Positive electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of batteries, in particular to a positive electrode material and a preparation method and application thereof. The positive electrode material comprises a composite lithium iron pyrophosphate material, the structural general formula of the composite lithium iron pyrophosphate material is Li4 (Fe1-xMx) a (P2O7) (PO4) 2, a is more than 2.5 and less than or equal to 3.2, X is more than or equal to 0.21 and less than or equal to 0.6, and M is a transition metal element; the transition metal element comprises at least one of a VB group element, a VIIB group element and a VIII group element. According to the invention, metal elements with high redox potential are added on the basis of the composite lithium iron pyrophosphate material to replace iron sites, so that the average voltage of the material is improved, meanwhile, the material structure is supported, a lithium ion migration channel is widened, and the energy density, low-temperature capacity and cycling stability of the material are improved, so that the low-temperature performance of the battery is further improved.

Description

Technical Field

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

[0002] With the rapid development of new energy, high-capacity lithium-ion power batteries have become a global hot topic in new energy and technology development. Cathode materials, a key component affecting lithium battery capacity, are currently a focus of technological breakthroughs. Lithium iron phosphate (LFP) materials have an olivine-type crystal structure, and lithium-ion migration relies on one-dimensional channels, resulting in poor low-temperature kinetic performance and a significant decrease in capacity retention at low temperatures, making them unsuitable for use in cold regions. Lithium manganese iron phosphate (LFP) materials optimize the voltage platform of LFP through manganese doping, but they suffer from significant composition-dependent defects. Excessive manganese doping can obstruct lithium-ion migration channels, significantly reducing the constant-current charge-to-charge ratio and causing a sharp increase in material impedance, leading to rapid capacity decay during battery cycling and affecting long-term service stability. While composite lithium pyrophosphate materials developed in recent years have improved structural stability, they still have inherent shortcomings. Their theoretical specific capacity is slightly lower than that of LFP and LFP, and their voltage platform is relatively low, resulting in a significant decrease in battery energy density, failing to meet the application requirements of high-energy-density scenarios.

[0003] Therefore, in order to address the shortcomings of existing iron-based cathode materials, there is an urgent need for a new cathode material that is low in cost, has excellent low-temperature performance, stable ion migration efficiency, low impedance, and high energy density. This has become an important research and development direction in the field of lithium-ion battery cathode materials. Summary of the Invention

[0004] In view of this, the present invention aims to at least partially solve one of the technical problems in the related art. To this end, the present invention provides a cathode material, its preparation method, and its application. By adding a metal element with a high redox potential to replace iron sites on a composite lithium iron pyrophosphate material, the average voltage of the material is increased, the lithium-ion migration channels are broadened, and the energy density, low-temperature capacity, and cycle stability of the material are improved.

[0005] To solve the above-mentioned technical problems, the present invention is implemented as follows: According to one aspect of the present invention, a cathode material is provided, comprising: a composite lithium iron pyrophosphate material; the composite lithium iron pyrophosphate material having the general structural formula Li4(Fe) 1-x M x ) a(P2O7)(PO4)2, wherein 2.5<a≤3.2, 0.21≤X≤0.6, and M is a transition metal element; the transition metal element includes at least one of Group VB, Group VIIB, and Group VIII elements.

[0006] In some of these embodiments, the range of a in the general formula of the cathode material includes 2.8 < a ≤ 3.

[0007] In some of these embodiments, the high-voltage plateau ratio Rx of the composite lithium iron pyrophosphate material satisfies the following relationship: 0.18≤Rx≤0.59; where Rx=1.02X-0.03, 0.21≤X≤0.6.

[0008] In some embodiments, the compaction density of the cathode material is 2.2 g / cm³. 3 ~3.2g / cm 3 .

[0009] In some of these embodiments, the particle size of the cathode material D90 ranges from 8 nm to 12 nm.

[0010] In some embodiments, the transition metal element includes at least one of nickel, manganese, cobalt, and vanadium.

[0011] In some embodiments, the cathode material further includes a carbon coating layer covering the surface of the composite lithium iron pyrophosphate material, optionally with a thickness of 1 nm to 20 nm.

[0012] According to another aspect of the present invention, the present invention provides a method for preparing the cathode material described in the above technical solution, comprising the following steps: a) Mixing lithium source, iron source, M source and organic acid to obtain a mixture; b) The mixture is mixed with a solvent and ball-milled to form a slurry, and the slurry is spray-dried to obtain a precursor powder; c) The precursor powder is subjected to microwave activation and calcination treatment, wherein M is a transition metal element.

[0013] In some of these embodiments, in step a), the lithium source includes at least one of lithium phosphate, lithium hydroxide, and lithium carbonate.

[0014] In some of these embodiments, in step a), the iron source includes at least one of ferric phosphate, ferric nitrate, and ferric oxide.

[0015] In some of these embodiments, in step a), the M source includes at least one of a nickel source, a manganese source, and a cobalt salt. Optionally, the M source includes at least one of nickel phosphate, nickel carbonate, nickel oxide, nickel nitrate, manganese phosphate, manganese nitrate, manganese dioxide, cobalt tetroxide, cobalt nitrate, and cobalt carbonate.

