Positive electrode slurry, positive electrode sheet and lithium-ion battery

Abstract

The present invention relates to the field of lithium-ion batteries, and particularly to a positive electrode slurry, a positive electrode sheet and a lithium-ion battery. The positive electrode slurry includes a positive electrode active material, a binder and a conductive agent; wherein, the binder includes polyvinylidene fluoride and a composite binder; the composite binder includes an α,β-ethylenically unsaturated nitrile compound, an ionic liquid and an organic acid ester; the conductive agent includes graphene, and the D50 of the graphene sheet diameter is 0.1 to 2.0 nm. The positive electrode slurry of the present invention is applied to the positive electrode sheet, which can significantly improve the compaction density of the positive electrode sheet and reduce the thickness of the positive electrode sheet, thereby significantly improving the energy density of the battery cell.

Description

Technical Field

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

[0002] In the battery core of a lithium battery, the thickness ratio of the positive electrode sheet usually exceeds 55%. Increasing the compaction density of the positive electrode sheet can effectively reduce the thickness of the positive electrode sheet, thereby improving the energy density of the battery core. However, due to the relatively low true density of the conductive agent and the binder, the use of the binder and the conductive agent in the positive electrode paste will reduce the overall compaction density of the positive electrode sheet, and thus affect the energy density of the battery core.

[0003] During the preparation process of the positive electrode sheet, it is necessary to roll the coated and dried electrode sheet to enhance the bonding strength between the active material and the foil, so as to prevent peeling during electrolyte immersion and battery use. The rolling of the electrode sheet can compress the volume of the battery core, improve the energy density of the battery core, reduce the porosity between the active material, the conductive agent and the binder inside the electrode sheet, reduce the resistance of the battery and improve the battery performance. However, when the electrode sheet is rolled, the electrode sheet will extend in all directions, and the active material bonded to the current collector will slip under the push of the horizontal pressure, thereby driving the extension of the current collector of the battery electrode sheet. If the rolling elongation rate of the electrode sheet is too large, it is easy to cause the electrode sheet to have wavy wrinkles, thereby affecting the conductivity of the positive electrode sheet. Summary of the Invention

[0004] In view of this, the present invention is committed to providing a positive electrode paste, a positive electrode sheet and a lithium-ion battery to solve the problems of relatively low compaction density of the positive electrode sheet and relatively high elongation rate of the electrode sheet after rolling in the prior art.

[0005] On the one hand, the present invention provides a positive electrode paste, which includes a positive electrode active material, a binder and a conductive agent;

[0006] Wherein, the binder includes polyvinylidene fluoride and a composite binder; the composite binder includes an α,β-ethylenically unsaturated nitrile compound, an ionic liquid and an organic acid ester;

[0007] The conductive agent includes graphene; the D50 of the graphene flake diameter is 0.1 - 2.0 nm.

[0008] In the positive electrode paste provided by the present invention, the binder includes polyvinylidene fluoride and a composite binder. By adding the composite binder, the flexibility of the electrode sheet can be increased, and the lithium iron phosphate particles can be more easily rearranged during rolling, reducing the extrusion of the aluminum foil of the positive electrode current collector, thereby reducing the rolling elongation rate. At the same time, in the positive electrode paste of the present invention, the conductive agent contains small-diameter graphene, which can significantly enhance the slip of lithium iron phosphate during rolling, thereby increasing the compaction density of the positive electrode sheet.

[0009] Optionally, the sheet diameter D50 of the graphene is 0.1 to 1.3 nm, and the number of layers of the graphene is 1 to 12 layers; based on the total mass of the positive electrode slurry, the mass fraction of the graphene is 0.05 to 0.3%.

[0010] Optionally, the conductive agent further includes conductive carbon black and / or carbon nanotubes.

