A positive electrode material, a preparation method and application thereof

Abstract

The application provides a positive electrode material, a preparation method and application thereof, and has the chemical formula of Li a Fe b (PO4) c @C; wherein 0.95<=a<=1.10, 0.95<=b<=1.05, and 0.95<=c<=1.05; and the primary particles of the positive electrode material are twinned crystals composed of multiple single crystals. The application provides a positive electrode material, a preparation method and application thereof, and can balance the processing performance and electrochemical performance of the positive electrode material, and improve the rate performance and cycle performance of the positive electrode material.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4, LFP) boasts advantages such as abundant raw materials, low cost, and long cycle life, making it one of the next-generation industrialized cathode materials for lithium-ion batteries. However, LFP materials exhibit low lithium-ion solid-phase transport rates, resulting in poor rate performance. This poses a significant challenge to the application of LFP materials in fields requiring high power performance.

[0003] By nano-sizing lithium iron phosphate (LFP), the particle size of primary LFP particles is reduced, shortening the lithium-ion transport distance in the LFP material and improving its rate performance. However, nano-sizing inevitably leads to a significant increase in the specific surface area of ​​the material, greatly increasing the difficulty of electrode fabrication. At the same time, the high specific surface area also increases gas generation during high-temperature storage of the battery cell, negatively impacting cell safety. Summary of the Invention

[0004] This invention proposes a cathode material, its preparation method, and its application, which can balance the processing performance and electrical performance of the cathode material, and improve the rate performance and cycle performance of the cathode material.

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution.

[0006] This invention proposes a lithium battery cathode material, the chemical formula of which is Li. a Fe b (PO4) c @C; where 0.95≤a≤1.10, 0.95≤b≤1.05, 0.95≤c≤1.05; the primary particles of the cathode material are twins composed of multiple single crystals.

[0007] In one embodiment of the present invention, the average particle size of the single crystal is 30-200 nm.

[0008] In one embodiment of the present invention, the average particle size of the primary particles is 100-400 nm.

[0009] In one embodiment of the present invention, the specific surface area of ​​the positive electrode material is 10-14 m². 2 / g.

[0010] In one embodiment of the present invention, the mass of C coating in the cathode material accounts for 1-2 wt% of the total mass of the cathode material.

[0011] In one embodiment of the present invention, the cathode material includes a doping element, which is selected from one or more of Ti, V, Mg, Nb, Ca, Al, Mn, Co or Ni, and the content of the doping element is 400-7000 ppm of the total mass of the cathode material.

[0012] In one embodiment of the present invention, the positive electrode material is a secondary particle formed by the aggregation of multiple twin crystals.

[0013] The present invention also provides a method for preparing the above-described lithium battery cathode material, comprising at least the following steps:

[0014] A predetermined amount of lithium source, iron source, phosphoric acid, carbon source, and dispersant are mixed, and pure water is added, then stirred and ground until homogeneous to obtain an intermediate; and

[0015] After the intermediate is spray-dried, it is sintered under a protective gas atmosphere to obtain the cathode material.

[0016] In one embodiment of the present invention, the sintering temperature is 680-750°C and the sintering time is 6-18 hours.

[0017] In one embodiment of the present invention, the intermediate further includes a substance containing a dopant element, wherein the dopant element is selected from one or more of Ti, V, Mg, Nb, Ca, Al, Mn, Co or Ni.

[0018] The present invention also provides a lithium-ion battery, comprising the lithium battery positive electrode material described above.

[0019] The present invention also provides an electrochemical device comprising the lithium-ion battery described above.

[0020] In summary, this invention proposes a cathode material, its preparation method, and its application, which can simultaneously improve the processing performance and electrochemical performance of the cathode material. It can reduce the specific surface area of ​​the cathode material, improve its processing performance, and ensure its overall rate performance. By coating the surface of the cathode material with a carbon layer, the growth of lithium iron phosphate grains can be suppressed, while simultaneously enhancing the conductivity between primary particles and on the surface electrons, thereby improving the rate performance and cycle performance of the cathode material. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1This is a flowchart of a method for preparing a positive electrode material according to the present invention.

[0023] Figure 2 This is a schematic diagram of the structure of a lithium-ion battery according to the present invention.

[0024] Figure 3 This is a scanning electron microscope image of the cathode material in this invention.

[0025] Figure 4 This is the Raman spectrum of the cathode material in this invention. Detailed Implementation

[0026] The following specific examples illustrate the implementation of the present invention. 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, and various 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.

