Water-based positive electrode slurry, water-based positive electrode plate and lithium ion secondary battery

Abstract

The invention discloses water-based positive electrode slurry, a water-based positive electrode plate and a lithium ion secondary battery, and relates to the field of lithium batteries. The water-based positive electrode slurry comprises a water-based positive electrode active material, the water-based positive electrode active material comprises a positive electrode active material and a coating layer coating the surface of the positive electrode active material, and the coating layer comprises an alkane compound containing borate radicals and / or graphene; the water-based positive electrode slurry further comprises an additive, and the additive comprises sodium dodecyl benzene sulfonate and / or sodium polyacrylate. The hydrophobic coating layer is formed on the surface of the positive electrode active material to block contact of water molecules, the structure deterioration of the positive electrode active material is avoided, the conductivity and lithium ion mobility of the positive electrode active material are improved, the coating layer and the additive are subjected to hydrophobic combination, the positive electrode active material is uniformly dispersed in the aqueous slurry, coagulation is avoided, and the lithium ion conductivity is improved. And the charge-discharge efficiency and the cycle performance of the prepared lithium ion secondary battery are effectively improved.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and particularly to an aqueous positive electrode slurry, an aqueous positive electrode sheet, and a lithium ion secondary battery. Background Art

[0002] In the stirring process of lithium ion positive electrode materials, compared with the organic slurry process, the water-based slurry process is favored by many lithium battery factories because it does not involve solvent recovery and environmental pollution problems, and has advantages such as low price and less environmental restriction on homogenization.

[0003] However, when using the water-based slurry process to prepare the positive electrode sheet of the battery, there are many influences on the positive electrode active material. First, in the existing preparation methods of water-based slurries, the density of the positive electrode active material is relatively large, the mass of substances per unit volume is also large, and its water solubility is poor, which makes it difficult for solid substances such as conductive agents and positive electrode active substances to be evenly dispersed and easily agglomerate, resulting in poor consistency, low stability, and easy sedimentation of the aqueous positive electrode slurry. The electrode sheets prepared therefrom are prone to uneven coating surface density and poor conductivity of the material electrode sheets, which in turn leads to low specific capacity and first efficiency of battery charge and discharge, and poor cycle performance. Second, water as a stirring solvent will cause the surface structure of the positive electrode active material to deteriorate and the material impedance to increase, thus affecting the cycle performance of the material. When the surface of the material reacts with water, Li+ / H+ ion exchange will occur, resulting in lithium dissolution, increasing the pH value of the slurry. During the cycling process of the material, hydrogen will promote the release of water during the thermal decomposition process, and hydrofluoric acid generated by the reaction of water with the electrolyte will cause the material performance to deteriorate rapidly.

[0004] Therefore, the above problems limit the application scope of water-based positive electrode slurries in lithium ion batteries, and it is urgent to find effective solutions to the problems that the surface of the water-based positive electrode active material is prone to side reactions with water and the positive electrode active material is unevenly dispersed in the water-based slurry system. Summary of the Invention

[0005] The present invention provides an aqueous positive electrode slurry, an aqueous positive electrode sheet, and a lithium ion secondary battery to solve the problem that the positive electrode active material in the aqueous slurry is prone to react with water, improve the electronic conductivity and lithium ion mobility of the active material, and at the same time promote the dispersion of the positive electrode active material in the aqueous slurry, and improve the specific capacity and cycle capacity retention rate of the prepared lithium ion secondary battery.

[0006] To solve the above technical problems, one of the purposes of the present invention is to provide an aqueous positive electrode slurry, which includes an aqueous positive electrode active material; the aqueous positive electrode active material includes a positive electrode active material and a coating layer coated on the surface of the positive electrode active material, the coating layer includes an alkane compound containing a borate group and / or graphene, and the ratio of the median value of the sheet diameter of the graphene to the particle size Dv50 of the positive electrode active material is less than 0.3.

[0007] This application prepares an aqueous positive electrode slurry as an aqueous system. To avoid the easy occurrence of Li+ / H+ ion exchange reaction due to the contact between the positive electrode active material and water, resulting in lithium dissolution, a hydrophobic coating layer is formed on the surface of the lithium cobaltate material, which can block the contact between the surface of the positive electrode active material and water molecules and prevent the deterioration of the structure of the positive electrode active material. Graphene has high conductivity and can reduce the resistivity of the aqueous positive electrode active material. Controlling the ratio of the sheet diameter of the coated graphene to the particle diameter of the positive electrode active material to be less than 0.3 can avoid problems such as uneven coating and poor coating adhesion between graphene and the positive electrode active material. Moreover, the borate group at the head of the alkane compound containing borate in the coating layer can react with the harmful residual lithium on the surface of the positive electrode active material to form lithium borate, and lithium borate is a good lithium ion conductor, which can reduce the resistance, improve the conductivity and lithium ion mobility of the positive electrode active material, and improve the specific capacity and cycle capacity retention rate of the prepared lithium ion secondary battery.

[0008] As a preferred solution, the median sheet diameter of the graphene is 2 - 4 μm.

