Preparation method for and use of cobalt hydroxide nanosheet

Abstract

The present disclosure belongs to the technical field of cobalt hydroxide preparation. Disclosed are a preparation method for and the use of a cobalt hydroxide nanosheet. The preparation method comprises: mixing first liquid caustic soda and water to prepare a base solution; under a protective atmosphere, adding in a parallel flow manner a cobalt salt solution, second liquid caustic soda and a silane coupling agent to the base solution so as to carry out a reaction, controlling the reaction pH to be 12-13 during the parallel flow process, and obtaining a cobalt hydroxide slurry after the reaction is completed; and carrying out solid-liquid separation on the cobalt hydroxide slurry, washing the obtained solid material, and drying and crushing same to obtain a cobalt hydroxide nanosheet.

Description

Preparation method and application of cobalt hydroxide nanosheets Technical Field

[0001] The present disclosure belongs to the technical field of cobalt hydroxide preparation, and particularly relates to a preparation method of cobalt hydroxide nanosheets and applications thereof. Background Art

[0002] Nickel-rich ternary materials are one of the cathode materials that currently meet the requirements of high energy density lithium-ion batteries. Nickel-rich ternary cathode materials include nickel-rich lithium nickel cobalt manganese oxide (LiNi x Co y Mn z O2) and lithium nickel cobalt aluminum oxide (LiNi x Co y Al z O2) materials typically have a nickel content greater than 50%. Due to their high nickel content, nickel-rich ternary cathode materials offer high reversible discharge specific capacity, but poor cycling performance, primarily due to unstable surface properties. Residual lithium on the surface of nickel-rich ternary materials, when exposed to air, easily forms impurities such as Li2CO3 and LiOH, increasing the alkalinity of the material particles. This not only complicates the subsequent coating process, but the insulating impurities also increase the interfacial impedance of the material.

[0003] As an excellent additive for ternary materials, cobalt hydroxide can react with the residual alkali on the surface of nickel-rich ternary materials during the secondary sintering process of the materials to form an active material coating layer. This not only reduces the contact area between the material and the electrolyte, reduces side reactions and surface deterioration, but also provides additional electrochemical properties, increases the charge and discharge capacity of the material, and reduces the interfacial impedance of the material.

[0004] Currently, cobalt hydroxide as a ternary material additive must meet the following requirements: 1. Co ≥ 62%; 2. Particle size D50 < 1 μm, and good sample dispersion. The smaller the cobalt hydroxide primary particles, the more conducive it is to forming a uniform coating layer, reducing contact between the ternary material and the electrolyte. Chinese invention patent application CN110233263A uses a pH of 9-12.5 as the synthesis conditions for cobalt hydroxide nanosheets. However, at this pH, cobalt hydroxide primary particles are typically 300-600 nm. Such larger primary particles can adversely affect the coating of the ternary material. During the coating process, the cobalt hydroxide nanosheets will be misaligned and stacked, resulting in incomplete coating of the ternary material surface and the presence of gaps. This is not conducive to the removal of residual alkali on the surface and will lead to performance degradation. For smaller primary particles, such as <150nm, harsh synthesis conditions are usually required. Chinese invention patent application document CN106186085A uses extremely low concentration of cobalt metal liquid to obtain cobalt hydroxide nanosheets with a particle size of 100-400nm, which undoubtedly leads to higher costs and lower reaction rates, which is not conducive to large-scale production.

[0005] Based on the above research, there is an urgent need to develop a method for synthesizing ultrafine cobalt hydroxide nanosheets under common pH conditions.

[0006] Summary of the Invention

[0007] The present disclosure aims to address at least one of the technical problems existing in the above-mentioned prior art. To this end, the present disclosure provides a method for preparing cobalt hydroxide nanosheets and its application. The method can produce cobalt hydroxide nanosheets with primary particles smaller than 150 nm at a pH of 12-13 and is suitable for large-scale production.

[0008] According to one aspect of the present disclosure, a method for preparing cobalt hydroxide nanosheets is proposed, comprising the following steps:

[0009] S1: mixing the first liquid caustic soda with water to prepare a base liquid, and adding a cobalt salt solution, a second liquid caustic soda, and a silane coupling agent to the base liquid in parallel under a protective atmosphere to react, controlling the reaction pH to 12-13 during the parallel flow process. After the reaction is completed, a cobalt hydroxide slurry is obtained;

[0010] S2: performing solid-liquid separation on the cobalt hydroxide slurry, and washing, drying, and crushing the obtained solid material to obtain the cobalt hydroxide nanosheets.

