Using combinatorial and high-throughput materials science methods, we have studied thin-film libraries of Sn 1−x Co x ͑0 Ͻ x Ͻ 0.6͒ and ͓Sn 0.55 Co 0.45 ͔ 1−y C y ͑0 Ͻ y Ͻ 0.5͒ alloy negative electrode materials for Li-ion batteries. Over one hundred compositions have been studied carefully by X-ray diffraction and electrochemical methods. The Sn 1−x Co x system is found to be amorphous for 0.28 Ͻ x Ͻ 0.43. For 0.43 Ͻ x Ͻ 0.6, the amorphous phase coexists with electrochemically inactive crystalline Co 3 Sn 2 . 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 ͓Sn 0.55 Co 0.45 ͔ 1−y C y ͑0 Ͻ y Ͻ 0.5͒ library, has a number of positive effects. First, all alloys with 0.05 Ͻ y Ͻ 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 Х 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.