Combinatorial study of Sn1-xCox (0 < x < 0.6) and [Sn0.55Co0.45](1-y)C-y (0 < y < 0.5) alloy negative electrode materials for Li-ion batteries

Using combinatorial and high-throughput materials science methods, we have studied thin-film libraries of Sn1-xCox(0 < x < 0.6) and [Sn0.55Co0.45](1-y)C-y(0 < y < 0.5) alloy negative electrode materials for Li-ion batteries. Over one hundred compositions have been studied carefully by X-ray diffraction and electrochemical methods. The Sn1-xCox system is found to be amorphous for 0.28 < x < 0.43. For 0.43 < x < 0.6, the amorphous phase coexists with electrochemically inactive crystalline Co3Sn2. Amorphous materials with x = 0.4 show a specific capacity of 650 mAh/g, but differential capacity, dQ/dV, vs potential is not stable vs cycling indicating irreversible atomic-scale changes in the alloy, most likely due to tin aggregation. Adding carbon to this system, for example in the [Sn0.55Co0.45](1-y)C-y (0 < y < 0.5) library, has a number of positive effects. First, all alloys with 0.05 < y < 0.5 are amorphous, with carbon directly incorporated within the amorphous phase. Second, the addition of carbon increases, not decreases, the specific capacity from about 670 +/- 15 mAh/g for y = 0.05 to 700 +/- 15 mAh/g for y = 0.4. Third, compositions with y congruent to 0.4 show differential capacity vs potential curves that do not change during charge-discharge cycling, indicating that such alloys are stable on the atomic scale and hence are extremely good candidates for long cycle life. Stability increases with carbon content up to y = 0.4. (c) 2005 The Electrochemical Society.