A positive electrode sheet, a method for manufacturing the same, and use thereof

Abstract

This invention relates to the field of battery technology, specifically to a positive electrode sheet, its preparation method, and its application. The positive electrode sheet includes: a positive electrode current collector; and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector along its thickness direction. The positive electrode active material layer includes a positive electrode active material and a composite lithium supplement agent. The composite lithium supplement agent includes Li5FeO4 and a conductive network structure layer at least partially coating the Li5FeO4, wherein the conductive network structure layer includes N-type carbon nanotubes and P-type carbon nanotubes. Compared with existing technologies, this invention constructs a "bulk-interface" hierarchical conductive channel within the electrode by compositing N-type CNTs and P-type CNTs with LFP, forming a built-in electric field to accelerate carrier separation / transport, while simultaneously introducing a lithium supplement agent to form an electron-ion-Li... + The three-dimensional collaborative transport network ultimately improves the electronic conductivity, lithium-ion diffusion rate, energy density, and cycle life of the cathode.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a positive electrode sheet, its preparation method, and its application. Background Technology

[0002] With the development of new energy battery technology, the demand for high-performance cathode materials has further increased. Lithium iron phosphate (LFP) has attracted widespread attention due to its advantages such as high safety, long cycle life, low cost, environmental friendliness, and good high-temperature stability. However, the electronic conductivity of LFP materials (10⁻⁶ Ω·cm) is a significant factor. -9 S / cm) and lithium-ion diffusion rate (10 -14 cm 2 The low efficiency ( / s) is one of the key issues limiting its commercial application, resulting in poor rate performance. In addition, the first irreversible capacity loss of LFP is more than 7%, which easily leads to SEI and consumes lithium source, resulting in a certain reduction in battery capacity and energy density.

[0003] To address the aforementioned technical issues, a traditional improvement method is to coat the LFP material with a carbon layer, such as carbon black or carbon nanotubes, to enhance its conductivity. However, in actual operation, carbon black primarily conducts electricity through point contact, which often fails under high-rate conditions. Furthermore, carbon nanotubes tend to agglomerate, making it impossible to optimize electron / ion transport while maintaining conductivity. Summary of the Invention

[0004] In view of this, the present invention aims to address, to a certain extent, the technical problems in related technologies. Therefore, the present invention provides a positive electrode sheet, its preparation method, and its application. By optimizing the conductive network design: combining N-type CNTs (electron acceptors) and P-type CNTs (electron donors) with LFP, a "bulk-interface" hierarchical conductive channel is constructed within the electrode, forming a built-in electric field to accelerate carrier separation / transport. Simultaneously, a lithium supplement agent is introduced to form an electron-ion-Li... + The three-dimensional collaborative transport network ultimately improves the electronic conductivity, lithium-ion diffusion rate, energy density, and cycle life of the cathode.

[0005] To solve the above-mentioned technical problems, the present invention is implemented as follows: According to one aspect of the present invention, a positive electrode sheet is provided, comprising: Positive current collector; A positive electrode active material layer is disposed on at least one surface of the positive electrode current collector along the thickness direction; the positive electrode active material layer includes a positive electrode active material and a composite lithium supplement agent; The composite lithium supplement includes Li5FeO4 and a conductive network structure layer that at least partially coats the Li5FeO4, wherein the conductive network structure layer includes N-type carbon nanotubes and P-type carbon nanotubes.

[0006] In some of these embodiments, the N-type carbon nanotubes include at least one of nitrogen-doped carbon nanotubes, phosphorus-doped carbon nanotubes, and arsenic-doped carbon nanotubes.

[0007] In some of these embodiments, the diameter of the N-type carbon nanotubes is 6 nm to 10 nm.

[0008] In some embodiments, the P-type carbon nanotubes include at least one of boron-doped carbon nanotubes and aluminum-doped carbon nanotubes.

[0009] In some of these embodiments, the diameter of the P-type carbon nanotubes is 7 nm to 13 nm.

[0010] In some embodiments, the thickness of the conductive network structure layer is 3 nm to 10 nm.

[0011] In some embodiments, the mass ratio of the N-type carbon nanotubes to the P-type carbon nanotubes in the conductive network structure layer is 1:(1~3).

[0012] In some of these embodiments, the positive electrode active material includes lithium iron phosphate.

[0013] In some embodiments, the decomposition voltage of the composite lithium supplement is 2.8V to 3.2V.

[0014] In some of these embodiments, the particle size of the Li5FeO4 is 30 nm to 70 nm.

[0015] In some embodiments, the specific surface area of ​​the Li5FeO4 is 37 m². 2 / g~43m 2 / g.

[0016] In some of these embodiments, the porosity of the positive electrode active material layer is 55% to 65%.

[0017] In some embodiments, the compaction density of the positive electrode active material layer is 2.3 g / cm³. 3 ~2.7g / cm 3 .

[0018] In some embodiments, the positive electrode active material layer further includes a binder.

[0019] In some embodiments, the adhesive includes at least one of polyvinylidene fluoride, polyimide, polyacrylonitrile, and polyvinyl butyral.

[0020] In some of these embodiments, the mass ratio of the positive electrode active material, the composite lithium supplement, and the binder in the positive electrode active material layer is (90~94):(3~7):(2~4).

[0021] According to another aspect of the present invention, the present invention provides a method for preparing the positive electrode sheet described above, comprising the following steps: a) N-type carbon nanotubes, P-type carbon nanotubes and a first solvent are mixed to obtain a suspension; then Li5FeO4 is added to the suspension and the mixture is ground to obtain a first slurry. b) Add a silane coupling agent to the first slurry obtained in step a) and react it, then freeze-dry it to obtain a composite lithium supplement; c) The composite lithium supplement obtained in step b) is mixed with the positive electrode active material and the second solvent to obtain a second slurry; the second slurry is then coated on at least one side of the positive electrode current collector along the thickness direction, and then subjected to a second freeze-drying and pressing to obtain a positive electrode sheet.

[0022] In some of these embodiments, the first solvent includes at least one of ethanol, methanol, and ethylene glycol.

[0023] In some of these embodiments, the first mixing is performed by ultrasonic dispersion for 20 to 40 minutes.

[0024] In some embodiments, the grinding method is ball milling, with a rotation speed of 250 rpm to 350 rpm and a time of 1.5 h to 2.5 h.

[0025] In some of these embodiments, the silane coupling agent includes at least one of 3-aminopropyltriethoxysilane, γ-aminopropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, and γ-mercaptopropyltrimethoxysilane.

[0026] In some embodiments, the amount of the silane coupling agent added is 4% to 6% of the total mass of the N-type carbon nanotubes and the P-type carbon nanotubes.

[0027] In some of these embodiments, the reaction is carried out at a temperature of 60°C to 80°C for a duration of 5 to 7 hours.

[0028] In some embodiments, the first freeze-drying temperature is -55°C to -45°C, the pressure is 0.08 Pa to 0.12 Pa, and the time is 11 h to 13 h.

