An experimental and analytical study on intermittent slot die coating of viscoelastic battery slurries.
Keywords Slot die coating, Intermittent coating, Viscoelastic fluid, Non-Newtonian fluid, Lithium-ion batteries
In times of increasing energy prices, local renewable energy sources are becoming more and more important for our everyday power supply. (1,2) Although photo-voltaic and wind power technologies are already highly efficient, (3,4) they are strongly dependent on their environmental conditions and are not always available. A reasonable approach is to use secondary batteries to store the electric energy provided by such transient sources. Characterized by large cycle stabilities and no memory effects, lithium-ion batteries (LIB) are suitable for use as local storage. (5)
Much experience with lithium-ion batteries has been gained by using them for small-scale mobile applications, such as mobile phones, notebooks, and digital cameras. (6) To date, the cost-intensive manufacturing process of suitable large-scale LIB cells prevents their extensive usage in other fields, such as electric mobility or stationary power supply. (4,6) One way to decrease these costs is to speed up the production lines. Figure 1 shows schematically the main process steps for stacked LIB cell production.
The typical LIB electrode film consists of active material particles, binder, and additives. (5,7-9) In the dry state, the electrode structure must be porous, to allow for a total penetration of the electrolyte as lithium-ion transport path to the active materials. To deliver current, the electrode needs to be a conductor and also connected to suitable metal foils as current arrestor. During manufacturing, the electrode components are mixed with a solvent and coated on the metal substrate as a liquid film. Here, the most precise and common coating method is the pre-metered slot die coating. (7) In a subsequent drying step, the solvent is evaporated, leaving the desired porous film structure. To increase adhesion and energy density, the electrodes are calendered to a desired porosity. (5,7,10) During the calendering step, the film and the coated part of the metal foil are stretched. Depending on the calendering pressure, this stretching can lead to undesired folds at the intersections between coated and uncoated areas of the substrate (Fig. 2, left). Amplified by super-elevations at the edge of the film (so-called heavy edges), (11) these folds can cause film-peeling and prevent precise electrode cutting in the cell assembly step. One way to prevent this critical defect is to cut off the uncoated substrate and the edge of the coating. Due to the need for an uncoated current arrester, this is not possible for continuously coated substrates. For this reason, some manufacturers coat intermittently and place the current arresters in the uncoated gaps between the film patterns (Fig. 2, right). This allows the critical edge region to be cut off, preventing problems during calendaring.
Nevertheless, coating intermittently reduces production stability. The process must repeatedly leave its steady state, which limits the coating speed. Since the process of intermittent coating is not yet scientifically understood, its parameters are generally adjusted empirically.
The aim of this work is to fill in this knowledge gap and provide scientific insight. In general, intermittent coating consists of two more or less steady states--coating and not coating--separated by two hard-to-control transitions. At the coating start, the liquid and (depending on the process concept) parts of the machinery must be accelerated. For slot die coating, a stable coating bead between die lips and coated substrate is crucial for a homogenous film. (12,13) Therefore, the coating bead must be established as fast as possible before the coating starts. In the second transition, the converse is true; all the liquid must be stopped and the coating bead removed quickly and free of residues. Making things worse, LIB slurries are characterized by viscoelastic behavior. (9) Although the effect of viscoelasticity on coating limits (14,15) and the start-up of a coating bead (16) have been mentioned in literature, a detailed investigation of the impact on repeatedly starting and stopping the coating process is still missing. In spite of the lack of scientific understanding, there are many technologies available in industry for intermittent LIB slot die coating. Some companies have developed special bypass valves to manipulate the feed, (17,18) while others also use slot dies with variable inner geometry. (19-24) A rapid build-up or break-up of the bead is often accomplished via a gap alternation. (25,26) However, all of these technologies are optimized empirically and none of them has been described theoretically.
In this work, we investigate the dominating and limiting mechanisms of intermittent coatings and analyze how they can be controlled. We have simplified the experimental set-up to its most basic form, in order to provide transferability to all industrial concepts. This explicitly excludes any process which avoids slurry leaking. Aiming for an easy process control, we took a closer look at the relation of system pressure and wet film thickness for various coating velocities.
For investigation of basic intermitting technologies, the experimental set-up was built with only the most essential equipment.
