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

Abstract

The invention provides a modified lithium iron phosphate material and a preparation method thereof, a positive electrode and a lithium ion battery. The preparation method comprises the following steps: mixing raw materials including a ferrous source, a phosphorus source and a metal salt solution to respectively obtain first slurry with high iron-phosphorus molar ratio and high total metal ion concentration and second slurry with low iron-phosphorus molar ratio and low total metal ion concentration; mixing raw materials including the first slurry (second slurry), a lithium source, a solvent and a carbon source, and sequentially performing ball milling, drying and calcining to respectively obtain first lithium iron phosphate with small particle size and second lithium iron phosphate with large particle size; mixing raw materials including first lithium iron phosphate, second lithium iron phosphate, a third carbon source, a fourth carbon source, a metal coating source and a third solvent to obtain a mixture, and sequentially grinding, drying and calcining the mixture to obtain the modified lithium iron phosphate material. Therefore, the problems that the cost is high, the carbon coating thickness is difficult to control and magnetic foreign matters exceed the standard in the preparation method of the lithium iron phosphate in the prior art are solved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion batteries, in particular to a modified lithium iron phosphate material, a preparation method thereof, a positive electrode and a lithium ion battery. BACKGROUND

[0002] Iron phosphate (FePO4) is a key precursor for preparing lithium iron phosphate (LiFePO4), and the quality of the iron phosphate directly affects the performance of the final positive electrode material. At present, the process route for synthesizing lithium iron phosphate by high-temperature carbon thermal reduction method with iron phosphate as the precursor has become the mainstream preparation method in the market due to its high technical maturity and stable product performance. Lithium iron phosphate is a positive electrode material based on an olivine-type crystal structure, and occupies a dominant position in power batteries (such as electric vehicles) and large-scale energy storage systems due to its outstanding comprehensive advantages. Its core advantages are: excellent safety: high decomposition temperature (about 350 DEG C), and thermal stability far superior to other systems, effectively reducing the risk of thermal runaway. Ultra-long cycle life: the cycle number is usually more than 2000 times, significantly prolonging the service life of the battery system. Outstanding cost advantage: the main raw materials iron (Fe) and phosphorus (P) are abundant in the earth's crust and low in price, and the overall cost competitiveness of the material system is strong. However, compared with high-nickel ternary (NCM / NCA) and other positive electrode materials, lithium iron phosphate also has the short board of relatively low energy density and slightly poor low-temperature performance. In order to overcome these limitations and improve its overall competitiveness, various modification technologies are widely used in the industry: ion doping (such as metal element doping): specific metal ions (such as magnesium, titanium, aluminum, etc.) are introduced during synthesis, aiming to optimize the crystal structure, improve the lithium ion diffusion rate and electronic conductivity, thereby improving the rate performance and capacity performance. Carbon coating: a layer of conductive carbon layer is coated on the surface of the particles, effectively enhancing the electronic conduction ability between particles, improving the conductivity of the material, and improving the rate performance. Nanocrystallization: reducing the particle size of primary particles, shortening the lithium ion diffusion path, and improving the reaction kinetics. Secondary calcination (two-burn process): by optimizing the calcination process (such as staged calcination), the particle growth and morphology are adjusted to achieve a better particle size distribution (particle size grading), and finally the powder compaction density is improved, which is crucial for improving the volumetric energy density of the battery.

[0003] Although the above modification strategies effectively improve the overall performance of lithium iron phosphate, the current specific methods still have some deficiencies to be solved: ① Doping cost and uniformity problem: for example, in order to realize titanium (Ti) doping, a large amount of titanium dioxide (TiO2) is added, and the commonly used titanium dioxide itself is relatively high in price, which significantly increases the raw material cost. The titanium dioxide added at the back end is not uniform on the one hand (doping is not uniform, the by-products formed are not uniform, dead lithium is formed, and the first charge is significantly reduced); ② Trade-off of carbon coating: although carbon coating can significantly improve the conductivity, too thick or too much carbon layer will occupy the electrode space, leading to a decrease in the compaction density, which is not conducive to the improvement of the volume energy density of the battery. Agglomeration is serious during grinding, and the particle size is difficult to control, a large amount of carbon source with high graphitization degree residual carbon, such as PEG, PVA, etc., which has a dispersing effect, needs to be added, but the carbon source cost increases dramatically, and the two-burned products are seriously layered; ③ Cost of conventional two-burn process: although the use of two-burn process to improve the particle size distribution and compaction density is effective, the additional calcination step means higher energy consumption cost and more complex production process, which leads to the corresponding increase in labor cost and equipment depreciation cost; ④ Magnetic problem: the by-products consume a lot of Li and P, including the volatilization of lithium during two-burning, which leads to the excess of Fe, causing the risk of exceeding the standard of magnetic foreign matter during one-burning and two-burning, which affects the performance of the finished product. SUMMARY

[0004] The main purpose of the present application is to provide a modified lithium iron phosphate material, a preparation method thereof, a positive electrode and a lithium ion battery, so as to solve the problems of high cost, difficulty in controlling the thickness of carbon coating and excessive magnetic foreign matter in the prepared lithium iron phosphate in the prior art.

[0005] In order to achieve the above purpose, according to one aspect of the present application, a preparation method of a modified lithium iron phosphate material is provided, the modified lithium iron phosphate material comprising Li x Fe y PO4and a carbon coating layer and Li x M2(PO4)3coating layer successively coated on the surface of Li y Fe a PO4, and the chemical formula of the modified lithium iron phosphate material is Li x Fe y PO4@C@Li aM2(PO4)3, wherein 0.95≤x≤1.05, 0.95≤y≤0.97, 1≤a≤5, M is selected from any one or more of Mn, Ti, Sc, Sn, Al, In, Sb, Bi and Zr; the preparation method comprises: step S1, mixing raw materials comprising a first ferrous source, a first phosphorus source and a first metal salt solution to obtain a first slurry; step S2, mixing raw materials comprising a second ferrous source, a second phosphorus source and a second metal salt solution to obtain a second slurry; step S3, mixing raw materials comprising the first slurry, a first lithium source, a first solvent and a first carbon source to obtain a first precursor, and sequentially performing first ball milling, first drying and first calcination on the first precursor to obtain a first lithium iron phosphate; step S4, mixing raw materials comprising the second slurry, a second lithium source, a second solvent and a second carbon source to obtain a second precursor, and sequentially performing second ball milling, second drying and second calcination on the second precursor to obtain a second lithium iron phosphate; step S5, mixing raw materials comprising the first lithium iron phosphate, the second lithium iron phosphate, a third carbon source, a fourth carbon source, a metal coating source and a third solvent to obtain a mixture, and sequentially performing grinding, third drying and third calcination on the mixture to obtain a modified lithium iron phosphate material; wherein the molar ratio of iron to phosphorus in the first slurry is higher than the molar ratio of iron to phosphorus in the second slurry, and the concentration of total metal ions in the first slurry is higher than the concentration of total metal ions in the second slurry; the average particle size of the first lithium iron phosphate is smaller than the average particle size of the second lithium iron phosphate.

[0006] By the above method, the doping element is introduced during the preparation of the precursor on the basis of fully utilizing the preparation process of the precursor iron phosphate, a cheap soluble doping source (metal salt solution) is used to reduce the cost and ensure uniform distribution, and the dehydration calcination process is omitted to reduce energy consumption. By adjusting the content of the main elements of the precursor, the iron-phosphorus ratio and the doping amount, the required large and small particle lithium iron phosphate is directly prepared by the first calcination and the second calcination by using the influence of the precursor on the growth of the lithium iron phosphate particles in the later stage. The first calcination and the second calcination respectively select high graphitization degree organic polymer carbon source and cheap high residual carbon content small molecule carbon source; the grinding uses mixed carbon source according to the particle ratio, which maximizes the improvement of the graphitization degree of the coated carbon and the performance of the material while reducing the cost, and controls the thickness of the carbon coating. The relationship between the grinding particle size, the proportion of the mixed carbon source and the size of the particle feed ratio is determined in the grinding stage, the process is cooperatively controlled, and the Li a M2(PO4)3(such as Li5Mn2(PO4)3, Li3Sc2(PO4)3, etc.) high-conductivity fast-ion-conductor coating layer improves the performance of the material. In addition, since the proportion of iron elements is reduced at the feed end, it is beneficial to inhibit the generation of magnetic substances phosphorus-iron compounds, and therefore the obtained lithium iron phosphate positive electrode material has lower magnetic foreign matter impurities and better compaction and electrical performance.

