Lithium iron phosphate positive plate and manufacturing method and application thereof

Abstract

The invention provides a lithium iron phosphate positive plate and a manufacturing method and application thereof. A specific flexible binder and an additive containing a flexible organic group are used in an active layer of the lithium iron phosphate positive plate; the density C of the double side faces of the lithium iron phosphate positive plate, the compaction density D of the double sides, the elongation E and the elongation F of the current collector are controlled to meet the specific relations that E is larger than or equal to (C-46) * 0.02 + (D-2.55) * 2 + 0.9 and F is larger than or equal to 1.1 + E * 2, C is larger than or equal to 46 mg / cm < 2 >, and D is larger than or equal to 2.55 g / cm < 3 >; the flexible binder and the additive can greatly improve the maximum compaction density of the pole piece, so that the pole piece with high compaction density can be obtained under high surface density, and in order to further solve the problem of high band breakage frequency, a specific high-elongation current collector is matched and used through the relation of C and D, so that the reject ratio of the manufacturing process can be effectively reduced, and the production efficiency can be improved; and the high-surface-density and high-compaction pole piece can meet the mass production feasibility.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a lithium iron phosphate positive electrode sheet and a manufacturing method and application thereof. Background Art

[0002] With the adjustment of the global energy structure and the rapid development of the new energy vehicle industry, the demand for high-performance lithium-ion batteries is growing. The realization of this goal is highly dependent on improving the overall energy density of lithium-ion batteries. Therefore, further improving battery energy density from multiple perspectives such as battery system design, material system optimization, and electrode manufacturing technology has become one of the important directions of current lithium-ion battery research and development.

[0003] From the perspective of the internal structure of the battery, the positive electrode sheet is one of the key components that affect the battery capacity and energy density. The positive electrode sheet is usually composed of a current collector (such as aluminum foil), active material, conductive agent and binder, and is prepared through a series of process steps such as coating, drying, and rolling. Among them, the coating surface density and compaction density, as two key parameters in the electrode manufacturing process, have a direct and important impact on the energy density of the battery. The coating surface density refers to the mass of active material coated per unit area. The higher the value, the more active material can be accommodated in the battery per unit volume or unit weight, which helps to improve the overall capacity of the battery. The compaction density reflects the degree of closeness between the active material particles. A higher compaction density is not only beneficial to improving the electronic conductivity and ion diffusion capacity of the electrode, but also can reduce the internal porosity of the electrode to a certain extent, thereby improving the energy utilization rate of the battery.

[0004] However, in actual production, there is a certain mutual constraint between coating area density and compaction density. When coating area density increases, the thickness of the active material layer increases, and the compression ratio required during rolling also increases accordingly. This will cause the electrode to be subjected to greater stress during the compaction process. At this time, achieving a high compaction density will make electrode processing more difficult, and the frequency of electrode breakage during rolling will increase significantly, seriously affecting process yield and production efficiency.

[0005] From the above, it can be seen that there is still a need for a technical solution that can achieve a simultaneous improvement in coating surface density and compaction density without sacrificing processing stability and consistency to ensure process yield. Summary of the Invention

[0006] In view of the problems existing in the prior art, the object of the present invention is to provide a lithium iron phosphate positive electrode sheet and its manufacturing method and use, wherein a specific flexible binder and an additive containing a flexible organic group are used in the active layer of the lithium iron phosphate positive electrode sheet, and the double-sided density C, double-sided compaction density D, elongation E of the lithium iron phosphate positive electrode sheet and the elongation F of the current collector are controlled to satisfy a specific relationship E≥(C-46)×0.02+(D-2.55)×2+0.9, and F≥1.1+E×2, wherein C≥46mg / cm 2 , D≥2.55g / cm 3 Flexible adhesives and additives can significantly increase the maximum compaction density of the electrode, so as to obtain a high compaction density electrode at a high coating surface density. In order to ensure the smooth progress of the roller pressing process at this time and solve the problem of high belt breakage frequency, the relationship between C and D is used to match a specific high elongation current collector, which can effectively reduce the process defect rate and improve production efficiency, so that high surface density and high compaction electrode can meet the feasibility of mass production.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a lithium iron phosphate positive electrode sheet, comprising a current collector and an active layer disposed on both sides of the current collector, the active layer comprising lithium iron phosphate, a conductive agent, a binder, and an additive; the binder comprising a main binder and a flexible binder, the flexible binder having a film elongation at least twice that of the main binder; and the additive containing a flexible organic group;

