Positive electrode slurry, positive electrode sheet and lithium ion battery

Abstract

The present application relates to the field of lithium ion batteries, in particular to a positive electrode slurry, a positive electrode sheet and a lithium ion battery. The positive electrode slurry comprises a positive electrode active material, a binder and a conductive agent; the binder comprises polyvinylidene fluoride and a composite binder, and the composite binder comprises an α,β-ethylenically unsaturated nitrile compound, an ionic liquid and organic acid ester; the conductive agent comprises graphene, the flake diameter D50 of the graphene being 0.1-2.0 nm Applying the positive electrode slurry to the positive electrode sheet can remarkably improve the compaction density of the positive electrode sheet and reduce the thickness of the positive electrode sheet, thereby remarkably improving the energy density of battery cells.

Description

Positive electrode slurry, positive electrode tab and lithium ion battery

[0001] The present application claims priority to the Chinese patent application No. 202410640574.9, filed on May 22, 2024, and entitled "Positive electrode slurry, positive electrode tab and lithium ion battery", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the field of lithium ion batteries, in particular to a positive electrode slurry, a positive electrode tab and a lithium ion battery. BACKGROUND

[0003] In the battery cell of a lithium battery, the thickness ratio of the positive electrode tab usually exceeds 55%. Increasing the compaction density of the positive electrode tab can effectively reduce the thickness of the positive electrode tab, thereby improving the energy density of the battery cell. However, due to the low true density of the conductive agent and the binder, the use of the binder and the conductive agent in the positive electrode slurry will reduce the compaction density of the positive electrode tab as a whole, thereby affecting the energy density of the battery cell.

[0004] During the preparation process of the positive electrode tab, the tab after drying is subjected to rolling to enhance the adhesion strength of the active material and the foil, so as to prevent peeling during electrolyte soaking and battery use. The tab rolling can compress the volume of the battery cell, improve the energy density of the battery cell, reduce the porosity between the active material, the conductive agent and the binder in the tab, and reduce the resistance of the battery to improve the performance of the battery. However, when the tab is rolled, the tab will extend in all directions, and the active material adhered to the current collector will slip under the push of the horizontal pressure, thereby driving the extension of the current collector of the battery tab. If the rolling extension rate of the tab is too large, it is easy to cause the tab to have a wavy wrinkle, thereby affecting the conductivity of the positive electrode tab.

[0005] SUMMARY

[0006] Therefore, the present application is devoted to providing a positive electrode slurry, a positive electrode tab and a lithium ion battery to solve the problems of low compaction density of the positive electrode tab and high extension rate of the tab after rolling in the prior art.

[0007] In one aspect, the present application provides a positive electrode slurry, which comprises a positive electrode active material, a binder and a conductive agent.

[0008] The binder comprises polyvinylidene fluoride and a composite binder. The composite binder comprises an α, β-olefinic unsaturated nitrile compound, an ionic liquid and an organic acid ester.

[0009] The conductive agent comprises graphene. The flake diameter D50 of the graphene is 0.1-2.0 nm.

[0010] In the positive electrode slurry provided by the application, the binder comprises polyvinylidene fluoride and a composite binder. By adding the composite binder, the flexibility of the electrode sheet can be increased, the lithium-iron particles can be more easily rearranged during rolling, the extrusion on the positive electrode current collector aluminum foil is reduced, and thus the rolling elongation is reduced. Meanwhile, in the positive electrode slurry of the application, the conductive agent contains small flake diameter graphene, which can significantly enhance the slip of lithium-iron during rolling, thereby increasing the compaction density of the positive electrode sheet.

[0011] Optionally, the flake diameter D50 of the graphene is 0.1-1.3 nm, the number of layers of the graphene is 1-12 layers, and the mass fraction of the graphene is 0.05-0.3% based on the total mass of the positive electrode slurry.

[0012] Optionally, the conductive agent further comprises conductive carbon black and / or carbon nanotubes.

[0013] Optionally, the mass fraction of the conductive agent is 0.1-2.5% based on the total mass of the positive electrode slurry, the mass fraction of the conductive carbon black is 0.1-1.6% based on the total mass of the positive electrode slurry, and the mass fraction of the carbon nanotubes is 0.1-0.9% based on the total mass of the positive electrode slurry.

