization of the silicon electrode due to stress. Li diffusion in the interior of the electrode plays an important role for a fundamental understanding and for an optimization of application relevant properties like charging/discharging rates, maximum capacity, self-discharge, and cycling stability. This contribution reports about the realization of Li diffusion experiments in electrochemically lithiated amorphous silicon. Li diffusion was investigated at room temperature using the technique of secondary ion mass spectrometry (SIMS) in depth profile and in line scan mode in combination with stable 6Li isotope tracers. Experiments were done on 200 nm thin Si electrodes with a Li+ insertion stage of about 30 % and 80 % of full capacity. SIMS line scan data were successfully obtained, which will allow to measure fast Li diffusion on electrodes over macroscopic distances in the mm range, which is not possible using SIMS depth profiling mode.