Thermal stability of ternary compounds in the Cu-Li-Sn system and phase transition of the Cu6Sn5 electrode: First-principles calculations and experiment

Cu₆Sn₅ is a promising alternative material for commercial anode in lithium ion batteries. In this study, the thermal stability of ternary compounds in the Cu-Li-Sn system and equilibrium phase transition of Cu₆Sn₅ upon charge-discharge reaction are investigated by means of first-principles calculations and key experiments. The ternary phase diagram for the Cu-Li-Sn system at 0 K constructed by first-principles calculations exhibits that CuLi₂Sn₂, CuLi₂Sn and Cu₂LiSn are thermodynamically stable, but Cu₆Li₃Sn₄ is thermodynamically metastable. In addition, calculated equilibrium reaction path and voltage curve of Cu₆Sn₅ electrode suggest that CuLi₂Sn₂ and Cu₂LiSn firstly form before the well-known CuLi₂Sn appears during lithiation. Vibrational free energy contribution is introduced to study thermal stability of ternary phases, which predicts that Cu₆Li₃Sn₄ is thermodynamically stable above 148 °C. In order to verify our calculations, phase equilibria of eleven ternary alloys at 120 and 180 °C are investigated by means of X-ray diffraction, scanning electron microscopy and electron probe microanalyses. It shows that Cu₆Li₃Sn₄ can decompose between 180 and 120 °C, which is consistent with our predictions in calculations. Thus, our study suggests that Cu₆Li₃Sn₄ is metastable at room temperature and CuLi₂Sn₂, CuLi₂Sn and Cu₂LiSn can form during the electrochemical lithiation of Cu₆Sn₅ electrode.