[0009] Those skilled in the art should understand that the median sheet diameter refers to arranging the graphene in ascending order of sheet diameter, and the median sheet diameter is the sheet diameter of the graphene when the cumulative volume ratio reaches 50% when arranged in the middle. Therefore, the median sheet diameter of the graphene being 2 - 4 μm means that the sheet diameter of 50% of the graphene is less than or equal to 2 - 4 μm.

[0010] As a preferred solution, the particle diameter Dv50 of the positive electrode active material is 12 - 15 μm.

[0011] This application controls the particle diameter Dv50 of the positive electrode active material to be 12 - 15 μm and selects graphene with a median sheet diameter of 2 - 4 μm. Since the particle diameter of the positive electrode active material is controlled within the above range and the sheet diameter of the graphene is smaller than that of the positive electrode active material, based on the particle diameter matching of the two, a dense coating layer can be formed on the surface of the positive electrode active material, making the coating layer fit tightly, achieving high-density coating, effectively blocking the penetration of water molecules, avoiding the problem that the particle diameter of the graphene is too large to form a tight coating on the surface of the positive electrode active material, and avoiding the problem of uneven coating.

[0012] As a preferred solution, the mass ratio of the positive electrode active material to the coating layer in the aqueous positive electrode material is (98 - 99.6):(0.4 - 2).

[0013] As a preferred solution, the mass ratio of the positive electrode active material to the coating layer in the aqueous positive electrode material is (99.2 - 99.4):(0.6 - 0.8).

[0014] As a preferred solution, the coating layer of the aqueous positive electrode active material includes graphene and an alkane compound containing borate with a mass ratio of (1 - 4):(1 - 4).

[0015] As a preferred embodiment, the mass ratio of the borate group-containing alkane compound to graphene is (3 - 4):(1 - 2).

[0016] The coating layer on the surface of the positive electrode active material of the present application uses the borate group-containing alkane compound and graphene in the above (3 - 4):(1 - 2) ratio. The borate group of the borate group-containing alkane compound reacts with the harmful residual lithium on the surface of the positive electrode active material to generate lithium ion conductor lithium borate, and cooperates with graphene with excellent conductivity for coating, which can simultaneously improve the lithium ion mobility and electron conductivity on the surface of the positive electrode active material, thereby reducing the electrode polarization phenomenon of the positive electrode active material, reducing the resistance value of the battery, and thus improving the first charge-discharge efficiency and cycle performance of the material.

[0017] As a preferred embodiment, the borate group-containing alkane compound is n-tetradecyl borate.

[0018] As a preferred embodiment, the positive electrode active material includes at least one of lithium cobaltate, lithium nickel cobalt manganate, lithium iron phosphate, and lithium manganate.

[0019] As a preferred embodiment, the graphene is reduced graphene.

[0020] In the present application, the median value of the sheet diameter of the graphene is preferably 2 - 4 μm, and the particle size Dv50 of the positive electrode active material is preferably 12 - 15 μm. This is because the graphene with the above sheet diameter can be coated more uniformly and densely on the surface of the positive electrode active material. And reduced graphene is preferably used. Under the action of a strong oxidant, oxygen-containing active functional groups are introduced onto the graphene sheet to obtain graphene oxide. By using chemical, thermal, or electrochemical methods, and under the action of a reducing agent (such as hydroiodic acid, sodium borohydride, hydrazine hydrate, etc.), some of the oxygen-containing groups on the surface of graphene oxide are reduced to generate reduced graphene. Therefore, the reduction degree of reduced graphene is high and the conductivity is better, which can further improve the electron conductivity of the positive electrode sheet.

[0021] As a preferred embodiment, the preparation method of the aqueous positive electrode active material is: mixing and grinding the positive electrode active material and the coating layer raw materials in proportion, and then sintering for 1 - 10 h under an inert gas or nitrogen atmosphere and at a temperature of 200 - 900 °C to obtain the aqueous positive electrode active material.

[0022] As a preferred embodiment, the sintering temperature is any one or any range value of two of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, and is preferably 500 - 800 °C.

[0023] As a preferred embodiment, the sintering time is any one or any range value of two of 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, preferably 4-8h.

[0024] As a preferred embodiment, the aqueous positive electrode paste further comprises an additive, and the additive comprises sodium dodecylbenzenesulfonate and / or sodium polyacrylate.

[0025] The aqueous positive electrode active material in the aqueous positive electrode paste of the present application forms a hydrophobic coating layer on the surface of the positive electrode active material, which can block the reaction between water molecules and the surface of the positive electrode active material and cause structural deterioration. The coating layer contains hydrophobic groups. At the same time, an additive is added. The additive is uniformly dispersed in the aqueous system by using hydrophilic groups. The hydrophobic groups of the additive can undergo hydrophobic binding with the hydrophobic groups of the coating layer of the aqueous positive electrode active material, uniformly disperse the positive electrode active material in the aqueous solution system, play the effect of delaying the sedimentation of the positive electrode active material, ensure the uniform coating density of the paste on the positive electrode plate, and improve the charge and discharge specific capacity and cycle performance of the prepared lithium ion secondary battery.

