Picosecond laser structuring of graphite anodes—Ablation characteristics and process scaling

Lithium-ion batteries are the dominant energy storage solution for many applications, such as portable consumer electronics or electromobility. However, the design of automotive lithium-ion batteries is subject to a conflict of interest between a high energy density, enabling a high range capability of an electric vehicle on the one hand and a high power density facilitating fast charging and discharging on the other hand.¹ Laser structuring is a novel process that alleviates this conflict of interests by facilitating lithium-ion diffusion through microscopic channels in the electrodes.^2–4 The channels can be realized as grid,^5,6 line,^7,8 or hole structures.^9,10 It has been demonstrated that laser electrode structuring significantly improves the fast charging capability of lithium-ion batteries without cell degradation¹¹ and extends their lifespan.^9,10 These benefits are particularly noticeable when using laser structuring on graphite anodes¹² and thick¹³ or highly compacted electrodes.¹⁴ Additionally, laser structuring can speed up the filling process of lithium-ion batteries by accelerating the electrolyte wetting of the electrodes.^8,15,16

The rise of electromobility is currently leading to the development of large production capacities for lithium-ion batteries, especially in the United States, Europe, and Asia.¹⁷ While laser drying of electrode coatings,^18,19 laser cutting of electrodes,^20,21 and laser welding of current collector foils^22,23 are established processes in battery production, laser electrode structuring has not progressed beyond the laboratory scale due to several production-related challenges. Firstly, the integration of laser structuring into the manufacturing chain of electrode production is complex and challenging, with the different integration options having various advantages and disadvantages.²⁴ Furthermore, the interdependencies between laser processing parameters and the resulting structure geometries are convoluted and, thus, complicate the process design. Process studies on laser electrode structuring of graphite anodes have been published for femtosecond pulsed²⁵ and picosecond pulsed laser radiation.²⁶ However, the influence of processing parameters such as the laser wavelength or the pulse repetition rate (PRR) remains ambiguous for picosecond pulsed laser structuring. Also, laser processing with pulse burst (PB) modes, known to yield a high material removal from other applications,^27–30 has not been examined for laser electrode structuring. The effect of PBs on the delamination width and maximum cutting speed in laser cutting of graphite–copper–graphite electrodes was investigated by Huang et al.³¹ However, cutting fundamentally differentiates from laser microdrilling as the process is governed by the need to penetrate the copper current collector, which possesses a significantly higher ablation threshold than graphite.³² Moreover, existing studies on laser structuring of battery electrodes have focused solely on ablation depth, with little attention paid to hole diameters. Lastly, a scale-up of laser electrode structuring to meet the throughput of industrial battery production lines is necessary to make the process economically viable.³³ Recently, the application of polygon scanning units,³⁴ roll-to-roll high-speed processing using hollow cylinders,³⁵ and direct laser interference patterning³⁶ have been proposed to increase the productivity of laser electrode structuring. Beam splitting, for example, using a diffractive optical element (DOE), is an attractive option for the simultaneous processing of large sample areas,^37–39 which has not yet been explored for laser structuring of electrodes.

This work deepens the knowledge of the ablation characteristics of laser electrode structuring by experimentally examining the interplay between laser parameters and the resulting product properties in a comprehensive process study. Special attention is paid to previously unregarded parameters, such as the laser wavelength or the laser PRR, including PBs as well as the electrode hole diameters. Furthermore, the feasibility of scaling up the laser electrode structuring process is evaluated by combining experimental and theoretical approaches. A DOE is initially applied for multibeam laser structuring of graphite anodes, and a modeling approach is used to evaluate the industrial process feasibility as a function of product and process parameters. The obtained results support the future implementation of laser electrode structuring in industrial battery production lines.

In this study, state-of-the-art commercial double-side coated graphite anodes (Customcells Itzehoe, Germany) on copper foil were used as a substrate for laser structuring. The properties of the calendered electrodes are listed in Table I.

