LIBs are electrochemical cells that convert chemical energy into electrical energy (and vice versa). They consist of negative and positive electrodes (anode and cathode, respectively), both of which are surrounded by the electrolyte and separated by a permeable polyolefin membrane (separator). An electrode consists of an electroactive material, as well as a binder material, which enables structural integrity while improving the interconnectivity within the electrode, adhesion to the current collector and the formation of the solid electrolyte interface (SEI) during the first battery cell cycles [ 9 ]. Lithium-ion battery cells are connected (either in series or in parallel) in battery modules. Then, battery modules with electrical, thermal and mechanical components are assembled into a battery pack. It should be noted that in this paper, either battery or the cell refers to a single LIB cell and neither to the module nor the battery pack (system).
This section first describes the production of LIBs according to the state-of-the-art from the perspective of series production. Then, three examples are used to illustrate the challenges of series production. In the next sections, the process of industrialization from lab to pilot to series production is explained and the possibilities and status of the use of artificial intelligence in battery cell production are discussed.
In summary, the quality of the production of a lithium-ion battery cell is ensured by monitoring numerous parameters along the process chain. In series production, the approach is to measure only as many parameters as necessary to ensure the required product quality. The systematic application of quality management methods enables this approach. If quality is not assured, scrap is produced, and this is associated with high costs. The reason for this is that the cost of a battery cell is dominated by the cost of the materials, which accounts for around 75% of the manufacturing costs [ 13 ]. The share of material costs in manufacturing is not expected to fall in the next few years. On the contrary, material costs are expected to rise due to a shortage and high demand of raw materials [ 16 ]. In addition, the production of a battery consists of many individual steps, and it is necessary to achieve high quality in every production step and to produce little scrap. In a long process chain with, for example, 25 process steps and a yield of 99.5% each, the cumulative yield is just 88% [ 17 ]. This highlights the economic relevance of scrap minimization. Thus, in the next section, challenges in industrial battery cell manufacturing with special attention to scrap reduction will be discussed.
Industrial manufacturing follows a systematic approach to quality assurance in which the methods, devices, frequency and scope (100% test vs. random sample) of the tests, etc., are defined in a control plan that combines in-line and laboratory testing. If quality deviations are found, the corresponding samples are subjected to a detailed laboratory analysis. Deviations in quality are processed according to quality management methods 8D or 5why. In the 8D method, a team is formed, the problem is described, and immediate action is taken to correct the error. In the event of a serious deviation, additionally, the 5why method can be used, in which the cause of the error is analyzed. The production parameter settings are adjusted until the specification values are restored. The products produced during this time are sorted according to the severity of the error.
After describing the manufacturing process of a lithium-ion battery cell, the methods of quality assurance will be briefly reported in this section. Quality generally indicates the extent to which a product meets the agreed requirements. The battery cell manufacturing process represents a quality chain in which the performance of both the product and the manufacturing process is examined. Methods of quality assurance in battery cell production have been demonstrated, for example, by Schnell and Reinhart, in which they proposed a quality gate concept for the complex production process [ 13 ]. Riexinger et al. also proposed a concept for the traceability of process parameters in the production of batteries, in which they addressed the measurement methods for individual process steps and the scope of testing [ 14 ]. Although numerous approaches and proposals for quality assurance have already been made, no standards have been established to date. This is due to the complexity of the manufacturing process (many process steps with intermediate products and different cycle times) and the different battery cell formats and designs (material combinations). The technical cleanliness of the production process plays a major role in the quality of the product. From the mass production manufacturer point of view, the cost of quality control (time to collect quality parameters and measuring equipment) must always be less than the cost of scrap rates. In addition, the effort toward quality assurance leads to higher quality products for which higher market prices can be achieved. To illustrate the process of quality assurance in industry, a comparison between the literature versus the industrial procedure is given using the example of the mixing and calendering process. Table 1 shows the quality parameters identified as important in the literature, as well as their measurement methods.