A syringe pump (Chemyx Nexus 6000) with stiff two-piece syringes (HSW Norm-Ject, 60 mL) is used for liquid supply. It provides a constant, pressure-independent volume flow, which was needed for the investigations in this work. A pumping system with a performance curve (e.g., progressing cavity pumps) would amplify pressure variations caused by the switching bypass valve and complicate the measurements. The piping was realized using stiff Teflon hoses connected via clamping rings (Swagelok, Hy-Lok). To analyze the fluid pressure, two pressure transducers (Omega PR25Y-V-10-M12) were installed. For an online and time-resolved evaluation, the data were collected by a microcontroller (MC, Arduino Uno rev.3). To detect any pressure variation in the upstream system, the first pressure transducer was located between the syringe pump and valve. The applied pneumatic bypass valve (Swagelok SS-43GXLS6MMA15S3) is the crucial part of this set-up and there are many possible layouts. Various companies have filed many patents in this field. Again, the complexity was kept to a minimum by integrating a basic 3-way pneumatic driven ball valve. The switching in the waste-pipe direction was spring supported and thus faster than the switching in the slot die direction. During every coating interruption, the liquid was directed through a waste pipe into a collecting tank. To investigate the effect of the local pressure drop, we applied different waste pipes of various lengths. During the coating phase, the bypass valve directed the liquid into the slot die. We applied a die with two cavities and two slots with slightly different dimensions (Table 1). The second mounted pressure transducer was integrated in the downstream cavity (Fig. 4).
As described elsewhere, (11,27) the slot die was mounted in the 8 o'clock position against a stainless steel backing roll (see Fig. 3). (28,29) On the roll, the coated wet film was profiled by means of a two-dimensional laser triangulation system (Keyence LJ-V7060). The resulting wet film profile was then correlated to the die pressure distribution.
To investigate the major mechanisms behind intermittent LIB coatings, we used a representative waterbased anode slurry. It consisted of a waterborne latex dispersion and carboxymethyl cellulose (CMC) binder system, with slightly flaked graphite as the active material. To describe the shear-thinning viscosity of the slurry, a power-law approach was applied:
[eta] = [Kappa][[??].sup.[epsilon]-1] (1)
The consistency factor and the power law exponent were fitted to [Kappa] = 59.4Pa [s.sup.[epsilon]] and [epsilon] = 0.37, respectively. Measurements with a rotational rheometer show the shear-thinning character of the slurry in the relevant range of shear rates (see Fig. 5, left). For a coating speed of [u.sub.w] = 5m/min and a gap of [h.sub.G] = 127 [micro]m, we expect a shear stress of approximately [??] = 6561 1/s. This results in a viscosity of [eta] = 1Pa s. Former measurements with a capillary rheometer confirm the shear-thinning trend of the recipe used for shear rates up to 100,000 1/s, as has been shown elsewhere. (29) To verify the applied pressure calculations, we also applied a comparable viscous Newtonian model system (silicon oil, Rotitherm[R] M 220, [eta] = 1 Pa s). In advance, the applied anode slurry shows a viscoelastic behavior as can be seen in the frequency sweep in Fig. 5 (right). This may result, for each intermitting transition state, in a dampening of any slurry acceleration or deceleration.
In the first series of experiments, we investigated the relationship between die pressure and coated wet film thickness. Because of the viscoelastic slurry behavior, we expected ramped profiles for the die pressure and each thickness profile. Intermittent electrode films were achieved simply by opening and closing the bypass valve. Since the process was found to be fully reproducible, each pattern signal of the pressure transducers should be comparable to a random laser-profiled pattern.
In further experiments, we examined the role of the waste pipe pressure drop. Apparently, the pressure level upstream from the bypass valve should remain constant between the coating and interruption phases. Otherwise, one would expect an additional pressure leveling or ramping for each pattern. To predict and adjust the needed steady waste pipe pressure drop, the system pressure during the steady coating phase was calculated. To do so, we combined available equations from literature.