[0009] The elongation of the current collector is F, the double-sided density of the lithium iron phosphate positive electrode sheet is C, the double-sided compaction density of the lithium iron phosphate positive electrode sheet is D, and the elongation of the lithium iron phosphate positive electrode sheet is E, then E≥(C-46)×0.02+(D-2.55)×2+0.9, and F≥1.1+E×2, where C≥46mg / cm 2 , D≥2.55g / cm 3 .

[0010] In the present invention, the size of the film elongation can show the softness of the binder. By introducing a flexible binder with higher film elongation into the positive electrode, the crystallinity of the main binder such as polyvinylidene fluoride (PVDF) is reduced, and the overall film elongation of the binder is improved. At the same time, an additive with a flexible group is added to synergistically improve the softness of the electrode. The additive exists in the positive electrode in the form of a liquid film, which can reduce the friction between the positive electrode material particles during the rolling process, thereby increasing the maximum (limit) compaction density of the electrode. At the same time, by using a current collector with both high tensile strength and high elongation, the maximum rolling pressure and electrode extension requirements required under high surface density and high compaction density can be met, effectively reducing the frequency of tape breakage of high compaction density electrodes during the rolling process under the process, thereby meeting the feasibility of mass production of high surface density and high compaction density positive electrode sheets.

[0011] It should be noted that the film elongation of the adhesive described herein refers to its elongation at break or tensile strength, an important parameter for measuring the adhesive's flexibility and ductility. It reflects the extent to which a material can stretch under stress and is a key indicator for evaluating adhesive performance.

[0012] The double-side density C of the present invention is ≥46 mg / cm 2 , for example, it can be 46 mg / cm 2 , 47mg / cm 2 , 48mg / cm 2 , 49mg / cm 2 、50mg / cm 2 , 51mg / cm 2 , 52mg / cm 2 , 53mg / cm 2 , 54mg / cm 2 、55mg / cm 2 , 56mg / cm 2 , 57mg / cm 2 , 58mg / cm 2 , 59mg / cm 2 、60mg / cm 2 、61mg / cm 2 or 62 mg / cm 2 wait.

[0013] The double-sided compaction density D of the present invention is ≥ 2.55 g / cm 3 , for example, it can be 2.55g / cm 3 , 2.58g / cm 3 , 2.6g / cm 3 , 2.62g / cm 3 , 2.65g / cm 3 , 2.68g / cm3 , 2.7g / cm 3 , 2.73g / cm 3 or 2.75g / cm 3 wait.

[0014] It should be noted that in the present invention, the two sides of the double-sided coated electrode are regarded as the same, that is, under normal circumstances, the coating thickness on one side is the same as the coating thickness on the other side, and each is equal to half of the total coating thickness; and the active layer thicknesses on both sides after rolling are the same, and each is equal to half of the total active layer thickness; similarly, the coating surface density on one side is regarded as the same as the coating surface density on the other side, and both are equal to half of the double-sided surface density C, and the compaction density on one side is regarded as the same as the compaction density on the other side, and both are equal to half of the double-sided compaction density D.

[0015] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0016] As a preferred technical solution of the present invention, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, carbon fibers or graphene.

[0017] Preferably, the primary binder comprises PVDF.

[0018] Preferably, the flexible binder includes copolymer-modified PVDF; the copolymer-modified PVDF includes PVDF-TFE.

[0019] Preferably, the film elongation of the main adhesive is 300% to 450%, for example, 300%, 330%, 350%, 380%, 400%, 420% or 450%, etc., and the film elongation of the flexible adhesive is 2 to 3 times the film elongation of the main adhesive, for example, 2 times, 2.1 times, 2.2 times, 2.3 times, 2.4 times, 2.5 times, 2.6 times, 2.7 times, 2.8 times, 2.9 times or 3 times, etc.

