Positive electrode material, preparation method thereof and electrochemical device

Abstract

The invention provides a positive electrode material, a preparation method thereof and an electrochemical device, and particularly relates to the technical field of battery materials. The positive electrode material comprises a first component, a second component and a third component, the first component comprises lithium iron phosphate, the second component comprises lithium phosphate, the third component comprises a carbon material, the lithium phosphate is at least partially distributed on the surface of the lithium iron phosphate, and the surface of the lithium iron phosphate and the surface of the lithium phosphate are coated with the carbon material. The double coating of the lithium iron phosphate and the carbon material significantly improves the conductivity of the lithium phosphate positive electrode material, and further improves the high rate capability of the lithium iron phosphate material.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a positive electrode material, a preparation method thereof, and an electrochemical device. Background Art

[0002] Since their introduction, lithium-ion batteries have garnered significant attention due to their high energy density, stable discharge voltage, and long operating life. Their research and application are receiving increasing attention. As a crucial component of lithium-ion batteries, cathode materials play a key role in battery safety, voltage, and capacity, directly determining their performance.

[0003] At present, the most widely studied cathode materials for lithium-ion batteries include transition metal oxides, such as lithium cobalt oxide (LiCoO2) and lithium manganese oxide (LiMnO2) with hexagonal layered structures, lithium permanganate (LiMn2O4) with spinel structure, and some new cathode materials such as polyanionic lithium iron phosphate (LiFePO4) and multi-element transition metal oxides such as lithium nickel cobalt manganese oxide (LiNi 1-x-y Co x Mn y O2, NCM), lithium nickel cobalt aluminum oxide (LiNi 1-x-y Co x Al y O2, NCA) etc. Among them, polyanionic lithium iron phosphate cathode materials have advantages over layered oxide and spinel structure cathode materials, such as high safety, stable platform, high theoretical specific capacity and low cost. This makes it one of the strong competitors for power battery cathode materials. However, lithium iron phosphate has only 3.6g·cm -3 The low theoretical density, coupled with its actual tap density and compaction density, cannot meet the demand for further increases in the volumetric energy density of power batteries. Furthermore, the structural characteristics of lithium iron phosphate result in low intrinsic conductivity, making it unsuitable for use under high-current charge and discharge conditions. Summary of the Invention

[0004] In view of the above shortcomings of the prior art, the present invention provides a positive electrode material, a preparation method thereof, and an electrochemical device to improve the problem that the volume energy density of lithium iron phosphate batteries cannot meet the requirements, thereby affecting the battery capacity and use at high rates.

[0005] To achieve the above-mentioned objectives and other related objectives, the present invention provides a positive electrode material, which includes: a first component, a second component and a third component, wherein the first component includes lithium iron phosphate, the second component includes lithium phosphate, and the third component includes a carbon material, the lithium phosphate is at least partially distributed on the surface of the lithium iron phosphate, and the carbon material is coated on the surface of the lithium iron phosphate and the lithium phosphate.

[0006] In one example of the present invention, the positive electrode material is composed of a compound of large-particle positive electrode material and small-particle positive electrode material, and the small-particle positive electrode material is filled in the gaps of the large-particle positive electrode material; wherein, the particle size D1 of the large-particle positive electrode material is 3500nm to 8660nm, and the particle size D2 of the small-particle positive electrode material is 189nm to 245nm.

[0007] In one example of the present invention, the volume ratio of the large-particle positive electrode material to the small-particle positive electrode material is 1:(2 to 3).

[0008] In one example of the present invention, the median particle size D50 of the positive electrode material is 500 nm to 1600 nm.

[0009] In one example of the present invention, the powder compaction density of the positive electrode material is 2.32 g·cm -3 to 2.57 g·cm -3 .

[0010] In one example of the present invention, the carbon material includes amorphous carbon or graphite, and the mass of the carbon material accounts for 0.9wt% to 2.0wt% of the total mass of the positive electrode material; and / or the mass of the lithium phosphate accounts for 6wt% to 7.5wt% of the mass of the lithium iron phosphate.

[0011] Another aspect of the present invention provides a method for preparing a positive electrode material, the method comprising at least the following steps:

[0012] Mixing a lithium source, an iron source, a phosphorus source and a carbon source uniformly in a solvent to prepare a mixed slurry;

[0013] After drying the mixed slurry, sintering and crushing it to obtain the positive electrode material;

[0014] The molar ratio of the lithium source, the iron source and the phosphorus source is Li:Fe:P=(1+x):1:(1+x / 3), x≥0.06; the prepared positive electrode material includes: a first component, a second component and a third component, the first component includes lithium iron phosphate, the second component includes lithium phosphate, and the lithium phosphate is at least partially distributed on the surface of the lithium iron phosphate; the third component includes a carbon material, and the carbon material is coated on the surface of the lithium iron phosphate and the lithium phosphate.

[0015] In an example of the present invention, the mass of the carbon source accounts for 5 wt % to 12 wt % of the total mass of the raw material.

[0016] In an example of the present invention, the x satisfies 0.06≤x≤0.09.

[0017] In one example of the present invention, the mixed slurry is dried, sintered, and crushed to obtain the positive electrode material, including: spray-drying the mixed slurry, sintering it in an inert atmosphere at a temperature of 700°C to 850°C for 6 to 12 hours, and after sintering, grinding and crushing to obtain the positive electrode material.

[0018] In one example of the present invention, the lithium source includes one or more of lithium carbonate, lithium hydroxide, and lithium acetate.

[0019] In one example of the present invention, the iron source includes one or more of ferrous oxalate, ferrous pyrophosphate, ferric phosphate, ferric oxide, and metallic iron.

[0020] In an example of the present invention, the phosphorus source includes one or more of iron phosphate, ammonium dihydrogen phosphate, and lithium phosphate.

[0021] In one example of the present invention, the carbon source includes one or more of glucose, polyethylene glycol, sucrose, citric acid, and graphite.

