Development of Novel Intermetallic Anode Materials for Improved Li-Ion Batteries

Abstract

Sn-based intermetallics are novel anode materials for lithium-ion batteries since their theoretical capacities (994 mAhg-1), associated with the formation of Li17Sn4, are much higher than those of traditional graphite-based anodes (372 mAhg-1). However, during the lithiation of Sn, these materials suffer from large volume changes of up to 185 % when formation of Li17Sn4 takes place. Consequently, the resulting pulverization of the electrode material leads to degradation of particle-to-particle contacts and drastic performance losses. One way to minimize the volume changes and retain electrode integrity is to develop new anode materials in which Li+ is stored by displacement or insertion reaction mechanisms. Cu and Sb are promising elements for new intermetallic Sn-based electrodes since their additions could lead to Li+uptake via these reactions. In addition, the combination of these two elements can improve the overall cell performance. Phase diagrams are essential tools for designing new electrode chemistries, since they can be used to predict the compositional changes and phase transformations of the electrode materials during lithiation and de-lithiation. In a simulation-guided design approach, CALPHAD-based thermodynamic descriptions of the multi-component systems are used to simulate phase formation and electromotive force (EMF) values during charge/discharge processes in the cell. However, for the development of the Gibbs free energy descriptions of the various phases, reliable thermodynamic, phase diagram, and electrochemical data are required as input data. In our group, consistent thermodynamic descriptions of the Li-Sn and Li-Sb binary systems were developed. Furthermore, the heat capacities of selected Li-X compounds from room temperature to 300°C were measured using differential scanning calorimetry. Half cells in coin cell geometry were constructed using electrodes containing LixSny and LixSby intermetallic compounds as active materials, which were synthesized from the pure metals by annealing in an inert atmosphere. The room temperature EMF values of the coin cells were compared to those predicted by the simulations. Post-mortem microstructural investigations (SEM and ex-situ XRD) of the electrodes were performed on cells which were galvanostatically cycled with potential limitation (GCPL). These investigations show that during cycling, the suppression of the LiSn and Li2Sn5 phases leads to significant improvements in capacity retention. This is attributed to the fact that the main volume changes during lithiation/de-lithiation of Sn occur in the phase region between pure Sn and Li2Sn5, and between Li2Sn5 and LiSn. Based on these data, prototype coin cells were assembled with Li17Sn4 as the anode material and de-lithiated manganese oxide spinels (LixMn2O4) as the cathode material. The results of the electrochemical testing of these cells are compared to the predictions from the thermodynamic descriptions of the cathode and anode materials systems.