Preparation method of negative electrode slurry, negative electrode plate and battery

Abstract

The invention provides a preparation method of negative electrode slurry, a negative electrode plate and a battery, and the preparation method of the negative electrode slurry prepared by the invention comprises the following steps: S1, preparing a first glue solution by using a first binder and water; carrying out dry powder mixing on a negative electrode main material and a first conductive agent, adding the first glue solution, and continuously mixing to obtain a first mixed system; s2, adding part of a second binder into the first mixed system, and mixing to obtain a second mixed system; s3, continuing to add the remaining second binder into the second mixed system to obtain a third mixed system; s4, adding a second conductive agent into the third mixed system, and mixing to obtain a fourth mixed system; and S5, adding a third binder into the fourth mixed system, mixing, and adjusting the solid content and viscosity to obtain the negative electrode slurry. The negative electrode slurry has the advantages of high uniform dispersion stability, no obvious settlement under long-time storage, small viscosity change, high stripping force of the prepared pole piece, and good cycle performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a method for preparing a negative electrode slurry, a negative electrode sheet and a battery. Background Art

[0002] At present, in order to pursue high energy density of batteries, silicon negative electrode materials are often used in the industry. However, silicon negative electrode materials have poor conductivity and huge volume expansion after charging and discharging, which makes the conductive network between materials easy to be destroyed, resulting in rapid failure in the cycle. Therefore, in addition to the conventional carbon black conductive agent, single-walled carbon nanotubes need to be added to ensure the conductive performance of the negative electrode. However, the simultaneous addition of multiple composite conductive agents greatly increases the difficulty of homogenizing the negative electrode slurry. The conductive agent is easy to agglomerate and difficult to disperse, especially single-walled carbon nanotubes, which have a high aspect ratio and high specific surface area. The homogenization process is very easy to agglomerate and difficult to disperse. Therefore, a suitable silicon negative electrode homogenization process is very important. At present, the homogenization process of silicon negative electrode systems of most manufacturers in the industry generally adds single-walled carbon nanotube conductive agents during the kneading stage. This process is very likely to cause the agglomeration of single-walled carbon tubes, increase the difficulty of homogenization, and cause large slurry particles and poor stability.

[0003] In view of this, it is of great significance to provide a new method for preparing negative electrode slurry to reduce the agglomeration of single-walled carbon nanotubes in the negative electrode slurry, improve the uniformity and dispersion stability of the slurry, and thus achieve the effect of improving the performance of the electrode sheet and optimizing the battery performance. Summary of the invention

[0004] In order to solve the problems and deficiencies in the prior art, the present invention provides a method for preparing a negative electrode slurry, a negative electrode sheet and a battery. The negative electrode slurry prepared by the present invention has high uniform dispersion stability, no obvious sedimentation after long-term storage, and small viscosity change. The slurry has low fineness and lower resistivity, so the electrode sheet prepared by the present invention has high stability, high peel strength and lower resistance, thereby further optimizing the cycle performance of the battery.

[0005] According to a first aspect of the present invention, there is provided a method for preparing a negative electrode slurry, comprising the following steps: S1. preparing a first glue solution by using a first binder and water; mixing a negative electrode main material and a first conductive agent by dry powder, adding the first glue solution thereto and continuing to mix to obtain a first mixed system; the first binder comprises at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose and lithium carboxymethyl cellulose; the negative electrode main material comprises a silicon-based material; the first conductive agent comprises at least one of conductive carbon black and conductive graphite; S2. adding part of the second binder to the first mixed system and mixing to obtain a second mixed system; the second binder comprises a polyacrylic acid glue solution; S3. continuing to add the remaining second binder to the second mixed system to obtain a third mixed system; S4. adding the second conductive agent to the third mixed system and mixing to obtain a fourth mixed system; the second conductive agent comprises single-walled carbon nanotubes; S5. adding the third binder to the fourth mixed system and mixing, adjusting the solid content and viscosity to obtain a negative electrode slurry; the third binder comprises at least one of styrene-butadiene rubber emulsion and styrene-propylene rubber emulsion.

[0006] In the prior art, when multiple conductive agents of various types are homogenized, the single-walled carbon tube conductive agent is unevenly dispersed, easily agglomerated, and the conductive agent coating effect is poor. In addition, the prior art mostly uses a single binder, which has poor bonding and coating effects on the main material, resulting in the slurry dispersion system being prone to uneven dispersion, instability, and easy agglomeration.

[0007] In the preparation method of the negative electrode slurry provided by the present invention, the negative electrode main material and the first conductive agent are pre-dry mixed by dry powder mixing. On the one hand, the conductive agent agglomerates can be fully broken and dispersed. On the other hand, dry powder stirring can achieve microscopic mixing, and deposit a layer of conductive agent deposition composed of fine dispersed particles on the surface of larger active material particles, thereby forming a good conductive network. The first conductive agent added in this step is a carbon black or graphite conductive agent. This type of conductive agent has good dispersibility whether it is dry powder mixing or wet powder mixing. However, conductive agents with high aspect ratio and high specific surface area such as single-walled carbon nanotubes have poor dispersibility when directly mixed with the negative electrode main material by dry or wet methods, which can easily cause agglomeration of single-walled carbon nanotubes, increase the difficulty of homogenization, and cause large slurry particles and poor stability.