[0026] As a preferred embodiment, the mass ratio of the aqueous positive electrode active material to the additive is (90-96):(0.2-1.5).

[0027] As a preferred embodiment, the additive comprises sodium dodecylbenzenesulfonate and sodium polyacrylate with a mass ratio of (3-7):(3-7).

[0028] As a preferred embodiment, the mass ratio of sodium dodecylbenzenesulfonate to sodium polyacrylate is (4-6):(4-6).

[0029] The additive of the present application uses sodium dodecylbenzenesulfonate and sodium polyacrylate to jointly promote the dispersibility of the aqueous positive electrode active material in the aqueous solvent system. When the ratio of the above two additives is controlled within the above range, the dispersibility of the aqueous positive electrode active material in the aqueous solvent is better.

[0030] As a preferred embodiment, the aqueous positive electrode paste comprises a positive electrode conductive agent, a positive electrode binder and water, and the mass ratio of the aqueous positive electrode active material, the positive electrode conductive agent, the additive, the positive electrode binder and water is (90-96):(2-4):(0.2-1.5):(2-5):(60-70).

[0031] As a preferred embodiment, the positive electrode conductive agent is at least one of conductive carbon black, carbon nanotubes, acetylene black, and graphene.

[0032] As a preferred embodiment, the positive electrode conductive agent comprises conductive carbon black and carbon nanotubes with a mass ratio of (2-3):(0.2-0.8).

[0033] As a preferred embodiment, the positive electrode binder is at least one of polyacrylate, polyvinylidene fluoride, polytetrafluoroethylene, and styrene-butadiene rubber.

[0034] To solve the above technical problems, a second object of the present invention is to provide an aqueous positive electrode sheet, which includes a current collector and a positive electrode material located on the surface of the current collector, and the positive electrode material is prepared by using the aqueous positive electrode slurry.

[0035] As a preferred embodiment, in the aqueous positive electrode material, the mass ratio of the aqueous positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is (90-96):(2-4):(2-5).

[0036] The aqueous positive electrode material on the aqueous positive electrode sheet of the present application is formed by coating the aqueous positive electrode slurry on the current collector and then drying. The drying uses the drying temperature and time that are conventional operations for positive electrode sheets in the art. Most of the additives and water will volatilize and overflow during the drying process, while the aqueous positive electrode active material, the positive electrode conductive agent, and the positive electrode binder can remain on the current collector. Therefore, the residual amounts of the additives and water are not specifically limited on the aqueous positive electrode sheet.

[0037] As a preferred embodiment, in the aqueous positive electrode material, the mass ratio of the aqueous positive electrode active material, the positive electrode conductive agent, and the positive electrode binder is (92-95):(2.5-3.5):(3-4).

[0038] As a preferred embodiment, the current collector is aluminum foil.

[0039] As a preferred embodiment, the thickness of the positive electrode material is 40-50 μm.

[0040] As a preferred embodiment, the method for preparing the positive electrode sheet includes the following steps: coating the aqueous positive electrode slurry on the current collector, and then performing drying, cold pressing, and slitting to obtain the positive electrode sheet.

[0041] As a preferred embodiment, the single-sided coating weight of the aqueous positive electrode slurry is 0.1-0.5 g / 1540.25 mm 2 .

[0042] To solve the above technical problems, a third object of the present invention is to provide a lithium-ion secondary battery, which includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, and the positive electrode sheet uses the above-mentioned aqueous positive electrode sheet.

[0043] As a preferred embodiment, the negative electrode sheet includes a negative electrode material, and the negative electrode material includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder with a mass ratio of (95-98):(0.5-1.5):(2-4).

[0044] As a preferred embodiment, the negative electrode material is prepared from a negative electrode slurry, and the negative electrode slurry comprises a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and water in a mass ratio of (95-98):(0.5-1.5):(2-4):(16-22).

[0045] As a preferred embodiment, the negative electrode active material is at least one of a carbon-based compound, a silicon-based compound, and a titanium-based compound.

[0046] As a preferred embodiment, the negative electrode active material is artificial graphite.

[0047] As a preferred embodiment, the negative electrode conductive agent is at least one of conductive carbon black, carbon nanotubes, acetylene black, and graphene.

[0048] As a preferred embodiment, the negative electrode binder is at least one of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0049] As a preferred embodiment, the negative electrode binder comprises styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 1:1.

[0050] As a preferred embodiment, the separator material is at least one of PE, PP, and polyimide film.

[0051] As a preferred embodiment, the electrolyte comprises a lithium salt and an organic solvent in a mass ratio of (6-10):(90-94), and the organic solvent is at least one of ethylene carbonate, diethyl carbonate, propylene carbonate, propyl propionate, and vinylene carbonate.

