Preparation method of a highly dispersed carbon nanotube conductive liquid

Abstract

The present invention provides a method for preparing a highly dispersed carbon nanotube conductive liquid, comprising the following steps: S1. Carbon nanotubes with a diameter not exceeding 15 nm are put into a dilution containing a dispersant and fully dispersed to obtain a first dispersion liquid, wherein the dispersant is a non-PVP dispersant; S2. Anhydrous piperazine is added to the first dispersion liquid and fully dispersed to obtain a grinding liquid; S3. The grinding liquid is subjected to sand grinding treatment, and the grinding medium for the sand grinding treatment is zirconium balls with a diameter of 0.7 - 1.4 mm; S4. The slurry obtained after S3 is subjected to high-pressure homogenization dispersion treatment, and the high-pressure homogenization dispersion pressure is 650 - 850 bar; S5. The slurry obtained after S4 is subjected to demagnetization and sieving treatment to prepare the carbon nanotube conductive liquid. The carbon nanotube conductive liquid prepared by the preparation method provided by the present invention has good dispersibility and good storage stability, and does not rely on the use of PVP dispersant, so that the above carbon nanotube conductive liquid can meet the application requirements of a high-voltage system.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and specifically relates to a method for preparing a highly dispersed carbon nanotube conductive liquid. Background Art

[0002] As a one-dimensional nanomaterial with a high aspect ratio, high conductivity, and high flexibility, carbon nanotubes (CNTs) have been increasingly widely used in the field of lithium batteries. Adding CNTs to the electrode materials of lithium-ion batteries can form an effective conductive network, greatly improving the capacity and cycling performance of the batteries. However, due to the high specific surface area and high aspect ratio of CNTs, it is extremely difficult to disperse CNTs. To fully utilize the role of carbon nanotubes, they can only be made into slurries, and in industrial applications, it is required that the mass content of carbon nanotubes in the carbon nanotube conductive slurry is above 1%, and it should have a storage life of at least several months.

[0003] In the currently common methods for preparing CNT conductive liquids, PVP (polyvinylpyrrolidone) is generally used as a dispersant for CNTs. However, under the ternary high-voltage system, the electrochemical stability of PVP itself is poor, which limits the application of CNT conductive liquids containing PVP in ternary electrodes. Otherwise, the side reactions that occur during the operation of the battery by PVP will affect the battery performance. However, for small-diameter CNTs, if a non-PVP dispersant is used for dispersion, there are problems of low dispersion efficiency and poor dispersion effect. As a result, the obtained CNT conductive liquid often has too high viscosity, poor storage stability, and is prone to reverse thickening. After reverse thickening, a series of processing problems such as filter clogging and difficult dispersion will occur. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a highly dispersed carbon nanotube conductive liquid, so as to obtain a carbon nanotube conductive liquid with good dispersibility and good storage stability without using PVP as a dispersant, in order to promote the application requirements of carbon nanotube conductive liquids in high-voltage systems.

[0005] According to one aspect of the present invention, there is provided a method for preparing a highly dispersed carbon nanotube conductive liquid, comprising the following steps: S1. Carbon nanotubes with a tube diameter not exceeding 15 nm are put into a diluent containing a dispersant and fully dispersed to obtain a first dispersion liquid, wherein the dispersant is a non-PVP dispersant; S2. Anhydrous piperazine is added to the first dispersion liquid and fully dispersed to obtain a grinding liquid; S3. The grinding liquid is subjected to sanding treatment, and the grinding medium for the sanding treatment is zirconia balls with a diameter of 0.7 - 1.4 mm; S4. The slurry obtained after S3 is subjected to high-pressure homogenization dispersion treatment, and the high-pressure homogenization dispersion pressure is 650 - 850 bar; S5. The slurry obtained after S4 is subjected to demagnetization and sieving treatment to prepare the carbon nanotube conductive liquid. It should be further noted that the above-mentioned "non-PVP dispersant" refers to other dispersants except polyvinylpyrrolidone (PVP). In the above preparation method, using anhydrous piperazine as a viscosity reducer can significantly reduce the viscosity of the conductive agent without affecting the conductive performance of the slurry. In addition, more importantly, due to the application of anhydrous piperazine, during the intermediate process of preparing the carbon nanotube (in this solution, specifically referring to small-diameter carbon nanotubes with a tube diameter not exceeding 15 nm) conductive agent, the processing performance of the slurry involved is significantly improved, manifested as the dispersibility and structural stability of the carbon nanotubes can be significantly improved. On this basis, the present invention further adopts specific sanding and high-pressure homogenization dispersion to conduct combined homogenization treatment on small-diameter carbon nanotubes. On the one hand, it can fully disperse the carbon nanotubes, and on the other hand, it can keep the specific morphology structure with the superiority of small-diameter carbon nanotubes intact. Among them, due to the improvement of the dispersibility of the carbon nanotubes, zirconia balls with a diameter of 0.7 - 1.4 mm are used as the grinding medium in the sanding process, and the morphology structure of the carbon nanotubes is not easily damaged by such a grinding medium. At the same time, the above-mentioned grinding medium can play a good pre-dispersion role on the slurry containing carbon nanotubes through sanding, which is beneficial to the subsequent high-pressure homogenization dispersion treatment. And due to the improvement of the structural stability of the carbon nanotubes and the pre-dispersion treatment by sanding, in the subsequent high-pressure homogenization process, it can be applied to a higher dispersion pressure, so as to improve the dispersion uniformity of the carbon nanotubes and shorten the corresponding dispersion time without damaging the morphology structure of the carbon nanotubes. In summary, the carbon nanotube conductive liquid prepared by the preparation method provided by the present invention has good dispersibility and good storage stability, and does not rely on the use of PVP dispersant, so that the above-mentioned carbon nanotube conductive liquid can meet the application requirements of high-voltage systems.

