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Article Influence of Laser-Generated Cutting Edges on the Electrical Performance of Large Lithium-Ion Pouch Cells

Tobias Jansen 1,*, Maja W. Kandula 1, Sven Hartwig 1, Louisa Hoffmann 2 , Wolfgang Haselrieder 3 and Klaus Dilger 1 1 Institute of Joining and Welding, Technische Universität Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany; [email protected] (M.W.K.); [email protected] (S.H.); [email protected] (K.D.) 2 Institute for High Voltage Technology and Electrical Power Systems, Technische Universität Braunschweig, Schleinitzstraße 23, 38106 Braunschweig, Germany; louisa.hoff[email protected] 3 Institute for Particle Technology, Technische Universität Braunschweig, Volkmaroder Straße 5, 38104 Braunschweig, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-531-391-95596



Received: 5 November 2019; Accepted: 26 November 2019; Published: 3 December 2019


Abstract: Laser cutting is a promising technology for the singulation of conventional and advanced electrodes for lithium-ion batteries. Even though the continuous development of laser sources, beam guiding, and handling systems enable industrial relevant high cycle times, there are still uncertainties regarding the influence of, for this process, typical cutting edge characteristics on the electrochemical performance. To investigate this issue, conventional anodes and cathodes were cut by a pulsed fiber laser with a central emission wavelength of 1059–1065 nm and a pulse duration of 240 ns. Based on investigations considering the pulse repetition frequency, cutting speed, and line energy, a cell setup of anodes and cathodes with different cutting edge characteristics were selected. The experiments on 9 Ah pouch cells demonstrated that the cutting edge of the cathode had a greater impact on the electrochemical performance than the cutting edge of the anode. Furthermore, the results pointed out that on the cathode side, the contamination through metal spatters, generated by the laser current collector interaction, had the largest impact on the electrochemical performance.

Keywords: production strategies; laser cutting; cell manufacturing; automotive pouch cells

1. Introduction

Due to continuing human-induced CO2 emissions, the global warming of the earth and associated negative consequences of extreme weather are steadily increasing [1]. The impact of climate change has increased awareness of the population in industrialized countries of environmentally friendly or carbon-neutral behavior. These interests are driving the development of new, greener, and more efficient technologies for the major CO2 producing sectors, which are electrical energy production and individual mobility. To reduce CO2 emissions in the individual mobility sector, electric mobility is considered a key technology [2,3]. In order to maintain the positive CO2 balance of electric mobility, the use of electrical energy from renewable energy sources is required, but also the production of cells must be made more efficient and, therefore, more ecological. In particular, a more efficient large-scale cell production can be achieved by reducing rejects by optimizing existing or developing new technologies, as material costs dominate over operating and investment costs [4]. Lithium-ion pouch cells are considered to be the most effective electrochemical technology. Due to their advantages regarding the high volumetric utilization of the installation space of the battery pack,

Batteries 2019, 5, 73; doi:10.3390/batteries5040073 www.mdpi.com/journal/batteries Batteries 2018, 4, x FOR PEER REVIEW 2 of 21

Lithium-ion pouch cells are considered to be the most effective electrochemical technology. Due to their advantages regarding the high volumetric utilization of the installation space of the battery Batteries 2019, 5, 73 2 of 20 pack, they are especially well suited for automotive batteries. Because of the possibility to customize the cell geometry, the pouch cell leads to less dead volume compared to conventional cylindrical they18,650 are especiallycells [5]. Besides, well suited the forhigh automotive volumetric batteries. energy densities Because ofon the battery possibility level topouch customize cells have the cellhigh geometry,volumetric the energy pouch cellof 466 leads WhL to less−1 and dead a specific volume energy compared density to conventional of 241 Whkg cylindrical−1 on cell 18650level (Cathode: cells [5]. Besides,LG Chem. the high NCM volumetric 111, Anode: energy LG densitiesChem. Graphite) on battery [6]. level Even pouch though cells cylindrical have high volumetric 18,650 cells energy have a 1 1 ofvolumetric 466 WhL− energyand a specific density, energy which density is about of 20% 241 Whkghigher− thanon cellthose level of pouch (Cathode: cells LG [5], Chem. the homogeneous NCM 111, Anode:mechanical LG Chem. behavior Graphite) during [6]. charging Even though and discharging cylindrical 18650 of pouch cells havecells alead volumetric to longer energy cycle density, lifetime, whichwhich is aboutis to be 20% considered higher than a long those term of pouchadvantage cells [ov5],er the the homogeneous higher energy mechanical density [7]. behavior For pouch during cells, chargingin general, and it discharging is fundamental of pouch to cut cells the lead endlessly to longer coated cycle electrodes lifetime, which and make is to be them considered suitable a for long the termfollowing advantage stacking over process. the higher It is energy imperative density to look [7]. more For pouch closely cells, at the in cutting general, process it is fundamental itself since each to cutcut the could endlessly lead to coated manufacturing electrodes errors and make in terms them of suitable contamination for the followingand cutting stacking edge quality process. [8]. It Due is imperativeto a large to looknumber more of closely processes, at the cuttingit is necessar processy itselfto optimize since each every cut could step leadto improve to manufacturing ecological errorsproductivity. in terms ofFigure contamination 1 shows the and evolution cutting edge of the quality accumulated [8]. Due toproduction a large number rejects of of processes, a conventional it is necessarycell production to optimize line schematically. every step to improve ecological productivity. Figure1 shows the evolution of the accumulated production rejects of a conventional cell production line schematically.

Figure 1. Influence of the reject rate of every single process on the total reject rate of a cell production line. Figure 1. Influence of the reject rate of every single process on the total reject rate of a cell production Asline. shown in Figure1, each process can contribute to the overall e fficiency of the production line. Due to the numerous processes of battery production, even small reject rates can lead to a high overallAs reject shown rate in and, Figure therefore, 1, each to process low utilization can contribute of raw to materials. the overall Already efficiency at a rejectof the rate production of only 1%line. perDue process, to the thenumerous accumulated processes rejects of battery at the endproduction, of the line even reach small almost reject 14%. rates Therefore, can lead to each a high process overall shouldreject berate at and, least therefore, in the range to low of a utilization 4σ reject rate, of raw respectively, materials. with Already a reject at a rate reject of underrate of 0.09% only 1% with per regardprocess, to ecological the accumulated production. rejects Investigating at the end and of improvingthe line reach the singulationalmost 14%. process Therefore, can contribute each process to accomplishingshould be at least these in high the standards.range of a 4σ reject rate, respectively, with a reject rate of under 0.09% with regardBesides to ecological the established production. shearing Investigating or die-cutting and (DIN improving 8588) [9], laserthe singulation cutting is becoming process can increasingly contribute commonto accomplishing in cell production these high lines standards. due to its process-immanent advantages over the contact-based singulationBesides method the [established10]. Especially shearing for very or thick die-cutting and fragile (DIN electrodes, 8588) [9], or purelaser lithium cutting metal is becoming anodes, laserincreasingly cutting is nocommon longer anin alternativecell production but state lines of thedue art to technology its process-immanent due to the lack advantages of contact andover the the associatedcontact-based lack ofsingulation mechanical method stress [ 11[10].]. In Especially the following for very section, thick we and have fragile given aelectrodes, summary or of thepure numerouslithium metal studies anodes, in this laser field cutting and laser is ablationno longer generally. an alternative but state of the art technology due to the lack of contact and the associated lack of mechanical stress [11]. In the following section, we have Stategiven of the a summary Art Laser Cuttingof the numerous of Electrodes studies in this field and laser ablation generally. The interactions between the material and the laser-generated photons during the cutting process is very dynamic and very complex due to a large number of possible and partly mutually dependent influencing factors. The key factors of a laser cutting plant are wavelength (λ), average power (Pavg), Batteries 2018, 4, x FOR PEER REVIEW 3 of 21

State of the Art Laser Cutting of Electrodes The interactions between the material and the laser-generated photons during the cutting Batteriesprocess2019 is ,very5, 73 dynamic and very complex due to a large number of possible and partly mutually3 of 20 dependent influencing factors. The key factors of a laser cutting plant are wavelength (λ), average

power 𝑃, spot size 𝑑, laser profile and Rayleigh length in focus, cutting speed 𝑣, the spot size d , laser profile and Rayleigh length in focus, cutting speed (v ), the number of passes, number of spotpasses, and cutting angle. In the case of pulsed laser beam sources,c the additional factors and cutting angle. In the case of pulsed laser beam sources, the additional factors are pulse peak are pulse peak power 𝑃, pulse energy 𝐸, pulse repetition frequency (𝑃𝑅𝐹), and pulse shape, power P , pulse energy (E ), pulse repetition frequency (PRF), and pulse shape, as well as pulse as well aspeak pulse duration (τ). TheP relevant material properties of the electrode are the composition of duration (τ). The relevant material properties of the electrode are the composition of the coating the coating (anode/cathode), collector material, coating thickness, collector thickness, absorption (anode/cathode), collector material, coating thickness, collector thickness, absorption coefficient, and coefficient, and degree of compaction of the electrode (Figure 2). degree of compaction of the electrode (Figure2).