[0052] As a preferred embodiment, the lithium salt is LiPF 6 。

[0053] As a preferred embodiment, the organic solvent comprises ethylene carbonate, diethyl carbonate, propylene carbonate, propyl propionate, and vinylene carbonate in a mass ratio of 20:30:20:28:2.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] 1. A coating layer is formed on the surface of the positive electrode active material in the aqueous positive electrode sheet of the present application. Both the alkane compound containing a borate group and graphene in the coating layer have hydrophobic groups, which can block the contact between the surface of the positive electrode active material and water molecules, avoid the deterioration of the structure of the positive electrode active material, and both the alkane compound containing a borate group and graphene can improve the conductivity, ensuring that the prepared lithium-ion secondary battery maintains a high specific capacity, Coulomb efficiency, and cycle performance.

[0056] 2. An additive containing hydrophilic groups and hydrophobic groups is added to the aqueous positive electrode slurry of the present application. The hydrophilic groups promote the dispersion of the additive in the aqueous slurry, and the hydrophobic groups of the additive can undergo hydrophobic bonding with the hydrophobic groups of the hydrophobic coating layer of the aqueous positive electrode active material, realizing the uniform dispersion of the positive electrode active material in the aqueous solution system, delaying the sedimentation of the positive electrode active material, and improving the charge-discharge specific capacity and cycle performance of the prepared lithium-ion secondary battery.

[0057] 3. The borate radical of the alkane compound in the aqueous positive electrode active material of the present application can react with the harmful residual lithium on the surface of the positive electrode active material to generate lithium borate, a lithium ion conductor. At the same time, it is coated in cooperation with graphene with excellent conductivity, synchronously improving the conductivity and lithium ion mobility of the positive electrode active material, so that the charge-discharge efficiency and cycle capacity retention rate of the prepared lithium-ion secondary battery are significantly improved. Detailed Embodiments

[0058] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0059] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention. In addition, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0060] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0061] In order to further illustrate the present invention, the present invention will be described in detail below in conjunction with embodiments, but they cannot be understood as limiting the scope of protection of the present invention. The raw materials used in the following embodiments and comparative examples of the present application can be obtained commercially unless otherwise specified, and the same raw materials are used in parallel experiments.

[0062] Example 1

[0063] A preparation method of an aqueous positive electrode slurry, comprising the following steps:

[0064] (1) Mix and grind lithium cobaltate and n-tetradecyl borate in a mass ratio of 99.6:0.4. The particle size Dv50 of lithium cobaltate is 14 μm. Subsequently, under an argon atmosphere, sinter at a temperature of 600 °C for 6 h to obtain lithium cobaltate coated with n-tetradecyl borate, which is the aqueous positive electrode active material;

[0065] (2) Mix 93.7 kg of the aqueous positive electrode active material, 0.3 kg of sodium dodecylbenzenesulfonate, 2.6 kg of conductive carbon black, 0.4 kg of carbon nanotubes and 3 kg of polyacrylate, add 65 kg of deionized water, and stir under the action of a vacuum mixer until the mixed system becomes a homogeneous and fluid aqueous positive electrode slurry.

[0066] Example 2

[0067] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 1. The difference is that in step (1), the mass ratio of lithium cobaltate to n-tetradecyl borate is 99.4:0.6.

[0068] Example 3

[0069] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 1. The difference is that in step (1), the mass ratio of lithium cobaltate to n-tetradecyl borate is 99.2:0.8.

[0070] Example 4

[0071] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 1. The difference is that in step (1), the mass ratio of lithium cobaltate to n-tetradecyl borate is 98:2.

[0072] Example 5

[0073] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference is that in step (1), n-tetradecyl borate is replaced with an equal amount of reduced graphene. The median value of the sheet diameter of the reduced graphene is 2 μm, and the median value of the sheet diameter means that the sheet diameter of 50% of the graphene particles with a cumulative volume is less than or equal to 2 μm. Specifically, it is purchased from the reduced graphene oxide rGO of the SmartCmeta brand. The particle size Dv50 of lithium cobaltate is 12 μm. At this time, the ratio of the median value of the sheet diameter of the reduced graphene to the particle size Dv50 of lithium cobaltate is less than 0.3.

[0074] Example 6

[0075] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference is that in step (1), n-tetradecyl borate is replaced with an equal amount of reduced graphene. The median diameter of the reduced graphene flakes is 3 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand. At this time, the ratio of the median diameter of the reduced graphene flakes to the particle size Dv50 of lithium cobaltate is less than 0.3.

[0076] Example 7

[0077] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference is that in step (1), n-tetradecyl borate is replaced with an equal amount of reduced graphene. The median diameter of the reduced graphene flakes is 4 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand. The particle size Dv50 of lithium cobaltate is 15 μm. At this time, the ratio of the median diameter of the reduced graphene flakes to the particle size Dv50 of lithium cobaltate is less than 0.3.

[0078] Example 8

[0079] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference is that in step (1), n-tetradecyl borate is replaced with an equal amount of a mixture of n-tetradecyl borate and reduced graphene at a mass ratio of 1:4. The median diameter of the reduced graphene flakes is 3 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand. At this time, the ratio of the median diameter of the reduced graphene flakes to the particle size Dv50 of lithium cobaltate is less than 0.3.