[0011] Optionally, based on the total mass of the positive electrode slurry, the mass fraction of the conductive agent is 0.1 to 2.5%; optionally, based on the total mass of the positive electrode slurry, the mass fraction of the conductive carbon black is 0.1 to 1.6%; the mass fraction of the carbon nanotubes is 0.1 to 0.9%.

[0012] Optionally, based on the total mass of the positive electrode slurry, the mass fraction of the binder is 1.0 to 3.0%, and the mass ratio of polyvinylidene fluoride to the composite binder is 19 to 4:1; optionally, based on the total mass of the composite binder, the mass fraction of the α,β-ethylenically unsaturated nitrile compound is 85 to 99%, the mass fraction of the ionic liquid is 0.5 to 10%, and the mass fraction of the organic acid ester is 0.5 to 10%.

[0013] Optionally, the α,β-ethylenically unsaturated nitrile compound is selected from at least one of acrylonitrile, α-halopropionitrile, and α-alkylacrylonitrile; 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-butyl-1-methylpyrrolidinium imide, N-butylpyridinium imide, 1-ethyl-3-methylimidazolium methyl phosphate, N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide salt, 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 esters, citrate esters, and aliphatic dibasic acid esters; the citrate ester is selected from at least one of tributyl citrate, acetyl tributyl citrate, isopropyl citrate, polyoxyethylene / polyoxypropylene ether monoester of citric acid and propylene glycol, alkyl glycoside monoester of citric acid, and glycerol stearate citrate; the aliphatic dibasic acid ester is selected from at least one of dioctyl adipate, dioctyl azelate, dioctyl sebacate, and dibutyl sebacate.

[0014] 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 materials.

[0015] The second aspect of the present invention provides a positive electrode sheet, which includes 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. The positive electrode active material layer includes the above-mentioned positive electrode slurry.

[0016] Optionally, the difference between the compaction density of the positive electrode sheet and the compaction density of the positive electrode active material powder in the positive electrode sheet is ≥ 0.25 g / cc, and the rolling elongation rate of the positive electrode sheet is ≤ 1.5%.

[0017] The third aspect of the present invention provides a lithium-ion battery, which includes a positive electrode sheet, and the positive electrode sheet is the above-mentioned positive electrode sheet.

[0018] Through the above technical solutions, the present invention provides a positive electrode slurry, a positive electrode sheet, and a lithium-ion battery. The positive electrode slurry includes a positive electrode active material, a binder, and a conductive agent. Among them, the binder includes a mixture of polyvinylidene fluoride and a composite binder. By adding the composite binder, the flexibility of the electrode sheet can be increased, the extrusion of the aluminum foil of the positive electrode current collector can be reduced, and the rolling elongation rate can be decreased. At the same time, the conductive agent contains small-diameter graphene. By adding the small-diameter graphene, the slip of lithium iron phosphate during rolling can be significantly enhanced, thereby increasing the compaction density of the positive electrode sheet.

[0019] Other features and advantages of the present invention will be described in detail in the subsequent specific implementation section. Specific Implementation

[0020] The present invention discloses a positive electrode slurry, a positive electrode sheet, and a lithium-ion battery. Those skilled in the art can draw on the content of this article and appropriately modify the process parameters to achieve it. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all regarded as included in the present invention. The methods and applications of the present invention have been described through preferred embodiments. Those related can obviously make changes or appropriate alterations and combinations to the methods and applications described in this article without departing from the content, spirit, and scope of the present invention to implement and apply the technology of the present invention.

[0021] In the description of the present invention, a list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A, B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then 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 may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0022] In the description of the present invention, the term "compacted density" is one of the reference indicators of the material energy density, and the specific calculation formula is: compacted density = areal density / (thickness of the electrode sheet after rolling - thickness of the current collector), unit: g / cc; the term "rolling elongation rate" is the percentage of the areal density △M of the electrode sheet after rolling to the areal density M before rolling, and the specific calculation formula is: δ = △M / M × 100%.