[0020] Preferably, the film elongation of the flexible adhesive is 700% to 900%, for example, 700%, 730%, 750%, 780%, 800%, 820%, 850%, 880% or 900%.

[0021] Preferably, the additive includes at least one of polyether-modified phosphate ester, polyester-modified phosphate ester, alcoholamine-modified phosphate ester, and alkyl-modified phosphate ester.

[0022] As a preferred technical solution of the present invention, taking the total mass of the lithium iron phosphate, conductive agent, binder and additives as 100%, the proportion of the flexible binder is recorded as A, and the proportion of the additive is recorded as B, then C≤(A+B)×2000+40.

[0023] Preferably, C≤62mg / cm 2 .

[0024] Preferably, the proportion of the lithium iron phosphate is 96% to 98.2%, such as 96%, 96.3%, 96.5%, 96.8%, 97%, 97.2%, 97.4%, 97.6%, 97.8%, 98% or 98.2%, etc.; the proportion of the conductive agent is 0.3% to 1.5%, such as 0.3%, 0.5%, 0.7%, 1%, 1.2%, 1.4% or 1.5%, etc.; The proportion of the main binder is 1.2% to 1.6%, for example, 1.2%, 1.3%, 1.4%, 1.5% or 1.6%, etc.; the proportion of the flexible binder A is 0.1% to 0.6%, for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or 0.6%, etc.; the proportion of the additive B is 0.2% to 0.5%, for example, 0.2%, 0.3%, 0.4% or 0.5%, etc.

[0025] As a preferred technical solution of the present invention, E≤(C-46)×0.02+(D-2.55)×2+1.

[0026] As a preferred technical solution of the present invention, the current collector includes a high-tensile and high-elongation aluminum foil, and the tensile strength of the high-tensile and high-elongation aluminum foil is ≥230MPa, for example, 230MPa, 240MPa, 250MPa, 260MPa, 270MPa, 280MPa, 290MPa, 300MPa, 310MPa, 320MPa, 330MPa, 340MPa or 350MPa, etc.; the elongation F is ≥4.5%, for example, 4.5%, 4.6%, 4.8%, 4.9%, 5%, 5.2%, 5.4%, 5.5%, 5.6%, 5.8% or 6%, etc.

[0027] It should be noted that, considering that the elongation of the actual tensile-resistant and high-elongation aluminum foil is not infinite, in order to meet the needs of practical applications, the present invention limits the maximum value of E, thereby limiting F to a value that is not unrealistically large, ensuring that actual products of high-tensile-resistant and high-elongation aluminum foil that can meet the elongation requirements can be matched.

[0028] Preferably, based on the mass of the high tensile strength and high elongation aluminum foil as 100%, the proportion of Al element is 99.1% to 99.5%, such as 99.1%, 99.2%, 99.3%, 99.4% or 99.5%, etc.; the proportion of Fe element is 0.4% to 0.5%, such as 0.4%, 0.42%, 0.44%, 0.46%, 0.48% or 0.5%, etc.; the proportion of Si element is 0.14% to 0.3%, such as 0.14 %, 0.16%, 0.18%, 0.2%, 0.22%, 0.24%, 0.26%, 0.28% or 0.3%, etc.; the proportion of Zn element is 0.01% to 0.1%, such as 0.01%, 0.03%, 0.05%, 0.08% or 0.1%, etc.; the proportion of Cu element is 0.001% to 0.01%, such as 0.001%, 0.003%, 0.005%, 0.008 or 0.01%, etc.

[0029] For example, compared with the high-tensile and high-elongation aluminum foil, a conventional aluminum foil has a tensile strength of 190-230 MPa, an elongation of 2.3%-3.3%, an Al element proportion of 99.6%-99.9%, a Fe element proportion of <0.30%, a Si element proportion of <0.15%, a Zn element proportion of <0.03%, and a Cu element proportion of <0.02%.