[0022] In an example of the present invention, in the prepared positive electrode material, the mass of the lithium phosphate accounts for 6wt% to 7.5wt% of the mass of the lithium iron phosphate, and the mass of the carbon material accounts for 0.9wt% to 2.0wt% of the total mass of the positive electrode material.

[0023] The present invention also provides an electrochemical device, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet includes the above-mentioned positive electrode material or the positive electrode material prepared by the above-mentioned preparation method.

[0024] The positive electrode material of the present invention comprises lithium iron phosphate, lithium phosphate and carbon material. Lithium phosphate as an ion conductor adheres to the surface of lithium iron phosphate to enhance the ionic conductivity of the lithium iron phosphate material. Carbon material has good conductivity. Coating a carbon layer on the surface of lithium iron phosphate and lithium phosphate can improve the electronic conductivity of the lithium iron phosphate material. The double coating of lithium iron phosphate with lithium phosphate and carbon material can improve the conductivity of the lithium iron phosphate positive electrode material, thereby facilitating the improvement of the high-rate performance of the lithium iron phosphate material.

[0025] The present invention uses a spray-drying-assisted carbothermal reduction method to prepare lithium phosphate-modified lithium iron phosphate materials with graded particle sizes by controlling the ratio of lithium, iron, and phosphorus sources and milling time. The graded particle size allows small particles to fill the gaps between larger particles, reducing the interparticle gaps and increasing the material's compaction density. This lithium phosphate-modified, high-compactness lithium iron phosphate material not only improves the battery's volumetric energy density but also helps maintain the battery's long-cycle performance and release capacity under high-current charge and discharge conditions.

[0026] The preparation method of the present invention has a simple process, and the raw materials used are common, abundant and easily available raw materials, do not contain precious metal elements, are easy to purchase, are inexpensive, and have low production costs, which is conducive to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 The following are scanning electron microscope (SEM) images of the positive electrode material of the present invention at different ratios;

[0029] Figure 2 is a flow chart of a method for preparing a positive electrode material according to one embodiment of the present invention;

[0030] Figure 3 X-ray diffraction (XRD) patterns of the positive electrode material of the present invention in one embodiment and a comparative example;

[0031] Figure 4 FIG. 4 is a particle size distribution diagram of the positive electrode material of the present invention in one embodiment. DETAILED DESCRIPTION

[0032] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. The details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other unless they conflict.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0034] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0035] As used herein, "plurality," "multiple," "multiple times," etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more than or equal to two.

[0036] Herein, the terms "preferred," "better," and "more preferred" are merely used to describe preferred implementations or examples and should not be construed as limiting the scope of protection of the present invention. If multiple "preferred" terms appear in a technical solution, each "preferred" term is considered independent unless otherwise specified and there are no contradictions or mutual constraints.

[0037] Herein, “further”, “further”, “particularly”, etc. are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of the present invention.

[0038] When referring to a numerical range herein, unless otherwise specified, the distribution of the values ​​within the numerical range is considered continuous and includes the two numerical endpoints (i.e., the minimum and maximum values) of the numerical range, as well as every value between the two numerical endpoints. When multiple numerical ranges are provided to describe a feature or characteristic, these numerical ranges may be combined.

[0039] In this article, D0: represents the particle size corresponding to when the cumulative particle size distribution of a sample reaches 0%. Its physical meaning is that the particles with a particle size smaller than it account for 0%;

[0040] D 10 : Indicates the particle size corresponding to when the cumulative particle size distribution number of a sample reaches 10%. Its physical meaning is that the particles with a diameter smaller than this account for 10%;

[0041] D 50 : Indicates the particle size corresponding to when the cumulative particle size distribution percentage of a sample reaches 50%. Its physical meaning is that particles with a diameter larger than it account for 50%, and particles with a diameter smaller than it also account for 50%. D50 is also called the median diameter or median particle size.

[0042] D 90 : Indicates the particle size corresponding to when the cumulative particle size distribution number of a sample reaches 90%. Its physical meaning is that the particles with a diameter smaller than this account for 90%;

[0043] D 100 : Indicates the particle size corresponding to the cumulative volume fraction of a sample reaching 100%. Its physical meaning is that particles with a particle size smaller than it account for 100%.

[0044] Lithium iron phosphate (LiFePO4), as a new type of lithium-ion battery cathode material, has the following advantages over common transition metal oxide cathode materials: large theoretical specific capacity, reaching 170mAh / g, and the actual specific capacity in industrial production can reach more than 145mAh / g; stable discharge voltage platform; long cycle life, which can reach more than 1,000 times; good high-temperature performance and thermal stability: wide operating temperature range (-20℃~75℃); stable structure, O and P are firmly bonded by strong covalent bonds, making it difficult for the material to evolve oxygen and decompose, and has excellent safety performance; good compatibility with most electrolyte systems and good storage performance; does not contain precious metals and toxic elements, the raw materials are abundant and easy to obtain, the price is low, it is environmentally friendly and pollution-free, and it is a truly green battery material.

[0045] However, lithium iron phosphate has two obvious disadvantages: first, low conductivity, large polarization during high-rate charge and discharge, and low actual specific capacity; second, low tap density, with a theoretical tap density of only 3.6 g / cm 3 The actual tap density is only 1.0g / cm 3 About, less than half of the tap density of LiCoO2. Under low compaction density, the contact area between particles is small, the internal resistance of the battery is large, and the battery polarization increases, which affects the capacity of the battery and its use under high rate conditions.

[0046] Based on this, the present invention provides a positive electrode material, a method for preparing the positive electrode material, and an electrochemical device containing the positive electrode material. The ionic conductivity of the lithium iron phosphate material is enhanced by modifying lithium phosphate on the surface of the lithium iron phosphate, and the electronic conductivity of the lithium iron phosphate material is improved by carbon coating. This composite coating significantly improves the conductivity of the lithium iron phosphate material and improves the high-rate performance of the lithium iron phosphate material.