[0080] Example 9

[0081] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference is that in step (1), n-tetradecyl borate is replaced with an equal amount of a mixture of n-tetradecyl borate and reduced graphene at a mass ratio of 2:3. The median diameter of the reduced graphene flakes is 3 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand. At this time, the ratio of the median diameter of the reduced graphene flakes to the particle size Dv50 of lithium cobaltate is less than 0.3.

[0082] Example 10

[0083] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference lies in that in step (1), n-tetradecyl borate is equivalently replaced by a mixture of n-tetradecyl borate and reduced graphene at a mass ratio of 3:2. The median diameter of the reduced graphene flakes is 3 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand. At this time, the ratio of the median diameter of the reduced graphene to the particle size Dv50 of lithium cobaltate is less than 0.3.

[0084] Example 11

[0085] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference lies in that in step (1), n-tetradecyl borate is equivalently replaced by a mixture of n-tetradecyl borate and reduced graphene at a mass ratio of 4:1. The median diameter of the reduced graphene flakes is 3 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand. At this time, the ratio of the median diameter of the reduced graphene to the particle size Dv50 of lithium cobaltate is less than 0.3.

[0086] Example 12

[0087] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 10. The difference lies in that the content of sodium dodecylbenzenesulfonate additive is increased to 0.6 kg. In step (2), 93.4 kg of aqueous cathode active material, 0.6 kg of sodium dodecylbenzenesulfonate, 2.6 kg of conductive carbon black, 0.4 kg of carbon nanotubes, and 3 kg of polyacrylate are mixed, and 65 kg of deionized water is added. Stirring is carried out under the action of a vacuum mixer until the mixed system becomes a homogeneous and flowable aqueous cathode slurry.

[0088] Example 13

[0089] A method for preparing an aqueous cathode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 10. The difference lies in that the content of sodium dodecylbenzenesulfonate additive is increased to 0.9 kg. In step (2), 93.1 kg of aqueous cathode active material, 0.9 kg of sodium dodecylbenzenesulfonate, 2.6 kg of conductive carbon black, 0.4 kg of carbon nanotubes, and 3 kg of polyacrylate are mixed, and 65 kg of deionized water is added. Stirring is carried out under the action of a vacuum mixer until the mixed system becomes a homogeneous and flowable aqueous cathode slurry.

[0090] Example 14

[0091] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 10. The difference is that the content of sodium dodecylbenzenesulfonate additive is increased to 1.2 kg. In step (2), 92.8 kg of aqueous positive electrode active material, 1.2 kg of sodium dodecylbenzenesulfonate, 2.6 kg of conductive carbon black, 0.4 kg of carbon nanotubes, and 3 kg of polyacrylate are mixed, and 65 kg of deionized water is added. Stir under the action of a vacuum mixer until the mixed system becomes a homogeneous and flowable aqueous positive electrode slurry.

[0092] Example 15

[0093] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that in step (2), sodium dodecylbenzenesulfonate is equally replaced by sodium polyacrylate.

[0094] Example 16

[0095] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that in step (2), sodium dodecylbenzenesulfonate is equally replaced by a mixture of sodium polyacrylate and sodium dodecylbenzenesulfonate in a mass ratio of 3:7.

[0096] Example 17

[0097] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that in step (2), sodium dodecylbenzenesulfonate is equally replaced by a mixture of sodium polyacrylate and sodium dodecylbenzenesulfonate in a mass ratio of 5:5.

[0098] Example 18

[0099] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that in step (2), sodium dodecylbenzenesulfonate is equally replaced by a mixture of sodium polyacrylate and sodium dodecylbenzenesulfonate in a mass ratio of 7:3.

[0100] Example 19

[0101] A preparation method of an aqueous positive electrode slurry, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that no sodium dodecylbenzenesulfonate or sodium polyacrylate additive is added to the aqueous positive electrode slurry. In step (2), sodium dodecylbenzenesulfonate is equally replaced by a mixture of sodium dodecylbenzenesulfonate and 3-hydroxyphenylphosphinylpropionic acid in a mass ratio of 3:7.

[0102] Comparative Example 1

[0103] A preparation method of an aqueous positive electrode slurry, which is different from Example 13 in that lithium cobaltate is not coated with n-tetradecyl borate and reduced graphene, and sodium dodecylbenzenesulfonate or sodium polyacrylate additive is not added to the aqueous positive electrode slurry. The preparation method includes the following steps:

[0104] Mix 94 kg of lithium cobaltate, 2.6 kg of conductive carbon black, 0.4 kg of carbon nanotubes and 3 kg of polyacrylate, add 65 kg of deionized water, and stir under the action of a vacuum mixer until the mixed system becomes a homogeneous and fluid aqueous positive electrode slurry.

[0105] Comparative Example 2

[0106] A preparation method of an aqueous positive electrode slurry, wherein the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference is that the ratio of the median diameter of the reduced graphene flakes to the particle size Dv50 of the lithium cobaltate is greater than 0.3. In step (1), n-tetradecyl borate is replaced with an equal amount of reduced graphene. The median diameter of the reduced graphene flakes is 6 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand.

