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

Abstract

The application discloses a lithium iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of batteries. 1+y Fe 1‑ x Pd x PO4F y / C;0

Description

TECHNICAL FIELD

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

[0002] Lithium ion battery refers to a secondary battery system in which two different lithium intercalation compounds are used as the positive electrode and the negative electrode respectively, and the structure of the lithium intercalation compound can reversibly insert and remove lithium ions. The material of the lithium ion battery is mainly composed of four parts: positive electrode material, negative electrode material, separator and electrolyte. Among them, the specific capacity of the positive electrode active material increases by 50%, and the energy density of the battery increases by 28% accordingly. It can be seen that the positive electrode material of the lithium ion battery is the key influencing factor to increase the energy density of the battery, improve the cycle performance stability and reduce the cost of the battery. The currently widely used positive electrode material has defects in performance and preparation cost, which cannot meet the demand of energy storage equipment and electric tools for power lithium ion batteries. Therefore, it is inevitable trend to develop new materials with good structural stability, abundant raw material resources and low preparation cost.

[0003] Goodenough's research group first proposed that LiFePO4 material with olivine structure has lithium intercalation / deintercalation performance, and LiFePO4 as a new type of lithium ion battery positive electrode material has been widely concerned. The main advantages of LiFePO4 material are wide raw material source, environmental friendliness, high theoretical specific capacity (170 mAh / g) and stable discharge platform. The more prominent advantage compared with layered material is that the phosphate ion of LiFePO4 material is a very stable anion group, which will not decompose oxygen during charging and discharging process, and the volume strain of the material itself is small. At the same time, LiFePO4 and charged product orthorhombic iron phosphate (FePO4) have similar crystal structures, so LiFePO4 material has the advantages of good safety stability, thermal stability and long cycle life, and is considered to be one of the most potential positive electrode materials for power lithium ion batteries.

[0004] Although LiFePO4 is used as the positive electrode material of lithium ion battery has great advantages, there are still many problems in the synthesis and practical process: the ferrous ion is easy to be oxidized in the synthesis process, and the agglomeration phenomenon is easy to occur during heat treatment; the lithium ion diffusion coefficient is relatively low, and the electrochemical performance is poor; the theoretical density is low, and the tap density is low. SUMMARY

[0005] The main purpose of the present application is to provide a lithium iron phosphate cathode material, a preparation method thereof and a lithium ion battery, so as to solve the problem that LiFePO4 as the positive electrode material of lithium battery still has poor electrochemical performance in actual use in the prior art.

[0006] In order to achieve the above object, according to one aspect of the present application, a lithium iron phosphate positive electrode material is provided, which has the following structural formula:

[0007] Li 1+y Fe 1-x Pd x PO4F y / C formula I;

[0008] In formula I, 0 < x ≤ 0.3, 0 < y ≤ 0.5;

[0009] Further, in formula I, 0.01 ≤ x ≤ 0.2, 0.01 ≤ y ≤ 0.3; preferably, 0.05 ≤ x ≤ 0.2, 0.1 ≤ y ≤ 0.3. The present application adopts the above structural ratio, which is beneficial to improve the electrochemical performance of the lithium iron phosphate material, especially the conductivity and structural stability.

[0010] Further, the palladium and fluorine co-doping in the structure of the lithium iron phosphate positive electrode material is in the lithium iron phosphate, and the outer surface of the lithium iron phosphate is coated with a carbon material.

[0011] The present application performs carbon coating modification ( / C) on LiFePO4. The carbon material has good conductivity, which can further improve the surface conductivity of LiFePO4. The present application selects specific metal cation palladium and fluorine to co-dope the carbon-coated lithium iron phosphate. The doped metal palladium ion can further improve the intrinsic conductivity and surface conductivity of the lithium iron phosphate. The small particle size of LiFePO4 can reduce the Li + migration distance, improve the diffusion rate of Li + , and the fluorine element can make the structure of the lithium iron phosphate material stable. The present application improves the overall conductivity of the lithium iron phosphate material through carbon coating surface modification and palladium and fluorine co-doping internal modification, and further improves the electrochemical performance.

