Iron phosphate with core-shell structure as well as preparation method and application of iron phosphate

Abstract

The invention discloses iron phosphate with a core-shell structure and a preparation method and application thereof, and relates to the technical field of lithium ion battery positive electrode materials, the iron phosphate with the core-shell structure comprises an inner core part, the inner core part is iron phosphate with a high iron-phosphorus ratio, the iron-phosphorus ratio of the inner core part is 0.965-0.985, the outer shell part is iron phosphate with a low iron-phosphorus ratio, and the iron-phosphorus ratio of the outer shell part is 0.965-0.985. The ratio of iron to phosphorus of the shell part is 0.955 to 0.965; the thickness of the shell part is 10 to 30 nm; the overall iron to phosphorus ratio of the iron phosphate with the core-shell structure is 0.965 to 0.980; in the invention, the inner core is converted at a high temperature (92-100 DEG C) to form an iron phosphate crystal structure with a high iron-phosphorus ratio, and the outer core is converted at a relatively low temperature (80-90 DEG C) to form an iron phosphate crystal structure with a low iron-phosphorus ratio, so that the iron phosphate with a core-shell structure, which has a relatively high internal iron-phosphorus ratio and a relatively low external iron-phosphorus ratio, is finally obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for lithium-ion batteries, and particularly relates to a kind of iron phosphate with a core-shell structure, its preparation method and application. Background Art

[0002] During the process of preparing lithium iron phosphate materials by the solid-phase method, under high temperature and strong reducing environment, the reaction between lithium iron phosphate and the carbon coating layer will become extremely violent, and side reactions are likely to occur on their interfaces to form impurity phases such as iron phosphide. On the one hand, iron phosphide has strong magnetism and there is a safety risk of piercing the battery separator; on the other hand, iron phosphide is easily corroded by the electrolyte, and the released Fe 3+ 、Fe 2+ will enter the electrolyte to catalyze the growth of the SEI film, resulting in the loss of active lithium and affecting the cycling performance of lithium iron phosphate batteries. Therefore, in the actual production process of lithium iron phosphate, it is very necessary to avoid the generation of iron phosphide.

[0003] Existing technical solutions for preparing lithium iron phosphate without or with low iron phosphide mainly include: Patent CN117963872A discloses a preparation method and application of a lithium iron phosphate material with a low iron phosphide content, and suppresses the generation of iron phosphide by controlling the atmosphere in the high-temperature sintering stage. Specifically, this technology uses a two-stage sintering method of low temperature (680 - 775 °C) and high temperature (776 - 820 °C) to prepare lithium iron phosphate, and increases the nitrogen flow rate in the high-temperature stage where iron phosphide is easily generated to reduce the relative content of reducing gases such as CO and H2 in the kiln to suppress the generation of iron phosphide.

[0004] Patent CN116161638A discloses a lithium iron phosphate material with low magnetic foreign matters and high tap density, its preparation method and application, and adopts an acid leaching and demagnetization method to remove iron phosphide based on the two-burn process. Specifically, first measure the content of iron phosphide in the intermediate product after the first sintering, then use acid solutions such as citric acid, acetic acid, and oxalic acid to remove magnetic foreign matters such as elemental iron and iron sulfide, and then add lithium source, phosphorus source, and carbon source according to the content of iron phosphide in the intermediate product after the first sintering for low-temperature secondary sintering to obtain a lithium iron phosphate material with magnetic foreign matters less than 100 ppb.

[0005] However, the above technologies mostly focus on improving in the preparation process of lithium iron phosphate, either creating an environment unfavorable for the formation of iron phosphide, such as low-temperature sintering, increasing the nitrogen flow rate to reduce the content of reducing gases, or eliminating the generated iron phosphide through methods such as acid dissolution and secondary sintering. The process is complex, the cost is high, and it is difficult to balance high capacity and high tap density. Summary of the Invention