[0007] Further, in the step S1, the molar ratio of the first ferrous source, the first metal salt solution and the first phosphorus source is 1:(0.050-0.100):(1.030-1.040); and / or, the first ferrous source is selected from any one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride and ferrous nitrate; and / or, the first phosphorus source is selected from any one or more of diammonium hydrogen phosphate, phosphoric acid and ammonium dihydrogen phosphate; and / or, the first metal salt solution is a sulfate solution, and the sulfate solution is selected from any one or more of titanium sulfate solution, manganese sulfate solution, scandium sulfate solution, zirconium sulfate solution, antimony sulfate solution and bismuth sulfate solution.

[0008] The above first metal salt solution in the first slurry can introduce a doping element in the preparation stage of the first lithium iron phosphate, on the one hand, the cost of the doping element is reduced, on the other hand, the distribution of the doping element is more uniform, forming uniform doping and uniform primary fast ion conductor coating; the precursor dihydrate dehydration calcination process is omitted, and the energy consumption cost is reduced. Controlling the molar ratio of the first ferrous source, the first metal salt solution and the first phosphorus source in the above range helps to control the content of the doping element in the first lithium iron phosphate from the source, thereby improving the performance of the first lithium iron phosphate.

[0009] Further, in the step S2, the molar ratio of the second ferrous source, the second metal salt solution and the second phosphorus source is 1:(0.025-0.050):(1.040-1.050); and / or, the second ferrous source is selected from any one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride and ferrous nitrate; and / or, the second phosphorus source is selected from any one or more of diammonium hydrogen phosphate, phosphoric acid and ammonium dihydrogen phosphate; and / or, the second metal salt solution is a sulfate solution, and the sulfate solution is selected from any one or more of titanium sulfate solution, manganese sulfate solution, scandium sulfate solution, zirconium sulfate solution, antimony sulfate solution and bismuth sulfate solution.

[0010] The above second metal salt solution in the second slurry can introduce a doping element in the preparation stage of the second lithium iron phosphate, on the one hand, the cost of the doping element is reduced, on the other hand, the distribution of the doping element is more uniform, forming uniform doping and uniform primary fast ion conductor coating; the precursor dihydrate dehydration calcination process is omitted, and the energy consumption cost is reduced. Controlling the molar ratio of the second ferrous source, the second metal salt solution and the second phosphorus source in the above range helps to control the content of the doping element in the second lithium iron phosphate from the source, thereby improving the performance of the second lithium iron phosphate.

[0011] Further, in the step S3, the D50 particle size of the first lithium iron phosphate is 600 nm to 1000 nm; and / or, the molar ratio of iron element to phosphorus element in the first lithium iron phosphate is 0.960 to 0.970; and / or, the ratio of the molar amount of iron element in the first slurry to the molar amount of lithium element in the first lithium source is 1:(0.515 to 0.525); and / or, the first lithium source is selected from any one or more of lithium carbonate, lithium hydroxide and lithium phosphate; and / or, the mass of the first carbon source is 4 wt% to 6 wt% of the mass of the modified lithium iron phosphate material; and / or, the first carbon source is selected from any one or more of polyethylene glycol, polyvinyl alcohol and polyvinylpyrrolidone; and / or, the first solvent is selected from any one or more of deionized water and ethanol; and / or, the solid content of the first precursor is 40% to 50%; and / or, the D50 particle size of the product obtained by the first ball milling is 300 to 400 nm; and / or, the temperature of the first drying is 200°C to 300°C; and / or, the temperature of the first calcination is 450°C to 550°C, and the time is 8h to 12h; and / or, the first calcination is performed in a protective atmosphere, and the protective atmosphere is nitrogen and / or argon.

[0012] Based on the influence factors of the main element content of the first lithium iron phosphate precursor and the characteristic influence of the doping elements on the growth of the first lithium iron phosphate particles at the back end, by controlling the ratio of the molar amount of iron element in the first slurry to the molar amount of lithium element in the first lithium source, the mass of the first carbon source to the mass of the modified lithium iron phosphate material, the type of the first carbon source, the D50 particle size of the product obtained by the first ball milling, the solid content of the first precursor, the temperature of the first drying and the temperature of the first calcination within the above ranges, a first lithium iron phosphate precursor with a higher iron to phosphorus ratio and different element doping amounts is obtained, and then a small particle lithium iron phosphate meeting the requirements is prepared after the first calcination, which is convenient for forming particle grading during subsequent grinding. In addition, by reducing the iron to phosphorus molar ratio in the first lithium iron phosphate precursor through the feeding ratio of the first slurry, and increasing the feeding ratio of lithium element by controlling the ratio of the molar amount of iron element in the first slurry to the molar amount of lithium element in the first lithium source to 1:(0.515 to 0.525) during the first ball milling, the loss of lithium element after the third calcination is avoided, and magnetic impurity phosphorus iron compounds are formed.

[0013] Further, in the step S4, the D50 particle size of the second lithium iron phosphate is 1500-2500 nm; and / or, the molar ratio of iron element to phosphorus element in the second lithium iron phosphate is 0.950-0.960; and / or, the ratio of the molar amount of iron element in the second slurry to the molar amount of lithium element in the second lithium source is 1:(0.515-0.525); and / or, the second lithium source is selected from any one or more of lithium carbonate, lithium hydroxide and lithium phosphate; and / or, the mass of the second carbon source is 4wt%-6wt% of the mass of the modified lithium iron phosphate material; and / or, the second carbon source is selected from any one or more of glucose, sucrose and citric acid; and / or, the second solvent is selected from any one or more of deionized water, ethanol; and / or, the solid content of the second precursor is 30%-40%; and / or, the D50 particle size of the product obtained by the second ball milling is 400-500 nm; and / or, the temperature of the second drying is 200°C-300°C; and / or, the temperature of the second calcination is 650°C-750°C, and the time is 8h-12h; and / or, the second calcination is carried out in a protective atmosphere, and the protective atmosphere is nitrogen and / or argon.

[0014] Based on the influence factors of the main element content of the second lithium iron phosphate precursor and the characteristic influence of the doping elements on the growth of the second lithium iron phosphate particles in the later stage, by controlling the ratio of the molar amount of iron element in the second slurry to the molar amount of lithium element in the second lithium source, the mass of the second carbon source to the mass of the modified lithium iron phosphate material, the type of the second carbon source, the solid content of the second precursor, the D50 particle size of the product obtained by the second ball milling, the temperature of the second drying and the temperature of the second calcination within the above ranges, a second lithium iron phosphate precursor with a higher iron-phosphorus ratio and different element doping amounts is obtained, and then a large-particle lithium iron phosphate meeting the requirements is prepared after the second calcination, which is convenient for forming a particle grading during subsequent grinding. In addition, by reducing the iron-phosphorus molar ratio in the second lithium iron phosphate precursor through the feeding ratio of the second slurry, and increasing the feeding proportion of lithium element by controlling the ratio of the molar amount of iron element in the second slurry to the molar amount of lithium element in the second lithium source to be 1:(0.515-0.525) during the second ball milling, the loss of lithium element after the third calcination is avoided, and magnetic impurity phosphorus-iron compounds are formed.

[0015] Further, in the step S5, the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate and the mass ratio of the third carbon source to the fourth carbon source are each independently n, and 1≤n≤9; the grinding particle size D50 of the product obtained by grinding the mixture and n satisfy the relationship: grinding particle size D50=(1+1 / n)μm.

[0016] The relationship between the grinding particle size, the proportion of mixed carbon sources and the proportion of large and small particle slurries is determined, which is convenient for the coordinated control of the preparation processes before and after.

[0017] Further, in the step S4, the third carbon source is selected from any one or more of glucose, sucrose and citric acid; and / or, the fourth carbon source is selected from any one or more of polyethylene glycol, polyvinyl alcohol and polyvinylpyrrolidone; and / or, the ratio of the total molar amount of the first ferrous source and the second ferrous source to the total molar amount of the total metal elements in the metal coating source is 1:(0.0025-0.005); and / or, the metal coating source is selected from any one or more of manganese oxide, scandium oxide, tin oxide, titanium oxide, aluminum oxide, indium oxide, antimony oxide, bismuth oxide and zirconium oxide; and / or, the third solvent is selected from any one or more of deionized water and ethanol; and / or, the solid content of the mixture is 40%-50%; and / or, the grinding is performed in a single-cylinder self-circulating manner; and / or, the temperature of the third drying is 200°C-300°C; and / or, the temperature of the third calcination is 780°C-820°C, and the time is 4h-8h; and / or, the third calcination is performed in a protective atmosphere, and the protective atmosphere is nitrogen and / or argon.