[0012] Further, the median particle size D50 of the lithium iron phosphate positive electrode material is 0.3-0.6 μm, and the morphology is spherical.

[0013] According to a second aspect of the present application, a preparation method of the above lithium iron phosphate positive electrode material is provided, which comprises the following steps:

[0014] Step S1: obtain a lithium source, an iron source, a palladium source, a phosphorus source, a fluorine source and a carbon source according to the element ratio;

[0015] Step S2: mix the lithium source, the iron source, the palladium source, the phosphorus source, the fluorine source and the carbon source to prepare a mixed powder;

[0016] Step S3: heat treat the mixed powder in an inert atmosphere to obtain the lithium iron phosphate positive electrode material.

[0017] Further, in step S1, the molar ratio of Li element in the lithium source, Fe element in the iron source, Pd element in the palladium source, P element in the phosphorus source, F element in the fluorine source and C element in the carbon source is (1+y):(1-x):x:1:y:(3-6); wherein, 0

[0018] Further, in step S1, the lithium source is selected from one or more of lithium hydroxide monohydrate, lithium oxide, lithium peroxide, lithium oxalate, lithium hydroxide, lithium carbonate, lithium phosphate, lithium nitrate and lithium chloride.

[0019] Further, the iron source is selected from one or more of iron powder, iron phosphate, ferrous oxalate, ferrous acetate and iron nitrate.

[0020] Further, the palladium source is selected from one or more of palladium on carbon, , and .

[0021] Further, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid and ammonium phosphate.

[0022] Further, the fluorine source is selected from one or more of lithium fluoride, iron fluoride and sodium fluoride.

[0023] Further, the carbon source is selected from one or more of citric acid, glucose, glycine, sucrose, ascorbic acid and polyvinyl alcohol.

[0024] The composite material prepared by the above-mentioned lithium source, iron source, palladium source, phosphorus source, fluorine source, carbon source and the like selected in the present application has better electrochemical performance; those skilled in the art can select from the prior art according to actual needs.

[0025] Further, in step S2, the lithium source, iron source, palladium source, phosphorus source, fluorine source, carbon source and organic solvent are mixed to form a gel; and then the gel is dried to prepare a mixed powder.

[0026] Further, the organic solvent includes an alcohol solvent, and the alcohol solvent is ethylene glycol.

[0027] Further, in step S2, the carbon source and iron source are dissolved in water, stirred until completely dissolved, the phosphorus source, lithium source, palladium source and fluorine source are added, stirred to form a mixed solution; ethylene glycol is added to the mixed solution, stirred to form a gel; and the gel is dried under vacuum and ground to obtain a mixed powder.

[0028] Further, in step S2, the carbon source and the iron source are dissolved in water, and stirred at a temperature of 60-100 DEG C until completely dissolved; the phosphorus source and the lithium source are added, and magnetically stirred at a temperature of 70-100 DEG C in a water bath for 1-4 hours to form a mixed solution; the palladium source, the fluorine source and the ethylene glycol are added to the mixed solution, and magnetically stirred at an oil bath temperature of 100-140 DEG C for 1-5 hours until a gel is formed.

[0029] The present application can realize the targeted dissolution and mixing of the raw materials according to the characteristics of the raw materials by heating and stirring the raw materials at different temperatures, so as to fully dissolve the raw materials and form a uniform gel state.

[0030] Further, in step S3, the mixed powder is first subjected to initial firing treatment in an inert atmosphere to obtain an initial firing product, and the initial firing product is ground to obtain a ground product; and then the ground product is calcined in an inert atmosphere to obtain the lithium iron phosphate positive electrode material.

[0031] Further, the initial firing treatment is performed at a temperature increasing rate of 1-5 DEG C / min and a temperature of 350-400 DEG C for 3.5-4.5 hours.

[0032] Further, the calcination is performed at a temperature increasing rate of 1-2 DEG C / min and a temperature of 650-800 DEG C for 6-12 hours.

[0033] The lithium iron phosphate material modified by the carbon coating, the palladium and fluorine co-doping and the iron phosphate material can be more stable and uniform, so as to improve the electrochemical performance of the modified lithium iron phosphate material.