[0008] Then, the first glue solution is added thereto for pre-kneading with the negative electrode main material. The first glue solution is a carboxymethyl cellulose thickener, which has hydrophilic and lipophilic groups, while the negative electrode main material is lipophilic but not hydrophilic, and cannot be well wetted by the solvent. Therefore, the pre-kneading and adding of this type of thickener is mainly to coat the surface of the negative electrode main material, make the negative electrode main material hydrophilic, improve the wettability of the negative electrode main material with the solvent, and then make the main material well wetted and dispersed, and suspend the main material at the same time. Then, the second binder is added to S2 and S3 in two parts. The second binder is a polyacrylic acid glue solution. Generally speaking, this type of glue solution is a water-soluble glue solution, and the effects of adding it in two steps are different. The front S2 is mainly to continue kneading the negative electrode main material, and its main function is to coat the surface of the negative electrode main material and the conductive agent, wet the negative electrode main material and the conductive agent, so that the negative electrode main material and the conductive agent form a good bond and coating. Later, S3 continues to add the remaining second binder. Since the first glue and the second binder both contain a large amount of solvents such as water, the solid content and viscosity of the entire system have been significantly reduced during the process of adding the remaining second binder in S3, and dispersion is relatively easy. Therefore, adding the remaining second binder in this process can suspend the entire system and ensure the stability of the system viscosity.

[0009] Therefore, after adding the remaining second binder, adding a second conductive agent with a high aspect ratio and high surface area such as single-walled carbon nanotubes in S4 can ensure that it can be effectively bonded by the second binder, thereby ensuring the firm bonding of the single-walled carbon nanotubes to the negative electrode main material.

[0010] Finally, the third adhesive is added. The third adhesive is a suspended emulsion, which is easy to demulsify under high shear force. In order to improve the mixing effect during the early stage of the slurry mixing, the stirring rate or the dispersion line speed is generally high. Therefore, adding it when the slurry is better dispersed in the later stage is beneficial to ensure the stability of the suspended emulsion and give full play to the role of the third adhesive.

[0011] In summary, it is precisely because the present invention designs a matching feeding sequence according to the properties of different raw materials that the binder and the conductive agent can be well coated on the surface of the negative electrode main material to form a good conductive network, so that the negative electrode sheet and the battery prepared by it show better performance. At the same time, such a feeding sequence makes the negative electrode main material, each conductive agent, and each binder more dispersed and stable in the system, and the suction and repulsion of each material in the slurry reach a better balance, effectively improving the dispersion stability of the slurry, so that it will not have obvious sedimentation under long-term storage, the solid content changes low, and the stability is high. At the same time, the feeding sequence of the first conductive agent and the second conductive agent in the present invention cannot be disrupted, and the disruption will affect the slurry dispersion effect and the conductive agent coating effect, resulting in large slurry particles, unstable viscosity, sedimentation, and conductive agent agglomeration problems; and the feeding sequence of the first binder, the second binder, and the third binder cannot be disrupted, and the disruption will affect the slurry dispersion effect, resulting in large slurry particles, unstable viscosity, and sedimentation problems. The change in the order of the third binder will cause it to have demulsification problems.

[0012] In addition, the conductive agent used in the present invention is a combination of carbon black or graphite and single-walled carbon nanotubes, which can take into account the difficulty of dispersion during the dispersion process as well as the stability of the slurry, conductivity and other properties. This is because conventional carbon black has good dispersion and low dispersion difficulty over graphite conductive agents, while the single-walled carbon nanotubes have poor dispersion after exceeding a certain proportion, which has a greater impact on the material properties. Therefore, the combination of the two is more conducive to the slurry to obtain better comprehensive performance. And the binder of the present invention also uses three types of binders to match, and these three binders have their own advantages and disadvantages. For example, the viscosity of the carboxymethyl binder is general, the brittleness is large, and the flexibility is poor. The pole piece is easy to crack during charging and discharging, and is greatly affected by the electrode ratio, pH value and other conditions; the carboxyl group of the polyacrylic acid binder is highly hydrophilic, and it is easy to react with the residual water in the battery, affecting the performance; the styrene-butadiene rubber styrene-acrylic rubber suspension emulsion is unstable and easy to demulsify, affecting the performance, etc. The present invention uses these three binders in combination, which can effectively reduce the disadvantageous performance of these binders, optimize the bonding, dispersion and stability of the binders, and has a more obvious effect on optimizing the performance of the negative electrode slurry.

[0013] It should also be noted here that if multi-walled carbon nanotubes are used in silicon-based negative electrode slurry, their adaptability is relatively low compared to single-walled carbon nanotubes, that is, they cannot improve the conductivity and other properties of silicon-based negative electrode materials very well, and affect the performance of silicon-based negative electrode sheets or batteries.

