Sustainable energy supply increasingly relies on batteries for intermediate or mobile energy storage. Within the battery electrodes, hierarchical structuring of the active material has the potential to enhance rate capability and specific capacity. The resulting electrode contains porous secondary particles, which consist of smaller primary particles. This contribution examines the relationship between the morphology of hierarchically structured electrodes and the cell performance.
We investigate a cell whose positive electrode consists of hierarchically structured NMC111. For this purpose, we use a continuum cell model. It includes electrochemical processes at different length scales: electronic and ionic transport at electrode level as well as within the secondary particles, the electrochemical reaction at the primary particle surfaces, and diffusion in the solid primary particles. Within this modeling framework, we systematically vary single morphological properties to understand how they affect the processes within the electrode and ultimately the cell performance.
Depending on the morphology of the hierarchically structured positive electrode different processes affect the performance of the battery cell [1]. The main limit for the cell performance is a low electronic conductivity within the secondary particles [2]. It causes a decreasing specific capacity with increasing discharge rate since inner regions of the secondary particles are hardly exploited. A high secondary particle porosity further reduces the electronic conductivity and aggravates the effect. In large primary particles, concentration gradients occur due to the slow diffusion and additionally compromise the cell performance. Thick electrodes experience a decreased salt concentration in the electrolyte. Consequently, reaction kinetics slow down resulting in a declining rate performance. Close to the current collector, very thick electrodes even suffer a total salt depletion of the electrolyte. In these regions, the electrochemical reaction fades, which causes a sudden drop in specific capacity. Due to the high inner surface area of hierarchically structured electrodes, variations of this property do not affect the cell performance. This understanding provides a guideline for the design of hierarchically structured electrodes which yield a high rate capability.

References
[1] Wagner, Amalia Christina, et al. "Hierarchical Structuring of NMC111-Cathode Materials in Lithium-Ion Batteries: An In-Depth Study on the Influence of Primary and Secondary Particle Sizes on Electrochemical Performance." ACS Applied Energy Materials 3.12 (2020): 12565-12574.
[2] Birkholz, Oleg. "Modeling transport properties and electrochemical performance of hierarchically structured lithium-ion battery cathodes using resistor networks and mathematical half-cell models." (2021).