[0014] Optionally, the mass fraction of the binder is 1.0-3.0% based on the total mass of the positive electrode slurry, and the mass ratio of the polyvinylidene fluoride to the composite binder is 19-4:1. Optionally, the mass fraction of the α, β-olefinic unsaturated nitrile compound is 85-99% based on the total mass of the composite binder, the mass fraction of the ionic liquid is 0.5-10%, and the mass fraction of the organic acid ester is 0.5-10%.

[0015] Optionally, the α, β-ethylenically unsaturated nitrile compound is selected from at least one of acrylonitrile, α-halogenated acrylonitrile and α-alkyl acrylonitrile; optionally, the ionic liquid is selected from at least one of 1-ethyl-3-methyl imidazolide, 1-butyl-3-methyl imidazolide, 1-ethyl-3-methyl imidazole tetrafluoroborate, 1-ethyl-3-methyl imidazole hexafluorophosphate, 1-butyl-1-methyl pyrrolidinium, N-butyl pyrrolidinium, 1-ethyl-3-methyl imidazole methylphosphonate, N-butyl-N-methyl pyrrolidinium bis(trifluoromethyl sulfone) imide, 1-ethyl-3-methyl imidazole dihydrogen ammonium salt, 1-ethyl-2, 3-dimethyl imidazole tetrafluoroborate and 1-ethyl-2, 3-dimethyl imidazole hexafluorophosphate; optionally, the organic acid ester is selected from at least one of fatty acid ester, citric acid ester and fatty diacid ester; the citric acid ester is selected from at least one of tributyl citrate, acetyl tributyl citrate, isopropyl citrate, propylene glycol polyoxyethylene / polyoxypropylene ether monolaurate, alkyl glycoside monolaurate and glycerol stearate citrate; the fatty diacid ester is selected from at least one of dioctyl adipate, dioctyl azelate, dioctyl sebacate and dibutyl sebacate.

[0016] Optionally, the positive electrode active material is selected from at least one of lithium iron phosphate, lithium manganese iron phosphate and nickel cobalt manganese ternary material.

[0017] The second aspect of the present application provides a positive electrode tab, which comprises a current collector, a conductive layer coated on the surface of the current collector and a positive electrode active material layer coated on the surface of the conductive layer, wherein the positive electrode active material layer comprises the positive electrode slurry described above.

[0018] Optionally, the difference between the compaction density of the positive electrode tab and the compaction density of the positive electrode active material powder in the positive electrode tab is ≥0.25 g / cc, and the rolling elongation of the positive electrode tab is ≤1.5%.

[0019] The third aspect of the present application provides a lithium ion battery, which comprises a positive electrode tab, wherein the positive electrode tab is the positive electrode tab described above.

[0020] Through the technical solution described above, the present application provides a positive electrode slurry, a positive electrode tab and a lithium ion battery, wherein the positive electrode slurry comprises a positive electrode active material, a binder and a conductive agent, the binder comprises a mixture of polyvinylidene fluoride and a composite binder, the addition of the composite binder can increase the flexibility of the tab, reduce the extrusion of the positive electrode current collector aluminum foil and reduce the rolling elongation; meanwhile, the conductive agent contains small flake diameter graphene, the addition of the small flake diameter graphene can significantly enhance the slip of iron lithium during rolling, thereby increasing the compaction density of the positive electrode tab.

[0021] Other features and advantages of the present application will be apparent from the detailed specification, which follows. DETAILED DESCRIPTION

[0022] The present application discloses a positive electrode slurry, a positive electrode sheet and a lithium ion battery, and those skilled in the art can refer to the content herein to appropriately improve the process parameters for implementation. It is particularly pointed out that all similar substitutions and changes are obvious to those skilled in the art, and they are considered to be included in the present application. The method and application of the present application have been described by the preferred embodiments, and the relevant personnel can obviously make changes or appropriate changes and combinations to the methods and applications described herein without departing from the content, spirit and scope of the present application, to realize and apply the present application technology.

