A method for demagnetizing conductive agent

Abstract

The present invention provides a method for demagnetizing a conductive agent, the demagnetizing method comprising the following steps: adding a conductive agent to a mixed solution of a solvent and a dispersant, after a first mixing and a second mixing, an additional solvent is added for a third mixing to obtain a conductive agent slurry; after the conductive agent slurry is demagnetized at least twice, the demagnetization of the conductive agent is completed. The present invention fully considers the intrinsic physical properties of the conductive agent, such as a large specific surface area, a small density, and a strong affinity with the solvent, and adds the solvent twice, and undergoes different preparation processes, so that the conductive agent can be fully dispersed. At the same time, through the wet demagnetization process, the demagnetization effect is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries and relates to a method for demagnetizing a conductive agent. Background Art

[0002] With the continuous promotion of electric vehicles, the manufacturing scale of lithium-ion power batteries has grown rapidly due to the excellent performance of lithium batteries such as high energy density, low internal resistance, long cycle life and low self-discharge, which has put forward higher requirements for the endurance of electric vehicles. According to the working principle of lithium-ion batteries, the charging and discharging process requires the joint participation of lithium ions and electrons. However, the conductivity of the positive and negative electrode materials currently used in lithium batteries cannot meet the requirements, so it is necessary to add an additional 1% to 2% of conductive agent to improve the conductivity of the electrode.

[0003] Currently, the commonly used conductive agents are generally conductive graphite, conductive carbon black and conductive carbon tubes. Since the specific surface area of ​​conductive carbon tubes is 1500-3000m 2 / g is very easy to agglomerate and is expensive. It is generally used in trace amounts or not used in the preparation of lithium batteries; the morphology of conductive graphite is irregular spherical or flaky, and the contact between it and the active material is point-to-point or point-to-face. At the same time, the anisotropy of graphite active materials and the flaky structure of graphite can be well stacked to increase the compaction density of the electrode, but the wrapping and gap filling effects of conductive graphite on active materials are relatively weak. Therefore, conductive graphite is generally less used in the design of electrode formulas; conductive carbon black is a multi-branched cluster and fibrous agglomerate formed by the agglomeration of primary nanoparticles. It is mostly in point-to-point contact with the active material and dispersed around the active material to form a multi-branched conductive network, which can improve the conductivity of the electrode and reduce the internal resistance of the battery, but the contact between particles is insufficient and the size of the primary aggregates is limited, resulting in a large internal resistance value. As the cycle proceeds, more side reactions occur inside the battery, causing the active material to separate from the matrix, resulting in an increase in the internal resistance of the battery; therefore, in actual use, conductive graphite and conductive carbon black need to be used in combination.

[0004] In addition, in the process of producing and preparing conductive agents, it is inevitable that compounds of elements such as Fe, Ni and Gr will be contained in trace amounts. These magnetic compounds are called magnetic substances. The presence of magnetic substances will seriously affect the self-discharge and voltage drop of the battery cell. Therefore, battery-grade conductive agents have extremely high requirements for the content of magnetic substances. The content of magnetic substances is ≤100ppb, and the content of magnetic substances in the slurry is ≤60ppb.

[0005] In the existing lithium battery production, negative pressure is usually used to suck the bagged conductive agent into the silo through the pipeline. In the pipeline, a high-strength magnetic bar (the strength of the magnetic bar is ≥5000Gs) is used to demagnetize the conductive agent. After the pulping process is completed, the slurry will be tested for the content of magnetic substances. After the slurry is beaten out of the mixing tank, it passes through a mixing iron removal vehicle, a coating iron removal device and then to coating. In this process, the slurry undergoes at least two iron removals. For example, a graphene composite conductive agent production equipment disclosed in CN209934913U requires a complex process flow and only undergoes simple demagnetization during the preparation process. The above-mentioned traditional demagnetization method is difficult to remove trace magnetic substances in the conductive agent. If the conductive agent is added to the slurry, the conductive agent will be dispersed and wrapped around the positive / negative electrode main material, and the magnetic substance will be difficult to remove at this time.

[0006] Based on the above research, it is necessary to provide a method for demagnetizing a conductive agent. The method is simple to operate and has an obvious demagnetizing effect. It can significantly remove the magnetic substances in the conductive agent before the conductive agent is slurried and used, thereby preventing the magnetic substances in the conductive agent from affecting the battery performance. Summary of the invention

[0007] The object of the present invention is to provide a method for demagnetizing a conductive agent, and in particular to a method for demagnetizing a battery-grade conductive agent. The demagnetization method fully considers the intrinsic physical properties of the conductive agent, so that the demagnetization effect is excellent, the content of magnetic substances in the conductive agent is greatly reduced, and the problem that trace magnetic substances in the conductive agent are difficult to remove, thereby affecting the self-discharge and voltage drop performance of the battery cell, is solved.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a method for demagnetizing a conductive agent, the demagnetization method comprising the following steps:

[0010] (1) adding a conductive agent to a mixture of a solvent and a dispersant, mixing once and twice, and then adding an additional solvent for a third mixing to obtain a conductive agent slurry;

[0011] (2) After the conductive agent slurry in step (1) is demagnetized at least twice, the demagnetization of the conductive agent is completed.

