Graphite pole piece compaction density test and battery maximum energy density evaluation method

Abstract

This invention discloses a method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of a battery cell, comprising the following steps: preparing graphite electrodes with gradient compaction densities; measuring the physical parameters of the graphite electrodes and selecting sample electrodes; calculating the porosity of the sample electrodes and testing the electrolyte wetting time; assembling a test battery; performing electrochemical and lithium plating tests on the test battery; determining the limiting compaction density; and evaluating the maximum energy density of the battery cell. This invention can quickly find the maximum compaction density of graphite anode materials without lithium plating or performance degradation, and based on this, accurately evaluate and predict the maximum energy density under the given battery cell process, thereby guiding battery cell design and shortening the development cycle.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery manufacturing processes and testing technology, and in particular to a method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells. Background Technology

[0002] In lithium-ion battery manufacturing, electrode compaction density is one of the key parameters determining the cell's energy density. For graphite anodes, increasing compaction density can effectively improve the electrode's volumetric energy density, thereby increasing the overall cell's energy density. However, higher compaction density is not always better. Excessively high compaction density can lead to graphite particle breakage, a sharp decrease in porosity, and difficulty in electrolyte wetting, resulting in severe lithium plating during charging. This not only reduces the battery's cycle life but also poses significant safety hazards.

[0003] Existing evaluation methods mostly rely on manufacturing full cells for long-cycle testing. This trial-and-error approach has a long development cycle, high material and labor costs, and makes it difficult to quickly and accurately determine the compaction density limit of a specific graphite material in the early stages of development. As a result, it is not possible to efficiently evaluate the maximum cell energy density that the material can achieve under current technology. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells. This method can quickly determine the maximum compaction density of graphite anode materials without lithium plating or performance degradation. Based on this, the maximum energy density of the battery cell under the given process can be accurately evaluated and predicted, thereby guiding battery cell design and shortening the R&D cycle.

[0005] To achieve the objectives of this invention, the following solution is adopted: A method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells includes the following steps: S1: Prepare multiple sets of graphite electrode sheets with different compaction densities; S2: Measure the total thickness H of all graphite electrodes. total and total mass M total Then calculate the compaction density PD, the formula is: PD=(M total -M foil ) / [S×(H total -H foil )], M foil Where S is the average mass of the current collector in the graphite electrode, and H is the surface area of ​​the graphite electrode. foil To determine the average thickness of the current collector, multiple sets of graphite electrodes were taken as sample electrodes in ascending order at predetermined compaction density intervals. S3: Calculate the porosity K for all sample electrodes. The formula is: K=PD / T, where PD is the compaction density and T is the true density of the coating solid phase. Then, test the electrolyte wetting time for all sample electrodes. S4: Assemble the sample electrode with the counter electrode to form a test cell; S5: Charge the test battery corresponding to each group of sample electrodes at a predetermined rate, and monitor the negative electrode potential during charging. S6: Select the test cells from S5 with the highest compaction density where the average negative electrode potential is greater than 0V when charged to 100%, and denote their compaction density as PD1. Select the test cells from S5 with the lowest compaction density where the average negative electrode potential is less than or equal to 0V when charged to 100%, and denote their compaction density as PD2. Calculate the limiting compaction density ρ of the graphite material being tested. max Its formula is: ρ max =(PD1+PD2) / 2; S7: Determine if ρ max If the electrolyte immersion time of the sample electrode is less than 150s, then proceed to S9; otherwise, proceed to S8. S8: Take the ratio of the current ρ in sequence max The PD of the electrode sheet of the first group of samples with lower compaction density is taken as the limiting compaction density ρ of the tested graphite material. max Then return to S7; S9: Calculation of electrode volume V based on ultimate compaction density 极片 Its formula is: V 极片 =(M total -M foil ) / ρ max +S×H foil +V 正极 V 正极 The volume of the counter electrode used in S4; S10: V 极片 Substitute the target cell design drawings into the calculation of the total cell volume V. 总 Then, the energy density is calculated based on the cell energy density calculation model.

