Modified lithium iron phosphate cathode material, preparation method thereof and lithium ion battery

Abstract

This application provides a modified lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery. The modified lithium iron phosphate cathode material is a secondary particle formed by primary particle stacking; the primary particles include first lithium iron phosphate particles, second lithium iron phosphate particles, and third lithium iron phosphate particles; the first lithium iron phosphate particles, second lithium iron phosphate particles, and third lithium iron phosphate particles have a D... 50 The sizes decrease sequentially, and d1 ≤ 2.5 μm; the core materials of the first, second, and third lithium iron phosphate particles all have the general chemical formula shown in formula (I): LiFe x M 1‑x PO4(I), where 0.980≤x≤0.995, and M is a doping element selected from one or more elements in Group IVB and Group VB. The above-mentioned modified lithium iron phosphate cathode material possesses both high compaction density and high lithium-ion transport rate, which can improve the rate performance and cycle performance of lithium-ion batteries.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4) is a cathode material based on olivine structure. With its high energy density, long cycle life and excellent safety performance, it has shown great application potential in electric vehicles, energy storage systems and portable electronic devices, and has become one of the cutting-edge fields of lithium-ion battery technology research and development.

[0003] Although various methods for preparing lithium iron phosphate (LFP) cathode materials have been developed in the industry, these methods still have certain limitations in improving the electrochemical performance and energy density of LFP cathode materials. For example, traditional preparation processes often cannot achieve precise control over the crystallinity of LFP cathode materials, which can easily lead to limited diffusion rates of lithium ions in the cathode material, thus affecting the electrode's activity and reaction efficiency. In addition, LFP cathode materials prepared using traditional methods often suffer from poor particle size distribution, resulting in low compaction density. This not only affects the stability and efficiency of the lithium-ion electrode during charge and discharge processes but also impacts the overall energy density of the lithium-ion battery, thereby limiting the overall performance improvement of lithium-ion batteries.

[0004] Therefore, researching and developing a modified lithium iron phosphate cathode material with high compaction density and high lithium-ion transport rate is of great significance for improving the energy density, electrochemical stability and cycle performance of lithium-ion batteries. Summary of the Invention

[0005] The main objective of this application is to provide a modified lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery, in order to solve the problems of low compaction density and low lithium-ion transport rate of lithium iron phosphate cathode materials in the prior art, and the resulting problems of low energy density, poor electrochemical stability, and poor cycle performance of lithium-ion batteries.

[0006] To achieve the above objectives, this application provides a modified lithium iron phosphate cathode material, which is a secondary particle formed by primary particle stacking; the primary particles include a first lithium iron phosphate particle, a second lithium iron phosphate particle, and a third lithium iron phosphate particle; the D of the first lithium iron phosphate particle, the second lithium iron phosphate particle, and the third lithium iron phosphate particle... 50The particles are denoted as d1, d2, and d3, respectively; where d3 < d2 < d1, and d1 ≤ 2.5 μm; the first lithium iron phosphate particle consists of a first core and a first carbon coating layer from the inside out; the second lithium iron phosphate particle consists of a second core and a second carbon coating layer from the inside out; the third lithium iron phosphate particle consists of a third core and a third carbon coating layer from the inside out; the materials of the first core, the second core, and the third core are the same, and all have the general chemical formula shown in formula (I): LiFe x M 1-x PO4(I), where 0.980≤x≤0.995, M is a dopant element selected from one or more elements in Group IVB and Group VB.

[0007] The modified lithium iron phosphate cathode material provided in this application is a secondary particle formed by the stacking of three primary particles with different particle sizes. The large-diameter first lithium iron phosphate particles provide solid structural support, the medium-diameter second lithium iron phosphate particles fill the gaps between the first lithium iron phosphate particles, and the small-diameter third lithium iron phosphate particles further fill the fine gaps, thereby effectively increasing the compaction density of the modified lithium iron phosphate cathode material and thus improving the energy density of the lithium-ion battery. The first, second, and third lithium iron phosphate particles each have a core and a carbon coating layer covering the surface of their cores. The presence of these three carbon coating layers constructs a continuous conductive network, improving the conductivity of the modified lithium iron phosphate cathode material. Simultaneously, it effectively improves the structural stability of the modified lithium iron phosphate cathode material, thereby enhancing the cycle performance of the lithium-ion battery. The materials of the first core, the second core, and the third core all possess the specific chemical formula shown in formula (I). Introducing specific types of doping elements into the lattice of lithium iron phosphate can improve the electronic structure of the modified lithium iron phosphate cathode material, optimize the lithium-ion diffusion path, thereby increasing the lithium-ion transport rate and consequently improving the initial coulombic efficiency and cycle performance of the lithium-ion battery. In summary, the modified lithium iron phosphate cathode material provided in this application possesses both high compaction density and high lithium-ion transport rate. Using it as a cathode active material in lithium-ion batteries can effectively improve the energy density, initial coulombic efficiency, electrochemical stability, and cycle performance of lithium-ion batteries.

[0008] Furthermore, based on the volume percentage of the modified lithium iron phosphate cathode material, the first lithium iron phosphate particles account for 40–60 vol%, the second lithium iron phosphate particles account for 20–30 vol%, and the third lithium iron phosphate particles account for 10–30 vol%; the compaction density of the modified lithium iron phosphate cathode material is 2.5–2.7 g / cm³. 3 The preferred concentration is 2.6–2.7 g / cm³. 3 .

[0009] Compared to other ranges, limiting the volume ratio of the first, second, and third lithium iron phosphate particles and the compaction density of the modified lithium iron phosphate cathode material to the above ranges is beneficial to improving the structural stability of the modified lithium iron phosphate cathode material, thereby improving the energy density and cycle stability of lithium-ion batteries.

[0010] Furthermore, in the modified lithium iron phosphate cathode material, the carbon element has a weight percentage of 1.3 to 1.5 wt%.

[0011] Compared to other ranges, limiting the carbon content in modified lithium iron phosphate cathode materials to the above range is beneficial to improving the conductivity and structural stability of modified lithium iron phosphate cathode materials, thereby improving the cycle performance of lithium-ion batteries.

[0012] Furthermore, in the modified lithium iron phosphate cathode material, 0.5≤d1-d2≤1.9; 0.2≤d2-d3≤0.9; preferably, d1 is 1.5~2.5μm, d2 is 0.6~1μm, and d3 is 0.1~0.4μm.

[0013] Compared to other ranges, limiting d1, d2, and d3 to the above range is beneficial to improving the gradation effect of modified lithium iron phosphate cathode materials, thereby improving the energy density, first efficiency, and cycle performance of lithium-ion batteries.

[0014] Furthermore, the doping element accounts for 0.1 to 3 wt% of the weight of the modified lithium iron phosphate cathode material; preferably, the doping element is selected from one or more of the group consisting of Ti, V, Zr and Nb.

[0015] Compared to other ranges and types, limiting the content and type of doping elements to the above range is beneficial to improving the electronic structure of modified lithium iron phosphate cathode materials and optimizing the diffusion path of lithium ions, thereby improving the first coulombic efficiency and cycle performance of lithium-ion batteries.

[0016] Furthermore, in the first lithium iron phosphate particle, the weight ratio of the first core to the first carbon coating layer is (1-5):(50-100); in the second lithium iron phosphate particle, the weight ratio of the second core to the second carbon coating layer is (1-5):(100-200); and in the third lithium iron phosphate particle, the weight ratio of the third core to the third carbon coating layer is (1-5):(200-300).

[0017] Compared to other ranges, limiting the weight ratio of the core to the carbon coating layer in the first, second, and third lithium iron phosphate particles to the above ranges is beneficial to improving the structural stability, compaction density, conductivity, and cycle performance of the modified lithium iron phosphate cathode material.

[0018] To achieve the above objectives, another aspect of this application provides a method for preparing the modified lithium iron phosphate cathode material provided in this application. The method includes: step S1, mixing a lithium source, an iron source, a phosphorus source, a carbon source, a first dispersant, and a first solvent, followed by a first grinding process to obtain a first mixed slurry; wherein the first dispersant is a water-soluble polymeric dispersant containing one or more groups selected from carboxyl, amide, and hydroxyl groups, and the number average molecular weight of the water-soluble polymeric dispersant is 10,000 to 100,000; step S2, mixing the first mixed slurry with a second dispersant, followed by a second grinding process and a first drying process to obtain a mixture; wherein the second dispersant is a nonionic polyether compound; the nonionic polyether compound... The compound has a number average molecular weight of 200–5000; in step S3, the mixture is divided into three parts, denoted as the first precursor, the second precursor, and the third precursor; the first precursor is subjected to a first sintering to obtain a first powder; in step S4, the second precursor is subjected to a second sintering to obtain a second powder; the temperature of the first sintering is higher than that of the second sintering; in step S5, the first powder, the second powder, the third precursor, the additive, and the second solvent are mixed, and then subjected to a third grinding, a second drying, and a third sintering in sequence to obtain a modified lithium iron phosphate cathode material; wherein, the additive is an oxide containing M, M is a dopant element, and the dopant element is selected from one or more elements in Group IVB and Group VB.