[0030] In a second aspect, the present invention provides a method for manufacturing the lithium iron phosphate positive electrode sheet according to the first aspect, the manufacturing method comprising:

[0031] Lithium iron phosphate, a conductive agent, a binder, and an additive are mixed to form a slurry; the binder includes a main binder and a flexible binder, the film elongation of the flexible binder is at least twice that of the main binder; and the additive contains a flexible organic group;

[0032] The slurry is coated on both sides of a current collector, the elongation of the current collector is F, the double-side density is controlled to be C, and then baked and rolled in sequence to obtain a lithium iron phosphate positive electrode sheet with a double-side compaction density of D. The elongation of the lithium iron phosphate positive electrode sheet is E, then E≥(C-46)×0.02+(D-2.55)×2+0.9, and F≥1.1+E×2, wherein C≥46mg / cm 2 , D≥2.55g / cm 3 .

[0033] As a preferred technical solution of the present invention, the preparation method of the lithium iron phosphate can be any one of a high-temperature solid-phase reaction method, a carbon thermal reduction method, a hydrothermal method, a sol-gel method or a co-precipitation method, or a combination of at least two thereof.

[0034] As a preferred technical solution of the present invention, the solvent in the slurry includes NMP (N-methylpyrrolidone).

[0035] Preferably, the solid content of the slurry is 57% to 70%, for example, 57%, 58%, 60%, 62%, 64%, 66%, 68% or 70%.

[0036] As a preferred technical solution of the present invention, the process of mixing lithium iron phosphate, a conductive agent, a binder and an additive to prepare a slurry includes: first mixing the main binder, the flexible binder and the solvent for 80 to 180 minutes, for example, 80 minutes, 100 minutes, 120 minutes, 140 minutes, 160 minutes or 180 minutes, etc., to obtain a binder glue; then adding the conductive agent to the binder glue and mixing for 30 to 60 minutes, for example, 30 minutes, 40 minutes, 50 minutes or 60 minutes, etc., to obtain a conductive glue; dividing the lithium iron phosphate into two parts, first adding the conductive glue to the conductive glue, and finally adding the conductive agent to the conductive glue. Add the first part and mix and stir for 15 to 30 minutes, for example, 15 minutes, 18 minutes, 20 minutes, 22 minutes, 24 minutes, 26 minutes, 28 minutes or 30 minutes; then add the second part and mix and stir for 60 to 120 minutes, for example, 60 minutes, 70 minutes, 80 minutes, 90 minutes, 100 minutes, 110 minutes or 120 minutes; while adding lithium iron phosphate, add the solvent and mix and stir; finally, add the additive and mix and stir for 30 to 60 minutes, for example, 30 minutes, 40 minutes, 50 minutes or 60 minutes to obtain the slurry.

[0037] As a preferred technical solution of the present invention, the total coating thickness of the double-sided coating is 300-460 μm, for example, 300 μm, 320 μm, 340 μm, 360 μm, 380 μm, 400 μm, 420 μm, 440 μm or 460 μm.

[0038] Preferably, the baking temperature is 85-115°C, such as 85°C, 90°C, 95°C, 100°C, 105°C, 110°C or 115°C, and the baking time is 2-6 minutes, such as 2 minutes, 3 minutes, 4 minutes, 5 minutes or 6 minutes.

[0039] In a third aspect, the present invention provides a battery comprising the lithium iron phosphate positive electrode sheet described in the first aspect.

[0040] It should be noted that due to space limitations and to avoid redundancy, the present invention does not exhaustively list all applicable point values ​​within the above numerical range, but is not limited to the listed values. Other unlisted values ​​within the above numerical range are also applicable.

[0041] Compared with the existing technical solutions, the present invention has at least the following beneficial effects:

[0042] In order to solve the problem in the prior art that high surface density and high compaction density cannot be taken into account at the same time, and the frequency of tape breakage during the pole piece rolling process is high, the present invention adopts a combination of flexible adhesives and additives to improve the maximum compaction density of the pole piece, ensuring that the compaction density is further improved at a high coating surface density. At the same time, by establishing a relationship between the double-side surface density of double-sided coating and the double-sided compaction density to match a specific high-tensile and high-elongation current collector, the problem of increased tape breakage frequency during rolling under high surface density and high compaction density is effectively improved, the process defect rate is effectively reduced and production efficiency is improved, so that high surface density and high compaction density lithium iron phosphate positive electrode pieces meet the feasibility of mass production. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0044] It should be apparent to those skilled in the art that the embodiments are only intended to help understand the present invention and should not be considered as specific limitations of the present invention.