[0027] It should be understood that the present invention can be implemented in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Unless otherwise specified, “%” and “parts” as shown in the following embodiments refer to “% by mass” and “parts by mass”, respectively.

[0028] The technical solution of the present invention will be further described in detail below with reference to several embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] This invention proposes a cathode material with the chemical formula Li. a Fe b (PO4) c @C; where 0.95≤a≤1.10, 0.95≤b≤1.05, and 0.95≤c≤1.05. The primary particles of the cathode material are twins composed of single crystals, and the average particle size of the single crystals constituting the primary particles is, for example, 30-200 nm, or 50-150 nm, while the average particle size of the resulting primary particles is, for example, 100-400 nm, or 150-250 nm. By controlling the average particle size of the single crystals, the processing performance is prevented from deteriorating due to excessively small average particle size, while the electrochemical performance is prevented from deteriorating due to excessively large average particle size, thus improving both the processing and electrochemical performance of the cathode material.

[0030] In one embodiment of the present invention, the specific surface area of ​​the primary particles of the cathode material is, for example, 10-14 m². 2 / g, for example, 11-13m 2 / g. By forming twins, the specific surface area of ​​the cathode material can be reduced, improving its processing performance. Controlling the size and specific surface area of ​​the twins can prevent poor processing performance due to a high specific surface area. If the twin size is too large, the number of single crystals forming the twins and the number of grain boundaries will be excessive, leading to a tendency towards non-spherical morphology and potentially deteriorating processing performance. In one embodiment of this invention, during the preparation of the cathode sheet, the cathode material can be in the form of primary particles or aggregates formed by twin aggregation; that is, the cathode material can also be in the form of secondary particles. Aggregating primary particles into secondary particles further reduces the specific surface area of ​​the cathode material, improving its processing performance.

[0031] In one embodiment of the present invention, the particle size of the primary particles of the positive electrode material is not greater than 1.0 μm, and the particle size distribution of the primary particles of the positive electrode material satisfies D0 = [0.15, 0.2] μm, D 10 = [0.2, 0.3] μm, D 50 = [0.3, 0.5] μm, D 90 = [0.5, 0.7] μm, D 99 = [0.7, 0.9] μm and D 100 = [0.9, 1.0] μm. Where D0, D 10 D 50 D 90 and D 100 These are the 0%, 10%, 50%, 90%, 99%, and 100% volumetric cumulative particle sizes, determined using a particle size analyzer. This means the primary particle size is 0.15-1.0 μm, indicating the absence of large particles in the primary particles of the cathode material, ensuring good overall rate performance. Simultaneously, the smallest particle size is greater than 0.15 μm, and there is a low concentration of fine powder, i.e., D... 10 The specific surface area is 0.2-0.3μm, thus ensuring a low specific surface area and further ensuring good processing performance.

[0032] Please see Figure 3 As shown, in one embodiment of the present invention, the morphology and size of the cathode material are characterized using a cross-section polisher-scanning electron microscope (CP-SEM). From... Figure 3As can be seen, the particle size of the cathode material is less than 1 μm, and the primary particle shape is, for example, rod-shaped, fibrous, or whisker-shaped, with an aspect ratio of, for example, 1-10 and a radial dimension of, for example, 0.1-0.4 μm, meaning the shortest cross-sectional dimension of the primary particle is, for example, 0.1-0.4 μm. By controlling the morphology of the cathode material, when applied to lithium-ion batteries, lithium ions tend to be transported along the shorter surface, thus improving the rate performance of the lithium-ion battery.

[0033] In one embodiment of the present invention, a carbon layer is uniformly and continuously coated on the surface of the cathode material. The thickness of the carbon layer is, for example, 2-15 nm, and the carbon coating amount is, for example, 1 wt%-2 wt% of the total mass of the cathode active material, or, for example, 1.1-1.4 wt%. By coating the cathode material with a carbon layer, the growth of lithium iron phosphate grains can be suppressed, while simultaneously enhancing the conductivity between primary particles and surface electrons, thereby improving the rate performance and cycle performance of the cathode material. By controlling the thickness of the carbon layer, problems such as excessively thick carbon layers leading to excessively large specific surface areas and processing difficulties are prevented, while problems such as poor conductivity due to excessively thin carbon layers are also avoided, thus achieving a balance between battery performance and processability.

