Tesla 18650 battery design analysis
The Tesla Model S battery module consists of 444pcs of 18650 batteries. The overall dimensions of the module are shown in Figure 1. The length of the module is approximately 68.8cm, the width is approximately 30cm, and the height is approximately 7.9cm. The total volume is approximately 0.163m3, surface area is approximately 0.5145㎡. The total mass of the module is about 25.4kg, and a single 18650 battery is about 47g. In the Tesla Model S, 16 modules are connected in series, with more than 7,100 batteries per vehicle.
Before disassembling the module, conduct an impedance test on the module. Use Hioki's BT3554 battery tester to test the impedance between the positive and negative terminals at a frequency of 1 kHz. The impedance of the module at 1 kHz at room temperature is 3.25 mΩ.
In order to better understand the thermal behavior of the module, several holes were drilled in the module casing and temperature sensors were placed inside the package and on the surface of the battery module. The temperature sensor arrangement is shown in Figure 2. The sensor with a blue border is directly connected to the battery, the sensor with a red border is installed in the drill hole on the module, and the black border indicates that the sensor is installed on the top of the current collecting plate.
Set the voltage range of the module to 15 V (2.5 V·6) ~ 25.2 V (4.2 V·6), and select the current 117.65A. The evolution of temperature distribution over time is shown in Figure 3. The position of the sensor in the upper right corner of the figure is indicated by color. It can be seen from the figure that the temperature peaks in the components of sensors 203 and 218 are 48.25°C and 47.14°C. Temperature minimums were detected at sensors 238 and 225, both around 27.88°C. Additionally, open plastic casings have a significant impact on temperature. The temperature on the left side is significantly higher than on the right side. The capacity test results show that the charging capacity is 222.706 Ah and the discharge capacity is 222.704Ah.
The module disassembly process is shown in Figures 4 and 5
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After disassembly, each battery was visually inspected and tested for 1kHz impedance and voltage using a Hioki BT3554. Voltage test range is -6 V to 6 V with 1 mV resolution; impedance range is up to 300 mΩ with 10μΩ resolution. The disassembled and damaged battery was processed, and the resistance test results are shown in Figure 6. Blue indicates lower impedance, while higher impedance is indicated in red. Significantly higher impedance was detected in one of the cells in the upper left corner. In general, uneven impedance distribution has no correlation with location. The average impedance at 1 kHz is 24.69 mΩ with a standard deviation of 0.39 mΩ.
The voltage test results are shown in Figure 7. The average voltage is 3.6 V and the standard deviation is 0.002 V. In the upper right corner, the 74 cells connected in parallel have a slightly higher voltage than the average, about 1 mV, possibly due to the previous use of a disabled BMS.
The disassembly process of the 18650 battery is shown in Figures 8 and 9.
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The specifications of the battery pole piece are shown in Figure 10. The total length of the negative electrode is 725 mm, the width is 60 mm, and the thickness is about 180 microns. Thickness is measured with a spiral micrometer. The tab is soldered to the uncoated copper foil on the left. The positive electrode is 665 mm long, 58 mm wide, and 140 microns thick. The pole lug is welded to an uncoated aluminum foil with a width of 5mm in the middle of the pole piece. There is an uncoated area on the outside of the negative electrode, approximately 6x7.5 cm2. The positive electrode is coated except for the middle tab. The total coating areas of the negative electrode and positive electrode are 780 c㎡ and 765.6?c㎡?respectively, and the excess positive electrode Overhang area of the negative electrode is approximately 1.88%.
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The quality analysis of each component of the battery is shown in Figure 11. The components of the battery are separated and then weighed. The electrode mass exceeds 73% of the battery weight. The mass of the battery before disassembly was 46.82 grams, and after disassembly it was 44.77 grams. This may be related to the evaporation and loss of electrolyte and the falling of scratched particles. The electrode including the current collector weighs 34.25 grams, including 15.4 grams for the negative electrode and 18.85 grams for the positive electrode. The casing, including the tabs and the tape used to secure the jelly roll, weighs approximately 8.8 grams. The diaphragm weighs approximately 1.72 grams. Therefore, assuming an average voltage of 3.7 V and a capacity of 3.1 Ah, the measured energy density can be calculated to be 244.98 Wh/kg.
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The electrode surface and cross-sectional morphology are shown in Figures 12 and 13. The electrode cross-sectional morphology shows that the thickness of the positive electrode is approximately 149μm. The aluminum foil is 14μm, and the thickness of the active material layer on one side is approximately 68μm. The measured thickness of the negative electrode is 229μm. The thickness of the copper foil is approximately 34μm, and the thickness of the active material layer on one side is approximately 97μm. Comparing the results of the micrometer measurement, this measurement result is questionable, especially the negative electrode and its copper foil value.
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The CT test morphology results of the battery are shown in Figure 14. The measured battery diameter is 18mm and the thickness of the casing is 0.32mm. The thickness of the electrode was studied, and approximately 40 layers of electrodes were measured, and the average negative electrode thickness was calculated to be 184μm with a standard deviation of 17μm. The thickness of the cathode is 167μm with a standard deviation of 25μm. Use a micrometer screw to measure the thickness of the negative electrode to 180μm, the positive electrode to 140μm, and the separator to 17μm. The discrepancy between the two measurements may be caused by the septum, which cannot be detected on CT scans.
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The void tortuosity τ of the electrode coating is calculated by the following formula
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Where x is a function of the specific void volume, total porosity and nominal density of the porous body; Y is a factor derived from mercury porosity measurements and surface area data; and ε represents the porosity of the sample. In Figure 15, the negative electrode particle distribution and its relative particle volume are expressed in percentage. The particle diameter ranges from 1μm to 10μm, with a peak value of 5μm, and the tortuosity is estimated to be τ=2. In Figure 16, the cathode particle distribution and its relative particle volume are expressed in percentage. The particle diameter ranges from 0.1μm to 1μm, with a peak value of 0.4μm, and the tortuosity is estimated to be τ=2.04.
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The overall parameter list of positive and negative pole pieces is as follows:
Assemble the positive and negative electrode plates into half cells and full cells. The electrochemical performance is shown in Figure 17-19.
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