[0018] In the grinding stage, different carbon sources with different properties are used, and the relationship between the grinding particle size, the mixing ratio of the carbon sources (the third carbon source and the fourth carbon source) and the mixing ratio of the large and small particle slurries is determined, so as to facilitate the synergistic control of the preparation processes before and after, that is, the small particle lithium iron phosphate is most seriously agglomerated in the grinding stage, and a carbon source with a dispersing effect is used, and the large particle lithium iron phosphate uses a conventional cheap and high residual carbon carbon source, and according to the actual particle size of the large and small particle lithium iron phosphate, the large and small particle lithium iron phosphate ratio and the feeding ratio of different carbon sources are determined, so as to form a dynamic ratio relationship, and even if the particle size of the lithium iron phosphate is too large or too small, it can also be rescued by adjusting the large and small particle slurry ratio, and the production flexibility is increased.

[0019] According to another aspect of the present application, a modified lithium iron phosphate material is provided, which is prepared by the above preparation method.

[0020] The modified lithium iron phosphate material obtained by the above preparation method has lower magnetic foreign matter impurities and better compaction and electrical properties.

[0021] According to still another aspect of the present application, a positive electrode is provided, which comprises a positive electrode material, and the positive electrode material is the modified lithium iron phosphate material described above.

[0022] The positive electrode comprising the modified lithium iron phosphate material of the present application has excellent compaction density and cycle performance and other electrical properties.

[0023] According to still another aspect of the present application, a lithium ion battery is provided, which comprises a positive electrode and a negative electrode, and the positive electrode is the positive electrode described above.

[0024] The lithium ion battery comprising the above positive electrode of the present application has excellent energy density, cycle performance and rate performance and other electrical properties.

[0025] By controlling the feeding ratio of ferrous source, phosphorus source and doped metal salt solution, the slurry with different doping contents and different iron-phosphorus ratios is prepared. After adding excess lithium source and suitable carbon source, lithium iron phosphate with different particle sizes is obtained. The lithium iron phosphate with different particle sizes is mixed with composite carbon source, metal coating source and third solvent according to the ratio, ground to the target particle size, dried, calcined and crushed to obtain the finished modified lithium iron phosphate material. Through the above method, the doping elements are introduced during the preparation of the precursor on the basis of fully utilizing the preparation process of the precursor iron phosphate. The cheap soluble doping source (metal salt solution) is used to reduce the cost and ensure uniform distribution, and the dehydration and calcination process is omitted to reduce the energy consumption. By adjusting the content of main elements, iron-phosphorus ratio and doping amount of the precursor, the influence of the precursor on the growth of the lithium iron phosphate particles is utilized, and the large and small particles of lithium iron phosphate required are directly prepared by the first calcination and the second calcination, which is convenient for the grading during the secondary grinding. The first calcination and the second calcination respectively use high graphitization degree organic macromolecular carbon source and cheap high residual carbon content small molecular carbon source; the grinding uses mixed carbon source according to the particle ratio, which maximizes the improvement of the graphitization degree of the coated carbon and the performance of the material while reducing the cost, and controls the thickness of the carbon coating. The relationship between the grinding particle size, the proportion of mixed carbon source and the feeding ratio of large and small particles is determined in the grinding stage, the process is cooperatively controlled, and the Li a M2(PO4)3 (such as Li5Mn2(PO4)3, Li3Sc2(PO4)3, etc.) fast ion conductor coating layer is used to improve the performance of the material. In addition, since the proportion of iron elements is reduced at the feeding end, it is beneficial to inhibit the generation of magnetic substances phosphorus iron compounds, and therefore, the obtained lithium iron phosphate positive electrode material has low magnetic foreign impurities and good compaction and electrical properties. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification. The illustrations in the drawings for the first embodiment of the application are used to explain the application and do not constitute an improper limitation of the application. In the drawings:

[0027] Figure 1 The SEM image of the small particle lithium iron phosphate positive electrode material B1 according to the first embodiment of the application is shown.

[0028] Figure 2 The SEM image of the large particle lithium iron phosphate positive electrode material B2 according to the first embodiment of the application is shown.

[0029] Figure 3 The SEM image of the finished lithium iron phosphate positive electrode material prepared according to the first embodiment of the application is shown. DETAILED DESCRIPTION

[0030] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other in the case of no conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0031] As analyzed in the background, the existing preparation method of lithium iron phosphate has problems of high cost, difficulty in controlling the thickness of carbon coating, and excessive magnetic foreign matter in the prepared lithium iron phosphate. In order to solve the technical problem, the present application provides a modified lithium iron phosphate material, a preparation method thereof, a positive electrode and a lithium ion battery.

[0032] In a typical embodiment of the present application, a preparation method of a modified lithium iron phosphate material is provided, the modified lithium iron phosphate material comprising Li x Fe y PO4and a carbon coating layer and a Li x Fe y PO4surface, and a Li a M2(PO4)3coating layer, the chemical formula of the modified lithium iron phosphate material being Li x Fe y PO4@C@Li a M2(PO4)3, wherein 0.95≤x≤1.05, 0.95≤y≤0.97, 1≤a≤5, and M is selected from any one or more of Mn, Ti, Sc, Sn, Al, In, Sb, Bi and Zr; the preparation method comprising: step S1, mixing raw materials comprising a first ferrous source, a first phosphorus source and a first metal salt solution to obtain a first slurry; step S2, mixing raw materials comprising a second ferrous source, a second phosphorus source and a second metal salt solution to obtain a second slurry; step S3, mixing raw materials comprising the first slurry, a first lithium source, a first solvent and a first carbon source to obtain a first precursor, and sequentially performing first ball milling, first drying and first calcination on the first precursor to obtain a first lithium iron phosphate; step S4, mixing raw materials comprising the second slurry, a second lithium source, a second solvent and a second carbon source to obtain a second precursor, and sequentially performing second ball milling, second drying and second calcination on the second precursor to obtain a second lithium iron phosphate; step S5, mixing raw materials comprising the first lithium iron phosphate, the second lithium iron phosphate, a third carbon source, a fourth carbon source, a metal coating source and a third solvent to obtain a mixture, and sequentially performing grinding, third drying and third calcination on the mixture to obtain the modified lithium iron phosphate material; wherein the molar ratio of iron to phosphorus in the first slurry is higher than the molar ratio of iron to phosphorus in the second slurry, and the concentration of total metal ions in the first slurry is higher than the concentration of total metal ions in the second slurry; the average particle size of the first lithium iron phosphate is smaller than the average particle size of the second lithium iron phosphate.

[0033] By controlling the feeding ratio of ferrous source, phosphorus source, and doped metal salt solution, slurries with different doping contents and different iron-phosphorus ratios are prepared; by adding excess lithium source and suitable carbon source respectively, lithium iron phosphate with different particle sizes is obtained after calcination; by mixing lithium iron phosphate with different particle sizes according to the ratio, composite carbon source, metal coating source, and third solvent, grinding to the target particle size, drying, calcining, and crushing, the finished modified lithium iron phosphate material is obtained. Through the above method, the doping element is introduced during the preparation of the precursor on the basis of fully utilizing the preparation process of the precursor iron phosphate, a cheap soluble doping source (metal salt solution) is used to reduce the cost and ensure uniform distribution, and the dehydration and calcination process is omitted to reduce energy consumption. By adjusting the content of the main elements of the precursor, the iron-phosphorus ratio, and the doping amount, the influence of the precursor on the growth of the lithium iron phosphate particles is utilized, and the required large and small particles of lithium iron phosphate are directly prepared through the first calcination and the second calcination, which is convenient for the formation of grading during secondary grinding. The first calcination and the second calcination respectively use high-graphitization organic high-molecular carbon source and cheap high-residual-carbon small-molecule carbon source; the grinding uses mixed carbon source according to the particle ratio, which maximizes the improvement of the carbon coating graphite degree and the material performance while reducing the cost, and controls the carbon coating thickness. The relationship between the grinding particle size, the mixed carbon source ratio, and the large and small particle feeding ratio is determined in the grinding stage, the process is cooperatively controlled, and the Li a M2(PO4)3 (such as Li5Mn2(PO4)3, Li3Sc2(PO4)3, etc.) fast ion conductor coating layer is used to improve the material performance. In addition, since the proportion of iron elements is reduced at the feeding end, it is beneficial to inhibit the generation of magnetic substances phosphorus-iron compounds, and therefore, the obtained lithium iron phosphate positive electrode material has lower magnetic foreign matter impurities and better compaction and electrical performance.

[0034] In an embodiment of the present application, in step S1, the molar ratio of the first ferrous source, the first metal salt solution, and the first phosphorus source is 1:(0.050-0.100):(1.030-1.040); and / or, the first ferrous source is selected from any one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride, and ferrous nitrate; and / or, the first phosphorus source is selected from any one or more of diammonium hydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate; and / or, the first metal salt solution is a sulfate solution, and the sulfate solution is selected from any one or more of titanium sulfate solution, manganese sulfate solution, scandium sulfate solution, zirconium sulfate solution, antimony sulfate solution, and bismuth sulfate solution.