[0016] In some of these embodiments, in step a), the organic acid includes at least one of citric acid and oxalic acid.

[0017] In some of these embodiments, in step b), the rotational speed of the sand mill is 1500 rpm to 2000 rpm, and the sand milling time is 6 h to 9 h.

[0018] In some of these embodiments, in step b), the solvent includes at least one of pure water, anhydrous ethanol, and isopropanol.

[0019] In some embodiments, in step b), the solid content of the mixture in the material after mixing the mixture with the solvent is 30% to 60%.

[0020] In some embodiments, step b) further includes introducing an inert gas during the spray drying process, the inert gas including at least one of nitrogen, helium, neon, and argon.

[0021] In some of these embodiments, in step b), the inlet temperature of the spray dryer is 150°C to 220°C, and the outlet temperature is 90°C to 110°C.

[0022] In some of these embodiments, in step c), the microwave power for microwave activation is 800W to 1000W, and the microwave heating time is 30s to 180s.

[0023] In some embodiments, step c) further includes introducing an inert gas for protection during the heat treatment process, the inert gas including at least one of nitrogen, helium, neon, and argon; In some embodiments, step c) includes a first calcination treatment, a second calcination treatment, and a third calcination treatment; the conditions for the first calcination treatment include: a heating rate of 1℃ / min to 3℃ / min, a calcination temperature of 140℃ to 160℃, and a holding time of 0.5h to 2h; the conditions for the second calcination treatment include: a heating rate of 4℃ / min to 6℃ / min, a calcination temperature of 300℃ to 350℃, and a holding time of 1h to 3h; the conditions for the third calcination treatment include: a heating rate of 4℃ / min to 6℃ / min, a calcination temperature of 500℃ to 650℃, and a holding time of 6h to 8h.

[0024] In some of these embodiments, in step a): the mixing process further includes adding a carbon source; Optionally, the carbon source includes carbon nanotubes and / or conductive carbon black; Optionally, the carbon source accounts for 0.1 wt% to 1.5 wt% of the total mass of the mixture.

[0025] According to another aspect of the present invention, a battery is provided, including a positive electrode sheet; 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 in the thickness direction, the positive active material layer including the positive electrode material described in the above technical solution or the positive electrode material prepared by the preparation method described in the above technical solution.

[0026] Implementing the technical solution of the present invention has at least the following beneficial effects: 1. This invention improves the average voltage of a composite lithium iron pyrophosphate material by adding a metal element with a high redox potential to replace the iron sites, while also providing structural support, widening the lithium-ion migration channels, and enhancing the material's energy density, low-temperature capacity, and cycle stability.

[0027] 2. In this embodiment of the invention, the present invention utilizes sand milling and spraying equipment for wet mixing, and microwave heating technology to activate the raw materials for tens of seconds or minutes, thereby increasing the crystallinity and fusion of the initial reaction of the material and improving the utilization rate of the raw materials. Then, high-temperature sintering is carried out. Studies have found that this method can improve the phase purity of the material, and the sintering temperature is lower, the particle strength is improved, the product specific capacity is higher, the residual alkali content is lower, and the electrode compaction is higher. When this material is applied to the battery cell, the energy density and cycle performance are improved.

[0028] 3. In the embodiments of the present invention, the present invention avoids the addition of materials from reacting violently at high temperatures due to multiple sintering, which can easily generate a large amount of gas and cause defects. By using different sintering temperatures to gradually increase the temperature, the addition of materials can be slowly and steadily decomposed and utilized, eliminating small gaps and neck growth, and preparing for rapid densification. When the temperature reaches the preset peak, the atomic diffusion is high enough at this temperature, causing the material to undergo significant volume diffusion, making it easier to achieve densification.

[0029] 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

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0036] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0037] 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.

[0038] Existing iron-based cathode materials, such as lithium iron phosphate, lithium manganese iron phosphate, and composite lithium iron pyrophosphate, all have inherent shortcomings in practical applications, limiting their use in the battery field. There is an urgent need for a new type of cathode material that is low-cost and possesses excellent low-temperature performance, stable ion migration efficiency, low impedance, and high energy density.

[0039] In summary, although existing technologies have conducted extensive research on the development of low-temperature lithium iron phosphate materials, the improvement effects are still not ideal. Through in-depth research, the inventors of this invention have discovered that by adding a metal element with a high redox potential to replace iron in a composite lithium iron phosphate material, the average voltage of the cathode material can be increased. Simultaneously, this provides structural support, widens the lithium-ion migration channels, and improves the material's energy density, low-temperature capacity, and cycle stability. Specifically, this invention adopts the following technical solution: According to one aspect of the present invention, a cathode material is provided, comprising: a composite lithium iron pyrophosphate material; the composite lithium iron pyrophosphate material having the general structural formula Li4(Fe) 1-x M x) a(P2O7)(PO4)2, wherein 2.5<a≤3.2, 0.21≤X≤0.6, and the transition metal element includes at least one of Group VB, Group VIIB, and Group VIII elements.