[0011] In some embodiments of the present disclosure, in step S1, the concentration of the first liquid caustic soda and / or the second liquid caustic soda is 100-400 g / L, and the pH of the base liquid is 11-13.

[0012] In some embodiments of the present disclosure, in step S1, the reaction is carried out in a reactor, the bottom liquid accounts for 30%-50% of the volume of the reactor, and the volume of the reactor is 50-10000L.

[0013] In some embodiments of the present disclosure, in step S1, the co-current process is accompanied by stirring, and the stirring speed is 200-700 r / min.

[0014] In some embodiments of the present disclosure, in step S1, the cobalt salt in the cobalt salt solution is at least one of cobalt chloride, cobalt sulfate or cobalt nitrate.

[0015] In some embodiments of the present disclosure, in step S1, the concentration of cobalt ions in the cobalt salt solution is 90-130 g / L, and the flow rate of the cobalt salt solution is 10-1200 L / h.

[0016] In some embodiments of the present disclosure, in step S1, the flow rate of the second liquid caustic soda is 5-800 L / h.

[0017] In some embodiments of the present disclosure, in step S1, the silane coupling agent is at least one of trimethoxymethylsilane, triethoxymethylsilane, or diethoxydiaminosilane.

[0018] In some embodiments of the present disclosure, in step S1, the amount of the silane coupling agent added is 0.05 wt% to 0.5 wt% of the target yield of cobalt hydroxide.

[0019] In some embodiments of the present disclosure, in step S1, the reaction temperature is 25-50° C.; and the reaction time is 1-5 h.

[0020] In some embodiments of the present disclosure, in step S1, a protective atmosphere is continuously introduced during the reaction, and the flow rate of the protective atmosphere is 0.5-30 L / min. Optionally, the protective atmosphere is one of nitrogen, helium, or argon.

[0021] In some embodiments of the present disclosure, in step S1, after the reaction is completed, the step further includes adding a first antioxidant to the reaction material for aging. After the raw materials are prepared, some dissolved oxygen is present in the system. During the precipitation process, the dissolved oxygen partially oxidizes the divalent cobalt hydroxide to trivalent cobalt, causing the material color to change from pink to gray. Adding the antioxidant for aging can reduce the oxidized trivalent cobalt to divalent cobalt, thereby ensuring sample purity.

[0022] In some embodiments of the present disclosure, in step S1, the aging time is 0.1-3 hours.

[0023] In some embodiments of the present disclosure, in step S1, the first antioxidant is at least one of ascorbic acid, citric acid, hydrazine hydrate, sodium borohydride, sodium sulfite or oleic acid.

[0024] In some embodiments of the present disclosure, in step S1, the amount of the first antioxidant added is 0.05-0.25 g per liter of the reaction material.

[0025] In some embodiments of the present disclosure, in step S2, a second antioxidant is added to the wash water during the washing process; the second antioxidant is at least one of ascorbic acid, citric acid, hydrazine hydrate, sodium borohydride, sodium sulfite, or oleic acid; and the amount of the second antioxidant added is 0.1-3 g / L. Adding the antioxidant during washing can reduce partially oxidized cobalt hydroxide, thereby ensuring sample purity.

[0026] In some embodiments of the present disclosure, in step S2, the equipment used for drying is one of a vacuum oven, a nitrogen furnace, a spray dryer or a flash dryer, wherein the drying temperature of the vacuum oven and / or the nitrogen furnace is 80-110°C, and the inlet air temperature of the spray dryer and / or the flash dryer is 150-180°C.

[0027] In some embodiments of the present disclosure, in step S2, the equipment used for the pulverization is a disc mill, a jet mill or a Chinese herbal medicine pulverizer.

[0028] In some embodiments of the present disclosure, in step S2, the washing is performed 1-3 times.

[0029] In some embodiments of the present disclosure, in step S2, the primary particle size of the cobalt hydroxide nanosheets is less than 150 nm.

[0030] The present disclosure also provides a coated ternary cathode material, which is obtained by coating the ternary cathode material with the cobalt hydroxide nanosheets obtained by the preparation method.