[0023] In the description of the present application, the list of items connected by the term "at least one of" or other similar terms means any combination of the listed items. For example, if the items A, B are listed, the phrase "at least one of A, B" means only A; only B; or A and B. In another example, if the items A, B, C are listed, the phrase "at least one of A, B, C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can include a single element or multiple elements. Item B can include a single element or multiple elements. Item C can include a single element or multiple elements.

[0024] In the description of the present application, the term "compaction density" is one of the reference indicators of the energy density of the material, and the specific calculation formula is: compaction density = areal density / (thickness of the electrode sheet after rolling - thickness of the current collector), unit: g / cc; the term "roll extension rate" is the percentage of the areal density of the electrode sheet after rolling △M and the areal density M before rolling, and the specific calculation formula is: δ = △M / M x 100%.

[0025] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The ranges or values should be construed to be roughly about the ranges or values. For ranges, the endpoints are included; for values, the value is included. These ranges or values can be combined with other ranges or values to form new ranges or values, which are also within the scope of the present application.

[0026] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0027] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0028] If not specifically stated, the "comprising" and "including" mentioned in the present application represent open type and can also be closed type. For example, the "comprising" and "including" can represent that other components not listed can also be included or can only include the listed components.

[0029] In order to solve the problem of low compaction density of the positive electrode sheet and high elongation after rolling in the prior art, the present application adopts the following technical scheme:

[0030] In one aspect, the present application provides a positive electrode slurry, which comprises a positive electrode active material, a binder and a conductive agent; wherein the binder comprises polyvinylidene fluoride and a composite binder; the composite binder comprises an α, β-ethylenically unsaturated nitrile compound, an ionic liquid and an organic acid ester; and the conductive agent comprises graphene, and the flake diameter D50 of the graphene is 0.1-2.0 nm.

[0031] In the positive electrode slurry of the present application, the binder comprises polyvinylidene fluoride (PVDF) and a composite binder, wherein the composite binder comprises an α, β-ethylenically unsaturated nitrile compound, an ionic liquid and an organic acid ester. By adding the composite binder, the flexibility of the electrode sheet can be increased, the lithium-iron particles can be more easily rearranged during rolling, the extrusion on the positive electrode current collector aluminum foil is reduced, and thus the rolling elongation is reduced. At the same time, in the positive electrode slurry of the present application, the conductive agent contains graphene, and the flake diameter D50 of the graphene is 0.1-2.0 nm. By adding small flake diameter graphene, the sliding of lithium-iron during rolling can be significantly enhanced, thereby increasing the compaction density of the positive electrode sheet.

[0032] In order to further enhance the sliding effect of lithium-iron, the flake diameter D50 of the graphene can be 0.1-1.3 nm, and the number of layers of the graphene can be 1-12 layers; for example, the flake diameter D50 of the graphene can be 0.2-0.8 nm, and the number of layers of the graphene can be 3-9 layers; as an exemplary embodiment of the present application, the flake diameter D50 of the graphene can be 0.2-0.6 nm, and the number of layers of the graphene can be 3-6 layers; optionally, the mass fraction of the graphene is 0.05-0.3% based on the total mass of the positive electrode slurry, and for example, the mass fraction of the graphene can be 0.09-0.25%.

[0033] The graphene used in the present application can be commercially available, for example, HZ013XG type from Hefei Haizhou New Material Technology Co., Ltd.

[0034] For example, the conductive agent can further comprise conductive carbon black and / or carbon nanotubes.

[0035] In the present application, the mass fraction of the conductive agent can be 0.1-2.5% based on the total mass of the positive electrode slurry; alternatively, the mass fraction of the conductive carbon black can be 0.1-1.6% based on the total mass of the positive electrode slurry; and the mass fraction of the carbon nanotube can be 0.1-0.9%. The appropriate content of the conductive agent can reduce the contact resistance of the electrode, accelerate the movement rate of the electron, and effectively improve the migration rate of the lithium ion in the electrode material, thereby improving the charge and discharge efficiency of the electrode. For example, the mass fraction of the conductive agent can be 0.3-1.0% based on the total mass of the positive electrode slurry, the mass fraction of the conductive carbon black can be 0.3-0.9%, and the mass fraction of the carbon nanotube can be 0.2-0.5%.