[0006] As an important precursor for preparing lithium iron phosphate, the microscopic morphology, crystal structure, and physical and chemical indexes of iron phosphate have a great influence on the reaction kinetics between lithium iron phosphate and the carbon coating layer during the solid-phase sintering process. For example, the iron-to-phosphorus ratio of iron phosphate has an important influence on the electrochemical performance of lithium iron phosphate and the content of iron phosphide. Selecting lithium iron phosphate with a high iron-to-phosphorus ratio as the precursor is beneficial to obtaining high-capacity lithium iron phosphate with smaller particle size and higher crystallinity, but at the same time, it will promote the side reaction between lithium iron phosphate particles and the carbon coating layer and promote the formation of iron phosphide. Therefore, improving at the iron phosphate end and constructing a unique precursor structure that is not prone to generating iron phosphide and helps maintain the compaction and electrical properties of lithium iron phosphate is of great significance for preparing high-capacity, high-compaction, iron-phosphide-free lithium iron phosphate materials.

[0007] In view of this, the present invention provides iron phosphate with a core-shell structure, its preparation method and application, which are specifically as follows: An iron phosphate with a core-shell structure, wherein the inner core part of the iron phosphate material with a core-shell structure is iron phosphate with a high iron-to-phosphorus ratio, and the outer shell part is iron phosphate with a low iron-to-phosphorus ratio; the iron-to-phosphorus ratio of the inner core iron phosphate part is 0.965 - 0.985, and the iron-to-phosphorus ratio of the outer core iron phosphate part is 0.955 - 0.965. The overall iron-to-phosphorus ratio of the iron phosphate material with a core-shell structure is 0.965 - 0.980.

[0008] Preferably, the primary particle size of the iron phosphate material with a core-shell structure is 60 - 200 nm, and the thickness of the outer core iron phosphate with a low iron-to-phosphorus ratio is 10 - 30 nm.

[0009] Preferably, the iron-to-phosphorus ratios of the inner core and the outer shell of the iron phosphate material with a core-shell structure are regulated by adjusting the crystal growth and development environment.

[0010] Preferably, the inner core iron phosphate is synthesized in an environment conducive to crystal growth and development, and the outer shell iron phosphate is synthesized in a relatively weak environment.

[0011] Preferably, the means for adjusting the crystal growth and development environment include, but are not limited to, the adjustment and combination of parameters such as reaction temperature, reaction pressure, reaction pH, reaction concentration, reaction ratio, stirring intensity, reaction duration, etc.

[0012] Preferably, the iron phosphate with a core-shell structure can be prepared by the following method: S1. Raw material preparation: Add appropriate amount of phosphoric acid to the solution dissolved with ferrous ions, and add appropriate amount of hydrogen peroxide solution to oxidize all ferrous ions into ferric ions, thus obtaining an iron salt solution. The molar concentration of ferrous ions in the iron salt solution is 0.5 - 2.0 mol / L, the molar ratio of iron element to phosphorus element in the iron salt solution satisfies n(Fe):n(H3PO4)=1:0.2 - 1:0.4, and the pH of the iron salt solution is 1.0 - 1.5. Add a pH regulator to the solution containing phosphate, monohydrogen phosphate or dihydrogen phosphate to obtain a phosphorus salt solution. The molar concentration of phosphorus element in the phosphorus salt solution is 0.5 - 2.0 mol / L, and the pH of the phosphorus salt solution is 3.0 - 4.0. The iron salt solution is prepared from raw materials such as ferrous sulfate, ferrous nitrate, ferrous chloride, iron powder, iron sheet, etc.; the phosphorus salt solution is prepared from raw materials such as ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, etc.

[0013] S2. Core reaction: Add appropriate amount of pure water into the reaction kettle as the bottom liquid, and add the iron salt solution and the phosphorus salt solution into the reaction kettle simultaneously at a certain feeding rate. After the feeding is completed, carry out the synthesis reaction for 10 - 30 min, then heat up to 92 - 100 °C, and keep it warm at this temperature for 120 - 180 min for the conversion reaction to obtain the high-iron-phosphorus ratio slurry A.

[0014] S3. Shell reaction: Transfer the slurry A described in step S2 into the reaction kettle as the core matrix material, control the temperature of the slurry in the kettle to be 80 - 90 °C, and add the iron salt solution and the phosphorus salt solution into the reaction kettle simultaneously at a certain feeding rate. After the feeding is completed, react for 60 - 180 min to obtain the iron phosphate slurry B with a high-iron-phosphorus ratio core and a low-iron-phosphorus ratio outer core.

