Positive active material, positive plate, battery and method for forming coating layer

Abstract

The invention provides a positive electrode active material, a positive plate, a battery and a method for forming a coating layer, and belongs to the technical field of batteries, the positive electrode active material comprises a positive electrode active material and the coating layer coating the surface of the positive electrode active material; the coating layer comprises a polymer as shown in a formula I, wherein at least one of R1 and R2 is a group containing lone pair electrons; the value range of m is 1 to 100000; the value range of n is 1 to 100000; the positive electrode active material contains a manganese element. According to the embodiment of the invention, the polymer as shown in the formula I is coated on the surface of the positive active material containing the manganese element, so that the positive active material can be isolated from the electrolyte through the coating layer, the dissolution of manganese is reduced through a physical limiting effect, and groups containing lone pair electrons can be complexed with manganese ions in the positive active material, so that the stability of the electrolyte is improved. And manganese dissolution is limited by chemical acting force, so that the dissolution problem of the positive electrode active material is reduced, and the cycle performance is improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode active material, a positive electrode sheet, a battery, and a method for forming a coating layer. Background Art

[0002] As the core of the new energy vehicle industry, the technological progress of power batteries directly affects the performance and market competitiveness of new energy vehicles. Due to the high energy density and relatively good safety of lithium-ion batteries, they have been widely used in the field of power battery technology. However, issues such as mileage anxiety, service life and safety are still the main obstacles to the development of new energy vehicles. By adding manganese elements to lithium iron phosphate, lithium manganese iron phosphate can increase the voltage platform by about 0.7V, increase the energy density by 10%-20%, and retain the safety and low cost characteristics of lithium iron phosphate. Therefore, lithium manganese iron phosphate batteries have become an important technical direction for power batteries.

[0003] Compared with lithium iron phosphate, the positive electrode of lithium manganese iron phosphate will cause manganese ions to dissolve and deposit on the surface of the negative electrode due to the Jan-Taylor effect of manganese (Mn), causing the SEI film (solid electrolyte interface) to continue to rupture and grow, which in turn leads to problems such as reduced coulombic efficiency and attenuated cycle performance. This phenomenon is particularly obvious at high temperatures.

[0004] At present, surface coating is usually used to inhibit the dissolution of manganese in lithium iron manganese phosphate. In the related art, metal oxides, fast ion conductors, graphene or other carbon-containing materials are usually used to coat lithium iron manganese phosphate. These coating materials are used to isolate the contact between lithium iron manganese phosphate and the electrolyte, and use physical restriction to inhibit the dissolution of manganese. However, since lithium iron manganese phosphate is a nano-scale particle, it is difficult to achieve uniform coating of each lithium iron manganese phosphate particle, resulting in poor effect of restricting manganese dissolution. Summary of the invention

[0005] The embodiments of the present application provide a positive electrode active material, a positive electrode sheet, a battery, and a method for forming a coating layer. The coating layer in the positive electrode active material can limit the dissolution of manganese through physical and chemical effects at the same time, thereby improving the cycle performance.

[0006] In a first aspect, an embodiment of the present application provides a positive electrode active material, including a positive electrode active substance, and a coating layer coated on the surface of the positive electrode active substance;

[0007] The coating layer comprises a polymer as shown in Formula I:

[0008]

[0009] Wherein, at least one of R1 and R2 is a group containing a lone pair of electrons; the value range of m is 1-100000; the value range of n is 1-100000;

[0010] The positive electrode active material contains manganese.

[0011] In some embodiments, R1 and R2 are independently selected from any one of a substituted or unsubstituted cyanoethoxy group, a substituted or unsubstituted carboxyl group, a substituted or unsubstituted hydroxyl group, a substituted or unsubstituted amino group, and a substituted or unsubstituted sulfonic acid group.

[0012] In some embodiments, R1 and R2 are independently selected from substituted or unsubstituted cyanoethoxy or substituted or unsubstituted carboxyl.

[0013] In some embodiments, R x and R y Any one independently selected from a group containing a conjugated aromatic hydrocarbon, a group containing a conjugated thioether, a group containing a conjugated amine and an aromatic hetero group.

[0014] In some embodiments, the positive electrode active material includes lithium manganese iron phosphate.

[0015] In some embodiments, the polymer is as shown in any one of Formula II-IV:

[0016]

[0017] The value range of m is 1-100000, and the value range of n is 1-100000.

[0018] In some embodiments, the polymer is formed by polymerizing a monomer represented by at least one of Formulas V-VIII:

[0019]

[0020] In some embodiments, 100≤m+n≤100000.