Pressure drop calculation
Starting from the upper pressure transducer (see Fig. 3) toward the coating bead, an expression for parabolic pipe flow is needed. The pressure drop in a pipe is given by the Hagen-Poiseuille expression. Modified with the representative shear rate for a power-law fluid in a circular pipe, (30) the expression becomes:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
Due to the pre-metered character of slot die coating, the flow rate could be substituted for using the die width, coating speed, and film thickness:
v = [w.sub.s] x [u.sub.w] x h (3)
The same approach applies for the flow inside the slot die. Combining the expression of pressure drop for a slot flow with its representative shear rate (30) yields the expression:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)
Describing the pressure drop in the coating gap is more complex. Due to a moving wall (the moving substrate), the Hagen-Poiseuille flow is superposed by a Couette flow. Durst (31) gives an expression for Newtonian fluids:
[DELTA][p.sub.G] = 1.34 [Ca.sup.3/4] [sigma]/h + [l.sub.D] (1 - 2h/[h.sub.G])(6[eta][u.sub.w][l.sub.D]/[h.sup.2.sub.G]) (5)
For 2h>[h.sub.G], Lee (32) solves an expression for a power law pressure drop inside a coating gap:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (6)
wherein the first summand describes the pressure beneath the downstream meniscus, known from the correlation by Ruschak. (33) Although this correlation was derived for Newtonian liquids, we assume only minor deviations, due to a small ho to h ratio and the overall small value of this part. The capillary number in the coating gap for power-law fluids (34) is approximately given by:
Ca = [Kappa] [u.sup.[epsilon].sub.w]/[sigma] x [h.sup.[epsilon]-1/sub.G] (7)
The resulting expression for the system pressure between pump and bypass valve consists of various pipe and slot terms as well as one gap term. Pipe bending and the transverse pressure drop inside the die cavities were neglected:
[DELTA][p.sub.System] = [summation over (i)] [DELTA][p.sub.P,i] + [summation over (j)] [DELTA][p.sub.s,j] + [DELTA][p.sub.G] (8)
For calculating the die pressure inside the downstream die cavity, only the pressure drops of the first slot and the coating gap need to be included:
[DELTA][p.sub.Die] = [DELTA][p.sub.s] + [DELTA][p.sub.G] (9)
After calculating the system and die pressure for various coating speeds via (8) and (9), the results were compared to the experimentally measured steady die and system pressures.
Results and discussion
To investigate the relationship between die pressure and wet film thickness, an electrode film was coated intermittently with [u.sub.w] = 5 m/min, [h.sub.g] = 127 [micro]pm, h = 120 [micro]m, [l.sub.c] = 60 mm and [l.sub.I] = 40 mm. In Fig. 6, the measured die pressure is plotted in relation to the resulting electrode pattern.
Figure 6 shows the periodically alternating die pressure (blue dotted line) for three consecutively coated patterns. Even though the length axis was shifted and the patterns are randomly combined with the film profile, the wet film thickness (green line) follows the pressure distribution exactly. In every coating phase, the die pressure reaches a plateau. Here, a steady state is reached in which the flow and, thus, the pressure drop in the die, is constant. The closing bypass valve at the end of the coating results in a decreasing volume flow and hence a decreasing pressure drop. Once the bypass valve is fully closed, the die pressure does not drop to zero immediately. Perhaps due to its viscoelastic behavior, or other effects like inertia or capillary flow, the electrode slurry releases its stored energy and expands. This leads to a continued but decreasing slurry flow leaking onto the substrate. The photographic image in Fig. 6 shows that this flow is beneath the coating limit; consequently, film defects appear. Most of the patents mentioned above cover some kind of technology to avoid this slurry leaking. They provide a sharp stop edge by either drawing back the slurry, manipulating the gap, or both. The leaking cannot be avoided by the use of a turning bypass valve alone, as was applied in this work. At the chosen process settings, the bypass valve was opened again before the slurry could fully expand. Hence, no steady-state die pressure in the gap phase is observed. Opening the bypass valve again does not increase the pressure abruptly. Again, this may be attributed to the viscoelastic slurry behavior and inertial flow resistance. The nearly resting and elastic slurry first has to be accelerated. After this ramping time, the wet film thickness increases toward the preset value. Remarkably, the slope of the die pressure for the onset (i.e., opening the valve) is less steep than for the offset (i.e., closing the valve). This may be attributed to the one-sided spring support of the valve.
