Heat capacities and an updated thermodynamic model for the Li-Sn system

Phase-pure Li17Sn4 and Li7Sn3 intermetallic compounds were synthesized using defined heat treatment procedures in specially designed Ta crucibles. The products were characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES) and powder X-ray diffraction (powder-XRD) techniques to determine their composition and phase purity. The heat capacities of the synthesized compounds were measured via the step method in sealed Ta crucibles using a Setaram C80 Tian-Calvet calorimeter. The experimentally determined heat capacities were then used to develop restricted Maier-Kelley models for each phase and to incorporate these data into the re-optimization of an existing thermodynamic description of the Li-Sn system. The new models for the intermetallic and liquid phases result in improved agreement between the calculated and experimental heat capacity, thermodynamic, electrochemical and phase diagram data. Since the re-optimized Gibbs free energy expressions are based on reliable heat capacity data, the models can now be used to predict emf values in the Li-Sn system at a variety of operation temperatures and active material compositions.