Simple evaluation method for flexibility of lithium battery electrode sheet

Abstract

The present application belongs to the technical field of lithium ion batteries, and particularly relates to a simple and convenient evaluation method for the flexibility of a lithium battery pole piece, comprising the following steps: (1) homogenously and stably dispersing a positive electrode active material, a conductive agent, a binder and a solvent into a uniform and stable positive electrode slurry; (2) coating the positive electrode slurry on the surface of a foil according to a double-sided area density of 340-400 g / m 2 ; (3) using a small roller press to obtain a finished electrode film with a certain thickness; (4) cutting out square interface flat electrode film A1, A2, A3 and A4 according to the conventional A5 paper size specification, placing the electrode film A1, A2, A3 and A4 flat on a smooth glass plate, using a plug gauge to measure the thickness of the electrode film A1, A2, A3 and A4, recording 15 data in the rolling direction, and using minitab software to draw a box plot. The method is simple and easy to operate, the data is accurate, and the flexibility of the pole piece is truly and effectively fed back.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a simple and convenient evaluation method for the flexibility of a lithium battery pole piece. BACKGROUND

[0002] In various key manufacturing processes of lithium batteries, electrode manufacturing technology is particularly important. After slurry coating, the main active material, conductive additive and binder are uniformly coated on the foil surface in a certain proportion. At this time, the porosity and interpenetrating network structure are very rich, and the micro transmission gap is large, so the van der Waals force between materials is very weak, and material falling is prone to occur. Therefore, the rolling process is particularly important, which compacts the loose material particles to a certain thickness, increases the van der Waals force and hydrogen bond interaction between materials and the material and foil, increases the peeling strength and shortens the lithium ion transmission path, reduces the bulk ohmic polarization and increases the electronic conductivity. Under the comprehensive action, the compaction performance of the pole piece is improved. At the same time, lithium battery engineers believe that the appropriate compaction density in the process can not only guarantee the flexibility of the pole piece, prevent the rolling, die cutting and winding of the broken belt, improve the production yield, but also match the appropriate electrolyte absorption performance to ensure that the pole piece is fully soaked and infiltrated. In terms of electrical performance, the appropriate compaction design has a significant impact on capacity, internal resistance, rate temperature rise and cycle life. Under the background and influence of electrode performance, the evaluation design and test method of the flexibility of the pole piece are particularly important.

[0003] At present, the operation instruction and patent for evaluating the flexibility of the pole piece in the enterprise are mostly macroscopic visual brittleness contrast, lack of quantitative index data representation, and the operation method is relatively complex, the execution efficiency is low, the equipment investment cost is high, and it is difficult to be widely applied. Among them, 1, the positive pole piece is folded once on the front and back surfaces, the material falling, forking and hand feeling pole piece rebound force at the fold are observed, and the brittleness of the pole piece is evaluated; 2, the pole piece is wrapped around a circular shaft with different diameters, and the flexibility of the pole piece is evaluated according to the pole piece indentation and material falling. In order to explore this problem, we found an interesting phenomenon in actual production, that is, the flat state of the positive pole piece is different under different compactions of the same lithium iron phosphate positive material and under the same compaction of different positive material films. When the brittleness of the pole piece is large, the material area state is uneven, and the texture is very hard. When the flexibility of the pole piece is excellent, the material area state is bright and flat, and the texture is soft. SUMMARY

[0004] The application aims to overcome the shortcomings of the prior art, and provide a simple and convenient evaluation method for the flexibility of the lithium battery pole piece.

[0005] In order to achieve the above-mentioned purpose, the technical scheme adopted by the application is as follows:

[0006] A simple evaluation method of lithium battery electrode flexibility, comprising the following steps:

[0007] (1) homogenously disperse the positive active material, conductive agent and binder into a uniform and stable positive slurry;

[0008] (2) coat the positive slurry of step (1) on the surface of the foil according to the double-sided area density of 340-400 g / m 2

[0009] (3) roll using a small roller press to obtain a finished electrode film of a certain thickness;

[0010] (4) cut out four square interface flat electrode films A1, A2, A3 and A4 according to the conventional A5 paper size specification;

[0011] Place the electrode films A1, A2, A3 and A4 flat on a smooth glass plate, respectively, and use a plug gauge to measure the gap thickness ΔH1, ΔH2, ΔH3 and ΔH4 of the electrode films A1, A2, A3 and A4, record 15 data in the rolling direction, and use minitab software to draw a box plot.

[0012] In a preferred example, the weight ratio between the positive active material, conductive agent and binder is 96.2-97.8: 0.5-1.5: 1-3.

[0013] In a preferred example, in step (1), the positive active material is lithium iron phosphate, the conductive agent is a combination of carbon black and carbon nanotubes, and the binder is PVDF, and the weight ratio between the lithium iron phosphate, carbon black, carbon nanotube and PVDF is 96.5: 0.5: 1: 2.

