Method for calculating liquid retention capacity of winding type lithium ion battery cell

Abstract

The invention relates to a method for calculating the liquid retention capacity of a winding type lithium ion battery cell, which relates to the field of lithium ion battery cells, and comprises the following steps: calculating the total theoretical pore volume V of the lithium ion battery cell, calculating the actual liquid absorption capacity M1 of a positive plate, the actual liquid absorption capacity M2 of a negative plate and the actual liquid absorption capacity M3 of a thin film; the method comprises the following steps: calculating the electrolyte quantity coefficient k required by pores among pole pieces after the pole pieces are wound into a roll core and the reserved pores of the whole battery cell, calculating the density rho of the electrolyte and the electrolyte consumption coefficient t of each charge and discharge in the long circulation process, and finally calculating the total electrolyte retention amount M. The method can accurately and simply calculate the electrolyte retention amount of the battery cell in the battery cell design stage, and can be used for calculating the electrolyte retention amount of the battery cell in the battery cell design stage. It is ensured that the battery cell obtains enough electrolyte support in the long-cycle process, the service life of the battery cell is prolonged, the performance of the battery cell is improved, and the method has high calculation precision and applicability and can be widely applied to design and production of the winding type lithium ion battery cell.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery cells, and more particularly to a method for calculating the liquid retention amount of a wound lithium-ion battery cell. Background Art

[0002] New energy battery cells have received increasing attention. Due to their excellent performance, low price, and good safety, they have been widely concerned. To further meet the needs of the public, new battery cells with high energy density and long cycle life are urgently needed. The electrolyte is continuously consumed during the cycling of the battery cell to supplement the loss of active lithium during the cycle, and its content greatly affects the performance of the battery cell. When the amount of injected electrolyte is too small, the positive and negative electrode plates and the separator are not fully wetted, affecting the capacity and internal resistance of the battery cell, and further affecting the electrical performance and cycle life, etc. Since there is not enough electrolyte, during the charge and discharge process of the battery cell, lithium ions cannot smoothly intercalate and deintercalate between the positive and negative electrodes, and then lithium crystal branches are formed. The lithium crystal branches will pierce the separator, causing short circuit between the positive and negative electrodes, seriously affecting the safety performance of the battery cell. At the same time, considering the overall energy density performance of the battery cell, it is not appropriate to add too much electrolyte. Therefore, an accurate method for calculating the liquid retention amount of the battery cell is needed.

[0003] In the prior art, for example, a method for calculating the liquid retention amount of the electrolyte in a battery cell is disclosed in Chinese Patent Application No. 202111524862.0. The disadvantage of this patent is that it ignores the thickness of the inner shell of the wound core and the thickness of the wound core, so that the liquid absorption amount at the bending part of the wound core is not fully considered. Summary of the Invention

[0004] Based on the above analysis, the present invention provides a method for calculating the liquid retention amount of a wound lithium-ion battery cell, and its specific implementation scheme is as follows:

[0005] A method for calculating the liquid retention amount of a wound lithium-ion battery cell includes the following steps:

[0006] S1: Measure the total theoretical pore volume V of the lithium-ion battery cell;

[0007] S2: Measure the actual liquid absorption amount M1 of the positive electrode plate, the liquid absorption amount M2 of the negative electrode plate, and the liquid absorption amount M3 of the thin film;

[0008] S3: Calculate the coefficient k of the electrolyte amount required for the pores between the electrode plates and the reserved pores of the overall battery cell after the electrode plates are wound into a wound core. Make the positive electrode plate, the negative electrode plate, and the separator into a wound core, measure the length, height, and thickness of the wound core before and after liquid absorption, calculate the ratio of the volume of the wound core before and after liquid absorption, the volume of the wound core = the length of the wound core * the height of the wound core * the thickness of the wound core, then calculate the ratio of the thickness of the inner shell of the package to the thickness of the wound core, the thickness of the inner shell of the package is the total thickness of the outer shell - 2 * the thickness of the shell, and then multiply the ratio of the volume of the wound core before and after liquid absorption by the ratio of the thickness of the inner shell of the package to the thickness of the wound core to obtain k;

