Synergistic Effects of Ge and Si on the Performances and Mechanism of the GexSi-x 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 rate. 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 GexSi(1-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 Wig is retained after 50 cycles, and is stabilized around 1020 (mA h)/g after 100 Cycles. Moreover, a limited irreversible capacity is lost oil 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, electrochernical 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 Li-15(Si-x/Ge-y)(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 Li-15(Six/Gey)(4) phase.