[0029] In some embodiments, the second solvent includes at least one of N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, and cyclohexanone.

[0030] In some embodiments, the second mixing process further includes the addition of a binder, preferably in a mass ratio of (90~94):(3~7):(2~4) of the positive electrode active material, the composite lithium supplementer and the binder.

[0031] In some of these embodiments, the coating thickness is 140 μm to 180 μm.

[0032] In some of these embodiments, the second freeze-drying temperature is -35°C to -25°C, the pressure is 8 Pa to 12 Pa, and the time is 7 h to 9 h.

[0033] In some of these embodiments, the compacted density is 2.3 g / cm³. 3 ~2.7g / cm 3 .

[0034] According to another aspect of the present invention, the present invention provides a battery comprising the positive electrode sheet described in the above technical solution or the positive electrode sheet prepared by the preparation method described in the above technical solution.

[0035] Implementing the technical solution of the present invention has at least the following beneficial effects: 1. This invention constructs a hierarchical "bulk-interface" conductive channel within the electrode by compositing N-type and P-type carbon nanotubes with lithium iron phosphate. This creates a built-in electric field that accelerates carrier separation / transport, improving electronic conductivity and lithium-ion diffusion rate. Traditional methods of improving conductivity by adding carbon nanotubes can only form randomly dispersed single electronic channels. However, this invention achieves synergistic conductivity through the heterojunction of N-type and P-type carbon nanotubes to create directional electron-hole dual channels. The N-type carbon nanotubes adsorb electrons to form high-mobility electronic channels, increasing bulk electronic conductivity by 10%. 4 P-type and N-type carbon nanotubes provide holes to construct a low-impedance interface transport layer, reducing the interface charge transfer impedance by 80%.

[0036] 2. This invention introduces a lithium replenishing agent to form an electron-ion-Li group. +The three-dimensional collaborative transport network improves lithium replenishment efficiency and solves the problem of side reactions of lithium replenishment agents, thereby improving cycle life: Traditional methods use free lithium replenishment agents with side reactions >20%, while the present invention introduces lithium replenishment agents through bonding and fixation, with side reactions <5%. Specifically, the present invention uses the stabilization mechanism of Li5FeO4-CNT chemical bonding. Fe-O- on the surface of Li5FeO4 and -COOH of CNT form Fe-OC covalent bonds and achieve functional complementarity. CNT can inhibit the contact between Li5FeO4 and electrolyte, reduce O2 / CO / CO2 side reactions, and at the same time, the decomposition products of Li5FeO4, such as Fe2O3 and Li2O, can also enhance the mechanical strength of the electrode.

[0037] 3. This invention suppresses particle breakage and improves structural stability through carbon nanotube encapsulation: Li5FeO4 lithium source undergoes initial charging decomposition (Li5FeO4→4Li + +4e - (+LiFeO2+O2↑), while the present invention, through a conductive network structure layer including N-type carbon nanotubes and P-type carbon nanotubes, can rapidly collect electrons, suppress the aggregation of Li5FeO4 decomposition products, realize multi-scale mass transfer optimization of the electrode, thereby significantly improving the structural stability of the positive electrode and effectively improving the energy density of the battery.

[0038] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0039] The present application will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0040] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0041] In the description of this application, "same chemical composition" should be interpreted broadly, that is, the main components of the two have the same chemical composition, or the two have substantially the same chemical composition, but may have errors or impurities within the acceptable range that can be understood by those skilled in the art.

[0042] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0043] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0044] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0045] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0046] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0047] Currently, the commercial application of lithium iron phosphate (LFP) materials is hampered by their low electronic conductivity and lithium-ion diffusion efficiency, which also leads to poor rate performance. Furthermore, LFP experiences an initial irreversible capacity loss of over 7%, easily forming an electron-electrode interphase (SEI) that consumes lithium, resulting in a reduction in battery capacity and energy density. To address these technical issues, a traditional improvement method is to coat the LFP material with a carbon layer, such as carbon black or carbon nanotubes, to improve its conductivity. However, in practice, carbon black's conductivity is mainly through point contact, which often fails under high-rate conditions. Carbon nanotubes, on the other hand, tend to agglomerate, failing to optimize electron / ion transport while maintaining conductivity.

[0048] Building upon this foundation, the inventors of this invention optimized the conductive network design by combining N-type and P-type carbon nanotubes with LFP to construct a hierarchical "bulk-interface" conductive channel within the electrode. This creates a built-in electric field that accelerates carrier separation / transport, improving electronic conductivity and lithium-ion diffusion rate. Simultaneously, a lithium replenishing agent is introduced and covalently bonded to the carbon nanotubes, forming an electron-ion-Li network. +The three-dimensional collaborative transport network, in which the lithium replenisher primarily functions as a pre-stored lithium source, decomposes during the battery's first charge, releasing additional Li₂. + This compensates for lithium loss during the first cycle of the battery, ensuring that the decomposition voltage of the lithium replenisher matches the voltage of the cathode material. At the same time, chemically bonded carbon nanotubes can suppress side reactions of the lithium replenisher, ultimately improving the electronic conductivity, lithium-ion diffusion rate, energy density, and cycle life of the cathode.

[0049] Specifically, the present invention adopts the following technical solution: According to one aspect of the present invention, a positive electrode sheet is provided, comprising: Positive current collector; A positive electrode active material layer is disposed on at least one surface of the positive electrode current collector along the thickness direction; the positive electrode active material layer includes a positive electrode active material and a composite lithium supplement agent; The composite lithium supplement includes Li5FeO4 and a conductive network structure layer that at least partially coats the Li5FeO4, wherein the conductive network structure layer includes N-type carbon nanotubes and P-type carbon nanotubes.

[0050] In this invention, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector along its thickness direction, preferably consisting of a positive current collector and a positive active material layer. In a specific embodiment of this invention, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0051] In specific embodiments of the present invention, the positive electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil can be used as the metal foil. The composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). The present invention does not have any special restrictions on the source of the positive electrode current collector; commercially available products or self-made products well known to those skilled in the art can be used.

[0052] In a specific embodiment of the present invention, the positive electrode active material layer includes a positive electrode active material and a composite lithium supplement, preferably composed of a positive electrode active material and a composite lithium supplement; wherein, the positive electrode active material includes lithium iron phosphate (LFP); the present invention uses the above-mentioned lithium iron phosphate as the positive electrode active material, which has the advantages of high safety, long cycle life and good stability. At the same time, this positive electrode active material also has a cost advantage compared with other types of positive electrode active materials, so it is suitable for new energy vehicles, large-scale energy storage power stations, low-speed electric vehicles and other fields.

[0053] In a specific embodiment of the present invention, the composite lithium supplement includes Li5FeO4 and a conductive network structure layer at least partially coating the Li5FeO4, wherein the conductive network structure layer includes N-type carbon nanotubes and P-type carbon nanotubes. The present invention constructs a "bulk-interface" hierarchical conductive channel within the electrode by combining N-type and P-type carbon nanotubes with the aforementioned lithium iron phosphate, forming a built-in electric field to accelerate carrier separation / transport and improve electronic conductivity and lithium-ion diffusion rate.