[0009] S4: Calculate the electrolyte density ρ and the coefficient t of electrolyte consumption per charge-discharge cycle during long-term cycling. The coefficient t of electrolyte consumption per charge-discharge cycle during long-term cycling = actual required electrolyte / theoretically required electrolyte. The actual required electrolyte is the actual electrolyte consumption after charge-discharge cycle experiments on the finished soft-pack battery cells with injected electrolyte. The theoretically required electrolyte = electrolyte consumption per cycle * designed life cycle number of the battery cells;

[0010] S5: The total liquid retention amount M of the battery cells = ((V * ρ + (M1 + M2 + M3))) / 2 * k * t.

[0011] By considering the changes in the size of the wound core and the packaging structure, the electrolyte conversion coefficient k can accurately reflect the distribution of the electrolyte inside the battery cells, which is crucial for optimizing the injection amount of the electrolyte and avoiding the influence of too much or too little electrolyte on the performance and safety of the battery cells. By calculating the difference between the actual required electrolyte and the theoretically required electrolyte, the coefficient t of electrolyte consumption per charge-discharge cycle during long-term cycling can reflect the consumption of the electrolyte in actual use. Considering the influence of the electrolyte consumption on the performance of the battery cells, the introduction of t makes the calculation of the liquid retention amount closer to the actual use conditions, which helps to predict the performance changes of the battery cells during long-term use.

[0012] In summary, the electrolyte conversion coefficient k and the coefficient t of electrolyte consumption per charge-discharge cycle during long-term cycling ensure the performance, safety, and life of the battery cells by accurately calculating the liquid retention amount of the electrolyte. The introduction of the electrolyte conversion coefficient k and the coefficient t of electrolyte consumption per charge-discharge cycle during long-term cycling in the liquid retention amount calculation method improves the scientificity and practicality of the battery cell design.

[0013] Preferably, in S1, the total theoretical pore volume V of the lithium-ion battery cells = pore volume V1 of the positive electrode sheet + pore volume V2 of the negative electrode sheet + pore volume V3 of the separator. The pore volume V1 of the positive electrode sheet = (volume of the positive electrode sheet - volume of the foil) * theoretical porosity. The foil refers to the metal material used as the support electrode material and conductor in the electrode sheet. The pore volume V2 of the negative electrode sheet = (volume of the negative electrode sheet - volume of the foil) * theoretical porosity. The pore volume V3 of the separator = length of the separator * width of the separator * thickness of the separator * separator porosity.

[0014] The porosities and material properties of different battery cells may vary. Therefore, by calculating the pore volume of each component separately, not only can the calculation accuracy be improved, but also it can adapt to the designs of various battery cells and ensure more accurate distribution of the electrolyte.

[0015] Preferably, the volume of the positive and negative electrode sheets = the rebound thickness after rolling of the positive and negative electrode sheets * the length and width of the positive and negative electrode sheets * the number of layers of the positive and negative electrode sheets, the volume of the foil = the length of the foil * the height of the foil * the thickness of the foil, the theoretical porosity = the volume of the added solid substances / the volume of the positive and negative electrode sheets, and the volume of the added solid substances = (the mass of the main active material of the electrode sheet + the mass of the additive + the mass of the binder) / (the density of the main active material of the electrode sheet + the density of the additive + the density of the binder).

[0016] The calculation methods of the volume of the positive and negative electrode sheets, the volume of the foil, and the theoretical porosity are defined here. The calculation methods can be used to adjust the proportions of the active material, the additive, and the binder. By clarifying the calculation methods of each parameter, the operability and accuracy of the method are enhanced.