[0031] According to the embodiments of the present disclosure, there are at least the following beneficial effects:

[0032] When the pH is in the range of 12-13, as the experiment progresses, the cobalt salt solution enters the reactor and comes into contact with the liquid caustic soda, resulting in a low local supersaturation. This causes the cobalt hydroxide to undergo both nucleation and growth. Consequently, reported inventions all show a tendency for cobalt hydroxide to grow, resulting in larger primary particles ranging from 300-600 nm. Furthermore, while cobalt hydroxide nanosheets with smaller primary particles can be synthesized using low-concentration cobalt salt solutions, this reduces synthesis efficiency and increases production costs. This process first adds a silane coupling agent as a surface inhibitor. This reduces primary particle growth even at a synthesis pH of 12-13, resulting in cobalt hydroxide nanosheets with primary particles less than 150 nm. This, in turn, forms a uniform coating on the surface of the positive electrode material, improving electrochemical performance. The surface inhibitor works by mixing the cobalt salt solution and the liquid caustic soda in the reactor during a cocurrent flow process, forming cobalt hydroxide nanosheets. At a pH of 12-13, the nanosheets come into contact with the reaction solution, where they grow slowly due to the influence of cobalt ions. This can lead to excessively large primary particles during prolonged liquid infusion. Here, a silane coupling agent is added as a surface inhibitor. During the liquid feeding process, the silane oxy group of the coupling agent will connect with the hydroxyl group on the surface of the cobalt hydroxide to form a hydrogen bond, and the organic functional group will be exposed as a hydrophobic group. The coupling agent is like a layer of water-proof film wrapped around the surface of the material, reducing the contact between the surface of the cobalt hydroxide and the cobalt ions in the mother liquor, and inhibiting the growth of primary particles from the root, and finally obtaining cobalt hydroxide nanosheets with primary particles less than 150nm. In addition, when the synthesis pH is lower than 12, the growth rate of cobalt hydroxide increases, which will cause the primary particle nanosheets to increase; when the synthesis pH is greater than 13, the cobalt hydroxide primary particle nanosheets will not decrease much, and a higher pH will lead to an increase in the amount of liquid alkali, resulting in an increase in the impurity content (Na + / K +) increases, increasing the washing cost, so the present disclosure controls the reaction pH at 12-13. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present disclosure is further described below with reference to the accompanying drawings and embodiments, wherein:

[0034] FIG1 is a SEM image of cobalt hydroxide nanosheets of Example 1 of the present disclosure;

[0035] FIG2 is a SEM image of cobalt hydroxide nanosheets of Example 2 of the present disclosure;

[0036] FIG3 is a SEM image of cobalt hydroxide nanosheets of Example 3 of the present disclosure;

[0037] FIG4 is a SEM image of cobalt hydroxide nanosheets of Example 4 of the present disclosure;

[0038] FIG5 is an SEM image of cobalt hydroxide nanosheets of Comparative Example 1 of the present disclosure;

[0039] FIG6 is an XRD pattern of cobalt hydroxide nanosheets of Examples 1-4 and Comparative Example 1 of the present disclosure;

[0040] FIG7 is an SEM image of a high-nickel ternary material coated with cobalt hydroxide nanosheets in Experimental Example 1 of the present disclosure;

[0041] FIG8 is an SEM image of the cobalt hydroxide nanosheet-coated high-nickel ternary material in Experimental Example 2 of the present disclosure. DETAILED DESCRIPTION

[0042] The following will clearly and completely describe the concept and technical effects of the present disclosure in conjunction with embodiments to fully understand the purpose, features and effects of the present disclosure.

[0043] Example 1

[0044] This example prepares a cobalt hydroxide nanosheet, and the specific process is as follows:

[0045] (1) 250 L of 400 g / L liquid caustic soda was mixed with pure water and injected into a 600 L reactor as a bottom liquid. The bottom liquid occupied about 41.67% of the volume of the reactor, and the pH of the bottom liquid was controlled at 12.4. Stirring was started (speed of 600 r / min). At 35° C., a cobalt chloride solution with a cobalt ion concentration of 130 g / L (flow rate of 60 L / h), a liquid caustic soda with a concentration of 400 g / L (flow rate of 24 L / h) and triethoxymethylsilane with a target yield of cobalt hydroxide of 0.05 wt% were added to the reactor in parallel to react. The liquid was fed for 3 h. During the parallel flow process, a nitrogen atmosphere (nitrogen flow rate of 10 L / min) was maintained and the reaction pH was controlled at 12.5. After the liquid was fed, ascorbic acid with a mother liquor volume of 0.05 g / L was added and aged for 2 h to obtain a cobalt hydroxide slurry.