[0019] The modified lithium iron phosphate cathode material prepared by the above-mentioned preparation method of this application exhibits a multi-level particle gradation effect, which can effectively improve the compaction density of the modified lithium iron phosphate cathode material and increase the lithium-ion transport rate. When the prepared modified lithium iron phosphate cathode material is used as a cathode active material in lithium-ion batteries, it can effectively improve the energy density, initial coulombic efficiency, electrochemical stability and cycle performance of lithium-ion batteries.

[0020] Further, in step S1, the molar ratio of lithium source, iron source and phosphorus source is (1.0~1.1):(0.95~1.0):(0.95~1.0); preferably, the lithium source is selected from one or more of the group consisting of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate and lithium dihydrogen phosphate; the iron source is selected from one or more of the group consisting of ferrous oxalate, ferrous sulfate, ferrous citrate, iron phosphate and iron oxide; and the phosphorus source is selected from one or more of the group consisting of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate and lithium dihydrogen phosphate.

[0021] Compared to other ranges and types, limiting the molar ratio of lithium, iron and phosphorus sources, as well as the types of lithium, iron and phosphorus sources, to the above ranges is beneficial to improving the electrochemical performance and structural stability of the prepared modified lithium iron phosphate cathode material.

[0022] Furthermore, the carbon source accounts for 5 to 15 wt% of the total weight of the lithium, iron, and phosphorus sources; preferably, the carbon source is selected from one or more of the group consisting of glucose, sucrose, starch, citric acid, and polyethylene glycol.

[0023] Compared to other ranges and types, limiting the amount and type of carbon source to the above range is beneficial to improving the conductivity and structural stability of the prepared modified lithium iron phosphate cathode material, thereby improving the cycle stability of lithium-ion batteries.

[0024] Further, the first dispersant accounts for 1 to 5 wt% of the total weight of the lithium source, iron source and phosphorus source; preferably, the first dispersant is selected from one or more of the group consisting of sodium polyacrylate, polyacrylamide and polyvinyl alcohol.

[0025] Compared to other ranges and types, limiting the amount and type of the first dispersant to the above range is beneficial to reducing the surface energy between the raw material particles during the first grinding process, inhibiting particle agglomeration, and thus improving the dispersion stability and dispersion uniformity of each component material in the first mixed slurry.

[0026] Further, the weight ratio of the first solvent to the total weight of the lithium source, iron source and phosphorus source is (0.5 to 3.0):1; preferably, the first solvent is selected from water and / or ethanol.

[0027] Compared to other ranges and types, limiting the amount and type of the first solvent to the above range is beneficial to improving the dispersion uniformity of each component material and improving the efficiency of the first grinding.

[0028] Further, in step S1, the first grinding speed is 800-2000 rpm, and the time is 1-5 hours; preferably, the D of the first mixed slurry... 50 The range is 0.5–1.5 μm.

[0029] Compared to other ranges, the rotational speed and time of the first grinding, and the D of the first mixed slurry... 50 Limiting the particle size to the above range is beneficial for obtaining a first mixed slurry with a more suitable particle size, which in turn is beneficial for obtaining a modified lithium iron phosphate cathode material with better gradation effect in the subsequent preparation process.

[0030] Further, in step S2, the second dispersant accounts for 1 to 5 wt% of the total weight of the lithium source, iron source and phosphorus source; preferably, the second dispersant is selected from one or more of the group consisting of polyoxyethylene ether, polyethylene glycol and polyvinylpyrrolidone.

[0031] Compared to other ranges and types, limiting the amount and type of the second dispersant to the above range is beneficial to reducing the viscosity of the first mixed slurry, inhibiting its agglomeration, improving its fluidity, and increasing the efficiency of the second grinding, thereby facilitating the acquisition of a stable and uniformly dispersed mixture.

[0032] Further, in step S2, the second grinding speed is 500-1000 rpm, and the time is 1-4 hours; preferably, the D of the mixture... 50 The thickness is 0.2–0.8 μm; preferably, the temperature of the first drying is 80–120 °C, and the time is 2–8 h.

[0033] Compared to other ranges, the first grinding speed, time, and D of the mixture... 50 Limiting the temperature and time of the first drying process to the above range is beneficial for obtaining a mixture with a more suitable particle size, which in turn is beneficial for obtaining a modified lithium iron phosphate cathode material with better gradation in the subsequent preparation process.

[0034] Further, in step S3, the weight ratio of the first precursor, the second precursor and the third precursor is (5-7):(1-3):(1-5).

[0035] Compared to other ranges, limiting the weight ratio of the first precursor, the second precursor, and the third precursor to the above range is beneficial for obtaining modified lithium iron phosphate cathode materials with better gradation effects.

[0036] Furthermore, the temperature of the first sintering is 730–780℃, the heating rate is 3–10℃ / min, and the time is 10–15h.

[0037] Compared to other ranges, limiting the temperature, heating rate, and time of the first sintering to the above range is beneficial to promoting the crystal growth of the first precursor, thereby making it easier to obtain a first powder with a more suitable particle size and a more stable structure.

[0038] Furthermore, in step S4, the temperature of the second sintering is 650–730°C, the heating rate is 3–10°C / min, and the time is 8–15 h.

[0039] Compared to other ranges, limiting the temperature, heating rate, and time of the second sintering to the above range is beneficial to promoting the crystal growth of the second precursor, thereby making it easier to obtain a second powder with a more suitable particle size and a more stable structure.

[0040] Further, in step S5, the weight ratio of the first powder, the second powder and the third precursor is (3-6):(1-3):(1-3).

[0041] Compared to other ranges, limiting the weight ratio of the first powder, the second powder, and the third precursor to the above range is beneficial for obtaining modified lithium iron phosphate cathode materials with better gradation effects.

[0042] Furthermore, the additive accounts for 0.5 to 5 wt% of the total weight of the lithium source, iron source, and phosphorus source; preferably, the doping element is selected from one or more of the group consisting of Ti, V, Zr, and Nb; preferably, the additive is selected from one or more of the group consisting of titanium dioxide, vanadium pentoxide, zirconium oxide, and niobium oxide.

[0043] Compared to other ranges and types, limiting the amount and type of additives to the above range is beneficial for introducing a more suitable amount of dopant elements, which is beneficial for improving the electronic structure of the prepared modified lithium iron phosphate cathode material, and thus beneficial for improving the electrochemical performance and cycle performance of lithium-ion batteries.

[0044] Further, the weight ratio of the second solvent to the total weight of the first powder, the second powder, the third precursor, and the additive is (0.5 to 2):1; preferably, the second solvent is selected from water and / or ethanol.

[0045] Compared to other ranges and types, limiting the amount and type of the second solvent to the above range is beneficial to improving the dispersibility and stability of each component material during the third grinding process.

[0046] Further, in step S5, the rotation speed of the third grinding is 800-2000 rpm, and the time is 1-5 hours; preferably, the D of the second mixed slurry obtained by the third grinding is... 50 The size ranges from 0.5 to 2 μm.

[0047] Compared to other ranges, the rotation speed and time of the third grinding and the D of the second mixed slurry are compared. 50 Limiting the particle size distribution to the above range helps to form a wider particle size distribution, which makes it easier to obtain modified lithium iron phosphate cathode materials with better gradation effect in the future.

[0048] Furthermore, the second drying temperature is 80–120℃, and the time is 3–6 h; the third sintering temperature is 700–760℃, the heating rate is 3–10℃ / min, and the time is 10–15 h.

[0049] Compared to other ranges, limiting the temperature and time of the second drying, and the temperature, heating rate and time of the third sintering to the above ranges is beneficial to promoting the doping of the dopant element M in the lithium iron phosphate lattice. It is also beneficial to control the crystal growth of the first powder, the second powder and the third precursor, and improve the gradation effect.

[0050] Another aspect of this application provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one side of its surface. The positive active material layer includes the modified lithium iron phosphate positive electrode material provided in this application.

[0051] The modified lithium iron phosphate cathode material provided in this application has both high compaction density and high lithium-ion transport rate. When used as a cathode active material in lithium-ion batteries, it can effectively improve the energy density, initial coulombic efficiency, electrochemical stability and cycle stability of lithium-ion batteries. Attached Figure Description

[0052] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0053] Figure 1 TEM images of the modified lithium iron phosphate cathode material prepared in Example 1 of this application are shown.

[0054] Figure 2 The SEM image of the modified lithium iron phosphate cathode material prepared in Example 1 of this application is shown.

[0055] Figure 3 The EDS diagram of the modified lithium iron phosphate cathode material prepared in Example 1 of this application is shown; wherein, Figure 3 Figure a shows the distribution of C element in the modified lithium iron phosphate cathode material prepared in Example 1; Figure b shows the distribution of O element in the modified lithium iron phosphate cathode material prepared in Example 1; Figure c shows the distribution of P element in the modified lithium iron phosphate cathode material prepared in Example 1; Figure d shows the distribution of Ti element in the modified lithium iron phosphate cathode material prepared in Example 1; and Figure e shows the distribution of Fe element in the modified lithium iron phosphate cathode material prepared in Example 1. Detailed Implementation

[0056] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0057] As described in the background section, existing lithium iron phosphate cathode materials suffer from low compaction density and low lithium-ion transport rate, resulting in low energy density, poor electrochemical stability, and poor cycle performance in lithium-ion batteries. To address these technical problems, this application provides a modified lithium iron phosphate cathode material, which is a secondary particle formed by primary particle stacking; the primary particles include a first lithium iron phosphate particle, a second lithium iron phosphate particle, and a third lithium iron phosphate particle; the first lithium iron phosphate particle, the second lithium iron phosphate particle, and the third lithium iron phosphate particle have a D... 50 They are denoted as d1, d2, and d3 respectively; where d3 < d2 < d1, and d1 ≤ 2.5 μm; the first lithium iron phosphate particle consists of a first core and a first carbon coating layer from the inside out; the second lithium iron phosphate particle consists of a second core and a second carbon coating layer from the inside out; the third lithium iron phosphate particle consists of a third core and a third carbon coating layer from the inside out; the materials of the first core, the second core, and the third core are the same, and all have the general chemical formula shown in formula (I), LiFe x M 1- x PO4 (I), where 0.980≤x≤0.995, M is a dopant element selected from one or more elements in Group IVB and Group VB.