[0017] Preferably, the liquid absorption amount M1 of the positive electrode sheet in S2 = the liquid absorption amount per unit of the positive electrode sheet Mc * the total volume Lc of the positive electrode sheet, the liquid absorption amount M2 of the negative electrode sheet = the liquid absorption amount per unit of the negative electrode sheet Ma * the total volume La of the negative electrode sheet, the liquid absorption amount M3 of the separator = the liquid absorption amount per unit of the separator Ms * the total volume Ls of the separator. The total volume Lc of the positive electrode sheet and the total volume La of the negative electrode sheet = the length of the positive and negative electrode sheets * the width of the positive and negative electrode sheets * the thickness of the electrode sheet after rolling and rebound liquid absorption. The total volume Ls of the separator = the length of the separator * the width of the separator * the thickness of the separator.

[0018] Due to the differences in the liquid absorption capabilities of the various components of the battery cell, calculating the liquid absorption amount per unit helps to maintain the consistency of battery cells in different batches. At the same time, the thickness change after liquid absorption is incorporated into the calculation, making the volume estimation more in line with the actual situation. Precise control of the liquid absorption amount makes the performance of each battery cell more stable during the production process, reducing performance fluctuations caused by uneven electrolyte distribution.

[0019] Preferably, the positive electrode sheet, the negative electrode sheet, and the separator are cut according to unit volume and grouped according to type. The small pieces after cutting are weighed and recorded as m1 before injection of the electrolyte after encapsulation. After injecting the electrolyte to completely submerge the positive electrode sheet, negative electrode sheet, and separator to be tested for full infiltration, the excess electrolyte is extracted and weighed and recorded as m2. Then the liquid retention amount m of this group = m2 - m1. The average value is calculated to obtain the final liquid retention amount m'. ma = m' / (the volume of the electrode sheet or separator after liquid absorption. The volume of the electrode sheet or separator after liquid absorption = the length of the electrode sheet or separator * the width of the electrode sheet or separator * the thickness of the electrode sheet or separator after liquid absorption. ma represents the liquid absorption amount per unit Mc of the positive electrode sheet or the liquid absorption amount per unit Ma of the negative electrode sheet or the liquid absorption amount per unit Ms of the separator, which is determined according to the specific experimental object).

[0020] By immersing the cut battery cell chips and measuring their weight changes, a more accurate actual liquid absorption amount can be obtained, rather than relying solely on theoretical calculations. This experimental method can reflect the actual adsorption situation of the electrolyte and is closer to the results in the actual production process than simply relying on theoretical calculations.

[0021] Preferably, the method of extracting the excess electrolyte is vacuum extraction.

[0022] Vacuum extraction can effectively eliminate the interference caused by bubbles or air generated during the electrolyte injection process, avoid inaccurate electrolyte injection volume due to air retention inside the battery cell, and can significantly improve the reliability and consistency of the measurement results. This is particularly important during the production of battery cells with high-precision requirements.

[0023] Preferably, the method of obtaining the liquid absorption amount in S2 can also be to measure the actual volumes of the positive electrode sheet, negative electrode sheet, and separator before and after soaking, and calculate the liquid absorption amount based on the volume difference of the materials before and after soaking.

[0024] The volume difference before and after soaking can intuitively reflect the degree of liquid absorption of the material. Especially in some porous materials, the infiltration of liquid usually causes its volume to expand. Directly measuring the volume change can provide an intuitive parameter related to the actual change of the material, avoiding the possible complexity in the mass measurement process, such as the need to consider factors such as moisture and solvent residue.

[0025] Preferably, when measuring the length, height, and thickness of the wound core before and after liquid absorption in S3, a measuring tool with an accuracy of 0.01 mm or less is used for measurement.

[0026] Using a high-precision measuring tool with an accuracy of 0.01 mm or less can greatly reduce the measurement error. This is crucial for the volume calculation of the battery cell and the evaluation of the liquid absorption amount, because the slight changes in the internal structure of the battery cell may directly affect the electrolyte distribution and the performance of the battery cell. Precise measurement can ensure the accurate capture of these changes in design and production, thereby improving the performance and stability of the entire battery cell system.

[0027] Preferably, the charge-discharge cycle experiment in S4 refers to a process of multiple cycle tests, and each experimental process includes charging, standing, and discharging steps.