The experiments were conducted using an ultrashort pulsed laser source (PicoBlade^TM3, Lumentum, USA) emitting laser radiation with a pulse duration of 10 ps at an infrared wavelength of 1064 nm. Visible and ultraviolet radiation at wavelengths of 532 nm and 355 nm was created with nonlinear optical elements inside the laser source, generating the second and third harmonics, respectively. Individual optical setups with different focusing lenses were used for each wavelength resulting in almost equal spot radii to enable a comparison of the processing results across different wavelengths. The beam waists were calculated based on beam propagation data provided by the laser source manufacturer and experimentally confirmed using the method proposed by Liu.⁴⁰ The laser powers were measured with a power meter (FieldMax II & PowerMax, Coherent, USA) after the scanning unit. Table II provides a summary of the laser system characteristics, while Fig. 1(a) schematically depicts the setup.

For the multibeam laser structuring of graphite anodes, a DOE (DBS-3x7-1064-0.1752, Topag Lasertechnik, Germany) creating a matrix of 3 × 7 subbeams was placed in the beam path [compare Fig. 1(b)]. A hole distance of approximately 312 μm on the substrate resulted. The beam matrices were positioned adjacent to each other until the whole area comprising 2 × 2 cm² was filled with drillings [compare Fig. 1(c)]. A final hole spacing of approximately 104 μm was realized by superimposing the beam matrix three times in each spatial direction, i.e., nine times in total. The DOE matrices did not initially overlap to avoid excessive heating of the workpiece in the processed area. An infrared (1064 nm) laser beam source (PicoBlade^TM3, Lumentum, USA) was used at its maximum power of 140 W on the workpiece and a PRR of 400 kHz resulting in pulse energies of 350 μJ. Considering the DOE efficiency stated by the manufacturer of 76.6 %, a laser power of approximately 107.2 W resulted. Thus, each subbeam contained a pulse energy of approximately 12.9 μJ. Eighty pulses per hole were applied. Graphite anodes were structured with similar laser processing parameters for benchmarking purposes using a singlebeam scanning strategy following a hole-by-hole approach. Five samples each were manufactured for singlebeam and multibeam laser structuring to allow statistical statements.

All laser-structured electrode surfaces were characterized with 3D laser-scanning confocal microscopy (LSM) (VK-X1000, Keyence, Japan). The confocal laser height measurement method and an objective with 20-fold magnification corresponding to a total magnification of 480 were applied. The hole diameters and depths were determined from the created topography files using a previously developed method for the automated characterization of laser-structured electrodes.⁴¹ In addition, digital light microscopy images (VHX-7000, Keyence, Japan) were taken for a qualitative visual inspection of the electrode surfaces. Scanning electron microscopy (SEM) (JSM-IT200, Jeol, Japan) was used to study the surface morphology and hole characteristics in detail. Electrodes processed for Sec. III B were tested regarding their mechanical integrity to assess potential damage to the material by excessive heat input. Pull-off tests were conducted using a universal tensile test machine (ProLine Z050, ZwickRoell, Germany) with a 1 kN load cell (Xforce HP, ZwickRoell, Germany). The anodes with a diameter of 15 mm were placed in the center of the sample holder between a top and a bottom die, equipped with adhesion tapes measuring 14 and 9 mm in diameter, respectively. The different diameters ensured a failure of the electrodes on their structured, upward-facing side. Based on previous studies,^24,42 the test procedure consisted of compression with a feed of 0.75 mm min⁻¹ until a pressure of 600 kPa was reached, a 30 s dwell phase at 600 kPa, and testing with a feed of 100 mm min⁻¹ until material failure.

The production costs of laser electrode structuring were estimated based on existing cost models for lithium-ion battery production.^43–45 Details of the cost calculation performed in this study can be found in the supplementary material (see Supplementary Material⁴⁶).

