[Battery Disassembly] Disassembly and characterization of Volkswagen ID series battery packs and soft pack batteries (2): LG 78Ah soft pack battery

In the previous article, we learned about the power battery pack design of the Volkswagen ID series electric vehicles, which adopts the traditional Cell to Module to Pack design. According to the needs of different cars, the number of modules can be changed. All ID series cars use a modular electric drive matrix ( MEB ) chassis, but the number of battery modules is different. This design can significantly reduce the manufacturing cost of electric vehicles. 24 pouch cells per battery module . There are two configurations : 8s3p and 12s2p . Each soft-pack battery has a rated voltage of 3.65V and a capacity of 78Ah , giving each module a total capacity of 6.83kWh . The weight of a battery module is 32 kg. Today, the editor will take you to learn about this LG 78Ah soft pack battery.

Abstract: The author disassembled and analyzed the Volkswagen ID.3 's large soft-pack battery with a nominal capacity of 78Ah to determine the technical level of industrial-scale batteries for automotive applications. The battery components were separated, geometrically measured, and weighed to quantify the volume and weight fraction of each component from the electrodes to the cell. Material samples on the electrodes were characterized by scanning electron microscopy (SEM) , elemental analysis and mercury porosimetry. After disassembly, the half-cells were assembled and evaluated in electrochemical tests. The results show that the battery is made of layered electrode layers stacked on top of each other. The positive electrode exhibits a bimodal particle distribution, and its active material is between NMC622 and NMC811 , ranging from Li 0.65 Mn 0.2 Co 0.15 O 2 . The negative active material is silicon-free graphite. Its active material mass accounts for more than 75% and volume accounts for more than 81% . The battery specific energy is 268 Wh kg -1 and the energy density is 674 Wh L -1 . In the charge rate test, the negative electrode is determined to be the limiting electrode. These results provide valuable insights into the technological status of automotive lithium-ion batteries and provide a reference for scientific research.

Preface

Research in academia and industry differs significantly. While many studies from laboratory to pilot scale have been published, less is known about how industry manufactures and designs commercial batteries because of the link between material selection, battery format, production process, and process and its impact on cost. There are numerous possibilities regarding the impact on quality and quality. The editor was very interested in commercial large-capacity batteries before entering the industry, and was curious about what the internal structure of these large-capacity batteries looked like. Laboratory-scale research is mainly focused on button-type, three-electrode batteries and small-sized pouch batteries (tens of mAh to several Ah levels) to obtain basic knowledge at the material and electrode levels. However, these batteries have little in common with industrial large-capacity batteries (tens to hundreds of Ah ). Low-capacity batteries typically benefit from excess electrolyte, optimal pressure, uniform pressure distribution, highly porous separators, and good heat dissipation compared to upgraded products. Furthermore, due to the high ratio of dead volume to active material (and the highly porous separator), they are virtually unaffected by evolved gases. Half-cell results in mAh cm -2 or mAh/g AM along with the previously mentioned advantages would make predictions in Wh kg -1 or Wh L -1 at the cell level unrealistic for industrial applications, especially When only measuring a few cycles. This article aims to introduce the characteristics of commercial batteries, not only to provide a realistic and quantitative reference for the academic community, but also to deepen people's understanding of batteries, a popular product, and their understanding of the transformation of laboratory batteries into applications. Therefore, we characterized, disassembled and analyzed an automotive pouch battery with a nominal capacity of 78 Ah extracted from a Volkswagen ID3 car. Electrochemical measurements for each electrode were determined by disassembling the fabricated half-cell. Conclusions were drawn on electrode design and battery structure. On this basis, assumptions are made about the production process used to manufacture the batteries. Additionally, the impact of the electrodes on the electrochemical performance of the overall battery is illustrated

Battery disassembly

The rated capacity of the battery shown in figure a below is 78Ah . For safety reasons, the battery is discharged to 3.0V in a constant current - constant voltage mode with a cut-off current of 1A to reduce its energy content. Then put the battery into the glove box. For this purpose, the transition chamber is purged with inert gas (not less than 800mbar ). Weigh the sample cell and place it on a rubber pad to prevent short circuit failure. Use a scalpel to open the battery. As shown in Figure b below , the cut is at the very edge of the deep-drawn soft cladding and is cut along three sides of the stacked geometry. The metal foil is cut in front of the lug weld. Remove the laminate from the packaging and peel off the adhesive strips. As shown in Figure c below , peel off the separator, positive electrode and negative electrode from the battery stack layer by layer. Figure d below is the positive electrode layer.