[0036] In an exemplary embodiment of the present application, the mass fraction of the binder can be 1.0-3.0% based on the total mass of the positive electrode slurry; for example, the mass fraction of the binder can be 1.2-2.0%, and the mass ratio of the polyvinylidene fluoride and the composite binder can be 19-4:1; alternatively, the mass fraction of the α,β-olefinic unsaturated nitrile compound can be 85-99% based on the total mass of the composite binder, the mass fraction of the ionic liquid can be 0.5-10%, and the mass fraction of the organic acid ester can be 0.5-10%. In an exemplary embodiment of the present application, the mass fraction of the α,β-olefinic unsaturated nitrile compound can be 90-99% based on the total mass of the composite binder, the mass fraction of the ionic liquid can be 0.5-5%, and the mass fraction of the organic acid ester can be 0.5-5%. In this embodiment, the positive electrode slurry can impart better flexibility to the positive electrode sheet, facilitate the rearrangement of the iron lithium particles, and greatly reduce the extrusion on the positive electrode current collector.

[0037] In an exemplary embodiment of the present application, the α,β-ethylenically unsaturated nitrile compound can be selected from at least one of acrylonitrile, α-halogenated acrylonitrile and α-alkyl acrylonitrile; optionally, the ionic liquid is selected from at least one of 1-ethyl-3-methylimidazolium imide, 1-butyl-3-methylimidazolium imide, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylpyrrolidinium imide, N-butylpyrrolidinium imide, 1-ethyl-3-methylimidazolium methylphosphonate, N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium dihydrogen ammonium salt, 1-ethyl-2,3-dimethylimidazolium tetrafluoroborate and 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate; optionally, the organic acid ester is selected from at least one of fatty acid ester, citric acid ester and fatty diacid ester; exemplarily, the citric acid ester can be selected from at least one of tributyl citrate, acetyl tributyl citrate, isopropyl citrate, propylene glycol polyoxyethylene / polyoxypropylene ether monolaurate of citric acid, alkyl glycoside monolaurate of citric acid and glycerol stearate citrate; the fatty diacid ester can be selected from at least one of dioctyl adipate, dioctyl azelate, dioctyl sebacate and dibutyl sebacate.

[0038] Exemplarily, the positive electrode active material can be selected from at least one of lithium iron phosphate, lithium manganese iron phosphate and nickel cobalt manganese ternary material.

[0039] Exemplarily, the positive electrode slurry of the present application further comprises a solvent, such as N-methylpyrrolidone.

[0040] A second aspect of the present application provides a positive electrode tab with high compaction density, which comprises a current collector, a conductive layer coated on the surface of the current collector and a positive electrode active material layer coated on the surface of the conductive layer, wherein the positive electrode active material layer comprises the above-mentioned positive electrode slurry.

[0041] The positive electrode tab of the present application has significantly improved compaction density and significantly reduced rolling elongation, and when applied to the production of battery cells, the energy density of the battery cells can be significantly improved.

[0042] Optionally, the difference between the compaction density of the positive electrode tab and the compaction density of the positive electrode active material powder in the positive electrode tab is ≥0.25 g / cc, and the rolling elongation of the positive electrode tab is ≤1.5%.

[0043] In an example embodiment of the present application, the preparation process of the positive electrode sheet can include: mixing a positive electrode active material, conductive carbon black and polyvinylidene fluoride to obtain a first material. N-methyl pyrrolidone is added to the first material, and kneading is performed to obtain a second material. Graphene, carbon nanotube paste and composite binder are added to the second material, and high-speed dispersion is performed to obtain a positive electrode slurry. The positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed. The positive electrode sheet is rolled to the designed thickness. The positive electrode sheet provided by the present application has mass production feasibility.

[0044] The third aspect of the present application provides a lithium ion battery, which comprises the positive electrode sheet as described above.

[0045] The present application is further described in detail by the following examples. The raw materials used in the examples can be obtained by commercial channels.