[0040] It should be further explained that the composite lithium iron pyrophosphate material exhibits a three-dimensional structure. The composition of the three-dimensional structure is more conducive to lithium ion migration. At the same time, the study found that this material has better low-temperature kinetic performance, which can improve the low-temperature capacity retention rate to a certain extent. In addition, the introduction of high-potential metal elements into this structure improves part of the redox plateau, significantly increases the average voltage of the material, improves the energy density of the material, and improves the cycle stability of the material.

[0041] In a specific embodiment of the present invention, 2.5 < a ≤ 3.2, preferably 2.8 < a < 3. It should be further explained that when a < 3, the material is in an iron-deficient state with some vacancies. The appearance of vacancies is beneficial to the widening of the lithium-ion migration channel and reduces the production of miscellaneous items, which can improve the cycle performance of the material. When a > 3.2, iron saturation will crowd out the original vacancies in the crystal structure, making the lithium-ion migration channel narrower or even blocked. At the same time, iron saturation may lead to an imbalance in the stress distribution inside the crystal lattice and a decrease in structural stability. At the same time, when a < 2.5, there are too many iron vacancies in the cathode material, which causes the original crystal lattice framework to lose support due to a large number of metal sites being vacant, resulting in a sharp increase in the degree of lattice distortion and a decrease in the continuity of ion migration.

[0042] In a specific embodiment of the present invention, the positive electrode material satisfies Rx = 1.02X - 0.03, where 0.21 ≤ X ≤ 0.6, and the range of Rx is 0.18 ≤ Rx ≤ 0.59. It should be further noted that Rx in the formula represents the proportion of high-voltage metal elements in the positive electrode material. When Rx < 0.18, it indicates that the proportion of high-voltage metal elements in the crystal structure is small, and the effect of improving energy density cannot be achieved. When Rx > 0.59, it indicates that the proportion of high-voltage metal elements in the crystal structure is large. During the charging and discharging process, the electronic transition ability of high-voltage metal ions is weak, the surrounding conductivity decreases, and some ions will produce Taylor effect, causing distortion of the surrounding lattice, hindering the ion migration path, resulting in a low constant current charge ratio and a decrease in specific capacity. Although the voltage is improved, the overall material energy density is not significantly improved, and the subsequent cycle performance is severely degraded. Therefore, high-voltage metals should not be added in excess.

[0043] In a specific embodiment of the present invention, the compaction density of the positive electrode material is 2.2 g / cm³. 3 ~3.2g / cm 3 For example, 2.2 g / cm³ 3 2.4g / cm 3 2.6g / cm 3 2.8g / cm 3 3g / cm 3 3.2g / cm 3 By controlling the compaction density of the cathode material within the range of the above two conditions, the active material particles can be firmly fixed in the electrode. This avoids the problem of the active material loosening and falling off due to volume expansion and contraction during charge and discharge cycles when the particles are at low compaction density. At the same time, it also avoids the risk of excessive internal stress in the electrode due to excessive compaction density, which could lead to particle breakage and crystal phase distortion.

[0044] In a specific embodiment of the present invention, the particle size of the positive electrode material D90 ranges from 8nm to 12nm, for example, 8nm, 9nm, 10nm, 11nm, 12nm, or any two of these ranges. Controlling the particle size of the positive electrode material D90 within the above range can increase the specific surface area. On the one hand, this can increase the contact area with the electrolyte, improve the wettability of the electrolyte, and reduce the interfacial resistance. On the other hand, it can provide more lithium-ion insertion / extraction active sites, significantly improving the charge / discharge efficiency and reaction rate of the battery.

[0045] In a specific embodiment of the present invention, the transition metal element includes at least one of nickel, manganese, cobalt, and vanadium. It should be further noted that in this application, there is no particular limitation on the type and raw material of the transition metal, as long as it has a high redox potential, can improve the average voltage of the material, support the cathode material, and improve the energy density of the material.

[0046] In a specific embodiment of the present invention, the cathode material further includes a carbon coating layer covering the surface of the composite lithium iron pyrophosphate material. The thickness of the carbon coating layer is 1nm to 20nm, for example, 1nm, 5nm, 10nm, 15nm, 20nm, or any two of these ranges. By controlling the carbon coating layer within the above range, it is possible to avoid discontinuity in the carbon layer when the carbon layer is too thin, thus preventing the formation of complete electron channels. Conversely, if the carbon coating layer is too thick, it may increase the resistance of lithium ions to passing through the carbon layer, which may reduce high-rate performance.

[0047] In summary, the cathode material of the present invention includes a composite lithium iron pyrophosphate material. Based on this, the present invention improves the average voltage of the material by adding some high redox metal elements to the composite lithium iron pyrophosphate material to replace the iron elements. At the same time, the added metal elements also play a supporting role in the structure of the material, widening the migration channels of lithium ions, and improving the energy density, low-temperature capacity and cycle stability of the material.

[0048] According to another aspect of the present invention, the present invention provides a method for preparing the cathode material described in the above technical solution, comprising the following steps: a) Mixing lithium source, iron source, M source and organic acid to obtain a mixture; b) The mixture is mixed with a solvent and ball-milled to form a slurry, and the slurry is spray-dried to obtain a precursor powder; c) The precursor powder is subjected to microwave activation and calcination treatment, wherein M is a transition metal element.