[0107] Comparative Example 3

[0108] A preparation method of an aqueous positive electrode slurry, wherein the reagents, equipment, and process parameters used in each step are the same as those in Example 2. The difference is that the ratio of the median diameter of the reduced graphene flakes to the particle size Dv50 of the lithium cobaltate is greater than 0.3. In step (1), n-tetradecyl borate is replaced with an equal amount of reduced graphene. The median diameter of the reduced graphene flakes is 4 μm, and it is specifically purchased as reduced graphene oxide rGO of the SmartCmeta brand. The particle size Dv50 of the lithium cobaltate is 10 μm.

[0109] Comparative Example 4

[0110] A preparation method of an aqueous positive electrode slurry, wherein the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that lithium cobaltate is not coated with n-tetradecyl borate and reduced graphene. In step (2), the aqueous positive electrode active material is replaced with an equal amount of lithium cobaltate, and the particle size Dv50 of the lithium cobaltate is 14 μm.

[0111] Comparative Example 5

[0112] A preparation method of an aqueous positive electrode slurry, wherein the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that sodium dodecylbenzenesulfonate or sodium polyacrylate additive is not added to the aqueous positive electrode slurry. In step (2), sodium dodecylbenzenesulfonate is replaced with an equal amount of the aqueous positive electrode active material.

[0113] Comparative Example 6

[0114] A method for preparing an aqueous positive electrode paste, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that sodium dodecylbenzenesulfonate or sodium polyacrylate additive is not added to the aqueous positive electrode paste. In step (2), sodium dodecylbenzenesulfonate is replaced with 3-hydroxyphenylphosphorylpropionic acid in equal amount.

[0115] Comparative Example 7

[0116] A method for preparing an aqueous positive electrode paste, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that sodium dodecylbenzenesulfonate or sodium polyacrylate additive is not added to the aqueous positive electrode paste. In step (2), sodium dodecylbenzenesulfonate is replaced with ethyl cellulose in equal amount.

[0117] Comparative Example 8

[0118] A method for preparing an aqueous positive electrode paste, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that sodium dodecylbenzenesulfonate or sodium polyacrylate additive is not added to the aqueous positive electrode paste. In step (2), sodium dodecylbenzenesulfonate is replaced with sodium dodecyl sulfate in equal amount.

[0119] Comparative Example 9

[0120] A method for preparing an aqueous positive electrode paste, where the reagents, equipment, and process parameters used in each step are the same as those in Example 13. The difference is that sodium dodecylbenzenesulfonate or sodium polyacrylate additive is not added to the aqueous positive electrode paste. In step (2), sodium dodecylbenzenesulfonate is replaced with a mixture of sodium dodecyl sulfate and 3-hydroxyphenylphosphorylpropionic acid with a mass ratio of 3:7 in equal amount.

[0121] Application Examples 1-19 and Comparative Application Examples 1-9

[0122] A method for preparing a lithium-ion secondary battery, comprising the following steps:

[0123] (1) Coating the aqueous positive electrode paste prepared in the example or comparative example on the positive electrode current collector aluminum foil, with a single-sided coating weight of about 0.2 g / 1540.25 mm 2 , and after drying, cold pressing, slitting, and welding the tab, a positive electrode plate is obtained, and the thickness of the positive electrode active material layer is about 41-46 μm;

[0124] (2) Mix artificial graphite, acetylene black, styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC) and deionized water in a mass ratio of 96:1:1.5:1.5:18, stir well, coat it on the negative current collector copper foil, and obtain the negative electrode plate after drying, cold pressing, slitting and welding the tab.

[0125] (3) Prepare a solution by mixing lithium salt LiPF 6 and non-aqueous organic solvents in a mass ratio of 8:92. The non-aqueous organic solvents include ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP) and vinylene carbonate (VC) in a mass ratio of 20:30:20:28:2 to obtain the electrolyte.

[0126] (4) Use a polyethylene (PE) porous polymer film as the separator. Stack the positive electrode plate, separator and negative electrode plate in sequence, with the separator in the middle of the positive electrode plate and the negative electrode plate to play a role in safety isolation. The positive electrode is led out by spot welding with an aluminum tab, and the negative electrode is led out by spot welding with a nickel tab, and then wind it to obtain the electrode assembly. Place the electrode assembly in the packaging case, inject the electrolyte, and obtain the lithium-ion secondary battery through the processes of encapsulation, formation and grading.

[0127] Performance detection test

[0128] (1) Specific capacity test: Transfer the lithium-ion secondary batteries prepared in the application example and the comparative application example to an environment of 25°C for charge and discharge tests. The voltage window is 3.0V - 4.58V, and the charge and discharge rate is 0.2C. The specific capacity is the ratio of the discharge specific capacity to the weight of the active material. The test results are shown in Table 1 below.