[0046] (2) The cobalt hydroxide slurry was filtered using a plate and frame filter press, and the filtered material was put into a slurry washing tank for slurry washing. During the slurry washing process, 0.2 g / L ascorbic acid was added to the washing water. After washing, the slurry was filtered and the filter cake was dried in a vacuum oven at 90°C.

[0047] (3) The dried filter cake is crushed using a Chinese herbal medicine crusher to obtain ultra-small particle size cobalt hydroxide nanosheets.

[0048] The cobalt hydroxide nanosheets obtained in this example are shown in Figure 1. The primary particle size is 90-110 nm, and the nanosheet thickness is approximately 30 nm, indicating a relatively small particle size. The XRD structure, shown in Figure 6, demonstrates a pure phase structure of cobalt hydroxide. Elemental content analysis revealed a cobalt content of 62.51%.

[0049] Example 2

[0050] This example prepares a cobalt hydroxide nanosheet, and the specific process is as follows:

[0051] (1) 250 L of 400 g / L liquid caustic soda and pure water were mixed and injected into a 600 L reactor as a bottom liquid. The bottom liquid occupied about 41.67% of the volume of the reactor. The pH of the bottom liquid was controlled at 12.4. Stirring was started (speed of 600 r / min). At 35° C., a cobalt chloride solution with a cobalt ion concentration of 130 g / L (flow rate of 60 L / h), a liquid caustic soda with a concentration of 400 g / L (flow rate of 24 L / h) and triethoxymethylsilane with a target yield of cobalt hydroxide of 0.05 wt% were added to the reactor in parallel to react. The liquid was added for 3 h. During the parallel flow process, a nitrogen atmosphere was maintained (nitrogen flow rate of 10 L / min) and the reaction pH was controlled at 12.5. After the liquid was added, no aging was performed to obtain a cobalt hydroxide slurry.

[0052] (2) The cobalt hydroxide slurry was filtered using a plate and frame filter press, and the filtered material was put into a slurry washing tank for slurry washing. During the slurry washing process, 0.2 g / L ascorbic acid was added to the washing water. After washing, the slurry was filtered and the filter cake was dried in a vacuum oven at 90°C.

[0053] (3) The dried filter cake is crushed using a Chinese herbal medicine crusher to obtain ultra-small particle size cobalt hydroxide nanosheets.

[0054] The cobalt hydroxide nanosheets obtained in this example are shown in Figure 2. The primary particle size is 90-110 nm, and the nanosheet thickness is approximately 30 nm, indicating a relatively small particle size. The XRD structure, shown in Figure 6, demonstrates a pure phase structure of cobalt hydroxide. Elemental content analysis revealed a cobalt content of 62.45%.

[0055] Example 3

[0056] This example prepares a cobalt hydroxide nanosheet, and the specific process is as follows:

[0057] (1) 250 L of 400 g / L liquid caustic soda was mixed with pure water and injected into a 600 L reactor as a bottom liquid. The bottom liquid occupied about 41.67% of the volume of the reactor, and the pH of the bottom liquid was controlled at 12.4. Stirring was started (speed of 600 r / min). At 35° C., a cobalt chloride solution with a cobalt ion concentration of 130 g / L (flow rate of 60 L / h), a liquid caustic soda with a concentration of 400 g / L (flow rate of 24 L / h), and triethoxymethylsilane with a target yield of cobalt hydroxide of 0.1 wt% were added to the reactor in parallel to react. The liquid was added for 3 h. During the parallel flow process, a nitrogen atmosphere (nitrogen flow rate of 10 L / min) was maintained and the reaction pH was controlled at 12.5. After the liquid was added, no aging was performed to obtain a cobalt hydroxide slurry.

[0058] (2) The cobalt hydroxide slurry was filtered using a plate and frame filter press, and the filtered material was put into a slurry washing tank for slurry washing. During the slurry washing process, 0.2 g / L ascorbic acid was added to the washing water. After washing, the slurry was filtered and the filter cake was dried in a vacuum oven at 90°C.

[0059] (3) The dried filter cake is crushed using a Chinese herbal medicine crusher to obtain ultra-small particle size cobalt hydroxide nanosheets.