[0045] Example 1

[0046] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet, comprising:

[0047] Weigh 0.07 kg of the main binder PVDF (film elongation is 320%) and 0.02 kg of the flexible binder PVDF-TFE (film elongation is 800%), mix and stir them in a double planetary mixer for 2 hours to completely dissolve them in 1.196 kg of solvent NMP to form a binder glue; under stirring conditions, add 0.025 kg of conductive carbon black and 0.020 kg of carbon nanotubes as conductive agents to the binder glue in sequence, mix and stir for 1 hour to fully disperse the conductive agent to obtain a conductive glue; then divide 4.85 kg of lithium iron phosphate into two parts, first add the first part and mix and stir for 0.5 hours, then add the second part and mix and stir for 1.5 hours, and during the addition of lithium iron phosphate, add 2.137 kg of solvent NMP at the same time, and finally add 0.015 kg of additive polyether modified phosphate (YTF6253 from Yite) and stir for 0.5 hours to make a positive electrode slurry with a solid content of 60%.

[0048] The prepared slurry was transferred to the storage tank of the extrusion coater and coated on both sides of the current collector by extrusion. The current collector was a high-tensile and high-elongation aluminum foil with a tensile strength of 236 MPa and an elongation of 4.5%. Based on the mass of the high-tensile and high-elongation aluminum foil as 100%, the proportion of Al element was 99.3%, the proportion of Fe element was 0.45%, the proportion of Si element was 0.17%, the proportion of Zn element was 0.05%, and the proportion of Cu element was 0.03%. The coating thickness on both sides was controlled to be 360 ​​μm, and the double-side density of the coating was controlled to be 54 mg / cm 2 Then bake at 100℃ for 3min, and then press the double-sided compaction density to 2.55g / cm 3 , 2.60g / cm 3 , 2.65g / cm 3 , 2.70g / cm 3 and 2.75g / cm 3 , and roller pressing is performed to obtain lithium iron phosphate positive electrode sheets with different compaction densities.

[0049] Example 2

[0050] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet. The manufacturing method reduces the double-sided density from 54 mg / cm 2 Adjusted to 62mg / cm 2 , except for the above, other conditions are exactly the same as those in Example 1.

[0051] Example 3

[0052] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet. The manufacturing method reduces the double-sided density from 54 mg / cm 2 Adjusted to 70mg / cm 2 Except for the above, other conditions are exactly the same as those in Example 1.

[0053] Example 4

[0054] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet, in which the mass of the flexible binder PVDF-TFE is adjusted from 0.02 kg to 0.005 kg, and the mass of the main binder PVDF is adjusted from 0.07 kg to 0.085 kg to keep the total mass of the main binder and the flexible binder unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0055] Example 5

[0056] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet, in which the mass of the flexible binder PVDF-TFE is adjusted from 0.02 kg to 0.01 kg, and the mass of the main binder PVDF is adjusted from 0.07 kg to 0.08 kg to keep the total mass of the main binder and the flexible binder unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0057] Example 6

[0058] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet, in which the mass of the flexible binder PVDF-TFE is adjusted from 0.02 kg to 0.03 kg, and the mass of the main binder PVDF is adjusted from 0.07 kg to 0.06 kg to keep the total mass of the main binder and the flexible binder unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0059] Example 7

[0060] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet, in which the mass of the flexible binder PVDF-TFE is adjusted from 0.02 kg to 0.04 kg, and the mass of the main binder PVDF is adjusted from 0.07 kg to 0.05 kg to keep the total mass of the main binder and the flexible binder unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0061] Example 8

[0062] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet. In the manufacturing method, the mass of the additive is adjusted from 0.015 kg to 0.005 kg, and the mass of other raw materials remains unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0063] Example 9

[0064] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet. The manufacturing method adjusts the mass of the additive from 0.015 kg to 0.01 kg, and keeps the mass of other raw materials unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0065] Example 10

[0066] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet. In the manufacturing method, the mass of the additive is adjusted from 0.015 kg to 0.025 kg, and the mass of other raw materials remains unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0067] Example 11

[0068] This embodiment provides a method for manufacturing a lithium iron phosphate positive electrode sheet. The manufacturing method adjusts the mass of the additive from 0.015 kg to 0.035 kg, keeping the mass of other raw materials unchanged. Except for the above, other conditions are exactly the same as those in Example 1.