[0046] Example 1

[0047] 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.2 parts of polyvinylidene fluoride are mixed to obtain a first material. N-methyl pyrrolidone is added to the first material, and kneading is performed to obtain a second material. 0.2 parts of graphene, 0.5 parts of carbon nanotube paste and 0.1 parts of composite binder are added to the second material, and high-speed dispersion is performed to obtain the positive electrode slurry of the present embodiment. In the present embodiment, the number of layers of graphene used is 12 layers, and the flake diameter D50 is 0.8 nm; based on the total mass of the composite binder, the composite binder used in the present embodiment includes 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methyl imidazole imide and 40wt% of tributyl citrate.

[0048] The prepared positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed.

[0049] Example 2

[0050] 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.2 parts of polyvinylidene fluoride are mixed to obtain a first material. N-methyl pyrrolidone is added to the first material, and kneading is performed to obtain a second material. 0.2 parts of graphene, 0.5 parts of carbon nanotube paste and 0.1 parts of composite binder are added to the second material, and high-speed dispersion is performed to obtain the positive electrode slurry of the present embodiment. In the present embodiment, the number of layers of graphene used is 12 layers, and the flake diameter D50 is 0.8 nm; based on the total mass of the composite binder, the composite binder used in the present embodiment includes 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methyl imidazole imide and 40wt% of tributyl citrate.

[0051] The prepared positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed.

[0052] Example 3

[0053] A first material is obtained by mixing 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.2 parts of polyvinylidene fluoride. N-methylpyrrolidone is added to the first material, and kneading is performed to obtain a second material. 0.2 parts of graphene, 0.5 parts of carbon nanotube slurry and 0.1 parts of a composite binder are added to the second material, and after high-speed dispersion, a positive electrode slurry of the present example is obtained. In the present example, the graphene used has 6 layers, and the flake diameter D50 is 0.2 nm; based on the total mass of the composite binder, the composite binder used in the present example includes 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methylimidazole imide and 40wt% of tributyl citrate.

[0054] The prepared positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed.

[0055] Example 4

[0056] A first material is obtained by mixing 95.6 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.2 parts of polyvinylidene fluoride. N-methylpyrrolidone is added to the first material, and kneading is performed to obtain a second material. 0.2 parts of graphene, 0.5 parts of carbon nanotube slurry and 0.5 parts of a composite binder are added to the second material, and after high-speed dispersion, a positive electrode slurry of the present example is obtained. In the present example, the graphene used has 6 layers, and the flake diameter D50 is 0.2 nm; based on the total mass of the composite binder, the composite binder used in the present example includes 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methylimidazole imide and 40wt% of tributyl citrate.

[0057] The prepared positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed.

[0058] Example 5

[0059] Mixing 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.2 parts of polyvinylidene fluoride to obtain a first material. Add N-methyl pyrrolidone to the first material and knead to obtain a second material. Add 0.2 parts of graphene, 0.5 parts of carbon nanotube slurry and 0.1 parts of composite binder to the second material, and after high-speed dispersion, the positive electrode slurry of the example is obtained. Among them, the graphene used in the example has 2 layers and a sheet diameter D50 of 0.2 nm; based on the total mass of the composite binder, the composite binder used in the example includes 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methyl imidazole imide and 40wt% of tributyl citrate.

[0060] The prepared positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed.

[0061] Example 6

[0062] Mixing 96 parts of lithium iron phosphate, 1.55 parts of conductive carbon black and 2.2 parts of polyvinylidene fluoride to obtain a first material. Add N-methyl pyrrolidone to the first material and knead to obtain a second material. Add 0.05 parts of graphene, 0.1 parts of carbon nanotube slurry and 0.1 parts of composite binder to the second material, and after high-speed dispersion, the positive electrode slurry of the example is obtained. Among them, the graphene used in the example has 2 layers and a sheet diameter D50 of 0.2 nm; based on the total mass of the composite binder, the composite binder used in the example includes 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methyl imidazole imide and 40wt% of tributyl citrate.

[0063] The prepared positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed.

[0064] Example 7

[0065] Mixing 96 parts of lithium iron phosphate, 0.5 parts of conductive carbon black and 2.2 parts of polyvinylidene fluoride to obtain a first material. Add N-methyl pyrrolidone to the first material and knead to obtain a second material. Add 0.3 parts of graphene, 0.9 parts of carbon nanotube slurry and 0.1 parts of composite binder to the second material, and after high-speed dispersion, the positive electrode slurry of the example is obtained. Among them, the graphene used in the example has 6 layers and a sheet diameter D50 of 0.6 nm; based on the total mass of the composite binder, the composite binder used in the example includes 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methyl imidazole imide and 40wt% of tributyl citrate.