[0047] See also Figure 1 In a first aspect, the present invention provides a positive electrode material, which includes a first component, a second component and a third component. The first component includes lithium iron phosphate (LiFePO4), the second component includes lithium phosphate (Li3PO4), and the third component includes a carbon (C) material. The lithium phosphate is distributed on at least a portion of the surface of the lithium iron phosphate, and the carbon material is coated on the surface of the lithium iron phosphate and the lithium phosphate. Among them, the carbon material can be graphite or amorphous carbon, and the amorphous carbon can be soft carbon or hard carbon, and can further be hard carbon. The lithium phosphate distributed on the surface of the lithium iron phosphate can enhance the ionic conductivity of the lithium iron phosphate material, and some lithium phosphate will enter the interior of the lithium iron phosphate to make up for the lithium loss in the lithium iron phosphate. The coating of the carbon material can improve the electronic conductivity of the lithium iron phosphate material. Compared with a single carbon coating layer, the composite coating of lithium phosphate and carbon significantly improves the conductivity of the lithium iron phosphate positive electrode material, thereby improving the high-rate performance of the lithium iron phosphate battery.

[0048] The inventors discovered in their research that the amount of lithium phosphate and carbon material coated affects the overall performance of the lithium iron phosphate composite material. If the coating amount is too little, the improvement effect is not obvious, and if the coating amount is too much, the performance of the lithium iron phosphate will be affected. Therefore, in this application, the mass of lithium phosphate distributed on the surface of the lithium iron phosphate accounts for 6wt% to 7.5wt% of the mass of the lithium iron phosphate, such as 6wt%, 7wt% or 7.5wt%, and the mass of the carbon material accounts for 0.9wt% to 2.0wt% of the total mass of the positive electrode material, such as 0.9wt%, 1.5wt% or 2.0wt%, and so on.

[0049] In one embodiment, the positive electrode material is a lithium iron phosphate composite positive electrode material with graded particle sizes formed by compounding large and small particles, wherein the small-particle positive electrode material is filled in the pores between the large-particle positive electrode material. The appropriate graded particle size can use small particles to fill the gaps between large particles to reduce the voids between particles and increase the material compaction density. In this embodiment, the particle size D1 of the large-particle positive electrode material is 3500nm to 8660nm, optionally, D1 is 4000nm, 6000nm or 8000nm, etc.; the particle size D2 of the small-particle positive electrode material is 189nm to 245nm, optionally, D2 is 190nm, 220nm or 240nm, etc.; the presence of large particles can improve the compaction density of the positive electrode material, and the addition of small particles makes full use of the voids between large particles, so that the compaction density of the positive electrode material is further improved. Furthermore, in the positive electrode material, the volume ratio of the large-particle positive electrode material to the small-particle positive electrode material is 1:(2 to 3). Preferably, the volume ratio of the large-particle positive electrode material to the small-particle positive electrode material is 1:2.43.

[0050] In one embodiment, the median particle size D50 of the positive electrode material is 500 nm to 1600 nm, optionally, D50 is 500 nm, 700 nm, 1000 nm, or 1600 nm, etc. D10 is 300 nm to 460 nm, optionally, D10 is 320 nm, 380 nm, or 460 nm, etc.; D90 is 1900 nm to 3900 nm, optionally, D90 is 2000 nm, 2500 nm, 3000 nm, or 3800 nm, etc. Further, the powder compaction density of the positive electrode material is 2.32 g·cm -3 to 2.57 g·cm -3 , close to the theoretical value (3.6g / cm 3 ), compared to the compaction density of conventional lithium iron phosphate 1.0g / cm 3 There has been significant improvement.

[0051] See also Figure 2 The second aspect of the present invention provides a method for preparing a positive electrode material, the preparation method comprising at least the following steps:

[0052] S1. Mixing a lithium source, an iron source, a phosphorus source, and a carbon source in a solvent to obtain a mixed slurry;

[0053] S2. After the mixed slurry is dried, sintered and crushed to obtain a positive electrode material.

[0054] The lithium source in step S1 includes, but is not limited to, lithium carbonate, lithium hydroxide, and lithium acetate. That is, the lithium source can be selected from any one of the above-listed substances, such as lithium carbonate, or any two or three of the above-listed substances, such as a combination of lithium carbonate and lithium acetate, or a combination of lithium carbonate, lithium hydroxide, and lithium acetate. Of course, the lithium source can also be selected from lithium salts that are not listed above and that can provide lithium ions.

[0055] The iron source includes one or more of ferrous oxalate, ferrous pyrophosphate, ferric phosphate, ferric oxide, and metallic iron. Optionally, the iron source is ferrous oxalate, or a combination of ferrous oxalate and ferric phosphate, or a combination of metallic iron, ferric oxide, and ferric phosphate, etc. The iron source includes but is not limited to the types of materials listed above.

[0056] The phosphorus source includes one or more of ferric phosphate, ammonium dihydrogen phosphate, and lithium phosphate. For example, the phosphorus source is ferric phosphate, or a combination of ammonium dihydrogen phosphate and lithium phosphate, or a combination of ferric phosphate, ammonium dihydrogen phosphate, and lithium phosphate. Phosphorus sources include, but are not limited to, the types of substances listed above.

[0057] The carbon source includes one or more of glucose, polyethylene glycol (PEG), sucrose, citric acid, and graphite. Optionally, the carbon source is glucose, or sucrose, or a composition of citric acid and polyethylene glycol, or a composition of graphite, sucrose, and polyethylene glycol, or the like. The carbon source can also be selected from other substances that can provide a carbon source in the art, which are not listed here one by one. It should be noted that when the lithium source, iron source, phosphorus source, and carbon source are two or more compositions, there is no restriction on the ratio in the composition, and they can be mixed in any proportion.

[0058] The ratio of the lithium source, iron source, and phosphorus source in step S1 is set according to the molar ratio of lithium, iron, and phosphorus in the desired positive electrode material. In one embodiment, the lithium source, iron source, and phosphorus source are weighed in a molar ratio of Li:Fe:P=(1+x):1:(1+x / 3). Optionally, the carbon source is weighed in a ratio of 5wt% to 12wt% of the total mass of the raw materials (lithium source, iron source, phosphorus source, and carbon source). Optionally, the mass of the carbon source accounts for 5wt%, 8wt%, or 12wt% of the total mass of the raw materials, etc.