[0034] Please see Figure 4 As shown, in one embodiment of the present invention, the cathode material is detected using a Raman spectrometer, and the detection depth of the Raman spectrometer is, for example, 10 nm. In the Raman spectrum of the cathode material, I 216 This indicates that the corresponding peak position in the Raman spectrum is 216±20 cm. -1 Peak intensity at I 1595 This indicates that the corresponding peak position in the Raman spectrum is 1595±20cm. -1 The peak intensity at 216±20 cm⁻¹. Specifically, in the Raman spectrum, this peak intensity is at 216±20 cm⁻¹. -1 The corresponding characteristic peak is that of lithium iron phosphate, 1595±20cm. -1 The corresponding characteristic peak is the G peak of carbon, which represents the sp of a fully graphitized structure. 2 Vibration. In one embodiment of the present invention, I 216 with I 1595 Satisfying the relation: 0≤I 216 / I 1595 ≤0.8, 0≤△I 216 / I 1595 ≤0.05, where △I 216 / I 1595 Indicating I in multiple tests 216 / I 1595 The deviation. I 216 / I 1595The smaller the ratio, the thicker the carbon layer; the larger the ratio, the thinner the carbon layer. This is indicated by the Ig in the Raman spectrum. 216 / I 1595 The ratio is used to determine the coating thickness of the carbon layer in the cathode material.

[0035] Please see Figure 4 As shown, in one embodiment of the present invention, I 1380 This indicates that the corresponding peak position in the Raman spectrum is at 1380±20 cm. -1 Peak intensity at 1380±20cm -1 The corresponding characteristic peak is the D peak of carbon, which represents the sp of disordered carbon. 3 Vibration. Through I 1380 / I 1595 The ratio is an important means of judging the degree of graphitization of the carbon coating layer. The smaller the ratio, the higher the degree of graphitization of the material, which leads to an increase in the primary particle size of the cathode material and a decrease in electrical performance. 1380 / I 1595 The larger the value, the lower the degree of graphitization, and the worse the conductivity and electrical performance of the cathode material. In one embodiment of the present invention, I 1380 with I 1595 Satisfies the relation: 0.6≤I 1380 / I 1595 ≤1, 0≤△I 1380 / I 1595 ≤0.05, △I 1380 / I 1595 Indicating I in multiple tests 1380 / I 1595 The deviation. Where, △I 1380 / I 1595 This indicates the uniformity of carbon coating at different locations. The smaller the value, the more uniform the carbon coating, which is more beneficial to cycle performance.

[0036] In one embodiment of the present invention, the cathode material further includes, for example, a doping element selected from one or more of Ti, V, Mg, Nb, Ca, Al, Mn, Co, or Ni, with a doping element content of 400-7000 ppm of the total mass of the cathode material. By adding a doping element to the cathode material, the electronic structure of lithium iron phosphate can be altered, its conductivity improved, and its electrochemical performance enhanced.

[0037] Please see Figure 1 As shown, the present invention also proposes a method for preparing a positive electrode material, which includes, but is not limited to, steps S100-S200.

[0038] Step S100: Mix the preset amounts of lithium source, iron source, phosphorus source, carbon source and dispersant, add pure water and stir and grind evenly to obtain an intermediate.

[0039] Step S200: After spray drying the intermediate, sinter it under a protective gas atmosphere to obtain the cathode material.

[0040] Please see Figure 1 As shown, in one embodiment of the present invention, in step S100, a preset amount of lithium source, iron source, phosphorus source, carbon source, and dispersant are mixed, and pure water is added and stirred and ground evenly to obtain an intermediate. The lithium source is selected, for example, from one or more combinations of lithium carbonate, lithium hydroxide, lithium acetate, lithium chloride, or lithium dihydrogen phosphate. The iron source is selected, for example, from one or more combinations of ferric phosphate, iron(II,III) oxide, iron(II,III) oxide, or ferrous oxalate. The phosphorus source is selected, for example, from one or more combinations of diammonium hydrogen phosphate, ferric phosphate, or ammonium dihydrogen phosphate. The carbon source is selected, for example, from one or more combinations of starch, glucose, sucrose, or polyvinyl alcohol. The dispersant is selected, for example, polyethylene glycol (PEG), specifically polyethylene glycol with a molecular weight of 2000. In this embodiment, the lithium source is selected, for example, from lithium carbonate; the iron source is selected, for example, from ferric phosphate; and ferric phosphate is also used as a phosphorus source to reduce the types of raw materials introduced and control the quality of the synthesized cathode material. The carbon source is selected, for example, from glucose; and the dispersant is selected as PEG2000. In this invention, the raw materials for synthesis may also include a substance containing a dopant element, which is selected from one or more of Ti, V, Mg, Nb, Ca, Al, Mn, Co or Ni. In this embodiment, the substance containing the dopant element is, for example, titanium dioxide (TiO2).

