Recent Advances in Upscaling Ultrafast Laser Structuring of High-Energy Electrodes for Lithium-ion Batteries

Nowadays, battery properties with regard of fast charging, high energy, and high power operation become more sophisticated, especially for automotive applications. In modern lithium-ion cells, composite electrodes, anodes and cathodes with typical film thickness of about 50 μm - 100 μm, are complex multi-material systems that are provided with defined material components, grain sizes, porosities, and pore size distributions down to the nanometer range. State-of-the-art batteries with pouch cell geometry for high energy applications consist of thick film electrode stacks with capacities of single battery pouch cells in the range of 20 - 100 Ah. It is undeniable that ultra-thick film composite electrodes (film thickness > 100 μm) will have to be installed for the next generation batteries and that enormous losses in the available power will have to be accepted if the electrode architecture is not adapted accordingly. Therefore, laser structuring of electrodes (3D battery concept) is getting worldwide increasing attention with regard to pushing lithium-ion battery electrochemical performances beyond state-of-the art technology [1-3]. ... mehrThe 3D battery concept takes a unique approach to realize ultra-thick film electrodes for high-energy batteries that can also be used for high power operation. Roughly speaking, anode structuring is boosting the charging properties while structuring of the cathodes is beneficial with regard to high power operation during discharging of the battery. In addition, structured electrodes - so-called 3D electrodes – may turn electrodes into superwicking for liquid electrolyte [4]. The latter one induces an enormous benefit for process reliability, processing costs, and battery lifetime. However, the bottleneck so far was the laser processing speed which needs to match the electrode coating speed of about 30 m/min. Promising from the perspective of upscaling laser processes is that after years of development, laser slitting, laser notching, and laser cutting of electrodes is finally being introduced in battery manufacturing [5]. The laser structuring of composite electrode is indeed more challenging than the cutting process due to the fact that the ablation process may not induce any structural or chemical modification of the active material particles. Furthermore, the ablation process should be stopped at the current collector surface which is either a thin aluminum (cathode) or copper (anode) foil with a typical thickness of about 20 μm or 8 μm, respectively. The thin current collector foils should not be damaged during ablation process. In addition, in case of electrode structuring the laser scan pathway per electrode footprint is much longer than for any respective electrode cutting process, which means that parallel laser processing via a multi-beam approach becomes necessary. To reach a suitable upscaling approach, a high power ultrafast laser source, an adapted design of the optical system as well as a suitable ablation pattern have to be considered. For the latter, three different types of structures have so far emerged: holes, lines, grids [6]. Simulations based on an electrochemical multi-scale model framework showed similar potential performance improvement for all three structure types methods [7]. In future studies, a multiphysics model must also take into account volume changes and induced mechanical stresses in order to be able to correctly assess influences on mechanical degradation and lithium-ion diffusion kinetics. It is worth mentioning that the type of pattern has an impact on several aspects of battery manufacturing and operation. Line patterns seem to be more powerful as capillary structures for electrode wetting with liquid electrolyte, while grid patterns are most suitable to counteract volume expansion or shrinkage of active materials during lithiation or delithiation. In case of the next generation anode material silicon, a volume change of about 300 % has to be considered and hereby 3D electrodes with grid patterns delivered the best electrochemical performance data [8].