[0058] The modified lithium iron phosphate cathode material provided in this application is a secondary particle formed by the stacking of three primary particles with different particle sizes. The large-diameter first lithium iron phosphate particles can provide solid structural support, the medium-diameter second lithium iron phosphate particles can fill the gaps between the first lithium iron phosphate particles, and the small-diameter third lithium iron phosphate particles can further fill the fine gaps, thereby effectively improving the compaction density of the modified lithium iron phosphate cathode material, and thus improving the energy density of lithium-ion batteries.

[0059] The first, second, and third lithium iron phosphate particles each have a core and a carbon coating layer covering the surface of their cores. The presence of the first, second, and third carbon coating layers can construct a continuous conductive network, improve the conductivity of the modified lithium iron phosphate cathode material, and also improve the structural stability of the modified lithium iron phosphate cathode material, thereby effectively improving the cycle performance of lithium-ion batteries.

[0060] The materials of the first core, the second core, and the third core are the same, and all have the specific chemical formula shown in formula (I). Compared with other types, introducing the doping elements of the above-mentioned specific type into the lattice of lithium iron phosphate can improve the electronic structure of the modified lithium iron phosphate cathode material, optimize the diffusion path of lithium ions, thereby increasing the lithium ion transport rate, and thus improving the first coulombic efficiency and cycle performance of lithium-ion batteries.

[0061] In summary, the modified lithium iron phosphate cathode material provided in this application has both high compaction density and high lithium-ion transport rate. When used as a cathode active material in lithium-ion batteries, it can effectively improve the energy density, first coulombic efficiency (hereinafter referred to as "first efficiency"), electrochemical stability and cycle performance of lithium-ion batteries.

[0062] In a preferred embodiment, based on the volume percentage of the modified lithium iron phosphate cathode material, the first lithium iron phosphate particles have a volume percentage of 40-60 vol%, the second lithium iron phosphate particles have a volume percentage of 20-30 vol%, and the third lithium iron phosphate particles have a volume percentage of 10-30 vol. The volume percentages of the first, second, and third lithium iron phosphate particles include, but are not limited to, the above ranges. Limiting them to these ranges is beneficial for improving the structural stability of the modified lithium iron phosphate cathode material, and also for increasing the compaction density of the modified lithium iron phosphate cathode material, thereby improving the energy density and cycle stability of the lithium-ion battery.

[0063] The modified lithium iron phosphate cathode material provided in this application has a high compaction density. In a preferred embodiment, the compaction density of the modified lithium iron phosphate cathode material is 2.5–2.7 g / cm³. 3 Compared to other ranges, limiting the compaction density of modified lithium iron phosphate cathode materials to the above range is beneficial to improving the structural stability of modified lithium iron phosphate cathode materials, thereby improving the energy density and cycle stability of lithium-ion batteries.

[0064] To further improve the structural stability of the modified lithium iron phosphate cathode material, thereby further improving the energy density and cycle stability of lithium-ion batteries, the preferred compaction density of the modified lithium iron phosphate cathode material is 2.6–2.7 g / cm³. 3 .

[0065] In order to further improve the conductivity and structural stability of the modified lithium iron phosphate cathode material, thereby further improving the cycle performance of lithium-ion batteries, in a preferred embodiment, the carbon content in the modified lithium iron phosphate cathode material is 1.3 to 1.5 wt%.

[0066] To further improve the gradation effect of the first, second, and third lithium iron phosphate particles, further increase the lithium-ion transport rate, and thus further improve the compaction density of the modified lithium iron phosphate cathode material, thereby further improving the energy density, first-efficiency performance, and cycle performance of the lithium-ion battery, in a preferred embodiment, the modified lithium iron phosphate cathode material has the following properties: 0.5 ≤ d1-d2 ≤ 1.9; 0.2 ≤ d2-d3 ≤ 0.9.

[0067] In a preferred embodiment, the first lithium iron phosphate particle has a d1 of 1.5–2.5 μm, the second lithium iron phosphate particle has a d2 of 0.6–1 μm, and the third lithium iron phosphate particle has a d3 of 0.1–0.4 μm. Compared to other ranges, limiting the average particle size of the first, second, and third lithium iron phosphate particles to the above ranges is beneficial for improving the gradation effect of the modified lithium iron phosphate cathode material, improving the lithium ion diffusion path, increasing the lithium ion transport rate, and also increasing the compaction density of the modified lithium iron phosphate cathode material, thereby improving the energy density, first-efficiency performance, and cycle performance of the lithium-ion battery.

[0068] In a preferred embodiment, the dopant element accounts for 0.1 to 3 wt% of the weight of the modified lithium iron phosphate cathode material. Compared to other ranges, limiting the dopant element content to the above range is beneficial to improving the electronic structure of the modified lithium iron phosphate cathode material, optimizing the lithium ion diffusion path, thereby improving the lithium ion transport efficiency, and further improving the first coulombic efficiency and cycle performance of the lithium-ion battery.

[0069] In order to further improve the electronic structure of modified lithium iron phosphate cathode material and optimize the diffusion path of lithium ions, thereby further improving the lithium ion transport efficiency and the first coulombic efficiency and cycle performance of lithium-ion batteries, in a preferred embodiment, the doping elements include, but are not limited to, one or more of the group consisting of Ti, V, Zr and Nb.

[0070] In a preferred embodiment, the weight ratio of the first core to the first carbon coating layer in the first lithium iron phosphate particle is (1-5):(50-100). This weight ratio includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the electrochemical activity and conductivity of the first lithium iron phosphate particle, as well as its structural stability, thereby improving the compaction density, conductivity, and cycle performance of the modified lithium iron phosphate cathode material.

[0071] In order to further improve the structural stability of the second lithium iron phosphate particles, and at the same time further improve the electrochemical activity and conductivity of the second lithium iron phosphate particles, thereby further improving the compaction density, conductivity and cycle performance of the modified lithium iron phosphate cathode material, in a preferred embodiment, the weight ratio of the second core to the second carbon coating layer in the second lithium iron phosphate particles is (1-5):(100-200).

[0072] In a preferred embodiment, the weight ratio of the third core to the third carbon coating layer in the third lithium iron phosphate particle is (1-5):(200-300). This weight ratio includes, but is not limited to, the range described above. Limiting it to this range is beneficial for improving the electrochemical activity and conductivity of the third lithium iron phosphate particle, as well as its structural stability, thereby improving the compaction density, conductivity, and cycle performance of the modified lithium iron phosphate cathode material.

[0073] The second aspect of this application also provides a method for preparing the modified lithium iron phosphate cathode material provided in this application. The method includes: step S1, mixing a lithium source, an iron source, a phosphorus source, a carbon source, a first dispersant, and a first solvent, followed by a first grinding process to obtain a first mixed slurry; wherein the first dispersant is a water-soluble polymeric dispersant; the water-soluble polymeric dispersant contains one or more groups selected from carboxyl, amide, and hydroxyl groups, and the number average molecular weight of the water-soluble polymeric dispersant is 10,000 to 100,000; step S2, mixing the first mixed slurry with a second dispersant, followed by a second grinding process and a first drying process to obtain a mixture; wherein the second dispersant is a nonionic polyether compound; the nonionic polyether compound... The number average molecular weight of the compound is 200-5000; Step S3, the mixture is divided into three parts, denoted as the first precursor, the second precursor and the third precursor respectively; the first precursor is subjected to a first sintering to obtain the first powder; Step S4, the second precursor is subjected to a second sintering to obtain the second powder; the temperature of the first sintering is higher than the temperature of the second sintering; Step S5, the first powder, the second powder, the third precursor, the additive and the second solvent are mixed, and then subjected to a third grinding, a second drying and a third sintering in sequence to obtain the modified lithium iron phosphate cathode material; wherein, the additive is an oxide containing M, M is a dopant element, and the dopant element is selected from one or more elements in Group IVB and Group VB.

[0074] In step S1 above, the introduction of the first dispersant of the specific type can reduce the surface energy between the raw material particles and inhibit particle agglomeration, thereby improving the dispersion stability and dispersion uniformity of each component material in the first mixed slurry. At the same time, it can improve the efficiency of the first grinding, making it easier to obtain a first mixed slurry with a suitable particle size, which is beneficial for forming a modified lithium iron phosphate cathode material with high compaction density in the subsequent preparation process.

[0075] In step S2, the introduction of the aforementioned specific type of second dispersant can reduce the viscosity of the first mixed slurry, improve its fluidity, and simultaneously increase the efficiency of the second grinding, inhibiting the agglomeration of the first mixed slurry, thereby facilitating the acquisition of a stable and uniformly dispersed mixture. The first and second grinding processes enable a more concentrated particle size distribution in the resulting mixture, improving its stability and consistency, and facilitating subsequent preparation steps.