[0015] S4. Drying and calcination: Carry out solid-liquid separation and washing on the slurry B described in step S3 to obtain a white filter cake C; the white filter cake C is obtained by drying and calcination dehydration treatment to obtain the core-shell structured iron phosphate material.

[0016] Preferably, in step S2, the feeding rate of the iron salt solution is (X / 30 - X / 60) L / min, and the feeding rate of the phosphorus salt solution is (X / 30 - X / 60) L / min.

[0017] Preferably, X is 40% of the volume of the reaction kettle. For example, when the volume of the reaction kettle used in step S2 is 10 L, the value of X is 4; when the volume of the reaction kettle used in step S2 is 2 m 3 ³, the value of X is 800.

[0018] Preferably, in step S2, the molar ratio of iron element in the iron salt solution and phosphorus element in the phosphorus salt solution input into the reaction kettle during the feeding process satisfies n(Fe):n(P)=0.95:1 - 1.05:1.

[0019] Preferably, in step S3, the molar ratio of iron element in the iron salt solution and phosphorus element in the phosphorus salt solution input into the reaction kettle during the feeding process satisfies n(Fe):n(P)=0.90:1 - 1:1.

[0020] Preferably, in step S3, the volume of the slurry A transferred into the reaction kettle for the shell reaction is 40 - 60% of the effective volume of the reaction kettle.

[0021] Preferably, during the shell reaction in step S3, the feeding rate of the iron salt solution is (Y / 60 - Y / 100) L / min, and the feeding rate of the phosphorus salt solution is (Y / 60 - Y / 100) L / min.

[0022] Preferably, Y is 20% of the volume of the reaction kettle. For example, when the volume of the reaction kettle used in step S3 is 10 L, the value of Y is 2; when the volume of the reaction kettle used in step S2 is 5 m 3 , the value of X is 1000.

[0023] Preferably, in step S3, during the shell reaction, the volume ratio of the iron salt solution input into the reaction kettle to the volume of the white slurry A used as the bottom liquid is 1:8 - 1:20.

[0024] Preferably, in step S4, the methods and equipment for solid-liquid separation and washing of the white slurry B are both common methods and equipment in the iron phosphate industry, and the methods and equipment for drying and calcining dehydration of the white filter cake C are also common methods and equipment in the iron phosphate industry.

[0025] On the other hand, the present invention discloses an application of the iron phosphate with a core-shell structure, and the iron phosphate with a core-shell structure is used to prepare a lithium iron phosphate material, and the lithium iron phosphate material has the characteristics of high capacity, high tap density and no iron phosphide.

[0026] Preferably, the lithium iron phosphate material is prepared by the following steps: mixing the iron phosphate with a core-shell structure with a lithium source and a carbon source, and obtaining the lithium iron phosphate material after sintering.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the present invention, the inner core is transformed at a high temperature (92 - 100 °C) to form a crystal structure of iron phosphate with a high iron - phosphorus ratio, while the outer core is transformed at a relatively low temperature (80 - 90 °C) to form a crystal structure of iron phosphate with a low iron - phosphorus ratio, finally achieving iron phosphate with a core - shell structure having a relatively high iron - phosphorus ratio inside and a relatively low iron - phosphorus ratio outside. The iron phosphate with a core - shell structure prepared by the present invention effectively inhibits the formation of iron phosphide during the sintering process of lithium iron phosphate by constructing an iron phosphate shell layer with a low iron - phosphorus ratio on the outer layer. The outer shell layer with a low iron - phosphorus ratio reduces the activity during the reaction with the carbon - coated layer, reducing the occurrence of side reactions, thereby improving the safety and cycle performance of the battery. At the same time, the iron phosphate with a core - shell structure combines the high - capacity characteristics of the inner core with a high iron - phosphorus ratio and the high safety and high cycle stability of the outer shell with a low iron - phosphorus ratio, which can improve the electrochemical performance of the lithium iron phosphate material.