starts with a high-temperature (HT) soaking step, where the wetting of the electrodes with electrolyte is enhanced in temperature chambers (approx. 45 °C). After this process, the batteries are transported to special racks and charged for the first time. During the first charge, the SEI is formed at the anode, which protects the anode from reactions with components of the electrolyte and influences the subsequent performance of the battery cell. Since this complex layer consists of different electrode and electrolyte decomposition products, the exact structure and formation of the SEI is still under investigation. During the initial charging process, gas is formed in larger cells and escapes under controlled conditions. After the first charging and the escape of the gas, a second or third electrolyte filling is carried out and the battery cell is finally sealed. Before closing with laser welding, the opening for the electrolyte filling is cleaned via laser cleaning. When the cell is finally sealed, a new leak test using helium is carried out and the final weight and dimensions are determined. The process for the wetting and formation of the cells takes 3–7 days [ 12 ]. In the following aging process, HT and RT (room temperature) resting takes place for continuous wetting, and then various current and voltage profiles are run through the battery cell. During aging, a process that can last for several weeks [ 12 ], the open-circuit voltage is measured to calculate the self-discharge rate, as elevated self-discharge values may indicate internal short circuits. In the final grading, the open-circuit voltage and internal resistance are measured. Afterwards, the battery cell housing is cleaned and wrapped with PET (polyethylene terephthalate) tape. Then, thickness measurements, together with dimensional and insulation tests are carried out. Finally, the cells are sorted according to their performance and thickness and transported to the outgoing warehouse. Cell finishing can account for up to 25% of factory floor space and requires a large amount of equipment, as each cell must go through this process [ 12 ].
The first step in the cell assembly is notching, where tabs are formed, or single sheets are cut out of the electrode web via laser cutting. The edge quality is examined regarding mechanical and thermal deformation. Moreover, the particle contamination on the electrode surface caused by cutting is checked. The next step is stacking the electrodes, where, although z-folding is currently widely used, it is being replaced by lamination stacking. Here, the separator is not folded around the electrodes but directly laminated on the anode, improving the speed and safety. The cell stack, commonly called a jelly roll, is then secured with tape to prevent the layers from slipping. The stacks are additionally heat-pressed and subsequently, the quality is determined by a thickness measurement. To determine the electrical insulation between the anodes and cathodes, an insulation test, the so-called Hi-Pot test, is performed. During the Hi-Pot test, high voltage direct current is applied to the cell stack to detect faults in the electrode production. The voltage applied is based on the cell design (e.g., number of electrodes in the jelly roll), generally in the range of 50–200 V. The electrical equivalent circuit diagram of the stack represents a circuit with a parallel resistor (R) and capacitor (C). The resistance of the stack is determined by the isolation of the separator. If a voltage is applied, the capacitance is changed. The voltage is applied for a time, for example, 5 s, and then the leakage current is measured (for example, less than 1 mA). If the leakage current is outside the tolerance, this indicates a defect. Typical defects are cracks in the separator, mismatching of electrode and separator or metallic contamination. After the Hi-Pot test, the anode and cathode tabs are bent, cut and joined together by ultrasonic welding. After this initial welding, protective tape is applied, and the cap is fixed by ultrasonic welding. The cap is also electrically insulated by tape, and another Hi-Pot test is performed. Next, the stack is wrapped with mylar tape for electrical insulation and pushed into the battery cell housing (can). To close the cell, the cap and can are laser-welded. After closing the battery cell, a helium test is performed as a leak test, and a Hi-Pot test is carried out again. Helium is a very volatile gas, and the pressure difference is used to verify the tightness of the battery cell. Before filling the electrolyte, the so-called baking, where vacuum drying is used to remove moisture and solvent residues, takes place. Battery cell baking and electrolyte filling are executed under clean (defined as the number of particles per m 3 ) and dry room conditions, since eliminating moisture is important to avoid degradation of the electrolyte. The clean room classes correspond to ISO 7 or ISO 8 classes, and the dew points in these rooms are between −15 °C and −60 °C. The filling of the electrolyte is carried out with a dosing lance, in which the precision of the metering is important, so that the battery cell housing is not contaminated with the electrolyte. The electrolyte filled in should be distributed as homogeneously as possible and the quantity measured gravimetrically. After filling, the opening is temporarily closed, and the cell finishing begins.