[0129] (2) Cycle performance test: Take 5 lithium-ion secondary batteries prepared in the application example and the comparative application example each, and repeat the charging and discharging of the lithium-ion secondary battery through the following steps, and calculate the cycle capacity retention rate of the lithium-ion secondary battery. Place the battery in an environment of 45°C for 60 minutes to reach a constant temperature, then charge it at a constant current and constant voltage of 1.0C to 4.55V, with a cut-off current of 0.05C, rest for 5 minutes, and discharge it at a constant current of 1.0C to 3.0V. Test the Coulomb efficiency of the first cycle and the cycle capacity retention rate. The cycle capacity retention rate = (discharge specific capacity of the 500th cycle / discharge specific capacity of the first cycle) × 100%, and the test results are shown in Table 1 below.

[0130] (3) Resistivity test: Use a resistivity measuring device (CIS Company) to measure the resistivity of the positive electrode plates prepared according to the application example and the comparative application example in the thickness direction. The measurement results are shown in Table 1 below.

[0131] Table 1 - Performance test results of the positive electrode plates or lithium-ion secondary batteries in the application example and the comparative application example

[0132]

[0133]

[0134]

[0135] In Application Examples 1 - 4 of the present application, lithium cobaltate was coated with n - tetradecyl borate, and in Application Examples 5 - 7, lithium cobaltate was coated with graphene. Both n - tetradecyl borate and graphene contain abundant hydrophobic groups, forming a hydrophobic material layer on the surface of lithium cobaltate, effectively inhibiting the problem that the reaction between lithium cobaltate and water in the water - based slurry easily occurs, leading to the deterioration of the surface structure. At the same time, it can combine with the hydrophobic groups of sodium dodecylbenzenesulfonate uniformly dispersed in the water - based positive electrode slurry, enabling the lithium cobaltate material to be uniformly dispersed in the aqueous slurry together, avoiding material aggregation, and effectively improving the specific capacity and cycle capacity retention rate of the material.

[0136] Among them, in Application Examples 1 - 4, as the coating amount of n - tetradecyl borate increased, the cycle capacity retention rate showed a trend of first increasing and then decreasing. The coating ratio of Application Example 2 was the best. This is because an appropriate amount of n - tetradecyl borate coating can prevent the reaction between the material surface and water, and combine with sodium dodecylbenzenesulfonate to improve the dispersibility of the positive electrode material, while excessive coating will hinder the migration of lithium ions.

[0137] Compared with Application Example 2, in Application Examples 5 - 7, the coating layer of the water - based positive electrode active material is reduced graphene, and the ratio of the sheet diameter of graphene to the particle diameter of lithium cobaltate is less than 0.3. In Comparative Application Example 2, the sheet diameter of graphene is relatively large, which is 6 μm, and in Comparative Application Example 3, the particle diameter of lithium cobaltate is relatively small, which is 10 μm. In Comparative Application Examples 2 - 3, the ratio of the sheet diameter of graphene to the particle diameter of lithium cobaltate is greater than 0.3. A too large ratio of the sheet diameter of graphene to the particle diameter of lithium cobaltate will lead to problems such as uneven coating and poor coating adhesion between graphene and the positive electrode active material, unable to effectively isolate the infiltration of water molecules, causing partial contact between lithium cobaltate and water and resulting in structural deterioration, affecting the cycle performance of the prepared lithium - ion secondary battery.

[0138] In addition, in Application Examples 8-11, lithium cobaltate was coated with n-tetradecyl borate and reduced graphene simultaneously. The borate group of n-tetradecyl borate can combine with the harmful residual lithium on the surface of the lithium cobaltate material to form lithium borate, which is a good lithium ion conductor. After being combined and coated with graphene with excellent conductivity, the material layer has both good electronic conductivity and lithium ion mobility. In Application Examples 10-11, by controlling the coating ratio of n-tetradecyl borate and reduced graphene, the specific capacity, initial Coulomb efficiency, and cycle capacity retention rate of the material can be further improved to a certain extent, and Application Example 10 has the best ratio.

[0139] By comparing the schemes of Application Examples 10 and 12-14, it can be seen that with the increase of sodium dodecylbenzenesulfonate, the initial Coulomb efficiency, discharge specific capacity, and cycle capacity retention rate of the prepared lithium ion secondary battery show a trend of first increasing and then leveling off, and Application Example 13 has the best performance. At the same time, in the aqueous cathode slurry of Application Examples 16-18, sodium polyacrylate and sodium dodecylbenzenesulfonate are added simultaneously as amphiphilic solvents, and the performance of the prepared lithium ion secondary battery is better than that of adding only sodium dodecylbenzenesulfonate or sodium polyacrylate. When the mass ratio of sodium polyacrylate to sodium dodecylbenzenesulfonate is 5:5, the specific capacity, Coulomb efficiency, and cycle capacity retention rate of the battery all reach the best.