[0028] 2. The iron phosphate with a core - shell structure provided by the present invention has a high iron - phosphorus ratio in the inner core part, which is beneficial to obtaining a lithium iron phosphate material with high capacity. During the sintering process of lithium iron phosphate, the precursor with a high iron - phosphorus ratio is conducive to forming a stable olivine structure and improving the crystallinity of the material, thereby enhancing the intercalation and deintercalation rate of lithium ions, and improving the charge - discharge efficiency and cycle life of the material.

[0029] 3. The iron phosphate with a core - shell structure provided by the present invention has a low iron - phosphorus ratio in the outer shell part, which is beneficial to inhibiting the generation of iron phosphide during the sintering process of lithium iron phosphate. The higher the iron - phosphorus ratio of the iron phosphate precursor, the easier it is to generate iron phosphide during the sintering process of lithium iron phosphate (4LiFePO4+9C→2Fe2P+Li4P2O7+9CO). Iron phosphide is generated from the side reaction between lithium iron phosphate and the carbon - coated layer. Constructing a precursor material with a low iron - phosphorus ratio on the particle surface ensures that the lithium iron phosphate in contact with the carbon - coated layer has relatively low crystallinity, which can significantly reduce the occurrence of the side reaction.

[0030] 4. The iron phosphate with a core - shell structure provided by the present invention has a low iron - phosphorus ratio in the outer shell part, which is beneficial to obtaining a lithium iron phosphate material with high tap density. During the sintering process of lithium iron phosphate, lithium ions decomposed from lithium sources such as lithium carbonate will first react with the outer layer of the precursor iron phosphate particles to form lithium iron phosphate, and then gradually penetrate from the outside to the inside until the reaction with the inner core of the precursor is complete. The lower the iron - phosphorus ratio of the outer layer of the precursor iron phosphate particles, the smaller the resistance for lithium ions to enter its inner core, the higher the reaction activity of the whole iron phosphate particle, and the more conducive to the formation of large - sized particles that help to improve the tap density of lithium iron phosphate.

[0031] 5. The provided iron phosphate with a core-shell structure has a simple preparation process and is suitable for large-scale production. Specifically, by simply adjusting or combining process parameters such as reaction temperature, reaction pressure, reaction pH, reaction concentration, reaction ratio, stirring intensity, and reaction duration, two different crystal growth and development environments can be created to separately control the iron-phosphorus ratio of the inner core and the outer iron phosphate part. The present invention only needs to synthesize iron phosphate as the inner core matrix in an environment conducive to crystal growth and development, and then transfer it to a relatively weaker environment for the shell reaction. Description of the Drawings

[0032] Figure 1 It is a cross-sectional view of the core-shell structured iron phosphate prepared in Example 1 of the present invention; Figure 2 It is an EPMA diagram of the Fe element of the core-shell structured iron phosphate prepared in Example 1 of the present invention; Figure 3 It is a cross-sectional EDS diagram of the core-shell structured iron phosphate prepared in Example 1 of the present invention; Figure 4 It is an XRD diagram of the lithium iron phosphate prepared from the iron phosphate prepared in Example 1 and Comparative Example 1 of the present invention; Figure 5 It is an electron probe microanalysis diagram of the Fe element corresponding to the internal cross-section of the iron phosphate prepared in Example 2 of the present invention; Figure 6 It is an electron probe microanalysis diagram of the Fe element corresponding to the internal cross-section of the iron phosphate prepared in Comparative Example 3 of the present invention. Detailed Description of the Invention

[0033] Example 1 A preparation method of an iron phosphate with a core-shell structure, comprising the following steps: S1. Raw material preparation: Dissolve ferrous sulfate in pure water, add phosphoric acid solution to adjust the pH value, and then slowly add hydrogen peroxide solution to oxidize all divalent iron ions into trivalent iron ions to obtain an iron salt solution with pH = 1.4 and an iron element concentration of 1.2 mol / L, where n(Fe):n(P) = 1:0.3 in this solution; at the same time, dissolve an appropriate amount of ammonium dihydrogen phosphate in pure water to prepare a phosphate salt solution with pH = 3.8 and a phosphorus element concentration of 1.2 mol / L.

