Synergistic Effects of Ge and Si on the Performances and Mechanism of the Ge_<i>x</i>Si_1–<i>x</i> Electrodes for Li Ion Batteries

While silicon is attractive due to its high capacity, germanium possesses a superior electronic and ionic conductivity, and is able to support much higher cycle rates. In the present paper, we investigate the electrochemical performances of GexSi1–x-based electrodes with x = 0, 0.25, 0.5, 0.75, and 1. The GexSi1–x samples are easily prepared by short ball milling. X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy–energy-dispersive X-ray spectrometry confirm that Si/Ge solid solutions are obtained with different ratios. We show that appropriate formulations of GexSi1–x electrodes can produce excellent cycling performances, with an increased capacity retention compared to that of Si-based electrodes only, and with higher capacity than that of Ge-based electrodes cycled in the same conditions. For Ge0.1Si0.9, a capacity of 1138 (mA h)/g is retained after 50 cycles, and is stabilized around 1020 (mA h)/g after 100 cycles. Moreover, a limited irreversible capacity is lost on the first discharge. The electrochemical data suggest that the copresence of Si and Ge in the same phase creates a synergistic beneficial effect on the GexSi1–x electrochemical performances. The electrochemical behavior of GexSi1–x is investigated for the first time by in situ XRD, and an original mechanism is highlighted with the reversible formation of a new Li15(Six/Gey)4 ternary phase at the end of discharge. Interestingly, a decrease of the cell parameter of this ternary phase is observed continuously from the end of discharge to the beginning of charge, which might be correlated to a loss of germanium of the lithiated Li15(Six/Gey)4 phase.

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