[0006] Furthermore, S1 includes: Graphite, conductive agent, thickener and binder are mixed in a predetermined ratio to form a slurry, which is then coated onto the current collector. After the slurry dries, a roller press is used to press the current collector by setting different pressures; Multiple groups of graphite electrode sheets with different compaction densities were prepared by slicing with a punching machine.

[0007] Furthermore, graphite, conductive agent, thickener and binder are mixed in a weight percentage of 95.5:1.5:1.5:1.5.

[0008] Furthermore, the thickener and binder are sodium carboxymethyl cellulose and styrene-butadiene rubber, respectively.

[0009] Furthermore, the interval described in S2 is in the range of 0.1 g / cm. 3 ~0.15g / cm 3 .

[0010] Furthermore, in S5, the AC impedance of the test cell corresponding to each group of sample electrodes is first tested, and then the test cell is charged.

[0011] Furthermore, in S5, the constant current ratio and polarization voltage are also monitored during charging.

[0012] Furthermore, the energy density calculated in S10 is the volumetric energy density E. V Its formula is: E V =C0×U / V 总 C0 is the rated capacity of the battery cell, and U is the nominal voltage of the battery cell.

[0013] The beneficial effects of this technical solution are as follows: 1. It abandons the traditional long-cycle trial-and-error method for full-cell batteries and significantly shortens the testing cycle by combining gradient electrode, electrochemical polarization and lithium plating boundary testing.

[0014] 2. By directly considering kinetic performance and the critical potential for lithium plating, the compaction density limit is scientifically defined, thus avoiding hidden damage caused by overpressure.

[0015] 3. By directly linking the compaction test at the electrode level with the energy density assessment at the cell level, researchers are provided with a clear design ceiling, which greatly improves the efficiency of introducing new materials and developing new cells. Attached Figure Description

[0016] Figure 1 A flowchart illustrating the main steps of an embodiment of this application is shown.

[0017] Figure 2 The curves showing the relationship between compaction density and porosity / electrolyte wetting time in embodiments of this application are illustrated.

[0018] Figure 3 A lithium plating test chart of an embodiment of this application is shown.

[0019] Figure 4 The diagram shows the relationship between the compaction density of the graphite electrode sheet in the embodiment of this application and the maximum energy density of the battery cell obtained from the evaluation. Detailed Implementation

[0020] To make the objectives, technical solutions and advantages of this application clearer, the implementation methods of this application will be described in detail below. However, the embodiments described in this application are only some embodiments of this application, and not all embodiments.

[0021] like Figures 1-4 The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells, as shown, is implemented according to the following steps: S1: Preparation of graphite electrode sheets with gradient compaction density: Graphite (negative electrode active material), conductive agent, thickener and binder are mixed in a predetermined ratio to form a slurry, which is then coated on both sides of the current collector. In this embodiment, graphite, conductive agent, thickener and binder are mixed in a weight percentage of 95.5:1.5:1.5:1.5. The conductive agent, thickener and binder are conductive carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber, respectively, and the current collector is copper foil. After the slurry is dried, the current collector is rolled by a roller press with different pressures, and then sliced ​​by a punching machine to prepare multiple sets of graphite electrode sheets with different compaction densities.

[0022] S2: Measure the physical parameters of the graphite electrodes and select sample electrodes: Measure the total thickness H of all graphite electrodes. total and total mass M total Then calculate the compaction density PD, the formula is: PD=(M total -M foil ) / [S×(H total -H foil )], M foil Where S is the average mass of the current collector in the graphite electrode, and H is the surface area of ​​the graphite electrode. foil To determine the average thickness of the current collector, multiple groups of graphite electrodes were taken as sample electrodes in increasing order at predetermined compaction density intervals, with the intervals ranging from 0.1 g / cm³. 3 ~0.15g / cm 3 The number of groups of sample electrodes is 5.

[0023] S3: Calculate the porosity of the sample electrode and test the electrolyte wetting time: Calculate the porosity K for all sample electrodes. The formula is: K=PD / T, where PD is the compaction density and T is the true density of the coating solid phase. Then, test the electrolyte wetting time for all sample electrodes and establish the corresponding curve of compaction density and porosity / electrolyte wetting time.

