Production test procedures
Lithium-ion cell production processes, starting with the active materials, always have high vertical integration rates. Therefore control processes are required in as many production steps as possible.
Lithium-ion cell production stages (bicells, pouch housings)
Anode band production
Housing in foil pouch
Evacuation and final sealing
Sequencing 15 production stages places extensive demands with regard to the quality of the product from the previous production stage. The individual stage yields do not merely add up to generate an overall yield; there is a multiplier effect! In reality, yields of 100 % rarely exist. This means that lower yields of 95 to 99 % have the following effect on the overall yield:
Yield per stage 99 %: Overall yield = (0.99)15 = 0.86 = 86 %
Yield per stage 98 %: Overall yield = (0.98)15 = 0.74 = 74 %
Yield per stage 97 %: Overall yield = (0.97)15 = 0.63 = 63 %
Yield per stage 96 %: Overall yield = (0.96)15 = 0.54 = 54 %
Yield per stage 95 %: Overall yield = (0.95)15 = 0.46 = 46 %
This example shows how essential testing and sorting stages are for the production process. Product material costs are more important than production costs in high-yield plants. Optimizing yield is therefore paramount for a plant to be operated economically.
This does not only apply to the above-mentioned battery production process with bicells, but to all processes. To ensure low failure rates during the battery warranty period, a great deal of control stages are required; they detect faults during the production process. Thin areas in separators can cause long-term effects such as increased self-discharging. It is possible that these are not detected by process control measures. Hence, reference samples are monitored to detect long-term effects.
Failure rates of < 1 ppm are called for internationally; this means that only every millionth battery should fail. The respective control measures control and secure the process. The lithium-ion cell production process can last up to two weeks. Detecting a manufacturing error as late as at the end of the process chain during the final inspection is the worst case scenario. However, the process still continues, and the cells produced during this time span need to be blocked. This can entail immense costs; therefore, manufacturers strive to implement extensive control stages in the process. The most important stages are described below.
19.2 Test procedures during coating
The essential foundation for continuously high cell quality is homogeneous electrode band coating.
Because the throughputs are very high, this is a very challenging task. For example, a small production line with a throughput of two million 20-Ah cells per year processes 26,000 km of anode and cathode band, respectively; this is equal to more than 14,000 m² flawless surface per day.
The coating parameters that are examined are layer thickness and inline surface quality. Measuring systems using radioactive radiation absorption or, of late, high-resolution laser systems provide reliable layer thickness measurements. Both procedures ensure that the desired control values are achieved. Measuring point positioning and density are key. As it is almost impossible to test the full cross-section, either measuring points close to each other are used or traversing measuring heads. For the traversing systems, it is important that their horizontal speed is concurrent with the plant's coating speed, which can reach up to 40 m/min. In an adequately stable process, these measurements are sufficient for testing the profile. The most common deviations are a tapered gradient in the profile’s coating thickness or so-called “chatter marks”, i.e., a rippled gradient, in machine direction.
The quality of the entire surface should be tested with camera systems after the dryer and before the winder. This enables detection of defects, contaminations, and cavities.
The most important part is handling the detected defects. Coating is a continuous process that is best not stopped. Coater stabilization can last up to one hour; it is not viable to stop the machines. Thus, the detected defects must be flagged and removed later in the process. Flagging can be performed by marking the defects on the non-coated edge with a color. Later, during electrode separation and before cell assembly, they can be removed. This requires complex production data control and linking of the individual process stages.
19.3 Test procedures during cell assembly
Cell body production
The electrode body can be manufactured with different technologies such as winding (round or prismatic) or stacking (bicell or electrodes coated on both sides). Irrespective of the production technology, it is important to provide full electrical isolation between the anode and the cathode. Also, each cathode part must face an anode part, isolated only by the separator. To ensure these requirements are met, it is necessary to test the finished electrode stack to verify that the anodes and the cathodes, including the conductor, are electrically separated and to check that the individual electrodes are correctly positioned.
Testing the cell body for short circuits
In the most harmless of cases, defective electrical anode-cathode separation causes increased self-discharging, while the worst case scenario is a direct short circuit of the electrode body. Then, not even cell forming is possible.