[0021] In a second aspect, an embodiment of the present application provides a positive electrode sheet, comprising the positive electrode active material as described above.

[0022] In a third aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet as described above.

[0023] In a fourth aspect, an embodiment of the present application provides a method for forming a coating layer, which is used to form a coating layer on the surface of a positive electrode active material, comprising:

[0024] Adding a polymer to a positive electrode slurry containing a positive electrode active material to obtain a first mixture;

[0025] Applying the first mixture to the surface of the positive electrode current collector, and baking the mixture so that the polymer is coated on the surface of the lithium manganese iron phosphate to form a coating layer;

[0026] Wherein, the positive electrode active material contains manganese element; the polymer is shown in formula I:

[0027]

[0028] Wherein, R1 and R2 are both groups containing lone pair electrons; the value range of m is 1-100000; the value range of n is 1-100000.

[0029] In some embodiments, the mass proportion of the polymer in the total amount of the polymer and the positive electrode active material is less than 10%.

[0030] In some embodiments, the mass proportion of the polymer in the total amount of the polymer and the positive electrode active material is 0.2%-5%.

[0031] In a fifth aspect, an embodiment of the present application provides another method for forming a coating layer, which is used to form a coating layer on the surface of a positive electrode active material, comprising:

[0032] Providing a positive electrode sheet containing a positive electrode active material;

[0033] Providing an electrolyte, and adding a polymerizable monomer into the electrolyte to obtain a composite electrolyte;

[0034] Assembling a positive electrode sheet and a composite electrolyte to form a battery;

[0035] Performing formation charging on the battery so that the polymerization monomers are polymerized on the surface of the positive electrode active material to generate polymers and form a coating layer;

[0036] Wherein, the positive electrode active material contains manganese element; the polymer is shown in formula I:

[0037]

[0038] Wherein, R1 and R2 are both groups containing lone pair electrons; the value range of m is 1-100000; the value range of n is 1-100000.

[0039] In some embodiments, the polymerized monomer accounts for less than 10% by weight in the composite electrolyte.

[0040] In some embodiments, the mass percentage of the polymerized monomer in the composite electrolyte is 0.5%-5%.

[0041] Beneficial effects of the embodiments of the present application:

[0042] In an embodiment of the present application, a positive electrode active material is provided, and the positive electrode active material includes a positive electrode active substance and a coating layer coated on the surface of the positive electrode active substance. The coating layer includes a polymer as shown in Formula I, and the polymer has a group containing a lone pair of electrons. The positive electrode active substance contains manganese. In the embodiment of the present application, by using a polymer as shown in Formula I to coat the surface of the positive electrode active substance containing manganese, the positive electrode active substance can be isolated from the electrolyte through the coating layer to reduce the dissolution of manganese by physical restriction, and the lone pair of electrons can be complexed with the manganese ions in the positive electrode active substance to limit the dissolution of manganese by chemical force, thereby reducing the dissolution problem of the positive electrode active substance and improving the cycle performance. DETAILED DESCRIPTION

[0043] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0044] In a first aspect, an embodiment of the present application provides a positive electrode active material, comprising a positive electrode active substance and a coating layer coated on the surface of the positive electrode active substance. The positive electrode active substance contains manganese.

[0045] The coating layer comprises a polymer as shown in Formula I:

[0046]

[0047] Wherein, R1 and R2 are both groups containing lone pair electrons; the value range of m is 1-100000; the value range of n is 1-100000.

[0048] For positive electrode active materials containing manganese, when used in batteries, they are easily affected by structural changes, electrolyte influences, and battery operating conditions, which can easily lead to manganese dissolution problems, which in turn cause battery capacity decay, reduced cycle performance, and reduced safety performance.

[0049] The positive electrode active material provided in the embodiment of the present application is coated with a coating layer, and the coating layer contains a polymer, and the polymer contains a group containing a lone pair of electrons. On the one hand, the coating layer can isolate the positive electrode active material from contact with the electrolyte, and reduce manganese dissolution through physical restriction; on the other hand, the group containing a lone pair of electrons can be complexed with the manganese ions in the positive electrode active material to limit manganese dissolution through chemical forces. That is, in the positive electrode active material provided in the embodiment of the present application, the coating layer can reduce the dissolution problem of the positive electrode active material through physical and chemical restriction, thereby reducing the problems of battery capacity attenuation, decreased cycle performance, and decreased safety performance.