[0054] In a specific embodiment of the present invention, the N-type carbon nanotubes preferably include at least one of nitrogen-doped carbon nanotubes, phosphorus-doped carbon nanotubes, and arsenic-doped carbon nanotubes, more preferably nitrogen-doped carbon nanotubes. It should be noted that the present invention selects pentavalent elements such as nitrogen, phosphorus, and arsenic to dop the carbon nanotubes. These elements can provide additional free electrons, becoming the main charge carriers, thereby enhancing the conductivity of the material. Furthermore, doping transforms the carbon nanotubes from intrinsic semiconductors into highly conductive N-type semiconductors, ensuring rapid electron transfer during the decomposition of the lithium replenisher and avoiding a decrease in lithium replenishment efficiency due to electron transport bottlenecks.

[0055] In a specific embodiment of the present invention, the diameter of the N-type carbon nanotubes is preferably 6 nm to 10 nm, for example, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any value between two of these. By controlling the diameter of the N-type carbon nanotubes within the above range, the present invention can avoid the following: if the diameter of the N-type carbon nanotubes is too small, the van der Waals forces between the carbon nanotubes will be too strong, resulting in a dense agglomeration and stacking phenomenon rather than a continuous three-dimensional network; at the same time, it can also avoid the following: if the diameter of the N-type carbon nanotubes is too large, the rigidity of the carbon nanotubes will be increased, making it difficult to flexibly interweave and form a dense network.

[0056] In a specific embodiment of the present invention, the P-type carbon nanotubes preferably include at least one of boron-doped carbon nanotubes and aluminum-doped carbon nanotubes, more preferably boron-doped carbon nanotubes. It should be noted that the present invention selects trivalent elements boron and aluminum to dope carbon nanotubes. These elements can create holes (positions lacking an electron) within the carbon nanotube lattice, becoming the main source of holes and thus affecting current conduction. Furthermore, doping yields P-type carbon nanotubes, which, together with N-type carbon nanotubes, construct a highly efficient, dead-zone-free conductive network. N-type carbon nanotubes can introduce free electrons, dominating electron conduction, while P-type carbon nanotubes introduce holes. The two form a miniature NP heterojunction at the contact interface, creating a built-in electric field and improving ion migration efficiency.

[0057] In a specific embodiment of the present invention, the diameter of the P-type carbon nanotubes is preferably 7nm to 13nm, for example, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, or any value between two of these. By controlling the diameter of the P-type carbon nanotubes within the above range, the present invention can form a conductive network with N-type carbon nanotubes of varying thicknesses. The thinner N-type carbon nanotubes fill the gaps between the thicker P-type carbon nanotubes, reducing the void ratio of the conductive network and avoiding delamination caused by excessive differences in the diameters of the two types of carbon nanotubes. This ensures uniform electron / ion transport within the network and accelerates the migration rate of lithium ions.

[0058] In a specific embodiment of the present invention, the thickness of the conductive network structure layer is preferably 3nm to 10nm, for example, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, or any value between two of these. By controlling the thickness of the conductive network structure layer within the above range, the present invention can form a continuous conductive shell through the interweaving of N-type carbon nanotubes and P-type carbon nanotubes, avoiding direct reaction between Li5FeO4 and the electrolyte, while retaining sufficient porosity to provide a fast diffusion channel for Li⁺, and also avoiding the compression of the positive electrode active material, thus ensuring sufficient energy density.

[0059] In a specific embodiment of the present invention, the mass ratio of N-type carbon nanotubes to P-type carbon nanotubes in the conductive network structure layer is preferably 1:(1~3), for example, 1:1, 1:2, 1:3, or any ratio between the two. By controlling the mass ratio of N-type carbon nanotubes to P-type carbon nanotubes within the above range, the present invention can optimize current-carrying self-balancing, maximize electron conduction efficiency, adapt to the diameter difference between N-type carbon nanotubes and P-type carbon nanotubes, and construct a dense and porous stable conductive network.

[0060] In a specific embodiment of the present invention, the decomposition voltage of the composite lithium replenishing agent is preferably 2.8V to 3.2V, for example, 2.8V, 2.9V, 3.0V, 3.1V, 3.2V, or any value between two of these. It should be noted that the initial delithiation voltage of the positive electrode active material after its first delithiation is between 3.0V and 3.5V. By controlling the decomposition voltage of the composite lithium replenishing agent within this range, it falls precisely within the interval where the positive electrode is about to initiate delithiation but has not yet fully initiated. This allows the composite lithium replenishing agent to release Li at the positive electrode. + Pre-decompose and release Li (or synchronously) + These Li + It can preferentially fill the lithium vacancies required for the formation of the SEI film in the negative electrode, or directly participate in the Li-coated cathode-negative electrode reaction. + Cycling to avoid the release of Li from the positive electrode + It is consumed by side reactions, thus improving lithium replenishment efficiency.

[0061] In a specific embodiment of the present invention, the Li5FeO4 is a lithium replenishing agent, connected to CNTs via Fe-OC bonds; the particle size of the Li5FeO4 is preferably 30nm~70nm, for example 30nm, 40nm, 50nm, 60nm, 70nm, or any value between two of these. It should be noted that by controlling the particle size of the lithium replenishing agent Li5FeO4 to the above-mentioned nanoscale, the present invention can increase the specific surface area and improve the lithium replenishment efficiency.

[0062] In a specific embodiment of the present invention, the specific surface area of ​​the Li5FeO4 is preferably 37 m². 2 / g~43m 2 / g, for example, 37m 2 / g、38m 2 / g、39m 2 / g、40m 2 / g、41m 2 / g、42m 2 / g、43m 2 / g, or a value between any two of them. This invention, by controlling the specific surface area of ​​Li5FeO4 within the above range, can shorten the Li... + Diffusion pathways accelerate reaction rates, prevent the formation of dead lithium, balance interfacial reactivity and side reaction suppression, and ensure battery stability.

[0063] In a specific embodiment of the present invention, the porosity of the positive electrode active material layer is preferably 55% to 65%, for example, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, or any value between two of these. By controlling the porosity of the positive electrode active material layer within the above range, the present invention can ensure sufficient wetting of the electrolyte and the Li +Rapid transmission solves the bottleneck of ion transmission at high rates.

[0064] In a specific embodiment of the present invention, the compaction density of the positive electrode active material layer is preferably 2.3 g / cm³. 3 ~2.7g / cm 3 For example, 2.3 g / cm³ 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 The value can be any value between the above two ranges. By controlling the compaction density of the positive electrode active material layer within the above range, this invention can ensure high energy density, guarantee electrochemical performance stability, and avoid damage to the active material and conductive network.

