Microstructuring of Electrodes for Li-Ion Batteries By Means of Laser Machining and Microembossing – a Promising Approach to Overcome Transport Limitations and to Improve Their Fast Charging Capability

In addition to costs, safety and durability, energy density and fast-charging capability are the key requirements for Li-ion batteries for automotive applications. To meet these requirements, new cell chemistries are being developed, such as silicon-containing anodes. Likewise, electrode designs with high mass loading, respectively high layer thickness and a high density are being researched. However, such high-energy electrodes exhibit significant transport resistances due to their design, which has a negative effect on the electrode kinetics and thus the fast-charging capability. In our presentation, we will use the example of NMC-based cathodes and graphite anodes to show how the electrode kinetics can be improved by systematically influencing the electrode microstructure and the electrode surface. Short and ultra-short pulsed near infrared laser processes as well as mechanical embossing methods using needle type tools are used. In doing so, near-surface “quasi 2D” modification and three-dimensional structuring of the electrodes active mass were investigated. We will demonstrate how a specific adjustment of the laser parameters e.g. energy per unit length allows a roughening of the electrode surface and a selective removal of the binder-conductive additive compound without damaging or significantly ablating the active material. This “quasi 2D” modification offers the advantage that the capacity is largely retained. Electrochemical analyses such as rate tests show an improvement of up to 25% in the fast-charging capability. Electrochemical impedance analyses using symmetrical cells underline these findings by revealing a reduced transport resistance. In tendency, the higher the laser energy input and surface roughness, the more pronounced the observed effects – with a loss of active material also being observed at high laser input. Comparatively, perforated (i.e. three-dimensionally microstructured) electrodes, are investigated. The results obtained are comparable to the surface-near modification. However, in the case of perforation by laser, about 10% of active material is removed, which is reflected in a corresponding loss of capacitance. Electrodes perforated by microembossing also show significantly improved electrode kinetics, but without material removal and capacity loss. This makes microembossing appear to be an interesting and alternative microstructuring process. Finally, the combination of both processes – “quasi 2D” plus 3D – is investigated to combine the beneficial effects of both. The differently processed electrodes were also subjected to various rapid aging tests such as 100 charge/discharge cycles with a current mode of C/3 as well as interspersed fast charging cycles. The modified graphite anodes exhibit a lower tendency to Li plating than unprocessed reference anodes, and the degradation is lower than for the reference electrodes. For instance, the perforated anodes exhibited a capacity retention of 96%. In addition to a significant improvement of the electrochemical performance, a clear enhancement of the electrolyte wetting is demonstrated. Droplet tests and rising height tests showed an improved wettability by a factor of 4 to 5, depending on the type of microstructuring. The process development and the electrochemical analyses were closely accompanied by microscopic and material analyses e.g. by means of white light interferometry, nano-CT, SEM and TEM as well as Raman spectroscopy to reveal the most important process-microstructure-property relationships. To sum it up, we believe that the presented microstructural modifications are very attractive – both, in terms of electrochemical performance as well as from a manufacturing point of view. Figure 1