Structural Changes in Copper-Tin Alloy Anodes Observed with X-Ray Microtomography

Abstract

Tin intermetallic alloy anodes for Li-ion batteries present a route for incorporating the high capacity of tin while potentially mitigating the attendant volumetric expansion issues. While initial cycling performance of these alloy anodes is encouraging, significant capacity fade is observed as cycle number increases. In an effort to better understand the coupling of microstructural changes and electrochemical stimuli for intermetallic alloy anodes a combined electrochemical and 3D microstructural investigation was performed for Cu-Sn alloy anodes. The tin intermetallic alloy Cu6Sn5 was produced by pressing and sintering copper and tin powders mixed in stoichiometric proportions. The resulting pellet electrodes measured 13 mm in diameter by 1 mm thick. Using these pellets as the working electrode, half cells were assembled using Li foil as the counter electrode and LiPF6 in diethyl carbonate as the electrolyte. The half cells were tested in a galvanostatic configuration. Lithiation and cycling of the pellet electrodes were performed within two voltage windows: 0.2-1.5 V vs. Li/Li+ and 0-1.5 V vs. Li/Li+. It has been proposed that distinct reaction products are formed in these two voltage windows. In the higher voltage window a lithiated Cu-Sn phase is formed, while in the lower voltage window Sn is fully lithiated and Cu is expelled from the alloy phase. Distinct structural changes are expected to occur in these voltage windows, particularly due to the volume change that occurs on full lithiation of Sn. To test for these structural changes synchrotron-based x-ray microtomography was performed on samples extracted from the pellet electrodes. Applying microtomography at a resolution of 1.3 μm, imaging of the full sample cross-section was achieved. This permitted observation of affected and unaffected sample regions. Samples from lithiated, cycled, and pristine electrodes were imaged and compared. Significant structural changes were observed in the tested electrodes, particularly those lithiated and cycled in the window of 0-1.5 V vs. Li/Li+. In this window pulverization of the active electrode is clearly observed at the working electrode interface that was closest to counter electrode in the half cells. These observations appear to support structural evolution mechanisms proposed in the literature.