[0049] This invention first mixes an iron source, a lithium source, an M source, and an organic acid using a ball mill, and then atomizes and dries them to obtain a precursor. It should be further noted that the purpose of adding the organic acid is to complex metal ions and achieve uniform dispersion of the raw materials.

[0050] In a specific embodiment of the present invention, the mixing process further includes adding a carbon source, which includes carbon nanotubes and / or conductive carbon black. The mass of the carbon source accounts for 0.1wt% to 1.5wt% of the total mass of the mixture. It should be further noted that adding a carbon source can improve the conductivity of electrons, reduce the internal resistance of the electrode, and at the same time reduce high-valence metal ions at high temperatures, thereby reducing the impurity content. By controlling the mass of the carbon source within the above range, the thickness of the carbon coating layer can be indirectly controlled.

[0051] In specific embodiments of the present invention, the lithium source includes at least one of lithium phosphate, lithium hydroxide, and lithium carbonate; in a preferred embodiment of the present invention, lithium phosphate is used as the lithium source. The iron source includes at least one of iron phosphate, iron nitrate, and iron oxide; in a preferred embodiment of the present invention, iron phosphate is used as the iron source. The M source includes a nickel source, a manganese source, and a cobalt salt; in a preferred embodiment of the present invention, the M source is selected from a manganese source, and the manganese source includes at least one of manganese phosphate, manganese nitrate, and manganese dioxide; in a preferred embodiment of the present invention, manganese phosphate is used as both the manganese and iron sources. In a preferred embodiment of the present invention, the organic acid is selected from at least one of citric acid and oxalic acid; in a preferred embodiment of the present invention, carbon nanotubes are used as the carbon source, and citric acid is used as the organic acid. In other preferred embodiments of the present invention, the nickel source includes at least one of nickel phosphate, nickel carbonate, nickel oxide, and nickel nitrate; and the cobalt salt includes at least one of cobalt tetroxide, cobalt nitrate, and cobalt carbonate.

[0052] In a specific embodiment of the present invention, the mixing process is preferably carried out in a sand mill to ensure that the raw materials are mixed evenly. In addition, the process also includes the addition of a solvent and a zirconium bead medium. The solvent includes at least one of pure water, anhydrous ethanol, and isopropanol. The sand mill medium preferably includes zirconium oxide balls. The diameter of the zirconium oxide balls is preferably 0.1 mm, and the ball-to-material ratio is preferably 4:1.

[0053] In a specific embodiment of the present invention, the mixing speed is preferably 1500 rpm to 2000 rpm, and the mixing time is preferably 6 h to 9 h, specifically 6 hours at 2000 rpm. This ensures that the above-mentioned raw materials are mixed evenly, which is beneficial to the smooth progress of subsequent steps.

[0054] In a specific embodiment of the present invention, in step a), the solid content of the mixture in the material after mixing the mixture with the solvent is 30% to 80%, for example, 30%, 50%, 60%, 70%, 80%, or any two of these ranges. By controlling the solid content of the mixture in the solvent within the above range, problems such as low grinding efficiency and precursor agglomeration that may be caused by too low solid content can be avoided, as well as problems such as uneven grinding and precursor cracking that may be caused by too high solid content.

[0055] In a specific embodiment of the present invention, after sand milling and mixing, the precursor needs to be spray-dried to obtain a precursor. The inlet temperature of the atomization drying is 150℃~220℃, specifically 150℃, 160℃, 180℃, 220℃, or any two of these ranges. The spray outlet temperature is maintained at 90℃~110℃, for example 90℃, 100℃, 110℃, 120℃, or any two of these ranges. By controlling the inlet and outlet temperatures of the spray drying within the above ranges, the drying efficiency of the droplets can be improved, while avoiding damage to the raw material properties caused by excessively high temperatures.

[0056] In a specific embodiment of the present invention, the spray drying process further includes the introduction of an inert gas, which includes at least one of nitrogen, helium, neon, and argon. By introducing an inert gas during spray drying, the oxidative deterioration of the mixed raw materials can be prevented, thereby affecting the subsequent battery performance.

[0057] In a specific embodiment of the present invention, the microwave power for microwave activation is 800W~1000W, and the microwave heating time is 30s~180s. It should be further noted that in this application, the raw materials are treated with microwave heating technology before high-temperature calcination. It should also be noted that in traditional heating, heating methods such as convection, conduction, or radiation are often relied upon. The heating of materials is usually gradually transferred from the outside to the inside. This may lead to problems such as temperature gradients inside the material, uneven reaction temperatures, and low energy utilization efficiency during the heating process. Compared with traditional heating, microwave heating achieves bulk heating through direct coupling between the microwave electromagnetic field and the material, so that heating is generated simultaneously throughout the material, achieving rapid and uniform heating. This results in synthesized materials with more uniform microstructure, fewer defects, and higher phase purity.