[0012] The present invention fully considers the intrinsic physical properties of the conductive agent, such as large specific surface area, small density, and strong affinity with the solvent. First, the solvent is added in two steps to prepare a slurry of the conductive agent. Due to the small density of the conductive agent, it is easy to float on the surface of the solvent. When the solvent is added at one time, there is too much solvent. During the mixing and dispersion process, the floating conductive agent will climb along the stirring shaft and the cylinder wall, causing the dispersibility of the conductive agent to decrease. Therefore, the present invention adds the solvent in batches. On the one hand, the conductive agent can be fully wetted. On the other hand, it can avoid the above phenomenon to the greatest extent in the actual production process, improve the dispersibility of the conductive agent, and enable the conductive agent to be evenly demagnetized in the subsequent demagnetization process; secondly, since the conductive agent that first contacts the solvent will affect the wettability of the solvent to the remaining conductive agent, the present invention takes into account the strong affinity of the conductive agent and the solvent, and through multiple mixing, the conductive agent can be further fully dispersed; in addition, the demagnetization of the present invention is carried out at least twice, which can remove a large amount of magnetic substances in the conductive agent.

[0013] The present invention makes the conductive agent into a slurry for wet demagnetization. Compared with dry demagnetization, due to the large specific surface area of ​​the conductive agent, the compounds containing trace elements such as Fe, Ni and Gr are wrapped by the conductive agent, resulting in a larger effective contact distance between these compounds and the magnetic rod. Since the magnetic field strength is inversely proportional to the square of the distance, the generated magnetic field force is proportional to the magnetic field strength at that position, resulting in poor demagnetization effect. On the other hand, due to the large specific surface area of ​​the conductive agent, there is a strong electrostatic effect inside the conductive agent, which affects the dry demagnetization effect.

[0014] The total number of demagnetization times in step (2) is more than 2 times, for example, it can be 2 times, 3 times, 4 times, 5 times, 6 times or 7 times, but is not limited to the listed values, and other integer values ​​not listed in the numerical range are also applicable.

[0015] The present invention does not specifically limit the type of the conductive agent, and those skilled in the art can reasonably select it according to needs. Exemplary conductive agents include any one of conductive graphite, conductive carbon black, conductive carbon nanotubes, graphene, acetylene black or Ketjen black, or a combination of at least two of them.

[0016] Preferably, the conductive graphite includes any one of KS-6, KS-15, SFG-6 or MX-15, or a combination of at least two thereof.

[0017] Preferably, the conductive carbon black includes any one of LITX 300, ECP600 JD or Super P Li, or a combination of at least two thereof.

[0018] Preferably, the conductive carbon tubes include SWCNTs.

[0019] Considering the physical properties of the conductive agent suitable for the demagnetization method of the present invention, a conductive agent with a larger specific surface area, a smaller density and a stronger affinity with the solvent is required. Therefore, the preferred conductive agent of the present invention includes conductive graphite and / or conductive carbon black.

[0020] Preferably, the stirring rate of the secondary mixing in step (1) is greater than the stirring rate of the primary mixing, and the dispersion rate of the secondary mixing is greater than the dispersion rate of the primary mixing.

[0021] The primary mixing process described in the present invention is a slow stirring and slow dispersing process, and the secondary mixing is a high-speed stirring and high-speed dispersing process. This is to further consider the physical properties of the conductive agent. The conductive agent that first contacts the solvent will affect the wettability of the solvent to the remaining conductive agent, thereby preventing the conductive agent just added during the primary mixing from floating on the solvent. If a larger stirring and dispersing rate is used, the conductive agent floating on the solvent will fly due to the airflow formed by the excessively fast stirring and dispersing speed, resulting in the loss of the conductive agent. In addition, slow stirring and slow dispersing during the primary mixing are beneficial to the full contact between the conductive agent and the solvent, thereby improving the wetting effect. The secondary mixing can be carried out at a stirring and dispersing rate greater than that of the primary mixing, because the conductive agent has been wetted by the primary mixing, and the secondary mixing can further disperse the conductive agent that was not fully dispersed in the primary mixing at a high-speed stirring and high-speed dispersing rate, thereby further improving the dispersibility of the conductive agent.

[0022] Preferably, the stirring rate of the three mixings in step (1) is greater than the stirring rate of the one mixing, and the dispersion rate of the three mixings is greater than the dispersion rate of the one mixing.

[0023] Preferably, the stirring rate of the tertiary mixing in step (1) is not lower than the stirring rate of the secondary mixing, and the dispersion rate of the tertiary mixing is not lower than the dispersion rate of the secondary mixing.

[0024] Preferably, the stirring rate of the tertiary mixing in step (1) is greater than the stirring rate of the secondary mixing, and the dispersion rate of the tertiary mixing is greater than the dispersion rate of the secondary mixing.

[0025] The stirring rate and dispersion rate of the tertiary mixing described in the present invention are greater than those of the secondary mixing. Since the third mixing is performed after the solvent is added again, the higher stirring and dispersion rates can further improve the dispersibility of the conductive agent, and the flying and climbing of the conductive agent will not occur; that is, the primary mixing, secondary mixing, and tertiary mixing described in the present invention are added in batches with the solvent, and the multiple mixing and multiple additions cooperate with each other to achieve sufficient dispersion of the conductive agent.

[0026] Preferably, the time of the third mixing in step (1) is not less than the time of the second mixing, and preferably is greater than the time of the second mixing.

[0027] Preferably, the time of the secondary mixing in step (1) is longer than the time of the primary mixing.

[0028] In order to coordinate the slow stirring and slow dispersion of the first mixing, the high-speed stirring and high-speed dispersion of the second mixing, and the higher-speed stirring and higher-speed dispersion of the third mixing, the corresponding times of the first mixing, the second mixing and the third mixing are gradually increased.

[0029] Preferably, the stirring rate of the primary mixing in step (1) is 10-20 rpm, for example, 10 rpm, 12 rpm, 14 rpm, 16 rpm, 18 rpm or 20 rpm, the dispersion rate is 200-300 rpm, for example, 200 rpm, 220 rpm, 240 rpm, 260 rpm, 280 rpm or 300 rpm, and the time is 10-15 min, for example, 10 min, 11 min, 12 min, 13 min, 14 min or 15 min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0030] Preferably, the stirring rate of the secondary mixing in step (1) is 30-40rpm, for example, it can be 30rpm, 32rpm, 34rpm, 36rpm, 38rpm or 40rpm, the dispersion rate is 1100-1300rpm, for example, it can be 1100rpm, 1150rpm, 1200rpm, 1250rpm or 1300rpm, and the time is 40-50min, for example, it can be 40min, 42min, 44min, 46min, 48min or 50min, but it is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable.