In this process study, the parameters wavelength, pulse repetition rate, fluence, and pulses per hole were varied, resulting in various laser powers and accumulated energies per hole. Figure 2 visualizes the influence of the laser processing parameters on the structure geometries. A constant pulse duration of 10 ps was applied, as a prior study indicated that varying the pulse duration within the picosecond regime has no noteworthy impact on laser electrode structuring.²⁶ Laser electrode structuring aims to create holes with a high aspect ratio, i.e., small hole diameters and large hole depths, to reduce the loss of electrode active material while decreasing the electrode tortuosity.^47,48 Consequently, the most desirable structure geometries can be found toward the bottom right of the representations in Fig. 2. Hole depths up to the coating thickness of approximately 70 μm (compare Table I) and hole diameters up to approximately 90 μm were created in the process study. Only a few outliers are present in Fig. 2, presumably resulting from the misclassification of very shallow holes by the automated geometry characterization.⁴¹ 

In the process study, it was found that the influence of the wavelength on the structure geometries was limited, as the color regimes largely overlap in Fig. 2(a). However, visible (532 nm) and ultraviolet (355 nm) wavelengths enabled higher aspect ratios for some parameter combinations as the absorption coefficient of graphite rises for decreasing wavelengths.⁴⁹ While the laser radiation is mainly absorbed by the graphite, the material ablation is governed by the evaporation of the binder additives possessing a significantly lower evaporation temperature than graphite.⁵⁰ Ultraviolet radiation yielded slightly lower hole diameters than longer wavelengths [compare Fig. 3(a)], which is attributed to increased absorption of radiation by the polymeric binders at low wavelengths.^51,52 A reduced heat transfer might result, as previously reported for laser material processing of carbon-fiber-reinforced polymers with low wavelengths.⁵³ A visual impact of different wavelengths on the electrodes’ microstructures or potential heat-affected zones was not noticed in SEM images (compare Supplementary Material⁴⁶).

Considering Fig. 2(b), it becomes evident that low wavelengths mainly improved the hole aspect ratios if combined with the PB mode. An increase in the ablation efficiency by the PB mode has been observed in previous studies with graphite anodes on copper foil,³¹ silicon,²⁷ or copper.²⁸ PRRs without the PB mode of 200 and 1000 kHz were compared, with the latter representing a potential route toward process scaling. However, lower hole depths were achieved at a pulse repetition rate of 1000 kHz than for 200 kHz, resulting in lower aspect ratios [compare Fig. 2(b)]. Hence, shielding effects, which are known from ultrashort pulsed laser processing of other materials, such as steel^54,55 and copper,⁵⁶ seem to have occurred at 1000 kHz. In SEM images, no microstructural differences were observed between 200 and 1000 kHz (compare Supplementary Material⁴⁶). The application of PBs with five pulses at 200 kHz theoretically enables the same processing speed as single pulses at 1000 kHz since the number of pulses with similar energy per time interval is equal. The holes created with the PB mode resulted in holes with a higher aspect ratio [compare Fig. 2(b)] in accordance with the literature.^28,30 Previous studies have attributed the effect to a laser-induced heat accumulation in the PB mode⁵⁷ or the reduction in particle/plasma shielding in PBs of three or more pulses.⁵⁸ Hence, the utilization of PBs with MHz intraburst spacing represents an attractive option for the process scaling of laser structuring without negative implication for the electrodes’ microstructures (compare Supplementary Material⁴⁶).

From Fig. 2(c), an increase in the hole diameters and depths with rising fluences is evident, as previously reported in the literature.^25,26 The fluences correlated approximately linearly with the hole diameters and depths, respectively [compare Figs. 3(a) and 3(c)]. Due to the nearly similar focal spot diameters throughout the process study (compare Table II), the fluences shown in Fig. 2(c) behave analogously to the applied pulse energies. Drilling with low fluences is known to yield a low heat input resulting in a small heat-affected zone and thus, small diameters from the laser processing of other materials such as metals⁵⁹ or carbon.^60,61 Following this, high aspect ratios were observed at low fluences in this study.

A rising number of pulses per hole increased both the hole diameters and depths [compare Fig. 2(d)]. In accordance with the literature,²⁶ an asymptotic progression was observed, especially for the hole depths at high numbers of pulses per hole [compare Figs. 3(b) and 3(d)]. The behavior is attributed to the cone-like shaped holes created during advancing drilling resulting in an enlarged surface area penetrated by the laser beam.^62,63 Hence, the effective fluence is decreased at large hole depths.