[0140] Compared with Application Example 13, in Comparative Application Example 1 and Comparative Application Example 4, the lithium cobaltate material was directly dispersed in the aqueous slurry, and the surface of the lithium cobaltate was not coated with a hydrophobic material layer, resulting in easy contact and reaction between the surface of the lithium cobaltate and water, which led to the destruction of the lithium cobaltate structure. The specific capacity and Coulomb efficiency of the prepared battery decreased to a certain extent, and the capacity decreased sharply during the cycle, with poor cycle stability. In Comparative Application Example 5, sodium dodecylbenzenesulfonate was not added to the aqueous cathode slurry, and the lithium cobaltate coated with a hydrophobic layer could not be effectively dispersed in water and was prone to sedimentation, resulting in uneven coating surface density of the prepared cathode electrode, leading to poor conductivity, specific capacity, first charge Coulomb efficiency, and conductivity of the battery. It shows that the combination strategy of hydrophobic coating of lithium cobaltate and sodium dodecylbenzenesulfonate can effectively reduce the resistivity of the cathode material, which is attributed to the fact that this combination can improve the uniformity of the cathode material slurry, resulting in the improvement of the initial Coulomb efficiency, discharge specific capacity, and cycle performance of the cathode active material.

[0141] Compared with Application Examples 13 and 15, the additives in Comparative Application Examples 6-8 are replaced by 3-hydroxyphenylphosphorylpropionic acid, ethyl cellulose, and sodium dodecyl sulfate respectively. It can be seen that sodium dodecylbenzenesulfonate and sodium polyacrylate perform better as additives in terms of cycle capacity retention rate. This is attributed to the fact that although the additives in Comparative Application Examples 6-8 can all undergo hydrophobic binding with coated lithium cobaltate, their dispersion stability and their effect on the structural stability of the coating layer are not as good as those of sodium dodecyl sulfate and sodium polyacrylate.

[0142] The specific embodiments described above define the purpose, technical solutions, and beneficial effects of the present invention. It is particularly pointed out that for those skilled in the art, any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An aqueous positive electrode slurry, characterized in that It comprises an aqueous positive electrode active material; the aqueous positive electrode active material comprises a positive electrode active material and a coating layer coated on the surface of the positive electrode active material, the coating layer comprises an alkane compound containing a borate group and / or graphene, and the ratio of the median sheet diameter of the graphene to the particle diameter Dv50 of the positive electrode active material is less than 0.

3.

2. The aqueous positive electrode slurry according to claim 1, characterized in that The median diameter of the graphene sheet is 2-4 μm; And / or, the particle size Dv50 of the positive electrode active material is 12-15 μm; And / or, the mass ratio of the positive electrode active material to the coating layer in the aqueous positive electrode active material is (98-99.6): (0.4-2).

3. The aqueous positive electrode slurry according to claim 1, characterized in that The coating layer of the aqueous positive electrode active material comprises graphene and a borate-containing alkane compound in a mass ratio of (1-4): (1-4); And / or, the borate-containing alkane compound is n-tetradecyl borate; and / or, the positive electrode active material comprises at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate and lithium manganese oxide; And / or, the graphene in the aqueous positive electrode active material is reduced graphene.

4. The aqueous positive electrode slurry according to claim 1, characterized in that The aqueous positive electrode active material is prepared by the following method: the positive electrode active material and the coating layer raw material are mixed and ground according to a certain proportion, and then sintered for 1-10 hours in an inert gas or nitrogen atmosphere at a temperature of 200-900° C. to obtain the aqueous positive electrode active material.

5. The aqueous positive electrode slurry according to claim 1, characterized in that The aqueous positive electrode slurry further includes an additive, wherein the additive includes sodium dodecylbenzene sulfonate and / or sodium polyacrylate.

6. The aqueous positive electrode slurry according to claim 5, characterized in that The mass ratio of the aqueous positive electrode active material to the additive is (90-96): (0.2-1.5); And / or, the additive comprises sodium dodecylbenzene sulfonate and sodium polyacrylate in a mass ratio of (3-7):(3-7).

7. The aqueous positive electrode slurry according to claim 5, characterized in that The aqueous positive electrode slurry also includes a positive electrode conductor, a positive electrode binder and water, and the mass ratio of the aqueous positive electrode active material, the positive electrode conductor, the additive, the positive electrode binder and the water is (90-96): (2-4): (0.2-1.5): (2-5): (60-70).

8. The aqueous positive electrode slurry according to claim 7, characterized in that The positive electrode conductive agent is at least one of conductive carbon black, carbon nanotubes, acetylene black and graphene; And / or, the positive electrode conductive agent comprises conductive carbon black and carbon nanotubes in a mass ratio of (2-3): (0.2-0.8); And / or, the positive electrode binder is at least one of polyacrylate, polyvinylidene fluoride, polytetrafluoroethylene, and styrene-butadiene rubber.

9. A water-based positive electrode sheet, characterized in that: It comprises a current collector and a positive electrode material located on the surface of the current collector, wherein the positive electrode material is prepared by using the aqueous positive electrode slurry as described in any one of claims 1-8.

10. A lithium ion secondary battery, characterized in that: It comprises a positive electrode plate, a negative electrode plate, an electrolyte and a separator, wherein the positive electrode plate is the aqueous positive electrode plate as claimed in claim 9.

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