[0031] Preferably, the stirring rate of the three mixings in step (1) is 30-40rpm, for example, 30rpm, 32rpm, 34rpm, 36rpm, 38rpm or 40rpm, the dispersion rate is 1100-1300rpm, for example, 1100rpm, 1150rpm, 1200rpm, 1250rpm or 1300rpm, and the time is 40-50min, for example, 40min, 42min, 44min, 46min, 48min or 50min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0032] Preferably, the step (1) further includes a premixing step of the solvent and the dispersant before the step (1) is performed.

[0033] Preferably, the stirring rate of the premix is ​​10-20rpm, for example, it can be 10rpm, 12rpm, 14rpm, 16rpm, 18rpm or 20rpm, the dispersion rate is 200-300rpm, for example, it can be 200rpm, 220rpm, 240rpm, 260rpm, 280rpm or 300rpm, and the time is 10-15min, for example, it can be 10min, 11min, 12min, 13min, 14min or 15min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0034] Preferably, in the conductive agent slurry of step (1), the mass ratio of the conductive agent, the dispersant and the total amount of solvent is (12-14):(5-6):(80-83), and the total amount of solvent includes the amount of solvent in the mixed solution of step (1) and the amount of additional solvent added, for example, it can be 12:5:82, 13:5:83, 14:6:83 or 14:6:80, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable, preferably 13.5:5.5:81.

[0035] When the amount of solvent added in the present invention is too much, a large amount of conductive agent will float on the surface of the solvent, making it difficult for the conductive agent to disperse. At the same time, too much solvent will also cause the solid content of the conductive agent to decrease, which is not convenient for the conductive agent ratio in the pulping process. If the amount of solvent added is too little, the conductive agent will agglomerate, reducing the dispersion degree of the conductive agent and affecting the subsequent demagnetization effect.

[0036] Preferably, the amount of additional solvent added in step (1) accounts for 20-30wt% of the total solvent amount in the conductive agent slurry, for example, it can be 20wt%, 22wt%, 24wt%, 26wt%, 28wt% or 30wt%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0037] When the amount of additional solvent added in the present invention is too much, the specific gravity of the solvent initially added during the first mixing and the second mixing will be smaller, the liquid level of the solvent will be too low, the stirring effect cannot be achieved, the conductive agent and the solvent cannot be fully wetted, and during the stirring process, the conductive agent floating on the surface of the solvent will form a flying phenomenon, which will reduce the dispersion effect of the first mixing and the second mixing, and affect the dispersion degree of the conductive agent in the conductive agent slurry; when the amount of additional solvent added is too little, the specific gravity of the solvent initially added during the first mixing and the second mixing will be larger, the conductive agent floating on the surface of the solvent will be higher than the effective stirring height of the stirring shaft, and the conductive agent will "climb" along the stirring shaft and the inner wall of the cylinder during the stirring process, and the conductive agent on the stirring shaft and the cylinder wall cannot be fully mixed into the slurry, thereby affecting the dispersion degree of the conductive agent.

[0038] Preferably, the dispersant in step (1) includes polyvinyl pyrrolidone series dispersants and / or KD series dispersants.

[0039] The types of polyvinyl pyrrolidone series dispersants of the present invention include any one of K10, K30 or K90 or a combination of at least two of them. Typical but non-limiting combinations include a combination of K10 and K30, and a combination of K30 and K90. The types of KD series dispersants include VOK-KD2 and / or VOK-KD4.

[0040] Preferably, the solvent in step (1) comprises any one of N-methylpyrrolidone, pure water, ethanol, ethyl methyl carbonate or ethylene sulfite, or a combination of at least two thereof. Typical but non-limiting combinations include a combination of N-methylpyrrolidone and ethanol, or a combination of ethyl methyl carbonate and ethylene sulfite, preferably water or N-methylpyrrolidone.

[0041] Preferably, the demagnetization in step (2) is a cyclic demagnetization, comprising: after the conductive agent slurry in step (1) is placed in a demagnetization device for demagnetization, it is transferred to a transfer tank to complete one demagnetization, and the demagnetization is cyclically performed once to complete the demagnetization of the conductive agent;

[0042] The present invention can realize the circulation between the transfer tank and the demagnetization equipment, thereby shortening the process flow of multiple demagnetizations. The setting of the transfer tank provides convenience for real-time monitoring of the content of magnetic substances in the conductive agent, so as to improve the uniformity of demagnetization.

[0043] Preferably, a stirring device is provided in the transfer tank in step (2).

[0044] Preferably, the rotation speed of the stirring device is 15-25 rpm, for example, 15 rpm, 20 rpm, 25 rpm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0045] The transfer tank of the present invention is provided with a stirring device, which can stir the demagnetized slurry evenly and enter the next cycle, thereby ensuring that mixing is performed again after each demagnetization, so that the conductive agent slurry can be evenly demagnetized.

[0046] Preferably, the total number of demagnetization times in step (2) is 2-4 times, preferably 2 times.

[0047] Preferably, the circulating demagnetization is achieved by circulating the conductive agent slurry in the transfer tank back to the demagnetization equipment through a diaphragm pump.

