High-Energy and High-Power Electrodes Screened by Microstructure Reconstruction and Model Simulation

Even though it is well-known that high-power and high-energy lithium ion cells require different electrode design rules, conventional analysis methods fail to quantify the key properties of their microstructures. To shed light on the differences between commercial high-power and high-energy cells, their electrode microstructures are analyzed in detail using focused ion beam and micro X-ray tomography. The cells under investigation are a 18650 LiFePO4/graphite power cell and a 18500 LiCoO2/graphite energy cell.

A three-dimensional reconstruction of anodes and cathodes allows for specifying their tailor-made microstructural characteristics. Beside the basic structural parameters (a) volume fractions, (b) surface areas, (c) average particle sizes and (d) tortuosity, a more advanced analysis yields additional information such as: (e) particle size distributions, (f) space-resolved tortuosity values (g) a detailed analysis of different secondary-particle classes.

Based on this array of microstructural information, the specific characteristics of all electrodes are identified with respect to cell performance. This newly gained insight is backed up by model simulations. These are based on a refined homogenized porous electrode model, which is parameterized with the structural parameters extracted from the reconstructions.

This combined approach gives a high-definition picture of tailor-made electrodes and facilitates understanding of rate-limiting mechanisms in high-power and high-energy electrodes. During charging and discharging, the electrochemically active zone spreads out over the entire electrode thickness within the high-power cell. Contrary, it is spatially restricted in both electrodes of the high-energy cell and migrates from the separator towards the current collector. Model simulations show, that this behavior is caused by (i) the lower electrode porosity and (ii) the larger electrode thickness of the high-energy cell. Furthermore, model simulations show, that the maximum discharge current of both cathodes (16C and 4C) is restricted by a depletion limitation of the electrolyte.  

 

Figure 1: Three-dimensional reconstructed microstructures of the high-power cell (a, c) and the high-energy cell (b, d). For the anodes (a, b), the graphite phase is shown in blue with the porosity appearing transparent. For the LiFePO₄ cathode (c), the two different agglomerate classes are shown in different shades of green, the carbon additive is shown in gray. For the LiCoO₂ cathode (d), the active material is also shown in green and the carbon in gray. The porosity in both cathodes appears transparent.