[0023] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range, between the endpoint values or individual point values of each range, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0024] If there is no special instruction, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0025] If there is no special instruction, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0026] If there is no special instruction, the "including" and "comprising" mentioned in the present application mean open-ended, and can also be closed-ended. For example, the "including" and "comprising" can mean that other components not listed can also be included or comprised, or only the listed components can be included or comprised.

[0027] In order to solve the problems of low compacted density of the positive electrode sheet and high elongation rate after rolling in the prior art, the present invention adopts the following technical solutions:

[0028] On the one hand, the present invention provides a positive electrode slurry, which includes a positive electrode active material, a binder, and a conductive agent; wherein, the binder includes polyvinylidene fluoride and a composite binder; the composite binder includes an α,β-ethylenically unsaturated nitrile compound, an ionic liquid, and an organic acid ester; the conductive agent includes graphene, and the sheet diameter D50 of the graphene is 0.1 - 2.0 nm.

[0029] In the positive electrode paste of the present invention, the binder includes polyvinylidene fluoride (PVDF) and a composite binder. Among them, the composite binder includes 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, and the lithium iron phosphate particles can be rearranged more easily during rolling, reducing the extrusion of the aluminum foil of the positive electrode current collector, thereby reducing the rolling elongation rate. At the same time, in the positive electrode paste of the present invention, the conductive agent contains graphene, and the D50 of the graphene sheet diameter is 0.1 to 2.0 nm. By adding small-diameter graphene, the slip of lithium iron phosphate during rolling can be significantly enhanced, thereby increasing the tap density of the positive electrode sheet.

[0030] In order to further enhance the slip effect of lithium iron phosphate, the D50 of the graphene sheet diameter can be 0.1 to 1.3 nm, and the number of graphene layers can be 1 to 12 layers; exemplarily, the D50 of the graphene sheet diameter can be 0.2 to 0.8 nm, and the number of graphene layers can be 3 to 9 layers; as an exemplary embodiment of the present invention, the D50 of the graphene sheet diameter can be 0.2 to 0.6 nm, and the number of graphene layers can be 3 to 6 layers; optionally, based on the total mass of the positive electrode paste, the mass fraction of the graphene is 0.05 to 0.3%; exemplarily, the mass fraction of the graphene can be 0.09 to 0.25%.

[0031] The graphene used in the present invention can be commercially available, for example, it can be purchased from the HZ013XG model of Hefei Haizhou New Material Technology Co., Ltd.

[0032] Exemplarily, the conductive agent may further include conductive carbon black and / or carbon nanotubes.

[0033] In the present invention, based on the total mass of the positive electrode paste, the mass fraction of the conductive agent can be 0.1 to 2.5%; optionally, based on the total mass of the positive electrode paste, the mass fraction of the conductive carbon black is 0.1 to 1.6%; the mass fraction of the carbon nanotubes is 0.1 to 0.9%. An appropriate content of the conductive agent can reduce the contact resistance of the electrode and accelerate the electron movement rate, while also effectively improving the migration rate of lithium ions in the electrode material, thereby improving the charge and discharge efficiency of the electrode. Exemplarily, based on the total mass of the positive electrode paste, the mass fraction of the conductive agent is 0.3 to 1.0%, the mass fraction of the conductive carbon black is 0.3 to 0.9%; the mass fraction of the carbon nanotubes is 0.2 to 0.5%.

[0034] In an exemplary embodiment of the present invention, based on the total mass of the positive electrode slurry, the mass fraction of the binder may be 1.0 to 3.0%; exemplarily, the mass fraction of the binder may be 1.2 to 2.0%; the mass ratio of polyvinylidene fluoride to the composite binder may be 19 to 4:1; optionally, based on the total mass of the composite binder, the mass fraction of the α,β-ethylenically unsaturated nitrile compound is 85 to 99%, the mass fraction of the ionic liquid is 0.5 to 10%, and the mass fraction of the organic acid ester is 0.5 to 10%. As an exemplary embodiment of the present invention, based on the total mass of the composite binder, the mass fraction of the α,β-ethylenically unsaturated nitrile compound is 90 to 99%, the mass fraction of the ionic liquid is 0.5 to 5%, and the mass fraction of the organic acid ester is 0.5 to 5%. In this embodiment, the positive electrode slurry can endow the positive electrode sheet with better flexibility, which is more conducive to the rearrangement of lithium iron phosphate particles and greatly reduces the extrusion of the positive electrode current collector.