[0014] Preferably, in S2, part of the second binder accounts for 40-70% by mass in the second binder; the remaining second binder accounts for 30-60% by mass in the second binder. Controlling the ratio of the second binder in the front and rear parts within the above range can make the slurry system more stable and easy to disperse during the entire mixing process, ensuring that the second binder and the first conductive agent and the first binder added previously can be better coated on the surface of the negative electrode main material, forming a more stable dispersion system and a tighter conductive network.

[0015] Preferably, in the negative electrode slurry, the input amount of each raw material is calculated based on the mass ratio of the negative electrode main material, the first conductive agent, the second conductive agent, the first binder, and the second binder of 93-98: 0.5-1.5: 0.02-0.1, 0.1-1: 1-3: 0.5-2. The input of each raw material in the above ratio can ensure that these materials are more stable in the slurry system, that is, the interaction force between the materials is well balanced, and at the same time, it can ensure that the obtained negative electrode slurry exhibits higher conductivity and dispersion stability, and optimize the performance of the pole piece and battery prepared therefrom.

[0016] Preferably, in S1, the solid content of the first glue solution is 1.4-1.8%; and in the process of adding the first glue solution, the mixing stirring rate is 10-35rpm, the mixing time is 30-60min, and the solid content of the first mixed system is controlled to be 75-82%. Controlling the first glue solution within the above solid content range can make the first binder best dissolved and dispersed evenly, and at the same time, the viscosity of the glue solution is appropriate, not too high or too low, so as to affect the pulping effect. And when the solid content of the first mixed system is added to 75-82%, it can ensure that the first binder and the negative electrode main material can be well pre-kneaded, so that the first binder can be effectively bonded and coated on the surface of the negative electrode main material, thereby improving the hydrophilicity of the negative electrode main material.

[0017] Preferably, in S1, the solvent used to prepare the first glue solution includes water.

[0018] Preferably, in S1, during the preparation of the first adhesive solution, the mixing and stirring rate is 10 to 35 rpm, the dispersion line speed is 10 to 20 m / s, and the mixing time is 3 to 6 hours. Within the above-mentioned mixing and stirring speed, dispersion line speed and mixing time range, it can be ensured that the preparation of the adhesive solution has a high time efficiency, and at the same time, it can be ensured that the first binder can be dissolved and dispersed uniformly as best. And the dispersion line speed is within the above-mentioned range, so that the molecular chain structure of the first binder will not be destroyed due to the excessively high line speed, and at the same time, the dispersion effect and time will not be affected due to the excessively low line speed.

[0019] Preferably, in S1, during the dry powder mixing of the negative electrode main material and the first conductive agent, the mixing stirring rate is 10 to 35 rpm and the mixing time is 15 to 60 min. Under a certain mixing stirring rate and mixing time, the three powder materials can be ensured to be mixed evenly on a macro scale. And the dispersion line speed is not set during this mixing process, because the dispersion paddle cannot be turned on due to excessive equipment load. Preferably, in S1, the negative electrode main material includes the first negative electrode main material and the second negative electrode main material; the first negative electrode main material includes at least one of artificial graphite, natural graphite, and mesophase carbon microbeads; the second negative electrode main material includes at least one of SiOx, SiC, and Si. The combination of these two negative electrode main materials is more conducive to taking into account the balance of energy density, rapid charging and discharging, and silicon-based volume expansion of silicon-based negative electrodes, so that the comprehensive electrochemical performance of silicon-based negative electrodes is better.

[0020] Preferably, in S2, the solid content of the second binder is 20-40%; and in the process of adding part of the second binder, the mixing stirring rate is 10-35rpm, the mixing time is 60-120min, and the solid content of the second mixed system is controlled to be 67-72%. The solid content of the second binder is 20-40%, which can ensure that the mixed system can be further diluted, and at the same time ensure that it can be evenly dispersed in the mixed system, and better bonded and coated on the surface of the negative electrode main material with other binders and conductive agents to form a more stable and dispersed slurry system. At the same time, the solid content of the second mixed system is controlled to be 67%-72%. This solid content range can ensure the optimal kneading state and ensure that the powder material is fully infiltrated, and the material will not be well infiltrated and poorly dispersed because the kneading state is too dry. At the same time, it will not be too wet to form a good kneading state, and the "kneading dough" effect cannot be achieved, resulting in small friction shear force between materials and poor dispersion.

[0021] Preferably, in S3, during the process of adding the remaining second binder, the mixing stirring rate is 10 to 35 rpm, the dispersion line speed is 10 to 20 m / s, and the mixing time is 60 to 120 min.

[0022] In the process of adding the second binder in two steps in S3 and S4, a certain mixing stirring rate and dispersion linear speed are maintained to ensure that the slurry can be dispersed as evenly as possible without destroying the structure of the second binder material, thereby achieving the best dispersion effect.

[0023] Preferably, in S4, during the process of adding the second conductive agent, the mixing stirring rate is 10 to 35 rpm, the dispersion linear velocity is 10 to 20 m / s, and the mixing time is 60 to 120 min. Under a certain mixing stirring rate and dispersion linear velocity, it is possible to ensure that the single-walled carbon nanotubes and the slurry can be evenly dispersed without interrupting the single-walled carbon nanotubes.