[0041] In one embodiment of the present invention, the molar ratio of iron, phosphorus, and lithium elements in the iron source, phosphorus source, and lithium source is, for example, (0.98-1):1:(1.0-1.1). In this embodiment, the molar ratio of iron, phosphorus, and lithium elements is, for example, 1:1:1.03. Specifically, for example, 100 kg of iron phosphate, 25.3 kg of lithium carbonate, 4.2 kg of glucose, 7.6 kg of PEG2000, and 0.1 kg of TiO2 are selected and added to 400 kg of pure water for stirring and grinding until homogeneous to obtain an intermediate.

[0042] Please see Figure 1 As shown, in one embodiment of the present invention, in step S200, the intermediate obtained in step S100 is dried, for example by spray drying, which avoids further crushing after drying and simplifies the manufacturing process. Sintering is then performed under a protective gas atmosphere to obtain the cathode material. In one embodiment of the present invention, the protective gas is, for example, nitrogen or argon, the sintering temperature is, for example, 680-750°C, and the sintering time is, for example, 6-18 hours. By controlling the raw material ratio, sintering temperature, and sintering time, the primary particle size of the cathode material is controlled, as well as the degree of graphitization of the carbon coating layer.

[0043] Please see Figure 2 As shown, the present invention also proposes a lithium-ion battery, comprising a positive electrode 10, a separator 20, a negative electrode 30, and an electrolyte 40. The separator 20 is located between the positive electrode 10 and the negative electrode 30, and the electrolyte 40 fills the space between the positive electrode 10, the separator 20, and the negative electrode 30. The positive electrode 10 comprises the aforementioned positive electrode material. The lithium-ion battery can be, for example, a primary lithium-ion battery or a secondary lithium-ion battery, and more specifically, a secondary battery. A secondary lithium-ion battery can be, for example, a pouch battery, a hard-case battery, or a cylindrical battery.

[0044] Please see Figure 2 As shown, in one embodiment of the present invention, the positive electrode sheet 10 includes the aforementioned positive electrode material, and may also include a positive electrode current collector, an adhesive, and a conductive agent. The positive electrode current collector can be, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. The surface of the positive electrode current collector is smooth, and fine textures may also be formed on its surface to improve the adhesion between the positive electrode active material and the positive electrode current collector. Besides 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, or non-woven fabric.

[0045] In one embodiment of the present invention, the adhesive is selected from one or more of the following: polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexanefluoropropylene (Polyhexafluoropropylene), or polymerized styrene-butadiene rubber (SBR). The conductive agent is selected from one or more of the following: conductive carbon black (Super P, SP), acetylene black, carbon nanotubes, and graphene.

[0046] In one embodiment of the present invention, the positive electrode current collector is, for example, selected from aluminum foil, with a thickness of, for example, 8-16 μm, and the positive electrode material is the aforementioned positive electrode material Li. a Fe b (PO4) c@C, the binder is selected from polyvinylidene fluoride (PVDF), and the conductive agent is selected from conductive carbon black. In one embodiment of the present invention, the positive electrode material, PVDF, and conductive carbon black are mixed, for example, at a weight ratio of 97:1.5:1.5, and an organic solvent is added and stirred until the system is homogeneous to obtain a positive electrode slurry. The organic solvent is selected, for example, N-methylpyrrolidone (NMP). The positive electrode slurry is uniformly coated onto an aluminum foil, and the aluminum foil is placed, for example, in a forced-air drying oven and dried at 120°C for 10 minutes. The dried aluminum foil is then rolled and cut to form a positive electrode sheet.

