The phase transformations and cycling performance of copper–tin alloy anode materials synthesized by sputtering

A sputtering technique was adopted to synthesize Sn–Cu thin film electrodes. Island Sn particles were obtained on the copper foil. Cu6Sn5 was spontaneously generated at the interface between Sn and Cu foil. To further improve the cycling stability, Cu source was introduced to increase the formation of Cu6Sn5 and to serve as a buffer during cycling. Moreover, the phase and elemental ratio of Sn and Cu varied in the synthesized electrode by alternately adjusting sputtering time for Sn and Cu. The cell synthesized by sputtering Sn for 5 min and Cu for 9 s alternately exhibited the best cycling stability. The 1st charge capacity of cell was 635 mA hg−1, and the 1st efficiency was even higher than 97%. The capacity remained higher than 500 mA hg−1 after 15 cycles. The phase transformation of cell was investigated through voltage profile, CV curve and in situ XRD analysis. The in situ XRD analysis confirmed that Cu6Sn5 could react with lithium directly without the presence of Li2CuSn during cycling. The reaction mechanism of Cu6Sn5 with lithium during cycling was demonstrated to be an alloying process, and the structure of Cu6Sn5 was thus a low-temperature monoclinic phase.

Research highlights▶ The morphology of electrode displays island structure on the surface of Cu foil by sputtering. ▶ The structural changes of copper–tin alloy during cycling could be recognized using in situ synchrotron X-ray diffraction technique. ▶ The reaction mechanism of copper–tin alloy was an alloying process, and the Cu6Sn5 alloy was considered as the low-temperature η′-Cu6Sn5.