[0035] In an exemplary embodiment of the present invention, the α,β-ethylenically unsaturated nitrile compound may be selected from at least one of acrylonitrile, α-halopropionitrile, and α-alkylacrylonitrile; 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-butyl-1-methylpyrrolidinium imide, N-butylpyridinium imide, 1-ethyl-3-methylimidazolium methyl phosphate, N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide salt, 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 esters, citrate esters, and aliphatic dicarboxylic acid esters; exemplarily, the citrate ester may be selected from at least one of tributyl citrate, acetyl tributyl citrate, isopropyl citrate, polyoxyethylene / polyoxypropylene ether monoester of citric acid and propylene glycol, alkyl glycoside monoester of citric acid, and glycerol stearate citrate ester; the aliphatic dicarboxylic acid ester may be selected from at least one of dioctyl adipate, dioctyl azelate, dioctyl sebacate, and dibutyl sebacate.

[0036] Exemplarily, the positive electrode active material may be selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, and nickel cobalt manganese ternary materials.

[0037] Exemplarily, the positive electrode slurry of the present invention further includes a solvent, such as N-methylpyrrolidone.

[0038] The second aspect of the present invention provides a positive electrode sheet with a high tap density. The positive electrode sheet includes 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. The positive electrode active material layer includes the above-mentioned positive electrode paste.

[0039] The positive electrode sheet of the present invention has a significantly improved tap density and a significantly reduced roll pressing elongation rate. When applied to the production of battery cells, it can significantly improve the energy density of the battery cells.

[0040] Optionally, in the present invention, the difference between the tap density of the positive electrode sheet and the tap density of the positive electrode active material powder in the positive electrode sheet is ≥0.25 g / cc, and the roll pressing elongation rate of the positive electrode sheet is ≤1.5%.

[0041] In an exemplary embodiment of the present invention, the preparation process of the positive electrode sheet may include: mixing positive electrode active material, conductive carbon black, and polyvinylidene fluoride to obtain a first material. Adding N-methylpyrrolidone to the first material and kneading to obtain a second material. Adding graphene, carbon nanotube slurry, and composite binder to the second material, and obtaining a positive electrode paste after high-speed dispersion; coating the positive electrode paste on aluminum foil by extrusion coating, and forming a positive electrode sheet after drying. The positive electrode sheet is roll-pressed to reach the designed thickness. The positive electrode sheet provided by the present invention has mass production feasibility.

[0042] The third aspect of the present invention provides a lithium-ion battery, which includes a positive electrode sheet, and the positive electrode sheet is the above-mentioned positive electrode sheet.

[0043] The present invention will be further described in detail below through examples. All raw materials used in the examples can be obtained through commercial channels.

[0044] Example 1

[0045] Mix 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-methylpyrrolidone to the first material and knead to obtain a second material. Add 0.2 part of graphene, 0.5 part of carbon nanotube slurry, and 0.1 part of composite binder to the second material, and obtain the positive electrode paste of this example after high-speed dispersion. Among them, the number of layers of graphene used in this example is 12 layers, and the particle size D50 is 0.8 nm; based on the total mass of the composite binder, the composite binder used in this example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide, and 40 wt% of tributyl citrate.

[0046] Coat the prepared positive electrode paste on aluminum foil by extrusion coating, and form a positive electrode sheet after drying.