[0076] In step S3, the mixture obtained in step S2 is divided into three parts, which are respectively referred to as the first precursor, the second precursor and the third precursor. The first precursor is subjected to a first sintering, which can improve the crystallinity of the first precursor and promote the formation of the first carbon coating layer, so as to obtain the first powder with a suitable particle size.

[0077] In step S4, the second precursor is subjected to a second sintering, which improves the crystallinity of the second precursor and promotes the formation of the second carbon coating layer, resulting in a second powder with a suitable particle size. Limiting the temperatures of the first and second sinterings within the aforementioned specific range promotes the formation of first and second lithium iron phosphate particles with different particle sizes.

[0078] In step S5, the mixture of the first powder, the second powder, the third precursor, the additive containing the specific dopant element M, and the second solvent is subjected to a third grinding process. This further optimizes the particle size distribution of the primary particles in the modified lithium iron phosphate cathode material, thereby increasing the compaction density of the obtained modified lithium iron phosphate cathode material. During the third sintering process, on the one hand, it promotes the doping of the dopant element M into the lithium iron phosphate lattice, improving the electronic structure of the obtained modified lithium iron phosphate cathode material and optimizing the lithium-ion diffusion path. On the other hand, it regulates the crystal growth of the first powder, the second powder, and the third precursor, promoting the formation of first, second, and third lithium iron phosphate particles with different particle sizes, thus improving the gradation effect.

[0079] In summary, the modified lithium iron phosphate cathode material prepared by the above-described preparation method of this application exhibits a multi-level particle gradation effect, which can effectively improve the compaction density of the modified lithium iron phosphate cathode material and increase the lithium-ion transport rate. When the prepared modified lithium iron phosphate cathode material is used as a cathode active material in lithium-ion batteries, it can effectively improve the energy density, initial coulombic efficiency, electrochemical stability and cycle performance of lithium-ion batteries.

[0080] In order to improve the electrochemical performance and structural stability of the prepared modified lithium iron phosphate cathode material, in a preferred embodiment, the molar ratio of lithium source, iron source and phosphorus source in step S1 is (1.0~1.1):(0.95~1.0):(0.95~1.0).

[0081] The lithium source used in this application may be any type commonly used in the art. In a preferred embodiment, the lithium source includes, but is not limited to, one or more of the group consisting of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, and lithium dihydrogen phosphate. The aforementioned types of lithium sources are readily available and inexpensive, which helps reduce production costs and improves the electrochemical performance of the resulting modified lithium iron phosphate cathode material.

[0082] The iron source used in this application may be any type commonly used in the art. In a preferred embodiment, the iron source includes, but is not limited to, one or more of the group consisting of ferrous oxalate, ferrous sulfate, ferrous citrate, ferric phosphate, and ferric oxide. These types of iron sources are readily available and inexpensive, and also contribute to improving the electrochemical performance of the resulting modified lithium iron phosphate cathode material.

[0083] The phosphorus source used in this application may be any type commonly used in the art. In a preferred embodiment, the phosphorus source includes, but is not limited to, one or more of the group consisting of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and lithium dihydrogen phosphate. The aforementioned phosphorus sources are readily available and inexpensive, and are also beneficial for improving the electrochemical performance of the resulting modified lithium iron phosphate cathode material.

[0084] In a preferred embodiment, the carbon source accounts for 5-15 wt% of the total weight of the lithium, iron, and phosphorus sources. The amount of carbon source used includes, but is not limited to, the above range. Limiting it within this range facilitates the formation of a first, second, and third carbon coating layer with more suitable thickness, which is beneficial for constructing a continuous conductive network. This, in turn, improves the conductivity of the modified lithium iron phosphate cathode material and enhances its structural stability, thereby improving the cycle stability of the lithium-ion battery.

[0085] In order to further improve the conductivity and structural stability of the modified lithium iron phosphate cathode material, thereby further improving the cycle performance of the lithium-ion battery, in a preferred embodiment, the carbon source includes, but is not limited to, one or more of the group consisting of glucose, sucrose, starch, citric acid and polyethylene glycol.

[0086] In a preferred embodiment, the first dispersant comprises 1 to 5 wt% of the total weight of the lithium source, iron source, and phosphorus source. The amount of the first dispersant includes, but is not limited to, the above range. Limiting it to the above range is beneficial to reduce the surface energy between the raw material particles during the first grinding process, inhibit particle agglomeration, and thus improve the dispersion stability and dispersion uniformity of the components in the first mixed slurry.

[0087] In order to further suppress the agglomeration of raw material particles during the first grinding process, thereby further improving the dispersion stability and dispersion uniformity of each component material in the first mixed slurry, in a preferred embodiment, the first dispersant includes, but is not limited to, one or more of the group consisting of sodium polyacrylate, polyacrylamide and polyvinyl alcohol.

[0088] In order to improve the dispersion uniformity of each component material and further improve the efficiency of the first grinding, in a preferred embodiment, the weight ratio of the first solvent to the total weight of the lithium source, iron source and phosphorus source is (0.5 to 3.0):1; preferably, the first solvent is selected from water and / or ethanol.

[0089] In a preferred embodiment, in step S1, the rotation speed of the first grinding is 800–2000 rpm, and the time is 1–5 h. The rotation speed and time of the first grinding include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial to improving the efficiency of the first grinding, thereby facilitating the acquisition of a first mixed slurry with a more suitable particle size.

[0090] In order to improve the efficiency of the first grinding and thus obtain a first mixed slurry with a more suitable particle size, in a preferred embodiment, a first grinding medium is used for the first grinding; the particle size of the first grinding medium is 0.4 to 1 mm.

[0091] In a preferred embodiment, the D of the first mixed slurry 50 The diameter is 0.5–1.5 μm. The D of the first mixed slurry... 50 Including but not limited to the above-mentioned range, limiting it to the above-mentioned range is beneficial to improving processing performance, increasing the efficiency of subsequent preparation processes, and facilitating the acquisition of modified lithium iron phosphate cathode materials with better gradation effects.

[0092] In a preferred embodiment, in step S2, the second dispersant accounts for 1 to 5 wt% of the total weight of the lithium source, iron source, and phosphorus source. The amount of the second dispersant includes, but is not limited to, the above range. Limiting it to the above range is beneficial to reduce the viscosity of the first mixed slurry, improve the efficiency of the second grinding, and suppress the agglomeration of the first mixed slurry, thereby facilitating the acquisition of a stable and uniformly dispersed mixture.

[0093] To further improve dispersion stability and flowability, in a preferred embodiment, the second dispersant includes, but is not limited to, one or more of the group consisting of polyoxyethylene ether, polyethylene glycol, and polyvinylpyrrolidone.

[0094] In a preferred embodiment, in step S2, the rotation speed of the second grinding is 500–1000 rpm, and the time is 1–4 hours. The rotation speed and time of the second grinding include, but are not limited to, the above-mentioned ranges. Limiting them to the above-mentioned ranges is beneficial to improving the efficiency of the second grinding, thereby facilitating the acquisition of a mixture with a more suitable particle size.

[0095] In order to improve the efficiency of the second grinding and thus obtain a mixture with a more suitable particle size, in a preferred embodiment, a second grinding medium is used for the second grinding; the particle size of the second grinding medium is 0.1 to 0.5 mm.

[0096] In a preferred embodiment, the D of the mixture 50 The micrometer diameter (D) of the mixture is 0.2–0.8 μm. 50 Including but not limited to the above-mentioned range, limiting it to the above-mentioned range is beneficial to improving processing performance, increasing the efficiency of subsequent preparation processes, and facilitating the acquisition of modified lithium iron phosphate cathode materials with better gradation effects.

[0097] In order to remove residual solvents from the mixture and improve its purity, thereby facilitating subsequent preparation processes, in a preferred embodiment, the first drying temperature is 80–120°C and the time is 2–8 hours.

[0098] In a preferred embodiment, in step S3, the weight ratio of the first precursor, the second precursor, and the third precursor is (5-7):(1-3):(1-5). The weight ratio of the first precursor, the second precursor, and the third precursor includes, but is not limited to, the above range. Limiting it to this range is beneficial for obtaining a modified lithium iron phosphate cathode material with better gradation, thereby improving the compaction density of the modified lithium iron phosphate cathode material and consequently improving the energy density of the lithium-ion battery.

[0099] In a preferred embodiment, the first sintering temperature is 730–780°C, the heating rate is 3–10°C / min, and the time is 10–15 h. The temperature, heating rate, and time of the first sintering are not limited to the above ranges. Limiting them to the above ranges is beneficial to improving the efficiency of the first sintering, promoting the crystal growth of the first precursor, and thus facilitating the acquisition of a first powder with a more suitable particle size and a more stable structure.

[0100] In a preferred embodiment, in step S4, the second sintering temperature is 650–730°C, the heating rate is 3–10°C / min, and the time is 8–15 h. The temperature, heating rate, and time of the second sintering include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial to improving the efficiency of the second sintering, promoting the crystal growth of the second precursor, and thus facilitating the acquisition of a second powder with a more suitable particle size and a more stable structure.