[0024] Preferably, in S5, the solid content of the third binder is 30-50%; and in the process of adding the third binder, the mixing stirring rate is 10-35rpm, the dispersion line speed is not higher than 7m / s, and the mixing time is 30-60min. Under a certain mixing stirring rate and dispersion line speed, the third binder can be guaranteed to have a good dispersion effect. In particular, the dispersion line speed is controlled to be no higher than 7m / s, so that the third binder will not be demulsified due to high shear, and the stability of the third binder and the overall slurry can be guaranteed.

[0025] Preferably, in S5, the solid content is adjusted to 45-55% and the viscosity is adjusted to 2500-6000 mPa·s.

[0026] According to a second aspect of the present invention, a negative electrode sheet is provided, comprising a negative electrode active material layer, wherein the negative electrode active material layer is prepared from a negative electrode slurry, and the negative electrode slurry is prepared by the above-mentioned negative electrode slurry preparation method. The negative electrode slurry prepared by the method provided by the present invention has high dispersion stability, high slurry fineness, and high conductivity, so the negative electrode sheet prepared therefrom has good conductivity, high peel strength, and can still maintain good structural stability after multiple charge and discharge cycles.

[0027] According to the third aspect of the present invention, a battery is provided, comprising the above-mentioned negative electrode sheet. Since the negative electrode sheet prepared by the present invention has good conductivity, high peel strength and excellent structural stability, the battery prepared therefrom has better cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 These are SEM images of the negative electrode sheets in Example 1 and Comparative Example 1 of the present invention.

[0029] Figure 2 It is a comparison chart of the cycle performance of the batteries in Example 1 and Comparative Example 1 at 45°C in the present invention. DETAILED DESCRIPTION

[0030] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present invention, rather than all embodiments.

[0031] Example 1

[0032] 1. Preparation of negative electrode slurry

[0033] The negative electrode slurry of this embodiment was prepared according to the following steps:

[0034] S1. A first adhesive having a solid content of 1.6% is prepared by using a first binder, carboxymethyl cellulose CMC, and water. During the preparation process, the stirring rate is 20 rpm, the dispersion line speed is 15 m / s, and the mixing time is 5 h; after dry powder mixing of the first negative electrode main material graphite, the second negative electrode main material SiC, and the first conductive agent conductive carbon black SP, during the dry powder mixing process, the mixing stirring rate is 15 rpm, and the mixing time is 30 min; then the first adhesive is added thereto and the mixing is continued to obtain a first mixed system, and the stirring rate during the mixing process is 20 rpm, the mixing time is 30 min, and the solid content of the first mixed system is controlled to be 75-82%;

[0035] S2. Adding part of the second binder polyacrylic acid glue PAA (solid content of 30%) to the first mixed system and mixing to obtain a second mixed system, and the stirring rate during the mixing process is 20 rpm, the mixing time is 90 min, and the solid content of the second mixed system is controlled to be 67 to 72%;

[0036] S3. Continue to add the remaining second binder polyacrylic acid glue PAA (solid content of 30%) to the second mixed system, and the stirring rate during mixing is 20 rpm, the dispersion line speed is 15 m / s, the mixing time is 90 min, and the third mixed system is obtained;

[0037] S4. The second conductive agent single-walled carbon nanotube SWCNT is added to the third mixed system and mixed, and the stirring rate during the mixing process is 20 rpm, the dispersion linear velocity is 15 m / s, and the mixing time is 90 min to obtain a fourth mixed system;

[0038] S5. The third binder styrene-butadiene rubber latex SBR (solid content 40%) was added to the fourth mixing system and mixed, and the solid content and viscosity were adjusted to 50.08% and 4510mpa·s, respectively. During the mixing process, the stirring rate was 20rpm, the dispersion line speed was 5.5m / s, the mixing time was 30min, and the vacuum degassing was performed at -85Kpa for 1h to obtain a negative electrode slurry;

[0039] Among them, in the negative electrode slurry prepared above, the feed ratio of each raw material is calculated based on the mass ratio of graphite: SiC: SP: SWCNT: CMC: SBR: PAA of 87.2: 8.96: 0.9: 0.04: 0.4: 1.7: 0.8, where the mass ratio of each raw material is a dry matter ratio, excluding solvent;

[0040] In S2 and S3, the mass proportion of part of the second binder PAA in the second binder PAA is 60%; the mass proportion of the remaining second binder PAA in the second binder PAA is 40%.

[0041] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0042] (1) Preparation of negative electrode sheet

[0043] The negative electrode slurry prepared above is coated on the negative electrode current collector (copper foil), dried and sliced ​​to obtain a negative electrode sheet. In addition, the SEM image of the negative electrode sheet prepared in this embodiment is as follows: Figure 1 shown.

[0044] (2) Preparation of positive electrode

[0045] The positive electrode material NCM811, the conductive carbon black SP, and the binder PVDF were added to (N-methylpyrrolidone) NMP in a mass ratio of 94:3:3 and mixed evenly to obtain a positive electrode slurry, and a positive electrode sheet was prepared using the obtained positive electrode slurry.

[0046] (3) Preparation of batteries

[0047] The positive electrode sheet, the separator and the negative electrode sheet are assembled, and an electrolyte (1 mol / L LiPF6 of ethylene carbonate EC+diethyl carbonate DEC+dimethyl carbonate DMC (volume ratio 1:1:1)) is injected, and after formation and constant volume, a battery is obtained.