[0006] Preferably, in S1, the addition amount of the carbon nanotubes is determined according to the mass content of the carbon nanotubes in the carbon nanotube conductive liquid being 3% - 5%.

[0007] Preferably, in S2, the addition amount of anhydrous piperazine is determined according to the mass of carbon nanotubes: the mass of anhydrous piperazine = 3 - 5: 0.005 - 0.1.

[0008] Preferably, in S4, high-pressure homogenization and dispersion treatment is carried out until the carbon nanotubes meet the size standard of 0.7μm < D 90 ≤ 1μm and fineness ≤ 3μm, then the high-pressure homogenization and dispersion treatment is completed. In the carbon nanotube conductive liquid, when the carbon nanotubes meet the above size characteristics, the carbon nanotubes can still exert their excellent electrical properties, and the carbon nanotube conductive liquid has better storage stability.

[0009] Preferably, during the sanding treatment in S3, the sanding speed is 600 - 800 RPM.

[0010] Preferably, the sanding treatment includes a rough grinding process and a fine grinding process. The grinding medium used in the rough grinding process is a rough grinding zirconia ball, and the grinding medium used in the fine grinding process is a fine grinding zirconia ball. The difference in diameter between the rough grinding zirconia ball and the fine grinding zirconia ball is △r, 0mm < △r ≤ 0.5mm. In the sanding treatment involved in the preparation method provided by the present invention, the rough grinding process and the fine grinding process are combined, and the grinding media used in the rough grinding process and the fine grinding process respectively meet a specific diameter difference, so as to improve the dispersion efficiency of carbon nanotubes, optimize the pre-dispersion effect of carbon nanotubes, and be able to reach the discharge standard within a shorter sanding treatment time, thereby further reducing the possibility of damage to the morphology and structure of carbon nanotubes by the sanding process, and further improving the dispersion uniformity and conductivity of the finally prepared carbon nanotube conductive liquid.

[0011] Preferably, during the sanding treatment in S3, the pump speed of the material feeding pump is 2 - 6 s / time.

[0012] Preferably, during the sanding treatment, the pump speed of the material feeding pump in the rough grinding process < the pump speed of the material feeding pump in the fine grinding process.

[0013] Preferably, the dispersant is a high molecular organic polymer formed by polymerization of one or several monomers selected from polyester monomers, polyether monomers, cyan monomers, and alkoxy monomers.

[0014] Preferably, the molecular structure of the dispersant simultaneously has an anchoring group suitable for using a high-crystallinity carbon material as the dispersion main body and a solvation chain for providing stable steric hindrance ability. This type of dispersant has good electrochemical stability and can play an excellent dispersion role in a dispersion system using a high-crystallinity carbon material as the dispersion main body.

[0015] Preferably, the dispersant includes at least one high molecular non-ionic dispersing material such as nitrile rubber, hydrogenated nitrile rubber, and styrene-acrylate polymer. Detailed implementation manners

[0016] To enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0017] Embodiment 1

[0018] 1. Preparation of carbon nanotube conductive liquid

[0019] According to different processes for preparing carbon nanotube conductive liquid, different treatment groups and control groups are set in this embodiment, which are respectively labeled as treatment groups 1A to 5A and control groups 1A to 8A.

[0020] Treatment group 1A

[0021] In this treatment group, the carbon nanotubes (CNT) used are CNTs with an average tube diameter of 10 nm and an average length of 10 μm. The above-mentioned materials are weighed according to the mass ratio of CNT: anhydrous piperazine: hydrogenated nitrile rubber: NMP = 4.3: 0.02: 1.1: 94.6. After the preparation of materials is completed, the carbon nanotube conductive liquid is prepared according to the following method:

[0022] S1. Add a dispersant to NMP, stir well to make the dispersant fully dissolve and form a homogeneous solution with the solvent, and then add CNT powder to it. The resulting solution is the first dispersion liquid.