[0047] Example 2

[0048] Mix 96 parts of lithium iron phosphate, 1 part of conductive carbon black, and 2.2 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.2 part of graphene, 0.5 part of carbon nanotube slurry, and 0.1 part of composite binder to the second material, and obtain the positive electrode slurry of this example after high-speed dispersion. Among them, the number of layers of graphene used in this example is 6 layers, and the particle size D50 is 0.6 nm; based on the total mass of the composite binder, the composite binder used in this example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide, and 40 wt% of tributyl citrate.

[0049] Apply the prepared positive electrode slurry on the aluminum foil by extrusion coating, and form a positive electrode plate after drying.

[0050] Example 3

[0051] Mix 96 parts of lithium iron phosphate, 1 part of conductive carbon black, and 2.2 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.2 part of graphene, 0.5 part of carbon nanotube slurry, and 0.1 part of composite binder to the second material, and obtain the positive electrode slurry of this example after high-speed dispersion. Among them, the number of layers of graphene used in this example is 6 layers, and the particle size D50 is 0.2 nm; based on the total mass of the composite binder, the composite binder used in this example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide, and 40 wt% of tributyl citrate.

[0052] Apply the prepared positive electrode slurry on the aluminum foil by extrusion coating, and form a positive electrode plate after drying.

[0053] Example 4

[0054] Mix 95.6 parts of lithium iron phosphate, 1 part of conductive carbon black, and 2.2 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.2 part of graphene, 0.5 part of carbon nanotube slurry, and 0.5 part of composite binder to the second material, and obtain the positive electrode slurry of this example after high-speed dispersion. Among them, the number of layers of graphene used in this example is 6 layers, and the particle size D50 is 0.2 nm; based on the total mass of the composite binder, the composite binder used in this example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide, and 40 wt% of tributyl citrate.

[0055] The prepared positive electrode slurry is coated on an aluminum foil by means of extrusion coating, and a positive electrode sheet is formed after drying.

[0056] Example 5

[0057] 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-methylpyrrolidone is added to the first material and kneaded to obtain a second material. 0.2 part of graphene, 0.5 part of carbon nanotube slurry and 0.1 part of composite binder are added to the second material, and the positive electrode slurry of this example is obtained after high-speed dispersion. Among them, the number of layers of graphene used in this example is 2 layers, and the particle size D50 is 0.2 nm; based on the total mass of the composite binder, the composite binder used in this example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide and 40 wt% of tributyl citrate.

[0058] The prepared positive electrode slurry is coated on an aluminum foil by means of extrusion coating, and a positive electrode sheet is formed after drying.

[0059] Example 6

[0060] 96 parts of lithium iron phosphate, 1.55 parts of conductive carbon black and 2.2 parts of polyvinylidene fluoride are mixed to obtain a first material. N-methylpyrrolidone is added to the first material and kneaded to obtain a second material. 0.05 part of graphene, 0.1 part of carbon nanotube slurry and 0.1 part of composite binder are added to the second material, and the positive electrode slurry of this example is obtained after high-speed dispersion. Among them, the number of layers of graphene used in this example is 2 layers, and the particle size D50 is 0.2 nm; based on the total mass of the composite binder, the composite binder used in this example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide and 40 wt% of tributyl citrate.

[0061] The prepared positive electrode slurry is coated on an aluminum foil by means of extrusion coating, and a positive electrode sheet is formed after drying.

[0062] Example 7

[0063] Mix 96 parts of lithium iron phosphate, 0.5 part of conductive carbon black, and 2.2 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.3 part of graphene, 0.9 part of carbon nanotube slurry, and 0.1 part of composite binder to the second material, and obtain the positive electrode slurry of this example after high-speed dispersion. Among them, the number of layers of graphene used in this example is 6 layers, and the particle size D50 is 0.6 nm; based on the total mass of the composite binder, the composite binder used in this example includes 30 wt% acrylonitrile, 30 wt% 1-ethyl-3-methylimidazolium imide, and 40 wt% tributyl citrate.