[0048] Example 2

[0049] 1. Preparation of negative electrode slurry

[0050] In the process of preparing the negative electrode slurry in this embodiment, the difference from that in embodiment 1 is that in S2 and S3, the mass proportion of part of the second binder PAA in the second binder PAA is 30%; the mass proportion of the remaining second binder PAA in the second binder PAA is 70%. The solid content and viscosity of the final negative electrode slurry are 48.33% and 4902 mpa·s, respectively, and the rest of the operations are the same as those in embodiment 1.

[0051] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0052] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0053] Example 3

[0054] 1. Preparation of negative electrode slurry

[0055] In the process of preparing the negative electrode slurry in this embodiment, the difference from that in embodiment 1 is that in S2 and S3, the mass proportion of part of the second binder PAA in the second binder PAA is 80%; the mass proportion of the remaining second binder PAA in the second binder PAA is 20%; and the solid content and viscosity of the final negative electrode slurry are 50.51% and 4204 mpa·s, respectively. The rest of the operations are the same as in embodiment 1.

[0056] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0057] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0058] Example 4

[0059] 1. Preparation of negative electrode slurry

[0060] In the process of preparing the negative electrode slurry in this embodiment, the difference from that in Example 1 is that in the negative electrode slurry, the feed ratio of each raw material is calculated based on the mass ratio of graphite: SiC: SP: SWCNT: CMC: SBR: PAA of 87.6: 9.06: 0.4: 0.04: 0.4: 1.7: 0.8, where the mass ratio of each raw material is the dry matter ratio, without solvent; and the solid content and viscosity of the final negative electrode slurry are 50.75% and 4680 mpa·s, respectively. The rest of the operations are the same as in Example 1.

[0061] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0062] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0063] Example 5

[0064] 1. Preparation of negative electrode slurry

[0065] In the process of preparing the negative electrode slurry in this embodiment, the difference from that in Example 1 is that in the negative electrode slurry, the feed ratio of each raw material is calculated based on the mass ratio of graphite: SiC: SP: SWCNT: CMC: SBR: PAA of 87.23: 8.96: 0.9: 0.01: 0.4: 1.7: 0.8, where the mass ratio of each raw material is the dry matter ratio, without solvent; and the solid content and viscosity of the final negative electrode slurry are 53.14% and 4927 mpa·s, respectively. The rest of the operations are the same as in Example 1.

[0066] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0067] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0068] Example 6

[0069] 1. Preparation of negative electrode slurry

[0070] In the process of preparing the negative electrode slurry in this embodiment, the difference from that in Example 1 is that in the negative electrode slurry, the feed ratio of each raw material is calculated based on the mass ratio of graphite: SiC: SP: SWCNT: CMC: SBR: PAA of 87.09: 8.96: 0.9: 0.15: 0.4: 1.7: 0.8, where the mass ratio of each raw material is the dry matter ratio, without solvent; and the solid content and viscosity of the final negative electrode slurry are 44.23% and 4763 mpa·s, respectively. The rest of the operations are the same as in Example 1.

[0071] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0072] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0073] Example 7

[0074] 1. Preparation of negative electrode slurry

[0075] The process of preparing the negative electrode slurry in this embodiment is different from that in embodiment 1 in that, in S1, during the process of adding the first glue solution, the solid content of the first mixed system is controlled to be 85%; and the final solid content and viscosity of the negative electrode slurry are 48.59% and 4805 mpa·s, respectively. The rest of the operations are the same as those in embodiment 1.

[0076] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0077] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0078] Example 8

[0079] 1. Preparation of negative electrode slurry

[0080] The process of preparing the negative electrode slurry in this embodiment is different from that in Example 1 in that, in S1, during the process of adding the first glue solution, the solid content of the first mixed system is controlled to be 63%; and the final solid content and viscosity of the negative electrode slurry are 50.78% and 5071 mpa·s, respectively. The rest of the operations are the same as in Example 1.

[0081] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0082] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0083] Example 9

[0084] 1. Preparation of negative electrode slurry

[0085] The process of preparing the negative electrode slurry in this embodiment is different from that in Example 1 in that, in S5, during the process of adding the third binder, the dispersion linear velocity is 9 m / s; and the final solid content and viscosity of the negative electrode slurry are 48.65% and 5206 mpa·s, respectively. The remaining operations are consistent with Example 1.

[0086] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0087] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0088] Example 10

[0089] 1. Preparation of negative electrode slurry

[0090] In the process of preparing the negative electrode slurry in this embodiment, the difference from that in embodiment 1 is that the first negative electrode main material is natural graphite, the first conductive agent is Ketjen black, and the third binder is styrene acrylic rubber latex; and the solid content and viscosity of the final negative electrode slurry are 49.71% and 4710 mPa·s respectively. The rest of the operations are the same as in embodiment 1.