[0065] In a specific embodiment of the present invention, the positive electrode active material layer preferably further includes a binder for bonding the positive electrode active material and the composite lithium supplementer to form a stable integral layer structure; the binder preferably includes at least one selected from polyvinylidene fluoride (PVDF), polyimide (PI), polyacrylonitrile (PAN), and polyvinyl butyral (PVB), more preferably polyvinylidene fluoride (PVDF). The present invention does not impose any special restrictions on the source of the binder; commercially available products well known to those skilled in the art can be used.

[0066] In a specific embodiment of the present invention, the preferred mass ratio of the positive electrode active material, the composite lithium replenishing agent, and the binder in the positive electrode active material layer is (90~94):(3~7):(2~4), for example, 90:7:3, 91:6:3, 92:5:3, 93:4:3, 94:3:3, or any ratio between two of these. By limiting the mass ratio of the positive electrode active material, the composite lithium replenishing agent, and the binder in the positive electrode active material layer to the above range, the present invention ensures that the positive electrode active material layer can efficiently compensate for the initial lithium loss while maintaining a high energy density. Simultaneously, the low proportion of binder ensures structural stability and reduces its interference with electrochemical performance.

[0067] The positive electrode provided by this invention constructs a "bulk-interface" hierarchical conductive channel within the electrode by compositing N-type and P-type carbon nanotubes with lithium iron phosphate. This creates a built-in electric field that accelerates carrier separation / transport, improving electronic conductivity and lithium-ion diffusion rate. Simultaneously, by introducing a lithium replenishing agent, an electron-ion-Li-ion exchange mechanism is formed. +The three-dimensional collaborative transport network improves lithium replenishment efficiency, solves the problem of side reactions of lithium replenishment agents, and thus improves cycle life. Furthermore, the carbon nanotube encapsulation method suppresses particle breakage and improves structural stability. Through the conductive network structure layer including N-type carbon nanotubes and P-type carbon nanotubes, electrons can be collected quickly and the aggregation of Li5FeO4 decomposition products can be suppressed, realizing multi-scale mass transfer optimization of the electrode, thereby significantly improving the structural stability of the positive electrode and effectively improving the energy density of the battery.

[0068] According to another aspect of the present invention, the present invention provides a method for preparing the positive electrode sheet described above, comprising the following steps: a) N-type carbon nanotubes, P-type carbon nanotubes and a first solvent are mixed to obtain a suspension; then Li5FeO4 is added to the suspension and the mixture is ground to obtain a first slurry. b) Add a silane coupling agent to the first slurry obtained in step a) and react it, then freeze-dry it to obtain a composite lithium supplement; c) The composite lithium supplement obtained in step b) is mixed with the positive electrode active material and the second solvent to obtain a second slurry; the second slurry is then coated on at least one side of the positive electrode current collector along the thickness direction, and then subjected to a second freeze-drying and pressing to obtain a positive electrode sheet.

[0069] This invention first involves mixing N-type carbon nanotubes, P-type carbon nanotubes, and a first solvent to obtain a suspension; then, Li5FeO4 is added to the suspension and the mixture is ground to obtain a first slurry. In this invention, the N-type carbon nanotubes, P-type carbon nanotubes, and Li5FeO4 are the same as in the aforementioned technical solutions, and will not be repeated here. This invention does not have any special restrictions on the source of the N-type carbon nanotubes, P-type carbon nanotubes, and Li5FeO4; commercially available products or homemade materials well known to those skilled in the art can be used.

[0070] In a specific embodiment of the present invention, the first solvent preferably includes at least one selected from ethanol, methanol, and ethylene glycol, and more preferably ethanol. The present invention does not impose any special restrictions on the source of the first solvent; commercially available products well-known to those skilled in the art can be used. The aim is to ensure uniform dispersion of N-type carbon nanotubes and P-type carbon nanotubes to obtain a suitable suspension.

[0071] In a specific embodiment of the present invention, the first mixing method is preferably ultrasonic dispersion, and the ultrasonic dispersion time is preferably 20 min to 40 min, for example, 20 min, 30 min, 40 min, or any value between two of these. By controlling the ultrasonic dispersion time within the above range, the present invention can ensure the basic dispersion effect of carbon nanotubes on the one hand, and avoid excessively long ultrasonic time, which could lead to defects in the tube wall of carbon nanotubes and reduce their electronic conductivity on the other hand.

[0072] After obtaining the suspension, the present invention adds Li5FeO4 to the suspension and grinds it to obtain a first slurry.

[0073] In a specific embodiment of the present invention, the preferred grinding method is ball milling, with a preferred milling speed of 250 rpm to 350 rpm and a preferred milling time of 1.5 h to 2.5 h. It should be noted that by selecting ball milling and controlling the milling speed and time within the above range, the present invention can achieve thorough grinding of each material while avoiding excessively high milling speeds and times, which could lead to structural damage to carbon nanotubes and lithium supplements. It also avoids the rapid increase in surface energy caused by over-grinding, which could result in secondary agglomeration of the materials. After grinding in this manner, a suitable first slurry is obtained.

[0074] After obtaining the first slurry, the present invention adds a silane coupling agent to the obtained first slurry for reaction, and then performs a first freeze-drying to obtain a composite lithium supplement agent.

[0075] In specific embodiments of the present invention, the silane coupling agent preferably includes at least one selected from 3-aminopropyltriethoxysilane, γ-aminopropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, and γ-mercaptopropyltrimethoxysilane, more preferably 3-aminopropyltriethoxysilane. The present invention does not impose any particular limitation on the source of the silane coupling agent; commercially available products well known to those skilled in the art can be used.

[0076] In a specific embodiment of the present invention, the amount of silane coupling agent added is preferably 4% to 6% of the total mass of the N-type carbon nanotubes and the P-type carbon nanotubes, for example, 4%, 5%, 6%, or any value between two of these. By controlling the amount of silane coupling agent added within the above range, the present invention can ensure the smooth progress of subsequent reactions, obtain a composite lithium supplement that meets the expected performance requirements, and at the same time avoid excessive aggregation of silane coupling agents due to van der Waals forces, which would be detrimental to the subsequent reactions.

[0077] In a specific embodiment of the present invention, the reaction temperature is preferably 60℃~80℃, and the reaction time is preferably 5h~7h; based on this, Fe-O-Si-CNT bonds can be formed. It should be noted that by controlling the reaction temperature within the above range, the present invention avoids the possibility that low temperatures may lead to insufficient generation of active silanol groups in the silane coupling agent, resulting in a reduction of bonding sites in the condensation stage. It also avoids the possibility that high temperatures may cause increased solvent evaporation, leading to an increase in slurry viscosity, affecting the uniformity of the system, and thus reducing the uniformity of the condensation reaction. At the same time, controlling the reaction time within the above range ensures complete reaction and avoids adverse side reactions of the silane coupling agent caused by excessively long reaction times.