[0066] The prepared positive electrode slurry is coated on an aluminum foil by extrusion coating, and after drying, a positive electrode sheet is formed.

[0067] Comparative Example 1

[0068] 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.3 parts of polyvinylidene fluoride were mixed to obtain a first material. N-methylpyrrolidone was added to the first material, and kneading was performed to obtain a second material. 0.2 parts of graphene and 0.5 parts of carbon nanotube slurry were added to the second material, and high-speed dispersion was performed to obtain the positive electrode slurry of the present comparative example. Among them, the number of layers of graphene used in the present comparative example was 15 layers, and the flake diameter D50 was 8 nm.

[0069] The prepared positive electrode slurry was coated on an aluminum foil by means of extrusion coating, and after drying, a positive electrode sheet was formed.

[0070] Comparative Example 2

[0071] 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.3 parts of polyvinylidene fluoride were mixed to obtain a first material. N-methylpyrrolidone was added to the first material, and kneading was performed to obtain a second material. 0.2 parts of graphene and 0.5 parts of carbon nanotube slurry were added to the second material, and high-speed dispersion was performed to obtain the positive electrode slurry of the present comparative example. Among them, the number of layers of graphene used in the present comparative example was 12 layers, and the flake diameter D50 was 0.8 nm.

[0072] The prepared positive electrode slurry was coated on an aluminum foil by means of extrusion coating, and after drying, a positive electrode sheet was formed.

[0073] Comparative Example 3

[0074] 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.2 parts of polyvinylidene fluoride were mixed to obtain a first material. N-methylpyrrolidone was added to the first material, and kneading was performed to obtain a second material. 0.2 parts of graphene, 0.5 parts of carbon nanotube slurry and 0.1 parts of a composite binder were added to the second material, and high-speed dispersion was performed to obtain the positive electrode slurry of the present comparative example. Among them, the number of layers of graphene used in the present comparative example was 15 layers, and the flake diameter D50 was 8 nm; based on the total mass of the composite binder, the composite binder used in the present comparative example included 30wt% of acrylonitrile, 30wt% of 1-ethyl-3-methylimidazolium and 40wt% of tributyl citrate.

[0075] The prepared positive electrode slurry was coated on an aluminum foil by means of extrusion coating, and after drying, a positive electrode sheet was formed.

[0076] Comparative Example 4

[0077] A first material was obtained by mixing 96 parts of lithium iron phosphate, 1 part of conductive carbon black and 2.2 parts of polyvinylidene fluoride. N-methyl pyrrolidone was added to the first material, and kneading was performed to obtain a second material. 0.2 parts of graphene, 0.5 parts of carbon nanotube slurry and 0.1 parts of a composite binder were added to the second material, and high-speed dispersion was performed to obtain the positive electrode slurry of the present comparative example. In the present comparative example, the graphene used had a layer number of 12 and a particle size D50 of 3 nm; and the composite binder used in the present comparative example included 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium and 40 wt% of tributyl citrate, based on the total mass of the composite binder.

[0078] The prepared positive electrode slurry was coated on an aluminum foil by means of extrusion coating, and after drying, a positive electrode sheet was formed.

[0079] The component proportions of the positive electrode slurries in Examples 1-7 and Comparative Examples 1-4 are shown in Table 1 (the content of each component in Table 1 is based on the total mass of the positive electrode slurry).

[0080] Table 1

[0081] Test Example 1

[0082] The compaction densities of the positive electrode sheets prepared from lithium iron phosphate, Examples 1-7 and Comparative Examples 1-4 were measured respectively, and the measurement results are shown in Table 2.

[0083] Table 2

[0084] As can be seen from Table 2, the compaction density of the positive electrode sheets prepared from Examples 1-7 can reach as high as 2.68 g / cc, which is significantly improved compared with the compaction densities of the positive electrode sheets prepared from Comparative Examples 1-4.