[0024] S4: Assemble the sample electrode with the counter electrode to form a test battery. In this embodiment, the counter electrode is a lithium metal or positive electrode.

[0025] S5: Perform electrochemical and lithium plating tests on all test batteries: Perform AC impedance tests on the test batteries corresponding to each group of sample electrodes, and then charge the test batteries at a predetermined rate. In this embodiment, the charging rate is 0.5C (the charging current value is equal to 0.5 times the rated capacity of the battery). During charging, monitor the constant current ratio and polarization voltage, and monitor the negative electrode potential through an in-situ reference electrode. By monitoring whether the negative electrode potential drops to 0V and / or disassembling and observing whether lithium plating occurs on the electrode surface after charging, find the compaction density threshold at which the lithium plating critical point occurs.

[0026] Next, in S6~S8, the ultimate compaction density ρ is determined. max In S9 and S10, the maximum energy density of the battery cell is evaluated.

[0027] S6: Select the test cells from S5 with the highest compaction density where the average negative electrode potential is greater than 0V when charged to 100%, and denote their compaction density as PD1. Select the test cells from S5 with the lowest compaction density where the average negative electrode potential is less than or equal to 0V when charged to 100%, and denote their compaction density as PD2. Calculate the limiting compaction density ρ of the graphite material being tested. max Its formula is: ρ max =(PD1+PD2) / 2.

[0028] S7: Determine if ρ max If the electrolyte immersion time of the sample electrode is less than 150s, proceed to step S9; otherwise, proceed to step S8.

[0029] S8: Take the ratio of the current ρ in sequence max The PD of the electrode sheet of the first group of samples with lower compaction density is taken as the limiting compaction density ρ of the tested graphite material. max Then return to S7.

[0030] S9: Calculation of electrode volume V based on ultimate compaction density 极片 Its formula is: V 极片 =(M total -M foil ) / ρ max +S×H foil +V 正极 V 正极 This refers to the volume of the counter electrode used in S4.

[0031] S10: V 极片 Substituting the target cell design drawings into the equations, and combining this with the volumes of the positive electrode, separator, and packaging materials, the total volume V of the cell can be calculated. 总 Then, the energy density is calculated based on the cell energy density calculation model.

[0032] Specifically, the calculated energy density is the volumetric energy density E. V Its formula is: E V =C0×U / V 总 C0 is the rated capacity of the battery cell, and U is the nominal voltage of the battery cell.

[0033] Optionally, the calculated energy density is the mass energy density E. m Its formula is: E m =C0×U / m, where m is the net weight of the battery cell.

[0034] The invention is further illustrated below with a test and evaluation example and a comparative example.

[0035] I. Examples 1. Preparation of graphite electrode sheets: Artificial graphite (average particle size Dv50 = 15 μm, specific surface area 1.2 m²) was used. 2 / g).

[0036] Double-sided coating was applied to 8μm copper foil, with the areal density on one side controlled at 10±0.5mg / cm². 2 The target value for the total areal density on both sides is 20±1 mg / cm³. 2 .

[0037] After roller pressing and drying, five groups of electrode sheets with different compaction density gradients were prepared, with measured compaction densities of 1.40 g / cm³. 3 1.55g / cm 3 1.65g / cm 3 1.75g / cm 3 1.85g / cm 3 The numbers are denoted as A1 to A5 respectively.

[0038] 2. Measure the physical parameters of the graphite electrode and verify the accuracy of the PD. Sampling preparation: After the electrode sheets subjected to gradient rolling are placed in a vacuum oven for 24 hours to release stress, they are cut into standard round sheets with a diameter of 16mm using a special punching machine. The calculated value of S is 2.011cm. 2 This size of disc has sufficient reaction area and can be directly assembled into the CR2430 coin cell casing as a working electrode.

[0039] Background measurement: Pre-measured parameters of the 8μm copper foil used, and the measured average thickness H of the blank copper foil. foil =8.0μm, average mass M of the same area foil =14.41mg.

[0040] Sample measurements: The total thickness of each graphite electrode was measured using a micrometer, and the average value of each group was taken as H.total .

[0041] The total mass of each graphite electrode was measured using a microbalance with a mass of 0.0001 g / L, and the average value of each group was taken as M. total .