In electrode production, the quality of the individual production steps is crucial. Besides the formulation, the quality of the electrode is determined by its uniformity, porosity and freedom from defects. Defects in electrode production can lead to lithium plating, cell swelling, overheating and poor electrochemical performance. The production of the electrode is subject to a complicated mechanism, with a mixing of solid particles, binder, solvents and the dynamic processes of solvent evaporation and solidification of the slurry on the metal foil. To increase the output of electrode production, wide coaters (up to 1400 mm) are used, which are operated at a high speed (up to 100 m/min). This parameter setting can lead to high quality challenges. Defects that can occur during coating and subsequent curing include particle agglomeration in the coating slurry, air bubbles on the coating layer caused by insufficient degassing and buffering of the slurry, and partial flaking of the dried slurry or wavy edges due to insufficient viscosity. Further information on the industrial view of electrode production can be found in Sheng et al. [ 11 ].
Electrode manufacturing starts with the reception of the materials in a dry room (environment with controlled humidity, temperature, and pressure). Powder materials are supplied in bags: big bags for the active material and mostly paper bags for the binder and the conductive material. The bags are transported on pallets by roller conveyor and elevator from the warehouse to the feeding area on the second floor. After cleaning in an air shower in the dry room, the bags are moved to the feeding station. The big bags are lifted by crane. All the materials are automatically dosed according to the weight ratios specified in the recipe and conveyed to the first floor. Active material and a conductive agent are added directly to the electrode slurry mixer, whereas the binder powder is first fed to another mixer to prepare the binder solution. After dry mixing of the active material and conductive agent, the binder solution is added to make a slurry for electrode coating. Although different alternatives are being studied for its replacement, NMP (N-Methyl-2-pyrrolidone) is the most utilized cathode slurry solvent, while deionized water is used for the anode. A homogenous electrode slurry is prepared via planetary mixer, which applies high shear forces. During slurry mixing, viscosity is one of the key quality parameters to control the mixing process for high-quality electrode coating. After mixing, the slurry is degassed and buffered in a tank for a maximum of one to two days. Prior to coating, the slurry is pumped to another buffer tank placed nearby the coating stations. Tandem coaters are state-of-the-art in mass production. Here, the substrate film is unwound and rewound only once. Two continuous ovens are placed one above the other so that coating and drying of the second side can follow directly after coating and drying of the first side. Slot dies are used for the application in mass production. Screw pumps precisely convey the required amount of slurry from a third, smaller feed tank to the slot die to ensure the specified loading. The wet electrode web is transferred to a drying area to evaporate the solvent by heat supply. The drying area has different temperature zones (below solvent evaporation temperature) to avoid rapid drying and thermal stresses on the film. Rapid solvent removal will cause surface cracks on the film. The toxic NMP solvent is recovered by condensation and then followed by a distillation process. After coating, the electrode coil is transported to calendering. Calendering is a rolling process with at least two counter-rotating, heatable rolls. The application of pressure reduces the thickness, which increases the volumetric energy density, and since particles are pressed into the substrate film, the pressure improves the electrical conductivity. Before and/or after calendering, the electrode web is slit into several smaller electrode coils or trimmed according to the battery cell design (e.g., prismatic, cylindrical or pouch) by slitting with roller knives. Calendering and slitting are often integrated in one machine.
Conventional processing of a lithium-ion battery cell consists of three steps: (1) electrode manufacturing, (2) cell assembly, and (3) cell finishing (formation) [ 8 10 ]. Although there are different cell formats, such as prismatic, cylindrical and pouch cells, manufacturing of these cells is similar but differs in the cell assembly step. The series production of prismatic cells is described below, and a schematic view for the manufacturing of a lithium-ion battery cell is given in Figure 1 , as a reference.