[0034] S2. Inner core reaction: Add 5 L of pure water as the bottom liquid into a 100 L reaction kettle, and simultaneously pump 40 L of the iron salt solution and 40 L of the phosphate salt solution into the reaction kettle at a feeding rate of 1 L / min. After the feeding is completed, carry out a synthesis reaction for 30 min, then raise the temperature to 98°C, and carry out a conversion reaction at 98°C for 150 min to obtain a high iron-phosphorus ratio slurry A. Take a small amount of the high iron-phosphorus ratio slurry A, wash, dry, and calcine it, and measure its iron-phosphorus ratio to be 0.978.

[0035] S3. Shell reaction: Transfer 40 L of the slurry A described in step S2 into a reaction kettle as the core matrix material. Control the temperature of the slurry in the kettle at 90 °C. Add 20 L of iron salt solution and 22 L of phosphate salt solution into the reaction kettle simultaneously at a feeding rate of 0.33 L / min. After the feeding is completed, react for 60 min to obtain the iron phosphate slurry B with a high iron-phosphorus ratio in the core and a low iron-phosphorus ratio in the outer core.

[0036] S4. Drying and calcination: Perform solid-liquid separation and washing on the slurry B described in step S3 to obtain a white filter cake C; the white filter cake C is subjected to drying and calcination dehydration treatment to obtain the core-shell structured iron phosphate material, and its iron-phosphorus ratio is measured to be 0.971.

[0037] Example 2 A preparation method of core-shell structured iron phosphate includes the following steps: S1. Raw material preparation: Dissolve ferrous sulfate in pure water, add phosphoric acid solution to adjust the pH value, and then slowly add hydrogen peroxide solution to oxidize all divalent iron ions into trivalent iron ions to obtain an iron salt solution with pH = 1.3 and an iron element concentration of 1.0 mol / L, where n(Fe):n(P) = 1:0.4 in this solution; at the same time, dissolve an appropriate amount of ammonium dihydrogen phosphate in pure water to prepare a phosphate salt solution with pH = 3.9 and a phosphorus element concentration of 1.0 mol / L.

[0038] S2. Core reaction: Add 5 L of pure water as the bottom liquid into a 100 L reaction kettle. Pump 40 L of iron salt solution and 40 L of phosphate salt solution into the reaction kettle simultaneously at a feeding rate of 0.67 L / min. After the feeding is completed, react for 20 min, then raise the temperature to 96 °C and keep it at this temperature for 160 min to obtain a high iron-phosphorus ratio slurry A. Take a small amount of the high iron-phosphorus ratio slurry A for washing, drying, and calcination, and its iron-phosphorus ratio is measured to be 0.970.

[0039] S3. Shell reaction: Transfer 40 L of the slurry A described in step S2 into a reaction kettle as the core matrix material. Control the temperature of the slurry in the kettle at 88 °C. Add 20 L of iron salt solution and 20 L of phosphate salt solution into the reaction kettle simultaneously at a feeding rate of 0.33 L / min. After the feeding is completed, react for 90 min to obtain the iron phosphate slurry B with a high iron-phosphorus ratio in the core and a low iron-phosphorus ratio in the outer core.

[0040] S4. Drying and calcination: Perform solid-liquid separation and washing on the slurry B described in step S3 to obtain a white filter cake C; the white filter cake C is subjected to drying and calcination dehydration treatment to obtain the core-shell structured iron phosphate material, and its iron-phosphorus ratio is measured to be 0.967.

[0041] Example 3 A preparation method of core-shell structured iron phosphate includes the following steps: S1. Raw material preparation: Dissolve an appropriate amount of ferrous chloride in pure water, add phosphoric acid solution to adjust the pH value, and then slowly add hydrogen peroxide solution to oxidize all divalent iron ions into trivalent iron ions, obtaining an iron salt solution with pH = 1.2 and iron element concentration of 1.5 mol / L, where n(Fe):n(P) = 1:0.35 in this solution; at the same time, dissolve an appropriate amount of monosodium hydrogen phosphate in pure water to prepare a phosphate salt solution with pH = 3.5 and phosphorus element concentration of 1.5 mol / L.