[0035] The above first metal salt solution in the first slurry can introduce a doping element in the preparation stage of the first lithium iron phosphate, on the one hand, the cost of the doping element is reduced, on the other hand, the doping element is more uniformly distributed, uniform doping and uniform primary fast ion conductor coating are formed; the precursor dihydrate dehydration calcination process is omitted, and the energy consumption cost is reduced. Controlling the molar ratio of the first ferrous source, the first metal salt solution and the first phosphorus source in the above range helps to control the content of the doping element in the first lithium iron phosphate from the source, thereby improving the performance of the first lithium iron phosphate.

[0036] In an embodiment of the present application, in step S2, the molar ratio of the second ferrous source, the second metal salt solution and the second phosphorus source is 1:(0.025-0.050):(1.040-1.050); and / or, the second ferrous source is selected from any one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride and ferrous nitrate; and / or, the second phosphorus source is selected from any one or more of diammonium hydrogen phosphate, phosphoric acid and ammonium dihydrogen phosphate; and / or, the second metal salt solution is a sulfate solution, and the sulfate solution is selected from any one or more of titanium sulfate solution, manganese sulfate solution, scandium sulfate solution, zirconium sulfate solution, antimony sulfate solution and bismuth sulfate solution. Preferably, the sulfate solution is titanium sulfate solution.

[0037] The above second metal salt solution in the second slurry can introduce a doping element in the preparation stage of the second lithium iron phosphate, on the one hand, the cost of the doping element is reduced, on the other hand, the doping element is more uniformly distributed, uniform doping and uniform primary fast ion conductor coating are formed; the precursor dihydrate dehydration calcination process is omitted, and the energy consumption cost is reduced. Controlling the molar ratio of the second ferrous source, the second metal salt solution and the second phosphorus source in the above range helps to control the content of the doping element in the second lithium iron phosphate from the source, thereby improving the performance of the second lithium iron phosphate.

[0038] In an embodiment of the present application, in step S3, the D50 particle size of the first lithium iron phosphate is 600-1000 nm; and / or, the molar ratio of iron elements to phosphorus elements in the first lithium iron phosphate is 0.960-0.970; and / or, the ratio of the molar amount of iron elements in the first slurry to the molar amount of lithium elements in the first lithium source is 1:(0.515-0.525); and / or, the first lithium source is selected from any one or more of lithium carbonate, lithium hydroxide and lithium phosphate; and / or, the mass of the first carbon source is 4wt%-6wt% of the mass of the modified lithium iron phosphate material; and / or, the first carbon source is selected from any one or more of polyethylene glycol, polyvinyl alcohol and polyvinylpyrrolidone; and / or, the first solvent is selected from any one or more of deionized water and ethanol; and / or, the solid content of the first precursor is 40%-50%; and / or, the D50 particle size of the product obtained by the first ball milling is 300-400 nm; and / or, the temperature of the first drying is 200°C-300°C; and / or, the temperature of the first calcination is 450°C-550°C, and the time is 8h-12h; and / or, the first calcination is performed in a protective atmosphere, and the protective atmosphere is nitrogen and / or argon.

[0039] Based on the influence factors of the main element content of the first lithium iron phosphate precursor and the characteristic influence of the doping elements on the growth of the first lithium iron phosphate particles at the back end, by controlling the ratio of the molar amount of iron elements in the first slurry to the molar amount of lithium elements in the first lithium source, the mass of the first carbon source to the mass of the modified lithium iron phosphate material, the type of the first carbon source, the D50 particle size of the product obtained by the first ball milling, the solid content of the first precursor, the temperature of the first drying and the temperature of the first calcination within the above ranges, a first lithium iron phosphate precursor with a higher iron-phosphorus ratio and different element doping amounts is obtained, and then a small particle lithium iron phosphate meeting the requirements is prepared after the first calcination, which is convenient for forming a particle grading during subsequent grinding. In addition, by reducing the iron-phosphorus molar ratio in the first lithium iron phosphate precursor through the feeding ratio of the first slurry, and increasing the feeding ratio of lithium elements by controlling the ratio of the molar amount of iron elements in the first slurry to the molar amount of lithium elements in the first lithium source to be 1:(0.515-0.525) during the first ball milling, the loss of lithium elements after the third calcination is avoided, and magnetic impurity phosphorus-iron compounds are formed.

[0040] In an embodiment of the present application, in step S4, the D50 particle size of the second lithium iron phosphate is 1500-2500 nm; and / or, the molar ratio of iron to phosphorus in the second lithium iron phosphate is 0.950-0.960; and / or, the ratio of the molar amount of iron in the second slurry to the molar amount of lithium in the second lithium source is 1:(0.515-0.525); and / or, the second lithium source is selected from any one or more of lithium carbonate, lithium hydroxide and lithium phosphate; and / or, the mass of the second carbon source is 4wt%-6wt% of the mass of the modified lithium iron phosphate material; and / or, the second carbon source is selected from any one or more of glucose, sucrose and citric acid; and / or, the second solvent is selected from any one or more of deionized water, ethanol; and / or, the solid content of the second precursor is 30%-40%; and / or, the D50 particle size of the product obtained by the second ball milling is 400-500 nm; and / or, the temperature of the second drying is 200°C-300°C; and / or, the temperature of the second calcination is 650°C-750°C, and the time is 8h-12h; and / or, the second calcination is performed in a protective atmosphere, and the protective atmosphere is nitrogen and / or argon.

[0041] Based on the influence factors of the main element content of the second lithium iron phosphate precursor and the characteristic influence of the doping elements on the growth of the second lithium iron phosphate particles in the later stage, by controlling the ratio of the molar amount of iron in the second slurry to the molar amount of lithium in the second lithium source, the mass of the second carbon source to the mass of the modified lithium iron phosphate material, the type of the second carbon source, the solid content of the second precursor, the D50 particle size of the product obtained by the second ball milling, the temperature of the second drying and the temperature of the second calcination within the above ranges, a second lithium iron phosphate precursor with a higher iron to phosphorus ratio and different element doping amounts is obtained, and then a large particle lithium iron phosphate meeting the requirements is prepared after the second calcination, which is convenient for forming a particle grading during subsequent grinding. In addition, by reducing the iron to phosphorus molar ratio in the second lithium iron phosphate precursor through the feeding ratio of the second slurry, and increasing the feeding ratio of lithium elements by controlling the ratio of the molar amount of iron in the second slurry to the molar amount of lithium in the second lithium source to be 1:(0.515-0.525) during the second ball milling, the loss of lithium elements after the third calcination is avoided, and magnetic impurity phosphorus iron compounds are not formed.

[0042] In an embodiment of the present application, in step S5, the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate and the mass ratio of the third carbon source to the fourth carbon source are each independently n, and 1≤n≤9; the grinding particle size D50 of the product obtained by grinding the mixture and n satisfy the relationship: grinding particle size D50=(1+1 / n)μm.

[0043] The relationship between the grinding particle size, the mixing ratio of the carbon sources and the mixing ratio of the large and small particle slurries is clear, which is convenient for the coordinated control of the preparation processes before and after.

[0044] In an embodiment of the present application, in step S4, the third carbon source is selected from any one or more of glucose, sucrose and citric acid; and / or, the fourth carbon source is selected from any one or more of polyethylene glycol, polyvinyl alcohol and polyvinylpyrrolidone; and / or, the ratio of the total molar amount of the first ferrous source and the second ferrous source to the total molar amount of the total metal elements in the metal coating source is 1:(0.0025-0.005); and / or, the metal coating source is selected from any one or more of manganese oxide, scandium oxide, tin oxide, titanium oxide, aluminum oxide, indium oxide, antimony oxide, bismuth oxide and zirconium oxide; and / or, the third solvent is selected from any one or more of deionized water and ethanol; and / or, the solid content of the mixture is 40%-50%; and / or, the grinding is performed in a single-cylinder self-circulation mode; and / or, the temperature of the third drying is 200°C-300°C; and / or, the temperature of the third calcination is 780°C-820°C, and the time is 4h-8h; and / or, the third calcination is performed in a protective atmosphere, and the protective atmosphere is nitrogen and / or argon.