[0058] In a specific embodiment of the present invention, in step b), the calcination treatment further includes the introduction of an inert gas for protection. The inert gas includes at least one of nitrogen, helium, neon, and argon. The purpose of introducing the inert gas is to inhibit the oxidation of metal ions, ensure the purity of the chemical composition of the material, and prevent moisture intrusion to avoid side reactions.

[0059] In a specific embodiment of the present invention, the calcination process includes: first, heating from room temperature to a first sintering temperature of 140℃~160℃ at a rate of 1℃ / min~3℃ / min, holding at this temperature for 0.5h~2h; then heating to a second sintering temperature of 300℃~350℃ at a rate of 4℃ / min~6℃ / min, holding at this temperature for 1h~3h; then heating to a third sintering temperature of 500℃~650℃ at a rate of 4℃ / min~6℃ / min, holding at this temperature for 6h~8h; and finally, naturally cooling to room temperature to complete the calcination process. It should be further noted that during the material forming process, some additives may react violently at high temperatures, easily generating large amounts of gas and causing defects. Using a stepped heating process with the first, second, and third sintering temperatures allows the additives to decompose and function slowly and steadily, eliminating small voids and neck growth, preparing for rapid densification. When the temperature reaches the preset peak, atomic diffusion is sufficiently high at this temperature, causing significant volume diffusion in the material, making densification easier to achieve.

[0060] The preparation method provided by this invention has simple process steps, mild and easy-to-control conditions, readily available and low-cost raw materials, and has broad application prospects.

[0061] According to another aspect of the present invention, a battery is provided, comprising a positive electrode sheet; 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 in the thickness direction, the positive active material layer comprising the positive electrode material described in the above-described technical solution or the positive electrode material 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 material described in the above-described technical solution, which will not be repeated here.

[0062] In a specific embodiment of the present invention, the positive electrode current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode active material layer is disposed on one or both of the two opposite surfaces of the positive electrode current collector. The positive electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil can be used as a metal foil; the composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer; 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.).

[0063] The present invention does not impose any special restrictions on the preparation method of the positive electrode sheet. The positive electrode sheet can be obtained by coating the positive electrode slurry onto the positive electrode current collector and then drying it, which is well known to those skilled in the art.

[0064] In a specific embodiment of the present invention, the positive electrode active material layer preferably includes a binder and a conductive agent in addition to the positive electrode material. The mass ratio of the positive electrode material, binder and conductive agent is preferably (70~99):(0.5~15):(0.5~15). The binder may include one or more of polyvinylidene fluoride (PVDF), sodium alginate, polyvinyl alcohol, polymethyl methacrylate, hydrogenated nitrile rubber, polytetrafluoroethylene, and polyacrylic acid, thereby improving the bonding strength between the positive electrode active material layer and the positive electrode current collector. The conductive agent may include one or more of conductive carbon black, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0065] In a specific embodiment of the present invention, the positive electrode material, binder and conductive agent are dissolved in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is then coated on the positive electrode current collector, and after drying and other processes, a positive electrode sheet can be obtained.

[0066] In a specific embodiment of the present invention, the battery preferably includes a negative electrode, an electrolyte, and a separator, in addition to the positive electrode. All of these can be made from raw materials well-known to those skilled in the art for preparing lithium-ion batteries, and the present invention does not impose any special limitations on them. The battery may also include an outer packaging, which can be used for encapsulation. This outer packaging can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell, or it can be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0067] The present invention does not impose any particular limitation on the shape of the battery, which can be cylindrical, square or other arbitrary shapes, and those skilled in the art can choose according to specific practical needs.

[0068] The present application will be specifically described below with reference to the embodiments, but the implementation and protection of the present invention are not limited thereto. The following embodiments are only some embodiments of the present application and are not intended to limit the present application.

[0069] Example 1 1. Synthesis of cathode material powder, target chemical formula: [Li4(Fe 0.5 Mn 0.5 ) 2.9 (P₂O₇)(PO₄)₂] First, the raw materials ferric phosphate, manganese phosphate, lithium phosphate, ferric nitrate, citric acid, carbon nanotubes, and water solvent are thoroughly mixed, with carbon nanotubes accounting for 0.5 wt% of the total mass of the mixture. The mixture is then milled in a sand mill, with 0.1 mm diameter zirconia balls added during the milling process. The solid content of the mixture is 33.3%, and the mass ratio of zirconia balls to the total powder is 4:1. The milling is carried out at a speed of 2000 rpm for 6 hours. After milling, the mixture is placed in a spray drying equipment with an inlet temperature of 150℃~160℃ and an outlet temperature of 90℃~95℃. Nitrogen gas is continuously introduced for protection during the drying process. After drying, the cathode material powder is obtained with a carbon coating thickness of 10 nm.

[0070] 2. Preparation of cathode materials: The aforementioned cathode material powder was activated by microwave at a power of 850 W for a reaction time of 60 s. The microwave-activated material was then transferred to a calcination furnace and calcined at 150 °C at a rate of 2 °C / min for 1 hour under a nitrogen atmosphere; calcined again at 350 °C at a rate of 5 °C / min for 2 hours; and finally calcined at 550 °C at a rate of 6 °C / min for 6 hours. After cooling, the calcined material was pulverized and sieved through a 400-mesh sieve to obtain the cathode material. The cathode material has an Rx of 0.48 and a compacted density of 3.0 g / cm³. 3 The D90 particle size ranges from 8nm to 12nm.

