Secondary lithium ion cells are now universally accepted as a viable key component for electrical energy storage in portable, entertainment, computing and telecommunication equipment required by todays information-rich, mobile society. Predominantly, the lack of suitable reversible negative electrode materials hinders the generation of new lithium ion batteries with distinctly higher charge densities and cycling stabilities.
Until now, a variety of (semi-)metal anodes have been tested, promising unrivalled capacities as well as safety due to more positive potentials against Li/Li+ compared to graphite. Sn-based intermetallic anodes are expected to deliver significant capacity increases (960 mAh/g) compared to graphite (372 mAh/g) although they suffer from large volume changes during lithiation/de-lithiation (L/D) which lead to electrode degradation and poor cycling stability. A significant improvement of cyclability can be achieved by using intermetallic alloys, e.g. Cu-Sn or Sb-Sn as anode materials [2015Nit].
The aim of the current project is to combine key experimental data (thermodynamic properties, phase diagram information, crystallographic data) with electrochemically derived data from prototype cells, an approach which was recently validated by Huggins in “Advanced batteries” [2008Hug]. Consistent thermodynamic descriptions of corresponding material systems are developed using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method [2007Luk] which is up to now the most advanced method for thermodynamic modeling and simulation of complex heterogeneous systems. This is also a key component of the new ICME (Integrated Computational Materials Engineering) method [2008Com], in which the use of computational thermodynamics helps bridge materials processing and materials structure to be able to predict materials properties. Our thermodynamic descriptions are critical for the design of new electrode materials with optimally tailored properties and allow the evaluation and even the prediction of internal cell reactions and open circuit voltages (OCV).
The preparation of phase-pure Li-Sn and Cu-Li-Sn alloys was done by metallurgical techniques adopting special pre-heat treatments and annealing routines. Subsequent characterization was performed by powder X-ray diffraction (XRD), ICP-OES and BSED imaging. The heat capacity (Cp) of selected phase-pure alloys was determined by differential scanning calorimetry (DSC) using a Tian-Calvet type arrangement. Due to the high reactivity of Li, both the synthesis and heat capacity measurements of the respective compounds required the use of tantalum crucibles as well as the manipulation inside an inert gas (Ar) glove box. The derived heat capacity data are implemented into the thermodynamic dataset which enables the temperature-dependent modeling of Gibbs energies and a reliable prediction of electrochemical data at various battery operation temperatures.
To verify our calculations, respective phase-pure alloys were used as active materials in battery test cells. Particularly, galvanostatic coulometry and cycling voltammetry were performed in suitable coin cell arrangements against Li/Li+. To facilitate the production of compact electrode materials, active materials were dispersed in N-Methyl-2-pyrrolidone (NMC) together with carbon black to improve conductivity and Polyvinylidene fluoride (PVDF) was added as a binder. Relevant lithiation steps at constant electrochemical potentials were investigated by ex-situ XRD and BSED imaging. For the first time, already lithiated multicomponent intermetallic alloys were used as anode materials so that in the initial cycling step Li was removed from the active Sn-based material. Corresponding effects on cell performance from this new approach will be discussed in detail.
[2015Nit] N. Nitta, F. Wu, J.T. Lee, G. Yushin. Materials Today 18 (2015): 252-264
[2008Hug] R.A. Huggins. Advanced Batteries. Materials Science Aspects (2008), New York: Springer
[2007Luk] H.L. Lukas, S.G. Fries, B. Sundman. Computational Thermodynamics: The Calphad Method. (2007), Cambridge University Press, United Kingdom
[2008Com] Committee on ICME. National Research Council (2008), Washington: National Academies