Process Alternatives in the Battery Production Prof. Dr.-Ing. Achim Kampker; Dr.-Ing. Peter Burggraf; Dipl.-Ing. Christoph Deutskens M.Eng.; Dipl.-Ing. Dipl.-Wirt. Ing. Heiner Heimes; Marc Schmidt Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University, SteinbachstraBe 19, 52074 Aachen, Germany
[email protected] Abstract
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The future demand for Lithium-Ion-Cells is definitely
going to increase. The main part accelerating the development is the cell production for electric cars. In order to gain more market share it is necessary to build up high-quality machines and connect them to an efficient process. Problematic for this target is the diverse process chain. An international survey showed that most companies are focused on just one specific operation; they cannot handle all steps with their own solutions. This paper presents a way to solve the problem by identifying, characterizing and assessing product alternatives along the whole process chain. In addition to that interdependences were detected and technology chains got written down in a last step.
I.
INTRODUCTION
Knowing about the finiteness of fossil energy sources as well as Germany's way stepping into a new area of diverse energy uses leads to a growing demand of Lithium-lon-Cells [I]. Car manufacturers try to develop new production concepts for the electro mobile production as well as improving the existing concepts. One of the most important aspects around the field of electric cars is the Lithium-Ion Battery: There are special requirements concerning the duration of life, performance characters, quality and safety. Nowadays the electric cars' breakthrough is retarded by the high purchase costs. The factor that drives costs up is definitely the battery (60% of the production costs, 40% of the costs in sum) [2]. In order to offer electric cars which are accepted by the end consumer the car manufactures are
forced to produce low cost and high quality performance batteries. The biggest influence concerning the total battery costs are the battery cells, they make out of 47% [3]. Hence, there are several challenges about producing the Lithium-Ion Cells. German manufactures aren't capable of producing Lithium-lon-Batteries in mass production. Up to the present the production process for Lithium-lon-Battery isn't yet automatized; in addition it is only restricted to limited quantities. Furthermore the end products oscillate along the result; the production processes are neither energy nor resources efficient. In contrast to the module- and packaging installation the Lithium-lon-Cell is recognized by diverse production technologies and similar fields of expertise. This is the reason machines and sites for the cell production are provided by different companies. An international survey asking more than 300 machine- and sites constructers pointed out that producing the whole process of Lithium-lon-Cells with their own machines is no option. Moreover they get provided by several experts who are focused on one field of the production process [4]. Hence, it's quite a challenge for the producers of Lithium-Ion-Cells to identify the right machines for every step in the production process, to choose the right production technology and to combine it all in order to find the perfect match. Furthermore there is no automated technology for the cell production so that mass production for Lithium-Ion-Cells is no option yet.
Fig. 1: Production process of a Lithium-Ian-Cell
978-1-4673-1372-8/12/$31.00 ©2012 IEEE
Mixing principle
Intensive mixer
Conventional mixer
Tempermg
With climate system
Without climate system
Loading
Continuous
Atmosphere
Slot die Diagonally
Heating
Slats
One-sided
Both-sided
Normal atmosphere
Protective gas
Conveyor belt
DeSSication type
Ejector
Levitation
One-sided
Both-sided
Circulation air
Fresh air
Heat exchanger
Length
Short
Long
Cooling
Cooling roll
Cooling section
Pre-treatment
Cleaning
Discharge
Number of roll pairs
Tempering
One
Without Two
Linear load
300 kN
500 kN
Web width
300 mm
500 mm
Roll material
Steel, chromed
Cast iron, chromed Installed
Drymg room Slitting
Steel, hardened and chromed Not installed
Rotary cutter
Laser beam
Cleaning
Surface
Edges
None
Cuttmg pnnclple
Punching
Wedge cutting
Laser cutting
Stacking principle
Z-Folding (one-sided)
Z-Folding (both-sided)
Gnpper technology Weldmg
Contactless (ultrasonic) Friction stir
Laser welding
Single stacking Vacuum
Induction roll cladding
welding
Pulse joining
LaYing procedure
Deep drawing foil
Folded foil
Pouch bag
Cleaning of the welds
Chlorine
Laser
Without
FOil closure
Impulse sealing
Filling Forming fo the Injector
Contact sealing
Injection
Heat sealing Inlaying of the cell stack
Wide
Slim
Fan nozzle
Metering
Fixed template
Location of the cell
Horizontal
Vertical
Contacting
Connection to the cell
Cell to the connection
Contacting way
Gripper
Spring contact probe
Damage allowed
Yes
nergetlc recovery
I
Rolling contract
Commabar Longitudinally
Atmosphere
I
Normal atmosphere
Application tool
GUide conduct
•.- -
Protective gas
Position of the coating Coating type
.