The cell body must be tested for short circuits at the end of the assembly at the latest. Short circuits can be caused by defective separators, uneven electrode cutting edges, wear and tear of the production machines, incorrect handling, or process stages such as ultrasonic conductor welding. Ultrasonic welding has the advantage that it results in very low weld transition resistance; however, it introduces mechanical energy into the electrode body. This can damage the separator in vulnerable locations, or the conductive electrode mass can be rattled loose from the collector and deposited between the electrodes, causing soft short circuits.
The test comprises a resistance measurement, either as an AC or DC measurement.
Standard resistance measuring bridges are used to measure AC resistance. At the beginning of the measurement, however, these measuring bridges do not provide constant measured values because the cell body has a high capacitive share in the overall resistance. Usually, measured values increase continuously at a decreasing speed until they reach a certain limit. If this level is very high, i.e., R > 1MOhm, it is assumed that the cell body does not have any detrimental short circuits. If the measured value is in the kOhm range or even below 1 kOhm, the cell body will not be able to take on more charge during forming. The flowing charge is not stored but transformed into heat by means of ohmic resistance.
Insufficient reproducibility is another disadvantage of AC measurement technology. It is negatively influenced by the measured sample’s moisture content. Water has a relatively high dielectric constant, which influences the capacitive share of the overall resistance. Electrode and separator moisture content therefore influences the absolute measurement value, so that measured values cannot be replicated if, for example, humidity levels change. AC resistance measurement is a simple and very quick procedure for detecting hard short circuits. The operator must be aware that the measured overall resistance has an ohmic and a capacitive share. This method is used for process control under consideration of the above-mentioned issues.
DC resistance measurement is another measurement method. Direct voltage is applied to the test specimen and the flowing current is measured. The condenser, consisting of the electrodes separated by the separator, must be charged first, similar to the AC resistance measurement method. The resulting limit current is independent of the capacity; it is almost the same as the sheer leakage current. Measured values are adjusted more quickly than with the AC method. Low voltages of < 20 V are sufficient for detecting direct short circuits, while higher DC voltages, up to 200 V, are used to detect soft short circuits. The latter cause the separator to “burn through” in thin areas. Handling higher voltages such as these requires special equipment.
The short circuit tests must have a clock speed that is compatible with that of the process. There is only a window of a few hundred milliseconds for measuring individual bicells during cell body assembly. If bicells are used in cell body design, there is the advantage that small cell body units can be tested for functionality during assembly and removed, if necessary. Testing and removing small individual parts decreases the overall process reject rate. After inserting the cell body into the housing and connecting the conductors, another short-circuit test during the production process must be carried out. Both the isolation of the electrodes from each other and contact to the housing are tested. It is not possible to forgo this test stage because changes to the electrode stack and the conductors can occur during cell body installation in the housing. Every time the cell body is handled, it can be damaged.
Testing cell body thickness
Testing electrode body thickness is necessary for two reasons: Before installation, it guarantees that the electrode body physically fits into the housing. The desired finished cell thickness must be met; this thickness must also comply with the tolerances specified in the data sheet. The measured value once more enables monitoring of coating on the electrode mass and, therefore, also of cell balancing. Coating thickness variations are multiplied in the electrode body because the coating comprises several layers. Coating irregularities of the cell body such as tapered coating profiles can thus be tested quickly. Technologies with a sufficient resolution such as mechanical, optical, or laser systems are suitable for these tests.
Production tolerances are taken into account when designing the cell by implementing an anode that is always slightly larger than the cathode. This surplus anode area is inactive during charge storage. Its size must strike a balance between providing optimum energy density and handling process tolerances.
The electrodes are pre-assembled from the coated bands or punched out as a whole. Before cell assembly, the correct electrode matrix position in the punched-out bands must be tested. Electrode position is tested inline to 100 % using camera systems.
Short circuit measurements and visual inspection that the electrodes are correctly positioned are essential test stages in the cell production process. Currently, surface contamination detection can be performed for a particle size of > 30 µm. Efforts are currently underway to improve small particle optical resolution during continuous operations. Electrode and band contaminations can be removed by cleaning. Very soft rotating brushes, connected to a suction system, are used to clean the surfaces.
