A long-cycle lithium iron phosphate thick electrode, a preparation method thereof, and a lithium-ion battery

Abstract

The present invention provides a long-cycle lithium iron phosphate thick electrode, a preparation method thereof, and a lithium ion battery. The lithium iron phosphate thick electrode includes a current collector and a first coating disposed on at least one surface of the current collector. A second coating and a lithium supplement layer are sequentially disposed on a surface of the first coating away from the current collector. The first coating includes small-particle lithium iron phosphate, and the second coating includes large-particle lithium iron phosphate. In the first coating and the second coating of the present invention, small-particle lithium iron phosphate and large-particle lithium iron phosphate are respectively contained. This structure of double-layer coating with large and small particles can effectively improve the liquid-phase transport efficiency and kinetic performance of the lithium iron phosphate thick electrode; at the same time, the lithium supplement layer can effectively make up for the loss of irreversible capacity in the thick electrode, improve the cycle performance of the electrode, and the arrangement of the lithium supplement layer on the outermost layer is also beneficial to the discharge of gas generated during the formation process, and cooperates with the first coating and the second coating to improve the rate performance and cycle performance of the lithium iron phosphate thick electrode at the same time.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and relates to a long-cycle lithium iron phosphate thick electrode, a preparation method thereof, and a lithium ion battery. Background Art

[0002] As the electrode thickness increases, the proportion of active materials increases significantly, thereby significantly improving the energy density of a single battery. Therefore, developing thick electrodes is of great significance for enhancing the battery energy density. However, as the electrode thickness increases, the liquid-phase lithium ion transport in the electrode sheet is blocked, which leads to an increase in battery internal resistance, a decrease in the utilization rate of active materials, and a significant attenuation of cycle performance and rate performance.

[0003] The main methods and strategies for improving the rate performance of thick electrodes are to develop high-rate positive and negative electrode materials, regulate the porous structure of thick electrodes, develop new electrolytes, etc. Among them, the design and regulation of the porous structure of thick electrodes are the most effective and widely used technical methods for improving the rate performance. For example, making holes in the thick electrode, and the construction of straight through pores formed after making holes can significantly improve the electrolyte infiltration rate of the thick electrode and improve its rate performance; such as Patent CN114400301A, which improves the infiltration effect of the electrolyte in the thick electrode by preparing a thick electrode and constructing a pore array in the electrode. However, compared with conventional electrodes, the irreversible capacity loss during the cycling of thick electrodes is serious, and the above methods have not been able to effectively reduce the capacity loss during cycling. Therefore, in order to improve the cycle performance of thick electrodes, the electrode sheet can be pre-lithiated.

[0004] Pre-lithiation technology is divided into negative electrode pre-lithiation and positive electrode pre-lithiation. Negative electrode pre-lithiation often uses lithium tape, lithium powder, lithium foil, etc. as lithium supplement agents. For example, Patent CN112786971A composes metallic lithium on at least one surface of the negative electrode sheet to obtain a pre-lithiated negative electrode sheet; however, lithium metals such as lithium powder and lithium tape are extremely sensitive to temperature and humidity in the environment and pose a large safety risk. Positive electrode pre-lithiation often uses Li2NiO2, Li5FeO4, Li2O, etc. as lithium supplement agents, which can deintercalate and intercalate lithium during the first charge to compensate for the active lithium consumed by the SEI, thereby improving the battery cycle performance; such as Patent CN114203990A, which adds lithium supplement agents such as Li2NiO2 and Li5FeO4 and positive electrode active materials into a solvent to prepare a positive electrode slurry, and then coats the positive electrode slurry on the surface of the current collector to prepare a lithium ion battery with a lithium supplement effect. However, the above positive electrode lithium supplement agents are similar to the negative electrode lithium supplement agents, are extremely sensitive to moisture, and generate a large amount of gas during deintercalation, and the effect of improving the battery cycle performance is limited.

[0005] In summary, preparing a lithium iron phosphate thick electrode with excellent rate performance and cycle performance is of great significance for the research and development of lithium ion batteries. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a long-cycle lithium iron phosphate thick electrode, a preparation method thereof, and a lithium-ion battery. The lithium iron phosphate thick electrode of the present invention is sequentially provided with a first coating, a second coating, and a lithium compensation layer on the surface of the current collector. The first coating and the second coating contain small-particle lithium iron phosphate and large-particle lithium iron phosphate respectively. This structure of double-layer coating with different particle sizes can effectively improve the liquid-phase transport efficiency and kinetic performance of the lithium iron phosphate thick electrode. At the same time, the lithium compensation layer can effectively make up for the loss of irreversible capacity in the thick electrode, improve the cycle performance of the electrode, and the lithium compensation layer arranged on the outermost layer is also beneficial to the discharge of gas generated during the formation process. Acting synergistically with the first coating and the second coating, it improves the rate performance and cycle performance of the lithium iron phosphate thick electrode at the same time.

[0007] In the present invention, "long cycle" means that the capacity retention rate of the battery is still not less than 80.6% after 5000 cycles at a rate of 0.5C / 0.5C; "thick electrode" means that the thickness of the electrode is not less than 100μm.

[0008] To achieve the purpose of this invention, the following technical solutions are adopted:

[0009] In the first aspect, the present invention provides a lithium iron phosphate thick electrode, which includes a current collector and a first coating provided on at least one surface of the current collector. The second coating and the lithium compensation layer are sequentially provided on the surface of the first coating away from the current collector. The first coating includes small-particle lithium iron phosphate, and the second coating includes large-particle lithium iron phosphate.

[0010] Preferably, the median particle size of the small-particle lithium iron phosphate in the first coating is 0.1 - 1μm.

