Enhancing the Performance of Lithium-Ion Batteries with Structured Electrodes
Authors: Kraft Ludwig , Kriegler Johannes , Habedank Jan Bernd , Jossen Andreas 
Published: December 2020

State-of-the-art electric vehicles (EVs) rely on lithium-ion batteries with high energy densities, offering a sufficient driving range and thereby gaining the customer’s acceptance. In order to further increase the energy density, the current trend goes towards larger cell formats and active materials incorporating higher specific capacities. Thicker and denser electrodes also offer an opportunity to increase the required energy density independently of the electrode’s composition, but face polarization issues if high current loads are applied. One major contributor are mass transport limitations in the liquid electrolyte as evoked during fast-charging scenarios in EVs.

This effect is especially pronounced in graphite anodes with flake-like particles, which result in highly tortuous pathways in the through-plane direction of the electrode. The thicker the electrodes, the less the utilization of the parts close to the current collector. Structuring of electrodes helps to overcome this limitation by providing enhanced transport pathways for the lithium-ions. In our case, a laser beam was used to induce hole-like structures in graphite anodes in a hexagonal pattern, depicted in Fig. 1 (a). NMC111/graphite coin cells comprising unstructured and structures anodes were built and tested in a discharge rate capability test, see Fig. 1 (b). The performance increase of cells comprising structured anodes can be clearly seen for discharge currents higher than 2.5 mA cm-2 or 1C.

The underlying mechanisms for the performance increase were identified by means of modeling and simulation. At low discharge rates, there is merely an influence of the structuring process, because the lithium-ion transport in the electrolyte is not limiting the discharge. For rising currents, lithium-ion gradients in the graphite anode build up and limit the discharge process. Structured anodes with a reduced tortuosity enhance the lithium-ion transport, decrease the resulting overpotentials and more capacity can be discharged from the cell until the cutoff voltage is reached. If the discharge current is further increased, the positive effect of laser-structuring fades and other limitations like electrolyte depletion in the cathode become more dominant. To conclude, there is a restricted C-rate window that can be positively affected by electrode structuring.

The same impact was detected for charging procedures, where decreased overpotentials resulted in a better fast charging performance for cells comprising structured graphite anodes. More capacity can be charged until a defined cutoff potential is reached, resulting in shorter charging times with an increased safety buffer. This makes hazardous lithium-plating less likely and extends the lifetime of lithium-ion batteries. To sum up, structured electrodes especially enhance the performance of high power and fast charging applications and should be considered for designing the next generation of lithium-ion batteries.

This work was financially supported by the German Federal Ministry of Education and Research (BMBF) in the project ExZellTUM II (03XP0081) and the follow-up project ExZellTUM III (03XP0255).

References:

Habedank et al. (2018), Increasing the Discharge Rate Capability of Lithium-Ion Cells with Laser-Structured Graphite Anodes: Modeling and Simulation, J. Electrochem. Soc. 165 (7) A1563-A1573.

Kraft et al. (2020), Modeling and Simulation of Pore Morphology Modifications using
Laser-Structured Graphite Anodes in Lithium-Ion Batteries, J. Electrochem. Soc. 167 013506.

Habedank et al. (2019), Enhanced Fast Charging and Reduced Lithium-Plating by Laser-Structured Anodes for Lithium-Ion Batteries, J. Electrochem. Soc. 166 (16) A3940-A3949.

Figure 1