Figure 2. Relevant primary process parameters and material properties of a continuous wave (cw) and pulsedFigure laser2. Relevant cutting primary process forprocess electrodes parame [10ters,12]. and material properties of a continuous wave (cw) and pulsed laser cutting process for electrodes [10,12]. For a simplified description of the influence of the primary process parameters in Figure2, the secondaryFor a parameterssimplified description energy density of the (ED) influence [13], intensity of the primary (I), pulse process fluence parameters (Hp), and number in Figure of 2, laser the pulsessecondary per surfaceparameters increment energy (n densityline)[13] (ED) for pulsed [13], intensity beam sources (I), pulse can fluence be used ( to𝐻 explain), and number the ablation of laser or cuttingpulses per behavior. surface Considering increment (𝑛 a certain) [13] wavelength, for pulsed beam average sources power, can and be used spot size,to explain these the parameters ablation canor cutting be adjusted behavior. by vc, PRFConsiderin, and τ.g The a secondarycertain wavelength, parameters average are defined power, by the and following spot size, equations: these 𝑣 𝑃𝑅𝐹, parameters can be adjusted by , and τ. The" secondary# parameters are defined by the Pavg j following equations: ED = (1) v d 2 c spot cm 𝐸𝐷 (1) 4 Pavg 4 P Pavg  W  𝐼 𝑓𝑜𝑟 𝑐𝑤 𝑎𝑛𝑑 𝐼 𝑓𝑜𝑟Peak 𝑝𝑢𝑙𝑠𝑒𝑑 l𝑎𝑠𝑒𝑟 𝑤𝑖𝑡ℎ𝑃 (2) I= f or cw and IP= f or pulsed laser with PPeak (2) π d2 π d2 ≈ PRF τ cm2 spot spot (3) 𝐻 " # EP j 𝑛 Hp = (3) 2 2 (4) π dspot cm The energy density describes the energy input per surface increment (cutting length times const. PRF (d + v ) spot size) on the material to be processed, dependingspot on theτ cuttingc speed, photonic power, and spot nline = (4) size. This parameter can be used to describe the vscalabilityc of the cutting speed for a defined laser/scannerThe energy system density and describes material the or energyto define input the pernecessary surface energy increment input (cutting for a quality length timescut and const. the spotcut-through size) on limit. the material Since the to ener be processed,gy density dependingrepresents the on average the cutting power speed, input photonic independent power, of and the spotpeak size. power, This the parameter intensity is can necessary be used to to further describe describe the scalability the cutting of process the cutting with speeda pulsed for laser a defined beam laser/scanner system and material or to define the necessary energy input for a quality cut and the cut-through limit. Since the energy density represents the average power input independent of the peak power, the intensity is necessary to further describe the cutting process with a pulsed laser beam Batteries 2019, 5, 73 4 of 20 Batteries 2018, 4, x FOR PEER REVIEW 4 of 21 source. In addition, the pulse fluence and the number of pulses, which hit per surface increment, source. In addition, the pulse fluence and the number of pulses, which hit per surface increment, must must be specified. Based on the energy density, the intensity, the pulse fluence, and the number of be specified. Based on the energy density, the intensity, the pulse fluence, and the number of hits per hits per area increment, the amount of material removal and the material removal behavior can be area increment, the amount of material removal and the material removal behavior can be derived. derived. Figure 3 schematically shows the number of hits per surface increment as a function of the Figure3 schematically shows the number of hits per surface increment as a function of the vc, PRF, τ, 𝑣, 𝑃𝑅𝐹, τ, and the 𝑑, as well as the effects on the electrode cutting edge characteristics. The and the dspot, as well as the effects on the electrode cutting edge characteristics. The ablation thresholds ablation thresholds a and b, as well as the heat-affected zone c shown in Figure 3, can be derived from a and b, as well as the heat-affected zone c shown in Figure3, can be derived from the mentioned the mentioned secondary parameters. secondary parameters.

Figure 3. Laser scanning microscope image of a laser-cut electrode (left); Ablation thresholds depending onFigure the number3. Laser of scanning hits, intensity, microscope pulse fluence, image energyof a laser-cut density, aselectrode well as the(left laser-); Ablation and corresponding thresholds plasma-intensity-profiledepending on the number (right of hits,): (a) intensity, cut-through pulse threshold, fluence, energy (b) ablation density, threshold, as well as and the (c) laser- thermic and influencingcorresponding threshold plasma-intensity-profile [10]. (right): (a) cut-through threshold, (b) ablation threshold, and (c) thermic influencing threshold [10]. As the type of energy input can lead to different removal mechanisms, a comparison of continuous waveAs (cw) the and type pulsed of energy systems input based can on lead the to energy different density removal is only mechanisms, conditionally a possible.comparison Here, of acontinuous distinction wave can be(cw) made and pulsed between systems thermal based (cw /onpulsed) the energy and athermaldensity is (pulsed) only conditionally dominant possible. removal processes.Here, a distinction The thermal can ablationbe made generally between proceeds thermal in(cw/pulsed) three successive and atherm phases,al and,(pulsed) in the dominant case of a continuousremoval processes. cut, also The in parallel thermal phases. ablation In thegenerally first phase, proceeds the photons in three are successive absorbed phases, by the surfaceand, in andthe penetratecase of a continuous a near-surface cut, area. also Inin theparallel following phases. second In the phase, first phase, the temperature the photons increases are absorbed at thesurface by the assurface a result and of penetrate the absorption, a near-surface and in deeper area. zones In the by following thermal conductivesecond phase, effects. the Thetemperature rising temperature increases leadsat the tosurface a transformation as a result of the stateabsorption, of matter and from in deeper solid to zones liquid by and thermal liquid conductive to gas, or directly effects. from The solidrising to temperature gas. In the thirdleads phase,to a transformation the penetration of depththe state increases of matter and from thus solid the melting to liquid or and evaporation liquid to zone,gas, or wherein directly the from material solid is to expelled gas. In inthe liquid third or phase, gaseous the form penetration from the depth kerf. The increases material and ejection thus canthe bemelting further or distinguishedevaporation zone, into wherein fusion, sublimation, the material andis expelled photochemical in liquid cuttingor gaseous/ablation. form from The athermal the kerf. removalThe material process ejection is characterized can be further by the distinguished fact that the duration into fusion, of the photonicsublimation, energy and input photochemical is too short forcutting/ablation. initiating heat The conduction. athermal This removal removal process process is ch canaracterized be achieved by withthe fact pulse that durations the duration of less of than the 10photonic ps. Furthermore, energy input due is to thetoo shortshort exposurefor initiating time ofheat pulses conduction. in the ps andThis fs removal range, the process spatial can extent be ofachieved the resulting with pulse plasma durations is negligibly of less small than [14 10]. Consideringps. Furthermore, the high due intensities,to the short athermal exposure processes time of canpulses be furtherin the ps distinguished and fs range, between the spatial sublimation extent cuttingof the andresulting photochemical plasma is ablation.negligibly small [14]. ConsideringFigure4 showsthe high the processesintensities, that athermal occur after processes the first can impact be offurther the photons distinguished on the processed between material.sublimation On cutting average, and the photochemical photons are absorbed ablation. in 10 fs, where the photonic energy is converted into thermalFigure energy 4 shows within the 100 processes fs by electron-electron that occur after relaxation the first impact of the electrodeof the photons systems on of the the processed covalent bonds.material. This On is average, followed the by photons an electron-phonon are absorbed relaxation in 10 fs, where after aboutthe photonic 1 to 10 ps,energy which is converted leads to a heatinto transferthermal ofenergy the electrons within 100 into fs the by lattice electron-electron structure. Parallel relaxation to this of process,the electrode the ablation systems begins, of the andcovalent after aboutbonds. 100 This ps, is thefollowed phonon-phonon by an electron-phonon relaxation leads relaxation to heat after conduction about 1 [ 15to ].10 ps, which leads to a heat transfer of the electrons into the lattice structure. Parallel to this process, the ablation begins, and after about 100 ps, the phonon-phonon relaxation leads to heat conduction [15]. Batteries 2019, 5, 73 5 of 20 Batteries 2018, 4, x FOR PEER REVIEW 5 of 21

FigureFigure 4.4. Time-scaled processes during a laser pulse material interaction, based on [16,17]. [16,17].