[0011] Preferably, the median particle size of the large-particle lithium iron phosphate in the second coating is 1 - 10μm, and the median particle size of the large-particle lithium iron phosphate is greater than that of the small-particle lithium iron phosphate.

[0012] Preferably, the lithium compensation layer includes a lithium compensating agent and a binder.

[0013] Preferably, the mass ratio of the lithium compensating agent to the binder in the lithium compensation layer is (30 - 50):(20 - 60).

[0014] Preferably, the lithium compensation layer further includes a conductive agent.

[0015] Preferably, the mass ratio of the lithium compensating agent, the conductive agent, and the binder in the lithium compensation layer is (30 - 50):(10 - 30):(20 - 60).

[0016] Preferably, the lithium compensating agent includes any one or at least two combinations of Li2NiO2, Li5FeO4, and Li2O.

[0017] Preferably, the total thickness of the first coating and the second coating is 100 to 1000 μm.

[0018] Preferably, the thickness ratio of the first coating to the second coating is 1:4 to 2:1.

[0019] Preferably, the thickness of the lithium supplement layer is 3 to 10 μm.

[0020] Preferably, the first coating and the second coating independently further include a conductive agent and a binder.

[0021] Preferably, the mass ratio of small particle lithium iron phosphate, binder and conductive agent in the first coating is (90 - 95):(1 - 2):(3 - 5).

[0022] Preferably, the mass ratio of large particle lithium iron phosphate, binder and conductive agent in the second coating is (92 - 96):(1 - 2):(1 - 2).

[0023] Preferably, the conductive agent in the first coating, the conductive agent in the second coating and the conductive agent in the lithium supplement layer independently include any one or a combination of at least two of carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers and graphene.

[0024] Preferably, the binder in the first coating, the binder in the second coating and the binder in the lithium supplement layer independently include polyvinylidene fluoride.

[0025] Preferably, the tap density of the lithium iron phosphate thick electrode is 2.4 to 2.7 g / cm 3 .

[0026] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate thick electrode according to the first aspect, the preparation method comprising:

[0027] Preparing a first slurry and a second slurry respectively using small particle lithium iron phosphate and large particle lithium iron phosphate, and preparing a lithium supplement slurry, coating the first slurry, the second slurry and the lithium supplement slurry on the surface of a current collector, and forming a first coating, a second coating and a lithium supplement layer which are sequentially stacked on at least one surface of the current collector to obtain the lithium iron phosphate thick electrode.

[0028] Preferably, coating the first slurry, the second slurry and the lithium supplement slurry on the surface of the current collector is carried out in the following manner:

[0029] Coating the first slurry and the second slurry on at least one surface of the current collector by a double-layer coating method, baking to obtain the first coating and the second coating, and coating the lithium supplement slurry on the surface of the second coating to obtain the lithium supplement layer.

[0030] Preferably, the method of coating the lithium - supplement slurry on the surface of the current collector includes slot - die coating, gravure coating or micro - gravure coating.

[0031] In a third aspect, the present invention provides a lithium - ion battery, and the positive electrode of the lithium - ion battery uses the lithium iron phosphate thick electrode according to the first aspect.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The lithium iron phosphate thick electrode of the present invention includes a current collector and a first coating, a second coating and a lithium - supplement layer sequentially arranged on the surface of the current collector. The first coating and the second coating respectively contain small - particle lithium iron phosphate and large - particle lithium iron phosphate. This structure of double - layer coating with different particle sizes can effectively improve the liquid - phase transport efficiency and kinetic performance of the lithium iron phosphate thick electrode, and greatly improve the rate performance of the lithium iron phosphate thick electrode. At the same time, the lithium iron phosphate thick electrode of the present invention also includes a lithium - supplement layer arranged on the surface of the second coating away from the current collector. The lithium - supplement layer can effectively make up for the irreversible capacity loss of the thick electrode and significantly improve the cycle performance of the thick electrode; the lithium - supplement layer is arranged on the surface of the thick electrode, and the gas generated during the formation process is easy to discharge, so that the gas can be efficiently discharged during the formation stage, preventing it from affecting the stability of the battery during subsequent cycling. The specific cooperation among the first coating, the second coating and the lithium - supplement layer in the present invention can improve the rate performance and cycle performance of the lithium iron phosphate thick electrode simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic structural diagram of a lithium iron phosphate thick electrode in a specific embodiment of the present invention.

[0035] Figure 2 is a schematic structural diagram of a device for preparing a lithium iron phosphate thick electrode in a specific embodiment of the present invention.

[0036] Figure 3 is a performance comparison diagram of the lithium iron phosphate thick electrodes of Examples 1 - 5 and Comparative Examples 1 - 5 of the present invention at different rates.

[0037] Figure 4 is a performance comparison diagram of the lithium iron phosphate thick electrodes of Examples 1 - 5 and Comparative Examples 1 - 5 of the present invention at different numbers of cycling.

[0038] Wherein, 1 - current collector; 2 - first coating; 3 - second coating; 4 - lithium - supplement layer; 5 - unwind device; 6 - double - layer simultaneous coating die; 7 - drying device; 8 - lithium - supplement coating device; 9 - rolling device; 10 - winding device. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] The technical solution of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0040] The development of thick electrodes is an important method and strategy to improve the energy density of lithium batteries. As the electrode thickness increases, the internal resistance of the battery increases significantly, its rate performance deteriorates rapidly, the irreversible reaction intensifies during the cycling process, and the battery life is significantly reduced. In order to simultaneously optimize the rate performance and cycling performance of thick electrodes, the present invention proposes a long-cycle lithium iron phosphate thick electrode, its preparation method and a lithium-ion battery, which have important value in the development and application of thick electrodes for lithium batteries.

