Negative pole piece as well as preparation method and application thereof

Abstract

The invention discloses a negative pole piece as well as a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The preparation method of the negative electrode plate comprises the following steps: S1, preparing negative electrode slurry; and S2, coating the surface of a current collector with the negative electrode slurry, and carrying out temperature increasing heating drying. According to the preparation method of the negative pole piece disclosed by the invention, by setting an increasing drying temperature curve, the constant-speed drying evaporation rate is relatively low, the migration amount of small-particle substances such as CMC and binders in the negative pole piece is minimum, the distribution of the binders on the upper layer and the lower layer of the negative pole piece is more uniform, and the binding power is greater.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a negative electrode sheet and a preparation method and application thereof. Background Art

[0002] The negative electrode of lithium-ion batteries plays a vital role in the battery. It is not only an important place for electrochemical reactions, but also directly affects the overall performance, safety and life of the battery. In the production and preparation of negative electrode sheets, the negative electrode slurry needs to be dried after coating. The baking step is an indispensable and crucial part of the coating process. It has the function of fully removing the solvent. In this process, it will also affect the uniformity of film formation. The adhesion and thickness of the film after forming are closely related to the coating and baking steps.

[0003] At present, the lithium battery industry mainly uses methods to improve the adhesion of negative electrode sheets in the coating process, such as improving the selection of binders, optimizing the formula, optimizing the slurry dispersion process, and optimizing the oven drying curve. At present, the drying mode adopted by lithium battery coating machines is mainly: air knife blowing suspension drying. When the coated wet film enters the oven, the temperature of the hot air in the cavity, the air humidity, the hot air flow rate, and the air knife contact method remain unchanged. The typical drying rate curve is "transition section-constant speed drying section-deceleration drying section-balance section". In the transition section, the coating film is cooled down first and then heated up. The leveling of the slurry mainly occurs in this stage. Due to the heat absorption of water or other solvents during vaporization, the drying rate in this stage is faster and the time is shorter. The solvent evaporation rate is constant in the constant speed drying section, but the specific evaporation rate varies due to the different distance, temperature, and time. Most of the water or other solvents are rapidly vaporized and evaporated in this stage, and the heat absorption and vaporization of the material are in a state of equilibrium. In the deceleration drying stage, the surface of the coating film is no longer wet, and part of the surface bound water begins to vaporize, but the water migration rate cannot keep up with the surface water vaporization rate, and the vaporization rate decreases. The moisture content in the electrode reaches a stable state in the balancing stage, which also marks the end of the drying process.

[0004] During the constant-rate drying stage, the liquid on the coating surface evaporates quickly to form an enriched layer. The liquid in the coating migrates from the inside to the surface due to capillary action. During this process, the binder molecules / conductive agent particles in the lower liquid layer will diffuse to the surface with the solvent and remain on the surface. When the temperature is high during the constant-rate drying stage, the evaporation rate of the material surface is faster, and small particles such as binders move to the surface, causing small particles such as binders to accumulate on the electrode at a faster rate and form an enriched layer on the surface, resulting in uneven distribution of adhesive / conductive agent within the thickness distribution range from the inside to the surface of the electrode, which will directly lead to a decrease in peel strength.

[0005] Patent document CN115458703A discloses a pole piece coating method, drying device and its use. After the current collector is coated with slurry, it is heated and dried and cooled and dried in turn. However, the negative pole piece usually uses water as a solvent, the boiling point of water is 100°C, and the molecular weight of hydroxymethyl cellulose (CMC) as a binder skeleton is only 60-70w, while the positive pole piece uses an organic solvent N-methylpyrrolidone (NMP), which has a boiling point of 202°C, and the molecular weight of polyvinylidene fluoride (PVDF) as a binder is 50-120w. The difference in the properties of the positive and negative electrode slurry components results in that when the drying method suitable for the positive pole piece is used for the negative pole piece, the binder skeleton CMC migrates outward during the drying process, resulting in uneven distribution of binder particles inside the negative pole piece, which directly leads to a decrease in peel strength. Summary of the invention

[0006] The object of the present invention is to provide a method for preparing a negative electrode sheet with less binder migration during the drying process and more uniform CMC distribution inside the negative electrode sheet; another object of the present invention is to provide a negative electrode sheet with more uniform CMC distribution inside the sheet.