The deposited energy per hole, calculated as the product of the pulse energy and the pulses per hole, predominantly correlated with the hole diameters. The characteristic can be seen as the color shade mainly changes vertically in Fig. 2(e). In contrast, data points of different depths at a certain diameter vary in color in Fig. 2(e). Hence, the hole depth was less affected by the deposited energy per hole than the hole diameters. Consequently, process strategies reducing the energy per hole are attractive for geometry optimization as they lead to larger hole aspect ratios and simultaneously allow the depth ablation efficiency to be increased, facilitating process scaling.

Figure 2(f) depicts the correlation of the hole geometries with the laser power, equivalent to the product of the pulse energy and the PRR. In this process study, high laser powers were achieved by using high single pulse fluences [compare Fig. 2(c)], resulting in unfavorable holes of large diameter and comparatively low depth.

In general, a certain depth of the laser-structured holes, typically only slightly less than the coating thickness, is desired to facilitate lithium-ion diffusion down to the current collector.⁴⁷ Hence, for the used material system of 70 μm thickness (compare Table I), hole depths above 56 μm corresponding to 80 % of the coating thickness can be considered as suitable. Furthermore, the hole diameter should be minimal, i.e., the aspect ratio as high as possible, to reduce the loss of active material. Holes with a depth over 56 μm and an aspect ratio over 1.5 can be found within the borders of the dotted ellipse in Fig. 2. One well-suited process parameter set from the parameter window is the combination of 532 nm wavelength, the 200 kHz PB mode, approximately 10.1 J cm⁻² fluence, and 80 pulses per hole (marked by a star symbol in Fig. 2). The obtained laser-structured graphite anode [compare Fig. 4(a)] possessed precise and uniform holes of (35.3 ± 2.3) μm in diameter and (61.2 ± 6.2) μm in depth. From Fig. 4(b) no burrs or surface elevations around the holes are visually discernible. In close-up images of the inner hole surfaces [compare Fig. 4(c)], no melt formations are visible due to the absence of a molten phase of graphite at ambient pressure.^64,65 Furthermore, partially perforated particles were not observed, indicating that the ablation process is governed by the expulsion of entire particles due to binder evaporation as previously suspected in the literature.^14,50 The behavior results in a stochastic process accounting for the large standard deviation observed in the hole geometries (compare Fig. 3).

Beam splitting is a promising approach for increasing the processing rate of laser electrode structuring. In this study, a reduction in the total processing time by approximately 88 % (from approximately 13.0 s to approximately 1.5 s for 4 cm²) compared to singlebeam structuring could be achieved by incorporating a DOE creating 21 subbeams. The time was not reduced to $1/21$ of its initial value as it features jump times between the matrix locations [compare Fig. 1(c)], during which the laser does not emit. The use of multibeam processing typically raises concerns regarding the potential for heat-induced stress to the substrate as larger powers are applied simultaneously to large areas. Thus, electrodes structured using singlebeam and multibeam scanning strategies with the same process parameters were examined in contrast (compare Fig. 5). For fast processing, the laser source was used at its maximum power of 140 W at 1064 nm and 400 kHz in the multibeam approach, resulting in a singlebeam pulse energy of 12.9 μJ considering the DOE efficiency (compare Sec. II D). Based on the outcomes of the conducted process study (compare Sec. III A), 80 pulses per hole were chosen. In Fig. 5(d), a regular pattern of dark spots can be noticed on the surfaces of multibeam processed electrodes, which were not apparent in singlebeam laser-structured electrodes [compare Fig. 5(a)] and resemble the orientation of the DOE beam matrices [compare Fig. 1(b)]. Yet, at high resolutions [compare the inset of Fig. 5(d)], no impact on the holes or surrounding surfaces was visually discernible. The analysis of the hole geometries using LSM between darker and brighter areas (compare Fig. 6) disclosed slightly larger hole diameters and depths in the dark regions. The effect could result from a locally increased heat accumulation during scanning at the edges between two adjacent beam matrices [compare Fig. 1(c)] or higher diffractive orders of the DOE. Although a DOE reduces the pulse energies effectively used for laser drilling (compare the stated DOE efficiency of 76.6 % in Sec. II D), nearly the full average power is transmitted through the DOE and penetrates the substrate.⁶⁶ The behavior might superficially ablate electrode additives such as binder or conductive additives,⁶⁷ which has been shown to result in electrochemical performance improvements.^68,69