[0047] Please see Figure 2 As shown, in one embodiment of the present invention, the negative electrode sheet 30 includes, for example, a negative electrode current collector, a negative electrode material, a binder, a conductive agent, and a thickener. The negative electrode current collector is selected from, for example, a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foamed copper current collector, or a stainless steel current collector. The negative electrode material is selected from, for example, a graphite-based material, a silicon-based material, lithium metal, or a composite material of graphite and silicon-based materials. The binder is selected from, for example, any one or more of polyvinylidene fluoride, polyamide, polypropylene, polyacrylate, polyethylene ether, polymethyl methacrylate, polyhexamethylene propylene, or styrene-butadiene rubber. The conductive agent is selected from, for example, any one or more of conductive carbon black, acetylene black, carbon nanotubes, and graphene. In one embodiment of the present invention, the negative electrode current collector is selected from, for example, copper foil; the negative electrode material is selected from, for example, artificial graphite; the conductive agent is selected from, for example, conductive carbon black; the binder is selected from, for example, styrene-butadiene rubber; and the thickener is selected from, for example, sodium carboxymethyl cellulose. In one embodiment of the present invention, artificial graphite, conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed, for example, at a mass ratio of 96.4:1:1.2:1.4, and deionized water is added. The mixture is then stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated onto a copper foil, and after drying, cold pressing, and slitting, a negative electrode sheet 30 is obtained. In one embodiment of the present invention, the negative electrode sheet 30 is, for example, a lithium metal sheet, or a lithium metal sheet with other metal mesh foils as a skeleton.

[0048] Please see Figure 2As shown, in one embodiment of the present invention, the electrolyte 40 includes, for example, an organic solvent and a lithium salt. The organic solvent is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl acetate (EA), dimethyl carbonate (DMC), or diethyl carbonate (DEC). The lithium salt is selected from, for example, one or more of lithium bis(fluorosulfonyl)imide (LiFSi), lithium difluorophosphate (LiPO2F2), lithium hexafluorophosphate (LiPF6), or lithium tetrafluoroborate (LiBF4). In one embodiment of the present invention, the lithium salt is selected from, for example, lithium hexafluorophosphate, and the organic solvent is selected from, for example, a mixture of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and ethyl acetate (EA). Ethyl carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and ethyl acetate (EA) are mixed, for example, in a volume ratio of 1:1:2:6. In an argon-atmosphere glove box with a water content of less than 10 ppm, thoroughly dried LiPF6 is dissolved in the mixed organic solvent and mixed evenly to obtain an electrolyte, wherein the concentration of LiPF6 is, for example, 1 mol / L.

[0049] Please see Figure 2 As shown, in one embodiment of the present invention, the separator 20 is, for example, a polyethylene (PE) membrane, a polypropylene (PP) membrane, a glass fiber membrane, or a polyethylene membrane. The thickness of the separator 20 is, for example, 9-15 μm. In one embodiment of the present invention, the positive electrode 10, the separator 20, and the negative electrode 30 are stacked sequentially, so that the separator 20 is positioned between the positive and negative electrodes to provide separation. The electrolyte 40 is filled between the positive electrode 10, the separator 20, and the negative electrode 30. In one embodiment of the present invention, the lithium-ion battery is, for example, a 2016 type coin cell battery, wherein the areal density of the positive electrode is 10 mg / cm³. 2 The compacted density is 2.0 g / cm³. 3 .

[0050] The present invention will be explained in more detail below by referring to embodiments, which should not be construed as limiting. Appropriate modifications can be made within the scope of the present invention, and all such modifications fall within the technical scope of the present invention.

[0051] Example 1

[0052] Preparation of the cathode material: 100g of iron phosphate, 25.3g of lithium carbonate, 4.2g of glucose, 7.6g of PEG2000, and 0.1g of TiO2 were added to 400g of pure water, stirred and ground until homogeneous. After spray drying, the mixture was sintered at 700℃ for 10h under a nitrogen atmosphere. The cathode material, LiFe(PO4)@C, consists of twinned primary particles. The average particle size of the single crystal in the primary particles is 97nm, the average particle size of the primary particles is 193nm, and the specific surface area is 12.2m². 2 / g.

[0053] Preparation of the positive electrode sheet: The positive electrode material LiFe(PO4)@C, the binder polyvinylidene fluoride, and the conductive agent acetylene black were mixed at a mass ratio of 97:1.5:1.5. After the positive electrode material, binder, and conductive agent were mixed evenly, the solvent N-methylpyrrolidone was added, and the mixture was stirred in a vacuum mixer until it became homogeneous and transparent, thus obtaining the positive electrode slurry. The positive electrode slurry was uniformly coated onto a 10 μm aluminum foil current collector, and after the current collector was dried at room temperature, it was transferred to an oven and dried at 120℃ for 10 h. Then, the positive electrode sheet was obtained by rolling and slitting.

[0054] Selection of negative electrode: Pure lithium metal sheet is selected as the negative electrode.