[0059] The inventors of this application discovered in their research that when x < 0.06, no significant lithium phosphate is formed in the prepared material. Analysis suggests that this may be because the excess lithium only compensates for losses during the calcination process and forms a non-stoichiometric lithium-rich lithium iron phosphate material, which is insufficient to form a lithium phosphate material. Therefore, x ≥ 0.06; alternatively, 0.06 ≤ x ≤ 0.09. When x > 0.09, the lithium iron phosphate surface is modified with excessive lithium phosphate. Since lithium phosphate itself has no electrochemical activity, the discharge capacity of the lithium iron phosphate-based battery decreases. Therefore, it is further preferred that 0.06 ≤ x ≤ 0.09. Within this range, an appropriate amount of lithium phosphate in the prepared positive electrode material is modified on the lithium iron phosphate surface, improving the conductivity of the lithium iron phosphate.

[0060] The mixed materials in step S1 can be mixed using conventional methods in the art. As an example, weighed lithium source, iron source, phosphorus source, and carbon source are placed in a mixing device such as a sand mill, and a solvent that does not chemically react with the above raw materials, such as deionized water, is added and mixed until uniformly mixed. It should be noted that the above solvent only serves as a medium, and there is no limit on the amount of addition thereof, as long as it can evenly disperse the raw materials.

[0061] In step S2, the mixed slurry is preferably dried using a spray dryer. After spray drying, loose fine powder or particles can be obtained, which is convenient for subsequent processing. After the mixed slurry is dried, the dried product is placed in a tube furnace, heated to 700-850°C under an inert atmosphere, and sintered at this temperature for 6-12 hours. In some embodiments, the inert atmosphere can be selected from nitrogen, argon or helium, preferably nitrogen, which is economical and environmentally friendly; the sintering temperature can be 700°C, 750°C, 800°C or 850°C, etc., and the sintering time is, for example, 6h, 8h, 10h or 12h, etc.

[0062] After sintering, the temperature is lowered to room temperature, the sintered product is taken out, and the target material is obtained by grinding and crushing. The grinding and crushing can be carried out in a conventional manner in the art, for example, grinding in a grinder for 3 to 9 hours, optionally 3 hours, 6 hours, or 9 hours. The specific grinding time can be selected according to the actual product requirements.

[0063] The product obtained after grinding and crushing is a LiFePO4 / (Li3PO4+C) composite positive electrode material composed of large and small particles, wherein the particle size D1 of the large particles is 3500nm~8660nm, and the particle size D2 of the small particles is 189nm~245nm; the volume ratio of the large-particle positive electrode material to the small-particle positive electrode material is 1:(2~3). The median particle size D50 of the positive electrode material is 500nm~1600nm. It should be noted that for the positive electrode material with the same raw material addition ratio, the median particle size D50 shows a downward trend with the increase of grinding time; under the same grinding time, the median particle size D50 of LiFePO4 / (Li3PO4+C) material shows a trend of first increasing and then decreasing with the increase of lithium source addition ratio. The reason for this may be that with the increase of lithium source addition ratio, non-stoichiometric lithium-rich lithium iron phosphate is formed, and the reaction activity of the material increases with the increase of lithium richness, which leads to the increase of particle size; when the lithium source is further increased, lithium iron phosphate with smaller particle size is formed, which reduces the median particle size of the material.

[0064] The present invention prepares lithium phosphate island-modified graded particle size lithium iron phosphate material by controlling the raw material addition ratio and grinding time. This unique particle size distribution improves the utilization rate of the gaps between particles, increases the compaction density of the lithium iron phosphate positive electrode material, and improves the volume energy density of the battery; in addition, the excellent ionic conductivity of lithium phosphate also improves the conductivity of the lithium iron phosphate material, improving the high-current charge and discharge performance of the battery.

[0065] A third aspect of the present invention provides an electrochemical device, which includes a positive electrode plate, and the positive electrode plate includes the above-mentioned positive electrode material or the positive electrode material prepared by the above-mentioned preparation method.

[0066] In one embodiment, the electrochemical device is a lithium-ion secondary battery, which includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The specific structure of the secondary battery is described in detail below:

[0067] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector. The positive electrode current collector is, for example, a foil formed by surface treatment of nickel, titanium, aluminum, silver, stainless steel or carbon. In addition to foil, the positive electrode current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam, etc. The thickness of the positive electrode current collector is, for example, 8μm to 15μm. In this embodiment, the positive electrode current collector is, for example, aluminum foil, and the thickness of the aluminum foil is, for example, 13μm. The positive electrode current collector has two oppositely disposed surfaces along its own thickness direction, and the positive electrode active material layer can be disposed on the surface of either side or on both surfaces. The positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent and a positive electrode binder, wherein the positive electrode active material is the positive electrode material described above in the present invention; the conductive agent and the binder are not specifically limited, and those skilled in the art can select according to actual needs.

[0068] As an example, the positive electrode binder is selected from any one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinylether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene or styrene butadiene rubber (SBR). The positive electrode conductive agent is selected from one or at least two of conductive carbon black (Super P), acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. The ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode active material layer can be set according to the conventional settings in the art.

[0069] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector is, for example, copper foil. The negative electrode current collector has two surfaces that face each other in the thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a thickener. The negative electrode active material, negative electrode conductive agent, negative electrode binder, and thickener can be selected from conventional materials in the art and are not specifically limited here.

[0070] As an example, the negative electrode material is selected from carbon and / or silicon negative electrode materials, wherein the carbon negative electrode material can be natural graphite, artificial graphite, soft carbon, hard carbon, etc.; the silicon negative electrode material can be elemental silicon, silicon oxide compounds, silicon carbon compounds, etc. The negative electrode conductive agent is selected from one or a combination of two or more of conductive carbon black, nano silver powder, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. The negative electrode binder is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), or a combination of multiple materials mixed in any proportion; the thickener can be sodium carboxymethyl cellulose (CMC-Na).