In order to completely disassemble the battery, cut the tabs from the battery soft bag, as shown in the picture below:

Electrode design

SEM images of the negative electrode ( a ) and positive electrode ( b ) are shown below . The negative electrode is mainly composed of flaky particles with a diameter of no more than 20 μm . The cathode image shows a bimodal distribution of spherical particles with diameters of approximately 3 μm and 9.5 μm . These secondary particles are composed of densely packed primary particles ( ? 1μm ). The four largest mass fractions in the negative electrode coating were measured by EDX : 86.4w% carbon, 6.4w% oxygen , 5.7w% fluorine and 1.1w% phosphorus . Elemental analysis confirmed the results of the EDX measurements, with a carbon content of 87% and a hydrogen content of 0.9% . The high carbon content and typical shape of the particles prove that graphite is the active material in the negative electrode. Neither EDX nor ICPAES found any evidence of silicon in the negative electrode ( ? 0.05 w% ). The high content of oxygen in the electrode layer indicates the presence of SEI as well as residual solvent from the electrolyte in the electrode. The hydrogen and phosphorus can be attributed to the electrolyte and SEI , but also to the binder. For the positive electrode, EDX shows that the active material is NMC , with a composition between NMC622 and NMC811 . Only the elements nickel, manganese and cobalt were evaluated and the results showed that nickel accounted for 65.1% by weight , cobalt accounted for 15.5% by weight , and manganese accounted for 19.4% by weight . ICP-AES included lithium content and measured 5.99 ± 0.05 w% . Nickel has the highest content, 33.53 ± 0.32w% . In addition, the cobalt content is 7.78 ± 0.08w% , which is lower than the manganese content ( 9.45 ± 0.1w% ). Combined with the metal molar mass, the cathode material was determined to be LiNi 0.65 Mn 0.2 Co 0.15 O 2 .

The electrodes are coated on both sides. The area of the negative electrode coating is 9.7 × 51.2cm 2 and the area of the positive electrode coating is 9.5 × 51cm 2 . The tabs are soldered on the opposite ends of the long electrode sizes. Therefore, there are three possibilities for electrode production: The first possibility is to use an intermittent coating process, with a layer of coating applied every 51.2 ( +0.8 ) cm for the negative electrode and every 51 ( +0.8 ) cm for the positive electrode, and then slit . The slitter allows the parallel production of several electrodes on one coating machine, as it can be cut mechanically or using a laser in the direction of material flow. The second possibility is a continuous coating process where the coating width is determined based on the electrode length. The third method is to continuously apply a coating twice the width of the electrode and cut it in the middle of the coating. Since the first method has the highest yield (and the second has the lowest yield), it is most likely to be adopted from a cost perspective. Regardless of the method, the electrodes are double-sided coated, with 12 μm copper foil used for the negative electrode and 14 μm aluminum foil used for the positive electrode. This can be done in parallel during the subsequent drying process or continuously during the intermediate drying process. The former process is superior in terms of output and energy consumption. The thickness of the negative electrode coating (single side) is 115.3μm , and the loading density is 18.1 mg cm -2 . The thickness of the single-sided positive electrode coating is 87.3μm , and the loading density is 27.9mg cm -2 . It should be considered that the weight and porosity of the coating will change after electrode production. During battery disassembly, low-boiling electrolyte solvents evaporate, leaving behind conductive salts and high-boiling solvents. In addition, a passivation layer (such as SEI ) is also formed during the formation process. Both increase the weight of the electrode and reduce the porosity of the electrode compared to its original state after production. In this case, the graph below shows the mercury porosimetry test results after cleaning the electrodes to reduce electrolyte residue. The negative electrode shows two normal distributions with peak values at pore diameters of approximately 10 -2 μm and 2 μm respectively . The pore volume distribution of the positive electrode has a broader relationship with the pore size. It has a peak at approximately 0.7 μm and extends toward lower pores as low as 0.1 μm on one side. The asymmetric pore size distribution of the cathode is assumed to result from the bimodal particle distribution.

Cell structure

The battery laminations are wrapped with five strips of tape and placed in a deep-drawn soft casing. The position is fixed by the left and right tabs at the seam. The battery stack consists of a single cell, starting from the separator, followed by the negative electrode, separator and positive electrode. The sequence ends again on the other side with the negative pole and finally with the diaphragm. The battery stack includes a total of 38 separators, 19 negative electrode sheets and 18 positive electrode sheets . The white residues of both the negative and positive electrodes only appear on one side of the sheet, and the resistance when manually separating the separator and electrode sheet is also different. Scanning electron microscopy images (below) show debris-like residue attached to one side of each electrode. Therefore, it is likely that the electrodes are cut by laser or mechanical methods, linked to the tabs, and then the monolithic electrode is laminated to the separator.