[0050] Among them, the group containing lone pairs of electrons refers to a chemical group that contains lone pairs of electrons in the molecular structure. Lone pairs of electrons refer to the non-bonding electrons that are not used to form covalent bonds in the outermost electron layer of atoms in addition to the bonding electrons used to form covalent bonds in the molecule. These unbonded electron pairs are called lone pairs of electrons. For example, hydroxyl, amino, formyl, carboxyl, cyano, etc.

[0051] It is understood that in the polymer represented by formula I, R x and R y They can be the same or different, and R1 and R2 can be the same or different.

[0052] In some embodiments, R1 and R2 are independently selected from any one of a substituted or unsubstituted cyanoethoxy group, a substituted or unsubstituted carboxyl group, a substituted or unsubstituted hydroxyl group, a substituted or unsubstituted amino group, and a substituted or unsubstituted sulfonic acid group.

[0053] The nitrogen atom in the cyanoethoxy group has a lone pair of electrons and can form a stable complex with manganese ions, keeping the structure unchanged under various environmental conditions. The carboxyl group can form a stable complex with manganese ions through various coordination forms such as monodentate coordination and bidentate bridging coordination, and polymers containing carboxyl groups usually have good solubility, which is convenient for coating positive electrode active materials. The oxygen atom in the hydroxyl group has a lone pair of electrons and can form a coordination effect with manganese ions, and the hydrogen atom in the hydroxyl group can also form hydrogen bonds with other molecules or groups, thereby enhancing the stability of the complex. The nitrogen atom in the amino group has a lone pair of electrons and can form a complex with manganese ions, and the amino group is alkaline and can accept protons to form ammonium salts, making the complex formed by the amino group and manganese ions more stable in an acidic environment. The sulfonic acid group has strong acidity and can form a stable sulfonate complex with manganese ions, thereby improving the restriction effect on manganese ions.

[0054] Further, R1 and R2 are independently selected from substituted or unsubstituted cyanoethoxy or substituted or unsubstituted carboxyl.

[0055] It can be understood that the cyanoethoxy group, carboxyl group, hydroxyl group, amino group and sulfonic acid group mentioned in the embodiments of the present application can be substituted or unsubstituted groups, and as long as they can have a lone pair of electrons, they can meet the requirements of the embodiments of the present application.

[0056] In some embodiments, R x and R y Independently selected from any one of a group containing conjugated aromatic hydrocarbons, a group containing conjugated thioethers, a group containing conjugated amines and an aromatic hetero group. The group containing conjugated aromatic hydrocarbons, the group containing conjugated thioethers, the group containing conjugated amines and the aromatic hetero group have good electrochemical activity, can provide capacity during the battery charge and discharge process, and reduce the problem of capacity reduction caused by the introduction of inactive substances.

[0057] In some embodiments, R x and R y Any one independently selected from substituted or unsubstituted phenyl, substituted or unsubstituted thienyl, substituted or unsubstituted pyrrolyl, substituted or unsubstituted anilino, substituted or unsubstituted phenothiazinyl, substituted or unsubstituted thianthrenyl, substituted or unsubstituted phenoxazinyl and substituted or unsubstituted carbazolyl.

[0058] In some embodiments, R x and R y Any one independently selected from a substituted or unsubstituted thienyl group, a substituted or unsubstituted pyrrolyl group, a substituted or unsubstituted anilino group and a substituted or unsubstituted carbazolyl group.

[0059] In some embodiments, the positive electrode active material includes lithium iron manganese phosphate. By adding manganese to lithium iron phosphate, the voltage platform of lithium iron manganese phosphate is increased by about 0.7V, the energy density can be increased by about 10-20%, and the safety and low cost characteristics of lithium iron phosphate are retained. By using lithium iron manganese phosphate as the positive electrode active material and coating it with a coating layer, while maintaining the advantages of high voltage platform, high energy density, safety and low cost, it can reduce the phenomenon of manganese dissolution, reduce the battery capacity attenuation, cycle performance degradation, safety performance reduction and other problems.

[0060] In some embodiments, the polymer is as shown in any one of Formula II-IV:

[0061]

[0062] The value range of m is 1-100000, and the value range of n is 1-100000.

[0063] The main chain of the polymer shown in formula II-IV contains polyaniline, polycarbazole and polythiophene segments, has good electrochemical activity, and can provide capacity during the charge and discharge process. The carboxyl and cyanoethoxy groups in its side chains can complex with manganese ions to inhibit manganese dissolution.

[0064] In some embodiments, the polymer is formed by polymerizing a monomer represented by at least one of Formulas V-VIII:

[0065]

[0066] The polymerizable monomers shown in formulas V-VIII can undergo electro-oxidative polymerization to form polymers similar to the polymers shown in formulas II-IV, which can provide capacity and inhibit manganese dissolution.