The basis for reducing scrap and, thus, lowering costs is mastering the process of cell production. The process of electrode production, including mixing, coating and calendering, belongs to the discipline of process engineering. Cell assembly with notching, stacking, filling, etc., is assigned to assembly technology. Cell finishing with charging of the battery to set the performance is covered by electrical engineering. All disciplines must work closely together to reduce production costs. The complexity of the battery manufacturing process, the lack of knowledge of the dependencies of product quality on process parameters and the lack of standards in quality assurance often lead to production over-engineering, high scrap rates and costly test series during industrialization [ 13 ]. In the next sections, selected examples from our expert experiences in series production will be presented, specifically, cases from the electrode, cell assembly and formation areas, based on which improvements in quality and a reduction of the scrap rates were obtained.
electrode manufacturing, slot die coating is a state-of-the-art production process due to its high precision and controllable flow behavior. Compared to other technologies, slot die coating has the advantages of a high coating speed combined with even thickness of the coating layer. When using slot die coating, surface defects rarely occur and the scaling work from pilot to series is feasible. The hydrodynamic behavior of the slurry and the coating parameters can be coordinated. Nevertheless, in coating, large-scale trial and error tests must be carried out through the industrialization of the process to set the optimal process conditions. This process is time-consuming and expensive; the development of a new battery cell goes through several sample phases (referIn the industrial process for, slot die coating is a state-of-the-art production process due to its high precision and controllable flow behavior. Compared to other technologies, slot die coating has the advantages of a high coating speed combined with even thickness of the coating layer. When using slot die coating, surface defects rarely occur and the scaling work from pilot to series is feasible. The hydrodynamic behavior of the slurry and the coating parameters can be coordinated. Nevertheless, in coating, large-scale trial and error tests must be carried out through the industrialization of the process to set the optimal process conditions. This process is time-consuming and expensive; the development of a new battery cell goes through several sample phases (refer Section 2.3 ), which take a total of 3 years from A-sample to D-sample. Once the product has been developed and industrialized, however, it takes about a month in electrode production to set the parameters of a new formulation. Therefore, more time must be spent on research and the establishment of a clear understanding of the relationship between the coating method and process parameters [ 18 19 ]. The selected example here shows how to proceed when a coating fault occurs in series production and what challenges still exist. The process chain of slot die coating, starting with mixing, and the challenge of setting the process parameters is not that simple. In serial production, it can be assumed that the process parameters are optimally set, and the slurry flowing into the slot die is homogeneously mixed and of good quality. Using filters (mesh and magnetic filters), foreign particles are removed, thus diminishing the presence of agglomerates. Despite the homogeneously mixed slurry, the filter system and the constant movement of the slurry through circulation, agglomerates can still appear on the slot die, leading to coating imperfections; when the slurry flows out of the slot die, the surface of the die can be irregularly wetted with slurry, thus resulting in the buildup of agglomerates. These agglomerates can block the outflow on the slot die and lead to a coating defect ( Figure 2 ).
2, time for failure detection and correction of 30 s and price of LFP (lithium iron phosphate) of 20 USD/kg [2 of electrode coating becomes unusable, meaning that with a load of 20 mg/cm2 with 96 wt.% CAM, approx. 5.76 kg of CAM were used in this area. At a price of 20 USD/kg for LFP, the scrap material cost is USD 115 for the single-layer coating. This example emphasizes the awareness on economic impact of scrap in electrode coating and how important it is to have a controllable and stable process.A defect in the coating directly causes high scrap costs since material costs are the main costs. The following calculation illustrates the costs that are incurred: two coating strips of 550 mm, coating speed 60 m/min, areal density of the electrode with 96 wt.% cathode active material (CAM) of 20 mg/m, time for failure detection and correction of 30 s and price of LFP (lithium iron phosphate) of 20 USD/kg [ 19 ]. At such a coating speed and 30 s (the time needed to detect and correct the coating failure), and a coating width of 2 × 550 mm, approx. 30 mof electrode coating becomes unusable, meaning that with a load of 20 mg/cmwith 96 wt.% CAM, approx. 5.76 kg of CAM were used in this area. At a price of 20 USD/kg for LFP, the scrap material cost is USD 115 for the single-layer coating. This example emphasizes the awareness on economic impact of scrap in electrode coating and how important it is to have a controllable and stable process.
The first measures carried out are the manual cleaning of the application tool at certain intervals, as well as the adjustment of the process parameters. The state of the art here is to first shut down the fault and, if necessary, stop production, and then to carry out a fault analysis with appropriate measures. In the following paragraphs, the procedures for investigation are explained and an overview of which scientific approaches are already available is provided.