[0069] Comparative Example 1

[0070] This comparative example provides a method for manufacturing a lithium iron phosphate positive electrode sheet, in which the current collector is replaced by a high-tensile and high-elongation aluminum foil with a conventional aluminum foil, wherein the conventional aluminum foil has a tensile strength of 210 MPa and an elongation of 3.3%. Based on the mass of the conventional aluminum foil as 100%, the Al element accounts for 99.65%, the Fe element accounts for 0.23%, the Si element accounts for 0.10%, the Zn element accounts for 0.01%, and the Cu element accounts for 0.01%. Except for the above, other conditions are exactly the same as those in Example 1.

[0071] Comparative Example 2

[0072] This comparative example provides a method for manufacturing a lithium iron phosphate positive electrode sheet, in which the mass of the flexible binder PVDF-TFE is adjusted from 0.02 kg to 0 kg, and the mass of the main binder PVDF is adjusted from 0.07 kg to 0.09 kg, that is, no flexible binder is used. Except for the above, other conditions are exactly the same as those in Example 1.

[0073] Comparative Example 3

[0074] This comparative example provides a method for manufacturing a lithium iron phosphate positive electrode sheet. In the manufacturing method, the mass of the additive is adjusted from 0.015 kg to 0 kg, that is, no additive is used. Except for the above, other conditions are exactly the same as those in Example 1.

[0075] Characterization and testing:

[0076] I. The lithium iron phosphate positive electrode sheets obtained in the examples and comparative examples were selected and tested for softness using a softness tester. The softness of the electrode sheet is characterized by the force (unit: mN) required to press down the same distance. The smaller the force, the softer the electrode sheet. Each positive electrode sheet was measured twice and the average result was taken. The results are shown in Table 1:

[0077] Table 1

[0078]

[0079]

[0080] In Table 1, “ / ” indicates that the positive electrode sheet with the double-sided compaction density could not be obtained and was not tested.

[0081] As can be seen from Table 1, the present invention can improve the flexibility of the electrode by optimizing the formula of flexible binder and additives, so that the maximum compaction density of the electrode can reach 2.75g / cm 3 Above (0.1g / cm2 higher than Comparative Example 1) 3 Comparative Example 1 has a maximum compaction density of only 2.65 g / cm 3 If we go higher, the pole piece has poor flexibility and the strip breaks every few dozen meters during the rolling process, making continuous rolling impossible.

[0082] Increasing the proportion of flexible adhesives and additives within an appropriate range will help improve the softness of the electrode.

[0083] Ⅱ. The lithium iron phosphate positive electrode sheets obtained in the examples and comparative examples were selected and the resistivity (unit: Ω·cm) of the electrode sheets was tested by a four-probe resistance tester. Two measurements were performed on each positive electrode sheet and the average value was taken. The test results are shown in Table 2:

[0084] Table 2

[0085]

[0086]

[0087] In Table 2, “ / ” indicates that the positive electrode sheet with the double-sided compaction density could not be obtained and was not tested.

[0088] It can be seen from Table 2 that the lithium iron phosphate positive electrode obtained by the present invention has improved softness and more uniform filling between particles after rolling at the same compaction density, which is beneficial to reducing the resistivity of the electrode sheet.

[0089] Ⅲ. Select the lithium iron phosphate cathode obtained in the examples and comparative examples to prepare a lithium-ion battery. The specific method is as follows:

[0090] 1) Cut the rolled positive electrode sheet into the required size;

[0091] 2) Preparation of negative electrode sheets: A negative electrode slurry was prepared according to a mass ratio of graphite: conductive agent SP: thickener CMC: binder SBR of 96.5:1.0:1.0:1.5; each raw material in the negative electrode slurry was added to a solvent according to the proportion of each raw material in the negative electrode slurry and stirred thoroughly to obtain a mixed slurry. The mixed slurry was then evenly coated on a copper foil, and the desired negative electrode sheet was obtained after drying, rolling, and cutting;

[0092] 3) The electrolyte used was a conventional lithium iron phosphate system electrolyte on the market, wherein the concentration of LiPF6 was 1.1 mol / L, and the mixed solvent components were ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 2.5:4.0:3.5.