[0028] A long-term charge-discharge cycle experiment, especially the addition of the standing stage, can help observe the trend of electrolyte consumption in the battery cell after multiple cycles. The standing stage simulates the stability of the battery cell in a non-working state, and the electrolyte consumption and other chemical reactions that may occur during the charging and discharging processes can also be captured in a timely manner. This comprehensive experimental design helps to evaluate the performance degradation or electrolyte loss of the battery cell after long-term use, ensuring the...

[0029] The beneficial effects of the present invention are as follows:

[0030] (1) The present invention particularly considers the microscopic changes in the expansion of the electrode sheet after liquid absorption in the wound battery cell, the influence of this change on the pore volume and the overall stress of the battery cell, and can better conform to the actual use situation of the battery cell.

[0031] (2) By introducing the coefficient k of the amount of electrolyte required for the pores between the electrode sheets and the overall reserved pores in the cell core after the electrode sheets are wound into the cell core and the coefficient t of the electrolyte consumed during each charge and discharge, the present invention can accurately predict the electrolyte demand of the cell during long-term cycling, thereby providing a theoretical basis for the cell and reducing the impact of electrolyte consumption on the cell performance.

[0032] (3) The present invention avoids the situations of excessive or insufficient electrolyte, ensures that the positive and negative electrode sheets and the separator of the cell are fully wetted, and avoids safety hazards such as the formation of lithium crystal branches and short circuits, further improving the safety and cycle life of the cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those skilled in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0034] Figure 1 It is a schematic flowchart of an embodiment of the method for determining the electrolyte injection amount of the cell provided by the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the specific embodiments described here are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0036] As Figure 1 shown, it is a schematic flowchart of an embodiment of the method for determining the electrolyte injection amount of the cell in the embodiment of the present application. The method for determining the electrolyte injection amount of the cell is applied to a wound lithium-ion cell. The calculation method of the liquid retention amount of the cell includes:

[0037] Wherein the lithium-ion cell model is a system with a wound cell structure, including a positive electrode sheet, a negative electrode sheet, and a separator.

[0038] S1, determine the theoretical pore volumes V of the positive and negative electrode sheets and the separator of the cell system, including the pore volume V1 of the positive electrode sheet, the pore volume V2 of the negative electrode sheet, and the pore volume V3 of the separator, and add the volumes of the three to obtain the total volume of the theoretical calculation.

[0039] Specifically, the pore volume V1 of the positive electrode sheet = (the volume of the positive electrode sheet - the volume of the foil) * the theoretical porosity, the pore volume V2 of the negative electrode sheet = (the volume of the negative electrode sheet - the volume of the foil) * the theoretical porosity, and the pore volume V3 of the separator = the length of the separator * the width of the separator * the thickness of the separator * the separator porosity. Among them, the volume of the positive and negative electrode sheets = the rebound thickness after rolling of the positive and negative electrode sheets * the length and width of the positive and negative electrode sheets * the number of layers of the positive and negative electrode sheets, the volume of the foil = the length of the foil * the height of the foil * the thickness of the foil, the theoretical porosity = the volume of the added solid substances / the volume of the positive and negative electrode sheets, and the volume of the added solid substances = (the mass of the main active material of the electrode sheet + the mass of the additive + the mass of the binder) / (the density of the main active material of the electrode sheet + the density of the additive + the density of the binder). Among them, the main active material of the positive electrode sheet generally uses materials such as nickel-cobalt-manganese oxide, lithium iron phosphate, and lithium manganate, the main active material of the negative electrode sheet generally uses materials such as natural graphite, artificial graphite, and silicon-based materials, the additive generally uses materials such as carbon black, conductive carbon fiber, and carbon nanotube, and the binder generally uses materials such as polyvinylidene fluoride and carboxymethyl cellulose.

[0040] S2. Measure the actual liquid absorption amount M1 of the positive electrode sheet, the liquid absorption amount M2 of the negative electrode sheet, and the liquid absorption amount M3 of the film.

