Microstructural adaptation of surfaces for the production of highly specialized functionalities is becoming more and more important in many industrial fields due to significantly enhanced product properties. One of these areas is the microstructure adaptation of lithium-ion battery electrodes, which can be improved in many different ways through the modification. However, in order to be able to scale up processes such as selective surface ablation or geometric structure adaptation, fundamental knowledge of process mechanisms as well as beam-matter interactions is necessary. In the present study, geometric structuring for microstructure adaptation of lithium-ion battery electrodes, were investigated using a fast IR measurement technique. With the help of these investigations, it could be shown how a potential ablation mechanism is taking place. This knowledge can support the transformation of such processes from the laboratory scale to a larger production scale. Composite electrodes were used as material, which consist of a large proportion of graphite and a small proportion of polymer binder.
Carbon fiber reinforced polymer (CFRP) offers the highest potential for lightweight applications due to its excellent weight to strength ratio in comparison to other materials. However, it is cost-intensive and therefore rarely used monolithically. This makes pretreatment and joining processes so important. Hybrid connections of CFRP and metals can be made by riveting, bolting, stir joining and by adhesive. For adhesive bonding, a pretreatment of the materials is necessary. A laser pretreatment has the advantage, that it is automatable and contactless. This paper deals with the pretreatment of CFRP with different laser wavelengths in near-infrared (NIR, 1030 and 1064 nm) and ultraviolet (UV, 355 nm) range. The influence of the processing direction relative to the fiber layer and the influence of the energy density was investigated with a short pulsed NIR infrared laser. In addition, the influence of different surface structures on the mechanical strength was investigated. The treated CFRP surfaces were examined with a light microscope and a scanning electron microscope. The tensile shear strengths were determined using CFRP and aluminum substrates, joined with a 2-part epoxy adhesive. As a comparison, UV laser treated specimens were also mechanically tested. An ultra-short pulsed NIR laser system was used to generate periodic structures on the CFRP to maximize the surface area without fiber damage and breakage. The investigations on the influence of the machining direction with NIR relative to the fiber layer showed insignificant differences in shear strengths. The variation of the energy density showed an influence on the ablation behavior of the CFRP matrix and the mechanical strength. The maximum strength with a cohesive failure in the adhesive was achieved with optimized short pulsed NIR laser parameter.
Lithium-ion batteries applied for example in electric driven vehicles aim towards increased energy density at high active mass loading per unit area. Therefore, calendering processes are used to densify the electrodes. However, a low porosity, especially in top layer of the material compound, leads in general to a low rate (dis-) charging capability due to hindered lithium-ion diffusion. This work proposes a laser surface treatment method of highly densified cathodes to reduce the apparent process limitation of ion diffusion. The surface treatment is done with a short pulsed infrared laser in the nanosecond regime. Depending on the provided energy density in the laser spot the electrochemical inactive matrix of the cathode can be ablated partially and most of the pores below the top layer get reopened. Cathodes with different high densities after calendering are laser treated and electrochemically analyzed. Highly densified cathodes with a porosity of 20% exhibit a distinct improvement of rate capability at C-rates higher than 2C in relation to cathodes without laser treatment. Explicitly, at high current rates of 5C the electrodes of 20% porosity show an improved capacity of more than 20%. In addition, at low current rates the results show no negative impact of the laser treatment. The results lead to the interpretation, that selective laser ablation enables an improved access of Li-ions into the active mass of the cathode. Keywords: Li-ion battery, cathodes, selective laser ablation, rate
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