Microstructural improvements in ultra-thick cathodes for high energy Li-ion batteries

Today, the extensive use of mobile electric devices leads to a growing demand of batteries with a high energy density. One possible strategy to enhance the energy density of current Lithium ion cells and to decrease the cost of the cell stack at the same time is to increase the mass loading of the electrodes to extreme values, yielding ultra-thick electrodes. However, ultra-thick electrodes suffer from several drawbacks, when compared to electrodes with mass loadings according to the state of the art. One of them is a reduced mechanical stability due to effects occurring in the electrode drying process1-2, e.g. binder migration, and another one is a low rate capability3-4, which is mainly attributed to a limited Lithium-ion transport5. These effects were evaluated by the comparison of NCM 622 cathodes with high and ultra-high thicknesses to the state of the art in terms of mechanical stability and electrochemical performance. To overcome the occurring transport limitations in ultra-thick cathodes and simultaneously improve their mechanical stability, we pursue two strategies to control the microstructure of the electrodes. Firstly, the distribution of passive materials in the electrode composite is examined by SEM-EDX, as well as by 3D microstructure resolved simulations6 based on stochastic microstructure modeling of the tomographic data7. By adjusting the mixing process we receive a more homogenous electrode structure with a better rate capability which also agrees with simulation results. As a further strategy, the drying process, which is a very critical step in the preparation of ultra-thick electrodes, is explored and it was shown that by optimized drying conditions the specific discharge capacity of ultra-thick electrodes could be increased by 70 % at a current density of 8 mA/cm2, which equals 1 C. References 1. C. C. Li, Y. W. Wang; Journal of the Electrochemical Society 158 (2011), A1361-1370. 2. B. G. Westphal, H. Bockholt, T. Günther, W. Haselrieder, A. Kwade; ECS Transactions 64, 22 (2015), 57-68. 3. H. Y. Tran, C. Täubert, M. Wohlfahrt-Mehrens; Progress in Solid State Chemistry 42 (2014), 118-127. 4. H. Zheng, J. Li, X. Song, G. Liu, V. Battaglia; Electrochimica Acta 71 (2012), 258-265. 5. H. Zheng, L. Tan,G. Liu, X. Song, V. S. Battaglia; Journal of Power Sources 208 (2012), 52–57. 6. T. Danner, M. Singh, S. Hein, J. Kaiser, H. Hahn, A. Latz; Journal of Power Sources 334, (2016), 191–201. 7. D. Westhoff, J. Feinauer, K. Kuchler, T. Mitsch, I. Manke, S. Hein, A. Latz and V. Schmidt Computational Materials Science 126 (2017), 453-467. Acknowledgement The presented work was financially supported by BMBF within the project HighEnergy under the reference number 03XP0073C/D/E