[0023] S2. Then add anhydrous piperazine to the first dispersion liquid and disperse it fully to obtain a grinding liquid.

[0024] S3. Sanding treatment: Inject the grinding liquid into a 90L self-circulating sanding system, and then perform subsequent sanding treatment

[0025] S3.1 Coarse grinding process: Use zirconium balls with an average diameter of 1.2 mm as the grinding medium to perform coarse grinding treatment on the grinding liquid at a sanding speed of 700 RPM. In the coarse grinding process, the pump speed of the material feeding pump is 3 s / time;

[0026] S3.2 Fine grinding process: Use zirconium balls with an average diameter of 0.7 mm as the grinding medium to perform fine grinding treatment on the grinding liquid at a sanding speed of 800 RPM. In the fine grinding process, the pump speed of the material feeding pump is 5 s / time.

[0027] S4. Perform high-pressure homogenization and dispersion treatment on the slurry obtained after S3 is completed. The high-pressure homogenization and dispersion pressure is 800 bar until the CNT in the slurry reaches D 90 = 1 μm and the fineness = 3 μm, and the high-pressure homogenization and dispersion treatment process is completed.

[0028] S5. Perform demagnetization and sieving on the slurry obtained after S4, and discharge the material to obtain the carbon nanotube conductive liquid of this treatment group.

[0029] Treatment group 2A

[0030] This treatment group prepares the carbon nanotube conductive liquid with reference to treatment group 1A. The difference from treatment group 1A is that in the raw materials for preparing the carbon nanotube conductive liquid, calculated by mass ratio, CNT: anhydrous piperazine = 3:0.2. Except for this, the materials used in this treatment group for preparing the carbon nanotube conductive liquid and other operation treatments are strictly the same as those in treatment group 1A.

[0031] Treatment group 3A

[0032] This treatment group prepares the carbon nanotube conductive liquid with reference to treatment group 1A. The difference from treatment group 1A is that in the raw materials for preparing the carbon nanotube conductive liquid, calculated by mass ratio, CNT: anhydrous piperazine = 5:0.003. Except for this, the materials used in this treatment group for preparing the carbon nanotube conductive liquid and other operation treatments are strictly the same as those in treatment group 1A.

[0033] Treatment group 4A

[0034] This treatment group prepares the carbon nanotube conductive liquid with reference to treatment group 1A. The difference from treatment group 1A is that in the high-pressure homogenization and dispersion treatment (S4) involved in preparing the carbon nanotube conductive liquid, the high-pressure homogenization and dispersion pressure is set to 650 bar. Except for this, the materials used in this treatment group for preparing the carbon nanotube conductive liquid and other operation treatments are strictly the same as those in treatment group 1A.

[0035] Treatment group 5A

[0036] This treatment group prepares the carbon nanotube conductive liquid with reference to treatment group 1A. The difference from treatment group 1A is that in the high-pressure homogenization and dispersion treatment (S4) involved in preparing the carbon nanotube conductive liquid, the high-pressure homogenization and dispersion pressure is set to 850 bar. Except for this, the materials used in this treatment group for preparing the carbon nanotube conductive liquid and other operation treatments are strictly the same as those in treatment group 1A.

[0037] Control group 1A

[0038] This control group prepares the carbon nanotube conductive liquid with reference to treatment group 1A. The difference from treatment group 1A is that in the process of preparing the carbon nanotube conductive liquid, the addition of anhydrous piperazine is omitted. Except for this, the materials used in this treatment group for preparing the carbon nanotube conductive liquid and other operation treatments are strictly the same as those in treatment group 1A.

[0039] Control group 2A

[0040] This control group prepared a carbon nanotube conductive liquid with reference to Treatment Group 1A. The difference from Treatment Group 1A is that during the preparation of the carbon nanotube conductive liquid, an equal mass of potassium hydroxide was used to replace anhydrous piperazine in the preparation of the carbon nanotube conductive liquid. Other than this, the materials used by this treatment group for preparing the carbon nanotube conductive liquid and other operation processes were strictly the same as those of Treatment Group 1A.

[0041] Control Group 3A

[0042] This control group prepared a carbon nanotube conductive liquid with reference to Treatment Group 1A. The difference from Treatment Group 1A is that during the preparation of the carbon nanotube conductive liquid, an equal mass of ethanolamine was used to replace anhydrous piperazine in the preparation of the carbon nanotube conductive liquid. Other than this, the materials used by this treatment group for preparing the carbon nanotube conductive liquid and other operation processes were strictly the same as those of Treatment Group 1A.

[0043] Control Group 4A

[0044] This control group prepared a carbon nanotube conductive liquid with reference to Treatment Group 1A. The difference from Treatment Group 1A is that during the preparation of the carbon nanotube conductive liquid, this control group omitted the sanding treatment (S3) process involved. Other than this, the materials used by this control group for preparing the carbon nanotube conductive liquid and other operation processes were strictly the same as those of Treatment Group 1A.

