Electrochemically enhanced Cu6Sn5 anodes with tailored crystal orientation and ordered atomic arrangements for lithium-ion battery applications

Abstract

Sn-based anodes are potential replacements for commercial carbon-based lithium-ion battery anodes. Moreover, alloying with Cu improves the cyclability of Sn. This study directly grows Cu6Sn5 crystals on polycrystalline Cu current collectors via a solid-liquid reaction with molten Sn alloys. The Cu6Sn5 crystals grown when a molten Sn-Cu alloy was used have a preferred orientation along the η-Cu6Sn5 (101). In this study, the crystals are tailored to grow along the η-Cu6Sn5 (2-12) preferred orientation by excluding Cu in the molten Sn alloy used as a starting material. The (2-12) oriented electrodes show significantly improved electrochemical properties, displaying a 50th cycle discharge capacity of 762 mAh g-1, a 55% increase over the (101) electrode under the same cycling conditions. The (101) electrode ceased to function at 1C and above, while the (2-12) electrode retained around 480 mAh g-1 at 2C. In addition, the (101) electrode shows the formation of 3-fold superstructures in the crystal lattice; while the (2-12) electrode shows atomic arrangements of a higher crystal symmetry, further facilitating the transportation of Li-ions. Density functional theory calculations confirm that Li-ions prefer to adsorb onto Sn in η-Cu6Sn5 and diffuse through the structure in a zig-zag pattern along the [111] channels with a low barrier energy of 0.705 eV.