ByBy meansmeans ofof thethe beambeam source used in this study, it is possible to generate generate pulses pulses in in the the ns ns range. range. DueDue toto thethe relativelyrelatively longlong pulsepulse durationduration of 240 ns,ns, we can only realize a thermallythermally affected affected cutting processprocess (thermal(thermal removalremoval process)process) withwith thisthis systemsystem [[14].14]. As a result of the composite structure structure and and thethe associatedassociated varyingvarying materialmaterial propertiesproperties overover thethe thickness of the electrode, it can be be assumed assumed that that severalseveral removalremoval mechanismsmechanisms taketake placeplace simultaneouslysimultaneously andand/or/or seriallyserially during cutting. The cutting cutting of of thethe porousporous coatingcoating willwill bebe characterizedcharacterized byby aa photochemicalphotochemical andand sublimationsublimation proportion, whereas whereas thethe cuttingcutting of the metallic metallic collector collector will will be be charac characterizedterized by by sublimation sublimation and and a fusion a fusion proportion. proportion. In Inaddition addition to tothe the direct direct interaction interaction between between laser laser and and material, material, a aplasma plasma which which correlates correlates to to the the intensityintensity distribution distribution of of the the laser laser will will also also interact intera withct with the electrode. the electrode. This can This lead can to materiallead to material removal orremoval thermal or loading thermal of loading the active of the material, active material in addition, in toaddition thermal toconduction thermal conduction effects. effects. TheThe laser-inducedlaser-induced plasmaplasma isis causedcaused byby thethe high intensities of the individual pulses, which which leads leads toto strongstrong oscillationsoscillations ofof thethe freefree electrons,electrons, enablingenabling them to knock bounded electrons out out of of neutrally neutrally chargedcharged atoms. atoms. The The avalanching avalanching increase increase of freeof free and and strong strong oscillating oscillating electrons electrons leads toleads a large to numbera large ofnumber free electrons of free andelectrons positively and positively charged species. charged The sp resultingecies. The high-energy resulting high-energy plasma absorbs plasma the photonsabsorbs ofthe the photons laser radiation of the laser by theradiation inverse by Bremsstrahlung the inverse Bremsstrahlung (IB) and the photoionization (IB) and the photoionization (PI). In this case, (PI). IB In is consideredthis case, IB to is be considered the main absorption to be the main mechanism. absorption The mechanism. photons in thisThe mechanismphotons in arethis absorbed mechanism by freeare electronsabsorbed as by they free collideelectrons with as neutralthey collide or ionized with neutra species.l or This ionized leads species. to an increase This leads in theto an energy increase of the in electronsthe energy and of thusthe electrons to an increase and thus in the to an degree increase of ionization in the degree and theof ionization temperature and of the the temperature plasma. Ifthe of temperaturethe plasma. andIf the density temperature of the plasmaand density rise above of the a pl certainasma rise level, above the plasma a certain can level, shield the the plasma area from can beingshield cut the from area the from laser being radiation. cut from This the effect laser is referred radiation. to asThis the effect plasma is shieldingreferred etoff ectas [the18]. plasma shieldingInvestigations effect [18]. on cw and pulsed laser beam cutting of electrodes have already been carried out in previousInvestigations studies. The on resultscw and showed pulsed thatlaserthe beam use cutting of single-mode of electrodes cw fiberhave lasers already made been it carried possible out to achievein previous very studies. high, industry-relevant The results showed cutting that speeds the us duee of tosingle-mode the high average cw fiber power lasers and made the it achievable possible 1 lowto achieve spot sizes. very Studies high, showedindustry-relevant that it was possiblecutting speeds to cut an due anode to the (120 highµm) withaverage 11,666 power mms and− and the a 1 cathodeachievable (130 lowµm) spot with sizes. 10,000 Studies mms− showedwith a that single-mode it was possible cw fiber to lasercut an at anode a wavelength (120 µm) in with the infrared 11,666 rangemms− (10701 and nm),a cathode an average (130 µm) power with of 10,000 5000 W, mms and−1 awith spot a size single-mode of 25 µm [cw19]. fiber This laser resulted at a inwavelength an energy 2 2 densityin the infrared of 2000 jcmrange− for (1070 the nm), cut-through an average limit power of the cathodeof 5000 W, and and an energya spot densitysize of 25 of 1714µm [19]. jcm− Thisfor theresulted anode. in Thean energy lower requireddensity of energy 2000 jcm density−2 for forthe the cut-through anode was limit probably of the duecathode to the and low an collector energy thicknessdensity of and 1714 the jcm lower−2 for degreethe anode. of compaction. The lower required In another energy study, density an anode for the (50 anodeµm) with was a probably collector 1 thicknessdue to the of low 30 µ collectorm was cut thickne with ass cutting and the speed lower of 2000degree mms of− compaction.using a single-mode In another cw study, Ytterbium an anode fiber 2 laser(50 µm) (1070 with nm, a 250collector W, and thickness 23 µm spot of 30 size). µm Thewas muchcut with lower a cutting required speed energy of 2000 density mms of−1 543 using jcm a− single-could bemode explained cw Ytterbium by the lower fiber materiallaser (1070 thickness nm, 250 and W, by and the 23 higher µm spot intensity size). [ 20The]. Basedmuch onlower these required results, furtherenergy investigationsdensity of 543 usingjcm−2 thecould same be explained system showed by the that lower compacted material (56thicknessµm) and and non-compacted by the higher 1 anodesintensity (70 [20].µm) Based with a collectoron these thicknessresults, further of 20 µ minvestigations could be cut atusing a speed the ofsame 5000 system mms− showed[21]. The that low 2 energycompacted density (56 µm) of 217 and jcm non-compacted− required for anodes the singulation (70 µm) with suggested a collector that thickness the collector of 20 thickness µm could was be cut at a speed of 5000 mms−1 [21]. The low energy density of 217 jcm−2 required for the singulation suggested that the collector thickness was the dominating speed-influencing parameter of the Batteries 2019, 5, 73 6 of 20 the dominating speed-influencing parameter of the electrode. Considering the strong influence of the collector thickness, as well as the intensity of the focused laser spot, it was plausible that an energy 2 1 density of 818 jcm− at 5000 mms− led to a cut-through of an anode (100 µm) with a current collector thickness of 10 µm by using a single-mode cw fiber laser (1070 nm, 450 W, 11 µm spot size) [22]. In principle, the investigations with cw laser systems showed that the achievable chamfer width (total ablated area) was less than 50 µm and increased with increasing energy densities [21]. The trend of these dependencies was also found in investigations with pulsed laser beam sources. In addition to this, the investigations showed that with pulsed beam sources in the nanosecond range, higher pulse repetition frequencies enabled higher cutting speeds and led to a smaller chamfer width [19]. By using an ns pulsed fiber laser (1070 nm, 100 W, 50 µm spot size, 500 kHz, and 30 ns), it was possible for Kronthaler et al. [23] to cut an anode (114 µm) with a collector thickness of 10 µm 1 at a speed of 1200 mms− . Using the same system parameter, a slightly higher cut-through limit of 1 1250 mms− could be achieved for a 124 µm thick cathode with a collector thickness of 20 µm [23]. 2 From the given parameters, an energy density of 160 jcm− resulted, for the cut-through limit, in an intensity per pulse of 3.4 108 Wcm 2 and a hit number of 20. Lutey et al. showed in their research × − that the number of hits per area increment caused an increase in the plasma shielding effect. With higher numbers of hits, a higher average power was needed to realize a cut. At 125 hits per area 2 increment (500 kHz), an energy density of 660 jcm− was needed, whereas, with a hit number of 5 2 (20 kHz), only an energy density of 352 jcm− sufficed [24]. Further results on laser cutting of electrodes showed that low energy densities and intensities were necessary for singulation for a smaller wavelength. At a wavelength of 1064 nm, an energy density of 448 jcm 2 and an intensity of 28.5 108 Wcm 2 were necessary [24], whereas, at 355 nm, − × − an energy density of 340 jcm 2 and an intensity of 1.7 108 Wcm 2 were sufficient to cut a 120 µm thick − × − anode [19]. These results could not be confirmed by studies with a laser in the green electromagnetic spectrum (532 nm, 1 ns, 6 W). For the singulation of an anode (130 µm), with the same collector thickness, an energy density of 560 jcm 2 and an intensity of 50.8 108 Wcm 2 were needed. This − × − could be attributed to the fact that the number of hits per surface increment of 165 increased the plasma shielding effect, and thus reduced the maximum cutting speed [25]. In order to realize the most dynamic and fast cutting processes possible in a cutting plant, the beam guidance on the workpiece is conventionally carried out by means of a remote scanner system. As a result of the low mass and the associated low inertia of the deflection mirrors, very high speeds and repetition accuracies can be achieved in the horizontal plane. Due to the varying distance of the electrode to the scanner system, a focus adjustment in the vertical direction is necessary. This adjustment can be produced by means of additional lenses (focus-shifter) or by a static f-theta objective. Considering the static beam refocusing, the advantage of an f-theta objective over a focus shifter is the higher repetition accuracy and wear-freedom. However, the dynamic focus by means of a focus shifter allows us to adjust the working field and to customize the focus in certain areas.

2. Experimental

2.1. Materials To investigate the influence of the laser process parameters on the properties of the cutting edge and the influence on the electrochemical performance, double-sided coated electrodes with industrially available material components were used. For the anode, the active material SMGA4 (91 wt.%; Hitachi, 1 Japan), with a specific capacity of 360 mAhg− , was coated on a 10 µm thick copper collector (Sumisho 3 Metallex, Japan). Subsequently, the coating was compacted to a density of 1.5 gcm− , which results in the porosity of 32.25% and a total thickness of 123 µm. On the cathode side, the active material 1 NMC 111 (90 wt.%; BASF, Germany), with a specific capacity of 165 mAhg− , was coated on 20 µm thick aluminum collector (Hydro Aluminum Rolled Products, Germany) and compacted to a degree 3 of 2.8 gcm− . The described compaction led to porosity of 31.35% and a total electrode thickness of Batteries 2019, 5, 73 7 of 20

Batteries143 µm. 2018 For, 4, x the FOR anode PEER REVIEW and cathode, a conductivity additive SFG6L (2 wt.% anode, 2 wt.% cathodes;7 of 21 Imerys, Switzerland), a carbon black C65 (2 wt.% anode, 4 wt.% cathodes; Imerys, Switzerland), and a Imerys,PVDF binderSwitzerland), (5 wt.% and anode, a PVDF 4 wt.% binder cathodes; (5 wt.% Solvay,anode, 4 Italy) wt.% were cathodes; utilized. Solvay, The Italy) cell manufacturing were utilized. Thewas cell carried manufacturing out with a was 27 µ mcarried thick out separator with a (Separion)27 µm thick and separator a conventional (Separion) LiPF and6 electrolyte a conventional (UBE LiPFIndustries6 electrolyte Ltd., Japan).(UBE Industries The conductive Ltd., Japan). salt LiPF The6 was conductive solved in salt a concentration LiPF6 was solved of one in molea concentration in a solvent ofconsisting one mole of in ethylene a solvent carbonate consisting (EC), of ethylene dimethyl carbonate carbonate (EC), (DMC), dimethyl and ethyl carbonate methyl (DMC), carbonate and (EMC), ethyl methylwith a volumetriccarbonate (EMC), ratio of with the solvent a volumetric components ratio of of the 1:1:1. solvent To suppress components the evolution of 1:1:1. ofTo gas suppress during the the evolutionfirst charging, of gas the during electrolyte the first contained charging, 2 wt.% the el ofectrolyte Vinylene contained Carbonate 2 (VC).wt.% Asof Vinylene further additives Carbonate for (VC).reducing As hydrogenfurther additives formation for at highreducing voltages, hydrogen the electrolyte formation contained at high 3 wt.%voltages, of cyclohexylbenzene the electrolyte contained(CHB). 3 wt.% of cyclohexylbenzene (CHB).

2.2.2.2. Analysis Analysis of of the the Cutting Cutting Edge Edge Characteristics Characteristics TheThe prescriptive prescriptive characteristics ofof thethe electrodeelectrode cutting cutting edge edge are are shown shown in in Figure Figure5a by5a meansby means of a ofmicrosection. a microsection. These These characteristics characteristics were determinedwere determined by light by microscopy light microscopy (VHX 2000 (VHX light microscope2000 light microscope(LM), Keyence, (LM), Osaka, Keyence, Japan) Osaka, and laser Japan) scanning and la microscopeser scanning (VK-X microscope Series 3D (VK-X Laser Scanning Series 3D Confocal Laser ScanningMicroscope Confocal (LSM), Microscope Keyence, Osaka, (LSM), Japan). Keyence, Here, Osaka, the parameters Japan). He chamferre, the widthparameters and heat-a chamferffected width zone and(HAZ) heat-affected were considered zone to(HAZ) be the were significant considered influencing to be factors the significant on the electrochemical influencing performance factors on and,the electrochemicaltherefore, investigated performance further. and, The therefore, HAZ is defined investigated as an areafurther. where The the HAZ active is materialdefined isas thermally an area wherestressed the but active not material removed. is Thethermally chamfer stressed width bu is characterizedt not removed. by The active chamfer material width removal is characterized and a melt byformation active material zone. removal and a melt formation zone.