[0034] Further, the inert atmosphere includes an argon atmosphere or a nitrogen atmosphere.

[0035] According to a third aspect of the present application, a positive electrode material is provided, which is the lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared by the method.

[0036] According to a fourth aspect of the present application, a lithium ion battery is provided, which includes the positive electrode material.

[0037] The carbon coating layer in the lithium iron phosphate positive electrode material can act as a conductive layer, which can improve the electrical conductivity of the particle surface of the LiFePO4 material; at the same time, the carbon coating layer can inhibit the grain growth of the material, so as to obtain particles with a smaller LiFePO4 particle size, reduce the migration distance of Li+ and improve the diffusion rate of Li+; and in the heat treatment process, the reducing property of carbon can effectively prevent Fe + from being oxidized to Fe3+. + from being oxidized to Fe3+.2+ Oxidation, and then a high-purity LiFePO4 cathode material is obtained; in the material, the co-doping of palladium and fluorine can improve the electronegativity of the LiFePO4 material, enhance the intrinsic conductivity and surface conductivity of the LiFePO4 material, effectively improve the electrochemical performance of the material, and the material is structurally stable. BRIEF DESCRIPTION OF DRAWINGS

[0038] The accompanying drawings, which form a part of the specification, are included to provide a further understanding of the application and are incorporated herein for explanation by illustrating a preferred embodiment of the present application.

[0039] Figure 1 The SEM image of the cathode material Li 1.2 Fe 0.85 Pd 0.15 PO4F 0.2 / C prepared in Embodiment 1 of the present application is shown. DETAILED DESCRIPTION

[0040] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0041] Since LiFePO4 is prone to oxidation in the synthesis process of ferrous ions and prone to agglomeration during heat treatment, the lithium ion diffusion coefficient is relatively low, the electrochemical performance is poor, the theoretical density is low, the tap density is low, and other defects exist in the synthesis and practical processes, the present application selects specific metal palladium and fluorine to co-dope carbon-coated modified lithium iron phosphate material, and the electrochemical performance of lithium iron phosphate is improved.

[0042] The present application provides a lithium iron phosphate cathode material, and the structural formula is as follows:

[0043] Li 1+y Fe 1-x Pd x PO4F y / C formula I;

[0044] In formula I, 0 < x ≤ 0.3, 0 < y ≤ 0.5; x is the mole number of palladium elements, y is the mole number of fluorine elements, 1+y is the mole number of lithium elements, 1-x is the mole number of iron elements, and the mole number of phosphorus elements is 1;

[0045] In the structure of Formula I above, the value of x can be any value from 0.01, 0.03, 0.05, 0.08, 0.10, 0.12, 0.15, 0.18, 0.20, 0.22, 0.25, 0.28, 0.3 or any value between any two; the value of y can be any value from 0.01, 0.03, 0.05, 0.08, 0.1, 0.12, 0.15, 0.18, 0.20, 0.22, 0.25, 0.28, 0.3, 0.32, 0.35, 0.38, 0.4, 0.42, 0.45, 0.48, 0.5 or any value between any two.

[0046] This invention modifies LiFePO4 with carbon coating ( / C). Carbon materials have good electrical conductivity, which can further improve the surface conductivity of LiFePO4. This invention selects specific metal cations, palladium and fluorine, for co-doping of carbon-coated lithium iron phosphate. The doped palladium ions can further improve the intrinsic conductivity and surface conductivity of lithium iron phosphate. The small particle size of LiFePO4 can reduce the Li-induced conductivity. + Migration distance, improve Li + The diffusion rate is improved, and the doped fluorine element can stabilize the structure of lithium iron phosphate material. This invention improves the overall conductivity of lithium iron phosphate material through carbon coating surface modification and palladium and fluorine co-doping internal modification. The two modification methods work synergistically to improve its electrochemical performance. The modified lithium iron phosphate material of this invention can achieve a maximum first charge specific capacity of 163 mAh / g and a maximum first discharge specific capacity of 157 mAh / g, with a first charge-discharge efficiency of 96%. Compared with the existing technology that only uses carbon coating modification, the first charge-discharge efficiency of lithium iron phosphate is improved by up to 3%, which is a very significant improvement.