[0048] Exemplarily, the cyclic demagnetization process of the present invention with a total of 2 demagnetization times includes: after the conductive agent slurry is demagnetized by the demagnetization equipment, it is transferred to the transfer tank through a pipeline, the magnetic material content of the conductive agent slurry in the transfer tank is tested, and then the conductive agent slurry is returned to the demagnetization equipment by a diaphragm pump for demagnetization again, and after demagnetization, it is transferred to the transfer tank through a pipeline, and the magnetic material content of the conductive agent slurry in the transfer tank is tested, completing the cyclic demagnetization process with a total of 2 demagnetization times.

[0049] Preferably, each demagnetization time is 30-60 minutes, for example, 30 minutes, 40 minutes, 50 minutes or 60 minutes, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0050] Preferably, the demagnetization equipment includes wet demagnetization equipment, exemplarily including a wet electromagnetic iron remover.

[0051] Preferably, after each demagnetization in step (2), the magnetic substance content of the conductive agent slurry in the transfer tank is tested.

[0052] After demagnetization, the present invention transfers the solution to a transfer tank, and samples are taken from the material intake port of the transfer tank to test the content of magnetic substances in the solution. That is, the present invention can monitor the content of magnetic substances after demagnetization. Since the content of magnetic substances in different conductive agents or different batches of conductive agents is different, the number of demagnetization times can be adjusted according to the conductive agents containing different magnetic substances, thereby avoiding unnecessary multiple demagnetizations and the problem of incomplete demagnetization due to too few demagnetization times.

[0053] Preferably, after the demagnetization of the conductive agent is completed in step (2), the conductive agent slurry is transferred to a storage tank for a vacuum defoaming step;

[0054] Preferably, the vacuum degree of the vacuum defoaming step is below -80KPa, for example, it can be -80KPa, -90KPa, -100KPa, -110KPa, -120KPa, -130KPa, -140KPa, -150KPa or -160KPa, and the time is 30-60min, for example, it can be 30min, 40min, 50min or 60min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0055] Preferably, the vacuum defoaming step is performed under stirring.

[0056] As a preferred technical solution of the demagnetization method of the present invention, the demagnetization method comprises the following steps:

[0057] (1) premixing the solvent and the dispersant for 10-15 minutes at a stirring rate of 10-20 rpm and a dispersion rate of 200-300 rpm to obtain a mixed solution of the solvent and the dispersant, adding the conductive agent to the mixed solution of the solvent and the dispersant, mixing for 10-15 minutes at a stirring rate of 10-20 rpm and a dispersion rate of 200-300 rpm, mixing for a second time at a stirring rate of 30-40 rpm and a dispersion rate of 1100-1300 rpm for 40-50 minutes, and adding additional solvent after the second mixing, mixing for a third time at a stirring rate of 30-40 rpm and a dispersion rate of 1100-1300 rpm for 40-50 minutes to obtain a conductive agent slurry;

[0058] The stirring rate, dispersion rate and time of the tertiary mixing are all greater than those of the secondary mixing, and the stirring rate, dispersion rate and time of the secondary mixing are all greater than those of the primary mixing;

[0059] In the conductive agent slurry, the mass ratio of the conductive agent, the dispersant and the total solvent is (12-14): (5-6): (80-83), the total solvent includes the amount of the solvent in the mixed solution and the amount of the additional solvent added, and the amount of the additional solvent added accounts for 20-30wt% of the total solvent in the conductive agent slurry;

[0060] (2) After the conductive agent slurry in step (1) is demagnetized in the wet demagnetization equipment for 30-60 minutes, it is transferred to a transfer tank to complete one demagnetization. The conductive agent slurry in the transfer tank is returned to the demagnetization equipment by a diaphragm pump and demagnetized again to achieve cyclic demagnetization. After each demagnetization, the magnetic substance content of the conductive agent slurry in the transfer tank is tested;

[0061] The conductive agent slurry in step (1) is demagnetized 2-4 times in total and then transferred to a storage tank, where it is vacuumed and defoamed for 30-60 minutes under stirring and a vacuum degree below -80 KPa.

[0062] Compared with the prior art, the present invention has the following beneficial effects:

[0063] (1) The present invention prepares the conductive agent into a slurry for wet demagnetization. Compared with dry demagnetization, the effective contact distance between the magnetic material and the magnetic rod can be reduced, the demagnetization effect can be improved, and the influence of the electrostatic effect between the conductive agents on the demagnetization effect during dry demagnetization can be avoided;

[0064] (2) The present invention fully considers the intrinsic physical properties of the conductive agent and adds the solvent in two steps to prepare the conductive agent slurry, thereby avoiding the problem of the floating conductive agent climbing up along the stirring shaft and the cylinder wall during the mixing and dispersion process, resulting in a decrease in dispersibility;

[0065] (3) The present invention takes into account the strong affinity between the conductive agent and the solvent. The conductive agent that contacts the solvent first will affect the wettability of the solvent to the remaining conductive agent. Therefore, the present invention can avoid the flying and climbing of the conductive agent through the primary mixing, secondary mixing and tertiary mixing with increasing stirring rate and dispersion rate, so that the primary mixing, secondary mixing and tertiary mixing are coordinated with the solvent added in batches to achieve full dispersion of the conductive agent.