[0067] In some embodiments, 100≤m+n≤100000. By setting the range of m+n within the above range, the polymer can maintain a certain strength and toughness, and has good solubility, which is convenient for forming a coating layer on the positive electrode active material.

[0068] In some embodiments, 100≤m+n≤50000. By setting the range of m+n within the above range, the strength, toughness and solubility of the polymer can be further guaranteed, thereby forming a good coating layer.

[0069] In a second aspect, an embodiment of the present application provides a positive electrode sheet, comprising the positive electrode active material as described above.

[0070] The beneficial effects of the positive electrode sheet provided in the embodiment of the present application are basically consistent with those of the positive electrode active material, and will not be elaborated here.

[0071] In a third aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet as described above.

[0072] The beneficial effects of the battery provided in the embodiments of the present application are basically consistent with those of the positive electrode active material and will not be elaborated here.

[0073] In a fourth aspect, an embodiment of the present application provides a method for forming a coating layer, which is used to form a coating layer on the surface of a positive electrode active material, comprising:

[0074] Adding a polymer to a positive electrode slurry containing a positive electrode active material to obtain a first mixture;

[0075] Applying the first mixture to the surface of the positive electrode current collector, and baking the mixture so that the polymer is coated on the surface of the lithium manganese iron phosphate to form a coating layer;

[0076] Wherein, the positive electrode active material contains manganese element; the polymer is shown in formula I:

[0077]

[0078] Wherein, R1 and R2 are both groups containing lone pair electrons; the value range of m is 1-100000; the value range of n is 1-100000.

[0079] In the embodiment of the present application, the polymer is added to the positive electrode slurry containing the positive electrode active material, so that the polymer is mixed with the positive electrode active material. Then, through the steps of coating and baking, the polymer is coated and combined on the surface of the positive electrode active material, so that a coating layer is formed on the surface of the positive electrode active material. The process of the preparation method is simple, no additional coating step is required, and efficiency can be improved.

[0080] In some embodiments, the mass proportion of the polymer in the total amount of the polymer and the positive electrode active material is less than 10%. The polymer can coat the positive electrode active material, thereby providing protection and limiting the dissolution of manganese. By setting the proportion of the polymer within the above range, the protective effect can be exerted while reducing the reduction in battery capacity and electrochemical performance caused by the excessive proportion of the polymer.

[0081] In some embodiments, the mass proportion of the polymer in the total amount of the polymer and the positive electrode active material is 0.2%-5%. By preferably setting the mass proportion of the polymer in the positive electrode active material to 0.2%-5%, the coating effect can be ensured, the uniformity of the coating can be improved, and a certain thickness can be ensured. At the same time, it can avoid introducing too much inactive material into the positive electrode active material, thereby ensuring the conductivity and energy density of the positive electrode active material.

[0082] In a fifth aspect, an embodiment of the present application provides another method for forming a coating layer, which is used to form a coating layer on the surface of a positive electrode active material, comprising:

[0083] Providing a positive electrode sheet containing a positive electrode active material;

[0084] Providing an electrolyte, and adding a polymerizable monomer into the electrolyte to obtain a composite electrolyte;

[0085] Assembling a positive electrode sheet and a composite electrolyte to form a battery;

[0086] Performing formation charging on the battery so that the polymerization monomers are polymerized on the surface of the positive electrode active material to generate polymers and form a coating layer;

[0087] Wherein, the positive electrode active material contains manganese element; the polymer is shown in formula I:

[0088]

[0089] Wherein, R1 and R2 are both groups containing lone pair electrons; the value range of m is 1-100000; the value range of n is 1-100000.

[0090] The embodiment of the present application obtains a composite electrolyte by adding a polymerized monomer to the electrolyte of the battery, and assembles the composite electrolyte with a positive electrode sheet containing a positive electrode active material to form a battery. During the formation and charging process of the battery, oxidation will occur on the surface of the positive electrode active material, so that the polymerized monomers in the composite electrolyte are oxidized and polymerized in situ on the surface of the positive electrode active material to generate a polymer. As the formation and charging continue, the polymer can be coated on the surface of the positive electrode active material to form a coating layer. The preparation method can utilize the oxidation generated on the surface of the positive electrode active material during the formation and charging process of the battery to polymerize the polymerized monomers in the composite electrolyte in situ on the surface of the positive electrode active material to form a uniform coating layer, thereby further improving the effect of limiting manganese dissolution, and the process is simple, the coating cost is low, and it is suitable for large-scale production applications.