[0042] S2. Core reaction: Add 10 L of pure water as the bottom liquid into a 200 L reactor, and simultaneously pump 80 L of the iron salt solution and 80 L of the phosphate salt solution into the reactor at a feeding rate of 1.2 L / min. After the feeding is completed, react for 25 min, then raise the temperature to 95°C and keep it at this temperature for 170 min to obtain the high-iron-phosphorus ratio slurry A. Take a small amount of the high-iron-phosphorus ratio slurry A, wash, dry, and calcine it, and measure its iron-phosphorus ratio to be 0.975.

[0043] S3. Outer shell reaction: Take 60 L of the slurry A described in step S2 and transfer it into the reactor as the core matrix material. Control the temperature of the slurry in the reactor to be 85°C, and simultaneously add 30 L of the iron salt solution and 32 L of the phosphate salt solution into the reactor at a feeding rate of 0.4 L / min. After the feeding is completed, react for 120 min to obtain the iron phosphate slurry B with a high-iron-phosphorus ratio core and a low-iron-phosphorus ratio outer shell.

[0044] S4. Drying and calcination: Perform solid-liquid separation and washing on the slurry B described in step S3 to obtain a white filter cake C; the white filter cake C is obtained by drying and calcination dehydration treatment to obtain the core-shell structured iron phosphate material, and its iron-phosphorus ratio is measured to be 0.969.

[0045] The above only discloses two specific embodiments of the present invention, but the present invention is not limited thereto. Any changes that can be thought of by those skilled in the art should fall within the protection scope of the present invention.

[0046] Comparative Example 1 The main difference between this comparative example and Example 1 is that the outer shell reaction is not carried out, and the specific steps are as follows: S1. Raw material preparation: Dissolve an appropriate amount of ferrous sulfate in pure water, add phosphoric acid solution to adjust the pH value, and then slowly add hydrogen peroxide solution to oxidize all divalent iron ions into trivalent iron ions, obtaining an iron salt solution with pH = 1.3 and iron element concentration of 1.0 mol / L, where n(Fe):n(P) = 1:0.4 in this solution. At the same time, dissolve an appropriate amount of ammonium dihydrogen phosphate in pure water to prepare a phosphate salt solution with pH = 3.9 and phosphorus element concentration of 1.0 mol / L.

[0047] S2, Core Reaction: Add 5 L of pure water as the bottom liquid into a 100 L reactor. Pump 40 L of iron salt solution and 40 L of phosphorus salt solution into the reactor simultaneously at a feeding rate of 0.67 L / min. After the feeding is completed, react for 20 min, then heat up to 96 °C and keep the temperature at this level for 160 min to obtain the high-iron-phosphorus ratio slurry A.

[0048] S3, Drying and Calcination: Separate the solid and liquid of the slurry A described in step S2 and wash it to obtain a white filter cake; the white filter cake is subjected to drying and calcination dehydration treatment to obtain the core-shell structured iron phosphate material, and its iron-phosphorus ratio is measured to be 0.971.

[0049] Comparative Example 2 The difference between this comparative example and Example 1 is as follows: In the core reaction of step S2, the temperature of the conversion reaction is 98 °C; in the shell reaction of step S3, the temperature of the slurry in the reactor is controlled at 98 °C. For the iron phosphate prepared in Comparative Example 2, the core reaction conditions are the same as the outer core reaction conditions, and the iron-phosphorus ratios of the core and outer core parts are similar. The iron-phosphorus ratio of the prepared iron phosphate is tested and measured to be 0.983.

[0050] Comparative Example 3 The difference between this comparative example and Example 1 is as follows: In the core reaction of step S2, the temperature of the conversion reaction is 90 °C; in the shell reaction of step S3, the temperature of the slurry in the reactor is controlled at 98 °C. For the iron phosphate prepared in Comparative Example 3, the core reaction conditions are lower than those of the outer core, and the iron-phosphorus ratio of the core part is lower than that of the outer core part, forming a core-shell material with a high iron-phosphorus ratio on the outer layer and a low iron-phosphorus ratio on the inner layer.