To explain the wetting behavior of the slurry, it is worth looking at its thixotropic rheological behavior, meaning that the viscosity is conditioned by the shear stress. With increasing shear stress, the viscosity initially falls, and after the shear stress decreases, the viscosity recovers. The deviations in the wetting of the slurry at the downstream meniscus are explained by Xiaoyu Ding et al. with the formation of a vortex when slurry is applied [ 20 ]. Figure 3 shows schematically how the downstream meniscus vortices form when slurry is applied through the slot die.
The decisive quality-determining parameters for the slot die coating are the distance between the slot die and the foil, the gap of the slot die and its shim geometry. Due to possible agglomerate formation on the downstream meniscus of the slurry, attention should be paid to the other parameters. It can be assumed that the return of the slurry due to the formation of vortices leads to the different wetting of the slot die and the formation of agglomerates. ( Figure 4
In the following paragraphs, the potential influences on the quality of the coating due to the formation of agglomerates and possible solutions are summarized. When composing the slurry, the rheological behavior should be examined in detail and the thixotropy influence should be taken into account so that the line speed is set to match the shear rate and the runback of the slurry to the downstream meniscus (different wetting due to vortex formation) is prevented as far as possible. In addition, the influence of the slot die itself should be investigated in more detail [ 20 ]. The surfaces of the slot nozzle can be adapted to influence the flow behavior. For example, the downstream lip can be treated with a lyophobic surface (solvent–repellent) and the upstream nozzle with a lyophilic surface (solvent–affine) [ 21 ]. In this way, the position of the contact line can be precisely determined, avoiding the drying out of slurry and, thus, the formation of agglomerates due to an irregular formation of the meniscus. From the point of view of series production, carrying out further investigations and examining the entire system, starting with the mixing up to the slurry application, will be useful.
cell assembly process, is used for the pole tab production for subsequent contacting of the electrode sheets. This technology is state-of-the-art to produce pole tabs, as the heat input is low and the cut quality is high. The laser cut must be optimally adjusted in its process parameters for different material combinations, such as copper foil (uncoated and slurry-coated) and aluminum foil (uncoated, slurry-coated and ceramic-coated). While laser cutting for slurry-coated metal foils (anode and cathode) has already been scientifically investigated numerous times [23,The next discussed industrial example is laser cutting, which, as part of theprocess, is used for the pole tab production for subsequent contacting of the electrode sheets. This technology is state-of-the-art to produce pole tabs, as the heat input is low and the cut quality is high. The laser cut must be optimally adjusted in its process parameters for different material combinations, such as copper foil (uncoated and slurry-coated) and aluminum foil (uncoated, slurry-coated and ceramic-coated). While laser cutting for slurry-coated metal foils (anode and cathode) has already been scientifically investigated numerous times [ 22 24 ] and rarely leads to quality problems in series production, laser cutting of cathodes with a ceramic layer (insulation layer) next to the cathode coating edge can pose a challenge. For this reason, it is worth taking a look at the process parameters. Figure 5 shows the irregularities on the cutting edge of the ceramic-coated aluminum foil. When the ceramic material is melted by the laser, an irregular cutting burr is formed, which is partly below the tolerance width and partly above the tolerance width. The occurrence of irregular melting during laser cutting of the ceramic coating can lead to subsequent short circuits in the battery.
The laser cut is determined by numerous process parameters, such as laser energy, frequency of the laser pulse, the wavelength and polarization of the laser beam, process gas, orientation and distance of the nozzle to the foil. The first step in analyzing the irregular cutting edge of the ceramic coating on the aluminum foil is to determine the laser process parameters. One cause of the formation of burrs can be, for example, the wrong polarization of the laser beam, in which the processing direction of the cut is selected transversely to the direction of oscillation of the laser beam. If the polarization direction is instead in line with the processing direction, a smooth and burr-free cut is made. The main setting parameters in the industrial process are the speed (10–60 m/min), laser power (100–500 kHz), pulse width and the distance between the beam and the foil. Images taken with a light microscope of a section of aluminum foil with a ceramic coating show clear differences in the heat-affected zone and structure of the laser cut when the parameters are varied. In addition to the setting of the laser parameters, the reason for an irregular cutting edge can also be caused by other process parameters relating to the cut material. The material properties that influence the cut edge are the layer thickness and quality of the aluminum foil (e.g., rolling direction), the loading of the ceramic layer (mg/cm2) and the resulting layer thickness after drying, and the degree of compaction after calendering. In addition, the quality of the laser cut can be influenced by the different material properties, such as the different thermal conductivity and crystalline structures of aluminum and ceramic. In the industrial coating process, coating on both sides is also common. This can also influence the cutting edge if, for example, there are deviations in the layer thickness on the A and B sides. Empirically observed, a high layer thickness of the ceramic coating combined with a low degree of compaction during calendering leads to more irregular cutting edges of the pole tabs during laser cutting.