[0042] Substitute the measured values ​​of each group of graphite electrodes into the formula described in S2 to calculate the PD value.

[0043] 3. The porosity K was calculated in groups, and the electrolyte wetting time was tested in groups. The physical parameters, calculation and test results of the graphite electrodes in each group are summarized in Table 1 below.

[0044] Table 1 Note: Porosity is calculated based on the true density of the coating solid phase, which is approximately 2.20 g / cm³.

[0045] Figure 2 The curve showing the relationship between compacted density and porosity / electrolyte wetting time is presented, with the horizontal axis representing compacted density in g / cm³. 3 The main vertical axis represents porosity (%), and the secondary vertical axis represents electrolyte wetting time (s). This figure visually illustrates the physical phenomenon that when the compaction density exceeds a certain limit, the collapse of the pore structure leads to a sharp deterioration in wetting dynamics.

[0046] 4. Electrochemical testing and lithium plating testing The graphite electrodes A1 to A5 were assembled with lithium metal sheets to form CR2430 three-electrode coin cells, and tested at 25±0.5℃.

[0047] In the AC impedance test, it was found that the charge transfer impedance (R) of the A4 group electrodes was... ct The impedance of group A5 increased by about 15% compared to group A3, while group A5, due to its porosity falling below 16%, was difficult to wet and its impedance increased significantly by 68% compared to group A3.

[0048] Lithium plating boundary determination: Charge to 100% using a 0.5C constant current and monitor the negative electrode potential (vs. Li / Li+).

[0049] Partial charts of lithium plating test are shown below. Figure 3 As shown, the horizontal axis represents the state of charge (SOC) in %, and the vertical axis represents the negative electrode potential (vs. Li / Li+) in V. The vertical axis at 0 marks the critical baseline for lithium deposition thermodynamics, visually confirming the accuracy of this method in capturing the critical point.

[0050] During the test, the lowest potential of groups A1 to A3 was maintained at 0.015V - 0.045V, which is a positive potential, and there was no lithium plating on the surface.

[0051] A4: The lowest potential drops to -0.002V, touching the 0V thermodynamic critical point. After disassembly, there are traces of silver-gray spots on the edge of the electrode (slight lithium plating).

[0052] A5: The lowest potential drops sharply to -0.052V, a lithium plating plateau appears, and after disassembly, the electrode surface is covered with a large amount of dead lithium.

[0053] Based on the above tests, this formulation system achieves a double-sided 20mg / cm² performance. 2 The ultimate compacted density at the surface density was determined to be 1.70 g / cm³. 3 Based on 1.70 g / cm³ 3 The compaction density was used to calculate the electrode volume, and the results were substituted into the preset VDA cell energy density calculation model to evaluate that the maximum volumetric energy density that this process can achieve is 725Wh / L.

[0054] Figure 4 A mapping diagram showing the relationship between the compaction density of graphite electrodes and the maximum energy density of the battery cell obtained from the evaluation is presented. The evaluation identifies the limiting compaction density point, as well as the safety evaluation zone and the lithium plating risk zone after exceeding the threshold.

[0055] II. Comparison Example 1. Using the exact same artificial graphite, slurry ratio, and coating process as in the example above, but without undergoing the gradient evaluation test of this invention, the electrode compaction density is uniformly rolled to 1.85 g / cm³ based on the industry's experience that high areal density corresponds to high compaction density. 3 That is, the same as group A5.

[0056] 2. Battery performance The electrode was assembled into a full cell and subjected to cycle testing (1C charging / 1C discharging), with a target of 100 cycles.

[0057] The battery maintained normal capacity (98.5% retention) during the first 30 cycles, but around the 40th to 45th cycle, the capacity plummeted, with the capacity retention dropping sharply to below 65%.

[0058] Disassembling the battery after it had experienced a water drop while it was fully charged revealed that the negative electrode surface was covered with a dense layer of dead lithium, and the pores were completely blocked by by-reaction products. This confirmed the effectiveness of blindly pursuing high compaction (1.85 g / cm³). 3 It crossed the lithium plating boundary, leading to serious hidden dangers.