[0060] The cobalt hydroxide nanosheets obtained in this example are shown in Figure 3. The primary particle size is 80-90 nm, and the nanosheet thickness is approximately 30 nm, indicating a relatively small particle size. The XRD structure, shown in Figure 6, demonstrates a pure phase structure of cobalt hydroxide. Elemental content analysis revealed a cobalt content of 62.64%.

[0061] Example 4

[0062] This example prepares a cobalt hydroxide nanosheet, and the specific process is as follows:

[0063] (1) 250 L of 400 g / L liquid caustic soda was mixed with pure water and injected into a 600 L reactor as a bottom liquid. The bottom liquid occupied about 41.67% of the volume of the reactor, and the pH of the bottom liquid was controlled at 12.4. Stirring was started (speed of 600 r / min). At 35° C., a cobalt chloride solution with a cobalt ion concentration of 130 g / L (flow rate of 60 L / h), a liquid caustic soda with a concentration of 400 g / L (flow rate of 24 L / h), and triethoxymethylsilane with a target yield of cobalt hydroxide of 0.5 wt% were added to the reactor in parallel to react. The liquid was added for 3 h. During the parallel flow process, a nitrogen atmosphere (nitrogen flow rate of 10 L / min) was maintained and the reaction pH was controlled at 12.5. After the liquid was added, no aging was performed to obtain a cobalt hydroxide slurry.

[0064] (2) The cobalt hydroxide slurry was filtered using a plate and frame filter press, and the filtered material was put into a slurry washing tank for slurry washing. During the slurry washing process, 0.2 g / L ascorbic acid was added to the washing water. After washing, the slurry was filtered and the filter cake was dried in a vacuum oven at 90°C.

[0065] (3) The dried filter cake is crushed using a Chinese herbal medicine crusher to obtain ultra-small particle size cobalt hydroxide nanosheets.

[0066] The cobalt hydroxide nanosheets obtained in this example are shown in Figure 4. The primary particle size is 80-90 nm, and the nanosheet thickness is approximately 30 nm, indicating a relatively small particle size. The XRD structure, shown in Figure 6, demonstrates a pure phase structure of cobalt hydroxide. Elemental content analysis revealed a cobalt content of 62.35%.

[0067] Comparative Example 1

[0068] This comparative example prepares a cobalt hydroxide nanosheet. The difference from Example 1 is that no silane coupling agent is added. The specific process is as follows:

[0069] (1) 250 L of 400 g / L liquid caustic soda was mixed with pure water and injected into a 600 L reactor as a bottom liquid. The bottom liquid occupied about 41.67% of the volume of the reactor and the pH of the bottom liquid was controlled at 12.4. Stirring was started (rotation speed of 600 r / min). A cobalt chloride solution with a cobalt ion concentration of 130 g / L (flow rate of 60 L / h) and a liquid caustic soda with a concentration of 400 g / L (flow rate of 24 L / h) were added to the reactor in parallel at 35° C. for reaction. The liquid was added for 3 h. During the parallel flow process, a nitrogen atmosphere (nitrogen flow rate of 10 L / min) was maintained and the reaction pH was controlled at 12.5. After the liquid was added, ascorbic acid with a mother liquor volume of 0.05 g / L was added and aged for 2 h to obtain a cobalt hydroxide slurry.

[0070] (2) The cobalt hydroxide slurry was filtered using a plate and frame filter press, and the filtered material was put into a slurry washing tank for slurry washing. During the slurry washing process, 0.2 g / L ascorbic acid was added to the washing water. After washing, the slurry was filtered and the filter cake was dried in a vacuum oven at 90°C.

[0071] (3) The dried filter cake is crushed using a Chinese herbal medicine crusher to obtain cobalt hydroxide nanosheets.

[0072] The cobalt hydroxide nanosheets obtained in this comparative example are shown in Figure 5 . The primary particle size is 200-400 nm, and the nanosheet thickness is 30-40 nm, indicating a relatively large particle size. The XRD structure, shown in Figure 6 , demonstrates a pure phase structure of cobalt hydroxide. Elemental content analysis revealed a cobalt content of 62.35%.