[0047] As one specific implementation method, the structure of the lithium iron phosphate cathode material is that palladium and fluorine are co-doped in lithium iron phosphate, and the outer surface of lithium iron phosphate is coated with carbon material.

[0048] In one specific implementation, the lithium iron phosphate cathode material has a particle size of 0.3μm~0.6μm and a spherical morphology.

[0049] As a specific implementation, in Formula I, 0.01≤x≤0.3, 0.01≤y≤0.5; alternatively, 0.01≤x≤0.2, 0.01≤y≤0.3; further alternatively, 0.05≤x≤0.2, 0.1≤y≤0.3; even further alternatively, 0.05≤x≤0.15, 0.15≤y≤0.25; for example, x=0.1, y=0.2. Adopting the above structural ratio is beneficial for improving the electrochemical performance of lithium iron phosphate materials, especially their conductivity and structural stability.

[0050] The application provides a preparation method of the lithium iron phosphate positive electrode material, and comprises the following steps:

[0051] Step S1: obtaining raw materials of lithium source, iron source, palladium source, phosphorus source, fluorine source and carbon source according to the molar ratio of each element in formula I;

[0052] Step S2: mixing the lithium source, iron source, palladium source, phosphorus source, fluorine source and carbon source to prepare a mixed powder;

[0053] Step S3: performing heat treatment on the mixed powder in an inert atmosphere to obtain the lithium iron phosphate positive electrode material.

[0054] The preparation method has the advantages of simple operation, easy availability of raw materials, small particle size of the prepared lithium iron phosphate modified material, and obvious improvement of the electrochemical performance of the material compared with unmodified lithium iron phosphate. 50 The particle size D50 is 0.3-0.6 μm, for example, 0.45 μm, the morphology is spherical, and the electrochemical performance of the material is obviously improved compared with unmodified lithium iron phosphate.

[0055] As a specific embodiment, in step S1, the molar ratio of Li element in the lithium source, Fe element in the iron source, Pd element in the palladium source, P element in the phosphorus source, F element in the fluorine source and C element in the carbon source is (1+y):(1-x):x:1:y:(3-6); wherein 0

[0056] The molar ratio of C elements in the carbon source is selected from any value in 3, 4, 5, 6 or a range value between any two; specifically, the molar ratio of Li elements in the lithium source, Fe elements in the iron source, Pd elements in the palladium source, P elements in the phosphorus source, F elements in the fluorine source and C elements in the carbon source can be selected as (1.1~1.3):(0.85~0.95):(0.05~0.15):1:(0.1~0.3):4.5. Under the above raw material ratio conditions, the first charge specific capacity, the first discharge specific capacity and the first discharge efficiency of the modified lithium iron phosphate material are obviously improved, such as the first charge-discharge efficiency reaches 95.5~96.2%; when the molar ratio of Li elements in the lithium source, Fe elements in the iron source, Pd elements in the palladium source, P elements in the phosphorus source, F elements in the fluorine source and C elements in the carbon source in the raw material formula is 1.2:0.9:0.1:1:0.2:4.5, the first charge specific capacity, the first discharge specific capacity and the first discharge efficiency of the modified lithium iron phosphate material are more improved, such as the first charge-discharge efficiency reaches about 96.2%.

[0057] The lithium source in the preparation method of the present application can be selected from one or more of lithium hydroxide monohydrate, lithium oxide, lithium peroxide, lithium oxalate, lithium hydroxide, lithium carbonate, lithium phosphate, lithium nitrate and lithium chloride; the iron source is selected from one or more of iron powder, iron phosphate, ferrous oxalate, ferrous acetate and iron nitrate; the palladium source is selected from one or more of palladium on carbon, 、 and The phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid and ammonium phosphate; the fluorine source is selected from one or more of lithium fluoride, iron fluoride and sodium fluoride; and the carbon source is selected from one or more of citric acid, glucose, glycine, sucrose, ascorbic acid and polyvinyl alcohol. The composite material prepared by the above-mentioned specific raw materials of the lithium source, the iron source, the palladium source, the phosphorus source, the fluorine source and the carbon source selected in the present application has better electrochemical performance; those skilled in the art can select from the prior art according to actual needs.