[0014] In a preferred example, in step (2), the foil is an aluminum foil with a thickness of 12 μm.

[0015] In a preferred example, in step (3), roll according to the compacted density of 2.30-2.60 g / cc.

[0016] Compared with the prior art, the present application studies the fitting gap discrete degree and distribution size of the electrode after rolling and the glass platform, uses minitab tool to draw a box plot to represent the difference in the flatness of the electrode, and further realizes the comparison of the quantitative data of the flexibility and brittleness of the electrode. The method of the present application is used to feedback the size of the lithium battery electrode flexibility, which is simple, accurate, real and effective. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 Box plot of gap thickness of lithium iron phosphate positive electrode film under gradient compaction in Example 1. DETAILED DESCRIPTION​

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Additional aspects and advantages of this invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It should be understood that the following description is merely illustrative and not intended to limit the invention.

[0019] The terms “comprising,” “including,” “comprising,” or any other variations thereof, as used herein, are intended to cover a non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0020] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0021] Example 1

[0022] The simple method for evaluating the flexibility of lithium battery electrodes in this embodiment includes the following steps:

[0023] (1) Lithium iron phosphate material A was homogenized and dispersed with solvent N-methylpyrrolidone according to the formula (96.5wt% lithium iron phosphate material + 0.5wt% carbon black + 1wt% carbon nanotubes + 2wt% PVDF) to prepare a uniform and stable positive electrode slurry;

[0024] (2) According to the double-sided surface density of 380g / m 2 The loading capacity is achieved by coating the positive electrode slurry from step (1) onto the surface of an aluminum foil with a thickness of 12 μm;

[0025] (3) Using a small roller press, the electrode films of a certain thickness are obtained by roller pressing at compaction densities of 2.30 g / cc, 2.40 g / cc, 2.50 g / cc and 2.60 g / cc respectively.

[0026] (4) Cut out electrode films A1, A2, A3, and A4 with flat four-sided interfaces according to the standard A5 paper size specifications;

[0027] Electrode films A1, A2, A3, and A4 were placed flat on a smooth glass plate. A plug gauge was used to measure the gap thickness ΔH1, ΔH2, ΔH3, and ΔH4 of electrode films A1, A2, A3, and A4, and 15 data points were recorded in the rolling direction. A box plot was drawn using Minitab software, as shown below. Figure 1 As shown;

[0028] Depend on Figure 1It can be seen from the analysis that, with the continuous increase of the compaction density, the gap thickness value ΔH of the positive plate after rolling also increases, and the geometric multiple is increasing. At the same time, the flexibility of the plate is decreasing, and the brittleness is increasing. Combined with the folding and bifurcation of the plate, it can be seen that when ΔH≤500μm, the compaction of the plate is in the best state, the upper shadow line is almost non-existent, the thickness consistency is good, which can ensure the high processing yield of the plate and the high energy density of the battery, when ΔH≤200μm, although the compaction flexibility is good, the compaction tightness of the particles is not enough, and the material advantage is not fully played; when ΔH is between 500-1000μm, the brittleness of the plate increases, the length of the upper and lower shadow lines increases, the range is large, the thickness consistency is poor, which easily leads to the increase of the production pass rate and the short circuit rate; when ΔH>1000μm, the upper shadow line is too long, the range is large, and the thickness consistency is poor, which proves that the compaction of the plate is actually too high, the brittleness is too large, and the material particles even have the phenomenon of crushing and cracking, and the flexibility cannot meet the production requirements, so the production is prone to frequent belt breakage, poor winding and coating, low product pass rate, and large scrap. Therefore, through the above mathematical modeling, the gap thickness value can be used to scientifically evaluate the flexibility of the plate, output the best compaction performance parameters, improve the production yield and battery performance, and reduce the research and development cost and improve the production efficiency.

[0029] Example 2

[0030] The simple evaluation method of the flexibility of the lithium battery plate in the embodiment includes the following steps:

[0031] (1) The lithium iron phosphate materials A, B, C and D are respectively uniformly dispersed with the solvent N-methyl pyrrolidone according to the formula (96.5wt% lithium iron phosphate material+0.5wt% carbon black+1wt% carbon nanotube+2wt% PVDF) to prepare uniform and stable positive electrode slurry;

[0032] (2) The positive electrode slurry of step (1) is coated on the surface of an aluminum foil with a thickness of 12μm according to the double-sided area density of 360g / m 2 with a load of 2.40g / cc to obtain four kinds of coated film pieces A, B, C and D;

[0033] (3) The four kinds of coated film pieces A, B, C and D of step (2) are rolled using a small roller press to obtain finished electrode film pieces with a certain thickness;

[0034] (4) According to the conventional A5 paper size specification, the electrode film pieces A2, B2, C2 and D2 with flat four-sided interface are cut out;

[0035] Place the electrode membrane pieces A2, B2, C2, D2 flat on a smooth glass plate, use a plug gauge to measure the thickness of the electrode membrane pieces A1, A2, A3, A4, record 15 data in the rolling direction, and use minitab software to draw a box plot.