[0041] Specifically, cut the positive electrode sheet, the negative electrode sheet, and the separator into pieces with a volume of 1 cm 3 in size, group them according to the type, with 5 pieces in each group, a total of 5 groups. Each group is encapsulated with an aluminum-plastic film, and the weight is weighed and recorded as m1. Inject electrolyte into the grouped positive electrode sheets, negative electrode sheets, and separators respectively and then encapsulate them. Then inject 10 - 20 ml of electrolyte and soak for 48 h. After ensuring that the electrode sheets or the separator are completely saturated with the liquid, vacuum pump out the excess electrolyte, weigh the weight and record it as m2. The liquid absorption amount of this group is recorded as m = m2 - m1. Calculate the average value of the 5 groups to obtain the final liquid retention amount m'. ma = m' / (the volume of the electrode sheet or the separator after liquid absorption). The volume of the electrode sheet or the separator after liquid absorption = the length of the electrode sheet or the separator * the width of the electrode sheet or the separator * the thickness of the electrode sheet or the separator after liquid absorption. ma represents the unit liquid absorption amount Mc of the positive electrode sheet, the unit liquid absorption amount Ma of the negative electrode sheet, or the unit liquid absorption amount Ms of the separator in the following text, which is determined according to the specific experimental object. Measure and obtain the unit liquid absorption amount Mc of the positive electrode sheet, the unit liquid absorption amount Ma of the negative electrode sheet, or the unit liquid absorption amount Ms of the separator.

[0042] The total volume Lc of the positive electrode sheet and the total volume La of the negative electrode sheet = the designed length of the positive and negative electrode sheets * the designed width of the positive and negative electrode sheets * the thickness of the electrode sheet after rolling and rebound and liquid absorption, and the total volume Ls of the separator = the length of the separator * the width of the separator * the thickness of the separator.

[0043] The liquid absorption amount M1 of the positive electrode sheet = the unit liquid absorption amount Mc of the positive electrode sheet * the total volume Lc of the positive electrode sheet, the liquid absorption amount M2 of the negative electrode sheet = the unit liquid absorption amount Ma of the negative electrode sheet * the total volume La of the negative electrode sheet, and the liquid absorption amount M3 of the separator = the unit liquid absorption amount Ms of the separator * the total volume Ls of the separator.

[0044] S3. Determine the coefficient k of the amount of electrolyte required for the pores between the electrode sheets and the reserved pores in the overall battery cell after the electrode sheets are wound into a core.

[0045] Specifically, use a vernier caliper with an accuracy of 0.01 mm or less to measure the length, height, and thickness of the core before and after liquid absorption. The volume of the core = the length of the core * the height of the core * the thickness of the core. Calculate the ratio of the volume of the core before and after liquid absorption, and then calculate the ratio of the thickness of the inner package shell to the thickness of the core. The thickness of the inner package shell is the minimum distance from the outer frame of the battery cell to the internal space. Then, multiply the ratio of the volume of the core before and after liquid absorption by the ratio of the thickness of the inner package shell to the thickness of the core to obtain the electrolyte conversion coefficient k.

[0046] S4. Determine the density ρ of the electrolyte and the coefficient t of the electrolyte consumed in each charge and discharge during the long cycle.

[0047] Specifically, the electrolyte density ρ is the actually measured electrolyte density. Inject an excessive amount of electrolyte into the finished soft-pack battery cell until the battery cell is completely submerged, and then conduct a charge and discharge cycle experiment at 25 °C and 0.1C. Obtain the electrolyte of the 1st cycle, 10th cycle, 20th cycle, 30th cycle, 40th cycle, 50th cycle, and 100th cycle through vacuum extraction. Then calculate the electrolyte consumed in every 10 cycles. The coefficient t of the electrolyte consumed in each charge and discharge during the long cycle = the actually required electrolyte - the theoretically required electrolyte. The actually required electrolyte = the mass of the electrolyte in the finished soft-pack battery cell before the charge and discharge cycle experiment - the mass of the electrolyte in the finished soft-pack battery cell after the charge and discharge cycle experiment. The theoretically required electrolyte = the electrolyte consumption per cycle * the designed life cycle number of the battery cell.