[0101] In order to obtain a modified lithium iron phosphate cathode material with better gradation effect, and thereby further improve the compaction density of the modified lithium iron phosphate cathode material, in a preferred embodiment, in step S5, the weight ratio of the first powder, the second powder and the third precursor is (3-6):(1-3):(1-3).

[0102] In a preferred embodiment, the additive accounts for 0.5 to 5 wt% of the total weight of the lithium source, iron source, and phosphorus source. The amount of additive used includes, but is not limited to, the above range. Limiting it to the above range is beneficial for introducing a more suitable amount of dopant elements, thereby improving the electronic structure of the modified lithium iron phosphate cathode material, increasing the lithium-ion transport rate, and thus improving the electrochemical performance and cycle performance of the lithium-ion battery.

[0103] In a preferred embodiment, the doping element includes, but is not limited to, one or more elements from the group consisting of Ti, V, Zr, and Nb. The types of doping elements include, but are not limited to, the range described above. Limiting them to this range is beneficial for improving the electronic structure of the modified lithium iron phosphate cathode material, increasing the lithium-ion transport rate, and thus improving the electrochemical performance and cycle performance of the lithium-ion battery.

[0104] In order to further improve the electronic structure of the modified lithium iron phosphate cathode material, further increase the lithium ion transport rate, and thus further improve the electrochemical performance and cycle performance of the lithium-ion battery, in a preferred embodiment, the additives include, but are not limited to, one or more of the group consisting of titanium dioxide, vanadium pentoxide, zirconium oxide and niobium oxide.

[0105] In order to improve the dispersibility and stability of the components in the third grinding process, in a preferred embodiment, the weight ratio of the second solvent to the total weight of the first powder, the second powder, the third precursor and the additive is (0.5 to 2):1; preferably, the second solvent includes, but is not limited to, water and / or ethanol.

[0106] In a preferred embodiment, in step S5, the rotation speed of the third grinding is 800–2000 rpm, and the time is 1–5 h. The rotation speed and time of the third grinding include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial to improving the efficiency of the third grinding, thereby facilitating the acquisition of a second mixed slurry with a more suitable particle size.

[0107] In order to improve the efficiency of the third grinding and promote the formation of a wider particle size distribution, thereby facilitating the subsequent acquisition of modified lithium iron phosphate cathode materials with better gradation effect, in a preferred embodiment, a third grinding medium and a fourth grinding medium are used for the third grinding; the particle size of the third grinding medium is 0.1-0.3 mm; the particle size of the fourth grinding medium is 0.3-0.5 mm; and the weight ratio of the third grinding medium to the fourth grinding medium is (5-7):(3-5).

[0108] To obtain a modified lithium iron phosphate cathode material with better gradation, in a preferred embodiment, the D of the second mixed slurry obtained by the third grinding... 50 The size ranges from 0.5 to 2 μm.

[0109] In order to remove residual solvent from the second mixed slurry, in a preferred embodiment, the second drying temperature is 80-120°C and the time is 3-6 hours.

[0110] In a preferred embodiment, the third sintering temperature is 700–760°C, the time is 10–15 h, and the heating rate is 3–10°C / min. The temperature, time, and heating rate of the third sintering are not limited to the above ranges. Limiting them to the above ranges is beneficial to improving the efficiency of the third sintering, promoting the doping of element M into the lithium iron phosphate lattice, improving the electronic structure of the obtained modified lithium iron phosphate cathode material, optimizing the lithium ion diffusion path, and also beneficial to controlling the crystal growth of the first powder, the second powder, and the third precursor, promoting the formation of first lithium iron phosphate particles, second lithium iron phosphate particles, and third lithium iron phosphate particles with different particle sizes, and improving the gradation effect.

[0111] A third aspect of this application also provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive and negative electrode. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface thereon, wherein the positive active material layer includes the modified lithium iron phosphate positive electrode material provided in this application. The modified lithium iron phosphate positive electrode material provided in this application has both high compaction density and high lithium-ion transport rate. Using it as a positive active material in lithium-ion batteries can effectively improve the energy density, initial coulombic efficiency, electrochemical stability, and cycle stability of lithium-ion batteries.

[0112] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0113] Example 1

[0114] A method for preparing a modified lithium iron phosphate cathode material specifically includes the following steps:

[0115] (1) 38g lithium carbonate, 183g ferrous oxalate, 117g ammonium dihydrogen phosphate, 44g glucose, 3.4g polyvinyl alcohol (number average molecular weight of 10,000) were mixed with 350g water to obtain a first mixture; wherein, the molar ratio of Li in the lithium source, Fe in the iron source and P in the phosphorus source was 1.02:1:1; the carbon source content was 13wt% and the polyvinyl alcohol content was 1wt% based on the total weight of the lithium source, iron source and phosphorus source; the first mixture was ground for 2h at 25℃ and 1000rpm to obtain D 50 The first mixed slurry has a diameter of 0.8 μm; the zirconium balls used in the first grinding process have a diameter of 0.5 mm and a ball-to-material ratio of 3:1.

[0116] (2) The mixed slurry obtained in step (1) is mixed with 3.4g of polyoxyethylene ether (number average molecular weight of 600) to obtain a second mixture; wherein the polyoxyethylene ether accounts for 1wt% of the total weight of the lithium source, iron source and phosphorus source; the second mixture is subjected to a second grinding at 25℃ and 800rpm for 3h, and then dried at 80℃ for 4h to obtain D. 50 The mixture is 0.3μm thick; the zirconium balls used in the second grinding process have a diameter of 0.3mm and a ball-to-material ratio of 5:1.

[0117] (3) Divide the mixture obtained in step (2) into three parts, which are respectively called the first precursor, the second precursor and the third precursor. The weight ratio of the first precursor, the second precursor and the third precursor is 5:1:4.

[0118] (4) The temperature is increased to 730°C at a heating rate of 5°C / min, and the first precursor is sintered at this temperature for 10 hours. After cooling, the first powder is obtained.

[0119] (5) The temperature is increased to 650°C at a heating rate of 5°C / min, and the second precursor is sintered at this temperature for 8 hours. After cooling, the second powder is obtained.

[0120] (6) Mix the first powder obtained in step (4), the second powder obtained in step (5), the third precursor obtained in step (3), 1.7g of titanium dioxide, and 200g of water to obtain a third mixture; wherein, the percentage of titanium dioxide in the total weight of lithium source, iron source, and phosphorus source is 0.5wt%; grind the third mixture for 2 hours at 25℃ and 800rpm to obtain D. 50The second mixed slurry has a thickness of 0.7 μm. The third grinding process uses two different diameter zirconium balls with diameters of 0.1 mm and 0.5 mm, with a weight ratio of 1:1. The ball-to-material ratio in the third grinding process is 4:1. The second mixture is dried at 100 °C for 3 h, then heated to 730 °C at a heating rate of 5 °C / min, and subjected to a third sintering at this temperature for 15 h. After cooling, the modified lithium iron phosphate cathode material is obtained.

[0121] TEM image of the modified lithium iron phosphate cathode material prepared in Example 1 is shown below. Figure 1 As shown, by Figure 1 It can be seen that the surface of the lithium iron phosphate core particles is coated with carbon. This carbon layer is the third carbon coating layer, and its thickness is about 10 nm.

[0122] The SEM image of the modified lithium iron phosphate cathode material prepared in Example 1 is shown below. Figure 2 As shown, by Figure 2 It can be seen that the particles are tightly packed together and exhibit a tertiary gradation.

[0123] The EDS diagram of the modified lithium iron phosphate cathode material prepared in Example 1 is shown below. Figure 3 As shown, where, Figure 3 Figure a shows the distribution of C element in the modified lithium iron phosphate cathode material; Figure b shows the distribution of O element; Figure c shows the distribution of P element; Figure d shows the distribution of Ti element; and Figure e shows the distribution of Fe element. Figure 3 Figures a, b, c, d, and e show that the modified lithium iron phosphate cathode material contains C, O, P, Ti, and Fe elements, and these elements are evenly distributed.

[0124] Example 2

[0125] A method for preparing a modified lithium iron phosphate cathode material specifically includes the following steps:

[0126] (1) 68.0g lithium acetate, 278.0g ferrous sulfate heptahydrate, 98g phosphoric acid, 44.4g sucrose, 13.3g polyacrylamide (number average molecular weight of 50,000) were mixed with 400g water to obtain a first mixture; wherein, the molar ratio of Li in the lithium source, Fe in the iron source and P in the phosphorus source was 1.03:1:1; the carbon source content was 10wt% and the polyacrylamide content was 3wt% based on the total weight of the lithium source, iron source and phosphorus source; the first mixture was ground for 2h at 25℃ and 1500rpm to obtain D 50The first mixed slurry is 1μm thick; the zirconium balls used in the first grinding process have a diameter of 0.7mm and a ball-to-material ratio of 3:1.

[0127] (2) The mixed slurry obtained in step (1) is mixed with 13.3g of polyvinylpyrrolidone (number average molecular weight of 1000) to obtain a second mixture; wherein, the percentage of polyvinylpyrrolidone in the total weight of lithium source, iron source and phosphorus source is 3wt%; the second mixture is subjected to a second grinding at 25℃ and 1000rpm for 3h, and then dried at 100℃ for 3h to obtain D. 50 The mixture is 0.7μm thick; the zirconium balls used in the second grinding process have a diameter of 0.5mm and a ball-to-material ratio of 5:1.

