The electrochemical reaction of Li with amorphous Si-Sn alloys

Si1-xSnx samples for 0<0.5 were prepared by magnetron sputtering using a combinatorial materials science approach. The room-temperature resistivity and X-ray diffraction (XRD) patterns of the samples were used to select materials having both an amorphous structure and good conductivity for further study. The reaction of lithium with amorphous Si0.66Sn0.34 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<4.4 in LixSi0.66Sn0.34. 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-Si0.66Sn0.34 starting material and a-Li4.4Si0.66Sn0.34 can be explained by the same local atomic arrangements as found in crystalline Si and Li-4.4 Si or Li-4.4 Sn, respectively. In fact, the in situ XRD patterns of a-LixSi0.66Sn0.34, for any x, can be well approximated by a linear combination of the patterns for x=0 and x=4.4. This suggests that predominantly only two local environments for Si and Sn are found at any value of x in a-LixSi0.66Sn0.44. However, based on differential capacity vs. potential results for Li/a-Si0.66Sn0.34 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/a-LixSi0.66Sn0.34 cells is caused by the energy dissipated during the changes in the local atomic environment around the host atoms. (C) 2002 The Electrochemical Society.