[0041] The specific embodiment part of the present invention provides a lithium iron phosphate thick electrode, and its structural schematic diagram is as Figure 1 shown. The lithium iron phosphate thick electrode includes a current collector 1 and a first coating 2 provided on at least one surface of the current collector 1. On the surface of the first coating 2 away from the current collector 1, a second coating 3 and a lithium supplement layer 4 are sequentially provided. The first coating 2 includes small-particle lithium iron phosphate, and the second coating 3 includes large-particle lithium iron phosphate.

[0042] The lithium iron phosphate thick electrode of the present invention includes a current collector 1, and a first coating 2, a second coating 3 and a lithium supplement layer 4 sequentially provided on the surface of the current collector 1. On the one hand, the first coating 2 and the second coating 3 respectively contain small-particle lithium iron phosphate and large-particle lithium iron phosphate. The particle size of lithium iron phosphate in the first coating 2 close to the surface of the current collector 1 is smaller, and the particle size of lithium iron phosphate in the second coating 3 away from the surface of the current collector 1 is larger. This structure of double-layer coating with large and small particles can effectively improve the liquid-phase transport efficiency and kinetic performance of the lithium iron phosphate thick electrode, and greatly improve the rate performance of the lithium iron phosphate thick electrode. On the other hand, the lithium iron phosphate thick electrode of the present invention further includes a lithium supplement layer 4 provided on the surface of the second coating 3 away from the current collector 1. The lithium supplement layer 4 can effectively make up for the irreversible capacity loss of the thick electrode and significantly improve the cycling performance of the thick electrode; at the same time, the lithium supplement layer 4 is provided on the surface of the thick electrode, and the gas generated during the formation process is easy to discharge, so that the gas can be efficiently discharged during the formation stage, preventing it from affecting the stability of the battery during the subsequent cycling process. The specific first coating 2, second coating 3 and lithium supplement layer 4 in the present invention cooperate with each other to simultaneously improve the rate performance and cycling performance of the lithium iron phosphate thick electrode.

[0043] In some embodiments, the median particle size of the small-particle lithium iron phosphate in the first coating 2 is 0.1 - 1 μm, for example, it can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm or 1 μm, etc.

[0044] In some embodiments, the median particle size of the large-particle lithium iron phosphate in the second coating 3 is 1-10 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm, etc., and the median particle size of the large-particle lithium iron phosphate is greater than the median particle size of the small-particle lithium iron phosphate.

[0045] It should be noted that in the present invention, the median particle size has the meaning well-known in the art, also known as the median diameter or average particle size D50, which is used to represent the average particle size of the active material. The physical meaning is that the particles smaller than this particle size account for 50% of the total volume of the particles, and the particles larger than this particle size also account for 50% of the total volume of the particles. The median particle size can be conveniently measured by a laser particle size analyzer.

[0046] In the present invention, the median particle sizes of the large-particle lithium iron phosphate and the small-particle lithium iron phosphate are further optimized. When the particle size of the large particles is relatively large, the solid-phase transport path of lithium ions in the second coating 3 increases significantly, the solid-phase diffusion rate of lithium ions deteriorates significantly, the kinetic performance decays rapidly, and in addition, the compaction density of the thick electrode is relatively low. When the particle size of the small particles is relatively small, the pores between the particles in the first coating 2 are relatively small, the liquid-phase transport resistance of the electrolyte is large, the liquid-phase ohmic impedance increases significantly, and at the same time, the relatively large specific surface area of the small particles results in a low compaction density and poor cycling performance of the thick electrode, and it is difficult to obtain a thick electrode with excellent rate performance and good cycling stability.

[0047] In some embodiments, the lithium supplement layer 4 includes a lithium supplement agent and a binder.

[0048] In some embodiments, the mass ratio of the lithium supplement agent to the binder in the lithium supplement layer 4 is (30-50):(20-60), where the selection range of the lithium supplement agent (30-50) can be, for example, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48 or 50, etc., and the selection range of the binder (20-60) can be, for example, 20, 25, 30, 35, 40, 45, 50, 55 or 60, etc.

[0049] In the present invention, when the lithium supplement layer 4 contains a lithium supplement agent and a large amount of binder, on the one hand, a large amount of binder can coat the surface of the lithium supplement agent, effectively avoiding the contact between the lithium supplement agent and air during the preparation of the thick lithium iron phosphate electrode, effectively suppressing the occurrence of side reactions on the surface of the lithium supplement agent or the electrode surface, improving the stability, and achieving longer cycles; on the other hand, since the lithium supplement layer 4 contains a relatively high content of binder, when the lithium supplement agent decreases or disappears after exerting its lithium supplement effect during the formation stage, the lithium supplement layer 4 can become a polymer film-like binder layer coating the surface of the thick lithium iron phosphate electrode, and the binder layer contains pores caused by the gas generation of the lithium supplement agent, effectively suppressing diaphragm wrinkling, while improving diaphragm wettability, and further improving the rate performance and cycle performance of the thick electrode. Therefore, when the lithium supplement layer 4 contains a lithium supplement agent and a binder with a mass ratio of (30-50):(20-60), it can improve the stability of prelithiation and optimize the battery interface effect.

[0050] In some embodiments, the lithium supplement layer 4 further includes a conductive agent.

[0051] In some embodiments, the mass ratio of the lithium supplement agent, the conductive agent and the binder in the lithium supplement layer 4 is (30-50):(10-30):(20-60). The selection range of the lithium supplement agent (30-50) can be, for example, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48 or 50, etc.; the selection range of the conductive agent (10-30) can be, for example, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 or 30, etc.; the selection range of the binder (20-60) can be, for example, 20, 25, 30, 35, 40, 45, 50, 55 or 60, etc.

