In the present work, we studied the electrochemical mechanism of the Si0.5Ge0.5 alloy as a realistic micron-sized electrode formulation using carboxymethyl cellulose (CMC) as the binder9. A combination of a large set of in situ and operando techniques were employed to investigate the structural evolution of Si0.5Ge0.5 during lithiation and delithiation processes: powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), Raman spectroscopy, and 7Li solid state nuclear magnetic resonance spectroscopy (NMR).
The results have presented a whole view of the structural modifications induced by the lithiation/delithiation processes. The Si0.5Ge0.5 amorphization was observed at the beginning of discharge. Further lithiation induces the formation of a-Lix(Si/Ge) intermediates and the crystallization of Li15(Si0.5Ge0.5)4 at the end of the discharge. At really low voltages a reversible process of overlithiation and formation of Li15+δ(Si0.5Ge0.5)4 was identified and related with a structural evolution of Li15(Si0.5Ge0.5)4. Upon charge, the c-Li15(Si0.5Ge0.5)4 was transformed into a-Lix(Si/Ge) intermediates. At the end of the process an amorphous phase assigned to a-SixGey was recovered. Thereby, it was demonstrated that Si and Ge are collectively active along the cycling process, upon discharge with the formation of a ternary Li15(Si0.5Ge0.5)4 phase (with a step of overlithiation) and upon charge with the rebuilding of the a-Si-Ge phase. This process is undoubtedly behind the enhanced performance of Si0.5Ge0.5 compared to a physical mixture of Si and Ge.
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