[0048] S5. Determine the final liquid retention amount M of the battery cell.

[0049] Specifically, consider the electrolyte required for the pores between the electrode sheets and the reserved space inside the battery cell and multiply it by the coefficient k. Finally, consider the coefficient t of the electrolyte consumed in each charge and discharge during the long cycle, and obtain the final liquid retention amount M = ((V * ρ + (Mc * Lc + Ma * La + Ms * Ls)) / 2 * k * t.

[0050] In this embodiment, by comprehensively considering the total pore volume of the positive electrode sheet, negative electrode sheet, and separator and the gap volume between the positive electrode sheet, negative electrode sheet, and separator, the amount of electrolyte injection for the wound lithium battery cell is calculated. It can accurately calculate the amount of electrolyte injection at the beginning of the design of the wound lithium battery cell.

[0051] Embodiment 2:

[0052] This embodiment relates to a method for obtaining the liquid absorption amount, mainly by measuring the actual volume difference of the positive electrode sheet, negative electrode sheet, and separator before and after soaking to calculate the liquid absorption amount.

[0053] Step 1: Volume measurement before soaking

[0054] Select the positive electrode sheet, negative electrode sheet and separator to be tested. Before putting the positive electrode sheet, negative electrode sheet and separator into the liquid, use a vernier caliper with an accuracy of 0.01 mm or less to measure the length, width and height of each material before soaking and calculate the volume of the material before soaking. The volume of the material before soaking = the length of the material before soaking * the width of the material before soaking * the height of the material before soaking.

[0055] Step 2: Soaking treatment

[0056] Inject electrolyte into the positive electrode sheet, negative electrode sheet and separator respectively and encapsulate them. Then inject the electrolyte and soak for 48 h. After the electrode sheet or separator is completely soaked with the liquid, vacuum pump out the excess electrolyte.

[0057] Step 3: Volume measurement after soaking

[0058] Use a vernier caliper with an accuracy of 0.01 mm or less to measure the length, width and height of each material after soaking and calculate the volume of the material after soaking. The volume of the material after soaking = the length of the material after soaking * the width of the material after soaking * the height of the material after soaking.

[0059] Step 4: Calculate the liquid absorption amount

[0060] Calculate the liquid absorption amount according to the volume difference of the material before and after soaking. The liquid absorption amount = the volume after soaking - the volume before soaking.

[0061] The above has introduced the embodiments of the present invention in detail. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those skilled in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A method for calculating the liquid retention capacity of a wound lithium-ion battery cell, characterized in that: The following steps are involved: S1: measuring the total theoretical pore volume V of the lithium-ion battery; S2: Measure the actual liquid absorption of the positive electrode sheet M1, the liquid absorption of the negative electrode sheet M2 and the liquid absorption of the film M3; S3: Calculate the electrolyte volume coefficient k required for the pores between the pole pieces and the reserved pores of the entire battery cell after the pole pieces are wound into a core, make the positive pole piece, the negative pole piece and the separator into a core, measure the length, height and thickness of the core before and after liquid absorption, calculate the ratio of the volume of the core before and after liquid absorption, the core volume = core length * core height * core thickness, then calculate the ratio of the thickness of the package inner shell to the thickness of the core, the thickness of the package inner shell is the total thickness of the outer shell - shell thickness * 2, and then multiply the ratio of the volume of the core before and after liquid absorption by the ratio of the thickness of the package inner shell to the thickness of the core to obtain k; S4: Calculate the electrolyte density ρ and the electrolyte consumption coefficient t for each charge and discharge during the long cycle, wherein the electrolyte consumption coefficient t for each charge and discharge during the long cycle = actual electrolyte required / theoretical electrolyte required, wherein the actual electrolyte required is the actual electrolyte consumption after the charge and discharge cycle experiment of the finished soft-pack battery cell injected with the electrolyte, and the theoretical electrolyte required = electrolyte consumption per cycle * designed life cycle of the battery cell; S5: The total liquid retention volume of the battery cell M = ((V*ρ+(M1+M2+M3))) / 2*k*t.