(b)

(a) (c)

FigureFigure 5. 5. AnalysisAnalysis of of the the prescriptive prescriptive cutting cutting edge edge characteristics: characteristics: (a (a) )Schematic Schematic microsection microsection of of a a laser-generatedlaser-generated cutting cutting edge, edge, 1. 1. Chamfer Chamfer width width (chw (chw),), 2. 2. The The heat-affected heat-affected zone zone (HAZ), (HAZ), 3. 3. Melt Melt formation,formation, 4. 4. Ablation, Ablation, 5. 5. Melt Melt superelevation, superelevation, αα ChamferChamfer angle; angle; (b (b) )LSM LSM image image of of a acutting cutting edge; edge; (c ()c ) AnalysisAnalysis of of a a cutting cutting edge edge by by LSM LSM data, data, based based on on [10]. [10].

ToTo measure thesethese characteristics, characteristics, LM LM and and LSM LSM images images were were taken taken of the upperof theside upper of the side electrodes of the electrodesat the cutting at the edges. cutting The edges. areas The of areas the heat-a of theff ectedheat-affected zone and zone the and chamfer the chamfer width couldwidth becould clearly be clearlyseparated separated by a combined by a combined measuring measuring method. Bymeth meansod. By of themeans LSM of topography the LSM topography images (Figure images5b,c), (Figurethe chamfer 5b,c), width the chamfer was measured. width was By subtractingmeasured. By the subtracting chamfer width the fromchamfer the entirewidth a fromffected the area entire (LM affected area (LM images), the heat-affected zone could be quantified. For the determination of contaminant products as a result of the laser-material interaction, SEM (FEI Quanta 650, Thermo Fisher, Waltham, MA, USA) and EDX (Oxford X-Max 80 mm², Oxford Instruments, Abingdon, England) images of the cut electrodes were taken. Batteries 2019, 5, 73 8 of 20 images), the heat-affected zone could be quantified. For the determination of contaminant products as a result of the laser-material interaction, SEM (FEI Quanta 650, Thermo Fisher, Waltham, MA, USA) and EDX (Oxford X-Max 80 mm2, Oxford Instruments, Abingdon, England) images of the cut electrodes Batteries 2018, 4, x FOR PEER REVIEW 8 of 21 were taken.

2.3. Cell Cell Format Format and and Manufacturing Manufacturing To evaluate the influenceinfluence of the cutting edge on the electrochemical performance, pouch cells with 15 compartments were built (total cell capacitycapacity about 9 Ah).Ah). The The relatively high number of compartments was was chosen chosen to to emphasize emphasize the the effect effect of of the the cutting cutting edge edge properties properties of of the the electrodes, electrodes, maximizing the effects effects of the cuttingcutting edgeedge characteristicscharacteristics on thethe electrochemicalelectrochemical performance.performance. The surface of the anodeanode coatedcoated withwith activeactive material material was was 16,484 16,484 mm mm²2 and, and, based based on on the the geometry geometry shown shown in in Figure 6a, gave a cutting edge to surface ratio of 0.030 mm1−1. The cathode is defined by an area of Figure6a, gave a cutting edge to surface ratio of 0.030 mm − . The cathode is defined by an area of 15,209 mm²2 and a ratio of cutting edge to the surface of 0.031 mm−1. For the anode, a circumferential 15,209 mm and a ratio of cutting edge to the surface of 0.031 mm− . For the anode, a circumferential overlap ofof 2.52.5 mm mm resulted resulted from from the the illustrated illustrated geometries geometries for the for anode the anode and the and cathode. the cathode. This overlap This overlapguaranteed guaranteed the total the stress total of thestress cathode of the ascathode a reference as a inreference these examinations in these examinations and ensured and the ensured correct thebalancing correct ofbalancing the compartment. of the compartment. By means means of of a az-folding z-folding process process (prototype (prototype plant, plant, Jonas Jonas & Redmann, & Redmann, Berlin, Berlin, Germany), Germany), the singularizedthe singularized electrodes electrodes were stacked were stacked alternating alternating between between a separator a separator to form an to electrode-separator- form an electrode- compositeseparator-composite (ECS). Subsequently, (ECS). Subsequently, the individual the individual collectors collectors of the electrodes of the electrodes were werejoined joined to a tab to a via tab ultrasonicvia ultrasonic welding welding (Ultraweld (Ultraweld F20, F20, Branson Branson Ultraschall, Ultraschall, Hannover, Hannover, Germany). Germany). For For this this purpose, purpose, the 15 single anode collectorscollectors werewere weldedwelded to to a a nickel nickel tab tab with with an an energy energy of of 200 200 j, andj, and cathodes cathodes collectors collectors to toan an aluminum aluminum tab withtab with an energy an energy of 100 jof at an100 oscillating j at an oscillating sonotrode amplitudesonotrode ofamplitude 30 µm. Subsequently, of 30 µm. Subsequently, the ECS was dried under vacuum for 120 °C for 16 h. In the following step, the ECS the ECS was dried under vacuum for 120 ◦C for 16 h. In the following step, the ECS was inserted wasinto theinserted pouch into bag the and pouch filled underbag and argon filled atmosphere under argon with atmosphere electrolyte andwith sealed. electrolyte Finally, and the sealed. filled Finally, the filled cells were tempered for 4 h at 60 °C to support the complete wetting of the electrodes. cells were tempered for 4 h at 60 ◦C to support the complete wetting of the electrodes. The finished Theassembled finished cell assembled is shown cell in Figure is shown6b [ 12in]. Figure 6b [12].

(a) (b)

Figure 6. CellCell format format and and pouch cell design: ( (aa)) Electrode Electrode format, format, ( (bb)) Complete Complete pouch pouch cell. cell.

2.4. Cell Diagnostic 2.4. Cell Diagnostic After assembly and wetting, the manufactured cells were placed in a climate chamber (WKM After assembly and wetting, the manufactured cells were placed in a climate chamber (WKM Inc., Lachendorf, Germany) at 20 C and connected to a battery tester (Series XCTS, Basytec Inc., Inc., Lachendorf, Germany) at 20 °C◦ and connected to a battery tester (Series XCTS, Basytec Inc., Asselfingen, Germany) with a fixed torque of 2.54 Nm. Due to the high capacity of the battery cells, Asselfingen, Germany) with a fixed torque of 2.54 Nm. Due to the high capacity of the battery cells, the tests required a high safety environment. Therefore, the climate chambers were equipped with a the tests required a high safety environment. Therefore, the climate chambers were equipped with a fire extinguishing system (Wagner Group Inc., Langenhagen, Germany). In the event of an accident, fire extinguishing system (Wagner Group Inc., Langenhagen, Germany). In the event of an accident, the climate chamber is flooded with nitrogen gas. Furthermore, an activated carbon filter (Stöbich technology Inc., Goslar, Germany) will filter the exhaust air in the pipe duct.

BatteriesBatteries 20182019, ,45, ,x 73 FOR PEER REVIEW 9 9of of 21 20

In our experiments, the cells were formed in two cycles. They were first charged and discharged the climate chamber is flooded with nitrogen gas. Furthermore, an activated carbon filter (Stöbich at 1/10 C, and in the second formation cycle with 1/2 C. Upper and lower cut-off voltages were 4.2 V technology Inc., Goslar, Germany) will filter the exhaust air in the pipe duct. and 2.9 V, respectively, for all charge-discharge cycles. To characterize the cells, a capacity test at 1/10 In our experiments, the cells were formed in two cycles. They were first charged and discharged C and a pulse test (1 C for 1 s) to determine the internal resistance were performed. After the at 1/10 C, and in the second formation cycle with 1/2 C. Upper and lower cut-off voltages were 4.2 V formation process, the cells were matured over eight days with a state of charge (SOC) of 50% at 20 and 2.9 V, respectively, for all charge-discharge cycles. To characterize the cells, a capacity test at 1/10 C C. Then, the aging of the cells began with a C-rate test with different discharge-rates from 1/5 to 2 C, and a pulse test (1 C for 1 s) to determine the internal resistance were performed. After the formation which lasted 20 cycles. Long-term cycling was then started at 1 C for 100 cycles. After this, the process, the cells were matured over eight days with a state of charge (SOC) of 50% at 20 C. Then, the cyclization was paused, and the internal resistance was measured in a pulse test before the C-rate test aging of the cells began with a C-rate test with different discharge-rates from 1/5 to 2 C, which lasted was repeated. These aging investigations were repeated periodically until at least 450 cycles were 20 cycles. Long-term cycling was then started at 1 C for 100 cycles. After this, the cyclization was reached. In this study, 5 cells per laser variation were analyzed, and only the long-term cycling was paused, and the internal resistance was measured in a pulse test before the C-rate test was repeated. considered. These aging investigations were repeated periodically until at least 450 cycles were reached. In this study, 5 cells per laser variation were analyzed, and only the long-term cycling was considered. 2.5. Laser Cutting Plant and Key Parameter 2.5. LaserThe laser Cutting cutting Plant plant and Keyused Parameter in this study was an in-house development and construction. Due to its Themodular laser cuttingstructure plant and used the process-immanent in this study was an ad in-housevantages development of the scanner and system, construction. it is suitable Due to forits modulara large number structure of anddifferent the process-immanent electrode formats. advantages The beam ofsource the scanner used was system, a nanosecond it is suitable pulsed for a fiberlarge laser number with of a dicentralfferent emission electrode wavelength formats. The of 1059–1065 beam source nm used (G4 wasPulsed a nanosecond Fiber Laser, pulsed SPI Lasers fiber UKlaser Ltd., with Southampton, a central emission UK). The wavelength average ofpower 1059–1065 of the nmfiber (G4 laser Pulsed was Fiber72 W Laser,with a SPIpeak Lasers pulse UK power Ltd., ofSouthampton, up to 13 kW UK).and an The M² average of <1.6. power Guidance of the and fiber focusing laser was of the 72 Wlaser with beam a peak were pulse performed power ofby up a 3- to axis13 kW laser and beam an M2 deflectionof <1.6. Guidance system and(AXIALSCAN focusing of the30/FOCUSSHIFTER, laser beam were performed Raylase byAG, a 3-axisWessling, laser Germany)beam deflection with a system working (AXIALSCAN field of 400 30× /FOCUSSHIFTER,400 mm². The first Raylase two dimensions AG, Wessling, of the Germany) scanner withwere a neededworking to field drive of the 400 spot400 over mm the2. The workpiece first two to dimensions create the cutout. of the scanner The third were dimension needed to was drive needed the spot to × ensureover the a workpiececonstant spot to createsize of the ~74 cutout. µm with The a third focus dimension depth of was0.6 mm needed on one to ensure level over a constant the entire spot workingsize of ~74 field.µm All with laser a focus cuts were depth made of 0.6 in mm one onpass. one The level fully over automated the entire handling working system field. All was laser carried cuts outwere by made simple in roll one to pass. roll Theand fullypick automatedand place operation handling (Figure system 7a), was controlled carried out by by an simple Arduino roll Mega to roll 2560.and pickSince and the place cut could operation only be (Figure realized7a), in controlled the focus by level an Arduinoof the laser Mega spot, 2560. a special Since negative the cut could form foronly the be positioning realized in theof focusthe electrode level of thehad laser to be spot, bu ailt special for each negative electrode form format for the positioning(Figure 7b). of The the positioningelectrode had of the to be electrode built for was each done electrode by negative format pressure (Figure7 b).on holes The positioning surrounding of the cutting electrode curve. was Evendone though by negative a cutting pressure on the on fly holes was surrounding possible with the the cutting used remote curve. Evenscanner though system, a cutting a static on cutting the fly operationwas possible was with used the to guarantee used remote constant scanner cutting system, speed a static over cutting the complete operation cutting was length used toand guarantee an easy formatconstant change. cutting speed over the complete cutting length and an easy format change.