The results presented above show the direct link between the die pressure and the deposited film. Thus, it is crucial to control this pressure for optimizing the film profile and holding back the expanding slurry. However, before optimizing the ramping via advanced intermittant technologies, one has to take a closer look at the upstream system pressure. For a minimized pattern ramping time, the pressure drop of waste pipe and slot die should be equalized. A disparity could provoke an additional leveling in the whole liquid. It might even prevent steady state for the coating and interruption phases, as seen later on. Hence, the expected system pressure during the coating phase must be calculated and the waste pipe adjusted accordingly. We assume a high impact of deviations in rheology and dimensions to the equations (2), (4), and (6). Therefore, we validate both the calculations for the shear-thinning anode slurry and for a Newtonian model system (as described above) using experimental measurements of system pressure drop [DELTA][p.sub.system] and die pressure drop [DELTA][p.sub.Die]. In Fig. 7, the results of calculated and measured pressures are plotted for various coatings speeds.
As described above, the gap and the film thickness were held constant. Therefore, a rising coating speed results in an increasing volume flow and, thus, an increasing pressure drop. For the Newtonian model system (Fig. 7, left), the calculations match the observed pressures in die and system quite well, thus confirming the calculation model. Toward higher coating speeds, the fluid pressure increases linearly. For the anode slurry (Fig. 7, right) with the shear-thinning fluid character, the pressure gradient decreases, due to velocity exponents smaller than unity in equations (4) and (6). The results in the chart confirm the model for the pressure drop of coating gap and die slot (blue dotted line and squares). The calculations match the measured die pressures for various coating speeds very well. In contrast, the pressure drop through the whole system seems to be underestimated. The measured pressure is about 10% higher than the calculated (green line and diamonds). This may be related to the neglected curves or unknown constrictions between the die and the second pressure transducer.
In the next step, the model was used to predict the steady-state pressure during the coating phase of the intermittent process. To show the impact of mismatching pressure drops on the intermittent process, we mounted three different waste pipes. The first one was designed to match the calculated pressure drop in the die direction, the second one had only 70% of the predicted pressure drop, and the third one, 150% of the desired value. In Fig. 8, the measured system pressures and the resulting film profiles are plotted.
The best coating is expected for equal pressure drops in slot die and waste pipe system (Fig. 8, top). The results show a constant pressure during the whole coating process, only interrupted slightly during the valve switching step. For this adjustment, the measured wet film thickness reaches the predicted plateau during the coating phase. Likewise, the system pressure and the film thickness also show a ramping at each coating start and stop. During the interruption phase, the expanding slurry inside the slot die leaks onto the substrate, causing small barring lines (see photographic image, Fig. 6). To avoid the ramping and leaking, additional equipment for a slurry draw back or an alternating gap would be needed (which is beyond the scope of this work). The drop in pressure after each switching may be attributable, as described above, to the viscoelastic slurry behavior and flow resisting effects, such as inertia, depending on the switching direction. The time needed to accelerate the slurry to a steady state depends on the slurry volume. According to this, it takes more time to accelerate the slurry in the coating die than in the waste pipe with a tenth of the die volume. For the coating direction, a steady coating bead also has to be developed. As described elsewhere, (16) this step is faster at higher coating speeds, smaller gap volumes, and higher slurry flow rates. The results underscore the influence of the bypass valve. It reduces the maximum resting slurry volume approximately to the inner volume of the slot die. Developments in industry even push the valves inside the die, (23,24) in order to reduce the amount of leaking by minimizing the relaxing slurry volume. The observed drop in pressure during the switching strongly depends on the slurry rheology and on the set-up dimensions. Especially for large volume production dies, this effect may be more distinct. Unless a feasible correlation is established, the influence of each slurry-equipment interaction has to be measured individually. For Newtonian liquids, where only inertia, capillary effects, and the developing contact line damp the flow, one would expect a shorter transition state.
In contrast to the optimum for equal pressure drops in slot die and waste, a too-low waste pipe pressure drop may even prevent the coating of a homogenous film (see Fig. 8, center). The measured system pressure alternates between the predicted coating and waste pipe pressures by slightly rising and falling. For the waste pipe lengths used here, the system does not reach any steady state. The pressure at the coating start rises too slowly to reach a stable coating bead within the first centimeters. Even after the entire coating period, the predicted film height was not reached. Thus, a much lower waste pipe pressure may prevent a closed film within the coating phase. Considering the conservation of mass, this could only be explained by a still rising trend, which is not visible for this short coating period. Hence, different system pressures not only affect the quality of the coating but definitely limit the whole process.