[0091] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0092] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0093] Embodiment 11

[0094] 1. Preparation of negative electrode slurry

[0095] In the process of preparing the negative electrode slurry in this embodiment, the difference from that in embodiment 1 is that the first negative electrode main material is mesophase carbon microspheres, the first conductive agent is conductive graphite, and the first binder is sodium carboxymethyl cellulose; and the solid content and viscosity of the final negative electrode slurry are 49.15% and 4201 mPa·s, respectively. The rest of the operations are the same as in embodiment 1.

[0096] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0097] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this embodiment is consistent with that in Embodiment 1.

[0098] Comparative Example 1

[0099] 1. Preparation of negative electrode slurry

[0100] The negative electrode slurry of this comparative example was prepared according to the following steps:

[0101] S1. The first adhesive solution having a solid content of 1.6% was prepared using carboxymethyl cellulose CMC and water. The stirring rate during the preparation process was 20 rpm, the dispersion line speed was 15 m / s, and the mixing time was 5 h;

[0102] S2. The second binder polyacrylic acid glue PAA (solid content of 30%) is added (added once) to the first glue to obtain a first mixed system, and the stirring rate during the mixing process is 20 rpm and the mixing time is 90 min;

[0103] S3. The second conductive agent single-walled carbon nanotube SWCNT is added to the first mixed system and mixed, and the stirring rate during the mixing process is 20 rpm, the dispersion linear velocity is 15 m / s, and the mixing time is 90 min to obtain a second mixed system;

[0104] S4. After dry powder mixing of the first negative electrode main material graphite, the second negative electrode main material SiC, and the first conductive agent conductive carbon black SP, during the dry powder mixing process, the mixing stirring rate is 15 rpm, and the mixing time is 30 min; then the second mixed system with a mass fraction of 45% is added thereto for mixing, and the stirring rate during the mixing process is 20 rpm, and the mixing time is 90 min, to obtain a third mixed system;

[0105] S5. The remaining 55% of the second mixed system was added to the third mixed system and continued to mix, and the stirring rate during the mixing process was 20 rpm, the dispersion linear velocity was 15 m / s, and the mixing time was 90 min to obtain a fourth mixed system;

[0106] S6. The third binder styrene-butadiene rubber latex SBR (solid content 40%) is added to the fourth mixing system and mixed, and the solid content and viscosity are adjusted to 49.45% and 4980mpa·s, respectively. During the mixing process, the stirring rate is 20rpm, the dispersion line speed is 5.5m / s, the mixing time is 30min, and the vacuum degassing is performed at -85Kpa for 1h to obtain a negative electrode slurry;

[0107] Among them, in the negative electrode slurry prepared above, the feed ratio of each raw material is calculated based on the mass ratio of graphite: SiC: SP: SWCNT: CMC: SBR: PAA of 87.2: 8.96: 0.9: 0.04: 0.4: 1.7: 0.8, where the mass ratio of each raw material is a dry matter ratio and does not contain a solvent.

[0108] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0109] The preparation of the negative electrode sheet, positive electrode sheet and battery in this comparative example is consistent with that in Example 1. The SEM image of the negative electrode sheet in this comparative example is as follows: Figure 1 shown.

[0110] Comparative Example 2

[0111] 1. Preparation of negative electrode slurry

[0112] The difference between the preparation of negative electrode slurry in this comparative example and Example 1 is that the second conductive agent single-walled carbon nanotubes are replaced with multi-walled carbon nanotubes, and the solid content and viscosity of the final negative electrode slurry are 48.36% and 5032 mpa·s respectively. The rest of the operations are the same as in Example 1.

[0113] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0114] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this comparative example is consistent with that in Example 1.

[0115] Comparative Example 3

[0116] 1. Preparation of negative electrode slurry

[0117] The difference between the preparation process of negative electrode slurry in this comparative example and that in Example 1 is that the order of adding the first binder CMC (first glue solution) is exchanged with the third binder (SBR emulsion), and the solid content and viscosity of the final negative electrode slurry are 42.51% and 5508 mpa·s, respectively. The rest of the operations are the same as in Example 1.

[0118] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0119] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this comparative example is consistent with that in Example 1.

[0120] Comparative Example 4

[0121] 1. Preparation of negative electrode slurry

[0122] The difference between the preparation of negative electrode slurry in this comparative example and Example 1 is that the addition order of the first conductive agent, conductive carbon black SP, is exchanged with the second conductive agent, single-walled carbon nanotube SWCNT, and the final solid content and viscosity of the negative electrode slurry are 46.57% and 4304 mPa·s, respectively. The rest of the operations are the same as in Example 1.

[0123] 2. Preparation of negative electrode sheet, positive electrode sheet and battery

[0124] The preparation of the negative electrode sheet, the positive electrode sheet and the battery in this comparative example is consistent with that in Example 1.