[0052] In the present invention, a relatively high content of conductive agent and binder can coat the surface of the lithium supplement agent. The three with a suitable content ratio can inhibit the occurrence of side reactions on the surface of the thick lithium iron phosphate electrode and improve the prelithium stability of the lithium supplement layer 4; at the same time, a relatively high content of conductive agent significantly improves the intercalation / deintercalation reaction rate of the lithium supplement agent, and cooperates with the specific first coating 2 and second coating 3, so that the Coulomb efficiency of the electrode after lithium supplementation is significantly improved.

[0053] In some embodiments, the lithium supplement agent includes any one or a combination of at least two of Li2NiO2, Li5FeO4 and Li2O.

[0054] In some embodiments, the total thickness of the first coating 2 and the second coating 3 is 100 to 1000 μm. For example, it can be 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, or 1000 μm, etc. Within this thickness range, it is beneficial to improve the energy density and cycling performance of the battery cell. If the thickness is too high, the battery polarization is large, and the cycle life is significantly attenuated.

[0055] In some embodiments, the thickness ratio of the first coating 2 to the second coating 3 is 1:4 to 2:1. For example, it can be 1:4, 1:3, 1:2, 1:1, 1.5:1, or 2:1, etc.

[0056] In the present invention, by controlling the thickness ratio of the first coating 2 to the second coating 3 within the range of 1:4 to 2:1, the pore structure of the thick electrode can be effectively regulated, the transport of the electrolyte in the thickness direction of the electrode can be promoted, and the kinetic performance and cycling stability of the thick electrode can be improved.

[0057] In some embodiments, the thickness of the lithium supplement layer 4 is 3 to 10 μm. For example, it can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm, etc. Selecting this thickness not only helps to exert the pre-lithiation effect, better combine with the first coating 2 and the second coating 3, and improve the cycling performance of the lithium iron phosphate thick electrode, but also helps to enhance its function of suppressing diaphragm wrinkling and improving diaphragm wettability after it becomes a binder layer during the formation stage, further improving the rate performance and cycling performance of the lithium iron phosphate thick electrode.

[0058] In some embodiments, the first coating 2 and the second coating 3 independently further include a conductive agent and a binder.

[0059] In the present invention, "independently" means that the selection of the two does not interfere with each other. Exemplarily, the first coating 2 and the second coating 3 independently further include a conductive agent and a binder, which means that the first coating 2 can contain a conductive agent, the second coating 3 can contain a binder, or both the first coating 2 and the second coating 3 can contain a conductive agent and a binder, and the selection of the two does not interfere with each other. And when both the first coating 2 and the second coating 3 contain a conductive agent and / or a binder, the types of the conductive agent and / or the binder in the first coating 2 can be the same as or different from those in the second coating 3.

[0060] In some embodiments, the mass ratio of small particle lithium iron phosphate, binder, and conductive agent in the first coating 2 is (90 - 95):(1 - 2):(3 - 5). The selection range (90 - 95) of the small particle lithium iron phosphate can be, for example, 90, 91, 92, 93, 94, or 95, etc.; the selection range (1 - 2) of the binder can be, for example, 1, 1.1, 1.2, 1.5, 1.8, or 2, etc.; the selection range (3 - 5) of the conductive agent can be, for example, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, or 5, etc.

[0061] In some embodiments, the mass ratio of large particle lithium iron phosphate, binder, and conductive agent in the second coating 3 is (92 - 96):(1 - 2):(1 - 2). The selection range (90 - 96) of the large particle lithium iron phosphate can be, for example, 90, 91, 92, 93, 94, 95, or 96, etc.; the selection range (1 - 2) of the binder can be, for example, 1, 1.1, 1.2, 1.5, 1.8, or 2, etc.; the selection range (1 - 2) of the conductive agent can be, for example, 1, 1.1, 1.2, 1.5, 1.8, or 2, etc.

[0062] In some embodiments, the conductive agent in the first coating 2, the conductive agent in the second coating 3, and the conductive agent in the lithium - supplementing layer 4 independently include any one or a combination of at least two of carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers, and graphene. For example, it can be a combination of carbon black and acetylene black, a combination of carbon fibers and graphene, a combination of Ketjen black and carbon nanotubes, or a combination of carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers, and graphene, etc.

[0063] In some embodiments, the binder in the first coating 2, the binder in the second coating 3, and the binder in the lithium - supplementing layer 4 independently include polyvinylidene fluoride (PVDF).

[0064] In some embodiments, the tap density of the lithium iron phosphate thick electrode is 2.4 - 2.7 g / cm 3 , for example, it can be 2.45 g / cm 3 , 2.5 g / cm 3 , 2.55 g / cm 3 , 2.6 g / cm 3 , 2.65 g / cm 3 , or 2.7 g / cm 3 etc.

[0065] It should be noted that after the preparation of the electrode in the battery, generally, drying and rolling operations are required. The tap density of the lithium iron phosphate thick electrode in the present invention refers to the tap density after the electrode is dried and rolled.

[0066] The specific implementation part of the present invention also provides a preparation method of the above-mentioned lithium iron phosphate thick electrode, and the preparation method includes:

[0067] Prepare a first slurry and a second slurry respectively using small-particle lithium iron phosphate and large-particle lithium iron phosphate, and prepare a lithium supplement slurry. Coating the first slurry, the second slurry and the lithium supplement slurry on the surface of the current collector 1 to form a first coating 2, a second coating 3 and a lithium supplement layer 4 stacked in sequence on at least one surface of the current collector 1, thus obtaining the lithium iron phosphate thick electrode.