If the waste pipe pressure drop is too high (Fig. 8, bottom), the effects are reversed. The system pressure alternates but stays constantly higher than predicted during the coating. After a short pressure drop during switching to the coating phase, the pressure remains too high. Furthermore, the resulting wet film is thicker than the adjusted gap.
This effect may only be explainable by the viscoelastic slurry behavior or inertial forces. The constant volume flux during the interruption places the whole liquid system under higher pressure. This causes the released slurry to excessively flow through the die and provoke a die swell at the outlet. The stored energy in the relatively large upstream pumping system seems to be enough to expand inside the whole pattern length. Again, this effect inhibits the intermittent process and emphasizes a prior pressure calculation. On the other hand, as a possible advantage, this effect could shorten the ramping at each coating start. A temporary limited expansion into the die might support the leveling at the starting edge.
Nevertheless, the pressure and the film height should show a trend toward their prediction, which is not visible in the present coating time.
To prove this assumption, additional experiments with longer coating and interruption phases are necessary. Thus, in Fig. 9, the results for different coating lengths of intermittent coatings with a too-high waste pipe pressure drop are plotted.
The chart in Fig. 9 shows that the applied liquid system compensates an overly long waste pipe very slowly. The predicted trend of falling pressure and film height during the coating phase is only observable for longer coatings in the range of 300 mm. Only for this long-term coating does the film finally reach its predicted film height.
These experiments were repeated for intermittent coatings with a too-low waste pipe pressure and the results plotted in Fig. 10.
Similar to the trend of too-high waste pipe pressures, the trend for too-low waste pipe pressures is only observable for longer coatings (Fig. 10, bottom). Even after 300 mm of coating, the preset film height was not reached.
If this long-term dampening and the short-term pressure drop during the switching is caused by viscoelasticity, they should not appear for Newtonian liquids. Thus, the experiments in Fig. 8 were redone with the Newtonian model system to first investigate the valve switching impact. The results are plotted with those of the LIB slurry in Fig. 11.
The system pressure of the Newtonian liquid shows quite the same curvature as the LIB slurry when reacting to the valve switching. Due to the different pressure levels for each liquid, it is difficult to compare their curvature in amount or detail. Yet, inertial forces seem to be the underlying cause of the damped acceleration after each switching state.
To investigate the long-term dampening effect on Newtonian liquids, the experiment in Fig. 10, bottom was repeated with the model system. The results are plotted in Fig. 12 in comparison to those of the LIB slurry.
Similar to the results of the LIB slurry, the system pressure of the Newtonian liquid increases during the whole coating phase. Thus, this effect is also not caused by slurry viscoelasticity but mainly by inertial forces. Other possible dampening sources may be enclosed air bubbles trapped in undercuts or elastic walls. However, the system was carefully bled during the filling operation. Moreover, the liquid-containing materials were selected to be very stiff, as described above.
In observing the too-low and too-high coatings in Fig. 8, two questions arise: First, where is the not casted liquid going to and, second, where is the excessive liquid coming from? The conservation of mass would seem to be violated, since the slurry is not as compressible as a gas. In order to disprove this, the waste pipe flux in each associated interruption phase has to compensate for the deviation in coated height.
This assumption could be proved by the comparison of applied and predicted liquid volume for the combined coating and intermittent phases ([V.sub.C] + [V.sub.I] = 0.144mL). The applied volume, therefore, was calculated implicitly from the measured system pressure. The obviously different calculations of waste pipe and coating direction are separated by the switching points.
As predicted for a too low waste pipe pressure, the volume flow during the interruption phase is far higher than for the coating phase (Fig. 13, top). When integrated for the whole period, the applied volume amounts to 0.148 mL, which deviates only 2.7% from the pre-set value. The observation is reversed for a too-low waste pipe pressure. The volume flow during the interruption phase is much smaller than during the coating and results in an applied volume of 0.158 mL and a deviation of 8.8%. Even with a critical look at the calculated values, the assumption of a compensating interruption phase could be confirmed.