[0125] Test Case

[0126] 1. Experimental Construction Method

[0127] The negative electrode slurries prepared in the above embodiments and comparative examples were tested or calculated for solid content, initial viscosity, slurry fineness, 24h sedimentation change, 24h viscosity, and resistivity; the coating peeling force, rolling peeling force, pole piece resistance, and pole piece rolling 24 rebound change rate of the negative electrode sheets prepared in the above embodiments and comparative examples were tested or calculated, and the coating of the single-walled carbon nanotubes on the pole pieces was observed under an electron microscope; and the capacity retention rate of the batteries prepared in the above embodiments and comparative examples was tested for 600 cycles. The specific test methods are as follows:

[0128] (1) The solid content of the negative electrode slurry is tested by a quick moisture meter; the viscosity is tested using a rotational viscometer; the fineness is tested using a scraper fineness meter; the test method for 24h sedimentation change is as follows: a. Take 200mL of slurry, record the sampling time, seal it with plastic wrap and store it in a mixing workshop for 24h; b. After standing, take three samples of the upper slurry to test the solid content; c. Pour out the slurry in the cup and take three samples of the slurry at the bottom of the cup to test the solid content; d. The solid content of the lower layer minus the solid content of the upper layer is the sedimentation change.

[0129] (2) The coating / rolling peeling force test method of the negative electrode sheet is as follows: Take the electrode sheet after coating / rolling and use a tensile machine to perform a 90° peeling test.

[0130] The electrode resistance test method is as follows: take the electrode after roller pressing and test it using a electrode resistance tester.

[0131] The test method for the 24-hour rebound change rate of the pole piece after rolling is as follows: take the pole piece after rolling and test the pole piece thickness every 2 hours until 24 hours, and the 24-hour rebound = (thickness at 24 hours - thickness at 0 hours) / 24.

[0132] (3) The test method for the capacity retention rate of the battery after 600 cycles is as follows: a. Stand still for 30 minutes; b. Charge at 1C constant current and constant voltage to 4.2V, with a cut-off current of 0.05C; c. Stand still for 30 minutes; d. Discharge at 1C constant current to 2.5V; e. Cycle a to d 600 times.

[0133] 2. Experimental results

[0134] (1) The relevant properties of the negative electrode slurries prepared in the above embodiments and comparative examples are shown in Table 1.

[0135] Table 1 Related performance results of negative electrode slurries prepared in various embodiments and comparative examples

[0136]

[0137]

[0138] (2) The relevant properties of the negative electrode sheets and batteries prepared in the above-mentioned embodiments and comparative examples are shown in Table 2.

[0139] Table 2 Relevant performance results of negative electrode sheets and batteries prepared in various embodiments and comparative examples

[0140]

[0141]

[0142] As can be seen from Tables 1 and 2, the negative electrode slurry prepared by the method of the present invention has high uniform dispersion stability, no obvious sedimentation under long-term storage, small viscosity change, low slurry fineness, and low resistivity. The negative electrode sheet prepared by the method has high stability, high coating and rolling peeling strength, and low resistance, thereby further improving the cycle performance of the battery. For details, please refer to the performance data of Examples 1 to 11 in Tables 1 and 2.

[0143] In Comparative Example 1, the order of adding materials was changed. Specifically, the second binder polyacrylic acid glue PAA was added at one time, and the single-walled carbon nanotubes were first mixed with the PAA+CMC mixed glue, and then mixed with the negative electrode main material and SP after dry mixing. The material was mixed twice, resulting in poor mixing effect of the materials, resulting in a decrease in the electrode peeling force, an increase in resistance, a high electrode rebound rate, and a decrease in battery cycle performance.

[0144] and Figure 1 , 2 The negative electrode SEM images and cycle performance test diagrams of Example 1 and Comparative Example 1 respectively show that the single-walled carbon nanotubes in Example 1 are coated on the main material surface in a spider web manner, and can form a good conductive network; while the single-walled carbon nanotubes in Comparative Example 1 are poorly coated, with only a few scattered on the main material surface, and cannot form a relatively good conductive network like in Example 1. Therefore, by using the preparation method provided by the present invention, the single-walled carbon nanotubes have better dispersion in the slurry, can form a better conductive network with other materials, and are more conducive to improving the dispersion stability of the slurry, thereby optimizing the electrode peeling force and battery cycle performance.

[0145] In Comparative Example 2, single-walled carbon nanotubes are replaced with multi-walled carbon nanotubes. Since multi-walled carbon nanotubes have low adaptability to the negative electrode slurry system of the present invention, they cannot improve the conductivity and other properties of the silicon-based negative electrode material, thereby affecting the performance of the silicon-based negative electrode sheet or battery.

[0146] The order of adding materials was also changed in Comparative Examples 3 and 4. Specifically, in Comparative Example 3, the order of adding the first binder CMC and the third binder SBR was swapped, and in Comparative Example 4, the order of adding the first conductive agent SP and the second conductive agent SWCNT was swapped. The swapping of the order of the binder or the conductive agent will lead to the deterioration of the negative electrode dispersion stability, resulting in a decrease in the peeling force of the negative electrode sheet and a decrease in the battery cycle performance.

[0147] Further comparing Example 1 with Examples 2 and 3, the feeding proportion of some PAA in S2 in Example 2 is lower, while the feeding proportion of some PAA in S2 in Example 3 is higher, both of which result in a decrease in the electrode sheet peeling force and the battery cycle performance. PAA is fed in two parts at a specific ratio, which also has a certain effect on the stability of the negative electrode slurry, thereby affecting the negative electrode sheet peeling force and the battery cycle performance.

