2, Oak Ridge National Laboratory, Oakridge, Tennessee, United States
In Situ neutron diffraction (ND) is a powerful technique that can be applied to investigate the phase transitions in Li-ion batteries owing to the higher depth of penetration of neutrons and higher neutron scattering cross-section of Li atoms, relative to that in X-ray scattering. The principal difficulty with in situ studies on Li-ion batteries is that neutrons will be incoherently scattered by H and 3Li7 in separator, electrolyte and the non-active components of the cell. We have successfully designed and demonstrated a cell, with single crystal Si sheets as casing material, for in situ electrochemical studies of Li-ion batteries. The objective of this work is to use this cell to understand the phase transitions upon lithiation of Si with columnar architecture. The columnar structure is designed to accommodate the large volume changes upon lithiation of Si. Hence, the real time evolution of structures in Si columns, upon lithiation, can provide insight on the performance of Si electrode. The electrodes were packaged inside the in situ cell and were subjected to full charge-discharge cycle while under neutron diffraction. Neutron diffraction patterns with high signal to noise ratio could be obtained. Interestingly, a peculiar phenomenon, where the diffraction peaks show a large change in peak intensity upon lithiation and with little change in the corresponding interplanar spacing, is observed. The results are analyzed in terms of a possible mechanism of lithiation of Si and the associated formation of Si islands embedded in amorphous lithiated Si, leading to a mosaic structure. The implications of these observations on designing better Si electrodes are discussed.