[0091] In some embodiments, the mass proportion of the polymerized monomer in the composite electrolyte is less than 10%. Controlling the polymerized monomer within the above range can enable the polymerized monomer to be oxidatively polymerized on the surface of the positive electrode active material to generate a polymer, and can maintain a certain fluidity of the composite electrolyte.

[0092] In some embodiments, the mass proportion of the polymerized monomer in the composite electrolyte is 0.5%-5%. Controlling the polymerized monomer within the above range enables the polymerized monomer to be oxidatively polymerized on the surface of the positive electrode active material to generate a polymer, forming a coating layer of suitable thickness, improving the efficiency of oxidative polymerization, and maintaining a certain fluidity of the composite electrolyte.

[0093] The following examples are further described in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods in the following examples that do not specify specific conditions are usually based on the conditions recommended by the manufacturer.

[0094] Example 1

[0095] (1) Preparation of positive electrode sheet:

[0096] Lithium manganese iron phosphate and conductive carbon black were stirred and mixed at a mass ratio of 95.2:0.5 for 1 hour, and then a polymer as shown in formula L1 was added and continued to be stirred and mixed for 4 hours, and then N-methylpyrrolidone was added and continued to be stirred and mixed for 3 hours to obtain a positive electrode intermediate material; wherein the polymer accounted for 1.5% of the total mass of the polymer and lithium manganese iron phosphate, and the revolution speed during the stirring and mixing process was 25 r / min, and the dispersion speed was 500 r / min;

[0097]

[0098] Where n = 1500;

[0099] Adding an aqueous solution of carbon nanotubes and positive polyvinylidene fluoride with a solid content of 5% to the positive electrode intermediate material, stirring and mixing for 3 hours, to obtain a positive electrode slurry with a solid content of 58%; wherein the mass ratio of lithium manganese iron phosphate, carbon nanotubes and positive polyvinylidene fluoride is 97.4:0.4:1.7;

[0100] The positive electrode slurry was coated on the surface of the carbon-coated aluminum foil with a coating thickness of 90 μm, and then dried at 120°C to obtain a single-sided surface density of 200 g / m 2 Positive electrode;

[0101] (2) Preparation of negative electrode sheet:

[0102] Graphite and conductive carbon black with a mass ratio of 96:1.8 were stirred and mixed for 1 hour to obtain a negative electrode intermediate material; wherein the revolution speed of the stirring and mixing was 20 r / min and the dispersion speed was 500 r / min;

[0103] Adding a negative electrode binder and an aqueous binder to the negative electrode intermediate material and stirring and mixing for 2 hours to obtain a negative electrode slurry with a solid content of 45%; wherein the negative electrode binder is a mixed solution of sodium carboxymethyl cellulose and deionized water, the solid content of which is 1.5%, and the mass ratio of sodium carboxymethyl cellulose to the aqueous binder is 0.4:1.8;

[0104] Deionized water was added to the negative electrode slurry and stirred for 2 h. The stirring speed was 20 r / min and the dispersion speed was 1500 r / min to obtain the negative electrode material. Then, the negative electrode material with a thickness of 50 μm was coated on the surface of the copper foil and dried at a temperature of 90 ° C to obtain a single-sided surface density of 100 g / m 2 The negative electrode;

[0105] (3) Preparation of electrolyte:

[0106] A non-aqueous solvent is prepared by using ethylene carbonate, dimethyl carbonate and diethyl carbonate in a volume ratio of 2:5:3, and in a sealed glove box with a water content of less than 10 ppm, lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide powders are dissolved in the non-aqueous solvent to prepare a lithium salt solution with a concentration of 1 mol / L to obtain an electrolyte;

[0107] (4) Preparation of lithium-ion batteries:

[0108] The positive electrode sheet, the separator and the negative electrode sheet are stacked to form a core package and placed in an aluminum shell. The aluminum shell containing the core package is baked. After baking, the electrolyte is injected into the aluminum shell. After high-temperature standing and formation, the electrolyte is injected for the second time. After standing and volume separation, a lithium-ion battery is obtained.

[0109] Example 2

[0110] In this embodiment, the polymer is also as shown in formula L1, but n=2000, and the other conditions remain the same as in embodiment 1.

[0111] Example 3

[0112] In this embodiment, the polymer is also as shown in formula L1, but n=2000, and the polymer accounts for 2% of the total mass of the polymer and lithium manganese iron phosphate, and the other conditions are consistent with those in embodiment 1.