[0045] Control Group 5A

[0046] This control group prepared a carbon nanotube conductive liquid with reference to Treatment Group 1A. The difference from Treatment Group 1A is that during the preparation of the carbon nanotube conductive liquid, this control group omitted the high-pressure homogenization and dispersion treatment (S4) process involved. Other than this, the materials used by this control group for preparing the carbon nanotube conductive liquid and other operation processes were strictly the same as those of Treatment Group 1A.

[0047] Control Group 6A

[0048] This control group prepared a carbon nanotube conductive liquid with reference to Treatment Group 1A. The difference from Treatment Group 1A is that the CNT used by this control group for preparing the carbon nanotube conductive liquid was CNT with an average tube diameter of 25 nm and an average length of 10 μm, and during the preparation of the carbon nanotube conductive liquid, the addition of anhydrous piperazine was omitted. Other than this, the materials used by this treatment group for preparing the carbon nanotube conductive liquid and other operation processes were strictly the same as those of Treatment Group 1A.

[0049] After completing the preparation of the above carbon nanotube conductive liquid, the slurry state of the prepared carbon nanotube conductive liquid was observed and recorded. And samples were taken from the above carbon nanotube conductive liquid, and after standing the samples for 7 days, the slurry state was observed and recorded.

[0050] 2. Preparation of Lithium-Ion Batteries

[0051] The carbon nanotube conductive liquids prepared by each treatment group and the control group were used to fabricate lithium-ion batteries according to the following method.

[0052] (1) Preparation of the positive electrode

[0053] In this treatment group, NCM523 was used as the positive active material of the positive electrode, polyvinylidene fluoride (PVDF) was used as the binder of the positive electrode, and SP and CNT in the polyvinylidene fluoride (PVDF) and carbon nanotube conductive liquid were used as the conductive agents of the positive electrode. The required materials were weighed according to NCM523:PVDF:SP:CNT = 98:1.2:0.5:0.3, where CNT was provided by the carbon nanotube conductive liquid. The above materials were mixed and stirred in a vacuum mixer until the system became homogeneous to obtain the positive electrode slurry; the positive electrode slurry was evenly coated on the positive current collector by extrusion coating, air-dried at room temperature and then transferred to an oven for further drying, and then cold-pressed and cut into the required specifications to obtain the positive electrode. During the process of coating the positive electrode slurry, the coating characteristics of the positive electrode slurry were observed and recorded.

[0054] 2) Preparation of the negative electrode: The negative active material graphite, conductive agent acetylene black, thickening agent CMC, and binder SBR were mixed according to a mass ratio of 96.2:0.8:1.2:1.8, and then deionized water as the solvent was added to the resulting mixed material, and stirred in a vacuum mixer until the system became homogeneous to obtain the negative electrode slurry; the negative electrode slurry was evenly coated on the negative current collector, air-dried at room temperature and then transferred to an oven for further drying, and then cold-pressed and cut into the required specifications to obtain the negative electrode.

[0055] 3) Assembly of the battery cell components: The above positive electrode, separator, and negative electrode were stacked in sequence to obtain the battery cell components after assembly.

[0056] 4) Injection of electrolyte, formation, and volume determination: The battery cell components were placed into the inner cavity of the battery case, and after drying, the electrolyte was injected into the inner cavity of the battery case; through processes such as vacuum packaging, standing, formation, and volume determination, the lithium-ion batteries of this treatment group were obtained. The electrolyte solvent used in this treatment group was ethylene carbonate, and the solute was lithium hexafluorophosphate (1M).

[0057] The lithium-ion batteries prepared according to the above method were subjected to DCR tests, and the specific test steps are shown in Table 1.

[0058] Table 1. Operation of DCR test steps

[0059]

[0060] 3. Performance characterization

[0061] The slurry states of the carbon nanotube conductive fluids prepared by each treatment group and the control group in this embodiment are shown in Table 2. The DCR growth rate of the lithium-ion battery after a certain period of time is calculated based on the DCR test results of the lithium-ion battery prepared according to this embodiment, and the results are shown in Table 3. In Table 2, the descriptions of the slurry states of the carbon nanotube conductive fluids involved correspond to the dispersion conditions of the carbon nanotube conductive fluids from good to poor as follows: watery, slightly thick watery, good fluidity, good fluidity but slightly thick, viscous but fluid, non-fluid.

