Modeling of Structural Changes in Cathodes of Lithium Ion Batteries Depending on State of Charge

Electrodes are responsible for the processes in a battery. On one hand to make the electrochemical reaction happen and on the other hand provide ion and electron conductivity. To find a material which provides the perfect environment for those two processes is a challenge and there is still a great potential of optimization. In order to improve these two processes, a three dimensional model of an electrode is applied. The structure is designed from porosity measurements and microscopies. This study focuses on changes of the dimension of the model based on charged and discharged electrodes.

Changes in pore and particle sizes have an influence on the diffusion coefficient, tortuosity and the Bruggeman factor. These parameters are influenced by the state of charge (SOC) of the battery.

A three dimensional model is made from focused ion beam scanning electron microscopy (FIB-SEM) images provided by Hutzenlaub et al. [1] and will be compared with a rebuild model from spheres on the bases of diffusion coefficient, tortuosity and the Bruggeman factor. This model is used as the basis to define the three parameters for the rebuilt model from spheres. The porosity changes will be determined from Hg-porosimetry. From scanning electron microscopy (SEM) images, the tortuosity will be calculated and in addition, a calculated porosity can be compared as a reference with the measurements.

Volume changes of the solid structure inside an electrode occur depending on the state of charge. The binder inside the electrode is considered to stay the same, as it is not an active participant of the electrochemical reaction. Diffusion coefficient, tortuosity and changes of the Bruggeman factor, depending on the SOC, will be determined and modeled.

Porosity measurements showed that a change of SOC changes the porosity significantly. The cathode porosity of a charged battery is smaller than of a discharged, the rate of influence on the here mentioned parameters will be the focus in this study. Porosity measurements of charged and discharged Li_x(Ni_yCo_1-y)_2-x0₂ cathodes showed comparable particle distribution with Li_xCoO₂ up to 12 µm [2]. With these results, a comparison between the porous structure from Hutzenlaub et al. [1] and the porosity measurements can be conducted. The representative model size is dependent on the porosity and resolution related with the particle sizes [3, 4].

The results of this study provide a three dimensional simulation model for porous electrode applicable for various batteries, based on measurements of porosity and SEM to calculate porosity and tortuosity, from which the Bruggeman factor results. This study will provide the opportunity to model representative porous cathodes for lithium ion batteries without FIB-SEM at different SOC.

[1]       T. Hutzenlaub, S. Thiele, R. Zengerle and C. Ziegler, Electrochemical and Solid State Letters, 15 (3) A33-A36 (2012).

[2]       D.E. Stephenson, E.M. Hartman, J.N. Harb, D.R. Wheeler, Journal of the electrochemical society, 154 (12) A1146-A1155 (2007).

[3]       J. Joos, T. Carraro, A. Wber, E. Ivers-Tiffée, Journal of Power Sources, 196, 7302-7307 (2011).

[4]       B. Rüger, J. Joos, A. Weber, T. Carraro, E. Ivers-Tifée, ECS Transactions, 25, 1211-1220 (2009).