[0113] Example 4

[0114] In this embodiment, the polymer is also as shown in formula L1, but n=2000, and the polymer accounts for 5% of the total mass of the polymer and lithium manganese iron phosphate, and the other conditions are consistent with those in embodiment 1.

[0115] Example 5

[0116] In this embodiment, the polymer is also as shown in formula L1, but n=2000, and the polymer accounts for 10% of the total mass of the polymer and lithium manganese iron phosphate, and the other conditions are consistent with those in embodiment 1.

[0117] Example 6

[0118] In this embodiment, the polymer is as shown in formula L2, m=1000, n=1000, and the polymer accounts for 2% of the total mass of the polymer and lithium manganese iron phosphate, and the other conditions are consistent with those in Example 1;

[0119]

[0120] Example 7

[0121] In this embodiment, the polymer is as shown in formula L3, m=1000, n=1000, and the polymer accounts for 2% of the total mass of the polymer and lithium manganese iron phosphate, and the other conditions are consistent with those in Example 1;

[0122]

[0123] Example 8

[0124] (1) Preparation of positive electrode sheet:

[0125] Lithium manganese iron phosphate and conductive carbon black were stirred and mixed at a mass ratio of 95.2:0.5 for 1 hour, and then N-methylpyrrolidone was added and stirred and mixed for 2 hours to obtain a positive electrode intermediate material; wherein, the revolution speed during the stirring and mixing process was 25 r / min and the dispersion speed was 500 r / min;

[0126] Adding an aqueous solution of carbon nanotubes and positive polyvinylidene fluoride with a solid content of 5% to the positive electrode intermediate material, stirring and mixing for 3 hours, to obtain a positive electrode slurry with a solid content of 60%; wherein the mass ratio of lithium manganese iron phosphate, carbon nanotubes and positive polyvinylidene fluoride is 97.4:0.4:1.7;

[0127] The positive electrode slurry was coated on the surface of the carbon-coated aluminum foil with a coating thickness of 90 μm, and then dried at 120°C to obtain a single-sided surface density of 200 g / m 2 Positive electrode;

[0128] (2) Preparation of negative electrode sheet:

[0129] Graphite and conductive carbon black with a mass ratio of 96:1.8 were stirred and mixed for 1 hour to obtain a negative electrode intermediate material; wherein the revolution speed of the stirring and mixing was 20 r / min and the dispersion speed was 500 r / min;

[0130] Adding a negative electrode binder and an aqueous binder to the negative electrode intermediate material and stirring and mixing for 2 hours to obtain a negative electrode slurry with a solid content of 50%; wherein the negative electrode binder is a mixed solution of sodium carboxymethyl cellulose and deionized water, the solid content of which is 1.5%, and the mass ratio of sodium carboxymethyl cellulose to the aqueous binder is 0.4:1.8;

[0131] Deionized water was added to the negative electrode slurry and stirred for 2 h. The stirring speed was 20 r / min and the dispersion speed was 1500 r / min to obtain the negative electrode material. Then, the negative electrode material with a thickness of 50 μm was coated on the surface of the copper foil and dried at a temperature of 90 ° C to obtain a single-sided surface density of 100 g / m 2 The negative electrode;

[0132] (3) Preparation of electrolyte:

[0133] A non-aqueous solvent is prepared by using ethylene carbonate, dimethyl carbonate and diethyl carbonate in a volume ratio of 3:4:3, and in a sealed glove box with a water content of less than 10 ppm, lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide powders are dissolved in the non-aqueous solvent to prepare a lithium salt solution with a concentration of 1 mol / L, and then a polymerized monomer as shown in formula L4 is added to the lithium salt solution to obtain an electrolyte; wherein the polymerized monomer accounts for 1% by mass in the electrolyte;

[0134]

[0135] (4) Preparation of lithium-ion batteries:

[0136] The positive electrode sheet, the separator and the negative electrode sheet are stacked to form a core package and placed in an aluminum shell. The aluminum shell containing the core package is baked. After baking, the electrolyte is injected into the aluminum shell. After high-temperature standing and formation, the electrolyte is injected again. After standing and dividing the capacity, a lithium-ion battery is obtained. In the formation process, 2.5A constant current charging is adopted for 5 hours, and then 3.9V constant voltage charging is adopted for 12 hours. The charge and discharge test range is 2.5V-4.2V.

[0137] Example 9

[0138] In this embodiment, the mass proportion of the polymerized monomer in the electrolyte is 2%, and the other conditions remain the same as those in Embodiment 6.