[0058] In step S2 of the preparation method of the present application, the lithium source, the iron source, the palladium source, the phosphorus source, the fluorine source, the carbon source and the organic solvent are mixed to form a gel; and then the gel is dried to prepare a mixed powder, wherein the organic solvent includes an alcohol solvent, and the alcohol solvent is preferably ethylene glycol. The step of preparing the gel in the process is not limited, as long as the raw materials are mixed and stirred to the gel state; the order of mixing the raw materials is not limited; and the step of preparing the gel into a mixed powder is not limited, as long as the gel can be prepared into a powder.

[0059] As a specific embodiment of step S2, the carbon source and the iron source can be dissolved in water, stirred until completely dissolved, and then the phosphorus source, the lithium source, the palladium source and the fluorine source are added and stirred to form a mixed solution; finally, the ethylene glycol is added into the mixed solution and stirred until a gel is formed; the obtained gel is vacuum dried and ground to obtain a mixed powder.

[0060] As a more specific embodiment of step S2, the carbon source and the iron source can be dissolved in water, stirred at 60-100℃ until completely dissolved; the phosphorus source and the lithium source are added, and magnetic stirring is performed at 70-100℃ under water bath for 1-4h to form a mixed solution; the palladium source, the fluorine source and the ethylene glycol are added into the mixed solution, and magnetic stirring is performed at 100-140℃ under oil bath for 1-5h until a gel is formed. In this preferred scheme, by limiting the mixing order of the materials, it is more conducive to the complete dissolution of each material. By heating and stirring the different raw materials at different temperatures, the raw materials can be mixed according to their characteristics, so that each raw material is fully dissolved and a uniform gel state is formed.

[0061] As a specific embodiment of step S3, the mixed powder can be first subjected to preliminary sintering treatment in an inert atmosphere to obtain a preliminary sintering product, the preliminary sintering product is ground to obtain a ground product; and then the ground product is calcined in an inert atmosphere to obtain a palladium and fluorine co-doped carbon-coated lithium iron phosphate material. By means of low-temperature preliminary sintering, cooling and grinding, and high-temperature calcination, a uniformly heat-treated lithium iron phosphate material with small particle size is obtained, which is beneficial to the electrical properties, and the carbon, palladium and fluorine can be fully combined with the lithium iron phosphate to achieve good carbon-coated modification effect, uniform co-doping of palladium and fluorine, etc., so that the final structure of the lithium iron phosphate material is more stable.

[0062] As a more specific embodiment of step S3, the preliminary sintering treatment conditions include: a temperature rising rate of 1-5℃ / min, a temperature of 350-400℃, and a time of 3.5-4.5h.

[0063] As a more specific embodiment of step S3, the calcination conditions include: a temperature rising rate of 1-2℃ / min, a temperature of 650-800℃, and a time of 6-12h. Under this calcination condition, the carbon-coated, palladium and fluorine co-doped and phosphoric acid iron column material can more stably and uniformly modify the lithium iron phosphate material, so as to improve the electrochemical performance of the modified lithium iron phosphate material.

[0064] The inert atmosphere in the above embodiments includes argon atmosphere or nitrogen atmosphere; in actual application, nitrogen can be selected.

[0065] The present application provides a positive electrode material, specifically a lithium ion battery polyanion positive electrode material, which can be prepared by the above method.

[0066] The application provides a lithium ion battery, a positive electrode material of which is the polyanion positive electrode material of the lithium ion battery.

[0067] The application will be further described in detail below in connection with specific examples, which should not be construed as limiting the scope of the application.