[0007] The present invention discloses a method for preparing a negative electrode sheet, comprising the following steps:

[0008] S1: preparing negative electrode slurry;

[0009] S2: coating the negative electrode slurry on the surface of the current collector and drying the negative electrode slurry by increasing the temperature step by step.

[0010] The current collector can be made of conventional lithium-ion battery current collector materials, such as copper foil, nickel foil, titanium foil, stainless steel foil, aluminum foil, etc.

[0011] Furthermore, in step S2, the temperature of the drying process is set to be from 70°C to 110°C, and the drying time is 1-2 minutes; the air volume during the drying process is 25-50m 3 / min.

[0012] Furthermore, in step S2, at least 8 levels of gradient heating are used in the temperature increasing drying; the step length of each level of heating is 5°C; and the residence time is 0.1-0.2min.

[0013] Furthermore, in the step S2, the wind frequency during the temperature increasing drying process is 20-23.

[0014] Furthermore, in step S1, the viscosity of the negative electrode slurry is 10000-15000 m·pas; in terms of mass percentage, the powder of the negative electrode slurry includes:

[0015]

[0016] The main active material of the negative electrode is selected from graphite, silicon-based materials (Si, SiO x , Si-C composite materials), tin-based materials (Sn, SnO2, Sn-C composite materials), metal oxides (TiO2, Fe2O3, MnO2), metal sulfides (MoS2, WS2), alloy materials (Sn-Co, Si-Ge), hard carbon, and soft carbon. One or more of them.

[0017] The conductive agent is selected from one or more of carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, and conductive polymers.

[0018] The water-based adhesive is selected from one or more of styrene-butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyacrylamide, and polyacrylic acid derivatives.

[0019] Furthermore, in step S1, the method for preparing the negative electrode slurry comprises the following steps:

[0020] Mix part of the water-based glue, the negative electrode active main material and the conductive agent evenly, add CMC, n-butanol, ethylene carbonate (EC) and water to knead, and scrape the material after kneading;

[0021] Add the remaining aqueous glue and water to adjust the viscosity and disperse the mixture to obtain the negative electrode slurry.

[0022] Furthermore, the amount of ethylene carbonate added is 1-3% of the mass of the powder; the mass of n-butanol added is 0.3-0.7%.

[0023] EC is added to make the surface of the negative electrode plate smoother and prevent edge chipping; the purpose of adding n-butanol is to reduce the surface gradient of the negative electrode slurry and prevent shrinkage holes from appearing during the coating process of the negative electrode slurry.

[0024] The present invention also discloses a negative electrode sheet, which is prepared by the preparation method described above.

[0025] Furthermore, the difference in weight loss between the upper layer and the lower layer of the coating of the negative electrode plate at 220-230° C. is ≤0.11%.

[0026] The coating of the negative electrode plate is formed by drying the positive electrode slurry.

[0027] Furthermore, the single-sided surface density of the negative electrode plate is 80-100 g / m 2 ; The bonding force of the negative electrode plate is 1.60-1.89N / 24mm.

[0028] The present invention also discloses an application of a negative electrode plate, and the negative electrode plate as described above is applied to the production of lithium-ion batteries.

[0029] The present invention discloses a method for preparing a negative electrode sheet, which sets an increasing drying temperature curve to reduce the constant drying evaporation rate, minimize the migration of small particles such as CMC and binder inside the negative electrode sheet, and make the upper and lower layers of the binder of the negative electrode sheet more evenly distributed and have greater bonding strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a temperature setting diagram corresponding to the oven sections in Example 1 and Comparative Examples 1-3 of the present invention;

[0031] Figure 2 It is a graph showing the change in moisture content in the negative electrode sheet corresponding to the oven sections in Example 1 and Comparative Examples 1-3 of the present invention;

[0032] Figure 3 It is a drying speed diagram of the negative electrode sheet corresponding to the oven sections in Example 1 and Comparative Examples 1-3 of the present invention;