[0066] (4) The present invention can monitor the content of magnetic substances after each demagnetization, so that the number of demagnetization times can be adjusted according to the conductive agent containing different magnetic substances, thereby avoiding unnecessary multiple demagnetizations and the problem of incomplete demagnetization due to too few demagnetization times. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 This is a SEM image of the conductive agent without demagnetization described in Example 1 of the present invention;

[0068] Figure 2 This is a process flow chart of the demagnetization method described in Example 1 of the present invention;

[0069] Figure 3 The present invention shows the content of magnetic substances in the conductive agent before demagnetization in Example 1, the conductive agent slurry before demagnetization in Example 1, the conductive agent slurry after demagnetization in Example 1, and the conductive agent slurry after demagnetization in Example 4, Example 5 and Comparative Example 1;

[0070] Figure 4 This is a SEM image of the conductive agent after demagnetization in Example 1 of the present invention;

[0071] Figure 5 This is an EDS spectrum of foreign matter on the electromagnetic filter screen after demagnetization in Example 1 of the present invention;

[0072] Figure 6 This is the SEM image of the conductive agent without demagnetization in Example 2 of the present invention. DETAILED DESCRIPTION

[0073] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0074] Example 1

[0075] This embodiment provides a demagnetization method, which includes the following steps:

[0076] (1) premixing the solvent and the dispersant for 12 minutes at a stirring rate of 15 rpm and a dispersion rate of 250 rpm to obtain a mixed solution of the solvent and the dispersant, adding the conductive agent to the mixed solution of the solvent and the dispersant, mixing for 15 minutes at a stirring rate of 15 rpm and a dispersion rate of 250 rpm, mixing for a second time at a stirring rate of 35 rpm and a dispersion rate of 1200 rpm for 40 minutes, and after the second mixing, adding additional solvent, mixing for a third time at a stirring rate of 40 rpm and a dispersion rate of 1250 rpm for 45 minutes to obtain a conductive agent slurry;

[0077] The stirring rate, dispersion rate and time of the tertiary mixing are all greater than those of the secondary mixing, and the stirring rate, dispersion rate and time of the secondary mixing are all greater than those of the primary mixing;

[0078] In the conductive agent slurry, the mass ratio of the conductive agent, the dispersant and the solvent is 13.5:5.5:81, and the amount of the additionally added solvent accounts for 25wt% of the amount of the solvent in the conductive agent slurry;

[0079] The conductive agent is Super P Li, the solvent is N-methylpyrrolidone, and the dispersant is polyvinylpyrrolidone K10;

[0080] (2) After the conductive agent slurry in step (1) is demagnetized in the wet electromagnetic iron remover for 45 minutes, it is transferred to a transfer tank to complete one demagnetization; a stirring device is provided in the transfer tank, and after the conductive agent slurry is stirred and mixed again, the conductive agent slurry in the transfer tank is transferred back to the wet electromagnetic iron remover by a diaphragm pump to be demagnetized again. After each demagnetization, the magnetic substance content of the conductive agent slurry in the transfer tank is tested;

[0081] After the above two demagnetizations are completed, the conductive agent slurry is transferred to a storage tank and defoamed for 40 minutes under vacuum at -80KPa while stirring;

[0082] The SEM of the conductive agent before demagnetization in this example is as follows Figure 1 As shown, the SEM of the conductive agent after demagnetization is as follows Figure 4 The flow chart of the demagnetization process is shown in Figure 2 As shown; the magnetic material content of the undemagnetized conductive agent, the undemagnetized conductive agent slurry and the demagnetized conductive agent slurry is as shown Figure 3 As shown; the EDS spectrum of foreign matter on the electromagnetic filter after demagnetization is as follows Figure 5 The content of each element is shown in the following table, which proves that the demagnetization method has achieved excellent demagnetization effect;

[0083]

[0084] Example 2

[0085] This embodiment provides a demagnetization method, which includes the following steps:

[0086] (1) premixing the solvent and the dispersant for 10 min at a stirring rate of 10 rpm and a dispersion rate of 300 rpm to obtain a mixed solution of the solvent and the dispersant, adding the conductive agent to the mixed solution of the solvent and the dispersant, mixing for 10 min at a stirring rate of 20 rpm and a dispersion rate of 300 rpm, mixing for a second time at a stirring rate of 30 rpm and a dispersion rate of 1100 rpm for 40 min, and after the second mixing, adding additional solvent, mixing for a third time at a stirring rate of 40 rpm and a dispersion rate of 1300 rpm for 50 min to obtain a conductive agent slurry;

[0087] The stirring rate, dispersion rate and time of the tertiary mixing are all greater than those of the secondary mixing, and the stirring rate, dispersion rate and time of the secondary mixing are all greater than those of the primary mixing;

[0088] In the conductive agent slurry, the mass ratio of the conductive agent, the dispersant and the solvent is 12:5:83, and the amount of the additionally added solvent accounts for 20wt% of the amount of the solvent in the conductive agent slurry;

[0089] The conductive agent is KS-6, the solvent is water, and the dispersant is polyvinyl pyrrolidone K30;

[0090] (2) After the conductive agent slurry in step (1) is demagnetized in the wet electromagnetic iron remover for 30 minutes, it is transferred to a transfer tank to complete a demagnetization; a stirring device is provided in the transfer tank, and after the conductive agent slurry is stirred and mixed again, the conductive agent slurry in the transfer tank is transferred back to the wet electromagnetic iron remover by a diaphragm pump to be demagnetized again. After each demagnetization, the magnetic substance content of the conductive agent slurry in the transfer tank is tested;

[0091] After the above two demagnetizations are completed, the conductive agent slurry is transferred to a storage tank and defoamed for 60 minutes under vacuum at -80KPa while stirring;

[0092] The SEM of the conductive agent before demagnetization in this example is as follows Figure 6 shown.