The LSM analysis of approximately 300 holes across different sample areas revealed no significant geometric differences between singlebeam and multibeam laser structuring (compare Fig. 7). In the latter, the holes possessed slightly lower average hole diameters [compare Figs. 7(a) and 7(c)] and depths [compare Figs. 7(b) and 7(d)]. Since the deviations were below the respective standard deviations, it is concluded that the use of a DOE for multibeam laser structuring did not impair the created structure geometries. Furthermore, similar hole shapes [compare Figs. 5(b) and 5(e)] and microstructures [compare Figs. 5(c) and 5(f)] were observed in high-resolution SEM images of singlebeam and multibeam structured electrodes, respectively. The results also render multibeam laser structuring feasible for the processing of other multimaterial systems, such as fuel cell diffusion membranes.⁶¹ 

Tear-off tests of the electrode coatings were conducted to evaluate a potential deterioration of the mechanical integrity of graphite anodes by the laser-induced heat input. The results presented in Table IV demonstrate that the tear-off stresses recorded for singlebeam and multibeam structured graphite anodes were remarkably similar. Indeed, the discrepancy between the tear-off stresses for the two configurations was considerably lower than the standard deviation observed for the five measured samples per configuration. Adhesion failures between the electrode coatings and the current collector foils were the predominant failure mode in both configurations, while cohesion failures barely occurred in the test. The absolute tear-off stresses exceeded previously reported values for battery electrodes,^24,42 which is attributed to material deviations, such as the particle size or the amount of binder additive. Furthermore, the low surface area used in this study (compare Sec. II E) could have resulted in higher absolute tear-off stresses. A reduction of the tear-off stresses in laser-structured electrodes compared to their unstructured counterparts, which was previously reported for nanosecond laser structuring of graphite anodes,²⁴ was not observed in this study (compare Table IV). Based on the tear-off tests, it can be inferred that multibeam electrode structuring with picosecond pulsed laser radiation did not negatively affect the mechanical electrode integrity.

For the industrialization of laser electrode structuring, the process has to approach the throughput in industrial battery production. The potential processing rates of laser electrode structuring in dependence on the laser power and the chosen hole distance were determined for quadratic and hexagonal hole patterns, respectively, using Eq. (3) (compare Fig. 8). In the calculation, it is assumed that the entire laser power is used for electrode structuring, which requires beam splitting to ensure an acceptable hole geometry (compare Sec. III B). Based on the findings from the conducted laser process study (compare Sec. III A), an accumulated energy per hole of 1 mJ was considered. Reducing the assumed energy per hole would linearly increase the obtained processing rates but result in shallower holes. Figure 8 reveals a high dependence of the achievable processing rates on the choice of the geometrical pattern and the hole distance. In the hexagonal hole pattern, the processing rate at a particular hole distance is lower than for the quadratic hole arrangement due to the smaller area associated with one hole [compare Fig. 1(d)]. However, the pattern design substantially influences the electrochemical performance in laser-structured electrodes.⁷⁰ Furthermore, an increased hole distance was shown to reduce the electrochemical benefits of laser structuring.^47,48 Hole distances of less than approximately 200 μm were experimentally proven to yield a high electrochemical performance increase for commercially relevant electrode materials.^9,10,71 Hence, the productivity of the laser structuring process on the one hand and the performance enhancement on the other hand are in conflict with each other and must be balanced.

