The Cu-Li-Sb system: A thermodynamic investigation on a materials system promising for lithium ion batteries

Highlights: • High temperature coulometric titration of four Cu-Sb alloys. • Three isothermal sections at 300 °C, 400 °C, and 600 °C. • Two vertical sections. • Liquidus projection. • Reaction scheme. Lithium containing intermetallic alloys have recently become the focus of many investigations regarding their potential application as electrode materials for lithium ion batteries. However the overwhelming increase in volume during lithiation prohibits a general use of these materials in state of the art batteries. The Cu-Li-Sb system has gained much attention in this respect, as the intermetallic compound η-Cu₂Sb showed remarkable cycling stabilities combined with a high charge capacity. To fully understand the reactions taking place upon charging/discharging of this material, a reliable phase diagram of the system is a prerequisite. Nevertheless almost no such data is available for the Cu-Li-Sb system. Especially reactions taking place at elevated temperatures become relevant regarding system malfunctions like e.g., thermal runaways of the battery cells. Therefore a substantial experimental investigation was conducted to enable the construction of three isothermal sections at 573 K, 673 K and 873 K. 83 alloy samples were thus metallurgically prepared and annealed at different temperatures. Analysis was done by powder X-ray diffraction. To validate the phase descriptions obtained from this data, electrochemical measurements were conducted. Coulometric titrations with four different Cu-Sb alloys (Cu concentrations = 80 at.%, 67 at.%, 50 at.%, 33 at.%) showed a good agreement with the phase fields expected from the powder X-ray diffraction data. 48 of the 83 samples were in addition analyzed by differential thermal analysis, which enabled the construction of two vertical sections (isopleths). These followed the lines connecting the binary alloy compositions Cu₆₆Sb₃₄ and Cu₅₀Sb₅₀ with Li. In addition a liquidus projection showing in- and mono-variant reactions, together with liquidus isopleths could be outlined. Finally a reaction scheme including all invariant reactions found within the system was developed.