[0068] In some embodiments, the coating of the first slurry, the second slurry and the lithium supplement slurry on the surface of the current collector 1 is carried out in the following manner:

[0069] Coat the first slurry and the second slurry on at least one surface of the current collector 1 by a double-layer coating method, and obtain the first coating 2 and the second coating 3 after baking. Coat the lithium supplement slurry on the surface of the second coating 3 to obtain the lithium supplement layer 4.

[0070] In some embodiments, the double-layer coating method is to use a double-layer simultaneous coating die head 6 for coating.

[0071] In some embodiments, the method of coating the lithium supplement slurry on the surface of the current collector 1 includes slot extrusion coating, gravure coating or microgravure coating.

[0072] In some embodiments, the preparation of the lithium supplement slurry is carried out in a sealed device, which can effectively avoid contact with air during the preparation process, and after discharging, the binder and the conductive agent are coated on the surface of the lithium supplement agent, further inhibiting the occurrence of surface side reactions and improving stability.

[0073] The specific implementation part of the present invention also provides a device for preparing a lithium iron phosphate thick electrode, and its structural schematic diagram is as Figure 2 shown. The device includes a unwind device 5, a double-layer simultaneous coating die head 6, a lithium supplement coating device 8, a rolling device 9 and a winding device 10 arranged in sequence.

[0074] In some embodiments, drying devices 7 are also arranged between the double-layer simultaneous coating die head 6 and the lithium supplement coating device 8, and between the lithium supplement coating device 8 and the rolling device 9.

[0075] In the present invention, the unwind device 5 unwinds the current collector 1, and simultaneously coats the first slurry and the second slurry on the surface of the current collector 1 through the double-layer simultaneous coating die head 6. After drying by the drying device 7, use the lithium supplement coating device 8 to coat the lithium supplement slurry on the surface of the second coating 3 formed by the second slurry, and dry again through the drying device 7 to form a lithium supplement layer 4 on the surface of the second coating 3. After rolling by the rolling device 9, it is wound to obtain the lithium iron phosphate thick electrode.

[0076] In some embodiments, the lithium supplement coating device 8 is a microgravure coating device. Microgravure coating is thinner and is a sealed coating, which is suitable for coating the lithium supplement slurry and is beneficial to isolating air, further enhancing the stability of the lithium supplement slurry.

[0077] The specific embodiment part of the present invention also provides a lithium-ion battery, and the positive electrode of the lithium-ion battery uses the above-mentioned lithium iron phosphate thick electrode.

[0078] The lithium-ion battery of the present invention has high rate performance and excellent cycling performance, and also has a high capacity retention rate after 5000 cycles.

[0079] Example 1

[0080] This example provides a lithium iron phosphate thick electrode, and its structural schematic diagram is as Figure 1 shown. The lithium iron phosphate thick electrode includes a current collector 1 and a first coating layer 2, a second coating layer 3, and a lithium supplement layer 4 sequentially arranged on one side surface of the current collector 1. The first coating layer 2 includes small particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 94:2:4, and the median particle size of the small particle lithium iron phosphate is 0.5 μm; the second coating layer 3 includes large particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 96:2:2, and the median particle size of the large particle lithium iron phosphate is 3 μm; the lithium supplement layer 4 includes lithium supplement agent Li5FeO4, carbon black, and PVDF with a mass ratio of 45:15:40;

[0081] The thickness of the first coating layer 2 is 100 μm, the thickness of the second coating layer 3 is 200 μm, the thickness of the lithium supplement layer 4 is 5 μm, and the compaction density of the lithium iron phosphate thick electrode is 2.3 g / cm 3 .

[0082] This example also provides a preparation method of the above-mentioned lithium iron phosphate thick electrode, including:

[0083] (1) Stir and disperse small particle lithium iron phosphate, PVDF, carbon black, and solvent N-methylpyrrolidone (NMP) to obtain a first slurry; stir and disperse large particle lithium iron phosphate, PVDF, carbon black, and NMP to obtain a second slurry; stir and disperse Li5FeO4, carbon black, PVDF, and NMP to obtain a lithium supplement slurry;

[0084] (2) Use a double-layer simultaneous coating die head 6 to coat the first slurry and the second slurry on one side surface of the current collector 1, respectively forming a first coating layer 2 with a thickness of 100 μm and a second coating layer 3 with a thickness of 200 μm on the surface of the current collector 1. After baking, coat the lithium supplement slurry on the surface of the second coating layer 3 to obtain a lithium supplement layer 4 with a thickness of 5 μm, and after drying, roll press to obtain a compaction density of 2.3 g / g / cm 3The lithium iron phosphate thick electrode.

[0085] Example 2

[0086] This example provides a lithium iron phosphate thick electrode, and its structural schematic diagram is as Figure 1 shown. The lithium iron phosphate thick electrode includes a current collector 1 and a first coating 2, a second coating 3, and a lithium supplement layer 4 that are sequentially arranged on one side surface of the current collector 1. The first coating 2 includes small particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 94:2:4. The median particle size of the small particle lithium iron phosphate is 0.5 μm; the second coating 3 includes large particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 96:2:2. The median particle size of the large particle lithium iron phosphate is 5 μm; the lithium supplement layer 4 includes a lithium supplement agent Li2NiO2, carbon black, and PVDF with a mass ratio of 45:15:40;

[0087] The thickness of the first coating 2 is 100 μm, the thickness of the second coating 3 is 200 μm, the thickness of the lithium supplement layer 4 is 5 μm, and the compaction density of the lithium iron phosphate thick electrode is 2.3 g / cm 3 .

[0088] In this example, the preparation method of the lithium iron phosphate thick electrode is the same as that of Example 1.