2. The method for calculating the liquid retention amount of a wound lithium-ion battery cell according to claim 1, characterized in that: The total theoretical pore volume V of the lithium-ion battery cell described in S1 = positive electrode sheet pore volume V1 + negative electrode sheet pore volume V2 + diaphragm pore volume V3, the positive electrode sheet pore volume V1 = (positive electrode sheet volume - foil volume) * theoretical porosity, the foil refers to the metal material used as supporting electrode material and conductive material in the electrode sheet, the negative electrode sheet pore volume V2 = (negative electrode sheet volume - foil volume) * theoretical porosity, the diaphragm pore volume V3 = diaphragm length * diaphragm width * diaphragm thickness * diaphragm porosity.

3. The method for calculating the liquid retention amount of a wound lithium-ion battery cell according to claim 2, characterized in that: The volume of positive and negative electrode sheets = the rebound thickness of the positive and negative electrode sheets after rolling * the length and width of the positive and negative electrode sheets * the number of positive and negative electrode sheet layers, the volume of foil = foil length * foil height * foil thickness, the theoretical porosity = the volume of added solid matter / the volume of positive and negative electrode sheets, the volume of added solid matter = (mass of the main material of the electrode active material + mass of the additive + mass of the binder) / (density of the main material of the electrode active material + density of the additive + density of the binder).

4. The method for calculating the liquid retention amount of a wound lithium-ion battery cell according to claim 1, characterized in that: The liquid absorption amount M1 of the positive electrode sheet in S2 = the unit liquid absorption amount Mc of the positive electrode sheet * the total volume Lc of the positive electrode sheet, the liquid absorption amount M2 of the negative electrode sheet = the unit liquid absorption amount Ma of the negative electrode sheet * the total volume La of the negative electrode sheet, the liquid absorption amount M3 of the diaphragm = the unit liquid absorption amount Ms of the diaphragm * the total volume Ls of the diaphragm, the total volume Lc of the positive electrode sheet and the total volume La of the negative electrode sheet = the length of the positive and negative electrode sheets * the width of the positive and negative electrode sheets * the thickness of the positive and negative electrode sheets after the rolling and rebound absorption of liquid, and the total volume Ls of the diaphragm = the length of the diaphragm * the width of the diaphragm * the thickness of the diaphragm.

5. The method for calculating the liquid retention amount of a wound lithium-ion battery cell according to claim 4, characterized in that: The positive electrode sheets, negative electrode sheets and diaphragms are cut according to unit volume and grouped according to type. The cut pieces are weighed after packaging and before liquid injection and recorded as m1. After the electrolyte is injected to completely cover the cut positive electrode sheets, negative electrode sheets and diaphragms to be tested for full infiltration, the excess electrolyte is extracted and weighed and recorded as m2. The liquid retention volume of this group is m=m2-m1, and the average value is calculated to obtain the final liquid retention volume m', ma=m' / (volume of the electrode sheet or diaphragm after liquid absorption), the volume of the electrode sheet or diaphragm after liquid absorption=length of the electrode sheet or diaphragm*width of the electrode sheet or diaphragm*thickness of the electrode sheet or diaphragm after liquid absorption, ma represents the unit liquid absorption volume Mc of the positive electrode sheet or the unit liquid absorption volume Ma of the negative electrode sheet or the unit liquid absorption volume Ms of the diaphragm, which is determined according to the specific experimental object.

6. The method for calculating the liquid retention amount of a wound lithium-ion battery cell according to claim 5, characterized in that: Excess electrolyte is extracted using vacuum extraction.

7. The method for calculating the liquid retention amount of a wound lithium-ion battery cell according to claim 1, characterized in that: When measuring the length, height and thickness of the core before and after absorbing liquid in S3, a measuring tool with an accuracy of 0.01 mm or less is used for measurement.

8. The method for calculating the liquid retention amount of a wound lithium-ion battery cell according to claim 1, characterized in that the charge and discharge cycle experiment in S4 refers to a multiple-cycle test process, and each test process includes charging, standing and discharging steps.

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