Early Identification of the Li15Si4 Phase Using in-Situ Synchrotron XRD

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