For structuring one side of an electrode of 1 m width and assuming a web speed of 1 m s⁻¹ in production,⁷² the required processing rate is 1 m² s⁻¹. For laser structuring of double-side coated electrodes at least two separate setups are necessary, doubling the required processing rate. Figure 8(b) shows that laser powers in the two-digit kilowatt regime are required in this scenario to achieve a hexagonal hole distance of less than approximately 500 μm. While ultrashort pulsed laser sources with kilowatt powers have recently transitioned from scientific to industrial accessibility, laser sources with multikilowatt average powers are expected to be commercially available within a decade.⁷³ Alternatively, parallelization of multiple beam sources of lower average power is a feasible approach to achieve the required processing rates. In the idealistic consideration of Fig. 8, the laser source is expected to emit radiation continuously, i.e., laser-off times during jumps between holes are disregarded. Hence, actual processing rates will be lower than those stated in Fig. 8. The challenges associated with high-speed beam deflection for ultrafast pulsed laser material processing are known from other applications^74,75 and have been previously discussed by Habedank et al. for laser electrode structuring.³⁴ 

The economic competitiveness of industrial-scale laser electrode structuring was assessed based on an exemplary production scenario assuming a factory located in Germany (see Supplementary Material⁴⁶). From the electrode throughput of 1 m s⁻¹ stated above and the material system used in this study (compare Table I), an annual cell production capacity of approximately 5.3 GW h results, which is in the range of currently projected small giga factories.¹⁷ In the scenario, total costs of approximately 1.96 $/kWh arise from laser anode structuring with plant acquisition and labor expenses being major contributors. Considering total costs of approximately 152 $/kWh for lithium-ion batteries manufactured in Germany,⁷⁶ this corresponds to a cost increase of approximately 1.3 %. The expenses for laser electrode structuring are expected to be inferior in other production sites such as the United States or China due to lower costs for labor and energy. Furthermore, falling prices for ultrashort-pulsed laser beam sources with high average power will promote a decrease in investment costs for laser structuring in the next years. Considering that laser structuring enables significant cell capacity increases at elevated current rates,^10,14 accelerated fast charging,^9,11 reduced lithium-plating,^10,11 and remarkably prolonged cell lifetimes,^9,10 and the associated costs appear economically bearable.

Despite its proven electrochemical benefits, laser electrode structuring is currently restricted to the laboratory scale due to limited process comprehension and scaling issues. In this publication, the results of a comprehensive experimental process study were presented, providing insights into interdependencies between laser processing parameters and the resulting structure geometries. Favorable results, i.e., high hole aspect ratios at an acceptable hole depth, were achieved by applying

• low wavelengths of 532 or 355 nm,

• a burst of five pulses at a burst repetition rate of 200 kHz, and

• pulse peak fluences of approximately 10 J cm⁻².

The challenges associated with the design and scale-up of laser electrode structuring have prompted the exploration of alternative processes with higher throughput, such as mechanical embossing.^77–79 Future research will need to focus on the comparative evaluation of laser structuring and processing alternatives. Benchmark studies should holistically consider product aspects, such as the electrochemical battery performance, mechanical electrode integrity, obtainable structure geometries, and production-related aspects, such as process flexibility, throughput, and precision.

This work has been funded by the German Federal Ministry of Education and Research (BMBF) under Grant No. 03XP03316B (LaserScale). The support of Lumentum Switzerland AG with expertise and laser system technology for electrode structuring is gratefully appreciated. The authors thank Philipp Senft and Nicolas Rodriguez for conducting the laser-scanning microscopy analysis of the electrode surfaces. The cost estimation of laser electrode structuring is based on preliminary works by Felix Dümig and Benedikt Eckardt, which are thankfully acknowledged.

The authors have no conflicts to disclose.

Lucas Hille: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Johannes Kriegler: Conceptualization (equal); Investigation (equal); Methodology (equal); Validation (equal); Writing – review & editing (equal). Andreas Oehler: Investigation (equal); Resources (equal); Validation (equal); Writing – review & editing (equal). Michalina Chaja: Investigation (equal); Resources (equal); Validation (equal); Writing – review & editing (equal). Sebastian Wagner: Investigation (equal); Writing – review & editing (equal). Michael F. Zaeh: Funding acquisition (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.