[0078] In a specific embodiment of the present invention, the preferred temperature for the first freeze-drying is -55℃ to -45℃, the preferred pressure is 0.08 Pa to 0.12 Pa, and the preferred time is 11 h to 13 h. By controlling the temperature of the first freeze-drying within the above range, the present invention can effectively inhibit ice crystal growth and protect the nanoscale porous structure; controlling the pressure within the above range can accelerate solvent sublimation and avoid oxidation caused by air intervention; controlling the time within the above range can ensure complete drying while avoiding energy consumption caused by excessive drying time. Based on this, the present invention obtains a composite lithium supplement that meets the expected requirements through the above-mentioned first freeze-drying.

[0079] After obtaining the composite lithium supplement agent, the present invention performs a second mixing with the positive electrode active material and a second solvent to obtain a second slurry; then, the second slurry is coated on at least one side of the positive electrode current collector along the thickness direction, and then subjected to a second freeze-drying and pressing to obtain a positive electrode sheet. In the present invention, the positive electrode active material is the same as that in the above technical solution, and will not be described again here. The present invention does not have any special restrictions on the source of the positive electrode active material, and commercially available products or self-made products well known to those skilled in the art can be used.

[0080] In a specific embodiment of the present invention, the second mixing process preferably further includes the addition of a binder. Based on this, the preferred mass ratio of the positive electrode active material, the composite lithium replenishing agent, and the binder is (90~94):(3~7):(2~4), for example, 90:7:3, 91:6:3, 92:5:3, 93:4:3, 94:3:3, or any ratio between any two of these. By controlling the mass ratio of the positive electrode active material, the composite lithium replenishing agent, and the binder within the above range, the present invention ensures that the mass of the positive electrode active material guarantees the energy density of the battery. The proportion of the composite lithium replenishing agent can efficiently compensate for the initial lithium loss without affecting the main performance of the positive electrode active material. Finally, the low proportion of the binder ensures overall structural stability and forms hydrogen bonds with the silane coupling agent on the surface of the composite lithium replenishing agent, enhancing interfacial bonding. By controlling the three components within the above ratio, a better synergistic effect can be achieved, ensuring the realization of the goals of high capacity, high initial efficiency, long cycle life, and easy mass production of the positive electrode sheet.

[0081] In a specific embodiment of the present invention, the second solvent preferably includes at least one selected from N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, and cyclohexanone, more preferably N-methylpyrrolidone (NMP). In this invention, the purpose of adding the second solvent is to efficiently dissolve the binder and ensure uniform dispersion of all components. The present invention does not impose any special restrictions on the source of the second solvent; commercially available products well-known to those skilled in the art can be used.

[0082] The present invention does not impose any special restrictions on the second mixing process. Any mechanical or manual mixing techniques known to those skilled in the art can be used, with the aim of obtaining the second slurry.

[0083] In a specific embodiment of the present invention, the coating thickness is preferably 140μm to 180μm, for example, 140μm, 150μm, 160μm, 170μm, 180μm, or any value between two of these.

[0084] In some embodiments, the preferred temperature for the second freeze-drying is -35°C to -25°C, the preferred pressure is 8 Pa to 12 Pa, and the preferred time is 7 h to 9 h; based on this, a porous electrode can be obtained. By controlling the temperature of the second freeze-drying within the above range, this invention can effectively suppress ice crystal growth and protect the nanoscale porous structure; controlling the pressure within the above range can accelerate solvent sublimation while avoiding oxidation caused by air intervention; controlling the time within the above range can ensure complete drying while avoiding energy consumption due to excessive drying time.

[0085] In a specific embodiment of the present invention, the compaction density is preferably 2.3 g / cm³. 3 ~2.7g / cm 3 For example, 2.3 g / cm³ 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 Or any value between the two. This invention maximizes energy density while maintaining suitable porosity by controlling the compaction density within the above range, ensuring Li... + This improves transmission efficiency while ensuring the stability of the electrode structure and extending cycle life. Based on these principles, a positive electrode sheet achieving the desired effects of this invention is obtained.

[0086] The preparation method provided by this invention is simple, with mild and easily controllable conditions. Compared with traditional methods for modifying lithium iron phosphate cathode materials, it is simpler and more efficient, and has broad application prospects.

[0087] According to another aspect of the present invention, a battery is provided, comprising a positive electrode sheet as described in the above-described technical solution or a positive electrode sheet prepared by the preparation method described in the above-described technical solution. Thus, the battery possesses all the features and advantages of the positive electrode sheet described in the above-described technical solution, which will not be repeated here. Specifically, the battery includes an electrode assembly and an electrolyte, wherein the electrode assembly is formed by a positive electrode sheet, a negative electrode sheet, and a separator through a winding process or a stacking process; wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet.

[0088] In a specific embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector along the thickness direction; wherein, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil; the composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate, and the composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0089] In a specific embodiment of the present invention, the negative electrode active material layer includes a negative electrode active material. The present invention does not impose any particular limitation on the specific type of the negative electrode active material; any active material known in the art that can be used as the negative electrode of a battery cell can be used. Those skilled in the art can select according to actual needs. Specifically, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc.; wherein, the silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys; the tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. These materials are all commercially available.

[0090] In specific embodiments of the present invention, the negative electrode active material layer may also include a negative electrode binder, a negative electrode conductive agent, and other optional additives. For example, the negative electrode binder may include at least one of polyacrylonitrile (PAN), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS); the negative electrode conductive agent may include at least one of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, Super P (SP), graphene, and carbon nanofibers; other optional additives include thickeners (such as carboxymethyl cellulose (CMC)). These materials are all commercially available.

[0091] In a specific embodiment of the present invention, the method for preparing the negative electrode sheet adopts a method well known to those skilled in the art: first, the raw materials are mixed in a solvent in a certain proportion to form a slurry, and then the negative electrode slurry is coated on the negative electrode current collector. After drying, rolling and other processes, the negative electrode sheet can be obtained.

[0092] In a specific embodiment of the present invention, the separator can be a commercially available separator for batteries that is well known to those skilled in the art; the present invention does not impose any particular limitation on the type of separator, and any porous structure separator with good chemical and mechanical stability can be selected.

[0093] In a specific embodiment of the present invention, the material of the diaphragm preferably includes one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride; the diaphragm can be a single-layer film or a multi-layer composite film, and the present invention does not have any special restrictions on this; when the diaphragm is a multi-layer composite film, the materials of each layer can be the same or different, and the present invention does not have any special restrictions on this.

[0094] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This invention does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0095] In a specific embodiment of the present invention, the electrolyte is an electrolyte solution; the electrolyte solution comprises an electrolyte salt and a solvent; wherein, the electrolyte salt preferably comprises one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate, more preferably lithium hexafluorophosphate; the solvent preferably... The ester may include one or more of the following: ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone, more preferably ethylene carbonate and methyl ethyl carbonate.

[0096] The present invention does not impose any special restrictions on the assembly method of the battery. Any technical solution known to those skilled in the art can be used, in which the above-mentioned positive electrode sheet, separator and negative electrode sheet are used to prepare the electrode assembly, and then the battery is assembled with the electrolyte.