[0139] Example 10

[0140] In this embodiment, the mass proportion of the polymerized monomer in the electrolyte is 5%, and the other conditions remain the same as those in Embodiment 6.

[0141] Embodiment 11

[0142] In this embodiment, the mass proportion of the polymerized monomer in the electrolyte is 10%, and the other conditions remain the same as those in Embodiment 6.

[0143] Example 12

[0144] In this embodiment, the polymerized monomer includes a first polymerized monomer and a second polymerized monomer, the first polymerized monomer is shown in Formula L4, the second polymerized monomer is shown in Formula L5, and the mass proportion of the polymerized monomer in the electrolyte is 2%, and the other conditions are consistent with those in Example 6;

[0145]

[0146] Example 13

[0147] In this embodiment, the polymerized monomer is as shown in formula L6, and the mass proportion of the polymerized monomer in the electrolyte is 2%, and the other conditions are consistent with those in embodiment 6;

[0148]

[0149] Embodiment 14

[0150] In this embodiment, the polymerizable monomer includes a first polymerizable monomer and a second polymerizable monomer, the first polymerizable monomer is as shown in Formula L6, the second polymerizable monomer is as shown in Formula L7, and the other conditions are consistent with those of Embodiment 9;

[0151]

[0152] Comparative Example 1

[0153] In this comparative example, no polymer was added, and the other conditions were the same as those in Example 1.

[0154] Comparative Example 2

[0155] In this comparative example, the polymer is as shown in formula L8, n=2000, and the other conditions are consistent with those in Example 3;

[0156]

[0157] Comparative Example 3

[0158] In this comparative example, the polymer is as shown in formula L9, n=2000, and the other conditions are consistent with Example 3;

[0159]

[0160] Comparative Example 4

[0161] In this comparative example, the polymer is as shown in formula L10, n=2000, and the other conditions are the same as those in Example 3;

[0162]

[0163] Comparative Example 5

[0164] In this comparative example, the polymerized monomer is as shown in formula L11, and the other conditions are consistent with those in Example 7;

[0165]

[0166] Comparative Example 6

[0167] In this comparative example, the polymerized monomer is as shown in formula L12, and the other conditions are consistent with those in Example 7;

[0168]

[0169] Comparative Example 7

[0170] In this comparative example, the polymerized monomer is as shown in formula L13, and the other conditions are consistent with those in Example 7;

[0171]

[0172] The batteries in Examples 1-14 and Comparative Examples 1-7 were subjected to cycle tests and rate performance tests respectively. The cycle test method is as follows: at an ambient temperature of 25°C, the battery is charged to 4.2V at a constant current of 1C (1C = 105A), then charged at a constant voltage to 0.05C, left for 30 minutes, discharged at a constant current of 1C to 2.5V, left for 30 minutes, and the charge and discharge cycle test is performed according to the above steps; the rate performance test method is as follows: at an ambient temperature of 25°C, the battery is charged to 4.2V at a constant current of 1C (1C = 105A), then charged at a constant voltage to 0.05C, left for 3 hours, discharged to 2.5V at 0.5C / 1C / 2C, and left for 30 minutes / 30 minutes / 60 minutes respectively. The results are shown in Table 1:

[0173] Table 1 Comparison of cycle performance and rate performance test results of different embodiments and comparative examples

[0174]

[0175]

[0176] As can be seen from Table 1, the capacity, rate performance and cycle performance of the batteries in Examples 1-14 of the present application are better than those in Comparative Documents 1-7. This is mainly because the embodiments of the present application form a coating layer on the surface of the positive electrode active material, and the coating layer can not only reduce manganese dissolution through physical restriction, but also can reduce manganese dissolution through chemical restriction by complexing with manganese ions in the positive electrode active material through the groups containing lone pair electrons in the coating layer. In addition, in the coating layer of the embodiment of the present application, the main chain of the polymer has good electrochemical activity, which can achieve coating while reducing the problem of capacity reduction caused by the introduction of inactive substances.

[0177] The batteries after cycles in Examples 1-14 and Comparative Examples 1-7 were disassembled, the powder on the surface of the negative electrode was scraped, aqua regia was added to the powder for digestion, and the manganese content was tested using an inductively coupled plasma emission spectrometer (ICP). The test results are shown in Table 2:

[0178] Table 2 Comparison of test results of manganese content on the negative electrode surface in different embodiments and comparative examples

[0179]

[0180]

[0181] As can be seen from Table 2, before the battery is cycled, the manganese content on the surface of the negative electrode sheet of Examples 1-14 and Comparative Examples 1-7 is not much different, but after 500 cycles, the manganese content on the surface of the negative electrode sheet of Comparative Examples 1-7 is significantly higher than that of Examples 1-14. This is mainly because the embodiments of the present application jointly limit the dissolution of manganese in the positive electrode active material through physical and chemical restrictions, thereby reducing the amount of manganese deposited on the negative electrode surface during the cycle.