[0073] XRD peak intensity ratio of Figure 5:

[0074] I(001) / I(100) can represent the ratio of the surface area to the side area of ​​the primary particle nanosheet. Since the (001) grain size of Examples 1, 2, 3, and 4 is close to that of Comparative Example 1 [the (001) grain size can represent the thickness of the primary particle], the side areas of the four samples are not much different. If I(001) / I(100) is smaller, the surface area of ​​the nanosheet is smaller. Among them, Examples 2, 3, and 4 show a smaller peak intensity ratio, and Example 1 is also much lower than Comparative Example 1. This shows that the silane coupling agent can effectively reduce the growth of cobalt hydroxide primary particles, thereby obtaining cobalt hydroxide nanosheets with ultra-small particle size.

[0075] Table 1 Peak intensity ratio of cobalt hydroxide in Example and Comparative Example

[0076] Test Example 1

[0077] This test example prepared a high-nickel ternary material coated with cobalt hydroxide nanosheets. The specific process is as follows:

[0078] (1) Take 500g of the cobalt hydroxide nanosheets and LiNi 0.92 Co 0.07 Mn 0.01 50 kg of O2 high-nickel ternary material was mixed in a three-dimensional mixer for 60 minutes.

[0079] (2) The mixed material was placed in a chamber furnace with oxygen and sintered at a temperature of 600°C for 8 hours. After sintering, the mixture was naturally cooled and sieved with a 300-mesh sieve to obtain LiNi coated with cobalt hydroxide nanosheets. 0.92 Co 0.07 Mn 0.01 O2 was kept aside, and the surface residual lithium of the coated and uncoated samples was tested.

[0080] (3) LiNi coated with cobalt hydroxide nanosheets 0.92 Co 0.07 Mn 0.01 O2 materials and uncoated LiNi 0.92 Co 0.07 Mn 0.01 O2 material was used as the positive electrode and lithium sheet as the negative electrode to assemble button batteries. The first-week charge and discharge capacity was tested at a current density of 0.1C on a Blue Electric test cabinet, and the capacity retention rate was also tested at a current density of 1C.

[0081] The coating effect of this test example is shown in Figure 7. The cobalt hydroxide nanosheets are evenly distributed on the surface of the ternary material, showing a good coating effect. The residual lithium detection is shown in Table 2. The residual lithium content in the uncoated sample is high. After coating with cobalt hydroxide, the residual lithium content is reduced. The electrochemical performance is shown in Table 3. The cobalt hydroxide coated sample exhibits a high first-cycle coulombic efficiency and enhanced cycle performance. The capacity retention rate is 93.1% after 25 cycles at a current of 1C.

[0082] Table 2 Test Example 1 Residual Lithium Detection

[0083] Table 3 Electrochemical performance of Test Example 1

[0084] Test Example 2

[0085] This test example prepared a high-nickel ternary material coated with cobalt hydroxide nanosheets. The specific process is as follows:

[0086] (1) Take 500g of cobalt hydroxide nanosheets and LiNi 0.92 Co 0.07 Mn 0.01 50 kg of O2 high-nickel ternary material was mixed in a three-dimensional mixer for 60 minutes.

[0087] (2) The mixed material was placed in a chamber furnace with oxygen and sintered at a temperature of 600°C for 8 hours. After sintering, the mixture was naturally cooled and sieved with a 300-mesh sieve to obtain LiNi coated with cobalt hydroxide nanosheets. 0.92 Co 0.07 Mn 0.01 O2 was kept aside, and the surface residual lithium of the coated and uncoated samples was tested.

[0088] (3) LiNi coated with cobalt hydroxide nanosheets 0.92 Co 0.07 Mn 0.01 O2 materials and uncoated LiNi 0.92 Co 0.07 Mn 0.01 O2 material was used as the positive electrode and lithium sheet as the negative electrode to assemble button batteries. The first-week charge and discharge capacity was tested at a current density of 0.1C on a Blue Electric test cabinet, and the capacity retention rate was also tested at a current density of 1C.

[0089] The coating effect of this test example is shown in Figure 8. The larger cobalt hydroxide nanosheets are misplaced and stacked on the surface of the ternary material, resulting in agglomeration and poor coating effect. The residual lithium detection is shown in Table 4. The residual lithium content in the uncoated sample is high. After coating with cobalt hydroxide, the residual lithium content is reduced, but it is still higher than that of the coated sample in Test Example 1. The electrochemical performance is shown in Table 5. The cobalt hydroxide coated sample exhibits a higher first-week coulombic efficiency and enhanced cycle performance. The capacity retention rate is 91.5% after 25 cycles at a current of 1C, which is lower than that of the coated sample in Test Example 1.