[0085] Test Example 2

[0086] The roll elongation rates of the positive electrode sheets prepared from lithium iron phosphate, Examples 1-7 and Comparative Examples 1-4 were measured respectively, and the measurement results are shown in Table 3.

[0087] Table 3

[0088] As can be seen from Table 3, the elongation rates of the positive electrode sheets prepared from Examples 1-7 after rolling are significantly lower than the elongation rates of the positive electrode sheets prepared from Comparative Examples 1-4 after rolling.

[0089] The above merely describes the preferred embodiments of the present application, and it should be pointed out that, for those skilled in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as falling within the protection scope of the present application.

Claims

1. A positive electrode slurry, wherein, The positive electrode slurry comprises a positive electrode active material, a binder and a conductive agent; The binder comprises polyvinylidene fluoride and a composite binder; the composite binder comprises an α, β-olefinic unsaturated nitrile compound, an ionic liquid and an organic acid ester; The conductive agent comprises graphene; the graphene has a sheet diameter D50 of 0.1-2.0 nm.

2. The positive electrode slurry of claim 1, wherein, The graphene has a sheet diameter D50 of 0.1-1.3 nm, and the graphene has a layer number of 1-12 layers. The mass fraction of the graphene is 0.05-0.3% based on the total mass of the positive electrode slurry.

3. The positive electrode slurry according to claim 1 or 2, wherein The conductive agent further comprises conductive carbon black and / or carbon nanotubes.

4. The positive electrode slurry of claim 3, wherein, The mass fraction of the conductive agent is 0.1-2.5% based on the total mass of the positive electrode slurry; the mass fraction of the conductive carbon black is 0.1-1.6%; and the mass fraction of the carbon nanotubes is 0.1-0.9%.

5. The positive electrode paste according to any one of claims 1 to 4, wherein, The mass fraction of the binder is 1.0-3.0% based on the total mass of the positive electrode slurry; and the mass ratio of the polyvinylidene fluoride and the composite binder is 19-4:

1. The mass fraction of the α, β-olefinic unsaturated nitrile compound is 85-99% based on the total mass of the composite binder; the mass fraction of the ionic liquid is 0.5-10%; and the mass fraction of the organic acid ester is 0.5-10%.

6. The positive electrode slurry according to any one of claims 1 to 5, wherein, The α, β-olefinic unsaturated nitrile compound is at least one selected from acrylonitrile, α-haloacrylonitrile and α-alkylacrylonitrile; The ionic liquid is at least one selected from 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-butyl-1-methylpyrrolidinium, N-butylpyrrolidinium, 1-ethyl-3-methylimidazolium methylphosphonate, N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium dihydrogen ammonium salt, 1-ethyl-2,3-dimethylimidazolium tetrafluoroborate and 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate; The organic acid ester is at least one selected from fatty acid ester, citric acid ester and fatty diacid ester; the citric acid ester is at least one selected from tributyl citrate, acetyl tributyl citrate, isopropyl citrate, propylene glycol polyoxyethylene / polyoxypropylene ether monostearate citrate, alkyl glycoside monostearate citrate and glycerol stearate citrate; and the fatty diacid ester is at least one selected from dioctyl adipate, dioctyl azelate, dioctyl sebacate and dibutyl sebacate.

7. The positive electrode slurry according to any one of claims 1 to 6, wherein, The positive electrode active material is at least one selected from lithium iron phosphate, lithium manganese iron phosphate and nickel-cobalt-manganese ternary material.

8. A positive electrode sheet, wherein, The positive electrode slurry comprises a positive electrode active material, a binder and a conductive agent; 9. The cathode sheet of claim 8, wherein, The difference between the compaction density of the positive electrode tab and the compaction density of the positive electrode active material powder in the positive electrode tab is ≥0.25 g / cc, and the roll elongation of the positive electrode tab is ≤1.5%.

10. A lithium-ion battery, wherein, The lithium-ion battery comprises a positive electrode sheet, and the positive electrode sheet is the positive electrode sheet according to claim 8 or 9. The lithium-ion battery comprises a positive electrode sheet, and the positive electrode sheet is the positive electrode sheet according to claim 8 or 9.

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