Interrupted
Vacuum
Cell to cell
Metering pump
No To the electrical network
None
Current
25A
50A
Interconnection
Individual circuits
Cell-Dropout System
Process temperature
Low
High
Puncture
Laser
Needle
Roiling of the cells
Yes
No
Location of the cell Storage temperature Aging time Weight
Vertical
Horizontal High
Low Two weeks
Temporal change Four weeks
Comparative measurement
Absolute measurement
Dimensions
Coordinate measurement machine
Calibre
Laser measurement
Density
Gas leak
Difference in weight
Difference in pressure
Capacity
High current discharge
Fig. 2: Morphological box
Normal current discharge
II.
DESCRIPTION OF THE PROCESS ALTERNATIVES
The production process of Lithium-lon-Cells is divided into three steps; the production of the electrode, the cell assembling as well as the formation and checking [5]. In the beginning it starts with mixing the active material in order to produce the coating paste. Afterwards the coating paste is applied to the copper and aluminium foils. Since the active material contains a high level of moisture stopping the adhesion on the cell, the coated foils are running through a drying section. Due to the expansion during the drying process and the high demands regarding the accuracy of the layer thickness and the surfaces, the drying section is connected to a calender. The calender compensates irregularities in thickness and compresses the material. During the slitting process the coated foils get cut lengthwise into thinner pieces due to the bigger dimension than the actual battery cell. In the following separating process the final sheets are created. For reaching the requested capacity, several sheets are stacked and the conductors are welded together. The cell stacks are packed into plastic coated aluminium foils to protect the cell chemistry against the environment in order to ensure its function. During the first loading process (called the "formation") chemical reactions are taking place; the SEI-Iayer is formed. Depending on the used material gas can spread out. The gas pocket, which is next to the cell, absorbs the gases and avoids bursting. After the formation process the gas pockets have to be removed and the cells have to be finally sealed. (Fig. 1) A closer look on the production process shows that each sub process is characterized by several technologies requiring different expert knowledge. In order to identify and describe all process alternatives, structured interviews with experts have been performed; machine and plant manufactures have been visited as well. The results were worked out and illustrated by a morphological box (Fig. 2). The identified alternatives have been described in detail. For instance, the separating process is shown below. Using endless foil offers great economic advantages e.g. it is possible to reach high velocities. In order to achieve a high throughput it is necessary to process the anode, cathode and
Laser cutti ng
separator foils "reel to reel". During the process of separating the geometries are designed in a way that the cell base area is always extended by the conductor. If the conductor of the anode is positioned in the upper right corner of the electrode, the conductor of the cathode has to be positioned in the upper left corner. The following alternatives can be used for the separation process: 1) Laser cutting 2) Stamping 3) Wedge cutting There are three ways to cut the previously slitted foils (Fig. 3). The first alternative is to cut the foils by using a robot controlled laser which evaporates the selected areas. The second alternative is to stamp the foils by using punch and die (shear cutting). The third alternative is the wedge cutting. The slitted foils are cut through all layers up to the anvil by using a blade with the shape of a cell. III.
EVALUATE THE PROCESS OPTIONS
In order to identity the process matching perfectly the own requirements it is necessary to rate the processes concerning general characteristics. Hence, several arguments were identified: Maturity, flexibility, time, costs as well as quality concerning processes. There are two kinds of costs: The investment and the business costs. The time is divided by process and set-up time. The quality isn't the same in every process step. Due to different values in one step, quality is evaluated differently. In several expert interviews there was a ranking from one to six (six: best in meeting the goals). The arguments got weighted; all information was transferred to a certain data base. Depending on one's own preferences it is now possible to weight the criteria differently.