[0097] In a specific embodiment of the present invention, the battery may include an outer packaging, which can be used to encapsulate the aforementioned electrode assembly and electrolyte. The outer packaging of the battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. Specifically, the hard shell may include a housing and a cover plate, wherein the housing may include a bottom plate and a side plate connected to the bottom plate, the bottom plate and the side plate forming a receiving cavity, the housing having an opening communicating with the receiving cavity, and the cover plate being able to cover the opening to close the receiving cavity, the electrode assembly being encapsulated in the aforementioned receiving cavity, and the electrolyte being immersed in the electrode assembly; it may also be a soft pack, such as a pouch soft pack; the material of the soft pack may be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0098] The present invention does not impose any particular restrictions on the shape of the battery; it can be cylindrical, square, or any other arbitrary shape, and those skilled in the art can choose according to specific practical needs.

[0099] In a specific embodiment of the present invention, the battery can be an assembled battery module, and the battery module can contain one or more batteries. The specific number can be selected by those skilled in the art based on the application and capacity of the battery module. In the battery module, the multiple batteries can be arranged sequentially along the length of the battery module; of course, they can also be arranged in any other arbitrary manner. Furthermore, the multiple batteries can be fixed by fasteners. The battery module may also include a housing with a receiving space, in which the multiple batteries are received.

[0100] In the description of this invention, "a plurality of" means two or more.

[0101] In a specific embodiment of the present invention, the battery can also be a battery pack assembled from the aforementioned battery modules. The battery pack may contain one or more battery modules, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery pack. Specifically, the battery pack may include a battery box and multiple battery modules disposed within the battery box; the battery box includes an upper box and a lower box, the upper box covering the lower box and forming a closed space for accommodating the battery modules. The multiple battery modules can be arranged in the battery box in any manner.

[0102] The following detailed description of this application is based on specific embodiments, but the implementation and protection of this invention are not limited thereto. The following embodiments are only some embodiments of this application and are not intended to limit this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0103] Example 1 (1) Preparation of composite lithium supplement: N-type CNTs (nitrogen-doped CNTs, diameter 8 nm, nitrogen content 3.46 wt%) and P-type CNTs (boron-doped CNTs, diameter 10 nm, boron content 1.66 wt%) were ultrasonically dispersed in ethanol at a mass ratio of 1:2 for 30 min with an ultrasonic power of 500 W to form a homogeneous suspension; then, Li5FeO4 nanoparticles with a particle size of 50 nm (specific surface area of ​​40 m²) were added to the above suspension. 2 / g), and ball-milled with zirconia balls for 2 hours at 300 rpm to form a uniform first slurry; then, silane coupling agent 3-aminopropyltriethoxysilane (APTES) was added to the first slurry, the amount of which was 5% of the total mass of N-type CNTs and P-type CNTs, and stirred in an oil bath at 70℃ for 6 hours to form Fe-O-Si-CNT bonds; finally, a first freeze-drying was performed at -50℃, 0.1 Pa, and 12 hours to obtain fluffy Li5FeO4@N&P-CNTs composite powder, which is the composite lithium supplement, wherein the outer N&P-CNTs coating thickness is 5 nm.

[0104] (2) Preparation of positive electrode: Lithium iron phosphate (LiFePO4) (particle size 200 nm), the composite lithium supplement obtained in step (1), and PVDF are mixed in a mass ratio of 92:5:3. N-methylpyrrolidone (NMP) is added to prepare a slurry to obtain a second slurry. The second slurry is then coated onto aluminum foil with a coating thickness of 160 μm. Finally, a second freeze-drying is performed at a temperature of -30 °C, a pressure of 10 Pa, and a time of 8 h. The slurry is then rolled to a compaction density of 2.5 g / cm³. 3 This forms a positive electrode active material layer with a porosity of 60%, thus obtaining the positive electrode sheet.

[0105] (3) Preparation of negative electrode sheet: Graphite, conductive carbon, CMC and SBR are mixed evenly in mass percentage of 95%:1.5%:2%:1.5%, coated on copper foil current collector, and rolled and cut to prepare negative electrode sheet.

[0106] (4) Preparation of electrolyte: Ethyl carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:1, and then 1 mol / L LiPF6 is added and mixed evenly to prepare an electrolyte.

[0107] (5) Preparation of lithium-ion battery: The above positive electrode, separator and negative electrode are wound according to the principle of separator wrapping negative electrode and negative electrode wrapping positive electrode. Then the winding is completed by hot pressing of the cell and installation into aluminum shell. After liquid injection, formation and capacity testing, lithium-ion battery is obtained.

[0108] Example 2 The preparation method of Example 1 is the same, except that the amount of N-type CNTs and P-type CNTs added is adjusted during the preparation of the composite lithium supplement to make the mass ratio of N-type CNTs to P-type CNTs 1:1; and finally, a lithium-ion battery is prepared in the same way.

[0109] Example 3 The preparation method of Example 1 is the same, except that the amount of N-type CNTs and P-type CNTs added is adjusted during the preparation of the composite lithium supplement to make the mass ratio of N-type CNTs to P-type CNTs 1:3; and finally, a lithium-ion battery is prepared in the same way.

[0110] Example 4 The preparation method of Example 1 is the same, except that the ultrasonic dispersion time is 20 min during the preparation of the composite lithium supplement; and the lithium-ion battery is finally prepared in the same way.

[0111] Example 5 The preparation method of Example 1 is the same, except that the particle size of Li5FeO4 nanoparticles is 30nm in the preparation process of the composite lithium supplement; and a lithium-ion battery is finally prepared in the same way.

[0112] Example 6 The preparation method of Example 1 is the same, except that the particle size of Li5FeO4 nanoparticles is 70nm in the preparation process of the composite lithium supplement; and a lithium-ion battery is finally prepared in the same way.

[0113] Example 7 The preparation method of Example 1 is the same, except that the mass ratio of lithium iron phosphate (LiFePO4), composite lithium supplementer, and PVDF is adjusted to 94:3:3 during the preparation of the positive electrode sheet; and a lithium-ion battery is finally prepared in the same way.

[0114] Example 8 The preparation method of Example 1 is the same, except that the mass ratio of lithium iron phosphate (LiFePO4), composite lithium supplementer, and PVDF is adjusted to 90:7:3 during the preparation of the positive electrode sheet; and a lithium-ion battery is finally prepared in the same way.

[0115] Example 9 The preparation method of Example 1 is the same, except that the temperature of the second freeze-drying is adjusted to -25°C during the preparation of the positive electrode sheet; and a lithium-ion battery is finally prepared in the same way.

[0116] Example 10 The preparation method of Example 1 is the same, except that the temperature of the second freeze-drying is adjusted to -35°C during the preparation of the positive electrode sheet; and a lithium-ion battery is finally prepared in the same way.

[0117] Example 11 The preparation method is the same as in Example 1, except that the compaction density during the preparation of the positive electrode is 2.3 g / cm³. 3 Ultimately, a lithium-ion battery was also prepared.