[0033] Figure 4 1 is a drying process diagram of the negative electrode sheet corresponding to the oven sections in Example 1 and Comparative Examples 1-3 of the present invention, wherein a is a transition section; b is a constant speed drying section; and c is a decreasing speed drying section;

[0034] Figure 5 is an average bonding force diagram of the negative electrode sheets prepared in Example 1 and Comparative Examples 1-3 of the present invention;

[0035] Figure 6 This is a graph showing the thermal weight loss test results of the upper and lower layers of the negative electrode sheet coating prepared in Comparative Example 1 of the present invention;

[0036] Figure 7 This is a graph showing the thermal weight loss test results of the upper and lower layers of the negative electrode sheet coating prepared in Comparative Example 2 of the present invention;

[0037] Figure 8 This is a graph showing the thermal weight loss test results of the upper and lower layers of the negative electrode sheet coating prepared in Example 1 of the present invention;

[0038] Fig. 9 It is a graph showing the thermal gravimetric test results of the upper and lower layers of the negative electrode sheet coating prepared in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0039] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] The negative electrode slurries of the embodiments and comparative examples were prepared by the following preparation method:

[0041] The powder ratio of the negative electrode slurry is as follows: 255kg of negative electrode active main material graphite (RF-6), 1.34kg of conductive agent conductive carbon black (Li-20), 9.345kg of water-based glue (PBA5), and 1.34kg of hydroxymethyl cellulose (CMC). The aqueous glue (PBA5) was added to deionized water to prepare 71.89 kg of liquid colloid with a solid content of 13%. The total amount of 60% (43.134 kg) of the aqueous glue was pre-kneaded with 255 kg of graphite and 1.34 kg of carbon black. After pre-kneading, 1.34 kg of CMC (molecular weight 60-70w), 5.34 kg of ethylene carbonate (EC), and 1.34 kg of n-butanol were added as auxiliary reagents for kneading and deionized water was added. After kneading until the solid content reached 65%, the material was scraped, and the remaining aqueous glue (28.756 kg) and deionized water were added for viscosity adjustment and dispersion (the linear speed was controlled at: 15.7-25 m / s, and the dispersion time was 120 min). The viscosity was adjusted to 10000-15000 m·pas, and finally a finished slurry with a solid content of 51.5±0.5% was obtained. In the process of preparing the negative electrode slurry, a total of 182.24 kg of deionized water was added (excluding the mass of deionized water used to prepare the aqueous glue).

[0042] The negative electrode slurry is coated on the surface of the current collector. In this embodiment, the current collector is copper foil, and the negative electrode single-side coating amount is 90g (solid content) / m 2 , double-sided: 180g / m 2 Four different drying curves are set: "high temperature-low temperature-high temperature", "low temperature-high temperature-low temperature", "temperature increasing step by step", and "temperature decreasing step by step". Under the condition that the coating speed and surface density remain unchanged, the air volume per minute is 25-50m 3 After 50m of stable production, take the pole pieces produced under each temperature structure condition for adhesion and thermogravimetric test. The oven used has 13 sections.

[0043] Example 1

[0044] That is, the drying curve of "temperature increasing step by step". The specific setting parameters of the oven are shown in Table 1.

[0045] Table 1 Oven setting parameters of Example 1

[0046]

[0047]

[0048] Through GB / T 39711-2020: Thermogravimetric analysis method for negative electrode materials for lithium-ion batteries, it was measured that the weight loss rate of the negative electrode sheet prepared in Example 1 was 0.55%.

[0049] Comparative Example 1

[0050] That is, the drying curve of "low temperature-high temperature-low temperature". The specific setting parameters of the oven are shown in Table 2.

[0051] Table 2 Oven setting parameters of comparative example 1

[0052]

[0053]

[0054] Through GB / T 39711-2020: Thermogravimetric analysis method for negative electrode materials for lithium-ion batteries, the weight loss rate of the negative electrode sheet prepared in Comparative Example 1 was measured to be 0.53%.

[0055] Comparative Example 2

[0056] That is, the drying curve of "temperature decreasing step by step". The specific setting parameters of the oven are shown in Table 3.

[0057] Table 3 Oven setting parameters of comparative example 2

[0058]

[0059]

[0060] Through GB / T 39711-2020: Thermogravimetric analysis method for negative electrode materials for lithium-ion batteries, the weight loss rate of the negative electrode sheet prepared in Comparative Example 2 was measured to be 0.46%.