[0045] In the grinding stage, different carbon sources with different properties are used, and the relationship between the grinding particle size, the mixing ratio of the third carbon source and the fourth carbon source, and the mixing ratio of the large and small particle slurries is determined, so as to facilitate the synergistic control of the preparation process. That is, the small particle lithium iron phosphate is most severely agglomerated in the grinding stage, and a carbon source with dispersing effect is used. The large particle lithium iron phosphate uses a conventional, inexpensive and high-residual carbon carbon source. According to the actual particle size of the large and small particle lithium iron phosphate, the large and small particle lithium iron phosphate ratio and the different carbon source feeding ratio are determined to form a dynamic ratio relationship. Even if the particle size of the lithium iron phosphate is too large or too small, it can also be rescued by adjusting the large and small particle slurry ratio, thereby increasing the flexibility of production.

[0046] In addition, the structure corresponding to the single-cylinder self-circulation mode is that the sand mill inlet is connected to the bottom of the mixing tank, and the sand mill outlet is connected to the top of the same mixing tank.

[0047] In another typical embodiment of the present application, a modified lithium iron phosphate material is provided, which is prepared by the above preparation method.

[0048] The modified lithium iron phosphate material obtained by the above preparation method has lower magnetic foreign matter impurities and better compaction and electrical properties.

[0049] In yet another typical embodiment of the present application, a positive electrode is provided, which comprises a positive electrode material, and the positive electrode material is the modified lithium iron phosphate material described above.

[0050] The positive electrode comprising the modified lithium iron phosphate material of the present application has excellent compaction density and cycle performance and other electrical properties.

[0051] In another typical embodiment of the present application, a lithium ion battery is provided, comprising a positive electrode and a negative electrode, wherein the positive electrode is the aforementioned positive electrode.

[0052] The lithium ion battery comprising the aforementioned positive electrode has excellent electrical properties such as energy density, cycle performance and rate capability.

[0053] The advantages of the present application will be illustrated in combination with specific examples.

[0054] Example 1

[0055] In this example, a carbon-composite lithium iron phosphate is prepared, and the specific steps are as follows:

[0056] 1) 1 mol of ferrous sulfate, 0.08 mol of titanium sulfate, and 1.035 mol of ammonium dihydrogen phosphate are used to prepare a dihydrate iron phosphate slurry A1 (first slurry) with a high iron-phosphorus ratio and high metal ion content;

[0057] 2) 1 mol of ferrous sulfate, 0.04 mol of titanium sulfate, and 1.045 mol of ammonium dihydrogen phosphate are used to prepare a dihydrate iron phosphate slurry A2 (second slurry) with a low iron-phosphorus ratio and low metal ion content;

[0058] 3) The dihydrate iron phosphate slurry A1 of step 1) is mixed with 0.5 mol of lithium carbonate and 8 g of polyethylene glycol at a target solid content of 45% and deionized water, and the grinding particle size D50 is 350 nm; the drying temperature is 290°C, the calcination temperature is 500°C, and the holding time in a nitrogen atmosphere is 10 h to obtain a preliminary small-particle lithium iron phosphate B1 (first lithium iron phosphate), and the SEM image thereof is as shown in Figure 1 ;

[0059] 4) The dihydrate iron phosphate slurry A2 of step 2) is mixed with 0.5 mol of lithium carbonate and 8 g of glucose at a target solid content of 35% and deionized water, and the grinding particle size D50 is 450 nm; the drying temperature is 290°C, the calcination temperature is 700°C, and the holding time in a nitrogen atmosphere is 10 h to obtain a preliminary large-particle lithium iron phosphate B2 (second lithium iron phosphate), and the SEM image thereof is as shown in Figure 2 ;

[0060] 5) The small particle lithium iron phosphate B1 of step 3) and the large particle lithium iron phosphate B2 of step 4) are put into deionized water according to the mass ratio of 8:2, and the mass ratio of polyethylene glycol to glucose is 8:2, with a target solid content of 45%, the total mass of the carbon source is 5% of the total mass of the large and small particle lithium iron phosphate, and manganese oxide is added according to the molar ratio of manganese to total iron source of 0.004:1. A single-cylinder self-circulation mode is used, that is, the sand mill feed port is connected with the bottom of the mixing tank, and the sand mill discharge port is connected with the top of the same mixing tank. According to the grinding particle size D50, the mass ratio of lithium iron phosphate B1 to lithium iron phosphate B2 n exists the following relationship: D50 = (1 + 1 / n) μm, the grinding end point particle size D50 = (1 + 1 / 4) = 1.25 μm is calculated, the drying temperature is 290°C, the calcination temperature is 800°C, and the finished carbon composite lithium iron phosphate is obtained after 5h of heat preservation in a nitrogen atmosphere, and the SEM diagram is shown in Figure 3 .

[0061] Example 2

[0062] The difference from example 1 is that "0.08 mol titanium sulfate" in step 1) is replaced with "0.1 mol titanium sulfate", and "0.04 mol titanium sulfate" in step 2) is replaced with "0.05 mol titanium sulfate", and finally carbon composite lithium iron phosphate is obtained.

[0063] Example 3

[0064] The difference from example 1 is that "0.08 mol titanium sulfate" in step 1) is replaced with "0.04 mol titanium sulfate", and "0.04 mol titanium sulfate" in step 2) is replaced with "0.02 mol titanium sulfate", and finally carbon composite lithium iron phosphate is obtained.

[0065] Example 4

[0066] The difference from example 1 is that "8g polyethylene glycol" in step 3) is replaced with "8g polyvinyl alcohol", and "8g glucose" in step 4) is replaced with "8g citric acid", and finally carbon composite lithium iron phosphate is obtained.

[0067] Example 5

[0068] The difference from example 1 is that "8g polyethylene glycol" in step 3) is replaced with "12g polyethylene glycol", and "8g glucose" in step 4) is replaced with "12g glucose", and finally carbon composite lithium iron phosphate is obtained.

[0069] Example 6

[0070] The difference from Example 1 is that "D50 is 350 nm" in step 3) is replaced by "D50 is 300 nm", and "D50 is 450 nm" in step 4) is replaced by "D50 is 500 nm", and finally carbon-composite lithium iron phosphate is obtained.

[0071] Example 7

[0072] The difference from Example 1 is that "D50 is 350 nm" in step 3) is replaced by "D50 is 450 nm", and "D50 is 450 nm" in step 4) is replaced by "D50 is 350 nm", and finally carbon-composite lithium iron phosphate is obtained.

[0073] Example 8

[0074] The difference from Example 1 is that "D50 is 350 nm" in step 3) is replaced by "D50 is 300 nm", and "D50 is 450 nm" in step 4) is replaced by "D50 is 500 nm", and finally carbon-composite lithium iron phosphate is obtained.

[0075] Example 9

[0076] The difference from Example 1 is that "D50 is 350 nm" in step 3) is replaced by "D50 is 300 nm", and "D50 is 450 nm" in step 4) is replaced by "D50 is 500 nm", and finally carbon-composite lithium iron phosphate is obtained.

[0077] Example 10

[0078] The difference from Example 1 is that in step 5), “mixing lithium iron phosphate B1 of step 3) with lithium iron phosphate B2 of step 4) in a mass ratio of 8:2, and polyethylene glycol with glucose in a mass ratio of 8:2” is replaced with “mixing lithium iron phosphate B1 of step 3) with lithium iron phosphate B2 of step 4) in a mass ratio of 0.8:1, and polyethylene glycol with glucose in a mass ratio of 0.8:1”; and in step 5), “calculating the grinding end particle size D50=(1+1 / n) μm according to the grinding particle size D50 and the mass ratio n of lithium iron phosphate B1 to lithium iron phosphate B2, D50=(1+1 / 4)=1.25 μm” is replaced with “calculating the grinding end particle size D50=(1+1 / n) μm according to the grinding particle size D50 and the mass ratio n of lithium iron phosphate B1 to lithium iron phosphate B2, D50=(1+1 / 0.8)=2.25 μm”, to obtain carbon-composite lithium iron phosphate finally.

[0079] Example 11

[0080] The difference from Example 1 is that in step 5), “mixing lithium iron phosphate B1 of step 3) with lithium iron phosphate B2 of step 4) in a mass ratio of 8:2, and polyethylene glycol with glucose in a mass ratio of 8:2” is replaced with “mixing lithium iron phosphate B1 of step 3) with lithium iron phosphate B2 of step 4) in a mass ratio of 10:1, and polyethylene glycol with glucose in a mass ratio of 10:1”; and in step 5), “calculating the grinding end particle size D50=(1+1 / n) μm according to the grinding particle size D50 and the mass ratio n of lithium iron phosphate B1 to lithium iron phosphate B2, D50=(1+1 / 4)=1.25 μm” is replaced with “calculating the grinding end particle size D50=(1+1 / n) μm according to the grinding particle size D50 and the mass ratio n of lithium iron phosphate B1 to lithium iron phosphate B2, D50=(1+1 / 10)=1.1 μm”, to obtain carbon-composite lithium iron phosphate finally.

