The Electrochemical Reaction of Li with Amorphous Si-Sn Alloys

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Abstract
Si1−xSi1−x SnxSnx samples for 0<x<0.50Si0.66Si0.66 Sn0.34Sn0.34 was then studied by electrochemical methods and by in situ XRD. The electrode material apparently remains amorphous throughout all portions of the charge and discharge profile, in the range 0<x<4.40LixLix Si0.66Si0.66 Sn0.34.Sn0.34. No crystalline phases are formed, unlike the situation when lithium reacts with tin. Using the Debye scattering formalism, we show that the XRD patterns of the a- Si0.66Sn0.34Si0.66Sn0.34 starting material and a­Li4.4Si0.66Sn0.34a­Li4.4Si0.66Sn0.34 can be explained by the same local atomic arrangements as found in crystalline Si and Li4.4Li4.4 Si or Li4.4Li4.4 Sn, respectively. In fact, the in situ XRD patterns of a- LixSi0.66Sn0.34,LixSi0.66Sn0.34, for any x, can be well approximated by a linear combination of the patterns for x=0x=0 and x=4.4.x=4.4. This suggests that predominantly only two local environments for Si and Sn are found at any value of x in a­Lixa­Lix Si0.66Si0.66 Sn0.44.Sn0.44. However, based on differential capacity vs. potential results for Li/a- Si0.66Si0.66 Sn0.34Sn0.34 there is no evidence for two-phase regions during the charge and discharge profile. Thus, the two local environments must appear at random throughout the particles. We speculate that the charge-discharge hysteresis in the voltage-capacity profile of Li/a­LixSi0.66Sn0.34Li/a­LixSi0.66Sn0.34 cells is caused by the energy dissipated during the changes in the local atomic environment around the host atoms. © 2002 The Electrochemical Society. All rights reserved.