[0068] Example 1 (preparation of Li 1.2 Fe 0.95 Pd 0.05 PO4F 0.2 / C material)

[0069] According to the molar ratio of Li:Fe:P:Pd:F: citric acid = 1:0.95:1:0.05:0.2:4.5, 0.2 mol of lithium carbonate, 0.38 mol of iron powder, 0.4 mol of phosphoric acid, 0.02 mol of palladium chloride, 0.08 mol of lithium fluoride and 1.8 mol of citric acid are weighed, and 0.6 mol of ethylene glycol is weighed. The citric acid and the iron powder are dissolved in water, and stirred at 90°C until completely dissolved. Then, the lithium carbonate and the phosphoric acid are added, and stirred at 90°C by magnetic force for 4 hours to form a mixed solution. Then, the ethylene glycol is added, and stirred at 120°C by magnetic force for 3 hours until a gel is formed. The gel is vacuum dried, and ground to obtain a mixed powder. The mixed powder is heated to 350°C at a rate of 2°C / min in a nitrogen-filled tube furnace for low-temperature initial firing for 4 hours. The obtained product is taken out, and ground again to be uniform. Then, the product is heated to 750°C at a rate of 2°C / min in a nitrogen-filled tube furnace for high-temperature calcination for 1 hour, and then cooled with the furnace. The obtained product is ground to obtain Li 1.2 Fe 0.95 Pd 0.05 PO4F 0.2 / C material, as shown in Figure 1

[0070] Example 2 (preparation of Li 1.2 Fe 0.9 Pd 0.1 PO4F 0.2 / C material)

[0071] ​Li, Fe, P, Pd, F, citric acid = 1 : 0.9 : 1 : 0.1 : 0.2 : 4.5, 0.2 mol of lithium carbonate, 0.36 mol of iron powder, 0.4 mol of phosphoric acid, 0.04 mol of palladium chloride, 0.08 mol of lithium fluoride and 1.8 mol of citric acid were weighed out, and 0.6 mol of ethylene glycol was further weighed out. The citric acid and the iron powder were dissolved in water, and stirred at 90°C until completely dissolved. Then, the lithium carbonate and the phosphoric acid were added, and stirred at 90°C by magnetic force for 4 hours to form a mixed solution. Subsequently, the ethylene glycol was added, and stirred at 120°C by magnetic force for 3 hours until a gel was formed. The gel was vacuum-dried, and ground to obtain a mixed powder. The mixed powder was heated at a rate of 2°C / min to 350°C in a nitrogen-filled tube furnace, and low-temperature primary sintering was performed for 4 hours. The obtained product was taken out, and ground again to be uniform. Then, the product was heated at a rate of 2°C / min to 750°C in a nitrogen-filled tube furnace, and high-temperature calcination was performed for 1 hour. The obtained product was ground after cooling to obtain a Li 1.2 Fe 0.9 Pd 0.1 PO4F 0.2 / C material.

[0072] Example 3 (Preparation of Li 1.2 Fe 0.85 Pd 0.15 PO4F 0.2 / C material)

[0073] Li, Fe, P, Pd, F, citric acid = 1 : 0.85 : 1 : 0.15 : 0.2 : 4.5, 0.2 mol of lithium carbonate, 0.34 mol of iron powder, 0.4 mol of phosphoric acid, 0.06 mol of palladium chloride, 0.08 mol of lithium fluoride and 1.8 mol of citric acid were weighed out, and 0.6 mol of ethylene glycol was further weighed out. The citric acid and the iron powder were dissolved in water, and stirred at 90°C until completely dissolved. Then, the lithium carbonate and the phosphoric acid were added, and stirred at 90°C by magnetic force for 4 hours to form a mixed solution. Subsequently, the ethylene glycol was added, and stirred at 120°C by magnetic force for 3 hours until a gel was formed. The gel was vacuum-dried, and ground to obtain a mixed powder. The mixed powder was heated at a rate of 2°C / min to 350°C in a nitrogen-filled tube furnace, and low-temperature primary sintering was performed for 4 hours. The obtained product was taken out, and ground again to be uniform. Then, the product was heated at a rate of 2°C / min to 750°C in a nitrogen-filled tube furnace, and high-temperature calcination was performed for 1 hour. The obtained product was ground after cooling to obtain a Li 1.2 Fe 0.85 Pd 0.15 PO4F 0.2 / C material.