[0055] Preparation of electrolyte: Battery-grade ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl acetate were mixed in a mass ratio of 1:1:2:6. In an argon atmosphere glove box with a water content of less than 10 ppm, fully dried LiPF6 was dissolved in the mixed organic solvent and mixed evenly to obtain the electrolyte. The concentration of LiPF6 was 1 mol / L.

[0056] Selection of diaphragm: Use a 12μm thick polypropylene diaphragm.

[0057] Battery fabrication: The coin cell was assembled in a glove box filled with dry argon gas. The positive electrode, separator, negative electrode and electrolyte were assembled into a coin cell of model 2016 and tested after standing for 12 hours.

[0058] Example 2

[0059] The cathode material was obtained at a sintering temperature of 700℃ and a sintering time of 12h. The cathode material LiFe(PO4)@C consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 52nm, the average particle size of the primary particles is 202nm, and the specific surface area is 13.0m². 2 / g, other operations are the same as in Comparative Example 1.

[0060] Example 3

[0061] The cathode material was obtained at a sintering temperature of 700℃ and a sintering time of 14h. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 33nm, the average particle size of the primary particles is 199nm, and the specific surface area is 13.4m². 2 / g, other operations are consistent with Example 1.

[0062] Example 4

[0063] The cathode material was obtained at a sintering temperature of 700℃ and a sintering time of 16 h. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 31 nm, the average particle size of the primary particles is 204 nm, and the specific surface area is 15.1 m². 2 / g, other operations are consistent with Example 1.

[0064] Example 5

[0065] The cathode material was obtained at a sintering temperature of 700℃ and a sintering time of 8 hours. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 147 nm, and the average particle size of the primary particles is 307 nm. The specific surface area is 11.3 m². 2 / g, other operations are consistent with Example 1.

[0066] Example 6

[0067] The cathode material was obtained at a sintering temperature of 700℃ and a sintering time of 6 hours. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 194 nm, the average particle size of the primary particles is 315 nm, and the specific surface area is 10.5 m². 2 / g, other operations are consistent with Example 1.

[0068] Example 7

[0069] The cathode material was obtained at a sintering temperature of 680℃ and a sintering time of 6 hours. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 191 nm, the average particle size of the primary particles is 322 nm, and the specific surface area is 9.8 m². 2 / g, other operations are consistent with Example 1.

[0070] Example 8

[0071] The cathode material was obtained at a sintering temperature of 680℃ and a sintering time of 8 hours. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 98 nm, the average particle size of the primary particles is 245 nm, and the specific surface area is 11.5 m². 2 / g, other operations are consistent with Example 1.

[0072] Example 9

[0073] The cathode material was obtained at a sintering temperature of 720℃ and a sintering time of 8 hours. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 96 nm, the average particle size of the primary particles is 392 nm, and the specific surface area is 10.3 m². 2 / g, other operations are consistent with Example 1.

[0074] Example 10

[0075] The cathode material was obtained at a sintering temperature of 750℃ and a sintering time of 8 hours. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 97 nm, the average particle size of the primary particles is 396 nm, and the specific surface area is 9.7 m². 2 / g, other operations are consistent with Example 1.

[0076] Example 11

[0077] The cathode material was obtained at a sintering temperature of 750℃ and a sintering time of 10h. The cathode material LiFe(PO4)@C consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 35nm, the average particle size of the primary particles is 153nm, and the specific surface area is 13.8m². 2 / g, other operations are consistent with Example 1.

[0078] Example 12

[0079] The cathode material was obtained at a sintering temperature of 750℃ and a sintering time of 12h. The cathode material, LiFe(PO4)@C, consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 34nm, the average particle size of the primary particles is 106nm, and the specific surface area is 14.2m². 2 / g, other operations are consistent with Example 1.

[0080] Example 13

[0081] The cathode material was obtained at a sintering temperature of 750℃ and a sintering time of 14h. The cathode material LiFe(PO4)@C consists of primary particles with a twinned structure. The average particle size of the single crystal in the primary particles is 31nm, the average particle size of the primary particles is 93nm, and the specific surface area is 14.5m². 2 / g, other operations are consistent with Example 1.

[0082] Example 14

[0083] The cathode material was obtained at a sintering temperature of 750℃ and a sintering time of 18h. The cathode material LiFe(PO4)@C is a secondary particle with a twinned structure, formed by the agglomeration of primary particles. The average particle size of the single crystal in the primary particles is 103nm, the average particle size of the primary particles is 211nm, and the specific surface area is 11.2m². 2 / g, other operations are consistent with Example 1.