Intermittent slot die coating is an important process step for manufacturing large-scale lithium-ion battery electrodes. Although there are many patents for intermittent film forming equipment, this part of coating science has not yet been covered by commonly available scientific research.
In this work, we established an experimental set-up to investigate the dominating and limiting processes behind intermittent slot die coating on a very simple basis. We controlled the flow of an LIB slurry through a precise syringe pump and interrupted it by switching a 3-way bypass valve between a waste-pipe and a slot die. The resulting system and die pressures then were correlated to the coated wet film profiles.
The results showed a direct relationship between coated wet film thickness and die pressure distribution. Hence, any film optimization requires a controlled die pressure. For casting a homogenous film, this pressure has to reach a steady state with a compulsory leveling at each start. To shorten this ramping period, we developed a non-Newtonian pressure calculation model for each steady state to align the system pressures of the coating and interruption phases. This resulted in an approximately constant system pressure and the predicted film thickness. In contrast, a different system pressure during the coating and interruption phases resulted in too-thin or too-thick wet films. Thus, the system pressure upstream of the bypass valve could not overcome the ramping within the coating length. The application of a Newtonian model system also showed this ramping behavior, although, here, it seems to be caused by omnipresent inertial forces.
Hence, intermittent slot die coating should always be accompanied by a prior pressure calculation.
Nomenclature Letters d Internal pipe diameter (mm) h Height (z-axis) ([micro]m) l Length (x-axis) (mm) p Pressure (mbar) u Speed (m/min) v Volume flow (mL/min) w Width (y-axis) (mm) Greek letters [gamma] Deformation (%) [??] Shear rate (1/s) [epsilon] Power law exponent (-) [Kappa] Consistency factor (Pa [s.sup.n]) [tau] Shear stress (Pa) Indices C Coating D Downstream lip G Gap I Interruption P Pipe S Slot W Web WP Waste-pipe
M. Schmitt ([mail]), R. Diehni, P. Scharfer, W. Schabel
Institute of Thermal Process Engineering, Thin Film
Technology, Karlsruhe Institute of Technology, Karlsruhe, Germany
Acknowledgments This work was supported by the "The Central Innovation Program SME" (ZIM), Funding-No. KF2840205FH3, by the German Federal Ministry for Economic Affairs and Energy (BMWi). Part of this work has been supported in the frame of the project Competence E (KIT) by the German Federal Ministry of Economics and Technology on the basis of a decision by the German Bundestag (Funding Nr. 03ET6016). The authors would like to thank all involved mechanics, assistants, and the students Miriam Vogt, Paul Kitz, Sandro Spiegel, Orhan Keskin, and Julian Klemens for their supporting work and building up of the experimental set-up. We specially thank Valentin Wenzel and Steffen Schmelzle (KIT-MVM-VM) for mixing support and Boris Bitsch (KIT-MVM-AM) for rheological measurements. We also want to thank our cooperation partners at KROENERT GmbH for many constructive discussions and our partners at TSE Trailer AG, Switzerland for the technical support.
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Table 1: Dimensions of the coating system and the applied slot die Parameter Value Unit System [d.sub.p,1] 4.0 mm [d.sub.p,2] 4.5 mm [d.sub.p,3] 4.8 mm l.sub.p,1] 94 mm l.sub.p,2] 82 mm l.sub.p,3] 67 mm Slot die [h.sub.G] 127 [micro]m [h.sub.S,1] 500 [micro]m [h.sub.S,2] 500 [micro]m [l.sub.D] 0.4 mm [l.sub.S,1] 18 mm [l.sub.S,2] 20 mm [W.sub.S] 12 mm
Please note: Some tables or figures were omitted from this article.
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|Author:||Schmitt, Marcel; Diehm, Ralf; Scharfer, Philip; Schabel, Wilhelm|
|Publication:||Journal of Coatings Technology and Research|
|Date:||Sep 1, 2015|
|Previous Article:||Investigation of the flow field in thin polymer films due to inhomogeneous drying.|
|Next Article:||Effect of shear strain in coating on the particle packing of gelled-clay particle dispersions during drying.|