[0074] Example 4 (Preparation of Li 1.1 Fe 0.85 Pd0.15 PO4F 0.1 / C material)

[0075] Example 4 differs from Example 3 in that the raw material ratio is different, 0.2 mol of lithium carbonate, 0.34 mol of iron powder, 0.4 mol of phosphoric acid, 0.06 mol of palladium chloride, 0.04 mol of lithium fluoride and 1.8 mol of citric acid are weighed according to the molar ratio of Li:Fe:P:Pd:F: citric acid = 1:0.85:1:0.15:0.1:4.5.

[0076] Example 5 (preparation of LiFePO4 / C material) 1.3 Fe 0.85 Pd 0.15 PO4F 0.3 / C material)

[0077] Example 5 differs from Example 3 in that the raw material ratio is different, 0.2 mol of lithium carbonate, 0.34 mol of iron powder, 0.4 mol of phosphoric acid, 0.06 mol of palladium chloride, 0.12 mol of lithium fluoride and 1.8 mol of citric acid are weighed according to the molar ratio of Li:Fe:P:Pd:F: citric acid = 1:0.85:1:0.15:0.3:4.5.

[0078] Comparative Example 1 (preparation of LiFePO4 / C material)

[0079] 0.2 mol of lithium carbonate, 0.4 mol of iron powder, 0.4 mol of phosphoric acid and 1.8 mol of citric acid are weighed according to the molar ratio of Li:Fe:P: citric acid = 1:1:1:4.5, and then 0.6 mol of ethylene glycol is weighed. The citric acid and iron powder are dissolved in water, and stirring is performed at 90°C until complete dissolution, and then lithium carbonate and phosphoric acid are added, and magnetic stirring is performed at 90°C for 4h to form a mixed solution; then ethylene glycol is added, and magnetic stirring is performed at an oil bath temperature of 120°C for 3h until a gel is formed; the gel is vacuum dried and then ground to obtain a mixed powder; the mixed powder is heated to 350°C at a rate of 2°C / min in a nitrogen-filled tube furnace for low-temperature initial firing for 4h; the obtained product is taken out and then ground again to uniformity, and then heated to 750°C at a rate of 2°C / min in a nitrogen-filled tube furnace for high-temperature calcination for 1h, and then the furnace is cooled, and the obtained product is ground to obtain the LiFePO4 / C material.

[0080] Comparative Example 2 (preparation of LiFePO4 / C material) 1.1 FePO4F 0.1 / C)

[0081] Li:Fe:P:F: citric acid = 1 : 1 : 1 : 0.1 : 4.5, 0.2 mol lithium carbonate, 0.4 mol iron powder, 0.4 mol phosphoric acid, 0.04 mol lithium fluoride and 1.8 mol citric acid were weighed, and 0.6 mol ethylene glycol was weighed. The citric acid and iron powder were dissolved in water, stirred at 90°C until completely dissolved, then lithium carbonate and phosphoric acid were added, and stirred at 90°C for 4h by magnetic stirring to form a mixed solution; then ethylene glycol was added, and stirred at 120°C in an oil bath for 3h by magnetic stirring until a gel was formed; the gel was vacuum dried and ground to obtain a mixed powder; the mixed powder was heated to 350°C at a rate of 2°C / min in a nitrogen-filled tube furnace for low-temperature initial firing for 4h; the obtained product was taken out and ground again to be uniform, then heated to 750°C at a rate of 2°C / min in a nitrogen-filled tube furnace for high-temperature calcination for 1h, and then cooled with the furnace; the obtained product was ground to obtain Li 1.1 FePO4F 0.1 / C material.