(a) (b)

FigureFigure 7. 7. LaserLaser cutting cutting plant: plant: (a (a) )Front Front view view of of the the la laserser cutting cutting plant; plant; ( (bb)) Negative Negative form. form. 3. Results and Discussion 3. Results and Discussion The presentation of the results has been divided into three sections. In the first section, the results The presentation of the results has been divided into three sections. In the first section, the results of the influence of the laser parameters on the cutting edge characteristics, chamfer width, and of the influence of the laser parameters on the cutting edge characteristics, chamfer width, and heat- heat-affected zone have been considered and discussed. The investigations focused on the pulse affected zone have been considered and discussed. The investigations focused on the pulse repetition repetition frequency, the cutting speed, and the pulse length, as well as the influence of the number of frequency, the cutting speed, and the pulse length, as well as the influence of the number of hits per surface increment, and intensity at a constant energy density. In the second section, the influence of Batteries 2019, 5, 73 10 of 20 hits per surface increment, and intensity at a constant energy density. In the second section, the influence of the cutting edge characteristics and corresponding laser process parameters on the electrochemical performance has been examined. Building on these results, the last section would present further investigations of the cutting process and the cutting edge, explaining the electrochemical behavior.

3.1. Influence of the Laser Process Parameters on the Cutting Edge Characteristics In a first step, we investigated the influence of the laser parameters on the cutting edge properties. For this purpose, the pulse repetition frequency (PRF) and the cutting speed (Vc) were deliberately varied with constant power and pulse duration. The resulting cut edges were evaluated according to the analysis methods presented. On the basis of the obtained data, models could be developed by means of the analysis program Design Expert 11, which describes the examined parameter space. The experimental design was made with a D-optimal strategy and comprised 13 experiments with five repeated measurements each. The response surface model was adapted to the measured values by fitting it to a quadratic polynomial of the form of Response = Intercept * + A + B + AB + AB2 + A2BA2 B2. The results for the anode (Figure8) showed that both parameters influenced the formation of the chamfer width and the heat-affected zone. In Figure8a model, it could be seen that the formation of the chamfer width steadily decreased with increasing PRF. This tendency could also be observed with increasing cutting speed. The smallest chamfer width for this model was obtained at the maximum 1 achievable speed of 700 mms− , with a pulse repetition frequency of 490 kHz. Considering the additional laser parameters, this resulted in a number of hits per area increment of 51 and an energy 2 density of 141 jcm− . The decrease in the chamfer width with increasing speed or decreasing energy density could be explained by the lower energy input per area. The dependence on energy density has already been confirmed by previous publications [24]. The decrease of the chamfer width with increasing pulse repetition frequency could be attributed to different mechanisms, which result from the adjusted mode of the energy input. Due to the constant average power and the constant pulse duration, the pulse peak power had to drop with increasing pulse repetition frequencies to reduce the energy per pulse. As a consequence, the intensity and the energy decreased with increasing pulse repetition frequency, and thus the width of the threshold intensity or the spot diameter, which led to an ablation, becoming smaller due to the Gaussian intensity distribution. Besides, the increased 1 number of hits of 360 (at 490 kHz and 100 mms− ) could lead to a more intensive harmonic laser-plasma interaction, which reduced the energy impinging on the target by shielding effects and thus reduced the energy density. In addition, the reduced pulsed peak power could lead to a smaller broadening of the plasma formation. Since the plasma was also involved in the removal of material and the thermal load on the surface, the proportion of the total material removal became less at higher frequencies. Literature regarding ns laser-induced breakdown spectroscopy describes threshold pulse fluences for 2 2 forming a plasma of 1.01 jcm− for aluminum and 1.46 jcm− for copper. As the smallest pulse fluence 2 at 490 kHz for this System was 3.45 jcm− , the plasma formation, in general, would always occur for the examined parameter space [26]. BatteriesBatteries 20182019, ,45, ,x 73 FOR PEER REVIEW 1111 of of 21 20

(a) (b)

FigureFigure 8. 8. ModelModel for for the the influence influence of of the the pulse pulse repetiti repetitionon frequency frequency and and cutting cutting speed speed on on the the anode anode cuttingcutting edge: edge: 72 72 w, w, 240 240 ns; ns; (a) ( aInfluence) Influence on on the the chamfer chamfer width width (Cubic (Cubic fitting: fitting: R² = R0.77,2 = 0.77,Adjusted Adjusted R² = 0.75,R2 = Predicted0.75, Predicted R² = 0.73); R2 = 0.73);(b) Influence (b) Influence on the on heat-affected the heat-affected zone zone (Cubic (Cubic fitting: fitting: R² = R 0.94,2 = 0.94, Adjusted Adjusted R² =R 0.93,2 = 0.93, Predicted Predicted R² = R0.93).2 = 0.93).

FromFrom the heat-aheat-affectedffected zone zone model model (Figure (Figure8b), 8b), it could it could be seen be seen that increasingthat increasing the PRF the could PRF reducecould reducethe heat-a theff heat-affectedected zone. The zone. cause The for cause the dependence for the dependence of the thermal of the loadthermal on the load PRF on could the PRF be explainedcould be explainedby the reasons by the given reasons above given for the above dependence for the ofdependence the chamfer of width the chamfer on the PRF. width The on correlation the PRF. of The the correlationHAZ with theof the cutting HAZ speed with showedthe cutting the oppositespeed showed behavior the toopposite the chamfer behavior width. to Asthe the chamfer cutting width. speed Asincreased, the cutting the HAZ speed increased increased, in the the range HAZ of increase 70–350 kHz.d in Thisthe couldrange beof explained70–350 kHz. by the This fact could that with be explainedincreasing by cutting the fact speed, that the with chamfer increasing width andcutting the kerfspeed, were the becoming chamfer steadily width smaller.and the Thiskerf meanswere becomingthat less material steadily wassmaller. removed This means for higher that cuttingless material speeds. was Due removed to the Gaussianfor higher intensity cutting speeds. distribution Due toand the the Gaussian increasing intensity speed, distribution the energy input and the profile incr changedeasing speed, to the the effect energy that theinput material profile which changed was to no thelonger effect ablated that the underwent material such which high was thermal no longer stress ablated that there underwent was an optical such high change. thermal This meansstress that that therethe final was product an optical cut change. with high This speed means contained that the afi largernal product active cut material with high area thatspeed is contained thermally stressed,a larger activewhich material would be area completely that is thermally removed stressed, at lower which speeds. would The investigations be completely on removed the cathode at lower were speeds. carried 1 Theout ininvestigations smaller parameter on the spacecathode (Figure were9a) carried with respectout in tosmaller the speed parameter (100–400 space mms (Figure− ) because 9a) with due 1 respectto the higher to the materialspeed (100–400 thickness mms of the−1) because collector due already to the at higher 455 mms material− , the cut-throughthickness of limitthe collector for high alreadyPRF was at reached. 455 mms The−1, the results cut-through for the formation limit for high of the PRF chamfer was reached. width as The a function results for of the the PRF formation and Vc ofshowed the chamfer similar width tendencies as a function as the anodic of the model.PRF and Both Vc showed with increasing similar tendencies PRF and with as the increasing anodic model. Vc, the Bothchamfer with width increasing decreased PRF significantly.and with increasing The results Vc, the showed chamfer that width the smallest decreased chamfer significantly. widths could The resultsonly be showed achieved that through the smallest the combination chamfer widths of low could energy only densities be achieved and high through PRF. the Possible combination causes forof lowthese energy dependencies densities could and high be transferred PRF. Possible from causes the explanations for these dependencies to the anode. could The be model transferred presented from for 2 thethe explanations development to of the the anode. heat-aff Theected model zone presented for cathodes for (Figurethe development9b) could be of adjusted the heat-affected with an R zoneof 0.93for cathodesand allowed (Figure a 92% 9b) reliable could be prediction. adjusted with With an increasing R² of 0.93 PRF,and allowed the HAZ a decreased92% reliable significantly prediction. until With it increasingapproached PRF, zero the at 490 HAZ kHz. decreased Here, the significantly influence of theuntil cutting it approached speed played zero only at a490 minor kHz. role. Here, Despite the influencethe low significance, of the cutting an speed increase played in the only cutting a minor speed role. led toDespite a reduction the low in thesignificance, HAZ. These an resultsincrease were in theopposite cutting to speed the results led to for a reduction the anode. in The the di HAZ.fference These in behavior results were was opposite explained to by the two results facts. for Firstly, the anode.the intensity The difference and energy in dibehaviorfference betweenwas explained the ablation by two threshold facts. Firstly, and the the thermal intensity stress and threshold energy differencewere smaller between for the the cathode ablation active threshold material and than th fore thethermal anode stress active threshold material. were Secondly, smaller the ablationfor the cathodethreshold active of the material cathode than active for material the anode was active higher material. than that Secondly, of the anode the activeablation material. threshold of the cathode active material was higher than that of the anode active material. BatteriesBatteries 20182019, ,45, ,x 73 FOR PEER REVIEW 1212 of of 21 20

(a) (b)

FigureFigure 9. 9. ModelModel for for the the influence influence of of the the pulse pulse repetition repetition frequency frequency and and cutting cutting speed speed on on the the cathode cathode cuttingcutting edge: edge: 72 72 w, w, 240 240 ns; ns; (a) ( aInfluence) Influence on on the the chamfer chamfer width width (Cubic (Cubic fitting: fitting: R² = R0.91,2 = 0.91,Adjusted Adjusted R² = 0.90,R2 = Predicted0.90, Predicted R² = 0.88); R2 = 0.88);(b) Influence (b) Influence on the on heat-affected the heat-affected zone zone (Cubic (Cubic fitting: fitting: R² = R 0.94,2 = 0.94, Adjusted Adjusted R² =R 0.93,2 = 0.93, Predicted Predicted R² = R0.92).2 = 0.92).

