Structural Changes in Alloy Anodes for Li-Ion Batteries

Tin (Sn) alloy electrodes show great potential for advancing battery performance due to the high capacity of tin. To realize this potential, the volumetric expansion during the lithiation process must be mitigated. One means of mitigating volumetric expansion of tin is to alloy it with copper to create Cu₆Sn₅. Such alloy electrodes retain some of the high capacity of tin, while attempting to accommodate volumetric changes with the addition of the malleable copper. Lithiation and delithiation tests were conducted with the Cu₆Sn₅ pellet electrodes to produce microstructural changes at the electrode surface. To observe and quantify these microstructural changes, x-ray microtomography was performed on electrode samples after electrochemical testing. The microtomography data was reconstructed into a 3D image, segmented, and the continuous phase size distribution (PSD) of each electrode sample was analyzed. The electrodes lithiated to 0 V vs Li/Li⁺ and then delithiated to 0.2 V vs. Li/Li⁺ showed the most substantial reduction in overall PSD compared to the other samples. This suggests that full lithiation of the Sn present in the alloy electrodes followed by partial delithiation of the Li_4.4Sn to Li₂CuSn can cause substantial microstructural changes related to volume expansion on lithiation and structural collapse upon delithiation. The electrodes fully lithiated to 0 V vs Li/Li+ and not delithiated show a higher overall phase size distribution, including all solid phases, than the pristine sample and the electrode samples that were partially lithiated to 0.2 V vs. Li/Li⁺ and delithiated to 1.5 V vs. Li/Li⁺. The higher overall phase size distribution that is shown by the sample that was fully lithiated and not delithiated is evidence of the significant volumetric expansion of the Cu₆Sn₅ compound due to lithiation. During this process of volumetric expansion, the phase size distribution of the Cu₆Sn₅/Sn phase is shown to decrease. When the volumetric expansion of the lithiated electrode samples and the volumetric contraction of the delithiated electrode sample are considered together, it can be inferred that the microstructural changes that are observed, such as the decrease in phase size distribution of the Cu₆Sn₅/Sn phase, can be attributed to the volumetric expansion and contraction of the compound during the lithiation and delithiation process.