Understanding the Lithiation/Delithiation Mechanism of Si1-XGex Alloys
Authors: Loaiza Laura Cristina , Salager Elodie , Louvain Nicolas , Boulaoued Athmane , Iadecola Antonella , Johansson Patrik , Stievano Lorenzo , Seznec Vincent , Monconduit Laure 
Published: February 2020

Lithium-ion batteries (LIBs) have an important place among energy storage devices due to their high capacity and good cyclability. However, the advancements in portable and transportation applications have extended the research towards new horizons, and today the development is hampered e.g. by the capacity of the electrodes employed. Silicon and germanium are among the considered modern anode materials as they can undergo alloying reactions with lithium while delivering high capacities. It has been demonstrated that silicon in its highest lithiated state can deliver up to ten times more capacity than graphite (372 mAh/g): 4200 mAh/g for Li22Si5 and 3579 mAh/g for Li15Si4, respectively1–3. On the other hand germanium presents a capacity of 1384 mAh/g for Li15Ge4
1, and a better electronic conductivity and Li ion diffusivity as compared to Si4. Nonetheless, the commercialization potential of Ge is limited by its cost. The synergetic effect of Si1-xGex alloys has been proven5, the capacity is increased compared to Ge-rich electrodes and the capacity retention is increased compared to Si-rich electrodes5, but the exact performance of this type of electrodes will depend on factors like specific capacity, C-rates, cost, etc. There are several reports on various formulations of Si1-xGex alloys with promising LIB anode performance1,5–8, with most work performed on complex nanostructures resulting from synthesis efforts implying high cost.

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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