[0128] (3) Divide the mixture obtained in step (2) into three parts, which are respectively called the first precursor, the second precursor and the third precursor. The weight ratio of the first precursor, the second precursor and the third precursor is 6:2:2.

[0129] (4) The temperature is increased to 750°C at a heating rate of 6°C / min, and the first precursor is sintered at this temperature for 12 hours. After cooling, the first powder is obtained.

[0130] (5) The temperature is increased to 680°C at a heating rate of 6°C / min, and the second precursor is sintered at this temperature for 10 hours. After cooling, the second powder is obtained.

[0131] (6) Mix the first powder obtained in step (4), the second powder obtained in step (5), the third precursor obtained in step (3), 2.2g of vanadium pentoxide, and 250g of water to obtain a third mixture; wherein, the percentage of vanadium pentoxide in the total weight of lithium source, iron source, and phosphorus source is 0.5wt%; grind the third mixture for 3 hours at 25℃ and 1200rpm to obtain D. 50 The second mixed slurry has a thickness of 1.2 μm. The third grinding process uses two different diameter zirconium balls with diameters of 0.1 mm and 0.3 mm, with a weight ratio of 6:4. The ball-to-material ratio in the third grinding process is 4:1. The second mixed slurry is dried at 110 °C for 2 h, then heated to 700 °C at a heating rate of 6 °C / min, and subjected to a third sintering at this temperature for 15 h. After cooling, the modified lithium iron phosphate cathode material is obtained.

[0132] Example 3

[0133] A method for preparing a modified lithium iron phosphate cathode material specifically includes the following steps:

[0134] (1) 108g lithium dihydrogen phosphate, 150g iron phosphate, 18g polyethylene glycol, 13g sodium polyacrylate (number average molecular weight of 100,000) were mixed with 350g water to obtain a first mixture; wherein, the molar ratio of Li in the lithium source, Fe in the iron source and P in the phosphorus source was 1.04:1:1; the carbon source content was 7wt% and the sodium polyacrylate content was 5wt% based on the percentage of the total weight of the lithium source, iron source and phosphorus source; the first mixture was ground for 3h at 25℃ and 1800rpm to obtain D 50 The first mixed slurry has a diameter of 1.3 μm; the zirconium balls used in the first grinding process have a diameter of 1 mm and a ball-to-material ratio of 3:1.

[0135] (2) The mixed slurry obtained in step (1) is mixed with 13g of polyethylene glycol (number average molecular weight of 5000) to obtain a second mixture; wherein, the percentage of polyethylene glycol in the total weight of lithium source, iron source and phosphorus source is 5wt%; the second mixture is subjected to a second grinding at 25℃ and 500rpm for 3h, and dried at 100℃ for 4h to obtain D. 50 The mixture is 0.4μm thick; the zirconium balls used in the second grinding process have a diameter of 0.3mm and a ball-to-material ratio of 5:1.

[0136] (3) Divide the mixture obtained in step (2) into three parts, which are respectively called the first precursor, the second precursor and the third precursor. The weight ratio of the first precursor, the second precursor and the third precursor is 7:1:2.

[0137] (4) The temperature is increased to 770°C at a heating rate of 8°C / min, and the first precursor is sintered at this temperature for 10 hours. After cooling, the first powder is obtained.

[0138] (5) The temperature is increased to 700°C at a heating rate of 8°C / min, and the second precursor is sintered at this temperature for 10 hours. After cooling, the second powder is obtained.

[0139] (6) Mix the first powder obtained in step (4), the second powder obtained in step (5), the third precursor obtained in step (3), 1.3g of zirconium oxide, and 200g of water to obtain a third mixture; wherein, the percentage of zirconium oxide in the total weight of lithium source, iron source, and phosphorus source is 0.5wt%; grind the third mixture for 4 hours at 25℃ and 1500rpm to obtain D. 50The second mixed slurry has a thickness of 1.5 μm. The third grinding process uses two different diameter zirconium balls with diameters of 0.2 mm and 0.5 mm, with a weight ratio of 7:3. The ball-to-material ratio in the third grinding process is 4:1. The second mixed slurry is dried at 90 °C for 4 h, then heated to 750 °C at a heating rate of 8 °C / min, and subjected to a third sintering at this temperature for 13 h. After cooling, the modified lithium iron phosphate cathode material is obtained.

[0140] Example 4

[0141] The difference from Example 1 is that in step (1), the amount of glucose is adjusted to 17g, so that the carbon source accounts for 5wt% of the total weight of the lithium source, iron source and phosphorus source; the remaining steps are the same as in Example 1.

[0142] Example 5

[0143] The difference from Example 1 is that in step (1), the amount of glucose is adjusted to 50.7g, so that the carbon source accounts for 15wt% of the total weight of the lithium source, iron source and phosphorus source; the remaining steps are the same as in Example 1.

[0144] Example 6

[0145] The difference from Example 1 is that in step (1), the amount of glucose is adjusted to 67.6g, so that the carbon source accounts for 20wt% of the total weight of the lithium source, iron source and phosphorus source; the remaining steps are the same as in Example 1.

[0146] Example 7

[0147] The difference from Example 1 is that in step (1), the amount of polyvinyl alcohol is adjusted to 20.3g, so that the percentage of polyvinyl alcohol in the total weight of lithium source, iron source and phosphorus source is 6wt%; the remaining steps are the same as in Example 1.

[0148] Example 8

[0149] The difference from Example 1 is that in step (1), the amount of polyvinyl alcohol is adjusted to 1.7g, so that the percentage of polyvinyl alcohol in the total weight of lithium source, iron source and phosphorus source is 0.5wt%; the remaining steps are the same as in Example 1.

[0150] Example 9

[0151] The difference from Example 1 is that in step (1), the rotation speed of the first grinding is adjusted to 2000 rpm, the time is adjusted to 5 hours, and the diameter of the zirconium balls used in the first grinding is adjusted to 1 mm, so that the D of the first mixed slurry is obtained. 50 The value is 1.5 μm; the remaining steps are the same as in Example 1.

[0152] Example 10

[0153] The difference from Example 1 is that in step (1), the rotation speed of the first grinding is adjusted to 500 rpm, the time is adjusted to 6 hours, and the diameter of the zirconium balls used in the first grinding is adjusted to 1.5 mm, so that the D of the first mixed slurry is obtained. 50 The value is 2μm; the remaining steps are the same as in Example 1.

[0154] Example 11

[0155] The difference from Example 1 is that in step (2), the amount of polyoxyethylene ether is adjusted to 1.7g, so that the percentage of polyoxyethylene ether in the total weight of lithium source, iron source and phosphorus source is 0.5wt%; the remaining steps are the same as in Example 1.

[0156] Example 12

[0157] The difference from Example 1 is that in step (2), the amount of polyoxyethylene ether is adjusted to 27.2g, so that the percentage of polyoxyethylene ether in the total weight of lithium source, iron source and phosphorus source is 8wt%; the remaining steps are the same as in Example 1.

[0158] Example 13

[0159] The difference from Example 1 is that in step (2), the rotation speed of the second grinding is adjusted to 500 rpm, the time is adjusted to 4 hours, and the diameter of the zirconium balls used in the second grinding is adjusted to 0.5 mm, so that the D of the obtained mixture is... 50 The value is 0.8 μm; the remaining steps are the same as in Example 1.

[0160] Example 14

[0161] The difference from Example 1 is that in step (2), the rotation speed of the second grinding is adjusted to 300 rpm, the time is adjusted to 5 hours, and the diameter of the zirconium balls used in the second grinding is adjusted to 1 mm, so that the D of the obtained mixture is... 50 The value is 1 μm; the remaining steps are the same as in Example 1.

[0162] Example 15

[0163] The difference from Example 1 is that in step (4), the temperature is raised to 780°C at a heating rate of 10°C / min and the first sintering is carried out at this temperature for 15 hours. Meanwhile, in step (5), the temperature is raised to 730°C at a heating rate of 10°C / min and the second sintering is carried out at this temperature for 8 hours. The remaining steps are the same as in Example 1.

[0164] Example 16

[0165] The difference from Example 1 is that in step (4), the temperature is increased to 700°C at a heating rate of 5°C / min, and the first sintering is carried out at this temperature for 8 hours; the remaining steps are the same as in Example 1.

[0166] Example 17

[0167] The difference from Example 1 is that in step (5), the temperature is increased to 750°C at a heating rate of 5°C / min, and the second sintering is carried out at this temperature for 6 hours; the remaining steps are the same as in Example 1.

[0168] Example 18

[0169] The difference from Example 1 is that in step (6), the amount of titanium dioxide used is 17g, and the percentage of titanium dioxide in the total weight of lithium source, iron source and phosphorus source is 5wt%; the remaining steps are the same as in Example 1.

[0170] Example 19

[0171] The difference from Example 1 is that in step (6), the amount of titanium dioxide used is 0.68g, and the percentage of titanium dioxide in the total weight of lithium source, iron source and phosphorus source is 0.2wt%; the remaining steps are the same as in Example 1.

[0172] Example 20

[0173] The difference from Example 1 is that in step (3), the weight ratio of the first precursor, the second precursor and the third precursor is adjusted to 7:3:5; the remaining steps are the same as in Example 1.