[0089] Example 3

[0090] This example provides a lithium iron phosphate thick electrode, and its structural schematic diagram is as Figure 1 shown. The lithium iron phosphate thick electrode includes a current collector 1 and a first coating 2, a second coating 3, and a lithium supplement layer 4 that are sequentially arranged on one side surface of the current collector 1. The first coating 2 includes small particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 94:2:4. The median particle size of the small particle lithium iron phosphate is 0.5 μm; the second coating 3 includes large particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 96:2:2. The median particle size of the large particle lithium iron phosphate is 3 μm; the lithium supplement layer 4 includes a lithium supplement agent Li2NiO2, carbon black, and PVDF with a mass ratio of 50:20:30;

[0091] The thickness of the first coating 2 is 150 μm, the thickness of the second coating 3 is 150 μm, the thickness of the lithium supplement layer 4 is 5 μm, and the compaction density of the lithium iron phosphate thick electrode is 2.3 g / cm 3 .

[0092] In this example, the preparation method of the lithium iron phosphate thick electrode is the same as that of Example 1.

[0093] Example 4

[0094] This example provides a lithium iron phosphate thick electrode, and its structural schematic diagram is asFigure 1 As shown, the lithium iron phosphate thick electrode includes a current collector 1, and a first coating 2, a second coating 3, and a lithium supplement layer 4 sequentially arranged on one side surface of the current collector 1. The first coating 2 includes small particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 94:2:4. The median particle size of the small particle lithium iron phosphate is 0.3 μm. The second coating 3 includes large particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 96:2:2. The median particle size of the large particle lithium iron phosphate is 5 μm. The lithium supplement layer 4 includes lithium supplement agent Li2NiO2, carbon black, and PVDF with a mass ratio of 50:20:30;

[0095] The thickness of the first coating 2 is 150 μm, the thickness of the second coating 3 is 150 μm, the thickness of the lithium supplement layer 4 is 10 μm, and the tap density of the lithium iron phosphate thick electrode is 2.3 g / cm 3 .

[0096] In this embodiment, the preparation method of the lithium iron phosphate thick electrode is the same as that of Example 1.

[0097] Example 5

[0098] This embodiment provides a lithium iron phosphate thick electrode, and its structural schematic diagram is as Figure 1 As shown, the lithium iron phosphate thick electrode includes a current collector 1, and a first coating 2, a second coating 3, and a lithium supplement layer 4 sequentially arranged on one side surface of the current collector 1. The first coating 2 includes small particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 94:2:4. The median particle size of the small particle lithium iron phosphate is 0.3 μm. The second coating 3 includes large particle lithium iron phosphate, binder PVDF, and conductive agent carbon black with a mass ratio of 96:2:2. The median particle size of the large particle lithium iron phosphate is 5 μm. The lithium supplement layer 4 includes lithium supplement agent Li2NiO2, carbon black, and PVDF with a mass ratio of 50:20:30;

[0099] The thickness of the first coating 2 is 150 μm, the thickness of the second coating 3 is 150 μm, the thickness of the lithium supplement layer 4 is 10 μm, and the tap density of the lithium iron phosphate thick electrode is 2.5 g / cm 3 .

[0100] In this embodiment, the preparation method of the lithium iron phosphate thick electrode is the same as that of Example 1.

[0101] Example 6

[0102] Except that the mass ratio of Li5FeO4, carbon black, and PVDF in the lithium supplement layer 4 is replaced with 63:21:16, the rest are the same as those in Example 1.

[0103] Example 7

[0104] Except that the mass ratio of Li5FeO4, carbon black, and PVDF in the lithium supplement layer 4 is replaced with 30:10:60, the rest is the same as in Example 1.

[0105] Example 8

[0106] Except that the median particle size of the small particle lithium iron phosphate is 1.5 μm, the rest is the same as in Example 1.

[0107] Example 9

[0108] Except that the median particle size of the large particle lithium iron phosphate is 11 μm, the rest is the same as in Example 1.

[0109] Comparative Example 1

[0110] Except for not setting the first coating 2 and the lithium supplement layer 4, and replacing the thickness of the second coating 3 with 305 μm, that is, only coating the second slurry with the same composition as in Example 1 but different thickness on the surface of the current collector 1, the rest is the same as in Example 1.

[0111] Comparative Example 2

[0112] Except for not setting the first coating 2 and the lithium supplement layer 4, and replacing the thickness of the second coating 3 with 305 μm, that is, only coating the second slurry with the same composition as in Example 2 but different thickness on the surface of the current collector 1, the rest is the same as in Example 2.

[0113] Comparative Example 3

[0114] Except for not setting the first coating 2 and the lithium supplement layer 4, and replacing the thickness of the second coating 3 with 305 μm, that is, only coating the second slurry with the same composition as in Example 3 but different thickness on the surface of the current collector 1, the rest is the same as in Example 3.

[0115] Comparative Example 4

[0116] Except for not setting the first coating 2 and the lithium supplement layer 4, and replacing the thickness of the second coating 3 with 310 μm, that is, only coating the second slurry with the same composition as in Example 4 but different thickness on the surface of the current collector 1, the rest is the same as in Example 4.

[0117] Comparative Example 5

[0118] Except for not setting the first coating 2 and the lithium supplement layer 4, and replacing the thickness of the second coating 3 with 310 μm, that is, only coating the second slurry with the same composition as in Example 5 but different thickness on the surface of the current collector 1, the rest is the same as in Example 5.

[0119] Comparative Example 6

[0120] Except for not setting the second coating 3, and replacing the thickness of the first coating 2 with 305 μm, that is, only coating the first slurry with the same composition as in Example 1 but different thickness on the surface of the current collector 1, the rest is the same as in Example 1.