[0093] Example 3

[0094] This embodiment provides a demagnetization method, which includes the following steps:

[0095] (1) premixing the solvent and the dispersant for 15 minutes at a stirring rate of 20 rpm and a dispersion rate of 200 rpm to obtain a mixed solution of the solvent and the dispersant, adding the conductive agent to the mixed solution of the solvent and the dispersant, mixing for 15 minutes at a stirring rate of 10 rpm and a dispersion rate of 200 rpm, mixing for a second time at a stirring rate of 40 rpm and a dispersion rate of 1200 rpm for 45 minutes, and after the second mixing, adding additional solvent, mixing for a third time at a stirring rate of 40 rpm and a dispersion rate of 1300 rpm for 50 minutes to obtain a conductive agent slurry;

[0096] The stirring rate, dispersion rate and time of the secondary mixing are all greater than those of the primary mixing;

[0097] In the conductive agent slurry, the mass ratio of the conductive agent, the dispersant and the solvent is 14:6:80, and the amount of the additionally added solvent accounts for 30wt% of the amount of the solvent in the conductive agent slurry;

[0098] The conductive agent is Super P Li, the solvent is ethyl methyl carbonate, and the dispersant is VOK-KD4;

[0099] (2) After the conductive agent slurry in step (1) is demagnetized in the wet electromagnetic iron remover for 60 minutes, it is transferred to a transfer tank to complete a demagnetization; a stirring device is provided in the transfer tank, and after the conductive agent slurry is stirred and mixed again, the conductive agent slurry in the transfer tank is transferred back to the wet electromagnetic iron remover by a diaphragm pump to be demagnetized again. After each demagnetization, the magnetic substance content of the conductive agent slurry in the transfer tank is tested;

[0100] After the above two demagnetizations are completed, the conductive agent slurry is transferred to a storage tank and defoamed for 30 minutes under vacuum at -50 KPa while stirring.

[0101] Example 4

[0102] This embodiment provides a demagnetization method, which is the same as that of Embodiment 1 except that step (2) is performed three times.

[0103] The magnetic material content of the conductive agent slurry after demagnetization in this embodiment is as follows: Figure 3 shown.

[0104] Example 5

[0105] This embodiment provides a demagnetization method, which is the same as that of Embodiment 1 except that step (2) is performed four times.

[0106] The magnetic material content of the conductive agent slurry after demagnetization in this embodiment is as follows: Figure 3 shown.

[0107] Example 6

[0108] This embodiment provides a demagnetization method, which is the same as Example 1 except that the stirring rate of the first mixing in step (1) is 35 rpm and the dispersion rate is 1200 rpm, so that the stirring rate and dispersion rate of the second mixing are the same as those of the first mixing.

[0109] Example 7

[0110] This embodiment provides a demagnetization method, which is the same as Example 1 except that the stirring rate of the secondary mixing in step (1) is 15 rpm and the dispersion rate is 250 rpm, so that the stirring rate and dispersion rate of the secondary mixing are the same as those of the primary mixing.

[0111] Example 8

[0112] This embodiment provides a demagnetization method, which is the same as Example 1 except that the stirring rate of the three mixings in step (1) is 35 rpm and the dispersion rate is 1200 rpm, so that the stirring rate and dispersion rate of the three mixings are the same as those of the second mixing.

[0113] Example 9

[0114] This embodiment provides a demagnetization method, which is the same as Example 1 except that the stirring rate of the three mixings in step (1) is 15 rpm and the dispersion rate is 250 rpm, so that the stirring rate and dispersion rate of the three mixings are smaller than those of the second mixing.

[0115] Example 10

[0116] This embodiment provides a demagnetization method, which is the same as that of Embodiment 1 except that the mass ratio of the conductive agent, the dispersant and the solvent in step (1) is 9.5:5.5:85.

[0117] Embodiment 11

[0118] This embodiment provides a demagnetization method, which is the same as that of Embodiment 1 except that the mass ratio of the conductive agent, the dispersant and the solvent in step (1) is 17.5:5.5:77.

[0119] Example 12

[0120] This embodiment provides a demagnetization method, which is the same as that of Embodiment 1 except that the amount of the additional solvent added in step (1) accounts for 15 wt % of the amount of the solvent in the conductive agent slurry.

[0121] Embodiment 13

[0122] This embodiment provides a demagnetization method, which is the same as that of Embodiment 1 except that the amount of the additional solvent added in step (1) accounts for 35 wt % of the amount of the solvent in the conductive agent slurry.

[0123] Comparative Example 1

[0124] This comparative example provides a demagnetization method, which is the same as Example 1 except that step (2) is performed only once and no cyclic demagnetization is performed;

[0125] The magnetic material content of the conductive agent slurry after demagnetization in this comparative example is as follows Figure 3 shown.

[0126] Comparative Example 2

[0127] This comparative example provides a demagnetization method, which comprises the following steps:

[0128] (1) premixing the solvent and the dispersant for 12 minutes at a stirring rate of 15 rpm and a dispersion rate of 250 rpm to obtain a mixed solution of the solvent and the dispersant, adding the conductive agent to the mixed solution of the solvent and the dispersant, and mixing for 100 minutes at a stirring rate of 35 rpm and a dispersion rate of 1200 rpm to obtain a conductive agent slurry;

[0129] In the conductive agent slurry, the mass ratio of the conductive agent, the dispersant and the solvent is 13.5:5.5:81;

[0130] The conductive agent is Super P Li, the solvent is N-methylpyrrolidone, and the dispersant is polyvinylpyrrolidone K10;

[0131] (2) After the conductive agent slurry in step (1) is demagnetized in the wet electromagnetic iron remover for 45 minutes, it is transferred to a transfer tank to complete one demagnetization; a stirring device is provided in the transfer tank, and after the conductive agent slurry is stirred and mixed again, the conductive agent slurry in the transfer tank is transferred back to the wet electromagnetic iron remover by a diaphragm pump to be demagnetized again. After each demagnetization, the magnetic substance content of the conductive agent slurry in the transfer tank is tested;

[0132] After the above two demagnetization processes, the conductive agent slurry was transferred to a storage tank and defoamed for 40 minutes under vacuum at -80 KPa while stirring.

[0133] Comparative Example 3

[0134] This comparative example provides a demagnetization method, which includes: directly demagnetizing the conductive agent Super P Li in a dry powder electromagnetic separator (model: KJLGL-350, output voltage 380V, magnetic flux density 1.5T) for 90 minutes, and mixing the conductive agent after demagnetization with N-methylpyrrolidone and polyvinylpyrrolidone K10 in a mass ratio of 81:13.5:5.5 to make a conductive agent slurry, and then testing the content of magnetic substances therein.