The separator itself is not stable because it has a thickness of 17.6 microns and an area of 10 × 51.7 cm 2 . In addition, lamination can halve the number of sheets that need to be stacked in the subsequent stacking process. Comparing the footprints of the electrodes shows that during the stacking process, the positioning accuracy must reach ±1 mm so that the positive electrode coating area is always covered by the negative electrode. In order to fix the position of the electrode sheets, the stacked layers can be subsequently laminated by increasing temperature and pressure. However, no evidence of this has been found so far.

The tabs are made of aluminum and nickel-plated copper, and the conductive cross-section of the tabs is 22.5mm 2 . The front and back of the pole ears have different embossed patterns. Unlike laser welding, this pattern is pressed into the metal using ultrasonic welding. Both patterns illustrate that welding is done in two steps. First, the tapered ends of the base foils ( cross-sectional area of 0.54 mm 2 for each negative electrode and 0.64 mm 2 for each positive electrode ) are welded together. Then, solder them together to the tabs. Finally, the electrode separator stack is inserted into the deep-drawn aluminum composite foil and sealed on both sides (at the left and right foils).

Compared to wound electrode - separator assemblies, the stacked and pouched cell formats enable one-step electrolyte injection and faster wetting. Therefore, we assume that the electrolyte is injected under vacuum and the cell is subsequently sealed, thereby achieving closed wetting. Possibly during or after the formation process, a degassing step was performed by cutting and sealing the cell to remove the air bladder to its final shape. The battery weight is 1.101 kg and the volume is 0.438 liters . The figure below shows the weight and volume proportions of the components in a battery. The electrode coatings containing active materials account for 75.7% and 81.6% of the battery weight respectively . In addition, the dead volume is approximately 1.4% , indicating high space utilization since pouch cells have advantages in terms of energy and packaging density.

Electrochemical properties

The positive and negative electrodes stripped from the battery core were assembled into multiple half-cells for charge and discharge cycle characterization. As shown in the figure below, each half-cell has experienced the first three cycles of formation. After testing, the positive electrode discharge surface capacity is about 4.3- 4.4mAh cm 2, the discharge surface capacity of the negative electrode is approximately 4.7-4.8mAh cm 2 .

The negative and positive charging capacities are 5.23mAh cm -2 and 5.02mAh cm -2 respectively . According to the surface capacity of the positive and negative electrodes, the N/P ratio is calculated to be 1.04 . Compared with the design strategies reported in the literature, this ratio is quite low. Due to the reduced potential and improved negative electrode capacity utilization, the available capacity of the battery is improved, but it also increases the risk of rapid capacity fading during operation because during operation A larger anode capacity window is used during cycling.

The original battery has a C/50 discharge capacity of 79.9Ah at 20 °C (picture below) and an energy retention rate of 295.4Wh . Based on the size and weight measurements, the specific energy of the battery was determined to be 268Wh kg -1 and the energy density was 674Wh L -1 . In comparison, due to the lower proportion of active material in button cells, the positive half-cell only reaches 8.25Wh kg -1 and 24.42Wh L -1 under the same conditions . Using graphite /NMC622 , a large hard-shell battery with a capacity of 22.01Ah ( C/5 ) showed lower characteristics of 138.26Wh kg -1 and 293.08Wh L -1 . Even using pouch cells at laboratory scale (based on graphite /NMC532 , 0.24Ah , C/20 ), only levels of 200Wh kg -1 and 390Wh L -1 are achieved. Therefore, the measured characteristic values of LG pouch batteries show the huge impact of battery structure and upgrading to high capacity.

In the subsequent charging current rate test (as shown in the figure below), the capacity retention rate of the half-cell was tested when the charging current increased. Although the NMC positive electrode can achieve quite stable capacity retention below 1C , it is quite stable at C/2 . At low rates, the negative electrode capacity will be reduced by two times. At a current rate of 1C , the negative electrode capacity is at its minimum and the negative potential is reached very early. Based on the above observations, it is primarily the negative electrode that limits the battery's charging capabilities. Therefore, during aggressive fast charging of the entire battery, a negative anode potential is reached, potentially causing lithium evolution to occur. Improvements in negative charging capabilities may increase the battery's overall fast-charging capabilities. The highest current density in the battery is located at the tapered end portion of the copper foil at the front of the tab on the negative side. Here, based on the cross-sectional area and cell capacity of 79.9Ah , the current density of the copper foil at 1C is 8.22 A mm -2 . On the positive side, aluminum is 7.05 A mm -2 at the same rate . At 1C , the tab conducts only 3.55A mm -2 .

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References

1.https://iopscience.iop.org/article/10.1149/1945-7111/ac4e11