[0121] Comparative Example 7

[0122] Except for not providing the lithium supplement layer 4, the rest is the same as in Example 1.

[0123] Comparative Example 8

[0124] Except for not providing the lithium supplement layer 4 and adding the lithium supplement agent in the lithium supplement slurry to the second slurry so that the second coating 3 includes the lithium supplement agent, the rest is the same as in Example 1.

[0125] The mass of the lithium supplement agent in the second coating 3 of this comparative example is equal to the mass of the lithium supplement agent in the lithium supplement layer 4 of Example 1.

[0126] I. Assembly of Lithium-Ion Battery

[0127] The lithium iron phosphate thick electrode prepared in the examples and comparative examples of the present invention is processed through die cutting and slitting processes to obtain the positive electrode, graphite is used as the negative electrode, 1M LiPF6 (EC:DMC:EMC = 5:3:2) is used as the electrolyte, and the processes of stacking and injecting electrolyte are carried out to obtain the lithium-ion battery.

[0128] II. Performance Testing

[0129] (1) Rate performance: At 25 °C, the capacity retention rate of the battery prepared with the lithium iron phosphate thick electrode is tested at different rates. The voltage range is 2.5 - 3.65V, and the rates are 0.5C, 1C, 2C, and 3C respectively. The calculation method of the capacity retention rate is to divide the discharge capacity at different rates by the discharge capacity at 0.33C to obtain the capacity retention rate at the corresponding rate. The test results are shown in Table 1 and Figure 3 as shown.

[0130] (2) Cycle performance: At 25 °C, the capacity retention rate of the battery prepared with the lithium iron phosphate thick electrode is tested at different cycle numbers. The voltage range is 2.5 - 3.65V, and the rate is 0.5C / 0.5C. The cycle numbers are 1000 cycles, 2000 cycles, 3000 cycles, and 5000 cycles respectively. The calculation method of the capacity retention rate is to divide the discharge capacity at the 1000th cycle, 2000th cycle, 3000th cycle, and 5000th cycle by the discharge capacity of the first cycle respectively to obtain the capacity retention rate at the corresponding cycle number. The test results are shown in Table 2 and Figure 4 as shown.

[0131] Table 1

[0132]

[0133] Table 2

[0134]

[0135]

[0136] As can be seen from the above Examples 1-9, on the surface of the current collector 1 of the lithium iron phosphate thick electrode of the present invention, a first coating layer 2, a second coating layer 3 and a lithium compensation layer 4 are sequentially provided. The first coating layer 2 and the second coating layer 3 contain small particle lithium iron phosphate and large particle lithium iron phosphate respectively. This structure of double-layer coating with large and small particles can effectively improve the liquid-phase transport efficiency and kinetic performance of the lithium iron phosphate thick electrode; at the same time, the lithium compensation layer 4 can effectively make up for the loss of irreversible capacity in the thick electrode, improve the cycle performance of the electrode, and the setting of the lithium compensation layer 4 on the outermost layer is also beneficial to the discharge of gas generated during the formation process, and cooperates with the first coating layer 2 and the second coating layer 3 to improve the rate performance and cycle performance of the lithium iron phosphate thick electrode at the same time.

[0137] Figure 1 and Figure 2 are respectively the rate performance comparison chart and the cycle performance comparison chart of Examples 1-5 and Comparative Examples 1-5. According to Figure 1 and Figure 2 it can be seen that the lithium iron phosphate thick electrode prepared by using the specific three-layer structure of the present invention can maintain a relatively high thickness and tap density while ensuring the rate performance and cycle performance. The lithium iron phosphate thick electrodes in Examples 1-5 have a high capacity retention rate at different rates, not less than 85% even at a high rate of 3C, and they even have long cycle performance, and the capacity retention rate is still about 90% after 5000 cycles; while in Comparative Examples 1-5, there is no structure of double-layer coating with large and small particles, nor does it contain the lithium compensation layer 4. After preparing thick electrodes with the same thickness and tap density, the rate performance is significantly reduced, and the capacity retention rate after 5000 cycles drops by nearly 10%, which is significantly worse than that of Examples 1-5.

[0138] Through the comparison between Example 1 and Comparative Example 6 in Table 1-2, it can be seen that when the lithium iron phosphate thick electrode does not contain the second coating layer 3, although the total thickness of the coating does not change, the overall electrode does not contain the structure of double-layer coating with large and small particles, which is not conducive to improving the liquid-phase transport efficiency and kinetic performance of the thick electrode, and is not conducive to exerting the synergistic effect with the lithium compensation layer 4. Therefore, the rate performance and cycle performance of Comparative Example 6 are significantly worse than those of Example 1.

[0139] By comparing Example 1 with Comparative Examples 7-8 in Table 1-2, it can be seen that without setting the lithium replenishing layer 4 or adding the lithium replenishing agent to the second coating 3, the rate performance and cycle performance of the lithium iron phosphate thick electrode cannot be effectively improved; in Comparative Example 7, no lithium replenishing layer 4 is set, and the irreversible capacity loss of the thick electrode cannot be compensated, and the cycle performance is extremely poor; although the same amount of lithium replenishing agent is added in Comparative Example 8, the lithium replenishing agent in Comparative Example 8 is located in the second coating 3, and it is impossible to inhibit the occurrence of surface side reactions through the coating effect of the binder and the conductive agent, and the gas generated during formation is not easy to be discharged, and no binder layer will be formed on the surface of the electrode to inhibit the wrinkling of the diaphragm and improve the wettability of the diaphragm, and the interface performance of the battery is significantly reduced; therefore, the rate performance and long cycle performance of Comparative Examples 7-8 are significantly worse than those of Example 1.