[0061] Comparative Example 3

[0062] That is, the drying curve of “high temperature-low temperature-high temperature”. The specific setting parameters of the oven are shown in Table 4.

[0063] Table 4 Oven setting parameters of comparative example 3

[0064]

[0065]

[0066] Through GB / T 39711-2020: Thermogravimetric analysis method for negative electrode materials for lithium-ion batteries, the weight loss rate of the negative electrode sheet prepared in Comparative Example 3 was measured to be 0.48%.

[0067] Performance testing and results analysis

[0068] The number of oven sections and the corresponding set temperatures of Example 1 and Comparative Examples 1-3 are plotted as Figure 1 , the number of oven stages of Example 1 and Comparative Examples 1-3 and the water content of the corresponding negative electrode sheets are plotted as Figure 2 .

[0069] like Figure 1-2 As shown, under the "gradually decreasing" drying curve of comparative example 2, the prepared negative electrode plate is basically dried in the 7th section of the oven. This is because the temperature in the early stage is too high, the drying rate is fast, and the pole plate skeleton is formed prematurely; the "high-low-high" drying curve of comparative example 3 has the same similar situation; the "low-high-low" drying curve of comparative example 1 and the "gradually increasing" drying curve of embodiment 1 are both basically dried before the 11th section of the oven, and the oven utilization rate is relatively high. The common point is that the early drying temperature is low.

[0070] The corresponding relationship between the number of oven sections, drying speed and temperature of Example 1 and Comparative Examples 1-3 is plotted as Figure 3 According to the characteristics of the drying speed of Example 1 and Comparative Examples 1-3, the drying process is divided into a transition section a-constant drying section b-reducing drying section c, such as Figure 4 shown.

[0071] like Figure 3-4 As shown, the transition section a of the drying curve of "gradually decreasing" in comparative example 2 has the highest drying rate, and the constant drying rate b is also the shortest, and the negative electrode sheet is dried the fastest, which also causes the internal binder to float and seriously affects the adhesion. The constant temperature drying section b of the drying curve of "gradually increasing" in example 1 is the longest, so the internal binder migration is not serious.

[0072] According to the test standard of GB / T2792-2014, the adhesion of the negative electrode sheets of Example 1 and Comparative Examples 1-3 was tested. The test results are shown in Table 5.

[0073] Table 5 Adhesion test results of Example 1 and Comparative Examples 1-3

[0074]

[0075] The average adhesion of the negative electrode sheets of Example 1 and Comparative Examples 1-3 is plotted as Figure 5 As shown in Table and Figure 5 As shown, the electrode sheet produced in Example 1 has the highest adhesion, with an average value of 1.797 N. The negative electrode sheet produced in Comparative Example 2 has the lowest adhesion, which is only 1.070 N.

[0076] Use a ceramic knife to scrape off the surface coating of the negative electrode plate. The thickness of the surface of the negative electrode plate is reduced by about 10-20μm. The obtained surface material is recorded as the upper layer. Place the portion of the negative electrode plate with the coating on a glass plate with double-sided tape and press lightly. Tear off the copper foil current collector and use a ceramic knife to scrape the material close to the bottom layer of the copper foil coating. When the thickness of the bottom layer of the coating is reduced by about 10-20μm, the obtained material is recorded as the lower layer.

[0077] The negative electrode sheets prepared in Example 1 and Comparative Examples 1-3, and the upper layer material and the lower layer material obtained by the above method were tested for thermal weight loss using a TGA4000 thermogravimetric analyzer. The test gas was nitrogen, the temperature was 30-600°C, and the heating rate was 5°C / min. The thermal weight loss results were as follows: Figure 6-9 shown.

[0078] CMC will increase the viscosity of the negative electrode slurry. When the viscosity is relatively high, its suspension force will be relatively large, and it will have a greater lifting force for the solids in the solution. This characteristic not only hinders the sedimentation of solids, but also plays a role in dispersing powders in the solution. As a dispersant and binder, when the difference in the CMC content between the upper and lower layers is too large, it will not only affect the dispersion of the powder, but also the dispersion of the powder in the part with less CMC content will be poor, the overall adhesion will be reduced, and the electrode impedance will increase accordingly. The dispersion of the powder in the part with more CMC content will be better, the adhesion will increase, and the electrode impedance will decrease accordingly.