In summary, it can be said that it is important not only to examine the properties of the materials and individual processes, but also to look at the relevant parameters of series production. Research work on laser cutting in battery cell production has so far mostly focused on uncoated and slurry-coated foils and their cut edges. It would be interesting to expand research on cathodes with a ceramic strip next to the coating edge. This is also of interest regarding solid-state batteries. It must be emphasized that the coating of the films, whether slurry, ceramic or dry coating, is always a mixture of substances consisting of many additives. On the one hand, the additives serve to adjust the mechanical properties of the batteries so that the cycle load remains low, and on the other hand, the additives serve to increase the performance of the battery. To achieve developments and improvements here, materials science and process technology should work closely together.
formation area, in which the first charging and testing of the battery cell, which can last several weeks, takes place. In this process step, a final decision is made whether the manufactured battery cell meets the requirements or whether it is scrap. Various quality parameters are measured for electrical characterization. Among other things, the open-circuit voltage (OCV), the internal resistance (IR) and the direct current internal resistance (DCIR) are measured. If the requirements are not met, rework is possible and the battery is re-submitted for HT or RT resting, for example. For process control and quality assurance, critical values and tolerance limits are defined for the electrical parameters to be measured. When measuring the electrical parameters, constant test conditions must be maintained. For example, cells that are exposed to different storage conditions (temperature deviation less than 5 k) show slight deviations in the open-circuit voltage (µV range) and self-discharge rate. Monitoring the temperature to correlate with test results is therefore recommended. If deviations in the open-circuit voltage measurement occur that lie outside the tolerance, a comprehensive analysis of the root cause is necessary. The measurement data are subject to a stochastic distribution. Errors correspond to outliers located at the edge of the distribution. In the event of an outlier detection, a broader data set is analyzed to identify correlations between outlier and production conditions (e.g., temperature). In this case, the batteries are removed from production and subjected to laboratory analysis. An example from series production is graphite powder residues on the separator. Due to deviation in the open-circuit voltage measurement, the cell was scrapped. After disassembly, the powder residues were visible during a visual inspection (The final example is related to thearea, in which the first charging and testing of the battery cell, which can last several weeks, takes place. In this process step, a final decision is made whether the manufactured battery cell meets the requirements or whether it is scrap. Various quality parameters are measured for electrical characterization. Among other things, the open-circuit voltage (OCV), the internal resistance (IR) and the direct current internal resistance (DCIR) are measured. If the requirements are not met, rework is possible and the battery is re-submitted for HT or RT resting, for example. For process control and quality assurance, critical values and tolerance limits are defined for the electrical parameters to be measured. When measuring the electrical parameters, constant test conditions must be maintained. For example, cells that are exposed to different storage conditions (temperature deviation less than 5 k) show slight deviations in the open-circuit voltage (µV range) and self-discharge rate. Monitoring the temperature to correlate with test results is therefore recommended. If deviations in the open-circuit voltage measurement occur that lie outside the tolerance, a comprehensive analysis of the root cause is necessary. The measurement data are subject to a stochastic distribution. Errors correspond to outliers located at the edge of the distribution. In the event of an outlier detection, a broader data set is analyzed to identify correlations between outlier and production conditions (e.g., temperature). In this case, the batteries are removed from production and subjected to laboratory analysis. An example from series production is graphite powder residues on the separator. Due to deviation in the open-circuit voltage measurement, the cell was scrapped. After disassembly, the powder residues were visible during a visual inspection ( Figure 6 ).