[0071] The separator is placed between the positive and negative electrodes to provide isolation. Examples of the separator include polyethylene (PE), polypropylene (PP), fiberglass, polyethylene, or composite films. The separator has a thickness of 9 to 18 μm, an air permeability of 180 to 380 s / 100 mL, and a porosity of 30 to 50%.

[0072] The electrolyte soaks into the positive and negative electrodes. During the battery's charge and discharge process, lithium ions are inserted and removed back and forth between the positive and negative electrodes, and the electrolyte acts as a conductor of lithium ions. The electrolyte can be any combination of conventional types in the art.

[0073] In one embodiment, the electrolyte includes an organic solvent and a lithium salt, wherein the organic solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC). The lithium salt may be selected from one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(trifluoromethanesulfonyl imide), LiTFSI (lithium bis(trifluoromethanesulfonyl imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalatoborate), LiBOB (lithium dioxalatoborate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorooxalatophosphate), and LiTFOP (lithium tetrafluorooxalatophosphate), or a combination thereof in any proportion. The molar concentration of the lithium salt in the electrolyte is 0.5 mol / L to 2 mol / L.

[0074] In other embodiments, some film-forming additives and functional additives that can improve battery performance may be added to the electrolyte, such as vinylene carbonate (VC), 1,3-propane sultone (PS), diethylene sulfate (DTD), etc. Those skilled in the art may select them according to actual needs.

[0075] The preparation process of lithium-ion secondary batteries is described below:

[0076] (1) Preparation of positive electrode sheet

[0077] The above-mentioned positive electrode materials, positive electrode conductive agents, and positive electrode binders are mixed in a certain mass ratio, and then the solvent N-methylpyrrolidone (NMP) is added in batches under high-speed stirring and stirred evenly to prepare a positive electrode slurry with a certain viscosity; the positive electrode slurry is then evenly coated on the positive electrode current collector aluminum foil, and after drying, rolling, and slitting, the positive electrode sheets are obtained.

[0078] (2) Preparation of negative electrode sheet

[0079] The above-mentioned negative electrode active material, negative electrode conductive agent, negative electrode binder and thickener are mixed in proportion, deionized water is added and stirred evenly under the stirring action of a vacuum mixer to obtain a negative electrode slurry; the negative electrode slurry is then evenly coated on the negative electrode current collector copper foil, dried at room temperature, and then transferred to an oven for drying, and cold pressed and cut to obtain a negative electrode sheet.

[0080] (3) Preparation of electrolyte

[0081] In an argon atmosphere glove box with a water content of <10 ppm, fully dried lithium salt (LiPF6) was dissolved in an organic solvent and mixed evenly to obtain an electrolyte, wherein the concentration of LiPF6 was 1 mol / L.

[0082] (4) Preparation of diaphragm

[0083] A 12 μm thick polypropylene (PP) or polyethylene (PE) porous polymer film is selected.

[0084] (5) Battery assembly:

[0085] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked or wound in sequence to form a bare battery cell; then they are wrapped with aluminum-plastic film, transferred to a vacuum drying oven and dried at 120°C, sealed after injecting electrolyte, and the electrolyte is formed to finally prepare a soft-pack battery (i.e., a lithium-ion secondary battery).

[0086] Those skilled in the art will understand that the above-described method for preparing a lithium-ion secondary battery is merely an example, and other commonly used methods in the art may be employed without departing from the disclosure of this application.

[0087] The present invention also provides an electronic device comprising at least one of the above-mentioned lithium-ion secondary batteries. The lithium-ion secondary battery can be used in the form of a single battery, a battery module or a battery pack to power the electronic device.

[0088] Electronic devices can be vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys and electric tools, etc. Vehicles are, for example, new energy vehicles, which can be pure electric vehicles, hybrid electric vehicles or extended-range vehicles, etc. Spacecraft include airplanes, rockets, space shuttles and spacecraft, etc. Electric toys include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys and electric airplane toys, etc. Electric tools include metal cutting electric tools, grinding electric tools, assembly electric tools and railway electric tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators and electric planers, etc. The electronic device includes the above-mentioned lithium ion secondary battery, and therefore includes the advantages of the above-mentioned lithium ion secondary battery, which will not be elaborated here.

[0089] The technical solutions of the present invention are described in detail below through several specific examples and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0090] Example 1

[0091] This embodiment provides a positive electrode material, which is LiFePO4 / (Li3PO4+C). The preparation method thereof is as follows:

[0092] (1) Lithium carbonate, iron phosphate and ammonium dihydrogen phosphate are weighed in a molar ratio of Li:Fe:P=1.06:1:1.02, and glucose as a carbon source is weighed in an amount of 10% of the total mass of the raw materials; the weighed raw materials are placed in a sand mill, and deionized water is used as a medium to mix the raw materials uniformly to prepare a mixed slurry.

[0093] (2) The mixed slurry is slowly injected into a spray dryer for drying, and then the dried product is placed in a tubular furnace, heated to 800°C under a nitrogen atmosphere, and sintered at this temperature for 10 hours; the sintered product is then ground in a grinder for 3 hours to obtain the target material.

[0094] The content of each component in the cathode material prepared in this example was determined using ICP (inductively coupled plasma) and X-ray diffraction (XRD) refinement. The residual carbon content of the finished product was determined using a carbon-sulfur analyzer. The test results showed that the molar ratio of the main elements in LiFePO4 / (Li3PO4+C) was Li:Fe:P = 1.060:0.993:1.051; the component mass ratio was LiFePO4:Li3PO4 = 94.72:6.28; and the carbon content was 1.50 wt%.

[0095] Example 2

[0096] The difference between this embodiment and embodiment 1 is that the raw materials are ground in the grinder for 6 hours, and the other steps remain unchanged.