FurtherFurther investigations investigations outside outside of of the the considered considered pa parameterrameter space space in in the the range range of of very very low low cutting cutting -1 speedsspeeds (50 mmsmms-1) andand veryvery high high energy energy densities, densities, respectively, respectively, showed showed that that on the on cathode the cathode and anode and anodecutting cutting edge, edge, either either no or no very or smallvery small HAZ HAZ could co beuld identified. be identified. This This was was caused caused by theby the slope slope of theof theintensity intensity distribution distribution and and the the very very high high energy energy input. input. As aAs result, a result, the areasthe areas that that were were previously previously only onlythermally thermally stressed stressed at lower at lower intensities intensities were subjectedwere subjected to material to material removal removal at higher at energy higher densities. energy 1 densities.Furthermore, Furthermore, the high energythe high density energy at density 50 mms at− 50led mms to an−1 led increase to an inincrease the ablation in the area,ablation since area, the sinceablation the thresholdsablation thresholds of the collector of the collector and active and material active dimaterialffer significantly. differ significantly. WithWith regard regard to to the the strong strong influence influence of of the the PRF PRF on on the the chamfer chamfer width width and and the the HAZ, HAZ, the the influence influence ofof the the pulse pulse duration duration and and the the pulse pulse peak peak power power on on different different PRF PRF was was investigated investigated in in a afurther further study. study. ForFor this this purpose, purpose, cuts cuts were were performed performed at at a a consta constantnt energy energy density density with with a a variation variation of of the the pulse pulse durationduration and and pulse pulse peak peak power. power. Pulse Pulse duration duration was was kept kept constant constant (240 (240 ns) ns) with with the the effect effect that that the the pulsepulse peak peak power decreaseddecreased withwith increasingincreasing frequency frequency (70 (70 kHz: kHz: 13 13 kW, kW, 102 102 kHz: kHz: 6 kW, 6 kW, 200 200 kHz: kHz: 2 kW, 2 kW,291 kHz:291 kHz: 1 kW, 1 kW, 403 403 kHz: kHz: 0.7 kW,0.7 kW, 490 490 kHz: kHz: 0.55 0.55 kW). kW). The The pulse pulse duration duration was was shortened shortened (240–20 (240–20 ns) ns)to keepto keep the the pulse pulse peak peak power power quasi quasi constant constant (70 kHz:(70 kHz: 13 kW, 13 kW, 102/ 200102/200/291/291 kHz: kHz: 10 kW, 10 kW, 403/ 490403/490 kHz: kHz:9 kW). 9 ThekW). results The results for the chamferfor the chamfer width and width the heat-aand theffected heat-affected zone derived zone from derived these experimentsfrom these experimentsare shown in are Figure shown 10. Thein Figure PRF variation 10. The PRF with vari constantation pulsewith durationconstant showedpulse duration the same showed tendencies the sameas in thetendencies previously as in presented the previously models presented in Figures models8 and9 .in Figures 8 and 9. AA reduction reduction of of the the pulse pulse length length with with a a quasi-constant quasi-constant pulse pulse peak peak power power led led to to a a larger larger chamfer chamfer widthwidth and and HAZ, HAZ, both both at at the the anode anode and and at at the the cath cathodeode (Figure (Figure 10).10). On On the the anode anode side, side, the the reduction reduction ofof the the pulse pulse length length led led to to a significant a significant enlargemen enlargementt of the of the heat-affected heat-affected zone zone for the for PRF the PRF102 and 102 200 and kHz.200 kHz.Due to Due the tohigher the higher PRF and PRF the and high the intensitie high intensities,s, the plasma the plasmaformed was formed of higher was of energy, higher leading energy, toleading enhanced to enhanced thermal thermalstress of stress the electrode of the electrode surface surfaceand, thus, and, potentially thus, potentially to a higher to a higher ablation. ablation. The reductionThe reduction of the of HAZ the HAZby the by increased the increased frequency frequency of the ns of laser the ns pulses laser was pulses thus was determined thus determined largely bylargely the low by thepulse low energy. pulse energy. Only by Only a much by a muchgreater greater reduction reduction of the of pulse the pulse length length of less of lessthan than 10 picoseconds,10 picoseconds, a cold a cold cutting cutting is possible is possible [14]. [14 ]. Batteries 2018, 4, x FOR PEER REVIEW 13 of 21 Batteries 2019, 5, 73 13 of 20

(a) (b)

Figure 10.10. InfluenceInfluence of of the the pulse pulse duration duration at diatff differenterent pulse pulse repetition repetition frequencies frequencies on the on chamfer the chamfer width andwidth heat-a andff heat-affectedected zone of zone the anodeof the (anodea) and ( thea) and cathode the cathode (b) cutting (b) cutting edge. edge.

The increasedincreased chamfer chamfer width width for for the the anode anode and and the cathodethe cathode at higher at higher pulse pulse peak powerspeak powers at higher at PRFhigher confirmed PRF confirmed the assumption the assumption that was previouslythat was pr madeeviously on the made anode on andthe anode cathode and model. cathode In addition model. toIn theaddition decrease to the in pulsedecrease peak in power, pulse peak the decrease power, the in pulse decrease energy in pu atlse higher energy PRF at led higher to a reductionPRF led to of a thereduction chamfer of width.the chamfer width. InIn thethe followingfollowing experiment,experiment, the the scalability scalability of of the the cutting cutting speed speed or or the the influence influence of of the the number number of hitsof hits at constantat constant energy energy density density was was investigated investigated (Figure (Figure 11). 11). The The results results for for the the anode anode showed showed that that at constantat constant energy energy density, density, the chamferthe chamfer width width increased increa atsed a reduced at a reduced rate. Arate. reason A reason for this for was this the was lower the intensitylower intensity and the and correlated the correlated intensity intensity distribution, distribution, as well as as well the loweras the energylower energy of the pulse,of the whichpulse, narrowedwhich narrowed the profile the ofprofile the material of the material removal threshold.removal threshold. As a result, As the a result, geometric the geometric distance between distance a fullbetween cut and a full the cut ablation and the of theablation active of material the active increased material (see increased Figure3 .).(see In Figure general, 3.). the In cuttinggeneral, kerf, the ascuttingBatteries well as2018kerf, the, 4 ,as entirex FOR well PEER areaas the REVIEW in whichentire area material in which was removed, material was became removed, smaller became as a result. smaller Because as a result.14 of of the 21 displacementBecause of the ofdisplacement the removal of thresholds, the removal a largerthreshol chamferds, a larger width chamfer was produced width was at lowerproduced speeds at lower for a specificspeeds for energy a specific density. energy density. The fluence resulting from the reduction of the average power to 33.3% was still 1.15 jcm−2 and was thus above the limit for the formation of plasma for aluminum. Despite the high number of hits, it could be assumed that shielding effects were negligible as the intensity was much lower, and the larger plasma formation led to increased removal of the active material. When the average energy was reduced to one-third of the maximum power, no cut could be made through the copper collector at a frequency of 490 kHz for the energy density being studied. Due to the strong reduction of the pulse energy and peak pulse power, the beam could no longer be coupled because of the low absorption of the copper. When cutting copper, it is first heated by the radiation until it oxidizes [10]. As soon as the material oxidizes, the radiation can be much better coupled into the material, and only then leads to the sufficiently high absorption of the laser radiation for a complete cut.

Figure 11. Influence of the intensity and the number of hits per area increment on the chamfer width at 2 aFigure constant 11. energy Influence density of the of intensity 328 jcm− and. the number of hits per area increment on the chamfer width at a constant energy density of 328 jcm–2. 2 The fluence resulting from the reduction of the average power to 33.3% was still 1.15 jcm− and was thusCathode above investigations the limit for the showed formation similar of plasma tendencies, for aluminum. which Despitewere much the high less number pronounced. of hits, itCompared could be assumedto the anode, that shieldingthe cathode eff ectscould were be cut negligible at 33% of as the the maximum intensity was average much power, lower, although and the largerthe cathode’s plasma formation aluminum led collector to increased was removaltwice as of thick the active as the material. anode’s Whencopper the collector. average energyThe results was basically showed that the necessary energy density could be used as a second parameter to define a cut-through limit.

3.2. Influence of the Cutting Edge Characteristics and the Process Parameters on the Electrochemical Performance Based on the knowledge gained from the experiments, a parameter study was developed to investigate the influence of the presented product and process properties on the electrochemical performance of the electrode, or the cell. In a first study, a cell was built with anodes and cathodes cut using the same laser and system parameters. The parameter configuration for this cell was referred to as V0 and served as a reference system for the variation of the parameter configuration. This parameter configuration was very reliable in terms of possible fluctuations in the focus position and the layer thickness. In a first limitation, it was examined whether the cutting edge on the anode side, or the cathode side, had a greater influence on the electrochemistry and, thus, the performance of the cell. In further experiments, only the cutting edge of the performance controlling electrode was examined. For the first experiments, cells with anodes (V1) and cathodes (V2) with a very large chamfer width were built. To produce this very large chamfer width, the electrodes were cut with a very high energy density and intensity. In the following, the parameters PRF (V3), Vc (V4), and τ (V5) were varied at a constant energy density at the electrode. Based on the parameter configurations shown in Table 1, it was possible to evaluate the previously presented cutting edges characteristics and process characteristics with regard to their influence on the electrochemical performance of the cell.

