Thermodynamic Database Development and Simulation of Novel Intermetallic Anode Materials for Improved Li-Ion Batteries
Authors: Cupid Damian Marlon , Li Dajian , Reichmann Thomas Ludwig , Fuertauer Siegfried , Beutl Alexander , Seifert Hans Jürgen , Flandorfer Hans 
Published: February 2020

Sn-based intermetallics are considered as promising anode materials for lithium ion batteries which can increase anode capacities by as much as 200% compared to traditional graphite anodes. However, these materials suffer from large volume strains on lithiation which result in pulverization, degradation of particle to particle contacts, and rapid electrode failure. One way to buffer the volume changes is by including additional elements which form a matrix phase for structure stabilization. Another possibility is to start with binary phases which provide a stable host lattice for the introduction of Li. Cu and Sb are promising elements for these two requirements, respectively, and the combination of these two elements with Sn is expected to lead to improved battery performance.

The electromotive force of an electrochemical cell is the difference between the chemical potential of lithium in the anode and cathode material. During lithiation and de-lithiation, compositional changes and phase transformations of the electrodes take place, resulting in changes in the chemical potential of lithium. Therefore, phase diagrams are essential tools for understanding and designing new battery materials. Thermodynamic, phase diagram, and electrochemical data are used as inputs to develop CALPHAD-based thermodynamic descriptions of the multi-component systems, which can then be used to predict open circuit voltages and coulometric titration curves. To be able to apply the simulations to battery operation temperatures, accurate experimental data on the heat capacities of the electrode materials are furthermore required. Therefore, the aim of the current work is to model the Gibbs energies of battery-relevant phases in the Cu-Li-Sb-Sn sub-systems using the CALPHAD approach and to use these models to predict battery performance parameters.

Key experiments in the Cu-Sn, Li-Sn, Li-Sb, and Cu-Li systems were performed to verify existing literature information and generate new thermodynamic and phase diagram data. In addition, the heat capacities of selected alloys were determined using scanning calorimetry (DSC). Based on a critical evaluation of the available experimental data, the Gibbs energy expressions for phases in the Cu-Li, Cu-Sn, Li-Sn and Li-Sb systems were modeled using the CALPHAD method. Calculations of coulometric titration curves performed using the new Gibbs energy models well reproduce the experimentally determined electrochemical data.

Electrochemical cycling (CCVV) of coin cells constructed using Sn and Cu-Sn active material arranged as half cells was also performed. Ex-situ XRD was used to characterize the intermittent anode compositions throughout the whole electrochemical lithiation. The results of these electrochemical tests were then compared to the open circuit voltages and coulometric titration curves calculated using our thermodynamic modeling of the multi-component systems. Furthermore, the electrochemical properties were calculated at a variety of temperatures to simulate cell behavior at different operation conditions.

This work has been funded by the German Research Foundation (DFG) priority program WenDeLIB under the project CU203/1-2 and the Helmholtz Association under the project VH-NG-1057.