[0135] The model of the wet electromagnetic iron remover used in the above embodiments and comparative examples is EMF-W250, the manufacturer is DAEBO MAGNETIC CO.LTD, the material of the electromagnetic filter is STS430, the filter size is Ф245×16H×26EA, and the DC voltage regulator Max DC is 215V×32A; the test method of the magnetic substance described in the above embodiments and comparative examples is carried out with reference to the "Test Method for Magnetic Substances" GB / T 24533-2009, and the magnetic removal rate shown in Table 1 is obtained, and the calculation method is: the content of the magnetic substance in the original conductive agent minus the content of the magnetic substance in the conductive agent slurry after demagnetization, the difference between the two / the content of the magnetic substance in the original conductive agent×100%=magnetic removal rate.

[0136] Table 1

[0137]

[0138]

[0139] From Table 1, we can see the following points:

[0140] (1) It can be seen from Examples 1-13 that the demagnetization method provided by the present invention substantially reduces the content of magnetic substances in the conductive agent by fully considering the intrinsic physical properties of the conductive agent, thereby achieving a better demagnetization effect; from Examples 1, 4-5 and in combination Figure 3 It can be seen that Example 1 performed demagnetization twice, Example 4 performed demagnetization three times, and Example 5 performed demagnetization four times. Although Examples 4-5 performed demagnetization more times, the demagnetization rate was not significantly improved, that is, the demagnetization rate of 3-4 times was equivalent to the effect of only performing demagnetization twice. The present invention preferably performs demagnetization twice; It can be seen from Examples 1 and 6-7 that the stirring and dispersing rate of the first mixing in Example 6 is too fast, which causes the conductive agent to disperse at a high speed, causing the conductive agent floating on the solvent to fly, resulting in the loss of the conductive agent, and is not conducive to full contact between the conductive agent and the solvent, which deteriorates the dispersion effect of the conductive agent and reduces the demagnetization effect. In Example 7, the secondary mixing is performed at a low speed dispersion, and the conductive agent that was not dispersed in the first mixing cannot be fully dispersed under high shear force, which deteriorates the dispersion effect of the conductive agent and reduces the demagnetization effect.

[0141] (2) It can be seen from Example 1 and Examples 8-9 that the tertiary mixing is preferably carried out at a higher dispersion rate and stirring rate than the secondary mixing, which is beneficial to the dispersion of the conductive agent and the improvement of the demagnetization rate; It can be seen from Example 1 and Examples 10-11 that when the content of the solvent in the conductive agent slurry is too little or too much, it is not conducive to the dispersion of the conductive agent and affects the subsequent demagnetization effect; It can be seen from Example 1 and Examples 12-13 that the amount of solvent added for the second time should be within a reasonable range to achieve better dispersion and demagnetization effects.

[0142] (3) It can be seen from Example 1 and Comparative Example 1 that only one demagnetization is performed, the demagnetization efficiency is low, and the magnetic material content in the conductive agent cannot be removed to the greatest extent. There will still be a large amount of magnetic material in the conductive agent that affects the performance of the conductive agent used in the battery; It can be seen from Example 1 and Comparative Example 2 that in Comparative Example 2, all the solvents are added at once during the dispersion of the conductive agent, and high-speed mixing is performed throughout the process, which will not only cause the conductive agent to climb along the stirring shaft and the cylinder wall, but also cause the conductive agent to fly, resulting in a decrease in the dispersibility of the conductive agent and a decrease in the demagnetization effect; It can be seen from Example 1 and Comparative Example 3 that Comparative Example 3 adopts dry demagnetization. Although the total demagnetization time is the same as that of Example 1, the magnetic material in the dry demagnetization is at a greater distance from the magnetic rod than in the wet demagnetization, and the demagnetization effect is poor. In addition, the strong electrostatic effect inside the conductive agent will also affect the demagnetization effect.

[0143] In summary, the present invention provides a method for demagnetizing a conductive agent, which fully considers the intrinsic physical properties of the conductive agent, achieves an excellent demagnetization effect, greatly reduces the content of magnetic substances in the conductive agent, and solves the problem that trace magnetic substances in the conductive agent are difficult to remove, thereby affecting the self-discharge and voltage drop performance of the battery cell.

[0144] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention are within the protection scope and disclosure scope of the present invention.

Claims

1. A method for demagnetizing a conductive agent, characterized in that: The demagnetization method comprises the following steps: (1) adding a conductive agent to a mixture of a solvent and a dispersant, mixing once and twice, and then adding an additional solvent for a third mixing to obtain a conductive agent slurry; (2) After the conductive agent slurry in step (1) is demagnetized at least twice, the demagnetization of the conductive agent is completed; The demagnetization in step (2) is a cyclic demagnetization, comprising: the conductive agent slurry in step (1) is placed in a demagnetization device for demagnetization, and then transferred to a transfer tank to complete a demagnetization, and the demagnetization is cyclically performed to complete the demagnetization of the conductive agent; The demagnetization equipment in step (2) includes a wet electromagnetic iron remover.

2. The demagnetization method according to claim 1, characterized in that: The stirring rate of the secondary mixing in step (1) is greater than the stirring rate of the primary mixing, and the dispersion rate of the secondary mixing is greater than the dispersion rate of the primary mixing.

3. The demagnetization method according to claim 1, characterized in that: In step (1), the stirring rate of the three mixings is greater than the stirring rate of the one mixing, and the dispersion rate of the three mixings is greater than the dispersion rate of the one mixing.