[0062] It can be seen from the data shown in Table 2 that compared with the carbon nanotube conductive fluids prepared by the control groups 1A - 5A, the carbon nanotube conductive fluids prepared by the treatment groups 1A - 5A have better fluidity and storage stability. And compared with the positive electrode slurries prepared using the carbon nanotube conductive fluids of the control groups 1A - 8A, the positive electrode slurries prepared using the carbon nanotube conductive fluids of the treatment groups 1A - 5A have more suitable viscosity and better coating construction characteristics. During the preparation of the carbon nanotube conductive fluids in the treatment groups 1A - 5A, for small-diameter CNTs (in this solution, specifically referring to CNTs with a diameter not exceeding 15 nm), anhydrous piperazine was used as a viscosity reducer. Based on the application of anhydrous piperazine, the dispersibility and structural stability of CNTs can be significantly improved. On this basis, specific sanding and high-pressure homogenization dispersion were further used to perform combined homogenization treatment on small-diameter CNTs. On the one hand, it plays a sufficient dispersing role for the carbon nanotubes, and on the other hand, the specific morphological structure that can exert the superiority of small-diameter carbon nanotubes remains intact. Among them, based on the improved dispersibility of CNTs, the CNTs are sanded, and the sanding process will not cause obvious damage to the morphological structure of CNTs, and can pre-disperse the CNTs well in the slurry. And based on the improved structural stability of CNTs and the pre-dispersion treatment by sanding, high-pressure homogenization dispersion treatment is further used to further disperse and homogenize the CNTs. Thus, the prepared carbon nanotube conductive fluid has good dispersibility and good storage stability.

[0063] By comparing Treatment Group 1A, Control Group 1A, and Control Group 6A, if the small-diameter CNT is used as the dispersion matrix, compared with Treatment Group 1A, Control Group 1A did not use anhydrous piperazine in the process of preparing the carbon nanotube conductive liquid, resulting in poor CNT dispersion. This is manifested as poor fluidity of the carbon nanotube conductive liquid, high viscosity of the corresponding positive electrode paste, obvious uneven particles, and poor coating performance. In addition, as can be seen from the data in Table 3, the DCR value measured for the lithium-ion battery prepared using the carbon nanotube conductive liquid of Control Group 1A is relatively high. The reason is that in the process of preparing the carbon nanotube conductive liquid in Control Group 1A, the structure of CNT was damaged by grinding, resulting in lower electrical performance of the prepared carbon nanotube conductive liquid. For Control Group 6A, the diameter of the CNT used is relatively large. Even without using anhydrous piperazine, a well-dispersed and storage-stable carbon nanotube dispersant can still be prepared. However, as can be seen from the data shown in Table 3, the DCR value of the lithium-ion battery prepared using the carbon nanotube conductive liquid provided by Control Group 6A is significantly high. The reason is that the diameter of the CNT contained in the carbon nanotube conductive liquid provided by Control Group 6A is relatively large, resulting in obvious deterioration of the corresponding conductive effect compared with the carbon nanotube conductive liquids prepared using small-diameter CNT, even though the prepared carbon nanotube conductive liquid has a good dispersion effect. The difference between Treatment Groups 1A to 3A lies in the ratio of CNT and anhydrous piperazine in the process of preparing the carbon nanotube conductive liquid. Through comparison, the comprehensive performance of the carbon nanotube conductive liquid prepared by Treatment Group 1 and the lithium-ion battery prepared using it is better. This shows that among Treatment Groups 1A to 3A, the ratio of CNT and anhydrous piperazine used in Treatment Group 1A is the best. In addition, through experimental verification, in the process of preparing the carbon nanotube conductive liquid, when feeding materials, if the mass ratio of CNT to anhydrous piperazine reaches the range of 3 - 5:0.005 - 0.1, the prepared carbon nanotube conductive liquid has better dispersion.

[0064] In addition, with the small-diameter CNT as the dispersion medium, although both Control Group 2A and Control Group 3A used viscosity reducers, as can be seen from the test results shown in Table 2 and Table 3, the dispersion effect of CNT in Control Group 2 and Control Group 3A is still relatively poor, manifested as poor fluidity and storage stability of the prepared carbon nanotube conductive liquid, and the DCR values of the lithium-ion batteries prepared using the carbon nanotube conductive liquids of Control Group 2A and Control Group 3A are also relatively high. This shows that for small-diameter CNT, not all viscosity reducers can effectively improve the dispersion uniformity and storage stability of CNT.

[0065] The test results show that the dispersibility of the carbon nanotube conductive fluids prepared in control group 4A and control group 5A is significantly poor, and the fluidity and storage stability of the slurries are both poor. During the preparation of the carbon nanotube conductive fluid in control group 4A, the sanding treatment was omitted, and during the preparation of the carbon nanotube conductive fluid in control group 5A, the high-pressure homogenization dispersion treatment was omitted. Thus, it can be shown that only when the sanding treatment and the high-pressure homogenization treatment are used in combination can the small-diameter CNTs be fully dispersed.

