In-situ synchrotron X-ray powder diffraction (SXRD) was performed on a Li-Si pouch cell on the high-energy beamline (I12) at Diamond Light Source. The high photon energies provided by the synchrotron allowed the beam to pass directly through the Li-Si pouch cell while still providing high resolution SXRD data. Furthermore, the large flux of the synchrotron beam allows high temporal resolution SXRD data to be collected. Fast acquisition time and high resolution SXRD patterns allows for accurate identification of the onset of Li15Si4 formation. Obrovac and Christensen [1] showed that the fully lithiated phase for silicon at room temperature is Li15Si4. They also showed that the Li15Si4 phase is formed below 50 mV (vs. Li), which typically occurs at the end of silicon lithiation. Li and Dahn [2] used in-situ XRD to show that the Li15Si4 phase does not occur until high capacity (> 3500 mAh/g) and low voltage (< 50 mV). The results discussed above clearly show that formation of the Li15Si4 phase can be capacity dependent. One could also theorize that the formation of Li15Si4 is voltage dependent. To further elucidate the effect of voltage on the formation of the Li15Si4 phase in-situ SXRD was performed on Li-Si cells at voltages below 50 mV. In-situ SXRD data is shown in Figure 1 as a function of intensity (left ordinate) and as a ‘waterfall plot’ vs. time (right ordinate). Figure 1 clearly shows that the silicon peak recedes as a function of time (and capacity), while the Li15Si4peak grows. To better quantify the formation of the Li15Si4 phase Figure 2 shows the evolution of the volume fraction of Li15Si4 as a function of time (lower plot abscissa) and capacity (upper plot ordinate). Figure 2 shows that the Li15Si4 phase can be observed almost immediately. As capacity increases the volume fraction of Li15Si4 continues to increase. These results show that the Li15Si4phase can be formed very early if the voltage at the silicon electrode is below 50 mV. References: [1] M. N. Obrovac and Leif Christensen, Electrochemical and Solid-State Letters, 7 (5) A93-A96, 2004 [2] J. Li and J. R. Dahn, Journal of The Electrochemical Society, 154 (3), A156-A161, 2007