[0182] The embodiments of the present application are introduced in detail above. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea. At the same time, for technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A positive electrode active material, characterized in that: It includes a positive electrode active material, and a coating layer coated on the surface of the positive electrode active material; The coating layer comprises a polymer as shown in Formula I: Wherein, at least one of R1 and R2 is a group containing a lone pair of electrons; the value range of m is 1-100000; the value range of n is 1-100000; The positive electrode active material contains manganese element.

2. The positive electrode active material according to claim 1, characterized in that R1 and R2 are independently selected from any one of a substituted or unsubstituted cyanoethoxy group, a substituted or unsubstituted carboxyl group, a substituted or unsubstituted hydroxyl group, a substituted or unsubstituted amino group and a substituted or unsubstituted sulfonic acid group.

3. The positive electrode active material according to claim 1, characterized in that R1 and R2 are independently selected from substituted or unsubstituted cyanoethoxy or substituted or unsubstituted carboxyl.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that: R x and R y Any one independently selected from a group containing a conjugated aromatic hydrocarbon, a group containing a conjugated thioether, a group containing a conjugated amine and an aromatic hetero group.

5. The positive electrode active material according to claim 4, characterized in that R x and R y Any one independently selected from substituted or unsubstituted phenyl, substituted or unsubstituted thienyl, substituted or unsubstituted pyrrolyl, substituted or unsubstituted anilino, substituted or unsubstituted phenothiazinyl, substituted or unsubstituted thianthrenyl, substituted or unsubstituted phenoxazinyl and substituted or unsubstituted carbazolyl.

6. The positive electrode active material according to claim 1, characterized in that The positive electrode active material includes lithium manganese iron phosphate.

7. The positive electrode active material according to any one of claims 1 to 3 or 5 to 6, characterized in that: The polymer is shown in any one of formula II-IV: The value range of m is 1-100000, and the value range of n is 1-100000.

8. The positive electrode active material according to any one of claims 1 to 3 or 5 to 6, characterized in that: The polymer is formed by polymerizing at least one of the monomers shown in Formulas V-VIII:

9. The positive electrode active material according to claim 7, characterized in that 100≤m+n≤100000.

10. A positive electrode sheet, characterized in that: The invention comprises the positive electrode active material as claimed in any one of claims 1 to 9.

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

12. A method for forming a coating layer, for forming a coating layer on the surface of a positive electrode active material, characterized in that: include: Adding a polymer to a positive electrode slurry containing a positive electrode active material to obtain a first mixture; Applying the first mixture to the surface of the positive electrode current collector, and baking the mixture so that the polymer is coated on the surface of the lithium manganese iron phosphate to form a coating layer; Wherein, the positive electrode active material contains manganese element; the polymer is as shown in Formula I: Wherein, R1 and R2 are both groups containing lone pair electrons; the value range of m is 1-100000; the value range of n is 1-100000.

13. The method for forming a coating layer according to claim 12, characterized in that: The mass proportion of the polymer in the total amount of the polymer and the positive electrode active material is less than 10%.

14. The method for forming a coating layer according to claim 12, characterized in that: The mass proportion of the polymer in the total amount of the polymer and the positive electrode active material is 0.2%-5%.

15. A method for forming a coating layer, for forming a coating layer on the surface of a positive electrode active material, characterized in that: include: Providing a positive electrode sheet containing a positive electrode active material; Providing an electrolyte, and adding a polymerizable monomer into the electrolyte to obtain a composite electrolyte; Assembling the positive electrode sheet and the composite electrolyte to form a battery; Performing formation charging on the battery so that the polymerized monomers are polymerized on the surface of the positive electrode active material to generate polymers to form a coating layer; Wherein, the positive electrode active material contains manganese element; the polymer is as shown in Formula I: Wherein, R1 and R2 are both groups containing lone pair electrons; the value range of m is 1-100000; the value range of n is 1-100000.

16. The method for forming a coating layer according to claim 15, characterized in that: The mass proportion of the polymerized monomer in the composite electrolyte is less than 10%.

17. The method for forming a coating layer according to claim 16, characterized in that: The mass proportion of the polymerized monomer in the composite electrolyte is 0.5%-5%.

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