Multiscale Investigation of Silicon Anode Li Insertion Mechanisms by Time-of-Flight Secondary Ion Mass Spectrometer Imaging Performed on an In Situ Focused Ion Beam Cross Section

Abstract

Considering its specific capacity, silicon is one of the most promising materials to replace graphite in lithium ion batteries anodes. However, its rapid capacity fading prevents its use in current batteries. Understanding lithiation and degradation mechanisms of silicon is important for improving its cyclability. In this work a novel approach is developed by using a focused ion beam implemented in the analysis chamber of a state-of-the-art time of flight secondary ion mass spectrometer. Detailed mapping of elements distribution, including lithium, inside a silicon particle or in the entire depth of the electrode, can thus be performed. During the first lithiation, a core–shell mechanism is observed and its evolution upon electrochemical cycling was examined. This mechanism is observed for all particles in the electrode, independently of their position. Cross analysis with Auger spectroscopy allowed Li concentration in the entire shell to be quantified. Fast lithiation paths getting through the pure silicon core have been evidenced by complementary scanning electron microscopy and transmission electron microscopy (TEM) analyses. Defects observed by TEM are supposed to contribute significantly in the Li diffusion inside the particle. This approach also provided evidence of lithium progressively trapped in Si particles after aging, in close relationship with capacity loss found for silicon anodes along cycling.