In situ conductivity, impedance spectroscopy, and ex situ Raman spectra of amorphous silicon during the insertion/extraction of lithium

The electrical conductivity and the impedance behavior of thin layers of amorphous silicon (a-Si), which are promising anode materials for lithium-ion batteries, were monitored in situ during the insertion/extraction of lithium in 1 M of a LiBOB (Li-bioxalato borate) propylene carbonate solution. In addition, Raman spectra of the same electrodes were recorded in situ and ex situ during lithiation/delithiation processes in the above-mentioned solutions. The conductivity of the a-Si electrode was increased by about 3.5 orders of magnitude during the course of lithium insertion. While the impedance response of these electrodes is complicated and cannot be resolved unambiguously, it is clear that the electrical conductivity influences strongly the electrodes' impedance: a similar dependence of the electrical conductivity and the impedance of these electrodes on the potential are measured. The intensity of the Raman signal dropped significantly upon lithiation and recovered at a potential of 0.523 V vs Li/Li⁺. It is suggested that the drop in the intensity of the Raman signal of the silicon electrodes upon their lithiation is due to changes in the optical skin depth of the a-Si, which occur upon the formation of the Li-Si alloy.