[0093] 4) Assemble the prepared positive electrode sheet, negative electrode sheet, separator and electrolyte, and then test the electrochemical performance.

[0094] The test method is as follows:

[0095] Rate discharge test: At 25°C, the battery was charged to 3.65V at a constant current of 0.5C, then charged to 0.05C at a constant voltage, and allowed to stand for 30 minutes. The battery was then discharged to 2.0V at constant currents of 0.33C, 0.5C, 1C, and 2C, respectively. The discharge capacity ratios (unit: %) at different rates were compared. The results are shown in Table 3.

[0096] Cycling test: at 25°C and 45°C, the battery was charged to 3.65V at a constant current of 1C, charged to 0.05C at a constant voltage, left to stand for 30 minutes, and then discharged to 2.0V at a constant current of 1C. The number of cycles was 1000. The capacity retention rate (unit: %) was compared. The results are shown in Table 4.

[0097] Table 3

[0098]

[0099] Table 4

[0100]

[0101]

[0102] In Tables 3 and 4, “ / ” indicates that the positive electrode sheet with the double-sided compaction density could not be obtained and was not tested.

[0103] Through the analysis of the rate and cycle performance of the examples and comparative examples (Table 3 and Table 4), the formula of using a flexible binder with an additive containing a flexible group proposed in the present invention can significantly improve the softness of the electrode. In combination with the use of high tensile strength and high elongation aluminum foil, the positive electrode designed under high surface density and high compaction has the feasibility of mass production. In addition, because the flexible formula helps to reduce the friction resistance of the electrode slipping between particles during the rolling process, the filling of large and small particles is more uniform, and the distribution consistency of the conductive agent and the binder in the electrode is better. The reduction of the electrode resistivity and the improvement of the battery rate discharge and cycle performance are also achieved, such as: 2.65g / cm 3 At higher compaction density, the capacity retention rate after 1000 cycles at 25°C and 45°C increased by 1.8% to 3.0%, and the capacity retention rate was 2.70 to 2.75 g / cm 3 There is no obvious deterioration in rate and cycle performance at higher compaction density.

[0104] In summary, the present invention has a double-sided density of 46 to 62 mg / cm 2The high surface density lithium iron phosphate positive electrode sheet is designed with a flexible binder and an additive containing a flexible group, and the current collector adopts high tensile strength and high elongation aluminum foil, which can increase the compaction density of the electrode by 0.1g / cm 3 The above, combined with continuous rolling without uninterrupted belting, makes stable mass production of high-areal-density and high-compact-density lithium iron phosphate cathode sheets feasible. Furthermore, the combination of flexible binders and additives can reduce the frictional resistance of inter-particle slip during the electrode sheet rolling process, resulting in more uniform filling of large and small particles and more consistent distribution of the conductive agent and binder in the electrode sheet, thereby improving the battery's rate and cycle performance.

[0105] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0106] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0107] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A lithium iron phosphate positive electrode sheet, characterized in that: The lithium iron phosphate positive electrode sheet includes a current collector and an active layer disposed on both sides of the current collector, wherein the active layer includes lithium iron phosphate, a conductive agent, a binder, and an additive; the binder includes a main binder and a flexible binder, wherein the film elongation of the flexible binder is at least twice that of the main binder; and the additive contains a flexible organic group; The elongation of the current collector is F, the double-sided density of the lithium iron phosphate positive electrode sheet is C, the double-sided compaction density of the lithium iron phosphate positive electrode sheet is D, and the elongation of the lithium iron phosphate positive electrode sheet is E, then E≥(C-46)×0.02+(D-2.55)×2+0.9, and F≥1.1+E×2, where C≥46mg / cm 2 , D≥2.55g / cm 3 .