[0071] Example 2 The preparation method provided in Example 1 is the same, except that the metal source M is changed to two metals, Mn and Co, with the general structural formula: [Li4(Fe 0.5 Mn 0.25 Co 0.25 ) 2.9 (P2O7)(PO4)2].

[0072] 1. Synthesis of cathode material powder, target chemical formula: Li4(Fe 0.5 Mn 0.25 Co 0.25 ) 2.9 (P₂O₇)(PO₄)₂ First, the raw materials iron phosphate, manganese phosphate, lithium phosphate, iron nitrate, citric acid, carbon nanotubes, cobalt nitrate, and water solvent are thoroughly mixed, with carbon nanotubes accounting for 1.5 wt% of the total mass of the mixture. The mixture is then milled in a sand mill, with 0.1 mm diameter zirconia balls added during the milling process. The solid content of the mixture is 60%, and the mass ratio of zirconia balls to powder is 4:1. The milling is carried out at a speed of 2000 rpm for 6 hours. After milling, the mixture is placed in a spray drying equipment with an inlet temperature of 210℃~220℃ and an outlet temperature of 100℃~110℃. Nitrogen gas is continuously introduced for protection during the drying process. After drying, the cathode material powder is obtained with a carbon coating thickness of 15 nm.

[0073] 2. Preparation of cathode materials: The aforementioned cathode material was activated by microwave at a power of 850 W for a reaction time of 60 s. The microwave-activated material was then transferred to a calcining furnace and calcined at 160 °C at a rate of 2 °C / min for 1 hour under a nitrogen atmosphere; calcined again at 320 °C at a rate of 4 °C / min for 3 hours; and finally calcined at 550 °C at a rate of 5 °C / min for 6 hours. After cooling, the calcined material was pulverized and sieved through a 400-mesh sieve to obtain the cathode material. The cathode material has an Rx of 0.49 and a compacted density of 3.2 g / cm³. 3 The D90 particle size ranges from 8nm to 12nm.

[0074] Example 3 The preparation method provided in Example 1 is the same, except that the metal source M is changed to two metals, Mn and Ni, with the general structural formula: Li4(Fe 0.5 Mn 0.25 Ni 0.25 ) 2.9 (P2O7)(PO4)2.

[0075] 1. Synthesis of cathode material powder, target chemical formula: Li4(Fe 0.5 Mn 0.25 Ni 0.25 ) 2.9 (P₂O₇)(PO₄)₂ First, the raw materials ferric phosphate, manganese phosphate, lithium phosphate, ferric nitrate, citric acid, conductive carbon black, nickel oxide, and water solvent are thoroughly mixed, with conductive carbon black accounting for 0.2 wt% of the total mass of the mixture. The mixture is then sand-milled in a sand mill, with 0.1 mm diameter zirconia balls added during the sand milling process. The solid content of the mixture is 60%, and the mass ratio of zirconia balls to the total powder is 3:1. The sand milling is carried out at a speed of 1500 rpm for 9 hours. After sand milling, the mixture is placed in a spray drying equipment with an inlet temperature of 180℃~190℃. Nitrogen gas is continuously introduced for protection during the drying process. After drying, the cathode material powder is obtained with a carbon coating thickness of 5 nm.

[0076] 2. Preparation of cathode materials: The aforementioned cathode material powder was activated by microwave at a power of 850 W for a reaction time of 60 s. The microwave-activated material was then transferred to a calcination furnace and calcined at 140 °C at a rate of 3 °C / min for 1.5 h under a nitrogen atmosphere; calcined again at 350 °C at a rate of 5 °C / min for 3 h; and finally calcined at 550 °C at a rate of 5 °C / min for 8 h. After cooling, the calcined material was pulverized and sieved through a 400-mesh sieve to obtain the cathode material. The cathode material has an Rx of 0.49 and a compacted density of 2.5 g / cm³. 3 The D90 particle size ranges from 8nm to 12nm.

[0077] Comparative Example 1 The preparation method provided in Example 1 is used, except that it does not include the M source, i.e., the chemical formula is Li4Fe. 2.9 (P2O7)(PO4)2.

[0078] 1. Synthesis of cathode material powder, target chemical formula: Li4Fe 2.9 (P₂O₇)(PO₄)₂ First, the raw materials iron phosphate, lithium phosphate, phosphoric acid, citric acid, and carbon nanotubes are thoroughly mixed in an aqueous solvent, with carbon nanotubes accounting for 0.5 wt% of the total mass of the mixture. The mixture is then milled in a sand mill, with 0.1 mm diameter zirconia balls added during the milling process. The solid content of the mixture is 33.3%, and the mass ratio of zirconia balls to powder is 4:1. The milling is carried out at a speed of 2000 rpm for 6 hours. After milling, the mixture is placed in a spray drying equipment with an inlet temperature of 210℃~220℃ and an outlet temperature of 100℃~110℃. Nitrogen gas is continuously introduced for protection during the drying process. After drying, cathode material powder is obtained with a carbon coating thickness of 10 nm.