[0036] Blank example

[0037] The evaluation method of the flexibility of the lithium battery pole piece in the present blank example comprises the following steps:

[0038] (1) Disperse the lithium iron phosphate material A according to the formula (96.5wt% lithium iron phosphate material + 0.5wt% carbon black + 1wt% carbon nanotube + 2wt% PVDF) with the solvent N-methyl pyrrolidone to prepare a uniform and stable positive electrode slurry;

[0039] (2) Coating the positive electrode slurry of step (1) on the surface of an aluminum foil with a thickness of 12μm according to the double-sided area density of 380g / m 2 ;

[0040] (3) Using a small roller press, roll according to the compaction density of 2.30g / cc, 2.40g / cc, 2.50g / cc, 2.60g / cc respectively to obtain a finished electrode membrane piece with a certain thickness;

[0041] (4) According to the conventional A5 paper size specification, cut out the electrode membrane pieces A1, A2, A3, A4 with a square interface flatness;

[0042] (5) Fold the electrode membrane pieces A1, A2, A3, A4 once respectively on the front and back, and record the phenomenon of pole piece folding: A1: fold mark is flat, without bifurcation and material falling, the folding hand feeling has no obvious rebound stress; A2: the fold mark is relatively flat, without bifurcation, with slight material falling, the folding hand feeling has a certain rebound stress; A3: the fold mark is uneven, with certain jagged bifurcation and material falling, the folding hand feeling has obvious rebound stress; A4: the fold mark is rough and peeling, with serious bifurcation and material falling, light observation leakage, folding hand feeling has obvious rebound stress, and the overall pole piece brittleness is large, which is the limit compaction.

[0043] Although this method objectively evaluates the brittleness phenomenon of the positive pole piece under different compactions, it lacks quantitative index measurement data, has large artificial subjective difference, and has large test error of different personnel operation. All the above affect the evaluation of the suitable use compaction of the pole piece and the output of the suitable compaction design, and the operation is complex, the accuracy is low, the representation is wide, and it is difficult to long-term improve the rolling electrode process processing capacity and output accurate design, and improve the production yield.

[0044] The above embodiments are only the preferred embodiments of the present application, and any simple modification, modification and alternative change made according to the technical essence of the present application to the above embodiments are within the scope of the technical scheme of the present application.

Claims

1. A simple method for evaluating the flexibility of lithium battery electrodes, characterized in that, Includes the following steps: (1) The positive electrode active material, conductive agent, binder and solvent are homogenized and dispersed into a uniform and stable positive electrode slurry; (2) According to the double-sided surface density of 340-400 g / m 2 The loading capacity is achieved by coating the positive electrode slurry from step (1) onto the surface of the foil. (3) Using a small roller press, the electrode films of a certain thickness were obtained by roller pressing at compaction densities of 2.30 g / cc, 2.40 g / cc, 2.50 g / cc and 2.60 g / cc respectively. (4) Cut out electrode films A1, A2, A3 and A4 with flat four-sided interfaces according to the standard A5 paper size specifications; Electrode films A1, A2, A3, and A4 were placed flat on a smooth glass plate. A plug gauge was used to measure the gap thickness ΔH1, ΔH2, ΔH3, and ΔH4 of the electrode films A1, A2, A3, and A4, recording 15 data points along the rolling direction. Box plots were then created using Minitab software. Analysis of the box plots showed that as the compaction density increased, the gap thickness ΔH of the positive electrode after rolling also increased. Simultaneously, the electrode's flexibility decreased, while its brittleness increased. Analysis of electrode folding and splitting revealed that when ΔH was ≤500μm, electrode compaction was optimal. When ΔH was ≤200μm, while compaction flexibility was good, particle compaction density was insufficient. When ΔH was between 500 and 1000μm, electrode brittleness increased. When ΔH > 1000μm, the actual electrode compaction was excessive, resulting in excessive brittleness.

2. The simple evaluation method for the flexibility of lithium battery electrode sheets according to claim 1, characterized in that, The weight ratio of the positive electrode active material, conductive agent, and binder is 96.2–97.8: 0.5–1.5: 1–3.

3. A simple method for evaluating the flexibility of lithium battery electrodes according to claim 2, characterized in that, Step (1): The positive electrode active material is lithium iron phosphate, the conductive agent is a combination of carbon black and carbon nanotubes, the binder is PVDF, and the weight ratio of lithium iron phosphate, carbon black, carbon nanotubes and PVDF is 96.5:0.5:1:

2.

4. A simple method for evaluating the flexibility of lithium battery electrodes according to claim 1, characterized in that, Step (2), the foil is an aluminum foil with a thickness of 12μm.

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