[0097] The contents of the components in the positive electrode material prepared in this embodiment were tested by ICP, XRD refinement and carbon-sulfur analyzer, and it was found that the amount ratio of the main elements in LiFePO4 / (Li3PO4+C) was Li:Fe:P=1.058:1.001:1.060; the component mass ratio was LiFePO4:Li3PO4=94.76:6.24; and the carbon content was 1.53wt%.

[0098] It should be noted that due to raw material loss or chemical changes during the preparation process, the ratio of the amount of substances between the elements in the actual product deviates from the ratio in the raw material, the same below.

[0099] Example 3

[0100] The difference between this embodiment and embodiment 1 is that the raw material is ground in the grinder for 9 hours, and the other steps remain unchanged.

[0101] The contents of the components in the positive electrode material prepared in this embodiment were tested by ICP, XRD refinement and carbon-sulfur analyzer, and it was found that the amount ratio of the main elements in LiFePO4 / (Li3PO4+C) was Li:Fe:P=1.055:0.993:1.058; the component mass ratio was LiFePO4:Li3PO4=94.73:6.27; and the carbon content was 1.54wt%.

[0102] Example 4

[0103] The difference between this embodiment and embodiment 1 is that lithium carbonate, iron phosphate and ammonium dihydrogen phosphate are weighed according to a molar ratio of Li:Fe:P=1.09:1:1.03, and the other steps remain unchanged.

[0104] The contents of the components in the positive electrode material prepared in this embodiment were tested by ICP, XRD refinement and carbon-sulfur analyzer, and it was found that the amount ratio of the main elements in LiFePO4 / (Li3PO4+C) was Li:Fe:P=1.086:1.001:1.082; the component mass ratio was LiFePO4:Li3PO4=93.31:6.69; and the carbon content was 1.47wt%.

[0105] Example 5

[0106] The difference between this embodiment and embodiment 4 is that the sintered product is ground in a grinder for 6 hours, and the other steps remain unchanged.

[0107] The contents of the components in the positive electrode material prepared in this embodiment were tested by ICP, XRD refinement and carbon-sulfur analyzer, and it was found that the amount ratio of the main elements in LiFePO4 / (Li3PO4+C) was Li:Fe:P=1.092:0.982:1.086; the component mass ratio was LiFePO4:Li3PO4=93.26:6.74; and the carbon content was 1.44wt%.

[0108] Example 6

[0109] The difference between this embodiment and embodiment 4 is that the sintered product is ground in a grinder for 9 hours, and the other steps remain unchanged.

[0110] The contents of the components in the positive electrode material prepared in this embodiment were tested by ICP, XRD refinement and carbon-sulfur analyzer, and it was found that in LiFePO4 / (Li3PO4+C), the ratio of the amount of main elements Li:Fe:P=1.087:1.002:1.093; the mass ratio of the components was LiFePO4:Li3PO4=93.28:6.72; and the carbon content was 1.45wt%.

[0111] Example 7

[0112] The difference between this embodiment and embodiment 1 is that lithium carbonate, iron phosphate and ammonium dihydrogen phosphate are weighed in a molar ratio of Li:Fe:P=1.12:1:1.03, the sintered product is ground in a grinder for 6 hours, and the other steps remain unchanged.

[0113] The contents of the components in the positive electrode material prepared in this embodiment were tested by ICP, XRD refinement and carbon-sulfur analyzer, and it was found that the amount ratio of the main elements in LiFePO4 / (Li3PO4+C) was Li:Fe:P=1.118:0.998:1.124; the component mass ratio was LiFePO4:Li3PO4=92.96:7.04; and the carbon content was 1.38wt%.

[0114] Comparative Example 1

[0115] The difference between this comparative example and Example 1 is that in this comparative example, lithium carbonate, iron phosphate and ammonium dihydrogen phosphate are weighed according to a molar ratio of Li:Fe:P=1:1:1, and the other steps remain unchanged.

[0116] Comparative Example 2

[0117] The difference between this comparative example and comparative example 1 is that the sintered product is ground in a grinder for 6 hours, and the other steps remain unchanged.

[0118] Comparative Example 3

[0119] The difference between this comparative example and comparative example 1 is that the sintered product is ground in a grinder for 9 hours, and the other steps remain unchanged.

[0120] Comparative Example 4

[0121] The difference between this comparative example and Example 1 is that lithium carbonate, iron phosphate and ammonium dihydrogen phosphate are weighed in a molar ratio of Li:Fe:P=1.03:1:1.01, and the other steps remain unchanged.

[0122] Comparative Example 5

[0123] The difference between this comparative example and comparative example 4 is that the sintered product is ground in a grinder for 6 hours, and the other steps remain unchanged.

[0124] Comparative Example 6

[0125] The difference between this comparative example and comparative example 4 is that the sintered product is ground in a grinder for 9 hours, and the other steps remain unchanged.

[0126] Scanning electron microscopy (SEM) was used to observe the micromorphology of the cathode materials prepared by grinding for 6 h under different raw material ratios. The test results are shown in Figure 1 , Figure 1 The morphologies of the cathode materials prepared with x = 0 (Comparative Example 2), x = 0.3 (Comparative Example 5), x = 0.6 (Example 2), and x = 0.9 (Example 5) are shown. The figures show that the cathode materials are composed of large and small particles, with the small particles filling the surface of the large particles. When x = 0.6 and x = 0.9, lithium phosphate is dotted on the surface of the lithium iron phosphate particles, and a thin film of carbon uniformly coats the surface of the lithium phosphate-modified lithium iron phosphate particles. When x = 0 and x = 0.3, the particle surface is smooth, with no obvious dotted lithium phosphate distribution.