[0079] Since the thermal decomposition temperature of CMC is 220-230°C, the mass loss of the upper and lower materials of the negative electrode sheet, i.e., the CMC content, is calculated within this temperature range. The difference in CMC content between the upper and lower materials of the positive electrode sheet prepared in Example 1 and Comparative Examples 1-3 is then compared. The results are shown in Table 6.

[0080] Table 6 Difference in CMC content between the upper and lower layers of the negative electrode sheets of Example 1 and Comparative Examples 1-3

[0081]

[0082] like Figure 6-9 As shown in Table 6, under the temperature setting of the "step-by-step increasing" drying curve of Example 1, the internal binder of the negative electrode plate migrates slowly and is evenly distributed inside the plate. The powder binder content close to the current collector side does not decrease, and its adhesion is the largest. Under the temperature design of Comparative Examples 2-3, the constant drying rate is short and the plate drying rate is fast, which affects the adhesion of the plate. By comparing the adhesion data, the adhesion of the negative electrode plate of Example 1 is the largest, which shows that the constant drying evaporation rate of Example 1 is the smallest, and the migration of small particles such as CMC and binder inside the plate is the least.

[0083] Adhesion tests and thermogravimetric analyses were performed on the negative electrode sheets prepared in Example 1 and Comparative Examples 1-3. Through the same negative electrode aqueous system slurry, the setting of the oven incremental drying curve in Example 1 can increase the adhesion of the negative electrode sheet by 35.83% compared with the adhesion of the sheet prepared in Comparative Example 1. The difference in CMC content between the upper and lower layers of the positive electrode sheet coating is only 0.11%, indicating that the distribution of small-sized materials such as CMC, binder, conductive agent, etc. inside the sheet is more uniform, and the sheet performance is better.

[0084] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing a negative electrode sheet, characterized in that: The following steps are involved: S1: preparing negative electrode slurry; S2: coating the negative electrode slurry on the surface of the current collector and drying the negative electrode slurry by increasing the temperature step by step.

2. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In step S2, the temperature of the drying is set to be from 70°C to 110°C, and the drying time is 1-2 minutes; the air volume during the drying process is 25-50m 3 / min.

3. The method for preparing a negative electrode sheet according to claim 2, characterized in that: In the step S2, at least 8 levels of gradient heating are used in the temperature increasing drying; the step length of each level of heating is 5°C; and the residence time is 0.1-0.2min.

4. The method for preparing a negative electrode sheet according to claim 3, characterized in that: In step S2, the wind frequency during the temperature increasing drying process is 20-23.

5. The method for preparing a negative electrode sheet according to claim 1, characterized in that: In the step S1, the viscosity of the negative electrode slurry is 10000-15000 m·pas; in terms of mass percentage, the powder of the negative electrode slurry includes:

6. The method for preparing a negative electrode sheet according to claim 5, characterized in that: In step S1, the method for preparing the negative electrode slurry comprises the following steps: Mix part of the water-based glue, the negative electrode active main material and the conductive agent evenly, add CMC, n-butanol, ethylene carbonate and water to knead, and scrape the material after kneading; Add the remaining aqueous glue and water to adjust the viscosity and disperse the mixture to obtain the negative electrode slurry.

7. A negative electrode plate, characterized in that: The preparation is obtained by the preparation method according to any one of claims 1 to 6.

8. The negative electrode sheet according to claim 1, characterized in that: The difference in weight loss between the upper layer and the lower layer of the coating of the negative electrode plate at 220-230° C. is ≤0.11%.

9. The negative electrode sheet according to claim 1, characterized in that: The single surface density of the negative electrode sheet is 80-100 g / m 2 ; The bonding force of the negative electrode plate is 1.60-1.89N / 24mm.

10. An application of a negative electrode sheet, characterized in that: The negative electrode sheet as described in any one of claims 8 to 9 is used in the production of lithium-ion batteries.

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