[0066] Table 2. Slurry state of carbon nanotube conductive fluid

[0067]

[0068]

[0069] Table 3. Statistics of DCR test results of lithium-ion batteries

[0070]

[0071] Example 2

[0072] 1. Preparation of carbon nanotube conductive fluid

[0073] In this example, the carbon nanotube conductive fluid was prepared by referring to the method provided by treatment group 1A in Example 1. Different treatment groups were set according to the discharge standards of the high-pressure homogenization dispersion treatment, and were respectively labeled as treatment group 2B and treatment group 3B. For intuitive comparison, Table 4 shows the discharge standards of the high-pressure homogenization dispersion treatment steps in the preparation of the carbon nanotube conductive fluid by treatment group 1A in Example 1, treatment group 2B, and treatment group 3B in this example. After the preparation of the carbon nanotube conductive fluid in this example was completed, the slurry state of the prepared carbon nanotube conductive fluid was observed and recorded. And samples were taken from the above carbon nanotube conductive fluid, and after standing for 7 days, the slurry state was observed and recorded.

[0074] Table 4. Discharge standards of the high-pressure homogenization dispersion treatment steps corresponding to each treatment group

[0075] Group Treatment Group 1A <![CDATA[The CNT reaches D 90 = 1 μm, fineness = 3 μm]]> Treatment Group 2B <![CDATA[The CNT reaches D 90 = 1 μm, fineness = 5 μm]]> Treatment Group 3B <![CDATA[The CNT reaches D 90 = 0.7 μm, fineness = 3 μm]]>

[0076] 2. Preparation of lithium-ion batteries

[0077] Using the carbon nanotube dispersants prepared by treatment group 2B and treatment group 3B, lithium-ion batteries were prepared according to the lithium-ion battery preparation method provided in Example 1. According to the DCR test method provided in Example 1, the DCR test was carried out on the lithium-ion batteries prepared in this example.

[0078] 3. Performance characterization

[0079] The slurry states of the carbon nanotube conductive liquids prepared by each treatment group and the control group in this embodiment are shown in Table 5. The DCR growth rate of the lithium-ion battery after a certain period of time is calculated based on the DCR test results of the lithium-ion battery prepared according to this embodiment, and the results are shown in Table 6. For intuitive comparison, the relevant data corresponding to Treatment Group 1A of Example 1 are loaded in Table 5 and Table 6. In the same way as the evaluation method of the slurry state in Test Example 1, in Table 5, the descriptions of the slurry states of the carbon nanotube conductive liquids involved correspond to the dispersion conditions of the carbon nanotube conductive liquids from good to poor as follows: watery, slightly thick watery, good fluidity, good fluidity but slightly thick, viscous but fluid, non-fluid. It can be seen from the test results that the carbon nanotube conductive liquids prepared by Treatment Group 1A, Treatment Group 2B, and Treatment Group 3B all have good fluidity and dispersion effects when they are just prepared. However, as the standing time of the prepared carbon nanotube conductive liquid prolongs, the differences among the slurry conditions of these three different carbon nanotube conductive liquids gradually increase. The fineness of the CNT used in Treatment Group 2B is relatively large, resulting in poor storage stability of the carbon nanotube conductive liquid prepared therefrom. After standing for 30 days, the viscosity of the carbon nanotube conductive liquid provided by Treatment Group 2B increases significantly, and the positive electrode slurry prepared by using it is prone to slight particle scratches during coating. The carbon nanotube conductive liquids provided by Treatment Group 1A and Treatment Group 3B respectively have good storage stability. After a long time of standing, the carbon nanotube conductive liquid can still maintain good dispersibility and fluidity. Among them, the D 90 of the CNT used in Treatment Group 1A is relatively large, resulting in a lower DCR value and a lower DCR growth rate of the lithium-ion battery prepared by using it.

[0080] Table 5. Slurry States of Carbon Nanotube Conductive Liquids

[0081]

[0082] Table 6. Statistics of DCR Test Results of Lithium-Ion Batteries

[0083]

[0084] Example 3

[0085] 1. Preparation of Carbon Nanotube Conductive Liquid

[0086] In this example, the carbon nanotube conductive liquid was prepared in the same way as that provided by treatment group 1A of Example 1. Different treatment groups were set according to different sanding processes in the sanding process, and were respectively labeled as treatment group 2C, treatment group 3C, and treatment group 4C. For intuitive comparison, Table 7 shows the specific settings of the sanding process in the preparation of the carbon nanotube conductive liquid for treatment group 1A of Example 1, treatment group 2C, treatment group 3C, and treatment group 4C of this example. After the preparation of the carbon nanotube conductive liquid in this example was completed, the slurry state of the prepared carbon nanotube conductive liquid was observed and recorded. And samples were taken from the above carbon nanotube conductive liquid, and the slurry state was observed and recorded after the samples were left standing for 7 days.

[0087] Table 7. Discharge standards of the high-pressure homogenization and dispersion treatment steps corresponding to each treatment group

[0088]

[0089]

[0090] 2. Preparation of lithium-ion batteries

[0091] Using the carbon nanotube dispersants prepared by treatment group 2C, treatment group 3C, and treatment group 4C, lithium-ion batteries were prepared according to the lithium-ion battery preparation method provided in Example 1. According to the DCR test method provided in Example 1, the DCR test was carried out on the lithium-ion batteries prepared in this example.