[0079] 2. Preparation of cathode materials: The aforementioned cathode material powder was activated by microwave at a power of 850 W for a reaction time of 60 s. The microwave-activated material was then transferred to a calcination furnace and calcined at 150 °C at a rate of 2 °C / min for 1 hour under a nitrogen atmosphere; calcined again at 350 °C at a rate of 5 °C / min for 2 hours; and finally calcined at 550 °C at a rate of 6 °C / min for 6 hours. After cooling, the calcined material was pulverized and sieved through a 400-mesh sieve to obtain the cathode material. The cathode material has an Rx of -0.03 and a compacted density of 2.5 g / cm³. 3 The D90 particle size ranges from 8nm to 12nm.

[0080] Comparative Example 2 The preparation method of the cathode material in this comparative example is generally the same as that in Example 1, except that: Without microwave activation, the aforementioned cathode material was directly transferred to a calcination furnace and calcined at 150°C at a rate of 2°C / min for 1 hour in a nitrogen atmosphere; then calcined at 350°C at a rate of 5°C / min for 2 hours; and finally calcined at 550°C at a rate of 5°C / min for 6 hours. After cooling, the calcined material was pulverized and sieved through a 400-mesh sieve to obtain the cathode material. The cathode material has an Rx of 0.48 and a compacted density of 3.0 g / cm³. 3 The D90 particle size ranges from 8nm to 12nm.

[0081] Comparative Example 3 The preparation method of the cathode material in this comparative example is generally the same as that in Example 1, except that: The aforementioned cathode material powder was activated by microwave at a power of 850 W for a reaction time of 60 s. The microwave-activated material was then transferred to a calcination furnace and calcined at 550 °C in a nitrogen atmosphere at a rate of 2 °C / min for 9 hours. After cooling, the calcined material was pulverized and sieved through a 400-mesh sieve to obtain the cathode material. The cathode material has an Rx of 0.48 and a compacted density of 2.8 g / cm³. 3 The D90 particle size ranges from 8nm to 12nm.

[0082] Performance testing: 1. Electrode fabrication and electrochemical performance testing: Using the final products prepared in Examples 1-3 and Comparative Examples 1-3 as positive electrode materials, to test the electrochemical performance of the lithium-ion positive electrode materials obtained in the examples and comparative examples, the positive electrode materials obtained in the examples or comparative examples, conductive carbon black, and binder polyvinylidene fluoride were mixed in a mass ratio of 90:5:5. N-methylpyrrolidone (NMP) was used as a solvent to form a slurry, which was then uniformly coated onto aluminum foil. After drying at a suitable temperature, the mixture was rolled and vacuum dried at 120°C for 12 hours to obtain a positive electrode sheet. The electrode sheet was cut, weighed, and placed in a glove box. In the glove box, using a sodium metal sheet as the negative electrode and a polypropylene porous membrane as the separator, sodium hexafluorophosphate (NaPF6) was dissolved in a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1. The electrolyte concentration was 1 mol / L. CR2032 button batteries were assembled in an argon-filled glove box.

[0083] 2. Electrochemical performance testing: Equipment: Xinwei Battery Testing System (CT-4008T); Test program: (1) Initial specific capacity test: Charge the capacitor to 4.05V at a constant current of 0.1C at 25℃, and discharge it to 2V at a constant current of 0.1C. Repeat the above process 3 times and record the initial specific capacity.

[0084] (2) -20℃ 0.2C discharge specific capacity (mAh / g) test: The battery was placed in a high and low temperature test chamber at a temperature of -20℃±0.5℃ for 2 hours. It was then charged at a constant current of 0.2C to 3.65V and discharged at a constant current to 2.5V. The discharge specific capacity was recorded. (3) Cyclic performance test: Charge to 3.65V with 1C constant current, discharge to 2.5V with 1C constant current, and record the discharge specific capacity as the first discharge specific capacity. Repeat the above steps and record the discharge specific capacity for the 100th time. Data processing: 100-cycle capacity retention (%) = (Specific capacity at the 100th discharge / Specific capacity at the 1st discharge) × 100%.

[0085] The test results are shown in Table 1 below.

[0086] Table 1 Test results for each parameter Analyze the results in Table 1: Example Analysis: Compared with Examples 1, 2, and 3, by adding two different metal elements with high redox potential, the experimental parameters such as 0.1C discharge specific capacity, 0.1C discharge average voltage, -20℃ low temperature retention rate, and 100-cycle capacity retention rate are not significantly different from those of Example 1. This indicates that under the protection of this invention, the cathode material can achieve the inventive purpose of this invention well by selecting different types and quantities of metal elements with high redox potential.

[0087] Comparative analysis: Combining the data from Comparative Example 1 and Example 1, when Comparative Example 1 was not doped with a metal element with a high redox potential, its average discharge voltage at 0.1C decreased significantly, and the low-temperature retention rate of the cathode material at -20℃ also decreased significantly.