[0051] Figure 1 and Figure 2 respectively show the internal sectional view and the corresponding Fe element electron probe microanalysis map (EPMA) of the core-shell structured iron phosphate prepared in Example 1 of the present invention. EPMA can accurately and precisely perform quantitative analysis on the micro-area chemical element distribution. As shown in the figure, the color depth of the outer layer part is significantly lower than that of the core part, indicating that the Fe element content in the outer layer part of the iron phosphate particles is relatively low, and the Fe element content in the core part is relatively high, that is, it shows that the prepared core-shell structured iron phosphate material has the structural characteristics of a relatively high iron-phosphorus ratio inside and a relatively low iron-phosphorus ratio outside.

[0052] Figure 3 Shows the EDS point scan map of the cross-section of the core-shell structured iron phosphate prepared in Example 1 of the present invention. At positions 1 and 3 inside the particles, the normalized mass contents of the Fe element are 31.09% and 30.87% respectively, which are significantly higher than those at positions 2 and 4 outside the particles, and the normalized mass contents of the Fe element are 25.24% and 25.10% respectively. This further illustrates that the iron phosphate prepared in the present invention has the structural characteristics of a relatively high iron-phosphorus ratio inside and a relatively low iron-phosphorus ratio outside.

[0053] Figure 4 The XRD patterns of the lithium iron phosphate prepared using the iron phosphate prepared in Example 1 of the present invention and Comparative Example 1 of the present invention as precursors are shown. As shown in the figure, after the core-shell structured iron phosphate prepared in Example 1 of the present invention is made into lithium iron phosphate, there are no characteristic peaks of impurity phases in its XRD pattern. In particular, the characteristic peak of iron phosphide cannot be observed in its XRD pattern. However, in the XRD pattern of the lithium iron phosphate prepared using the iron phosphate prepared in Comparative Example 1 of the present invention as a precursor, the characteristic peak belonging to the (111) crystal plane of iron phosphide can be clearly observed at the position of 40.2°-40.3° (PDF#83-2337). This shows that under the same conditions, the iron phosphate with a core-shell structure having a low iron-to-phosphorus ratio in the outer shell can effectively inhibit the formation of iron phosphide foreign matters during the sintering process of lithium iron phosphate.

[0054] The lithium iron phosphates prepared using the iron phosphates prepared in Example 1 and Comparative Example 1 of the present invention as precursors were tested for the content of magnetic foreign matters, tap density, and coin cell electrochemical performance. As shown in Table 1 below, after the core-shell structured iron phosphate prepared in the example of the present invention is made into lithium iron phosphate, the highest tap density of the powder can reach 2.563 g / cc, the discharge capacities at 0.5C and 1C can reach 148.4 mAh / g and 142.3 mAh / g respectively, and the content of (Fe-Li) in the magnetic foreign matters is only 0.127 ppm at the lowest, and the contents of other foreign matters are also lower than those of the comparative example. This shows that using the core-shell structured iron phosphate with a low iron-to-phosphorus ratio in the outer layer and a high iron-to-phosphorus ratio in the inner layer as a precursor can prepare a lithium iron phosphate material with a low iron phosphide content, high tap density, and high capacity.

[0055] Table 1 Test results of the performance of lithium iron phosphate

[0056] Figure 5 The electron probe microanalysis (EPMA) diagram of the Fe element corresponding to the internal cross-section of the iron phosphate prepared in Example 2 is shown. As shown in the figure, the color depth of the outer layer part is significantly lower than that of the inner core part, indicating that the Fe element content in the outer layer part of the iron phosphate particles is relatively low, and the Fe element content in the inner core part is relatively high, that is, it shows that the core-shell structured iron phosphate material prepared in Example 2 of the present invention has the structural characteristics of a relatively high iron-to-phosphorus ratio inside and a relatively low iron-to-phosphorus ratio outside.

[0057] Figure 6The electron probe microanalysis (EPMA) diagram of the Fe element corresponding to the internal cross-section of the iron phosphate prepared in Comparative Example 3 of the present invention is shown. As shown in the figure, the color depth of the outer layer part is significantly higher than that of the inner core part, indicating that the content of the Fe element in the outer layer part of the iron phosphate particles is relatively high, and the content of the Fe element in the inner core part is relatively low. That is to say, the core-shell structured iron phosphate material prepared in Comparative Example 3 of the present invention has the structural characteristics of a relatively low iron-to-phosphorus ratio in the interior and a relatively high iron-to-phosphorus ratio in the outer layer.