[0081] Example 12

[0082] The difference from Example 1 is that in step 1), “1.035 mol of ammonium dihydrogen phosphate” is replaced with “1.040 mol of ammonium dihydrogen phosphate”; and in step 2), “1.045 mol of ammonium dihydrogen phosphate” is replaced with “1.050 mol of ammonium dihydrogen phosphate”, to obtain carbon-composite lithium iron phosphate finally.

[0083] Example 13

[0084] The difference from Example 1 is that in step 5), “adding manganese oxide according to a molar ratio of manganese:total iron source=0.004:1 at the same time” is replaced with “adding zirconium oxide according to a molar ratio of zirconium:total iron source=0.005:1 at the same time”, to obtain carbon-composite lithium iron phosphate finally.

[0085] Example 14

[0086] The difference from Example 1 is that the "manganese oxide is added simultaneously according to the molar ratio of manganese: total iron source = 0.004:1 in step 5) is replaced by "zirconium oxide is added simultaneously according to the molar ratio of zirconium: total iron source = 0.0025:1", and finally carbon composite lithium iron phosphate is obtained.

[0087] Example 15

[0088] The difference from Example 1 is that the "manganese oxide is added simultaneously according to the molar ratio of manganese: total iron source = 0.004:1 in step 5) is replaced by "zirconium oxide is added simultaneously according to the molar ratio of zirconium: total iron source = 0.002:1", and finally carbon composite lithium iron phosphate is obtained.

[0089] Example 16

[0090] The difference from Example 1 is that the "calcination temperature is 500°C in step 3) is replaced by "calcination temperature is 550°C"; the "calcination temperature is 700°C in step 4) is replaced by "calcination temperature is 650°C"; the "calcination temperature is 800°C in step 5) is replaced by "calcination temperature is 780°C", and finally carbon composite lithium iron phosphate is obtained.

[0091] Example 17

[0092] The difference from Example 1 is that the "calcination temperature is 500°C in step 3) is replaced by "calcination temperature is 400°C"; the "calcination temperature is 700°C in step 4) is replaced by "calcination temperature is 600°C"; the "calcination temperature is 800°C in step 5) is replaced by "calcination temperature is 760°C", and finally carbon composite lithium iron phosphate is obtained.

[0093] Comparative Example 1

[0094] The difference from Example 1 is that the "0.08 mol titanium sulfate" in step 1) is deleted, and finally carbon composite lithium iron phosphate is obtained.

[0095] Comparative Example 2

[0096] The difference from Example 1 is that the "8g polyethylene glycol" in step 3) is replaced by "8g glucose"; the "and the mass ratio of polyethylene glycol to glucose is 8:2." in step 5) is replaced by "glucose", and finally carbon composite lithium iron phosphate is obtained.

[0097] Comparative Example 3

[0098] The difference from Example 1 is that:

[0099] 2) 1 mol ferrous sulfate, 0.04 mol titanium sulfate, 1.045 mol ammonium dihydrogen phosphate to prepare a low iron-phosphorus ratio, low metal ion content of iron phosphate dihydrate slurry A2 (second slurry);

[0100] 4) The iron phosphate dihydrate slurry A2 of step 2) is mixed with 0.5 mol lithium carbonate, 8 g glucose, and 35% target solid content with deionized water, and the grinding particle size D50 is 450 nm; the drying temperature is 290°C, the calcination temperature is 700°C, and the holding time in a nitrogen atmosphere is 10 h to obtain a preliminary large particle lithium iron phosphate B2 (second lithium iron phosphate);

[0101] 5) The large particle lithium iron phosphate B2 of step 4) and polyethylene glycol and glucose are mixed according to a mass ratio of 8:2, with a target solid content of 45%, and the total mass of the carbon source is 5% of the total mass of the large particle lithium iron phosphate, and manganese oxide is added according to a molar ratio of manganese: total iron source = 0.004:1. A single-cylinder self-circulation method is used, i.e. the sand mill feed port is connected to the bottom of the mixing tank, and the sand mill discharge port is connected to the top of the same mixing tank to control the final particle size D50 = 1.25 μm, the drying temperature is 290°C, the calcination temperature is 800°C, and the holding time in a nitrogen atmosphere is 5 h, and then the product carbon composite lithium iron phosphate is obtained after crushing.

[0102] Comparative Example 4

[0103] The difference from Example 1 is:

[0104] 1) 1 mol ferrous sulfate, 0.08 mol titanium sulfate, 1.035 mol ammonium dihydrogen phosphate to prepare a high iron-phosphorus ratio, high metal ion content of iron phosphate dihydrate slurry A1 (first slurry);

[0105] 3) The iron phosphate dihydrate slurry A1 of step 1) is mixed with 0.5 mol lithium carbonate, 8 g polyethylene glycol, and 45% target solid content with deionized water, and the grinding particle size D50 is 350 nm; the drying temperature is 290°C, the calcination temperature is 500°C, and the holding time in a nitrogen atmosphere is 10 h to obtain a preliminary small particle lithium iron phosphate B1 (first lithium iron phosphate);

[0106] 5) The small particle lithium iron phosphate B1, polyethylene glycol and glucose in step 3) are put into deionized water according to a mass ratio of 8:2 with a target solid content of 45%, the total mass of the carbon source is 5% of the total mass of the small particle lithium iron phosphate, and manganese oxide is added according to a molar ratio of manganese: total iron source = 0.004:1. A single-cylinder self-circulation mode is adopted, that is, the sand mill feed port is connected with the bottom of the mixing tank, and the sand mill discharge port is connected with the top of the same mixing tank, the control end particle size D50 = 1.25 μm, the drying temperature is 290°C, the calcination temperature is 800°C, and the product carbon composite lithium iron phosphate is obtained after crushing for 5h in a nitrogen atmosphere.

[0107] Comparative Example 5

[0108] The difference from Example 1 is that the "manganese oxide is added according to a molar ratio of manganese: total iron source = 0.004:1" in step 5) is deleted, and the final carbon composite lithium iron phosphate is obtained.

[0109] Comparative Example 6

[0110] 1) 1 mol of ferrous sulfate, 0.04 mol of titanium sulfate, and 1.045 mol of ammonium dihydrogen phosphate are used to prepare a low iron-phosphorus ratio and low metal ion content dihydrate iron phosphate slurry A1 (first slurry);

[0111] 2) 1 mol of ferrous sulfate, 0.08 mol of titanium sulfate, and 1.035 mol of ammonium dihydrogen phosphate are used to prepare a high iron-phosphorus ratio and high metal ion content dihydrate iron phosphate slurry A2 (second slurry);

[0112] 3) The dihydrate iron phosphate slurry A1 in step 1) is mixed with 0.5 mol of lithium carbonate and 8 g of glucose in deionized water according to a target solid content of 35%, and the grinding particle size D50 is 450 nm; the drying temperature is 290°C, the calcination temperature is 700°C, and the final product of the preliminary large particle lithium iron phosphate B1 (first lithium iron phosphate) is obtained after 10h of heat preservation in a nitrogen atmosphere;

[0113] 4) The dihydrate iron phosphate slurry A2 in step 2) is mixed with 0.5 mol of lithium carbonate and 8 g of polyethylene glycol in deionized water according to a target solid content of 45%, and the grinding particle size D50 is 350 nm; the drying temperature is 290°C, the calcination temperature is 500°C, and the final product of the preliminary small particle lithium iron phosphate B2 (second lithium iron phosphate) is obtained after 10h of heat preservation in a nitrogen atmosphere;

[0114] 5) The large particle lithium iron phosphate B1 of step 3) and the small particle lithium iron phosphate B2 of step 4) are put into deionized water with a mass ratio of 8:2, and a mass ratio of polyethylene glycol to glucose of 8:2, with a target solid content of 45%, and the total mass of the carbon source is 5% of the total mass of the large and small particle lithium iron phosphate, and manganese oxide is added according to a molar ratio of manganese:total iron source = 0.004:1. A single-cylinder self-circulation mode is used, i.e. the sand mill feed port is connected to the bottom of the mixing tank, and the sand mill discharge port is connected to the top of the same mixing tank. The grinding particle size D50, the mass ratio of lithium iron phosphate B1 to lithium iron phosphate B2 n exists the following relationship: D50 = (1 + 1 / n) pm, the grinding end point particle size D50 = (1 + 1 / 4) = 1.25 pm is calculated, the drying temperature is 290°C, the calcination temperature is 800°C, and the finished product carbon composite lithium iron phosphate is obtained after crushing for 5h in a nitrogen atmosphere.

