Mechanically Alloyed Sn‐Fe(‐C) Powders as Anode Materials for Li‐Ion Batteries: I. The Sn2Fe ‐ C System
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The authors have prepared intermetallic phases and mixtures of such phases in the Sn-Fe-C Gibbs triangle by mechanical alloying methods or by direct melting. This second paper in a three-part series focuses on the intermetallic phases in the binary Sn-Fe system, Sn{sub 2}Fe, SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5}. Using in situ X-ray diffraction and electrochemical methods, the authors study the reversible reaction of Li with these materials. Li/Sn-Fe cells made from annealed powders have reversible capacities of 600, 50, 20 mAh/g, respectively, for Sn{sub 2}Fe, SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5}. Li/Sn-Fe cells made from the same materials, but after high-impact ballmilling, show reversible capacities of 650, 320, 200, and 150 mAh/g. Specific capacities of 804, 676, 582, and 557 mAh/g are expected for Sn{sub 2}Fe, SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5} if all compounds react fully with Li to form Li{sub 4.4}Sn and Fe. In situ X-ray diffraction experiments on the ballmilled materials confirm the formation of /Li{sub 4}Sn during discharge but also show that in the cases of SnFe, Sn{sub 2}Fe{sub 3}, and Sn{sub 3}Fe{sub 5} at least 50% of the starting phase remains unreacted. Structural considerations suggest that as themore » Fe:Sn ratio increases, Fe atoms may form a impenetrable skin on the surface of particles or grains, as Li reacts with the Sn-Fe compounds. This skin prevents the full reaction of the intermetallic with Li, leading to an observed capacity which is lower than expected. High-impact ballmilling reduces particle and grain size, so the effect of the skin is less than for the annealed powders and higher capacities are obtained. As the Fe content in the Sn-Fe intermetallics increases, the cycle life of the materials improves, presumably because there is more Fe per Sn and because the formed Fe and residual starting material act as a matrix to hold the Sn and Li-Sn alloys together during cycling. The authors give an example of a material with a volumetric capacity of 1200 mAh/cm{sup 3} showing stable cycling for over 80 cycles.« less