Table 1. Set laser parameters to investigate the impact on electrochemical performance.

Parameter Unit V0 V1 V2 Vc mm s–1 300 50 300 PRF kHz 70 70 70 Anode τ ns 240 240 240 ED j cm–2 328 1968 328 IPeak W cm–2 3.02 × 108 3.02 × 108 3.02 × 108 Batteries 2019, 5, 73 14 of 20 reduced to one-third of the maximum power, no cut could be made through the copper collector at a frequency of 490 kHz for the energy density being studied. Due to the strong reduction of the pulse energy and peak pulse power, the beam could no longer be coupled because of the low absorption of the copper. When cutting copper, it is first heated by the radiation until it oxidizes [10]. As soon as the material oxidizes, the radiation can be much better coupled into the material, and only then leads to the sufficiently high absorption of the laser radiation for a complete cut. Cathode investigations showed similar tendencies, which were much less pronounced. Compared to the anode, the cathode could be cut at 33% of the maximum average power, although the cathode’s aluminum collector was twice as thick as the anode’s copper collector. The results basically showed that the necessary energy density could be used as a second parameter to define a cut-through limit.

3.2. Influence of the Cutting Edge Characteristics and the Process Parameters on the Electrochemical Performance Based on the knowledge gained from the experiments, a parameter study was developed to investigate the influence of the presented product and process properties on the electrochemical performance of the electrode, or the cell. In a first study, a cell was built with anodes and cathodes cut using the same laser and system parameters. The parameter configuration for this cell was referred to as V0 and served as a reference system for the variation of the parameter configuration. This parameter configuration was very reliable in terms of possible fluctuations in the focus position and the layer thickness. In a first limitation, it was examined whether the cutting edge on the anode side, or the cathode side, had a greater influence on the electrochemistry and, thus, the performance of the cell. In further experiments, only the cutting edge of the performance controlling electrode was examined. For the first experiments, cells with anodes (V1) and cathodes (V2) with a very large chamfer width were built. To produce this very large chamfer width, the electrodes were cut with a very high energy density and intensity. In the following, the parameters PRF (V3), Vc (V4), and τ (V5) were varied at a constant energy density at the electrode. Based on the parameter configurations shown in Table1, it was possible to evaluate the previously presented cutting edges characteristics and process characteristics with regard to their influence on the electrochemical performance of the cell.

Table 1. Set laser parameters to investigate the impact on electrochemical performance.

Parameter Unit V0 V1 V2 –1 Vc mms 300 50 300 PRF kHz 70 70 70 τ ns 240 240 240 Anode ED jcm–2 328 1968 328 –2 8 8 8 IPeak Wcm 3.02 10 3.02 10 3.02 10 × × × nline - 17 103 17 –1 Vc mms 300 50 300 100 300 PRF kHz 70 70 490 490 490 τ ns 240 240 240 240 20 Cathode ED jcm–2 328 1968 328 328 328 –2 8 8 8 8 8 IPeak Wcm 3.02 10 3.02 10 0.17 10 0.1 10 2.09 10 × × × × × nline - 17 103 120 362 120

Cutting speed (Vc), Pulse repetition frequency (PRF), Pulse duration (τ), Energy density (ED), Intensity (IPeak), Number of laser pulses per surface increment (nline).

The evaluation of the electrochemical performance showed that the cutting edge of the cathode exerted the greater influence. Therefore, the influence of cut edge characteristics and process configurations at the cathode was investigated. In the following, the characteristics chamfer width and heat-affected zone have been shown for the examined parameter configurations V0 to V5. The comparison of the features in Figure 12 showed that the total affected area consisting of the heat-affected zone and chamfer width was the largest for the reference parameter V0 for both the anode and the cathode. The largest chamfer width with almost identical characteristics showed the Batteries 2018, 4, x FOR PEER REVIEW 15 of 21

nline - 17 103 17 Vc mm s–1 300 50 300 100 300 PRF kHz 70 70 490 490 490 τ ns 240 240 240 240 20 Cathode ED j cm–2 328 1968 328 328 328 IPeak W cm–2 3.02 × 108 3.02 × 108 0.17 × 108 0.1 × 108 2.09 × 108 nline - 17 103 120 362 120

Cutting speed (Vc), Pulse repetition frequency (PRF), Pulse duration (τ), Energy density (ED),

Intensity (IPeak), Number of laser pulses per surface increment (nline).

The evaluation of the electrochemical performance showed that the cutting edge of the cathode exerted the greater influence. Therefore, the influence of cut edge characteristics and process configurations at the cathode was investigated. In the following, the characteristics chamfer width and heat-affected zone have been shown for the examined parameter configurations V0 to V5. The comparison of the features in Figure 12 showed that the total affected area consisting of the heat-affected zone and chamfer width was the largest for the reference parameter V0 for both the anode and the cathode. The largest chamfer width with almost identical characteristics showed the Batteries 2019, 5, 73 15 of 20 parameters V1 for the anode and V2 for the cathode. The further investigations with the parameters V3 to V5 showed, on the cathode side, the smallest influenced area with an average chamfer width parametersof 70 µm to V180 forµm. the anode and V2 for the cathode. The further investigations with the parameters V3 to V5 showed, on the cathode side, the smallest influenced area with an average chamfer width of 70 µm to 80 µm.

Figure 12. Response to the parameters, shown in Table1.

The investigation ofFigure the 12. influence Response ofto the the parameters, presented shown cutting in Table edge 1. characteristics on the electrochemical performance was carried out by means of the cyclization routine defined in Section 2.4. The resultThe investigation of the diagnosis of ofthe the influence electrochemical of the performancepresented cutting is shown edge in thecharacteristics form of normalized on the cyclizationelectrochemical curves performance in Figure 13 .was The carried cyclization out curvesby means showed of the that cyclization in comparison routine to defined the reference in Section (V0), the2.4. cathodeThe result cutting of the edge diagnosis (V2) exertedof the electrochemi a significantlycal performance greater influence is shown on the in cyclethe form stability of normalized than the anodecyclization (V1). curves In this case,in Figure the mean 13. The value cyclization for all cells, curves with showed the anode that (V1), in liedcomparison on the mean to the value reference of the reference(V0), the cathode cells (V0) cutting with a edge similar (V2) standard exerted deviationa significantly after greater 350 cycles. influence The cells on the with cycle the stability cathode than (V2) werethe anode far above (V1). this In this value case, and the thus mean had value significantly for all cells, higher with cycle the stability.anode (V1), All lied tests on to the determine mean value the influenceof the reference of the chamfercells (V0) width with a (V3–V5) similar standard on the cathode deviation side after showed 350 thatcycles. with The low cells chamfer with the width, cathode no improvement(V2) were far above of the this cycle value stability and thus could had be signific achievedantly compared higher cycle to V2. stability. Since the All cells tests V2 to todetermine V5 had higherthe influence cycle stability of the chamfer than the width reference (V3–V5) cells on V0, the the cathode statement side showed could be that made with that low the chamfer heat-aff width,ected zoneno improvement on the cathode of the side cycle had stability a greater could influence be achiev on theed electrochemicalcompared to V2. stabilitySince the of cells the V2 cell to than V5 had the chamferhigher cycle width. stability Furthermore, than the the reference cycling curvescells V0, of cellsthe statement V3 and V5 could were nearlybe made identical that the after heat-affected 350 cycles. Fromzone this,on the it couldcathode be side deduced had a that greater the pulse influence duration on the or pulseelectrochemical peak power stability as process of the parameters cell than didthe not exert a significant influence on the electrochemical performance. The cells with cathodes cut at constant line energy at reduced speed (V4) showed greater cycle stability after 350 cycles than the V3 and V5 cells. Despite the optimized process control, the cycle stability of the cells V4 was below that of V2. The cells V3 to V5 showed a greater capacity drop than the cells from the series V2, although they had a small chamfer width and no heat-affected zone. This was an indication that in addition to the previously known and analyzed product features, another, previously unrecognized feature, influenced the electrochemical performance. Batteries 2018, 4, x FOR PEER REVIEW 16 of 21 chamfer width. Furthermore, the cycling curves of cells V3 and V5 were nearly identical after 350 cycles. From this, it could be deduced that the pulse duration or pulse peak power as process parameters did not exert a significant influence on the electrochemical performance. The cells with cathodes cut at constant line energy at reduced speed (V4) showed greater cycle stability after 350 cycles than the V3 and V5 cells. Despite the optimized process control, the cycle stability of the cells V4 was below that of V2.