19.4 Electrolyte dosing
During so-called activation or electrolyte filling, the metered electrolyte amount must be checked. Both electrolyte overdosage and underdosage must be prevented. Underdosage means that the cell does not have sufficient electrolyte to achieve the specified performance characteristics. Purely volumetric control of the metered electrolyte amount is not recommended because gas cavities are also captured during the measurement. This is why gravimetric control is the preferred procedure, i.e., weighing the cell before and after metering.
Forming is, on the one hand, the cell's “electric birth” and, on the other hand, the first extensive electrical test step. It comprises the first charging of the cell. Also, the required protective layers such as the solid electrolyte interface are formed and unwanted contaminations are removed electrochemically.
Initial charging of the cell uses a considerable amount of energy to form the protective layers and for nonrecurring secondary reactions. This means that the first charging and discharging cycle introduces much more energy into the cell than it takes out. This overcapacity quickly decreases with the number of charging cycles. The charging capacity is higher in the first cycle than in the second cycle. The charge is irreversibly consumed during these forming processes; it is used to form protective layers (solid electrolyte interface, SEI), to silently burn residual water, and to oxidize contaminations. The discharging capacities of the first and second cycle, on the other hand, are very similar.
The charge factor approximates 100 % after several charging and discharging cycles. High-quality lithium polymer batteries have charge factors of 99.5 % or more. If the charge factor changes, this is a sign of slowly creeping-in process changes. There is no other measurement parameter that gives a better indication of the overall process status. It is a sum parameter that reflects the following qualities: dispersion, coating, cell body assembly, separator, electrolyte filling, electrolyte distribution, and active material utilization.
These three parameters are tested in one test step; the cell only needs to be handled and connected one time. The parameter for discharging capacity in the data sheet is checked. Charging efficiency provides information about the individual cell’s electrochemical system status (such as wetting, purity, process stability) and, if need be, about short circuits.
It makes sense to also check internal resistance in the testing device. Internal resistance is determined by the voltage drop during the short discharging current pulse (< 1 s) in accordance with Ohm’s law. The pulse duration must be so short that the diffusion effects do not influence the measurement and only pure internal resistance is measured.
19.6 Final inspection after ripening
Forming is frequently followed by ripening, also known as test storage. During test storage, the charged batteries are matured for several days at increased temperatures. During maturing, the final distribution of electrolytes occurs as well as the conversion of residual byproducts and contaminations. The battery stabilizes and possible self-discharging is forced.
After test storage, the next step in the process is the final extraction and sealing, if required. These steps are the last mechanical stages of cell production. An OCV (open circuit voltage) measurement after test storage determines whether the battery has soft short circuits or has been damaged during final sealing. The OCV values measured after final sealing are compared to the OCV values measured after forming. Internal resistance is measured again and the final mechanical dimensions are checked.
Cell grading, i.e., classification, can now take place based on measurement data acquired during the process. It is performed in compliance with customer's specifications, e.g., cell grading in accordance with internal resistances.
19.7 Reference sample monitoring
Aside from quality control, reference samples are monitored to obtain aging data and to verify long-term series quality.
This means that during production, batteries are taken out of the production line at certain time intervals and for each product change. The removed batteries are stored semi-charged and permanently inspected at increasing time intervals, e.g., at the time of removal, after one month, after three, six, nine, 12 months, and then every six months.
Each cell’s state of charge at the time of storage is known from the previous reference sample measurements. The first discharging determines the still existing charge and, therefore, the self-discharging rate. By subsequently fully charging and discharging the cell, changes in storage capacity caused by age can be identified. The second discharging reflects irreversible aging. At the end of the testing program, the cell is charged to a defined state and stored again. Simultaneously, internal cell resistance is measured and recorded. This reference sample monitoring generates real-time data on aging and self-discharging. Because reference sample storage quickly increases during the course of production, the number of reference samples must be determined realistically.
- 1.Lithium Ion Cell Production Processes, Ralph Brodd, Kazuo Tagawa, Advances in Lithium-Ion Batteries, 2002, pp. 267 – 288Google Scholar
- 2.Production Processes for Fabrication of Lithium-Ion Batteries, Kazou Tagawa, Ralph J. Brodd, Lithium-Ion Batteries, 2009, pp. 181–194Google Scholar
- 4.Fig. 19.1, 19.2, 19.3: Courtesy of Kemet Electronics Italia S.r.l (Arcotronics), I-40037 Sasso Marconi, ItalyGoogle Scholar