Abstract
Laser structuring of battery materials such as electrodes, current collectors, and separator materials becomes more and more a versatile tool for a flexible designing of battery performance with regard to high rate capability, reduced impedance and diffusion over-potential, enhanced battery lifetime, and a tremendous improvement of electrode wetting with liquid electrolyte. Especially structuring of thick film composite electrodes with film thickness up to 300 µm is of great interest for combining high energy and high power applications. Laser ablation by using ultrafast laser radiation ensures a high aspect ratio structuring without damaging or modifying the material properties of the active material, inactive compounds, and current collector. Thermal induced material modification and laser-induced material splatter on top of the electrode surfaces have been so far a major problem by using conventional nanosecond lasers. However, in recent years high power OEM-type ultrafast laser beam sources became available for industrial production. Those lasers provide powers of several 100 W up to the kW-regime with repetition rates of several tenfold MHz, which makes high-speed parallel material processing feasible. Finally, after introducing high-speed nanosecond laser cutting of electrodes almost one decade ago, we forecast that with regard to recent technical approaches quite soon high-speed electrode ultrafast laser structuring will entering the pilot line level. For this purpose, at KIT, parallel processing of cathode and anode materials with large foot print area is being developed in a roll-to-roll (R2R) environment in order to meet the process speed requirements of cell production. Furthermore, we are developing thick film electrode concepts for anodes made of graphite and silicon-graphite and cathodes based on Lithium-Nickel-Manganese-Cobalt-Oxide (NMC) with varying nickel content and water-based slurries. Electrode thicknesses in the range of 100-300 µm were synthesized for subsequent laser processing, cell assembly, and electrochemical characterization such as galvanostatic measurements, galvanostatic intermittent titration technique, cyclic voltammetry, and electrochemical impedance spectroscopy. Large area laser structuring in R2R environment for generation of three-dimensional (3D) electrode architectures with increased active surface area, enhanced mechanical integrity for high energy materials such as Si-graphite anodes and an overall boost in electrochemical performances on pouch cell level will be presented. Laser-induced breakdown spectroscopy (LIBS) was applied for elemental mapping of entire electrodes in order to characterize binder migration in ultra-thick film electrodes, and in post-mortem studies to evaluate degradation processes in unstructured, damaged, and structured thick film electrodes with large footprint (pouch cell level). Post-mortem studies show that significant chemical modifications across from electrodes with macroscopic surface defects could be detected while laser structured electrodes provide less inhomogeneity in lithium distribution in comparison to cells with unstructured electrodes. Furthermore, an inhomogeneity in mechanical load on cells could be identified as a main reason for cell degradation. LIBS is presented as a perfect tool to rapidly characterize and identify starting points for cell degradation on coin cell and pouch cell level. In summary, ultrafast laser structuring of electrodes provides a new manufacturing tool for next generation battery production to overcome current limitations in electrode design and cell performances.