[0127] X-ray diffraction (XRD) analysis of cathode materials prepared with different raw material ratios and milled for 6 hours revealed that when x < 0.06, the cathode materials lacked significant lithium phosphate diffraction peaks. This is because the excess lithium only compensated for losses during calcination, forming a non-stoichiometric lithium-rich lithium iron phosphate material, insufficient to form a lithium phosphate material. When x ≥ 0.06, a significant Li3PO4 diffraction peak appeared in the XRD pattern of the prepared material, indicating the formation of a Li3PO4-modified LiFePO4 / (Li3PO4+C) material. Figure 3 The XRD patterns of the positive electrode materials prepared when x = 0 (Comparative Example 2) and x = 0.06 (Example 2) are shown. It can be clearly seen from the figure that when x = 0, there is no characteristic diffraction peak of lithium phosphate, but when x = 0.6, characteristic diffraction peaks of lithium phosphate begin to appear. Furthermore, it can be seen from the figure that the XRD pattern of the positive electrode material prepared using the preparation method of the present invention has no impurity peaks, indicating that the material is of high purity, free of impurities, and has good crystallinity.

[0128] The particle size of the positive electrode materials prepared in Examples 1 to 7 and Comparative Examples 1 to 6 was tested using a particle size laser particle size analyzer. The test results are shown in Tables 1 and Figure 4 , Figure 4 The particle size distribution diagram of the positive electrode material prepared in Example 5 is shown. It can be seen from the figure that the positive electrode material prepared in this embodiment is composed of a compound of large particles and small particles, wherein the particle size of the small-particle positive electrode material is approximately normally distributed, and the particle size of the large-particle positive electrode material is also similarly normally distributed. The particle size of the small-particle positive electrode material is 189nm~245nm, and the particle size of the large-particle positive electrode material is 3500nm~8660nm; the volume ratio of the large-particle positive electrode material to the small-particle positive electrode material is 1:2.43.

[0129] The positive electrode materials prepared in Examples 1 to 7 and Comparative Examples 1 to 6 are assembled into a battery. The assembly process is as follows:

[0130] (1) Preparation of positive electrode sheet: The prepared positive electrode material, conductive agent conductive carbon black (SP), and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90:5:5, and then solvent N-methylpyrrolidone (NMP) was added in batches under high-speed stirring and stirred evenly to prepare positive electrode slurry; the positive electrode slurry was then evenly coated on the positive electrode current collector aluminum foil, dried at 120°C for 240 minutes, and rolled and cut to obtain positive electrode sheets;

[0131] (2) Preparation of negative electrode sheet: metal lithium sheet is used as negative electrode;

[0132] (3) Diaphragm: 12 μm thick polypropylene (PP) porous polymer film;

[0133] (4) Preparation of electrolyte: In an argon atmosphere glove box with a water content of <10 ppm, EC, DEC, and PC were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried lithium salt (LiPF6) was dissolved in the organic solvent and mixed to obtain an electrolyte, wherein the concentration of LiPF6 was 1 mol / L.

[0134] (5) Battery assembly: The above-mentioned positive electrode sheet, separator, metal lithium sheet and electrolyte were assembled into C2430 button cells in a glove box under a pure argon atmosphere.

[0135] The button cells assembled in Examples 1 to 7 and Comparative Examples 1 to 6 were subjected to electrical performance tests using a Shenghong Electric Co., Ltd. battery performance test system (equipment model: BTS05 / 10C8D-HP). The test results are shown in Tables 1 and 2. The test method is as follows:

[0136] (1) Secondary battery rate test: In a 25°C oven, activate at 0.1C for 3 cycles and then continue with 1C, 2C, 3C, 5C, 7C, and 10C rate cycles.

[0137] (2) Secondary battery cycle test: In an oven at 25°C, cycle charge and discharge are performed in the range of 2.5V to 3.75V at a current of 1C, and the discharge capacity of each cycle is recorded. The test is stopped when the discharge capacity is less than or equal to 80% of the discharge capacity of the first cycle, and the number of cycles is recorded. Here, less than 80% of the discharge capacity of the first cycle refers to the first value less than 80% of the discharge capacity of the first cycle that appears after the last cycle of cycle charge and discharge.

[0138] (3) Powder compaction density test: The positive electrode materials prepared in each embodiment and comparative example were tested at room temperature and a pressure of 160 MPa.

[0139] (4) First cycle Coulomb efficiency test:

[0140] First-cycle coulombic efficiency: the ratio of the discharge capacity in grams to the charge capacity in grams during the first constant current and constant voltage charge and discharge at 0.1C.

[0141] (5) Volume energy density test:

[0142] The mass specific energy when the volume energy density is 0.1C is multiplied by the electrode compaction density, where the electrode compaction density is the compaction density of the positive electrode material coated on the current collector to form the positive electrode sheet, and the electrode compaction density = surface density / (thickness of the electrode after rolling - thickness of the current collector); the mass specific energy is directly output in the power-on test software.

[0143] Table 1: Particle size distribution and compaction density of positive electrode materials of Examples 1 to 7 and Comparative Examples 1 to 6

[0144]

[0145] Table 2: Electrical properties of batteries assembled from Examples 1 to 7 and Comparative Examples 1 to 6

[0146]

[0147] Combine Figure 1 、 Figure 3 From Tables 1 and 2, it can be concluded that when the grinding time is constant, within the range of 0.06≤x≤0.09, the particle size of Li3PO4 is concentrated in the range of 10-60nm, and the particle size of LiFePO4 is concentrated in the range of 100nm-1.3μm (the average value of at least 200 particles randomly selected and photographed by SEM is calculated using Nano Measurer particle size measurement software). The nano-sized Li3PO4 particles are distributed in an island-like manner on the surface of the LiFePO4 particles, and the ionic conductivity of the positive electrode material is more significantly improved compared to the simple mixture of Li3PO4 and LiFePO4. XRD characterization shows that when x<0.06, the prepared material has no obvious characteristic diffraction peaks of lithium phosphate. This is because the excess lithium only compensates for the loss during the roasting process and forms a non-stoichiometric lithium-rich lithium iron phosphate material, but is insufficient to form a lithium phosphate material. When x≥0.06, the XRD spectrum of the prepared material shows an obvious Li3PO4 diffraction peak, indicating the formation of Li3PO4-modified LiFePO4 / (Li3PO4+C) material. An appropriate amount of Li3PO4 modification can improve the conductivity of the LiFePO4 material, thereby improving the rate performance, cycle performance and volume energy density of lithium iron phosphate. When x>0.09 (x=0.12), the Li3PO4 particles formed on the surface of LiFePO4 further increase, but because Li3PO4 has no electrochemical activity, excessive Li3PO4 will cause the discharge capacity, cycle life and volume energy density of the lithium iron phosphate battery to decrease.