The particles were then analyzed via EDX to examine the source of contamination. The results can be seen in Table 3 . The main element is carbon with 82.37 At%, which suggests that graphite particles are the cause of damage to the separator and short circuits.
The adhesion of graphite to the copper foil is the subject of many investigations. Graphite has poor adhesive properties because it is a soft and greasy material [ 25 ]. As an example, a primer layer is applied to increase the adhesion of the active material. Diehm et al. investigated the reduction of the binder in the anode slurry in order to increase the conductivity by reducing the proportion of the inactive components [ 26 ]. The adhesion could be increased by applying a carbon layer to the copper foil. Likewise, Lee and Oh increased the adhesion of the active material to the copper foil, as well as the performance of the cell by applying a graphene/polyvinylidene fluoride conductive adhesive layer [ 27 ]; however, applying a primer layer means an additional production step, and the costs that can be saved by reducing the scrap rate must be weighed against the effort required. Whether it is worth the effort to implement quality improvement measures and reduce the scrap rate is an individual company decision. The establishment of quality measures must be calculated as a business case and depends on, among other things, the cell chemistry, i.e., the raw material costs, personal costs, country-specific energy costs and supply chain. Regarding the example of a primer layer on the copper foil, it must be decided whether the coating is performed in-house, or a coated foil is purchased. If a coated foil is purchased, it is 30% more expensive than an uncoated one. The reduction in the scrap rate through this measure must then be compared with the costs for raw materials, energy, personnel, etc. Nevertheless, the application of a primer layer can be advantageous, as it creates a defined surface condition for the application of the slurry and the quality can be increased considerably. In addition to the possibility of using a primer to improve adhesion, the mechanical modification of the foil surface was also suggested by Babaiee et al. and Zhang et al. [ 28 29 ]. It was found that increased roughness improves adhesion, but partially impairs the electrochemical properties. Reducing the roughness by polishing the surface could improve the wetting of the slurry and improve conductivity and corrosion resistance.
The examples of surface treatment for examining adhesion and conductivity show that there is potential here to increase the quality of the film coating. Due to the individual slurry compositions of the manufacturers, however, tests must always be carried out. The quality assurance of the coating process should consist of several levels, such as a pre-process level, which ensures the quality of the materials used and the quality of the manufacturing equipment; a process-integrated level, which ensures the quality of the process and the surfaces; and a post-process level, which ensures the quality of the coated foil. The fact that the active material sometimes does not adhere to the foil as intended is therefore a problem and should be investigated further.
Graphite powder sometimes does not adhere properly, and the particles may result in contamination of the battery. The peel-off of the graphite powder from the layer can also be caused, for example, by the cutting process of the electrode. The graphite powder on the separator in Figure 6 slightly damaged it and led to micro-short circuits. A higher level of self-discharge was also found in these batteries compared to others. Extraordinarily, the particle residue only became visible in the formation by measuring the open-circuit voltage and self-discharge rate. Since quality check-up is accomplished in earlier stages of production steps (e.g., after the electrodes have been stacked and laminated), a Hi-Pot test was carried out. However, the damage to the separator did not cause any measurement deviations during the Hi-Pot test. It is therefore assumed that the production error became significant only when the electrolyte was filled in and the first charge was carried out. One explanation for this could be that the damage to the separator was caused by mechanical stress when charging the battery for the first time, since they are mechanically fixed in workpiece carriers. The mechanical tension counteracts the swelling of the electrodes and prevents the housing from warping. It is possible that due to the pressure that occurs in the battery during charging, the graphite particles were pressed deeper into the separator and permanently damaged it. Based on this explanation, it is possible that the graphite particles are only discovered when measuring the short-circuit current and the self-discharge in the formation. The required technical cleanliness in battery production must be also emphasized. There should be no particles that could damage the battery and lead to short circuits. From a quality point of view, it is interesting to carry out tests in which errors are introduced in different sections of the production process and to determine which method of quality measurement is suitable for error detection. It should also be noted that this error could only occur in serial production, since the mechanical fixation of batteries is handled less strictly in pilot lines and the focus there is placed on other process parameters.
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