[0092] 3. Performance characterization

[0093] The slurry states of the carbon nanotube conductive liquid prepared in each treatment group and the control group of this embodiment are shown in Table 8. The DCR growth rate of the lithium ion battery after a certain period of time is calculated based on the DCR test results of the lithium ion battery prepared in this embodiment, and the results are shown in Table 9. For intuitive comparison, the relevant data corresponding to the treatment group 1A of Example 1 are loaded in Tables 8 and 9. The evaluation method of the slurry state is the same as that in Test Example 1. In Table 8, the description of the carbon nanotube conductive liquid slurry state involved corresponds to the dispersion of the carbon nanotube conductive liquid from good to bad as follows: water-like, water-like and slightly thick, good flow, good flow but slightly thick, viscous but fluid, and no fluidity. From the test results, it can be seen that compared with the carbon nanotube conductive liquids prepared by treatment group 1A and treatment group 4C, the carbon nanotube conductive liquids prepared by treatment group 2C and treatment group 3C have significantly poorer dispersibility and significantly higher viscosity. The coating characteristics of the positive electrode slurries prepared by using the above two carbon nanotube conductive liquids are poor, and the DCR values measured by the corresponding lithium-ion batteries further prepared are high. Treatment group 2C lacks a fine grinding process in the process of preparing the carbon nanotube conductive liquid, while treatment group 3C lacks a coarse grinding process in the process of preparing the carbon nanotube conductive liquid. The test results show that the lack of fine grinding or coarse grinding in the process of preparing the carbon nanotube conductive liquid cannot fully CNT, resulting in a large number of clustered / entangled CNT agglomerates in the slurry, which has a negative impact on the slurry stability and DCR growth. Although the carbon nanotube conductive liquid prepared in treatment group 4C has good dispersibility and fluidity, after 7 days of standing, the carbon nanotube conductive liquid in treatment group 4C is more viscous than the carbon nanotube conductive liquid in treatment group 1A, which shows that the storage stability of the carbon nanotube conductive liquid prepared in treatment group 1A is better. Compared with treatment group 1A, in the fine grinding process of preparing the carbon nanotube conductive liquid in treatment group 4C, the particle size of the grinding medium selected is larger, so that the size of the CNT in the prepared carbon nanotube conductive liquid is larger, thereby reducing the initial DCR of the lithium ion battery prepared by using the carbon nanotube conductive liquid, but also making the dispersibility of CNT worse, the storage stability of the carbon nanotube conductive liquid is reduced, and then the DCR growth rate of the lithium ion battery prepared by using the carbon nanotube is increased.

[0094] Table 8. Slurry state of carbon nanotube conductive liquid

[0095]

[0096] Table 9. DCR test results of lithium-ion batteries

[0097]

[0098] Example 4

[0099] 1. Preparation of carbon nanotube conductive liquid

[0100] In this example, the carbon nanotube conductive liquid was prepared in the same manner as that provided by Treatment Group 1A of Example 1. Different treatment groups were set according to the different dispersants used, and were respectively labeled as Treatment Group 2D, Treatment Group 3D, and Control Group 1D. For intuitive comparison, Table 10 shows the types of dispersants used in Treatment Group 1A of Example 1, Treatment Group 2D, Treatment Group 3D, and Control Group 1D of this example during the preparation of the carbon nanotube conductive liquid. Among them, the NBR type dispersant refers to the nitrile rubber type dispersant, the HBR type dispersant refers to the hydrogenated nitrile rubber type dispersant, and the PVP type dispersant refers to the polyvinylpyrrolidone type dispersant. After the preparation of the carbon nanotube conductive liquid in this example was completed, the slurry state of the prepared carbon nanotube conductive liquid was observed and recorded. And samples were taken from the above carbon nanotube conductive liquid, and the slurry state was observed and recorded after the samples were left standing for 7 days.

[0101] Table 10. Discharge Standards of the High-Pressure Homogenization and Dispersion Treatment Steps Corresponding to Each Treatment Group

[0102] Group Dispersant Treatment Group 1A NBR type Treatment Group 2D HNBR type Treatment Group 3D Polyether type Control Group 1D PVP type

[0103] 2. Preparation of Lithium-Ion Batteries

[0104] Using the carbon nanotube dispersants prepared by Treatment Group 2D, Treatment Group 3D, and Control Group 1D, lithium-ion batteries were prepared according to the lithium-ion battery preparation method provided in Example 1. The high-voltage cycle performance tests were carried out on the lithium-ion batteries prepared in this example and the lithium-ion batteries prepared by Treatment Group 1A of Example 1. The specific test steps are shown in Table 11.