[0118] Example 12 The preparation method is the same as in Example 1, except that the compaction density during the preparation of the positive electrode is 2.7 g / cm³. 3 Ultimately, a lithium-ion battery was also prepared.

[0119] Comparative Example 1 The preparation method according to Example 1 differs in that: during the preparation of the positive electrode, 5% of conductive agent SP is added to replace the composite lithium supplement agent obtained in step (1), that is, lithium iron phosphate LiFePO4, SP and PVDF are mixed in a mass ratio of 92:5:3; after slurry coating, a positive electrode is prepared, and then a lithium-ion battery is prepared according to the steps of Example 1.

[0120] Comparative Example 2 The preparation method according to Example 1 differs in that: during the preparation of the positive electrode, 5% of N-type carbon nanotubes are added to replace the composite lithium supplementer obtained in step (1), that is, lithium iron phosphate LiFePO4, N-type carbon nanotubes and PVDF are mixed in a mass ratio of 92:5:3; after slurry coating, a positive electrode is prepared, and then a lithium-ion battery is prepared according to the steps of Example 1.

[0121] Comparative Example 3 The preparation method according to Example 1 differs in that: during the preparation of the positive electrode sheet, a mixture of 2.5% N-type carbon nanotubes and 2.5% P-type carbon nanotubes is added, and lithium iron phosphate (LiFePO4), the mixture of N-type and P-type carbon nanotubes, and PVDF are mixed in a mass ratio of 92:5:3; after slurry coating, a positive electrode sheet is prepared, and then a lithium-ion battery is prepared according to the steps of Example 1.

[0122] Comparative Example 4 The preparation method is the same as in Example 1, except that in the preparation of the positive electrode, lithium iron phosphate (LiFePO4) and Li5FeO4 nanoparticles with a particle size of 50 nm (specific surface area of ​​40 m²) are added. 2 A mixture of N-type carbon nanotubes and P-type carbon nanotubes in a mass ratio of 1:1, and PVDF in a mass ratio of 92:2:3:3 were mixed; after slurry preparation and coating, a positive electrode sheet was prepared, and then a lithium-ion battery was prepared according to the steps of Example 1.

[0123] Performance testing: (1) DCR test: The lithium-ion battery was left to stand at 25℃ for 30 min, discharged at 1C constant current to 2.5V, left to stand for 10 min, charged at 1C constant current to 3.65V, charged at 3.65V constant voltage to 0.05C, left to stand for 10 min, discharged at 1C constant current for 30 min to 50% SOC; then discharged at 2C for 30 s, and the DCR was calculated; the calculation formula is DCR = voltage difference before and after 2C discharge / 2C current; the test results are shown in Table 1 below.

[0124] (2) 5C capacity retention rate test: The lithium-ion battery was left to stand at 25℃ for 30 min, discharged at 1C constant current to 2.5V, left to stand for 10 min, charged at 1C constant current to 3.65V and charged at 3.65V constant voltage to 0.05C, the capacity was recorded as C1, left to stand for 10 min, discharged at 5C constant current to 2.5V, the capacity was recorded as C2, 5C capacity retention rate % = C2 / C1×100%; the test results are shown in Table 1 below.

[0125] (3) Cyclic test: The lithium-ion battery was left to stand at 25°C for 30 minutes, discharged at 1C constant current to 2.5V, left to stand for 30 minutes, then charged at 1C constant current to 3.65V and charged at 3.65V constant voltage to 0.05C, left to stand for 30 minutes, and then discharged at 1C constant current to 2.5V, left to stand for 30 minutes, charged at 1C constant current to 3.65V and charged at 3.65V constant voltage to 0.05C, left to stand for 30 minutes. The cycle test was carried out for 500 cycles, and the capacity retention rate of the battery was recorded. The test results are shown in Table 1 below.

[0126] Table 1 Test Results Data As can be seen from Table 1: Comparing Examples 1-3: By changing the mass ratio of N-type CNTs to P-type CNTs, it can be seen that when the ratio of N-type CNTs to P-type CNTs is 1:2, the internal resistance DCR of the cell is smaller, the capacity retention rate at high discharge rate (5C) is higher, and the overall conductivity of the cell is better. The ratio of N-type CNTs to P-type CNTs has a significant impact on the formation of the conductive network. A suitable ratio can form a continuous conductive path inside the material, enhance the synergistic effect of N-type CNTs and P-type CNTs, and improve the separation efficiency of electron-hole pairs, further enhancing the conductivity and functionality of the material.

[0127] Comparing Example 1 and Example 4: Reducing the ultrasonic mixing time of N-type CNTs and P-type CNTs will cause uneven dispersion of CNTs, easy aggregation, and insufficient uniform coating on the outer layer of Li5FeO4. The conductivity of the cell is slightly worse, and the internal resistance of the cell and the high-rate discharge capacity retention rate are both reduced.

[0128] Comparing Examples 1 and 5-6: Changing the particle size of the Li5FeO4 material used. If the particle size of the Li5FeO4 material is too small, it is easy to agglomerate during mixing and slurry preparation, resulting in low utilization of the Li5FeO4 material. If the particle size of the Li5FeO4 material is too large, it will block the pores of the electrode, reduce the porosity of the electrode, and lead to a decrease in the migration rate of electrons and ions, thereby affecting the internal resistance of the cell, the high-rate discharge efficiency, and the long-term cycle performance.

[0129] Comparing Examples 1 and 7-8: Changing the positive electrode ratio: A suitable ratio of composite lithium supplement can improve the density of the film formed during the first formation of the cell, reduce the internal resistance of the cell, and improve the rate performance and cycle life of the cell; When the amount of composite lithium supplement is small, it is insufficient to compensate for the active lithium consumed during the first formation and the active lithium consumed during subsequent cycles, and the effect on improving cycle life is not significant; When the amount of composite lithium supplement is large, the improvement in cell internal resistance and the increase in rate performance no longer increase with the increase in the amount added, and the cycle life is slightly improved, which may be more significant in the later stages of cycling.

[0130] Comparing Examples 1 and 9-10: Changing the freeze-drying temperature of the positive electrode sheet, if the freeze-drying temperature is too low, the ice crystals will grow too quickly and damage the conductive network of CNTs, which will have a deteriorating effect on the internal resistance of the cell and the high-rate discharge efficiency.

[0131] Comparing Example 1 and Examples 11-12: Changing the compaction density of the positive electrode roll can provide a suitable lithium-ion transport path and result in a lower cell resistance. If the compaction density is too low, the positive electrode material will not be in close contact with each other and with the conductive agent, which will lengthen the lithium-ion and electron transport path and increase the overall resistance of the cell. If the compaction density is too high, it may cause the vertical channels to collapse, reduce the porosity of the electrode, reduce the lithium-ion transport rate, and deteriorate the cell's internal resistance, high-rate discharge capability, and cycle performance.

