The Effect of Wavelength on the Laser Structuring of Graphite-Based Anodes for Lithium-Ion Batteries

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The use of laser-based processes in battery manufacturing is becoming more and more common [1]. The majority of the research and applications in this field focus on pulsed nanosecond lasers, primarily due to a lower capital cost. There are potential applications for shorter pulse length lasers in battery manufacture due to the change in ablation regime, from a thermal to a cold ablation process that results in significantly less damage to the structure of the batteryIn this research, a femtosecond laser has been used to cut and structure graphite anodes for coin cell batteries. The lasers fundamental emission wavelength is in the infra-red region at 1029nm and it has a maximum average power of 40W. The same laser is also capable of operating at the second (515nm) and third (343nm) harmonics, with a maximum average power of 20W and 10W respectively. This allowed for a direct comparison of how modifying the wavelength of the laser affects the structuring of the anode material, in this instance a graphite coating with an SBR-PVDF binder, with a specific focus on any amorphisation of the crystalline structure which may occur during the structuring process. A further comparison of optimised processes at each wavelength, with respect to time, are presented.A reduction in amorphization at all wavelengths was observed for both cutting and structuring graphitic material when compared to longer pulse length lasers. These findings could allow the use of lasers in creating 3D structured electrodes without the loss in performance of the final cell that is observed when using nanosecond lasers [3]. It has been shown that the creation of structures in the electrode material of a lithium ion battery can improve a cells stability under higher C-rates due to an improvement in ion diffusion kinetics by increasing the surface area of the active material [4]. As well as this, structuring can aid in the wetting of electrolytes into active materials, this is especially prevalent when looking into thick film electrodes such as those over 50µm [5].It has been shown that the specific capacity of a cell with a thick-film NMC electrode drops with increasing thickness, even for C rates as low as C/5 [1]. Laser structuring helps alleviate the problem by increasing the surface area of the active material and while this is done by removing some of said material, it can still lead to an increase in the effective capacity of a constructed cell. While some studies have investigated the machining of high aspect ratio micro-channels, the effect of wavelength on laser structuring of anodes with ultrafast lasers has not been thoroughly investigated until now.With more information on how various laser based structuring techniques effects the electrode material further improvements can be made not only to the individual performance of cells but the manufacturing process of lithium ion batteries in general.[1] W. Pfleging, “A review of laser electrode processing for development and manufacturing of lithium-ion batteries,” Nanophotonics, vol. 7, no. 3, pp. 549–573, Feb. 2018, doi: 10.1515/nanoph-2017-0044.[2] T. Jansen, M. W. Kandula, S. Hartwig, L. Hoffmann, W. Haselrieder, and K. Dilger, “Influence of Laser-Generated Cutting Edges on the Electrical Performance of Large Lithium-Ion Pouch Cells,” Batteries, vol. 5, no. 4, p. 73, Dec. 2019, doi: 10.3390/batteries5040073.[3] M. Mangang, H. J. Seifert, and W. Pfleging, “Influence of laser pulse duration on the electrochemical performance of laser structured LiFePO 4 composite electrodes,” J. Power Sources, vol. 304, pp. 24–32, 2016, doi: 10.1016/j.jpowsour.2015.10.086.[4] P. Smyrek, Y. Zheng, H. J. Seifert, and W. Pfleging, “Post-mortem characterization of fs laser-generated micro-pillars in Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 electrodes by laser-induced breakdown spectroscopy,” 2016, no. March 2016, p. 97361C, doi: 10.1117/12.2210815.[5] W. Pfleging and J. Pröll, “A new approach for rapid electrolyte wetting in tape cast electrodes for lithium-ion batteries,” J. Mater. Chem. A, vol. 2, no. 36, pp. 14918–14926, 2014, doi: 10.1039/C4TA02353F.