[0115] Material performance test:

[0116] Button cell production:

[0117] The carbon composite lithium iron phosphate materials prepared in the examples and comparative examples are used for the positive electrode, and then used with auxiliary materials and auxiliary materials to make button cells to analyze the button cell performance. In addition to the positive electrode material, other auxiliary materials and auxiliary materials required for button cell production are any one of the auxiliary materials or auxiliary materials that can be purchased on the market.

[0118] The carbon composite lithium iron phosphate material, SP conductive agent, and PVDF binder are weighed and mixed according to a mass ratio of 8:1:1, and a CR2032 type button half cell is made in an argon protective atmosphere.

[0119] Button cell test:

[0120] The finished button half cell is tested for charging and discharging at room temperature, with a voltage range of 2.5-4.0V.

[0121] Table 1 is the charging and discharging data and compaction density of the batteries obtained from the lithium iron phosphate positive electrode materials prepared in the above examples and comparative examples. In the table below: first efficiency% = first discharge capacity / first charge capacity x 100%, 0.2C discharge specific capacity: the discharge specific capacity obtained by charging and discharging at 0.2C after the first charging and discharging test at 0.2C, 1C discharge specific capacity: the discharge specific capacity obtained by charging and discharging at 1C after the 0.2C charging and discharging test, 2C discharge specific capacity: the discharge specific capacity obtained by charging and discharging at 2C after the 1C charging and discharging test.

[0122] The average particle size of the first lithium iron phosphate and the average particle size of the second lithium iron phosphate: the particle size is tested by using a Malvern Mastersizer 3000 particle size instrument according to GB / T 19077-2016 Particle Size Analysis-Laser Diffraction Method Part 1: General Principles.

[0123] The molar ratio of iron elements to phosphorus elements in the first lithium iron phosphate and the molar ratio of iron elements to phosphorus elements in the second lithium iron phosphate: the iron content is determined according to the test method specified in Appendix A of GB / T 33822-2017: Determination of Iron Content-Potentiometric Titration Method, the phosphorus content is determined according to the test method specified in Appendix B of GB / T 33822-2017: Determination of Phosphorus Content-Quinoline Molybdenum Ketone Gravimetric Method, and then the iron-phosphorus ratio is calculated.

[0124] Table 1 lists the charge-discharge data and compaction density data of the lithium iron phosphate positive electrode materials prepared in the examples and comparative examples. Table 2 lists the average particle size of the first lithium iron phosphate, the average particle size of the second lithium iron phosphate, the molar ratio of iron elements to phosphorus elements in the first lithium iron phosphate, and the molar ratio of iron elements to phosphorus elements in the second lithium iron phosphate.

[0125] Table 1

[0126]

[0127]

[0128] From the above description, it can be seen that the above-mentioned examples of the present application realize the preparation of lithium iron phosphate materials with excellent performance. When the obtained materials are used as lithium ion battery positive electrode materials and lithium ion batteries are prepared, the corresponding batteries show better performance.

[0129] Specifically, compared with Example 1, Example 2 appropriately increases the addition amount of the titanium content of the prepared large and small particles, and the influence on the compaction and electrical properties of the finished product is not obvious; Example 3 excessively reduces the addition amount of the titanium content of the prepared large and small particles, so that the growth of the small particles is slightly weakened during the second firing, the proportion of large particles increases, the finished product grading effect is slightly worse, the compaction is reduced, and the titanium content is reduced, and the rate performance is also reduced; Example 4 changes the carbon source when preparing large and small particles, and the performance of the final product is not obviously affected because the same type of carbon source is replaced and the content is not changed; Example 5 excessively increases the addition amount of the carbon source when preparing large and small particles, which causes the growth of large and small particles to be inhibited to a certain extent, and the grinding efficiency of the mixed large and small particles increases, but the particle size test result reaches the target value, and the actual particle size is small due to the excessive carbon layer coating, and at the same time, the excessive carbon coating affects the subsequent fast ion conductor coating bonding strength, so the compaction and electrical properties of the finished product are reduced; Example 6 appropriately adjusts the grinding particle size when preparing large and small particles, and the electrical properties and compaction are not obviously affected; Example 7 excessively increases the grinding particle size when preparing small particles, and at the same time, excessively reduces the grinding particle size when preparing large particles, which will cause the particle size of small particles to be too large and the particle size of large particles to be too small, the grading effect of the mixed large and small particles is weakened, and the electrical properties and compaction are adversely affected; Examples 8 and 9 appropriately adjust the ratio of large and small particles, and the performance of the finished product is not obviously affected; Example 10 excessively reduces the proportion of small particles, and Example 11 excessively increases the proportion of small particles, which will cause the grading effect to be poor, the compaction to be reduced, and at the same time, the proportion of small particles with a high iron-phosphorus ratio is excessively increased, so that the lithium element is more likely to volatilize and lose after high-temperature sintering, the proportions of iron and phosphorus elements are increased, and magnetic impurity phosphorus-iron compounds are easily generated; Example 12 appropriately increases the phosphorus content when preparing large and small particles, and the performance of the finished product is not obviously affected; Example 13 appropriately changes the type of metal elements added after mixing large and small particles, and Example 14 appropriately changes the addition amount of metal elements added after mixing large and small particles, and the performance of the finished product is not obviously affected; Example 15 excessively reduces the addition amount of metal elements added after mixing large and small particles, which will reduce the coating effect of the fast ion conductor, and the roundness and grading effect will be affected, so the compaction and electrical properties are reduced; Example 16 appropriately adjusts the sintering temperature of the prepared large and small particles and the sintering temperature after mixing large and small particles, and the performance of the finished product is not obviously affected; Example 17 excessively reduces the sintering temperature of the prepared large and small particles and the sintering temperature after mixing large and small particles, the small particles are poorly formed, the growth size of the large particles does not reach the target value, the final product grading is poor, and therefore the compaction and electrical properties are reduced.

[0130] Compared with Example 1, in Comparative Example 1, no titanium source is doped during the synthesis of iron phosphate, and the particles of the small particles will grow excessively during the first and second firings, which on the one hand causes difficulty in second grinding, and on the other hand makes it difficult to form particle grading during the second firing, which adversely affects the compaction and electrical properties.

[0131] Compared with example 1, the comparative example 2 replaces the mixed carbon source with single glucose, lacks high molecular polymers as the carbon source, and affects the graphitization degree of residual carbon and the dispersibility of small particles, which will affect the electrical performance and compaction of the finished product.

[0132] Compared with example 1, the comparative example 3 uses large particles prepared by single lower iron-phosphorus ratio, and the proportion of large particles is too high, and the compaction and electrical performance, especially the rate performance, are significantly reduced.

[0133] Compared with example 1, the comparative example 4 uses small particles prepared by single higher iron-phosphorus ratio, and the proportion of small particles is too high, the specific surface energy is larger, the temperature during the second firing is higher, the small particles are easy to over-fire, which will cause the finished product to have poor grading and unbalanced proportion of large and small particles, which will affect the electrical performance and compaction of the finished product, and at the same time, due to the increase of the proportion of lithium and phosphorus during feeding, the lithium element is more likely to volatilize and lose after the small particles are sintered at high temperature during the second firing, which will increase the proportion of iron and phosphorus elements, and it is easy to generate magnetic impurities of phosphorus-iron compounds.

[0134] Compared with example 1, the comparative example 5 does not add coated metal elements, on the one hand, the secondary doping of metal elements is not carried out, and on the other hand, the coating of fast ion conductors such as Li5Mn2(PO4)3 and Li3Sc2(PO4)3 cannot be formed, the conductivity is poor, and the roundness of the particles will also be affected, so the compaction is significantly reduced.

[0135] Compared with example 1, the comparative example 6 exchanges the formula and subsequent proportioning of large and small particles, which increases the proportion of large particles after mixing, but the grinding particle size after mixing of large and small particles is reduced, which reduces the efficiency and increases the cost, and although the particle size can reach the required range through grinding, the small particles formed by grinding and crushing of large particles are irregular in shape, which is not conducive to secondary coating, and the grading and coating of the finished product are poor, and the electrical performance and compaction are significantly reduced.