[0088] Comparative analysis: Combining the data from Comparative Examples 2 and 3 with those from Example 1, the 0.1C discharge capacity, 0.1C discharge average voltage, -20℃ low temperature retention rate, and 100-cycle capacity retention rate (%) of Comparative Examples 2 and 3 all decreased.

[0089] In summary, ensuring that the structural formula meets the requirements of the general formula parameters protected by this invention can effectively achieve the inventive objective of this invention.

[0090] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0091] 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.

[0092] 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.

[0093] 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 material, characterized by, The composite lithium iron pyrophosphate material; a general structural formula of the composite lithium iron pyrophosphate material is Li4(Fe 1-x M x ) a (P2O7)(PO4)2, wherein, 2.5 < a < 3.2, 0.21 < X < 0.6, M is a transition metal element; the transition metal element includes at least one of group VB elements, group VIIB elements and group VIII elements.

2. The positive electrode material of claim 1, wherein, In a general structure of the positive electrode material, a ranges from 2.8 to 3; And / or, the high-voltage platform proportion Rx of the composite lithium iron pyrophosphate material satisfies the relationship: 0.18≤Rx≤0.59; wherein Rx=1.02X-0.03, 0.21≤X≤0.

6.

3. The positive electrode material of claim 1, wherein, The compacted density of the positive electrode material is 2.2 g / cm 3 3.2 g / cm 3 ; And / or, the D90 particle size of the positive electrode material ranges from 8nm to 12nm.

4. The positive electrode material of claim 1, wherein, The transition metal element includes at least one of nickel, manganese, cobalt, and vanadium.

5. The cathode material of claim 1, wherein, The positive electrode material further includes a carbon coating layer coated on the surface of the composite lithium iron pyrophosphate material. Optionally, the thickness of the carbon coating layer ranges from 1nm to 20nm.

6. A method for producing the positive electrode material according to any one of claims 1 to 5, characterized by, The method comprises the following steps: a) mixing a lithium source, an iron source, an M source, and an organic acid to obtain a mixture; b) mixing the mixture with a solvent and performing sand milling to obtain a mixed slurry, and performing spray drying on the mixed slurry to obtain a precursor powder; c) performing microwave activation and calcination treatment on the precursor powder; wherein M is a transition metal element.

7. The preparation method according to claim 6, characterized in that, In step a): The lithium source includes at least one of lithium phosphate, lithium hydroxide, and lithium carbonate; And / or, the iron source includes at least one of iron phosphate, iron nitrate, and iron oxide; And / or, the M source includes at least one of a nickel source, a manganese source, and a cobalt salt; optionally, the M source includes at least one of nickel phosphate, nickel carbonate, nickel oxide, nickel nitrate, manganese phosphate, manganese nitrate, manganese dioxide, tricobalt tetroxide, cobalt nitrate, and cobalt carbonate; And / or, the organic acid includes at least one of citric acid and oxalic acid.

8. The preparation method according to claim 6, characterized in that, In step b): The rotation speed of the sand mill ranges from 1500rpm to 2000rpm, and the sand milling time ranges from 6h to 9h; And / or, the solvent includes at least one of anhydrous ethanol, pure water, and isopropyl alcohol; And / or, in the material after the mixture is mixed with the solvent, the solid content of the mixture in the material ranges from 30% to 60%; And / or, in the spray drying, the temperature ranges from 150℃ to 220℃, and the outlet temperature ranges from 90℃ to 110℃; the spray drying process further includes the introduction of an inert gas, and the inert gas includes at least one of nitrogen, helium, neon, and argon; And / or, in step c): The microwave power of the microwave activation ranges from 800w to 1000w, and the microwave heating time ranges from 30s to 180s; And / or, the calcination treatment is performed in an inert gas, and the inert gas includes at least one of nitrogen, helium, neon, and argon; And / or, the calcination treatment includes first calcination treatment, second calcination treatment, and third calcination treatment; The first calcination treatment has conditions including a temperature rising rate ranging from 1℃ / min to 3℃ / min, a calcination temperature ranging from 140℃ to 160℃, and a holding time ranging from 0.5h to 2h; the second calcination treatment has conditions including a temperature rising rate ranging from 4℃ / min to 6℃ / min, a calcination temperature ranging from 300℃ to 350℃, and a holding time ranging from 1h to 3h; and the third calcination treatment has conditions including a temperature rising rate ranging from 4℃ / min to 6℃ / min, a calcination temperature ranging from 500℃ to 650℃, and a holding time ranging from 6h to 8h.

9. The preparation method according to claim 6, characterized in that, In step a), the mixing process further comprises adding a carbon source. Optionally, the carbon source comprises carbon nanotubes and / or conductive carbon black. Optionally, the carbon source has a mass of 0.1wt%-1.5wt% of the total mass of the mixture.

10. A battery, characterized by The positive electrode plate comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side surface of the positive electrode current collector in the thickness direction, wherein the positive electrode active material layer comprises the positive electrode material according to any one of claims 1-5 or the positive electrode material prepared by the preparation method according to any one of claims 6-9.

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