Stamping Fig. 3: Process alternative cutting principle
Wedge cutting
4
Cutting principle
2
4,8
3,8
2
4,6
2,3
3,2
3,3
Investment costs
Operational costs
Process time
Tooling time
0,6
0,4
0,9
0,1
4
Degree of maturity
2,6
Flexibility
Wei htin Stamping Cutting principle
Wed e cuttin
Costs
Time
2 4
2
Laser cuttin
4 Quality
Wei htin
Edge geometry
Thermal load
Accuracy
Process stability
Deflection
Fouling
0,17
0,17
0,17
0,17
0,17
0,17
Stamping Cutting principle
2
Wed e cuttin
2
Laser cuttin
4
2
2
Fig, 4: Rating of the process step formation
Figure 4: Rating of the process step formation shows the electrode sheets' separation. The assigned scores are based on the following justifications. Degree of maturity
In contrast to punching and wedge cuts, laser technology for separating foils is relatively new. The technology has high potential; the degree of maturity is far worse than the one of the other technologies though. [6] Flexibility
The steering by a robot provides the laser with an unlimited flexibility. Due to the fact that the material is liquefied and vaporized during the cutting process, the pollution has to be absorbed by an air filtering system. The pollution must not reach the substrate surface. Hence, the air filtering system has to be installed close to the cutting edge which unfortunately reduces the flexibility of the whole system. The two other technologies have almost no flexibility; for every new electrode shape a new cutting tool has to be designed and produced. [6] Costs
The costs of a laser cutting system are about three to five times higher than the ones for a comparable punching machine. This disadvantage is mitigated very quickly because the laser has very low operational costs: The total tool costs are only 60% of a punching machine with the same production capacity. The operating and capital costs of a production plant with a wedge cutting system are the lowest though [7]. Time
The punching process is the fastest with a cycle time of about 0.2 seconds [8]. It is closely followed by the wedge cutting. The laser cutting process is the slowest one. It has to be considered that the limiting time factor refers to the handling of the punched sheets and not to the actual cutting process. Since the laser causes almost no wear, maintenances are not often necessary. During the laser cutting process many dirt particles are formed; they have to be disposed. Therefore, the
exhaust filters have to be cleaned periodically. The regular exchange and the preparation of the cutting tools needs considerably more time during the punching and wedge cutting process. The punching tool withstands about 500.000 strokes. The edges of the wedge cutting process have to be outpointed and replaced in shorter intervals. This leads to the worst rating especially concerning the tooling time. [6] In order to evaluate the quality there are six criteria: • Angle geometry Describes how clean the cut is done e.g. deburring It is absolutely necessary to avoid edges because they could slide between the sheets and lead directly to a short circuit. • Thermal impact • Accuracy Describes the form tolerance of the process • Stability in process Shows the balance between accuracy and rejects • Set Shows if the cells are bended during the process It is of high importance because the separator could be bridged and so a short circuit would occur immediately • Pollution Describes how much pollution occurs and how big the risk is to sneak right up to the electrodes' surface. The cleanest cut and so the best angle geometry is reachable with pressing. The wedge cut provides the worst results [8]. The thermal impact being dangerous for the material is very high while working with lasers. With other methods thermal impact is just caused by the friction during the heating process. The material gets evaporated; the outlying district is heating up. If you behold the accuracy of the angle geometry the situation is very similar with the exception that the stamping (1 /lm) limits the scale at the upper end. The
stamping is the most stable process regarding the following aspects: simplicity, enormous experience and long tools' durability. Concerning laser methods there are a lot of problems with evaporation flaring the laser beam. The wedge cut leads to an unstable process because of surface deterioration. A short circuit can just occur while pressing or wedge cutting. Working without pressure the laser shows the best results. On the other hand the pollution is one of its problems though; the laser is liquefying and evaporating the substrate. Avoiding this there are several ways to draw. [9] As an example for the separation process the results of the technology assessment are presented in a star diagram (Fig. 5). This diagram illustrates all advantages and disadvantages in just one figure: You are able to select the application-specific optimized procedure. [v.