[0064] Apply the prepared positive electrode slurry onto the aluminum foil by extrusion coating, and form the positive electrode sheet after drying.

[0065] Comparative Example 1

[0066] Mix 96 parts of lithium iron phosphate, 1 part of conductive carbon black, and 2.3 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.2 part of graphene and 0.5 part of carbon nanotube slurry to the second material, and obtain the positive electrode slurry of this comparative example after high-speed dispersion. Among them, the number of layers of graphene used in this comparative example is 15 layers, and the particle size D50 is 8 nm.

[0067] Apply the prepared positive electrode slurry onto the aluminum foil by extrusion coating, and form the positive electrode sheet after drying.

[0068] Comparative Example 2

[0069] Mix 96 parts of lithium iron phosphate, 1 part of conductive carbon black, and 2.3 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.2 part of graphene and 0.5 part of carbon nanotube slurry to the second material, and obtain the positive electrode slurry of this comparative example after high-speed dispersion. Among them, the number of layers of graphene used in this comparative example is 12 layers, and the particle size D50 is 0.8 nm.

[0070] Apply the prepared positive electrode slurry onto the aluminum foil by extrusion coating, and form the positive electrode sheet after drying.

[0071] Comparative Example 3

[0072] Mix 96 parts of lithium iron phosphate, 1 part of conductive carbon black, and 2.2 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.2 parts of graphene, 0.5 parts of carbon nanotube slurry, and 0.1 part of composite binder to the second material, and obtain the positive electrode slurry of this comparative example after high-speed dispersion. Among them, the number of layers of graphene used in this comparative example is 15 layers, and the particle size D50 is 8 nm; based on the total mass of the composite binder, the composite binder used in this comparative example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide, and 40 wt% of tributyl citrate.

[0073] Coat the prepared positive electrode slurry on the aluminum foil by extrusion coating, and form a positive electrode plate after drying.

[0074] Comparative Example 4

[0075] Mix 96 parts of lithium iron phosphate, 1 part of conductive carbon black, and 2.2 parts of polyvinylidene fluoride to obtain the first material. Add N-methylpyrrolidone to the first material and knead to obtain the second material. Add 0.2 parts of graphene, 0.5 parts of carbon nanotube slurry, and 0.1 part of composite binder to the second material, and obtain the positive electrode slurry of this comparative example after high-speed dispersion. Among them, the number of layers of graphene used in this comparative example is 12 layers, and the particle size D50 is 3 nm; based on the total mass of the composite binder, the composite binder used in this comparative example includes 30 wt% of acrylonitrile, 30 wt% of 1-ethyl-3-methylimidazolium imide, and 40 wt% of tributyl citrate.

[0076] Coat the prepared positive electrode slurry on the aluminum foil by extrusion coating, and form a positive electrode plate after drying.

[0077] The component ratios of the positive electrode slurries in Examples 1 to 7 and Comparative Examples 1 to 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).

[0078] Table 1

[0079]

[0080]

[0081] Test Example 1

[0082] Measure the tap density of the positive electrode plates prepared from lithium iron phosphate, Examples 1 to 7, and Comparative Examples 1 to 4 respectively, and the measurement results are shown in Table 2.

[0083] Table 2

[0084]

[0085] As can be seen from Table 2, the highest tap density of the positive electrode sheets prepared in Examples 1 to 7 can reach 2.68 g / cc, which is significantly improved compared to the tap density of the positive electrode sheets prepared in Comparative Examples 1 to 4.

[0086] Test Example 2

[0087] The rolling elongation rates of the positive electrode sheets prepared from lithium iron phosphate, Examples 1 to 7, and Comparative Examples 1 to 4 were measured respectively, and the measurement results are shown in Table 3.

