Synergistic Effects of Ge and Si on the Performances and Mechanism of the Ge_xSi_1–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 rates. In the present paper, we investigate the electrochemical performances of Ge x Si 1– x -based electrodes with x = 0, 0.25, 0.5, 0.75, and 1. The Ge x Si 1– 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 Ge x Si 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 Ge 0.1 Si 0.9, 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 Ge x Si 1– x electrochemical performances. The electrochemical behavior of Ge x Si 1– 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 (Si x /Ge y ) 4 phase.