[0084] Comparative Example 1

[0085] The cathode material was obtained at a sintering temperature of 750℃ and a sintering time of 20h. The cathode material, LiFe(PO4)@C, consists of single-crystal primary particles with an average particle size of 105nm and a specific surface area of ​​15.3m². 2 / g, other operations are consistent with Example 1.

[0086] Comparative Example 2

[0087] The cathode material was obtained at a sintering temperature of 680℃ and a sintering time of 20h. The cathode material, LiFe(PO4)@C, is a single-crystal primary particle with an average particle size of 211nm and a specific surface area of ​​14.1m². 2 / g, other operations are consistent with Example 1.

[0088] In this invention, lithium-ion batteries were prepared using different cathode materials in Examples 1-14 and Comparative Examples 1-2, and the rate performance of the lithium-ion batteries was tested. The test results are shown in Table 1.

[0089] In one embodiment of the present invention, the rate performance test involves constant current and constant voltage charging at 0.1C within the 2.0-3.75V range, with a charging cutoff current of 0.05C, followed by discharging at 0.1C. Then, constant current and constant voltage charging is performed again at 0.3C, with a charging cutoff current of 0.05C, followed by constant current discharging at 1C. The 1C discharge capacity divided by the 0.1C discharge capacity gives the rate performance.

[0090] Table 1. Performance test results of lithium-ion batteries in Examples 1-14 and Comparative Examples 1-2

[0091]

[0092] Please refer to Table 1. Combining Examples 1-14 and Comparative Examples 1-2, a cathode material with twinned primary particles was selected. Compared to cathode materials with single-crystal primary particles, the unit responsible for electrochemical performance in the cathode material is the single crystal. Twins influence the manufacturing process of lithium-ion batteries, thus simultaneously possessing good processing performance and electrochemical performance, improving the rate performance of the prepared lithium-ion battery. Referring to Examples 1-14, if the average particle size of the single crystal is too large, its electrochemical performance will deteriorate; if the average particle size of the single crystal is too small, it will, to some extent, worsen the processing performance. Therefore, by controlling the synthesis conditions of the cathode material and keeping the size of the single crystal within a certain range, both processing performance and electrochemical performance can be balanced. Furthermore, if the twin size is too small, the specific surface area of ​​the cathode material is high, resulting in poor processing performance; if the twin size is too large, there are too many single crystals forming the twins, leading to too many grain boundaries and a tendency towards non-spherical morphology, which also deteriorates processing performance and affects the rate performance of the lithium-ion battery. When primary cathode material particles agglomerate into secondary particles, the specific surface area of ​​the cathode material is further reduced, thereby further improving processing performance.

[0093] This invention selects some embodiments and comparative examples to study the influence of the particle size distribution of the cathode material on the performance of lithium-ion batteries. The results are shown in Table 2.

[0094] Table 2. Performance test results of lithium-ion batteries in some embodiments and Comparative Example 2

[0095]

[0096] Please refer to Table 2. In conjunction with the examples and Comparative Example 2, the cathode material obtained by this invention has a total particle size of less than 1.0 μm, with no large particles, resulting in better overall rate performance of the lithium-ion battery. The smallest particle size of the cathode material is greater than 0.15 μm, with less fine powder, ensuring a low specific surface area and good processing performance. Furthermore, the aspect ratio of the cathode material is 1-10. During operation, lithium ions tend to be transported along the shorter surface, improving the rate performance of the lithium-ion battery. The larger overall particle size maintains a larger aspect ratio, further reducing the specific surface area and improving the processing performance of the cathode material.

[0097] This invention selects some embodiments and comparative examples to study the influence of the thickness of the carbon coating layer and the degree of graphitization of the cathode material on the performance of lithium-ion batteries. The results are shown in Table 3.