2. The lithium iron phosphate positive electrode sheet according to claim 1, characterized in that: The conductive agent includes at least one of conductive carbon black, carbon nanotubes, carbon fibers or graphene; Preferably, the primary binder comprises PVDF; Preferably, the flexible binder comprises copolymer-modified PVDF; the copolymer-modified PVDF comprises PVDF-TFE; Preferably, the film elongation of the main adhesive is 300% to 450%, and the film elongation of the flexible adhesive is 2 to 3 times that of the main adhesive; Preferably, the film elongation of the flexible adhesive is 700% to 900%; Preferably, the additive includes at least one of polyether-modified phosphate ester, polyester-modified phosphate ester, alcoholamine-modified phosphate ester, and alkyl-modified phosphate ester.

3. The lithium iron phosphate positive electrode sheet according to claim 1 or 2, characterized in that: Taking the total mass of the lithium iron phosphate, the conductive agent, the binder and the additive as 100%, the proportion of the flexible binder is recorded as A, and the proportion of the additive is recorded as B, then C≤(A+B)×2000+40; Preferably, the lithium iron phosphate accounts for 95.8% to 98.2%, the conductive agent accounts for 0.3% to 1.5%, the main binder accounts for 1.2% to 1.6%, the flexible binder accounts for A for 0.1% to 0.6%, and the additive accounts for B for 0.2% to 0.5%; Preferably, C≤62mg / cm 2 .

4. The lithium iron phosphate positive electrode sheet according to any one of claims 1 to 3, characterized in that: E≤(C-46)×0.02+(D-2.55)×2+1.

5. The lithium iron phosphate positive electrode sheet according to any one of claims 1 to 4, characterized in that: The current collector comprises a high-tensile-strength and high-elongation aluminum foil, wherein the high-tensile-strength and high-elongation aluminum foil has a tensile strength of ≥230 MPa and an elongation F of ≥4.5%; Preferably, based on the mass of the high-tensile and high-elongation aluminum foil as 100%, the proportion of Al element is 99.1% to 99.5%, the proportion of Fe element is 0.4% to 0.5%, the proportion of Si element is 0.14% to 0.3%, the proportion of Zn element is 0.01% to 0.1%, and the proportion of Cu element is 0.001% to 0.01%.

6. A method for manufacturing a lithium iron phosphate positive electrode sheet according to any one of claims 1 to 5, characterized in that: The manufacturing method comprises: Lithium iron phosphate, a conductive agent, a binder, and an additive are mixed to form a slurry; the binder includes a main binder and a flexible binder, the film elongation of the flexible binder is at least twice that of the main binder; and the additive contains a flexible organic group; The slurry is coated on both sides of a current collector, the elongation of the current collector is F, the double-side density is controlled to be C, and then baked and rolled in sequence to obtain a lithium iron phosphate positive electrode sheet with a double-side compaction density of D. The elongation of the lithium iron phosphate positive electrode sheet is E, then E≥(C-46)×0.02+(D-2.55)×2+0.9, and F≥1.1+E×2, wherein C≥46mg / cm 2 , D≥2.55g / cm 3 .

7. The method for manufacturing a lithium iron phosphate positive electrode sheet according to claim 6, characterized in that: The solvent in the slurry includes NMP; Preferably, the solid content of the slurry is 57% to 70%.

8. The method for manufacturing a lithium iron phosphate positive electrode sheet according to claim 6 or 7, characterized in that: The process of mixing lithium iron phosphate, a conductive agent, a binder and an additive to form a slurry includes: first mixing and stirring the main binder, a flexible binder and a solvent for 80 to 180 minutes to obtain a binder glue; then adding the conductive agent to the binder glue and stirring for 30 to 60 minutes to obtain a conductive glue; dividing the lithium iron phosphate into two parts, first adding the first part to the conductive glue and stirring for 15 to 30 minutes, then adding the second part and stirring for 60 to 120 minutes, adding the solvent and stirring at the same time during the addition of lithium iron phosphate, and finally adding the additive and stirring for 30 to 60 minutes to obtain the slurry.

9. The method for manufacturing a lithium iron phosphate positive electrode sheet according to any one of claims 6 to 8, characterized in that: The total coating thickness of the double-sided coating is 300 to 460 μm; Preferably, the baking temperature is 85-115° C. and the baking time is 2-6 minutes.

10. A battery, characterized in that: The battery comprises the lithium iron phosphate positive electrode sheet according to any one of claims 1 to 5.

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