[0059] III. Examples and Comparison with Control Examples The compaction density is 1.70 g / cm³. 3 The graphite electrodes were assembled into a full cell and subjected to the same cycle test as the control example. The results are shown in Table 2.

[0060] Table 2 It can be seen that the tested graphite material has a compacted density of 1.70 g / cm³. 3 The manufactured battery exhibits good performance, reliability, and safety, although its compaction density is 1.85 g / cm³. 3 The battery has a higher cell volumetric energy density, but its porosity is not ideal, its stability is poor, and it is completely unusable.

[0061] The above are only some of the embodiments listed in this application and are not intended to limit this application.

Claims

1. A method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells, characterized in that, Includes the following steps: S1: Prepare multiple sets of graphite electrode sheets with different compaction densities; S2: Measure the total thickness H of all graphite electrodes. total and total mass M total Then calculate the compaction density PD, the formula is: PD=(M total -M foil ) / [S×(H total -H foil )], M foil Where S is the average mass of the current collector in the graphite electrode, S is the surface area of ​​the graphite electrode, and H is the average mass of the current collector in the graphite electrode. foil To determine the average thickness of the current collector, multiple sets of graphite electrodes were taken as sample electrodes in ascending order at predetermined compaction density intervals. S3: Calculate the porosity K for all sample electrodes. The formula is: K=PD / T, where PD is the compaction density and T is the true density of the coating solid phase. Then, test the electrolyte wetting time for all sample electrodes. S4: Assemble the sample electrode with the counter electrode to form a test cell; S5: Charge the test battery corresponding to each group of sample electrodes at a predetermined rate, and monitor the negative electrode potential during charging. S6: Select the test cells from S5 with the highest compaction density where the average negative electrode potential is greater than 0V when charged to 100%, and denote their compaction density as PD1. Select the test cells from S5 with the lowest compaction density where the average negative electrode potential is less than or equal to 0V when charged to 100%, and denote their compaction density as PD2. Calculate the limiting compaction density ρ of the graphite material being tested. max Its formula is: ρ max =(PD1+PD2) / 2; S7: Determine if ρ max If the electrolyte immersion time of the sample electrode is less than 150s, then proceed to S9; otherwise, proceed to S8. S8: Take the ratio of the current ρ in sequence max The PD of the electrode sheet of the first group of samples with lower compaction density is taken as the limiting compaction density ρ of the tested graphite material. max Then return to S7; S9: Calculation of electrode volume V based on ultimate compaction density 极片 Its formula is: V 极片 =(M total -M foil ) / ρ max +S×H foil +V 正极 V 正极 The volume of the counter electrode used in S4; S10: V 极片 Substitute the target cell design drawings into the calculation of the total cell volume V. 总 Then, the energy density is calculated based on the cell energy density calculation model.

2. The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells according to claim 1, characterized in that, S1 includes: Graphite, conductive agent, thickener and binder are mixed in a predetermined ratio to form a slurry, which is then coated onto the current collector. After the slurry dries, a roller press is used to press the current collector by setting different pressures; Multiple groups of graphite electrode sheets with different compaction densities were prepared by slicing with a punching machine.

3. The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells according to claim 2, characterized in that, Graphite, conductive agent, thickener and binder are mixed in a weight percentage of 95.5:1.5:1.5:1.

5.

4. The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells according to claim 2, characterized in that, The thickener and binder are sodium carboxymethyl cellulose and styrene-butadiene rubber, respectively.

5. The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells according to claim 1, characterized in that, The interval specified in S2 is in the range of 0.1 g / cm. 3 ~0.15g / cm 3 .

6. The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells according to claim 1, characterized in that, In S5, the AC impedance of the test cell corresponding to each group of sample electrodes is first tested, and then the test cell is charged.

7. The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells according to claim 1, characterized in that, In S5, constant current ratio and polarization voltage are also monitored during charging.

8. The method for testing the compaction density of graphite electrodes and evaluating the maximum energy density of battery cells according to claim 1, characterized in that, The energy density calculated in S10 is the volumetric energy density E. V Its formula is: E V =C0×U / V 总 C0 is the rated capacity of the battery cell, and U is the nominal voltage of the battery cell.

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