[0098] Table 3. Performance test results of lithium-ion batteries in some embodiments and comparative examples 1-2

[0099]

[0100] Please refer to Table 3. In conjunction with the examples and comparative examples 1-2, when the carbon coating layer on the surface of the cathode material is too thick, the specific surface area of ​​the cathode material increases, increasing the processing difficulty. Conversely, if the carbon coating layer is too thin, the conductivity of the cathode material is poor, resulting in poor rate performance of the lithium-ion battery. In the Raman spectrum of the cathode material, 216±20 cm⁻¹... -1 The peak at 1595±20 cm⁻¹ is the peak of lithium iron phosphate, while the peak at 1595±20 cm⁻¹ is the peak of lithium iron phosphate. -1 The peak at that location is the G peak of the carbon layer, I 216 / I 1595 The smaller the ratio, the thicker the carbon layer; the larger the ratio, the thinner the carbon layer. 1380±20cm -1 For the D peak of the carbon layer, I 1380 / I 1595 The smaller the value, the higher the degree of graphitization, leading to larger particle size and reduced electrical properties. 1380 / I 1595 The larger the value, the lower the degree of graphitization, resulting in poorer conductivity and electrical performance. Therefore, in this embodiment, the sintering temperature and time of the cathode material during the preparation process are controlled to regulate I. 1380 / I 1595 The ratio of [value] improves the electrical and processing properties of the cathode material.

[0101] In summary, this invention proposes a cathode material, its preparation method, and its application. The primary particles of the cathode material are twins composed of single crystals. By controlling the average particle size of the single crystals and primary particles, the invention prevents the processing performance from deteriorating due to excessively small average particle size of the single crystals, and simultaneously prevents the electrochemical performance from deteriorating due to excessively large average particle size of the single crystals, thus improving both the processing and electrochemical performance of the cathode material. The formation of twins reduces the specific surface area of ​​the cathode material, improving its processing performance. Furthermore, controlling the size and specific surface area of ​​the twins prevents poor processing performance caused by a high specific surface area. The primary particles in the cathode material have a uniform particle size distribution, with no large particles, ensuring good overall rate performance. The low amount of fine powder further guarantees a low specific surface area, ensuring good processing performance. By controlling the morphology of the cathode material, lithium ions tend to transport along shorter surfaces during application, thus improving the rate performance of lithium-ion batteries. By coating the surface of the cathode material with a carbon layer, the growth of lithium iron phosphate grains can be suppressed, while the conductivity between primary particles and on the surface electrons is enhanced, thereby improving the rate performance and cycle performance of the cathode material.

[0102] The above description is merely a preferred embodiment of this application and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept. For example, technical solutions formed by replacing the above-mentioned features with technical features with similar functions disclosed in this application (but not limited to) each other.

[0103] Apart from the technical features described in the specification, the other technical features are known to those skilled in the art. To highlight the innovative features of this invention, the other technical features will not be described in detail here.

Claims

1. A lithium battery cathode material, characterized in that, The chemical formula of the cathode material having a carbon coating is Li a Fe b (PO4) c @C; where 0.95≤a≤1.10, 0.95≤b≤1.05, 0.95≤c≤1.05; the primary particles of the cathode material are twins composed of multiple single crystals; the average particle size of the single crystal is 30-200nm; the average particle size of the primary particles is 100-400nm; wherein the surface of the cathode material has a carbon coating layer with a thickness of 2-15nm, and the mass of the carbon layer in the cathode material accounts for 1-2wt% of the total mass of the cathode material.

2. The lithium battery cathode material according to claim 1, characterized in that, The specific surface area of ​​the cathode material is 10⁻¹⁴ m². 2 / g.

3. The lithium battery cathode material according to claim 1, characterized in that, The cathode material includes doping elements, which are selected from one or more of Ti, V, Mg, Nb, Ca, Al, Mn, Co or Ni, and the content of the doping elements is 400-7000 ppm of the total mass of the cathode material.

4. The lithium battery cathode material according to claim 1, characterized in that, The cathode material is a secondary particle formed by the aggregation of multiple twin crystals.

5. A method for preparing the lithium battery cathode material as described in any one of claims 1-4, characterized in that, At least the following steps are included: A predetermined amount of lithium source, iron source, phosphoric acid, carbon source, and dispersant are mixed, and pure water is added, then stirred and ground until homogeneous to obtain an intermediate; and After the intermediate is spray-dried, it is sintered under a protective gas atmosphere to obtain the cathode material.

6. The method for preparing the lithium battery cathode material according to claim 5, characterized in that, The sintering temperature is 680-750℃, and the sintering time is 6-18h.

7. The method for preparing the lithium battery cathode material according to claim 5, characterized in that, The intermediate also includes a substance containing a dopant element, which is selected from one or more of Ti, V, Mg, Nb, Ca, Al, Mn, Co or Ni.

8. A lithium-ion battery, characterized in that, Including the lithium battery cathode material as described in any one of claims 1-4.

9. An electrochemical device, characterized in that, Including the lithium-ion battery as described in claim 8.

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