[0090] Table 4 Test Example 2 Residual Lithium Detection

[0091] Table 5 Electrochemical performance of Test Example 2

Claims

1. A method for preparing cobalt hydroxide nanosheets, characterized in that: The following steps are involved: S1: mixing the first liquid alkali with water to prepare a base liquid, and adding a cobalt salt solution, a second liquid alkali and a silane coupling agent to the base liquid in parallel under a protective atmosphere to react, controlling the reaction pH to 12-13 during the parallel flow process, and obtaining a cobalt hydroxide slurry after the reaction is completed; S2: performing solid-liquid separation on the cobalt hydroxide slurry, and washing, drying and crushing the obtained solid material to obtain the cobalt hydroxide nanosheets.

2. The preparation method according to claim 1, characterized in that: In step S1, the concentration of the first liquid caustic soda and / or the second liquid caustic soda is 100-400 g / L, and the pH of the base liquid is 11-13.

3. The preparation method according to claim 1, characterized in that: In step S1, the reaction is carried out in a reactor, the bottom liquid accounts for 30%-50% of the volume of the reactor, and the volume of the reactor is 50-10000L.

4. The preparation method according to claim 1, characterized in that: In step S1, the parallel flow process is accompanied by stirring, and the stirring speed is 200-700 r / min.

5. The preparation method according to claim 1, characterized in that: In step S1, the cobalt salt in the cobalt salt solution is at least one of cobalt chloride, cobalt sulfate or cobalt nitrate.

6. The preparation method according to claim 1, characterized in that: In step S1, the concentration of cobalt ions in the cobalt salt solution is 90-130 g / L, and the flow rate of the cobalt salt solution is 10-1200 L / h.

7. The preparation method according to claim 1, characterized in that: In step S1, the flow rate of the second liquid caustic soda is 5-800 L / h.

8. The preparation method according to claim 1, characterized in that: In step S1, the silane coupling agent is at least one of trimethoxymethylsilane, triethoxymethylsilane or diethoxydiaminosilane.

9. The preparation method according to claim 1, characterized in that: In step S1, the amount of the silane coupling agent added is 0.05wt%-0.5wt% of the target yield of cobalt hydroxide.

10. The preparation method according to claim 1, characterized in that: In step S1, the reaction temperature is 25-50°C; the reaction time is 1-5h.

11. The preparation method according to claim 1, characterized in that: In step S1, a protective atmosphere is continuously introduced during the reaction, and the flow rate of the protective atmosphere is 0.5-30 L / min.

12. The preparation method according to claim 1, characterized in that: In step S1, after the reaction is completed, the step further includes adding a first antioxidant to the reaction material for aging.

13. The preparation method according to claim 12, characterized in that: In step S1, the aging time is 0.1-3h.

14. The preparation method according to claim 12, characterized in that: In step S1, the first antioxidant is at least one of ascorbic acid, citric acid, hydrazine hydrate, sodium borohydride, sodium sulfite or oleic acid.

15. The preparation method according to claim 12, characterized in that: In step S1, the amount of the first antioxidant added is 0.05-0.25 g per liter of the reaction material.

16. The preparation method according to claim 1, characterized in that: In step S2, a second antioxidant is added to the washing water during the washing process; the second antioxidant is at least one of ascorbic acid, citric acid, hydrazine hydrate, sodium borohydride, sodium sulfite or oleate acid; and the added amount of the second antioxidant is 0.1-3 g / L.

17. The preparation method according to claim 1, characterized in that: In step S2, the equipment used for drying is one of a vacuum oven, a nitrogen furnace, a spray dryer or a flash dryer, wherein the drying temperature of the vacuum oven and / or the nitrogen furnace is 80-110°C, and the air inlet temperature of the spray dryer and / or the flash dryer is 150-180°C.

18. The preparation method according to claim 1, characterized in that: In step S2, the equipment used for pulverizing is a disc mill, a jet mill or a Chinese herbal medicine pulverizer.

19. The preparation method according to claim 1, characterized in that: In step S2, the primary particle size of the cobalt hydroxide nanosheets is less than 150 nm.

20. A coated ternary positive electrode material, characterized in that: The ternary positive electrode material is prepared by coating the cobalt hydroxide nanosheets prepared by the preparation method according to any one of claims 1 to 19.

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