[0140] By comparing Example 1 with Examples 6-7 in Table 1-2, it can be seen that the lithium replenishing agent, the conductive agent and the binder in the lithium replenishing layer 4 of the present invention can suppress the occurrence of side reactions on the surface of the lithium iron phosphate thick electrode with a suitable content ratio, and improve the pre-lithium stability of the lithium replenishing layer 4; the binder content in Example 6 is relatively small, which is not conducive to the formation of a coating film on the surface of the lithium replenishing agent and reducing the contact with the air on the one hand, and is not conducive to the formation of a binder layer in the lithium replenishing layer 4 during the formation stage on the other hand, affecting the stability of the thick electrode; the binder content in Example 7 is relatively high, which will cause the binder film layer to pass, affecting the deintercalation reaction rate of the lithium replenishing agent, and at the same time causing a larger surface resistance. Therefore, compared with Examples 6-7, Example 1 has better rate performance and cycle performance.

[0141] By comparing Example 1 with Examples 8-9 in Table 1-2, it can be seen that the size of the small-particle lithium iron phosphate in the first coating 2 and the size of the large-particle lithium iron phosphate in the second coating 3 will affect the electrochemical performance of the thick electrode; the median particle size of the small-particle lithium iron phosphate in Example 8 is relatively large, which will lead to a decrease in the bottom solid phase diffusion rate and an increase in polarization; the median particle size of the large-particle lithium iron phosphate in Example 9 is relatively large, which will lead to an increase in the surface diffusion path and an increase in the internal resistance of the electrode; therefore, the rate performance of the lithium iron phosphate thick electrode in Example 1 is the best.

[0142] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention are within the protection scope and disclosure scope of the present invention.

Claims

1. A lithium iron phosphate thick electrode, characterized in that, The lithium iron phosphate thick electrode includes a current collector and a first coating provided on at least one surface of the current collector. On the surface of the first coating away from the current collector, a second coating and a lithium supplement layer are sequentially provided. The first coating includes small particle lithium iron phosphate, and the second coating includes large particle lithium iron phosphate; The lithium supplement layer includes a lithium supplement agent, a binder, and a conductive agent, and the mass ratio of the lithium supplement agent, the conductive agent, and the binder is (36-50):(14-30):(20-50); The median particle size of the small particle lithium iron phosphate in the first coating is 0.2 μm to 0.6 μm; The median particle size of the large particle lithium iron phosphate in the second coating is 1 to 8 μm; The thickness ratio of the first coating to the second coating is 1:1 to 1:

2.

2. The lithium iron phosphate thick electrode according to claim 1, wherein The mass ratio of the lithium supplement agent to the binder in the lithium supplement layer is (36-50):(20-50).

3. The lithium iron phosphate thick electrode according to claim 1, characterized in that, The lithium supplement agent includes any one or a combination of at least two of Li2NiO2, Li5FeO4, and Li2O.

4. The lithium iron phosphate thick electrode according to claim 1, characterized in that, The total thickness of the first coating and the second coating is 100 to 1000 μm.

5. The lithium iron phosphate thick electrode according to claim 1, characterized in that, The thickness of the lithium supplement layer is 3 to 10 μm.

6. The lithium iron phosphate thick electrode according to claim 1, wherein, The first coating and the second coating also independently include a conductive agent and a binder.

7. The lithium iron phosphate thick electrode according to claim 6, characterized in that, The mass ratio of the small particle lithium iron phosphate, the binder, and the conductive agent in the first coating is (90-95):(1-2):(3-5).

8. The lithium iron phosphate thick electrode according to claim 6, wherein The mass ratio of the large particle lithium iron phosphate, the binder, and the conductive agent in the second coating is (92-96):(1-2):(1-2).

9. The lithium iron phosphate thick electrode according to claim 6, wherein The conductive agent in the first coating, the conductive agent in the second coating, and the conductive agent in the lithium supplement layer independently include any one or a combination of at least two of carbon black, acetylene black, Ketjen black, carbon nanotubes, carbon fibers, and graphene.

10. The lithium iron phosphate thick electrode according to claim 6, characterized in that, The binder in the first coating, the binder in the second coating, and the binder in the lithium supplement layer independently include polyvinylidene fluoride.

11. The lithium iron phosphate thick electrode according to claim 1, wherein The tap density of the lithium iron phosphate thick electrode is 2.4 to 2.7 g / cm 3 .

12. A method for preparing a lithium iron phosphate thick electrode according to any one of claims 1-11, characterized in that, The preparation method includes: Preparing a first slurry and a second slurry using small particle lithium iron phosphate and large particle lithium iron phosphate respectively, and preparing a lithium supplement slurry. Coating the first slurry, the second slurry, and the lithium supplement slurry on the surface of the current collector to form a first coating, a second coating, and a lithium supplement layer that are sequentially laminated on at least one surface of the current collector, thereby obtaining a lithium iron phosphate thick electrode.

13. The preparation method according to claim 12, characterized in that, Coating the first slurry, the second slurry, and the lithium supplement slurry on the surface of the current collector is carried out in the following manner: Coating the first slurry and the second slurry on at least one surface of the current collector by a double-layer coating method, and baking to obtain the first coating and the second coating. Coating the lithium supplement slurry on the surface of the second coating to obtain the lithium supplement layer.

14. The preparation method according to claim 12, characterized in that, The method of coating the lithium supplement slurry on the surface of the current collector includes slit extrusion coating, gravure coating, or microgravure coating.

15. A lithium-ion battery, characterized in that, The positive electrode of the lithium ion battery uses the lithium iron phosphate thick electrode according to any one of claims 1-11.

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