[0088] Table 3

[0089] Project Rolling elongation Example 1 1.5％ Example 2 1.4％ Example 3 1.4％ Example 4 1.0％ Example 5 1.2％ Example 6 1.5％ Example 7 1.1％ Comparative Example 1 1.5％ Comparative Example 2 1.8％ Comparative Example 3 1.8％ Comparative Example 4 1.6％

[0090] As can be seen from Table 3, the elongation rate of the positive electrode sheets prepared in Examples 1 to 7 after rolling is significantly lower than that of the positive electrode sheets prepared in Comparative Examples 1 to 4 after rolling.

[0091] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A positive electrode paste, characterized in that, The positive electrode paste includes a positive electrode active material, a binder, and a conductive agent; Among them, the binder includes polyvinylidene fluoride and a composite binder; the composite binder includes an α,β-ethylenically unsaturated nitrile compound, an ionic liquid, and an organic acid ester; the organic acid ester is selected from at least one of fatty acid esters, citrate esters, and aliphatic dicarboxylate esters; Based on the total mass of the composite binder, the mass fraction of the α,β-ethylenically unsaturated nitrile compound is 85-99%, the mass fraction of the ionic liquid is 0.5-10%, and the mass fraction of the organic acid ester is 0.5-10%; The conductive agent includes graphene; the D50 of the graphene sheet diameter is 0.1-2.0 nm, and the number of layers of the graphene is 1-12 layers.

2. The positive electrode paste according to claim 1, wherein The D50 of the graphene sheet diameter is 0.1-1.3 nm; Based on the total mass of the positive electrode paste, the mass fraction of the graphene is 0.05-0.3%.

3. The positive electrode paste according to claim 1, characterized in that The conductive agent further includes conductive carbon black and / or carbon nanotubes.

4. The positive electrode paste according to claim 3, characterized in that, Based on the total mass of the positive electrode paste, the mass fraction of the conductive agent is 0.1-2.5%; the mass fraction of the conductive carbon black is 0.1-1.6%; the mass fraction of the carbon nanotubes is 0.1-0.9%.

5. The positive electrode paste according to claim 1, characterized in that, Based on the total mass of the positive electrode paste, the mass fraction of the binder is 1.0-3.0%; the mass ratio of polyvinylidene fluoride to the composite binder is 19-4:

1.

6. The positive electrode paste according to claim 1, wherein The α,β-ethylenically unsaturated nitrile compound is selected from at least one of acrylonitrile, α-halopropionitrile, and α-alkylacrylonitrile; 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-butyl-1-methylpyrrolidinium imide, N-butylpyridinium imide, 1-ethyl-3-methylimidazolium methyl phosphate, N-butyl-N-methylpyrrolidine bis(trifluoromethanesulfonyl)imide salt, 1-ethyl-3-methylimidazolium dihydrogen ammonium salt, 1-ethyl-2,3-dimethylimidazolium tetrafluoroborate, and 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate; The citrate ester is selected from at least one of tributyl citrate, acetyl tributyl citrate, isopropyl citrate, polyoxyethylene / polyoxypropylene ether monoester of citric acid and propylene glycol, alkyl glucoside monoester of citric acid, and glycerol stearate citrate ester; the aliphatic dicarboxylate ester is selected from at least one of dioctyl adipate, dioctyl azelate, dioctyl sebacate, and dibutyl sebacate; 7. The positive electrode paste according to claim 1, characterized in that, The positive electrode active material is selected from at least one of lithium iron phosphate, lithium manganese iron phosphate, and nickel cobalt manganese ternary materials.

8. A positive electrode sheet, characterized in that, The positive electrode plate includes 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, and the positive electrode active material layer includes the positive electrode paste according to any one of claims 1-7.

9. The positive electrode sheet according to claim 8, wherein The difference between the tap density of the positive electrode plate and the tap density of the positive electrode active material powder in the positive electrode plate is ≥0.25 g / cc, and the rolling elongation rate of the positive electrode plate is ≤1.5%.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode sheet, and the positive electrode sheet is the positive electrode sheet described in claim 8 or 9.

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