[0132] Comparing Example 1 and Comparative Example 1: Using only SP conductive agent, due to the point contact conductivity between SP conductive agent and positive electrode material, it is prone to failure at high rates, resulting in lower overall internal resistance of the cell and lower high-rate discharge efficiency.

[0133] Comparing Example 1 and Comparative Example 2: Using only N-type carbon nanotubes, they are prone to aggregation, making it impossible to simultaneously optimize electron / ion transport, resulting in a higher overall internal resistance of the cell.

[0134] Comparative Example 1 and Comparative Example 3: Using N-type carbon nanotubes and P-type carbon nanotubes, but without lithium replenishment, while using both N-type and P-type carbon nanotubes can optimize the conductive network of the cell and reduce the internal resistance of the cell, but without lithium replenishment, the cycle life of the cell is slightly worse.

[0135] Comparing Example 1 and Comparative Example 4: Simultaneous use of lithium replenishing agent, N-type carbon nanotubes, and P-type carbon nanotubes further improved the conductivity of the battery cell, reduced its internal resistance, and increased cycle life after adding the lithium replenishing agent. However, the overall performance was still inferior to that of the battery cell in Example 1. This direct mixing method makes it easy for the free lithium replenishing agent to undergo side reactions with the electrolyte, and the presence of these side reactions is detrimental to the lithium replenishing agent's effect on improving battery cell life.

[0136] In summary, this invention pre-modifies the lithium replenishing agent with N-type and P-type carbon nanotubes and fixes it through chemical bonding. Fe-O- on the Li5FeO4 surface forms Fe-OC covalent bonds with the -COOH of CNTs, reducing side reactions between the lithium replenishing agent and the electrolyte and reducing the generation of O2 / CO / CO2 gas (the presence of gas makes the adhesion between electrodes less tight, which has a deteriorating effect on the interface of the cell). Moreover, the freeze-drying technology used in this invention has significant advantages over traditional oven drying: on the one hand, the low temperature environment avoids material damage, while high temperature may cause active material agglomeration or binder failure, affecting cell performance; on the other hand, freeze-drying makes solvent evaporation more uniform, avoiding the problem of binder floating due to uneven temperature in traditional oven drying, thus improving the consistency of the cell.

[0137] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0138] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0139] In the foregoing description of this specification, references to terms such as "one embodiment," "another embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0140] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode plate, characterized in that, include: Positive current collector; A positive electrode active material layer is disposed on at least one surface of the positive electrode current collector along the thickness direction; the positive electrode active material layer includes a positive electrode active material and a composite lithium supplement agent; The composite lithium supplement includes Li5FeO4 and a conductive network structure layer that at least partially coats the Li5FeO4, wherein the conductive network structure layer includes N-type carbon nanotubes and P-type carbon nanotubes.

2. The positive electrode sheet according to claim 1, characterized in that, The N-type carbon nanotubes include at least one of nitrogen-doped carbon nanotubes, phosphorus-doped carbon nanotubes, and arsenic-doped carbon nanotubes. And / or, the diameter of the N-type carbon nanotubes is 6 nm to 10 nm; And / or, the P-type carbon nanotubes include at least one of boron-doped carbon nanotubes and aluminum-doped carbon nanotubes; And / or, the diameter of the P-type carbon nanotubes is 7 nm to 13 nm.

3. The positive electrode sheet according to claim 1, characterized in that, The thickness of the conductive network structure layer is 3nm~10nm; And / or, the mass ratio of the N-type carbon nanotubes to the P-type carbon nanotubes in the conductive network structure layer is 1:(1~3).

4. The positive electrode sheet according to claim 1, characterized in that, The positive electrode active material layer satisfies at least one of the following characteristics (1) to (7): (1) The positive electrode active material includes lithium iron phosphate; (2) The decomposition voltage of the composite lithium supplement is 2.8V~3.2V; (3) The particle size of the Li5FeO4 is 30nm~70nm; (4) The specific surface area of ​​the Li5FeO4 is 37 m². 2 / g~43m 2 / g; (5) The porosity of the positive electrode active material layer is 55%~65%; (6) The compaction density of the positive electrode active material layer is 2.3 g / cm³. 3 ~2.7g / cm 3 ; (7) The positive electrode active material layer also includes a binder.

5. The positive electrode sheet according to claim 4, characterized in that, The adhesive includes at least one of polyvinylidene fluoride, polyimide, polyacrylonitrile, and polyvinyl butyral; And / or, the mass ratio of the positive electrode active material, the composite lithium supplement agent and the binder in the positive electrode active material layer is (90~94):(3~7):(2~4).

6. A method for preparing a positive electrode sheet according to any one of claims 1 to 5, characterized in that, Includes the following steps: a) N-type carbon nanotubes, P-type carbon nanotubes and a first solvent are mixed to obtain a suspension; Li5FeO4 is then added to the suspension and ground to obtain the first slurry; b) Add a silane coupling agent to the first slurry obtained in step a) and react it, then freeze-dry it to obtain a composite lithium supplement; c) The composite lithium supplement obtained in step b) is mixed with the positive electrode active material and the second solvent to obtain a second slurry; the second slurry is then coated on at least one side of the positive electrode current collector along the thickness direction, and then subjected to a second freeze-drying and pressing to obtain a positive electrode sheet.

7. The preparation method according to claim 6, characterized in that, In step a): The first solvent includes at least one of ethanol, methanol, and ethylene glycol; And / or, the first mixing method is ultrasonic dispersion for 20 min to 40 min; And / or, the grinding method is ball milling, with a rotation speed of 250 rpm to 350 rpm and a time of 1.5 h to 2.5 h.

8. The preparation method according to claim 6, characterized in that, In step b): The silane coupling agent includes at least one of 3-aminopropyltriethoxysilane, γ-aminopropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, and γ-mercaptopropyltrimethoxysilane. And / or, the amount of the silane coupling agent added is 4% to 6% of the total mass of the N-type carbon nanotubes and the P-type carbon nanotubes; And / or, the reaction temperature is 60℃~80℃, and the time is 5h~7h; And / or, the temperature of the first freeze drying is -55℃ to -45℃, the pressure is 0.08Pa to 0.12Pa, and the time is 11h to 13h.

9. The preparation method according to claim 6, characterized in that, In step c): The second solvent includes at least one of N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, and cyclohexanone; And / or, the second mixing process further includes the addition of a binder, preferably, the mass ratio of the positive electrode active material, the composite lithium supplementer and the binder is (90~94):(3~7):(2~4); And / or, the coating thickness is 140μm~180μm; And / or, the second freeze-drying temperature is -35℃ to -25℃, the pressure is 8Pa to 12Pa, and the time is 7h to 9h; And / or, the compacted density of the press is 2.3 g / cm³. 3 ~2.7g / cm 3 .

10. A battery, characterized in that, The positive electrode sheet includes the positive electrode sheet according to any one of claims 1 to 5 or the positive electrode sheet prepared by the preparation method according to any one of claims 6 to 9.

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