[0148] Comparing Example 1 with Example 4, the first conductive agent SP in Example 4 accounts for too little, which will cause poor dispersion of the slurry and affect the performance of the electrode and the battery.

[0149] Comparing Example 1 with Examples 5 and 6, the proportion of the second conductive agent SWCNT in Example 5 is too small, and the proportion of the second conductive agent SWCNT in Example 6 is too large. Too little or too much SWCNT is not conducive to taking into account both the conductivity and stability of the slurry, thereby affecting the peeling force of the negative electrode sheet and the cycle performance of the battery.

[0150] Comparing Example 1 with Examples 7 and 8, in S1 of Example 7, during the process of adding the first glue solution, the solid content of the first mixed system was 85%, which was too high; in S2 of Example 8, during the process of adding part of the second binder, the solid content of the second mixed system was 63%, which was too low. Both of the above situations will cause the dispersion of the binder in the slurry to decrease, thus affecting the stability of the slurry and having a certain impact on the negative electrode sheet and battery performance.

[0151] Comparing Example 1 with Example 9, in S5 of Example 9, during the process of adding part of the third binder, the dispersion linear velocity is 9 m / s. If it is too large, it will cause SBR demulsification and affect its stability, thereby affecting the slurry stability and affecting the performance of the negative electrode sheet and the battery.

[0152] By comparing Example 1 with Examples 10 and 11, it can be found that when certain types of raw materials are replaced, the negative electrode slurry still has good performance, and thus the negative electrode sheet and battery finally prepared also have good performance.

[0153] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solutions of the present invention can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present invention.

Claims

1. A method for preparing a negative electrode slurry, characterized in that: The steps include: S1. A first adhesive is prepared by using a first binder and water; after the negative electrode main material and the first conductive agent are dry-powder mixed, the first adhesive is added thereto and continued to mix to obtain a first mixed system; The first binder includes at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose; the negative electrode main material includes a silicon-based material; The first conductive agent includes at least one of conductive carbon black and conductive graphite; S2. adding part of the second binder to the first mixed system and mixing to obtain a second mixed system; The second adhesive includes polyacrylic acid glue; S3. Continue to add the remaining second binder to the second mixed system to obtain a third mixed system; S4. adding a second conductive agent to the third mixed system and mixing to obtain a fourth mixed system; the second conductive agent comprises single-walled carbon nanotubes; S5. adding the third binder to the fourth mixed system and mixing, adjusting the solid content and viscosity to obtain a negative electrode slurry; The third adhesive includes at least one of styrene-butadiene rubber emulsion and styrene-propylene rubber emulsion.

2. The method for preparing the negative electrode slurry according to claim 1, characterized in that: In S2, the mass proportion of the part of the second binder in the second binder is 40-70%; the mass proportion of the remaining second binder in the second binder is 30-60%.

3. The method for preparing the negative electrode slurry according to claim 1, characterized in that: In the negative electrode slurry, the input amount of each raw material is calculated based on the mass ratio of the negative electrode main material, the first conductive agent, the second conductive agent, the first binder, and the second binder of 93-98: 0.5-1.5: 0.02-0.1, 0.1-1: 1-3: 0.5-2.

4. The method for preparing the negative electrode slurry according to claim 1, characterized in that: In S1, the solid content of the first glue solution is 1.4-1.8%; In the process of adding the first glue solution, the mixing stirring rate is 10-35 rpm, the mixing time is 30-60 min, and the solid content of the first mixed system is controlled to be 75-82%.

5. The method for preparing the negative electrode slurry according to claim 1, characterized in that: In S2, the solid content of the second binder is 20-40%; In the process of adding the second binder, the mixing stirring rate is 10-35 rpm, the mixing time is 60-120 min, and the solid content of the second mixed system is controlled to be 67-72%.

6. The method for preparing the negative electrode slurry according to claim 5, characterized in that: In the step S3, during the process of adding the remaining second binder, the mixing stirring rate is 10 to 35 rpm, the dispersion line speed is 10 to 20 m / s, and the mixing time is 60 to 120 min.

7. The method for preparing the negative electrode slurry according to claim 1, characterized in that: In the step S4, during the process of adding the second conductive agent, the mixing stirring rate is 10 to 35 rpm, the dispersion linear speed is 10 to 20 m / s, and the mixing time is 60 to 120 min.

8. The method for preparing the negative electrode slurry according to claim 1, characterized in that: In S5, the solid content of the third binder is 30-50%; In the process of adding the third binder, the mixing stirring rate is 10 to 35 rpm, the dispersion line speed is not higher than 7 m / s, and the mixing time is 30 to 60 min.

9. The method for preparing the negative electrode slurry according to claim 1, characterized in that: In S5, the solid content is adjusted to 45-55%, and the viscosity is adjusted to 2500-6000 mPa·s.

10. A negative electrode sheet, characterized in that: The invention comprises a negative electrode active material layer, wherein the negative electrode active material layer is prepared from a negative electrode slurry, and the negative electrode slurry is prepared by the method for preparing a negative electrode slurry according to any one of claims 1 to 9.

11. A battery, characterized in that: Comprising the negative electrode sheet as claimed in claim 10.

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