[0174] Example 21

[0175] The difference from Example 1 is that in step (3), the weight ratio of the first precursor, the second precursor and the third precursor is adjusted to 3:1:1; the remaining steps are the same as in Example 1.

[0176] Example 22

[0177] The difference from Example 1 is that in step (6), the temperature is increased to 760°C at a heating rate of 10°C / min, and the third sintering is carried out at this temperature for 10 hours; the remaining steps are the same as in Example 1.

[0178] Example 23

[0179] The difference from Example 1 is that in step (6), the temperature is increased to 800°C at a heating rate of 5°C / min, and the third sintering is carried out at this temperature for 8 hours; the remaining steps are the same as in Example 1.

[0180] Comparative Example 1

[0181] A method for preparing a modified lithium iron phosphate cathode material specifically includes the following steps:

[0182] (1) Mix 38g lithium carbonate, 180g ferrous oxalate, 115g ammonium dihydrogen phosphate, 44g glucose and 350g water to obtain a mixture; wherein the molar ratio of Li in the lithium source, Fe in the iron source and P in the phosphorus source is 1.02:1:1; the carbon source accounts for 13 wt% of the total weight of the lithium source, iron source and phosphorus source;

[0183] (2) Grind the mixture obtained in step (1) at 25℃ and 1000 rpm for 3 hours until D is reached. 50 The diameter of the zirconium balls used in the grinding process was 0.3 μm, resulting in a mixed slurry. The diameter of the zirconium balls used in the grinding process was 0.5 mm, and the ball-to-material ratio was 5:1.

[0184] (3) The mixed slurry obtained in step (2) was dried at 80°C for 5 hours, and then heated to 730°C at a heating rate of 5°C / min. It was then sintered at this temperature for 10 hours. After cooling, the modified lithium iron phosphate cathode material was obtained.

[0185] Comparative Example 2

[0186] The difference from Example 1 is that the first dispersant sodium polyacrylate was not introduced in step (1); the remaining steps are the same as in Example 1.

[0187] Comparative Example 3

[0188] The difference from Example 1 is that in step (2), the second dispersant polyoxyethylene ether was not introduced; the remaining steps are the same as in Example 1.

[0189] Comparative Example 4

[0190] The difference from Example 1 is that in step (6), titanium dioxide was not introduced as an additive; the rest of the steps are the same as in Example 1.

[0191] The modified lithium iron phosphate cathode material prepared in Comparative Example 4 did not introduce any doping elements.

[0192] The average particle size and volume fraction of the first lithium iron phosphate particles, the second lithium iron phosphate particles, and the third lithium iron phosphate particles, as well as the chemical formulas of the materials of the first core, the second core, and the third core in the modified lithium iron phosphate cathode materials prepared in Examples 1 to 23 and Comparative Examples 1 to 4, are shown in Table 1.

[0193] Table 1

[0194]

[0195] The modified lithium iron phosphate cathode materials prepared in all the above embodiments and comparative examples of this application were tested as follows:

[0196] (1) The carbon content and compaction density were measured using the method specified in the national standard GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries";

[0197] (2) The content of doped elements was determined by the method specified in the national standard GB / T 33822-2017 "Nano Lithium Iron Phosphate".

[0198] The test results are shown in Table 2.

[0199] Table 2

[0200]

[0201] Application Examples

[0202] The lithium-ion battery was assembled according to the following method: (1) Preparation of positive electrode sheet: The positive electrode active material (the modified lithium iron phosphate positive electrode material prepared in Examples 1 to 23 and Comparative Examples 1 to 4 of this application were used as the positive electrode active material), conductive carbon black (SP), polyvinylidene fluoride difluoride (PVDF) binder and N-methylpyrrolidone (NMP) were mixed evenly in a weight ratio of 1:0.2:0.2:1.5 to obtain a positive electrode slurry; the positive electrode slurry was coated on the surface of a 3.8 μm thick aluminum foil, and then dried at 80°C for 12 h to obtain a positive electrode sheet with a thickness of 153.8 μm; (2) Lithium sheet was used as negative electrode sheet; (3) Lithium-ion battery assembly: The assembly was carried out in a glove box filled with argon gas, using the positive electrode sheet prepared above as the positive electrode and the lithium sheet as the negative electrode; 1 mol / L LiPF6 (using ethylene carbonate and diethyl carbonate in a weight ratio of 1:1) as the electrolyte; a Celgard 2400 membrane as the separator; and a lithium-ion battery was assembled.

[0203] The lithium-ion batteries assembled in all the above embodiments and comparative examples were subjected to charge-discharge tests and cycle performance tests. The specific test conditions are as follows: (1) Charge-discharge tests were conducted at 25°C, 1C rate, and a voltage range of 2 to 4V. The specific capacity of the lithium-ion battery at 25°C, the coulombic efficiency of the first cycle (first efficiency), and the specific capacity of the 1C discharge were measured. (2) 500-cycle tests were conducted at 45°C, 1C rate, and a voltage range of 2 to 4V. The capacity retention rate after 500 cycles was recorded.

[0204] The test results are shown in Table 3.

[0205] Table 3

[0206]

[0207] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0208] Comparing Example 1 and Comparative Example 1, it can be seen that, compared with the ungraded lithium iron phosphate material, the modified lithium iron phosphate cathode material provided in this application is a secondary particle formed by the stacking of three primary particles with different particle sizes. The large-diameter first lithium iron phosphate particles can provide solid structural support, the medium-diameter second lithium iron phosphate particles can fill the gaps between the first lithium iron phosphate particles, and the small-diameter third lithium iron phosphate particles can further fill the small gaps, thereby effectively improving the compaction density of the modified lithium iron phosphate cathode material, and thus improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0209] Comparing Example 1 with Comparative Examples 2 and 3, it can be seen that introducing the first dispersant and the second dispersant during the preparation of modified lithium iron phosphate cathode material can effectively improve the dispersion stability and dispersion uniformity of each component material, thereby facilitating the obtaining of modified lithium iron phosphate cathode material with better gradation effect in subsequent preparation processes, and thus improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0210] Comparing Example 1 and Comparative Example 4, it can be seen that, compared with lithium iron phosphate materials without doping elements, introducing the above-mentioned specific types of doping elements into the lattice of lithium iron phosphate can improve the electronic structure of the modified lithium iron phosphate cathode material, optimize the diffusion path of lithium ions, thereby increasing the lithium ion transport rate, and thus improving the first coulombic efficiency and cycle performance of lithium-ion batteries.

[0211] Comparing Examples 1 to 6, it can be seen that, compared with other ranges, limiting the amount of carbon source to the preferred range described above in this application is beneficial to forming a first carbon coating layer, a second carbon coating layer, and a third carbon coating layer with more suitable thickness, which is beneficial to constructing a continuous conductive network, thereby improving the conductivity of the modified lithium iron phosphate cathode material. At the same time, it is also beneficial to improve the structural stability of the modified lithium iron phosphate cathode material, which in turn is beneficial to improving the energy density, rate performance, and cycle performance of lithium-ion batteries.

[0212] Comparing Examples 1 to 3, Examples 7 and 8, it can be seen that, compared with other ranges, limiting the amount of the first dispersant within the above range is beneficial to reducing the surface energy between the raw material particles and inhibiting particle agglomeration. This is beneficial to improving the dispersion stability and dispersion uniformity of each component material in the first mixed slurry, which is beneficial to forming a modified lithium iron phosphate cathode material with high compaction density in the subsequent preparation process, and thus beneficial to improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0213] Comparing Examples 1 to 3, Examples 9 and 10, it can be seen that, compared to other ranges, the rotational speed and time of the first grinding, as well as the D of the first mixed slurry, are significantly improved. 50 Limiting the particle size to the above range is beneficial for obtaining a first mixed slurry with a more suitable particle size, improving processing performance, and thus beneficial for obtaining modified lithium iron phosphate cathode materials with better gradation effect, which in turn is beneficial for improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0214] Comparing Examples 1 to 3 and Examples 11 and 12, it can be seen that, compared with other ranges, limiting the amount of the second dispersant within the above range is beneficial to reducing the viscosity of the first mixed slurry, inhibiting the agglomeration of the first mixed slurry, improving fluidity, and obtaining a stable and uniformly dispersed mixture. This is beneficial to obtaining a modified lithium iron phosphate cathode material with better gradation effect in the subsequent preparation process, and thus beneficial to improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0215] Comparing Examples 1 to 3, Examples 13 and 14, it can be seen that, compared to other ranges, the rotational speed and time of the second grinding, as well as the D of the mixture, are significantly improved. 50 Limiting the particle size to the above range is beneficial for obtaining a mixture with a more suitable particle size, improving processing performance, and thus for obtaining a modified lithium iron phosphate cathode material with better gradation effect, which in turn is beneficial for improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0216] Comparing Examples 1 to 3 and Examples 15 to 17, it can be seen that, compared with other ranges, limiting the temperature, heating rate and time of the first sintering and the second sintering to the above range is beneficial to promoting the crystal growth of the first precursor and the second precursor, thereby facilitating the obtaining of modified lithium iron phosphate cathode materials with better gradation effect in subsequent preparation processes, and thus improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0217] Comparing Examples 1 to 3, Examples 18 and 19, it can be seen that limiting the amount of additives to the above range, compared with other ranges, is beneficial to introduce a more suitable amount of dopant elements, thereby improving the electronic structure of the prepared modified lithium iron phosphate cathode material, increasing the lithium ion transport rate, and thus improving the electrochemical performance and cycle performance of lithium-ion batteries.