Claims

1. A core-shell structured iron phosphate, characterized in that, Comprising: A core part, where the core part is iron phosphate with a high iron-to-phosphorus ratio, and the iron-to-phosphorus ratio of the core part is 0.965 - 0.985; A shell part, where the shell part is iron phosphate with a low iron-to-phosphorus ratio, and the iron-to-phosphorus ratio of the shell part is 0.955 - 0.965; the thickness of the shell part is 10 - 30 nm; The overall iron-to-phosphorus ratio of the iron phosphate with the core-shell structure is 0.965 - 0.

980.

2. The iron phosphate with a core-shell structure according to claim 1, wherein, The primary particle size of the iron phosphate with the core-shell structure is 60 - 200 nm.

3. A preparation method of core-shell structured iron phosphate, characterized in that, Including the following steps: S1. Raw material preparation: Prepare an iron salt solution, where the molar concentration of ferrous ions in the iron salt solution is 0.5 - 2.0 mol / L, the molar ratio of iron element to phosphorus element is n(Fe):n(H3PO4) = 1:0.2 - 1:0.4, and the pH is 1.0 - 1.5; Prepare a phosphorus salt solution, where the molar concentration of phosphorus element in the phosphorus salt solution is 0.5 - 2.0 mol / L, and the pH is 3.0 - 4.0; S2. Core reaction: Add pure water as the bottom liquid into the reaction kettle, add the iron salt solution and the phosphorus salt solution into the reaction kettle simultaneously. After the feeding is completed, carry out the synthesis reaction for 10 - 30 min, heat up to 92 - 100 °C, and carry out the conversion reaction for 120 - 180 min to obtain the slurry A with a high iron-to-phosphorus ratio; S3. Shell reaction: Transfer the slurry A to the reaction kettle, control the temperature at 80 - 90 °C, add the iron salt solution and the phosphorus salt solution into the reaction kettle simultaneously. After the feeding is completed, react for 60 - 180 min to obtain the slurry B; S4. Drying and calcination: Carry out solid-liquid separation and washing on the slurry B in step S3 to obtain a white filter cake C; the white filter cake C is obtained by drying and calcination dehydration treatment to obtain the iron phosphate material with the core-shell structure.

4. The preparation method of a core-shell structured iron phosphate according to claim 3, characterized in that, In step S2: the feeding rate of the iron salt solution is X / 30 - X / 60 L / min, the feeding rate of the phosphorus salt solution is X / 30 - X / 60 L / min, where X is 40% of the volume of the reaction kettle; the molar ratio of iron element to phosphorus element is n(Fe):n(P) = 0.95:1 - 1.05:

1.

5. The preparation method of a core-shell structured iron phosphate according to claim 3, characterized in that, In step S3: the volume of the slurry A is 40% - 60% of the effective volume of the reaction kettle.

6. The preparation method of a core-shell structured iron phosphate according to claim 5, characterized in that, In step S3: the feeding rate of the iron salt solution is (Y / 60 - Y / 100) L / min, the feeding rate of the phosphorus salt solution is (Y / 60 - Y / 100) L / min, where Y is 20% of the volume of the reaction kettle; The molar ratio of iron element to phosphorus element is n(Fe):n(P) = 0.90:1 - 1:

1.

7. The preparation method of a core-shell structured iron phosphate according to claim 3, characterized in that, In step S3, the volume ratio of the iron salt solution to the volume of the slurry A is 1:8 - 1:

20.

8. Application of a core-shell structured iron phosphate, characterized in that, The iron phosphate with the core-shell structure is used for preparing a lithium iron phosphate material.

9. Use of an iron phosphate with a core-shell structure according to claim 8, characterized in that, The lithium iron phosphate material is prepared by the following steps: mix the iron phosphate with the core-shell structure with a lithium source and a carbon source, and obtain the lithium iron phosphate material after sintering.

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