4. The demagnetization method according to claim 1, characterized in that: The stirring rate of the tertiary mixing in step (1) is not lower than the stirring rate of the secondary mixing, and the dispersion rate of the tertiary mixing is not lower than the dispersion rate of the secondary mixing.

5. The demagnetization method according to claim 1, characterized in that: The stirring rate of the tertiary mixing in step (1) is greater than the stirring rate of the secondary mixing, and the dispersion rate of the tertiary mixing is greater than the dispersion rate of the secondary mixing.

6. The demagnetization method according to claim 1, characterized in that: The time of the third mixing in step (1) is not less than the time of the second mixing.

7. The demagnetization method according to claim 1, characterized in that: The time of the third mixing in step (1) is longer than the time of the second mixing.

8. The demagnetization method according to claim 1, characterized in that: The time of the secondary mixing in step (1) is longer than the time of the primary mixing.

9. The demagnetization method according to claim 1, characterized in that: The stirring rate of the primary mixing in step (1) is 10-20 rpm, the dispersion rate is 200-300 rpm, and the time is 10-15 min.

10. The demagnetization method according to claim 1, characterized in that: The stirring rate of the secondary mixing in step (1) is 30-40 rpm, the dispersion rate is 1100-1300 rpm, and the time is 40-50 min.

11. The demagnetization method according to claim 1, characterized in that: The stirring rate of the three mixings in step (1) is 30-40 rpm, the dispersion rate is 1100-1300 rpm, and the time is 40-50 min.

12. The demagnetization method according to claim 1, characterized in that: The step (1) also includes a premixing step of the solvent and the dispersant before the step (1) is performed.

13. The demagnetization method according to claim 12, characterized in that: The stirring rate of the premix is ​​10-20 rpm, the dispersion rate is 200-300 rpm, and the time is 10-15 min.

14. The demagnetization method according to claim 1, characterized in that: In the conductive agent slurry of step (1), the mass ratio of the conductive agent, the dispersant and the total solvent is (12-14):(5-6):(80-83), and the total solvent includes the solvent in the mixed solution of step (1) and the additional amount of solvent added.

15. The demagnetization method according to claim 1, characterized in that: The amount of the additional solvent added in step (1) accounts for 20-30 wt % of the total amount of solvent in the conductive agent slurry.

16. The demagnetization method according to claim 1, characterized in that: The dispersant in step (1) includes polyvinyl pyrrolidone series dispersants and / or KD series dispersants.

17. The demagnetization method according to claim 1, characterized in that: The solvent in step (1) includes any one of N-methylpyrrolidone, pure water, ethanol, ethyl methyl carbonate or ethylene sulfite, or a combination of at least two thereof.

18. The demagnetization method according to claim 1, characterized in that: A stirring device is arranged in the transfer tank.

19. The demagnetization method according to claim 1, characterized in that: The total number of demagnetization times in step (2) is 2-4 times.

20. The demagnetization method according to claim 1, characterized in that: The total number of demagnetization times in step (2) is 2 times.

21. The demagnetization method according to claim 1, characterized in that: The circulating demagnetization is achieved by circulating the conductive agent slurry in the transfer tank to the demagnetization equipment through a diaphragm pump.

22. The demagnetization method according to claim 1, characterized in that: Each demagnetization process takes 30-60 minutes.

23. The demagnetization method according to claim 1, characterized in that: After each demagnetization in step (2), the magnetic substance content of the conductive agent slurry in the transfer tank is tested.

24. The demagnetization method according to claim 1, characterized in that: After the demagnetization of the conductive agent is completed in step (2), the conductive agent slurry is transferred to a storage tank for a vacuum defoaming step.

25. The demagnetization method according to claim 24, characterized in that: The vacuum degree of the vacuum defoaming step is below -80KPa, and the time is 30-60min.

26. The demagnetization method according to claim 24, characterized in that: The vacuum defoaming step is carried out under stirring.

27. The demagnetization method according to claim 1, characterized in that: The demagnetization method comprises the following steps: (1) premixing the solvent and the dispersant for 10-15 minutes at a stirring rate of 10-20 rpm and a dispersion rate of 200-300 rpm to obtain a mixed solution of the solvent and the dispersant, adding the conductive agent to the mixed solution of the solvent and the dispersant, mixing for 10-15 minutes at a stirring rate of 10-20 rpm and a dispersion rate of 200-300 rpm, mixing for a second time at a stirring rate of 30-40 rpm and a dispersion rate of 1100-1300 rpm for 40-50 minutes, and adding additional solvent after the second mixing, mixing for a third time at a stirring rate of 30-40 rpm and a dispersion rate of 1100-1300 rpm for 40-50 minutes to obtain a conductive agent slurry; The stirring rate, dispersion rate and time of the tertiary mixing are all greater than those of the secondary mixing, and the stirring rate, dispersion rate and time of the secondary mixing are all greater than those of the primary mixing; In the conductive agent slurry, the mass ratio of the conductive agent, the dispersant and the total solvent is (12-14): (5-6): (80-83), the total solvent includes the amount of the solvent in the mixed solution and the amount of the additional solvent added, and the amount of the additional solvent added accounts for 20-30wt% of the total solvent in the conductive agent slurry; (2) After the conductive agent slurry in step (1) is demagnetized in a wet electromagnetic iron remover for 30-60 minutes, it is transferred to a transfer tank to complete one demagnetization. The conductive agent slurry in the transfer tank is returned to the demagnetization equipment by a diaphragm pump and demagnetized again to achieve cyclic demagnetization. After each demagnetization, the magnetic substance content of the conductive agent slurry in the transfer tank is tested; The conductive agent slurry in step (1) is demagnetized 2-4 times in total and then transferred to a storage tank, where it is vacuumed and defoamed for 30-60 minutes under stirring and a vacuum degree below -80 KPa.

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