[0148] From the particle size test results of the materials obtained with different grinding times and raw material addition ratios, it can be seen that the median particle size D 50Within the range of 0.50 to 1.6 μm. When the grinding time was 3 to 9 hours, for the LiFePO4 / (Li3PO4+C) material with the same raw material addition ratio, the median particle size of the material showed a decreasing trend with increasing grinding time. At the same grinding time, the median particle size of the LiFePO4 / (Li3PO4+C) material showed a trend of first increasing and then decreasing with increasing lithium source addition ratio. This is because when x ≤ 0.06, non-stoichiometric lithium-rich lithium iron phosphate is formed, and the material's reactivity increases with increasing lithium richness, resulting in an increase in particle size. When 0.06 ≤ x ≤ 0.09, smaller Li3PO4 ion conductor materials appeared, reducing the material's median particle size. When x > 0.09, the smaller Li3PO4 particles in the material increased further, further reducing the material's particle size.

[0149] The cathode material of the present invention modifies the surface of lithium iron phosphate with an appropriate amount of lithium phosphate to enhance the ionic conductivity of the lithium iron phosphate material. Coating a carbon layer on the surface of lithium iron phosphate and lithium phosphate can improve the electronic conductivity of the lithium iron phosphate material, thereby improving the conductivity of the lithium iron phosphate cathode material, which is beneficial to improving the high-rate performance of the lithium iron phosphate material. In addition, by controlling the raw material addition ratio and grinding time, a carbon thermal reduction method assisted by spray drying is used to obtain a lithium iron phosphate-based cathode material with a graded particle size. This unique particle size distribution improves the utilization rate of the gaps between particles, improves the compaction density of the lithium iron phosphate-based cathode material, and improves the volume energy density of the battery. The excellent ionic conductivity of lithium phosphate also improves the electrical conductivity of the lithium iron phosphate material, improves the long cycle performance of the lithium iron phosphate battery and the release of capacity under high current charge and discharge conditions. Therefore, the present invention effectively overcomes some practical problems in the prior art and has high utilization value and use significance.

[0150] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A positive electrode material, characterized in that include: A first component includes lithium iron phosphate; A second component includes lithium phosphate, wherein the lithium phosphate is at least partially distributed on the surface of the lithium iron phosphate; and The third component includes a carbon material, and the carbon material is coated on the surface of the lithium iron phosphate and the lithium phosphate.

2. The positive electrode material according to claim 1, characterized in that The positive electrode material is composed of a large-particle positive electrode material and a small-particle positive electrode material; wherein the particle size D1 of the large-particle positive electrode material is 3500nm to 8660nm, and the particle size D2 of the small-particle positive electrode material is 189nm to 245nm; and / or the volume ratio of the large-particle positive electrode material to the small-particle positive electrode material is V1:V2=1:(2 to 3).

3. The positive electrode material according to claim 2, characterized in that The median particle size D50 of the positive electrode material is 500 nm to 1600 nm; and / or the powder compaction density of the positive electrode material is 2.32 g·cm -3 to 2.57 g·cm -3 .

4. The positive electrode material according to claim 1, characterized in that The carbon material includes amorphous carbon or graphite, and the mass of the carbon material accounts for 0.9wt% to 2.0wt% of the total mass of the positive electrode material; And / or, the mass of the lithium phosphate accounts for 6 wt % to 7.5 wt % of the mass of the lithium iron phosphate.

5. A method for preparing a positive electrode material, characterized in that: At least the following steps are included: Mixing a lithium source, an iron source, a phosphorus source and a carbon source uniformly in a solvent to prepare a mixed slurry; After drying the mixed slurry, sintering and crushing it to obtain the positive electrode material; Wherein, the molar ratio of the lithium source, the iron source and the phosphorus source is Li:Fe:P=(1+x):1:(1+x / 3), x≥0.06; The prepared positive electrode material comprises: A first component includes lithium iron phosphate; A second component includes lithium phosphate, wherein the lithium phosphate is at least partially distributed on the surface of the lithium iron phosphate; and The third component includes a carbon material, and the carbon material is coated on the surface of the lithium iron phosphate and the lithium phosphate.

6. The preparation method according to claim 5, characterized in that The x satisfies 0.06≤x≤0.09; And / or, the mass of the carbon source accounts for 5wt% to 12wt% of the total mass of the raw material.

7. The preparation method according to claim 5, characterized in that The mixed slurry is dried, sintered, and crushed to obtain the positive electrode material, comprising: The mixed slurry is spray-dried and then sintered at 700° C. to 850° C. for 6 to 12 hours under an inert atmosphere. After sintering, the mixed slurry is ground and crushed to obtain a positive electrode material.

8. The preparation method according to claim 5, characterized in that Include one or more of the following: The lithium source includes one or more of lithium carbonate, lithium hydroxide, and lithium acetate; The iron source includes one or more of ferrous oxalate, ferrous pyrophosphate, ferric phosphate, ferric oxide, and metallic iron; The phosphorus source includes one or more of iron phosphate, ammonium dihydrogen phosphate, and lithium phosphate; The carbon source includes one or more of glucose, polyethylene glycol, sucrose, citric acid, and graphite.

9. The preparation method according to claim 5, characterized in that In the prepared positive electrode material, the mass of the lithium phosphate accounts for 6wt% to 7.5wt% of the mass of the lithium iron phosphate, and the mass of the carbon material accounts for 0.9wt% to 2.0wt% of the total mass of the positive electrode material.

10. An electrochemical device, characterized in that The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises the positive electrode material according to any one of claims 1 to 4, or the positive electrode material prepared by the preparation method according to any one of claims 5 to 9.

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