[0136] Table 2

[0137]

[0138]

[0139] From the above description, it can be seen that the above-mentioned embodiments of the present application achieve the following technical effects:

[0140] By controlling the dosing ratio of ferrous source, phosphorus source, and doped metal salt solution, slurries with different doping contents and different iron-phosphorus ratios are prepared; by adding excess lithium source and appropriate carbon source respectively, lithium iron phosphate with different particle sizes is obtained after calcination; by mixing lithium iron phosphate with different particle sizes according to the ratio, composite carbon source, metal coating source, and third solvent, grinding to the target particle size, drying, calcining, and crushing, the finished modified lithium iron phosphate material is obtained. Through the above method, the doping elements are introduced during the preparation of the precursor on the basis of fully utilizing the preparation process of the precursor iron phosphate, the cheap soluble doping source (metal salt solution) is used to reduce the cost and ensure uniform distribution, and the dehydration and calcination process is omitted to reduce the energy consumption. By adjusting the content of the main elements of the precursor, the iron-phosphorus ratio, and the doping amount, the influence of the precursor on the growth of the lithium iron phosphate particles is utilized, and the required large and small particles of lithium iron phosphate are directly prepared through the first calcination and the second calcination, which is convenient for the grading during the secondary grinding. The first calcination and the second calcination respectively use high-graphitization organic high-molecular carbon source and cheap high-residual carbon small-molecule carbon source; the grinding uses mixed carbon source according to the particle ratio, which maximizes the improvement of the carbon coating graphitization degree and the material performance while reducing the cost, and controls the carbon coating thickness. The relationship between the grinding particle size, the mixed carbon source ratio, and the large and small particle dosing ratio is determined in the grinding stage, the process is synergistically controlled, and the Li a M2(PO4)3 (such as Li5Mn2(PO4)3, Li3Sc2(PO4)3, etc.) fast ion conductor coating layer is used to improve the material performance. In addition, since the proportion of iron elements is reduced at the dosing end, it is beneficial to inhibit the generation of magnetic substances phosphorus iron compounds, therefore, the obtained lithium iron phosphate positive electrode material has lower magnetic foreign impurities and better compaction and electrical performance.

[0141] The above is only a preferred embodiment of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a modified lithium iron phosphate material, characterized in that, The modified lithium iron phosphate material includes Li x Fe y PO4 and Li were sequentially coated x Fe y Carbon coating on PO4 surface and Li a The M2(PO4)3 coating layer, wherein the chemical formula of the modified lithium iron phosphate material is Li x Fe y PO4@C@Li a M2(PO4)3, wherein 0.95≤x≤1.05, 0.95≤y≤0.97, 1≤a≤5, and M is selected from any one or more of Mn, Ti, Sc, Sn, Al, In, Sb, Bi and Zr; The preparation method includes: Step S1: Mix the raw materials including the first ferrous source, the first phosphorus source and the first metal salt solution to obtain the first slurry; Step S2: The raw materials including the second ferrous source, the second phosphorus source and the second metal salt solution are mixed to obtain the second slurry; Step S3: Mix the raw materials including the first slurry, the first lithium source, the first solvent and the first carbon source to obtain the first precursor. Then, subject the first precursor to first ball milling, first drying and first calcination to obtain the first lithium iron phosphate. Step S4: Mix the raw materials including the second slurry, the second lithium source, the second solvent and the second carbon source to obtain the second precursor. Then, subject the second precursor to a second ball milling, a second drying and a second calcination to obtain the second lithium iron phosphate. Step S5: Mix the raw materials including the first lithium iron phosphate, the second lithium iron phosphate, the third carbon source, the fourth carbon source, the metal coating source and the third solvent to obtain a mixture. Then, grind, dry and calcine the mixture in sequence to obtain the modified lithium iron phosphate material. Wherein, the molar ratio of iron to phosphorus in the first slurry is higher than that in the second slurry, and the concentration of total metal ions in the first slurry is higher than that in the second slurry; The average particle size of the first lithium iron phosphate is smaller than that of the second lithium iron phosphate.

2. The preparation method according to claim 1, characterized in that, In step S1, the molar ratio of the first ferrous source, the first metal salt solution, and the first phosphorus source is 1:(0.050~0.100):(1.030~1.040); And / or, the first ferrous source is selected from any one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride, and ferrous nitrate; And / or, the first phosphorus source is selected from any one or more of diammonium hydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate; And / or, the first metal salt solution is a sulfate solution, wherein the sulfate solution is selected from any one or more of titanium sulfate solution, manganese sulfate solution, scandium sulfate solution, zirconium sulfate solution, antimony sulfate solution and bismuth sulfate solution.

3. The preparation method according to claim 1, characterized in that, In step S2, the molar ratio of the second ferrous source, the second metal salt solution, and the second phosphorus source is 1:(0.025~0.050):(1.040~1.050); And / or, the second ferrous source is selected from any one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride, and ferrous nitrate; And / or, the second phosphorus source is selected from any one or more of diammonium hydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate; And / or, the second metal salt solution is a sulfate solution, wherein the sulfate solution is selected from any one or more of titanium sulfate solution, manganese sulfate solution, scandium sulfate solution, zirconium sulfate solution, antimony sulfate solution and bismuth sulfate solution.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step S3, the D50 particle size of the first lithium iron phosphate is 600nm to 1000nm. And / or, the molar ratio of iron to phosphorus in the first lithium iron phosphate is 0.960 to 0.970; And / or, the molar ratio of iron in the first slurry to the molar ratio of lithium in the first lithium source is 1:(0.515~0.525); And / or, the first lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, and lithium phosphate; And / or, the mass of the first carbon source is 4 wt% to 6 wt% of the mass of the modified lithium iron phosphate material; And / or, the first carbon source is selected from any one or more of polyethylene glycol, polyvinyl alcohol, and polyvinylpyrrolidone; And / or, the first solvent is selected from any one or more of deionized water and ethanol; And / or, the solid content of the first precursor is 40% to 50%; And / or, the D50 particle size of the product obtained by the first ball milling is 300-400 nm; And / or, the temperature of the first drying is 200℃~300℃; And / or, the temperature of the first calcination is 450℃~550℃, and the time is 8h~12h; And / or, the first calcination is carried out in a protective atmosphere, said protective atmosphere being nitrogen and / or argon.

5. The preparation method according to any one of claims 1 to 4, characterized in that, In step S4, the D50 particle size of the second lithium iron phosphate is 1500-2500 nm. And / or, the molar ratio of iron to phosphorus in the second lithium iron phosphate is 0.950 to 0.960; And / or, the molar ratio of iron in the second slurry to the molar ratio of lithium in the second lithium source is 1:(0.515~0.525); And / or, the second lithium source is selected from any one or more of lithium carbonate, lithium hydroxide, and lithium phosphate; And / or, the mass of the second carbon source is 4 wt% to 6 wt% of the mass of the modified lithium iron phosphate material; And / or, the second carbon source is selected from any one or more of glucose, sucrose and citric acid; And / or, the second solvent is selected from any one or more of deionized water and ethanol; And / or, the solid content of the second precursor is 30% to 40%; And / or, the D50 particle size of the product obtained by the second ball milling is 400-500 nm; And / or, the temperature of the second drying is 200℃~300℃; And / or, the second calcination temperature is 650℃~750℃, and the time is 8h~12h; And / or, the second calcination is carried out in a protective atmosphere, said protective atmosphere being nitrogen and / or argon.

6. The preparation method according to claim 1, characterized in that, In step S5, the mass ratio of the first lithium iron phosphate to the second lithium iron phosphate and the mass ratio of the third carbon source to the fourth carbon source are each independently n, 1≤n≤9; The grinding particle size D50 of the product obtained by grinding the mixture satisfies the following relationship with n: grinding particle size D50 = (1 + 1 / n) μm.

7. The preparation method according to claim 1, characterized in that, In step S4, the third carbon source is selected from any one or more of glucose, sucrose, and citric acid; And / or, the fourth carbon source is selected from any one or more of polyethylene glycol, polyvinyl alcohol, and polyvinylpyrrolidone; And / or, the ratio of the total molar amount of the first ferrous source and the second ferrous source to the total molar amount of the total metal elements in the metal-coated source is 1:(0.0025~0.005); And / or, the metal coating source is selected from any one or more of manganese oxide, scandium oxide, tin oxide, titanium oxide, aluminum oxide, indium oxide, antimony oxide, bismuth oxide and zirconium oxide; And / or, the third solvent is selected from any one or more of deionized water and ethanol; And / or, the solid content of the mixture is 40% to 50%; And / or, the grinding is performed using a single-cylinder self-circulation method; And / or, the temperature of the third drying is 200℃~300℃; And / or, the third calcination temperature is 780℃~820℃, and the time is 4h~8h; And / or, the third calcination is carried out under a protective atmosphere, which is nitrogen and / or argon.

8. A modified lithium iron phosphate material, characterized in that, The modified lithium iron phosphate material is prepared by the preparation method described in any one of claims 1 to 7.

9. A positive electrode, comprising a positive electrode material, characterized in that, The cathode material is the modified lithium iron phosphate material as described in claim 8.

10. A lithium-ion battery, comprising a positive electrode and a negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 9.

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