[0218] Comparing Examples 1 to 3, Examples 20 and 21, it can be seen that, compared with other ranges, limiting the weight ratio of the first precursor, the second precursor and the third precursor to the above range is beneficial to obtaining a modified lithium iron phosphate cathode material with better gradation effect, thereby improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0219] Comparing Examples 1 to 3, Examples 22 and 23, it can be seen that, compared with other ranges, limiting the temperature, time and heating rate of the third sintering to the above range is beneficial in two ways: firstly, it promotes the doping of the dopant element M in the lattice of lithium iron phosphate, improves the electronic structure of the prepared modified lithium iron phosphate cathode material, and optimizes the diffusion path of lithium ions; secondly, it is beneficial in controlling the crystal growth of the first powder, the second powder and the third precursor, and improving the gradation effect of the prepared modified lithium iron phosphate cathode material, thereby improving the energy density, rate performance and cycle performance of lithium-ion batteries.

[0220] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0221] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A modified lithium iron phosphate cathode material, characterized in that, The modified lithium iron phosphate cathode material is a secondary particle formed by primary particle stacking; the primary particles include a first lithium iron phosphate particle, a second lithium iron phosphate particle, and a third lithium iron phosphate particle; the D of the first lithium iron phosphate particle, the second lithium iron phosphate particle, and the third lithium iron phosphate particle... 50 They are denoted as d1, d2, and d3, respectively. Wherein, d3 < d2 < d1, and d1 ≤ 2.5 μm; The first lithium iron phosphate particle comprises, from the inside out, a first core and a first carbon coating layer; the second lithium iron phosphate particle comprises, from the inside out, a second core and a second carbon coating layer; the third lithium iron phosphate particle comprises, from the inside out, a third core and a third carbon coating layer; the materials of the first core, the second core, and the third core are the same, and all have the general chemical formula shown in formula (I): LiFe x M 1-x PO4 (I), where 0.980≤x≤0.995, M is a dopant element selected from one or more elements in Group IVB and Group VB.

2. The modified lithium iron phosphate cathode material of claim 1, wherein, Based on the volume percentage of the modified lithium iron phosphate cathode material, the first lithium iron phosphate particles account for 40-60 vol%, the second lithium iron phosphate particles account for 20-30 vol%, and the third lithium iron phosphate particles account for 10-30 vol%; and / or, The compaction density of the modified lithium iron phosphate positive electrode material is 2.5-2.7 g / cm 3 ; and / or, The modified lithium iron phosphate cathode material contains 1.3 to 1.5 wt% carbon.

3. The modified lithium iron phosphate cathode material of claim 1 or 2, wherein, In the modified lithium iron phosphate cathode material, 0.5 ≤ d1 - d2 ≤ 1.9; and / or, 0.2 ≤ d² - d³ ≤ 0.9; and / or, The doping element accounts for 0.1–3 wt% of the weight of the modified lithium iron phosphate cathode material; and / or, The doping element is selected from one or more of the group consisting of Ti, V, Zr and Nb.

4. The modified lithium iron phosphate cathode material of claim 3, wherein, In the modified lithium iron phosphate cathode material, d1 is 1.5–2.5 μm, d2 is 0.6–1 μm, and d3 is 0.1–0.4 μm.

5. The modified lithium iron phosphate cathode material of claim 1 or 2, wherein, In the first lithium iron phosphate particle, the weight ratio of the first core to the first carbon coating layer is (1-5):(50-100); and / or, In the second lithium iron phosphate particle, the weight ratio of the second core to the second carbon coating layer is (1-5):(100-200); and / or, In the third lithium iron phosphate particle, the weight ratio of the third core to the third carbon coating layer is (1-5):(200-300).

6. A method of producing the modified lithium iron phosphate positive electrode material according to any one of claims 1 to 5, characterized by, The preparation method includes: Step S1: The lithium source, iron source, phosphorus source, carbon source, first dispersant and first solvent are mixed and then ground to obtain a first mixed slurry; wherein, the first dispersant is a water-soluble polymeric dispersant containing one or more groups selected from carboxyl, amide and hydroxyl groups, and the number average molecular weight of the water-soluble polymeric dispersant is 10,000 to 100,000. Step S2: The first mixed slurry is mixed with the second dispersant, and then subjected to a second grinding and a first drying process to obtain a mixture; wherein the second dispersant is a nonionic polyether compound; the number average molecular weight of the nonionic polyether compound is 200 to 5000. Step S3: Divide the mixture into three parts, denoted as the first precursor, the second precursor, and the third precursor; perform a first sintering on the first precursor to obtain the first powder. Step S4: The second precursor is subjected to a second sintering to obtain a second powder; the temperature of the first sintering is higher than the temperature of the second sintering. Step S5: Mix the first powder, the second powder, the third precursor, the additive, and the second solvent, and sequentially perform a third grinding, a second drying, and a third sintering to obtain the modified lithium iron phosphate cathode material; wherein, the additive is an oxide containing M, and M is a doping element selected from one or more elements in Group IVB and Group VB.

7. The method of claim 6, wherein the modified lithium iron phosphate cathode material is prepared by the steps of: mixing a lithium source, an iron source, a phosphate source, and a modifier source to form a mixture; and heating the mixture to form the modified lithium iron phosphate cathode material. In step S1, the molar ratio of the lithium source, the iron source, and the phosphorus source is (1.0–1.1):(0.95–1.0):(0.95–1.0); and / or, The lithium source is selected from one or more of the group consisting of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, and lithium dihydrogen phosphate; and / or, The iron source is selected from one or more of the group consisting of ferrous oxalate, ferrous sulfate, ferrous citrate, ferric phosphate, and ferric oxide; and / or, The phosphorus source is selected from one or more of the group consisting of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, and lithium dihydrogen phosphate; and / or, The carbon source comprises 5-15 wt% of the total weight of the lithium source, the iron source, and the phosphorus source; and / or, The carbon source is selected from one or more of the group consisting of glucose, sucrose, starch, citric acid, and polyethylene glycol; and / or, The first dispersant comprises 1-5 wt% of the total weight of the lithium source, the iron source, and the phosphorus source; and / or, The first dispersant is selected from one or more of the group consisting of sodium polyacrylate, polyacrylamide, and polyvinyl alcohol; and / or, The weight ratio of the first solvent to the total weight of the lithium source, the iron source, and the phosphorus source is (0.5–3.0):1; and / or, The first solvent is selected from water and / or ethanol.

8. The method for preparing the modified lithium iron phosphate cathode material according to claim 7, characterized in that, In step S1, the first grinding speed is 800–2000 rpm, and the time is 1–5 hours; and / or, D50 of the first mixed slurry is 0.5 to 1.5 μm. 50 0.5 to 1.5 μm.

9. The method for preparing the modified lithium iron phosphate cathode material according to claim 6, characterized in that, In step S2, the second dispersant comprises 1-5 wt% of the total weight of the lithium source, the iron source, and the phosphorus source; and / or, The second dispersant is selected from one or more of the group consisting of polyoxyethylene ether, polyethylene glycol, and polyvinylpyrrolidone; and / or, In step S2, the second grinding speed is 500–1000 rpm, and the time is 1–4 hours; and / or, The mixture's D 50 The thickness is 0.2–0.8 μm; and / or, The first drying temperature is 80-120℃, and the time is 2-8 hours.

10. The method for preparing the modified lithium iron phosphate cathode material according to claim 6, characterized in that, In step S3, the weight ratio of the first precursor, the second precursor, and the third precursor is (5-7):(1-3):(1-5); and / or, The first sintering temperature is 730-780℃, the heating rate is 3-10℃ / min, and the time is 10-15h.

11. The method for preparing the modified lithium iron phosphate cathode material according to claim 10, characterized in that, In step S4, the second sintering temperature is 650–730°C, the heating rate is 3–10°C / min, and the time is 8–15 h.

12. The method for preparing the modified lithium iron phosphate cathode material according to any one of claims 6 to 11, characterized in that, In step S5, the weight ratio of the first powder, the second powder, and the third precursor is (3-6):(1-3):(1-3); and / or, The additive comprises 0.5–5 wt% of the total weight of the lithium source, the iron source, and the phosphorus source; and / or, The doping element is selected from one or more elements chosen from the group consisting of Ti, V, Zr, and Nb; and / or, The additive is selected from one or more of the group consisting of titanium dioxide, vanadium pentoxide, zirconium oxide, and niobium oxide; and / or, The weight ratio of the second solvent to the total weight of the first powder, the second powder, the third precursor, and the additive is (0.5–2):1; and / or, The second solvent is selected from water and / or ethanol.

13. The method for preparing the modified lithium iron phosphate cathode material according to claim 12, characterized in that, In step S5, the third grinding speed is 800–2000 rpm, and the time is 1–5 hours; and / or, The third grinding obtained the second mixed slurry D 50 is 0.5-2 μm; and / or, The second drying temperature is 80–120°C, and the time is 3–6 hours; and / or, The third sintering temperature is 700–760℃, the heating rate is 3–10℃ / min, and the time is 10–15h.

14. A lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one surface thereon, characterized in that, The positive electrode active material layer comprises the modified lithium iron phosphate positive electrode material according to any one of claims 1 to 5.

Images