CHAINS CONCERNING PROCESS ALTERNATIVES
Next to the detailed observation of separate process steps, analyzing the whole process (building technology chains) plays an important role for a high quality production line [[ 0]. [n order to choose the best combination regarding just one alternative the consequences between the process steps have to be given. Hence, the interdependences have to be pointed out; the effects should be described with regard to already discussed criteria as well. The impact can be either very negative, negative, very positive or positive. To ensure a systematic proceeding the evaluation can be controlled by a general question algorithm. I.
[f you choose inert gas while compounding (due to the quality) the cleanness will be ensured; it is not possible that particles can influence following process steps anymore. It is the same for opposite direction.
2.
[f you choose a laser while slitting (because of quality
reasons), the pollution of the surface will decline while separating. You have the option to specialize handling the dirt caused by laser cuts. 3. [f you choose the formation at a high temperature (due to quality reasons), the life cycle of the whole cell will be raised by cleaning it with chlorine. Hence, it is now sure that the lifetime concerning capacity is nearly the same like the one of the seal weld. All existing approaches to form technology chains do not take into account the three difficulties named above, because other process steps, which only have minimal influence on the mentioned connections, are between the alternatives. Nevertheless, the interdependences, which influence several consecutive process steps, play an important role with regard to the efficiency and the quality of the cells. Furthermore, these interdependences also influence the investment needs. Since there are only approaches considering interdependences between consecutive process steps, a new methodology considering multiple steps has to be developed. [n addition it has to fulfill the dissected market structure of battery production. V.
SUMMARY
This article describes the challenges for plant and cell manufactures to place their products in the developing Lithium-Ion-Market. In order to reach this target the procedure and the results for identifying alternative production technologies were presented; there are various alternatives throughout the whole process chain. Furthermore the alternatives have been evaluated regarding the five criteria: degree of maturity, flexibility, costs, time and quality. Due to the clarity the results have been presented by a star diagram. In order to complete the basics for constructing efficient technology chains, many interdependences along the process chain have been detected by using the standardized procedure. Up to now, there is no methodology to collect and
Degree of maturity
-Stamping Quality
Flexibility
-
Wedge cutting
-Laser cutting
Fig. 5: Production process of a Iithium-ion-cell
utilize such data and to construct a continuous process chain. REFERENCES
[I] [2]
B. Mayer. "Lithium-ion race pics up," Automotive News Europe, vol.
13, June 2008.
BMU, "Konzept eines Programms zur MarkteinfUhrung von Elektrofahrzeugen, Berlin," September 2009, p. 6. [3] Thomas Schlick, Guido Hertel, Bernhard Hagemann, Eric Maiser, "Zukunftsfeld Elektromobilitat," Roland Berger Strategy Consultants, May 2011, p. 17. [4] Intern study, Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University, 2011. [5] Jiirgen Fleischer, " Produktionstechnische Herausforderungen bei der Herstellung von Li-Ionen Batterien fOr Automotive Anwendungen,", November 2011, pp. 9-10. [6] Several expert interviews, Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen University, 2012. [7] G. Hagen, "Contouring of Electrode Foils for Li-Ion-Batteries", Aachen, March 2011, p. 21 [8] G. Hagen, "Contouring of Electrode Foils for Li-Ion-Batteries", Aachen, March 2011, pp. 11-20 [9] Ernst Barenschee, "Herausforderungen bei der Serienfertigung grossformatiger Li-Ionenzellen fOr automobile Anwendungen", Frankfurt, October 20II, p. 22 [10] H. Diiff, 1. Jurklies, B. Schuster, (2004): "A New Software Tool Assists Generation and Evaluation of Process Chains within the Mould and Die Manufacturing", South Africa: COMA Stellenbosch., p. 272.