Batteries 2019, 5, 73 16 of 20

Figure 13. Capacity fading during the long-time cyclization of the investigated electrode/ cell configuration. Figure 13. Capacity fading during the long-time cyclization of the investigated electrode/cell 3.3.configuration. Further Investigations of the Cutting Process and the Electrode Surface to Explain the Electrochemical Behavior The cells V3 to V5 showed a greater capacity drop than the cells from the series V2, although In addition to the physical characteristics of the cutting edge, it was found that the different they had a small chamfer width and no heat-affected zone. This was an indication that in addition to parameters led to a different degree of flying sparks, as shown in Figure 14. This could be recorded the previously known and analyzed product features, another, previously unrecognized feature, by imaging techniques. The sparking could lead to contamination of the electrode and thus affect influenced the electrochemical performance. the electrochemical performance. The images showed that with increased pulse repetition frequency (V0 V3), the distance of the spark flight increased by 2.5 times. This could be explained by the 3.3. Further→ Investigations of the Cutting Process and the Electrode Surface to Explain the Electrochemical Behaviorincreased number of hits if the cutting speed remained the same. If the number of hits on the active material, as well as on the solid and molten collector, increased, the number and acceleration of the ablationIn addition products to wouldthe physical increase characteristics too. A reduction of the of cutting the cutting edge, speedit waswith found constant that the line different energy parameters(V4) showed led a to strong a different reduction degree of the of sparks.flying spar Theks, lower as shown level ofin theFigure spark 14. formation This could was be duerecorded to the bylower imaging energy techniques. and intensity The persparking pulse, could as well lead as theto contamination reduced travel of speed. the electrode The profile and of thus the affect sparkling the electrochemicalflight was very performance. similar to the The sparking images profile showed at low that pulse with repetitionincreased frequencypulse repetition and traversing frequency speed (V0 →(V2). V3), Since the distance variation of V2the was spark cut flight with increased a lower pulse by 2.5 repetition times. This frequency could be than explained variation by the V4, increased this could numberlead to lessof hits contamination if the cutting of speed the electrode remained and the thus same. explain If the the number impairment of hits of on the the performance active material, of V4 ascompared well as on to the V2. solid and molten collector, increased, the number and acceleration of the ablation productsIn addition would increase to the assessment too. A redu ofction the flying of the sparks, cutting the speed recordings with co couldnstant strengthen line energy the (V4) assumptions showed amade strong in reduction the previous of the sections sparks. concerning The lower the level formation of the spark of the formation plasma. A was qualitative due to the comparison lower energy of the andimages intensity in Figure per pulse,14 showed as well that as V0, the V2, reduced and V5 trav showedel speed. a clearly The profile purple-colored of the sparkling plasma flight formation, was verywhereas, similar for to the the other sparking parameters—V3 profile at low and pulse V4, re thepetition plasma frequency presumably and traversing lied below speed the ablation (V2). Since gas variationphase, and V2 thus was wascut with significantly a lower pulse smaller. repetition This also frequency explained than why variation at shorter V4, pulse this could lengths lead or to higher less contaminationpulse peak powers, of the theelectrode material and removal thus explain and the th formatione impairment of the of the HAZ performance at a constant of energyV4 compared density towas V2. greater. In addition to the assessment of the flying sparks, the recordings could strengthen the assumptions made in the previous sections concerning the formation of the plasma. A qualitative comparison of the images in Figure 14 showed that V0, V2, and V5 showed a clearly purple-colored plasma formation, whereas, for the other parameters—V3 and V4, the plasma presumably lied below Batteries 2018, 4, x FOR PEER REVIEW 17 of 21

the ablation gas phase, and thus was significantly smaller. This also explained why at shorter pulse lengths or higher pulse peak powers, the material removal and the formation of the HAZ at a constant Batteries 20182019,, 45,, x 73 FOR PEER REVIEW 1717 of of 21 20 energy density was greater. the ablation gas phase, and thus was significantly smaller. This also explained why at shorter pulse lengths or higher pulse peak powers, the material removal and the formation of the HAZ at a constant energy density was greater.

(V0) (V2)

(V0) (V2)

(V3) (V4)

(V3) (V4)

(V5)

Figure 14. Optical process investigations regarding the flyingflying sparks.

The changechange in in spark spark travel travel might might lead lead to a changeto(V5) a change in the degreein the ofdegree contamination of contamination of the electrodes of the and,electrodes thus, and, in addition thus, in to addition the cut edgeto the quality, cut ed adverselyge quality, aff ectadversely the electrochemical affect the electrochemical performance. Figure 14. Optical process investigations regarding the flying sparks. Theperformance. contaminations The contaminations of the electrode of werethe electrode analyzed were by SEManalyzed/EDX by images SEM/EDX and are images shown and in are Figure shown 15. Contamination products in the form of metal spatters could be identified on all electrodes tested. in FigureThe change 15. Contamination in spark travel products might inlead the to form a change of metal in thespatters degree could of contamination be identified ofon theall The reference cathode had the strongest metal spatter contaminations. The lowest contamination electrodeselectrodes tested.and, thus, The referencein addition cathode to the had cut the ed stgerongest quality, metal adversely spatter contaminations.affect the electrochemical The lowest occurred with the parameter variations V2 and V4. performance.contamination The occurred contaminations with the parameterof the electrode variations were analyzedV2 and V4. by SEM/EDX images and are shown These results showed that fewer sparks formation led to less contamination of the electrode surface in Figure 15. Contamination products in the form of metal spatters could be identified on all in the area of the cutting edge. Based on the results of the electrochemical diagnosis and the analysis of electrodes tested. The reference cathode had the strongest metal spatter contaminations. The lowest the cutting edges, it could be assumed that the contaminations on the cathode surface exerted a greater contamination occurred with the parameter variations V2 and V4. influence on the electrochemical performance than the heat-affected zone and the chamfer width.

+ (V0) (V2) + (V0) (V2) Figure 15. Cont. Batteries 2019, 5, 73 18 of 20 Batteries 2018, 4, x FOR PEER REVIEW 18 of 21

(V3) (V4)

(V5)

Figure 15. SEM/EDXSEM/EDX analysis of the cathode surface next to the cutting edge:edge: Contamination of the surface by molten aluminum splashessplashes are marked turquoise: 1. Electrode surface, 2. Cut Cut zone. zone.

4. Conclusions These results showed that fewer sparks formation led to less contamination of the electrode surfaceBy in means the area of a pulsedof the cutting nanosecond edge. fiber Based laser, on electrodesthe results could of the be electrochemical cut without significantly diagnosis aandffecting the analysisthe cell cycleof the stability. cutting edges, The influence it could be of theassumed laser parametersthat the contam or theinations cutting on edge the cathode showed surface only a exertedsmall influence a greater on influence the examined on the variations. electrochemical Considering performance a linear than behavior, the heat-affected the parameter zone V1 and led tothe a chamfertheoretical width. number of cycles of 1670 until the cells reached a state of health (SOH) of 80%. The best parameter V2 resulted in a cycle number of 1760. Since the capacity drop tended to decrease, it could 4.be Conclusions assumed that the cells would also reach higher cycles up to a SOH of 80%. ByThe means examined of a characteristics—chamferpulsed nanosecond fiber width laser, andelectrodes heat-a ffcouldected be zone—could cut without be significantly adjusted by affectingmeans of the the cell pulse cycle duration, stability. the The pulse influence repetition of the frequency, laser parameters and the or cutting the cutting speed. edge In principle,showed only the aresults small showedinfluence that on thethe chamferexamined width variations. decreased Cons withidering decreasing a linear energybehavior, density the parameter or with increasing V1 led to acutting theoretical speed. number Furthermore, of cycles with of 1670 a constant until the pulse cell durations reached and a state energy of density,health (SOH) higher of pulse 80%. repetition The best parameterfrequencies V2 could resulted result in ina cycle a smaller number chamfer of 1760. width. Since This the wascapacity probably drop duetended to the to decrease, fact that bothit could the bedecrease assumed of thethat energy the cells and would the decrease also reach of higher the intensity cycles up led to to a a SOH geometric of 80%. shift of the removal and the thermalThe examined load threshold. characteristics—chamfer This shift was probably width explainedand heat-affected by the Gaussian zone—could intensity be adjusted profile and by meansthe corresponding of the pulse slopeduration, of the the intensity pulse repetition in relation frequency, to the spot and size, the ascutting well asspeed. by the In intensityprinciple, andthe resultsenergy-dependent showed that formation the chamfer of thewidth plasma. decreased The experiments with decreasing with energy different density pulse durationsor with increasing with the cuttingsame pulse speed. energy Furthermore, showed, for with different a constant pulse repetition pulse duration frequencies and and energy constant density, energy higher density, pulse that repetitionthe material frequencies removal and could the result thermal in load,a smaller among cham thefer pure width. laser-material This was probably interaction, due depended to the fact on that the bothplasma the or decrease the intensity. of the Theenergy analysis and the of thedecrease size of of the the heat-a intensityffected led zone to a revealedgeometric that shift it wasof the essentially removal andinfluenced the thermal by the load pulse threshold. repetition This frequencies. shift was The prob causeably ofexplained this dependency by the Gaussian could be explainedintensity profile by the and the corresponding slope of the intensity in relation to the spot size, as well as by the intensity and energy-dependent formation of the plasma. The experiments with different pulse durations with Batteries 2019, 5, 73 19 of 20 same assumptions that have been made previously for the formation of the chamfer width. Both the anode and the cathode showed similar tendencies. In general, the anode could be cut much faster than the cathode due to the double material thickness of the collector, so that the parameter space for the model development in terms of speed for the cathode turned out smaller than for the cathode. Investigations to evaluate the influence of the chamfer width and the heat-affected zone on the electrochemical performance of large-sized multicompartment pouch cells showed that the HAZ had a greater influence than the chamfer width. Furthermore, the results showed that contamination products in the form of metal spatter influenced the electrochemical performance more than the width of the chamfer and probably also the HAZ. It could be shown by imaging techniques that with very low speeds and high intensities and pulse energies with a moderate number of hits, metal spatters could be reduced. Furthermore, it was possible to reduce the melting spatters even for a high number of hits per surface increment with a low traversing speed, intensity, and pulse energy. The results showed, so far, that the formation of the metal spatters was a function of the number of hits per surface increment, the speed of travel, the intensity, and the energy per pulse.

5. Outlook The results showed that the electrochemical performance of the electrodes, or the cells, was essentially affected by contamination products and less by the chamfer width or heat-affected zone investigated in this study. Based on this knowledge, it is imperative to find out which contaminations and concentrations are to be considered critical with regard to the electrochemical performance. Furthermore, the pulsed ns laser system should be used to investigate which parameters lead to which contaminations and whether the cw laser system offers process-inherent advantages with regard to the contamination of the electrode surface. On the basis of these further investigations, unknown new laser systems could be designed process-safe to fulfill the minimum requirement of four Sigma rejects.

Author Contributions: T.J. and M.W.K. wrote and edited the main parts of the paper, S.H. analyzed the REM/EDX data. L.H. analyzed the electrochemical data. W.H. analyzed the electrochemical data and administered the funding acquisition. K.D. administered the research project. All authors contributed to scientific discussions. Funding: The results were generated in the project DaLion (03ET6089) funded by the Federal Ministry of Economic Affairs and Energy. The authors express their gratitude for the financial support by the Ministry and for the project management by the ProjektträgerJülich. Acknowledgments: Hereby I would like to thank my student assistant Julien Essers for his technical support. Conflicts of Interest: The authors declare no conflict of interest.

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