[0105] Table 11. Operation of the High-Voltage Cycle Performance Test Steps

[0106]

[0107] 3. Performance Characterization

[0108] The slurry states of the carbon nanotube conductive liquids prepared by each treatment group and the control group in this embodiment are shown in Table 12. For intuitive comparison, the relevant data corresponding to Treatment Group 1A of Example 1 are loaded in Table 12. The test results of the high-voltage cycling characteristics of the lithium-ion batteries are shown in Table 13. The dispersants used in the preparation of the carbon nanotube conductive liquids tested in this example all have an anchoring group suitable for a highly crystalline carbon material as the dispersion main body and a solvation chain for providing stable steric hindrance ability at the same time. Using these types of dispersants can exert an excellent dispersing effect on CNTs, making these carbon nanotube conductive liquids have good dispersibility, fluidity, and storage stability. The positive electrode slurries prepared using them can also achieve good coating effects. However, in the DCR test of lithium-ion batteries, referring to Table 13, it can be clearly seen that under high-voltage working conditions, the capacity of the lithium-ion battery corresponding to Control Group 1D using PVP as the dispersant decreased significantly as the number of cycling increased, that is, its capacity retention rate was low. The reason is that PVP has poor electrochemical stability in the ternary high-voltage system, thus having an adverse impact on the cycling characteristics of the lithium-ion battery containing it.

[0109] Table 12. Slurry States of Carbon Nanotube Conductive Liquids

[0110]

[0111] Table 13. Statistical Results of Test on High-Voltage Cycling Characteristics of Lithium-Ion Batteries

[0112] Source of Carbon Nanotube Conductive Liquid Test Results of High-Pressure Circulation Characteristics Treatment Group 1A Capacity Retention Rate of 89.5% after 500 cycles at 45 0.5C / 0.5C Treatment Group 2D Capacity Retention Rate of 88.2% after 500 cycles at 45 0.5C / 0.5C Treatment Group 3D Capacity Retention Rate of 90.3% after 500 cycles at 45 0.5C / 0.5C Control Group 1D Capacity Retention Rate of 67.3% after 500 cycles at 45 0.5C / 0.5C

[0113] 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 has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A preparation method of a highly dispersed carbon nanotube conductive liquid, characterized in that, It includes the following steps: S1. Carbon nanotubes with a tube diameter not exceeding 15 nm are put into a dilution solution containing a dispersant and fully dispersed to obtain a first dispersion solution, wherein the dispersant is a non-PVP dispersant; S2. Anhydrous piperazine is added to the first dispersion solution and fully dispersed to obtain a grinding solution; S3. The grinding solution is subjected to sand grinding treatment, and the grinding medium for the sand grinding treatment is zirconium balls with a diameter of 0.7 - 1.4 mm; S4. The slurry obtained after S3 is completed is subjected to high-pressure homogenization dispersion treatment, and the high-pressure homogenization dispersion pressure is 650 - 850 bar; S5. The slurry obtained after S4 is completed is subjected to demagnetization and sieving treatment to prepare the carbon nanotube conductive liquid.

2. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 1, characterized in that: In S1, the addition amount of the carbon nanotubes is determined according to the mass content of the carbon nanotubes in the carbon nanotube conductive liquid being 3% - 5%.

3. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 2, wherein: In S2, the addition amount of the anhydrous piperazine is determined according to the mass of the carbon nanotubes: the mass of the anhydrous piperazine = 3 - 5:0.005 - 0.

1.

4. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 1, characterized in that: In the step S4, the high-pressure homogenization and dispersion treatment is carried out until the carbon nanotubes meet the size standard of 0.7μm < D 90 ≤ 1μm and the fineness ≤ 3μm, then the high-pressure homogenization and dispersion treatment is completed.

5. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 1, wherein: During the sand grinding treatment in S3, the sand grinding speed is 600 - 800 RPM.

6. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 5, wherein: The sand grinding treatment includes a rough grinding process and a fine grinding process. The grinding medium used in the rough grinding process is rough grinding zirconium balls, and the grinding medium used in the fine grinding process is fine grinding zirconium balls. The difference in diameter between the rough grinding zirconium balls and the fine grinding zirconium balls is △r, where 0 mm < △r ≤ 0.5 mm.

7. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 6, characterized in that: During the sand grinding treatment in S3, the pump speed of the material feeding pump is 2 - 6 s / time.

8. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 7, wherein: During the sand grinding treatment process, the pump speed of the feeding pump in the rough grinding process < the pump speed of the feeding pump in the fine grinding process.

9. The preparation method of the highly dispersed carbon nanotube conductive liquid according to any one of claims 1 to 8, characterized in that: The dispersant is a high molecular organic polymer polymerized from one or several monomers among polyester monomers, polyether monomers, cyan monomers, and alkoxy monomers.

10. The preparation method of the highly dispersed carbon nanotube conductive liquid according to claim 9, characterized in that: The molecular structure of the dispersant simultaneously has an anchoring group suitable for using a high-crystallinity carbon material as the dispersion main body and a solvation chain for providing a stable steric hindrance ability.

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