[0082] Comparative Example 3 (preparation of Li 1.2 FePO4F 0.2 / C)

[0083] Li:Fe:P:F: citric acid = 1 : 1 : 1 : 0.2 : 4.5, 0.2 mol lithium carbonate, 0.4 mol iron powder, 0.4 mol phosphoric acid, 0.08 mol lithium fluoride and 1.8 mol citric acid were weighed, and 0.6 mol ethylene glycol was weighed. The citric acid and iron powder were dissolved in water, stirred at 90°C until completely dissolved, then lithium carbonate and phosphoric acid were added, and stirred at 90°C for 4h by magnetic stirring to form a mixed solution; then ethylene glycol was added, and stirred at 120°C in an oil bath for 3h by magnetic stirring until a gel was formed; the gel was vacuum dried and ground to obtain a mixed powder; the mixed powder was heated to 350°C at a rate of 2°C / min in a nitrogen-filled tube furnace for low-temperature initial firing for 4h; the obtained product was taken out and ground again to be uniform, then heated to 750°C at a rate of 2°C / min in a nitrogen-filled tube furnace for high-temperature calcination for 1h, and then cooled with the furnace; the obtained product was ground to obtain Li 1.2 FePO4F 0.2 / C material.

[0084] Comparative Example 4 (preparation of Li 1.3 FePO4F 0.3 / C material)

[0085] According to the molar ratio of Li:Fe:P:F: citric acid = 1:1:1:0.3:4.5, 0.2 mol of lithium carbonate, 0.4 mol of iron powder, 0.4 mol of phosphoric acid, 0.12 mol of lithium fluoride and 1.8 mol of citric acid are weighed, and 0.6 mol of ethylene glycol is weighed. The citric acid and iron powder are dissolved in water, stirred at 90°C until completely dissolved, then the lithium carbonate and phosphoric acid are added, and stirred at 90°C for 4h by magnetic force to form a mixed solution; then the ethylene glycol is added, and stirred at 120°C in an oil bath by magnetic force for 3h until a gel is formed; the gel is vacuum dried and ground to obtain a mixed powder; the mixed powder is heated to 350°C at a rate of 2°C / min in a nitrogen-filled tube furnace for low-temperature initial firing for 4h; the obtained product is taken out and ground again, and then heated to 750°C at a rate of 2°C / min in a nitrogen-filled tube furnace for high-temperature calcination for 1h, and then cooled with the furnace; the obtained product is ground to obtain Li 1.3 FePO4F 0.3 / C material.

[0086] Table 1

[0087]

[0088] As can be seen from Table 1, the first charge specific capacity of the fluorine and palladium co-doped carbon-coated lithium iron phosphate material prepared in Examples 1-5 of the present application reaches 159.4-163.3 mAh / g, the first discharge specific capacity reaches 152.9-157.1 mAh / g, and the first charge efficiency reaches 95.5%-96.2%; while the first charge specific capacity of the lithium iron phosphate material prepared in Comparative Examples 1-4 reaches 149.9-154.3 mAh / g, the first discharge specific capacity reaches 139.7-149.1 mAh / g, and the first charge efficiency reaches 93.2%-95.6%; in comparison, the first charge and discharge efficiency of the fluorine and palladium co-doped carbon-coated lithium iron phosphate material prepared in Examples 1-5 of the present application is increased by about 3%, which has a relatively obvious effect in the field of research and development technology.

[0089] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application are used to distinguish similar objects, and do not necessarily have to describe a specific order or sequence. It should be understood that the terms thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that described herein.

[0090] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, 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 lithium iron phosphate cathode material, characterized in that, Its structural formula is as follows: Li 1+y Fe 1-x Pd x PO4F y / C of Formula I; In Formula I, 0 < x ≤ 0.3, 0 < y ≤ 0.5; the outer surface of the lithium iron phosphate is coated with a carbon material.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, In Formula I, 0.01 ≤ x ≤ 0.2, 0.01 ≤ y ≤ 0.

3.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, In Formula I, 0.05 ≤ x ≤ 0.2, 0.1 ≤ y ≤ 0.3; and / or, in the structure of the lithium iron phosphate cathode material, palladium and fluorine are co-doped in the lithium iron phosphate; and / or, the median particle size D50 of the lithium iron phosphate cathode material is 0.3 - 0.6 μm, and the morphology is spherical.

4. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, The method includes the following steps: Step S1: Obtain a lithium source, an iron source, a palladium source, a phosphorus source, a fluorine source, and a carbon source according to the element ratios; Step S2: Mix the lithium source, the iron source, the palladium source, the phosphorus source, the fluorine source, and the carbon source to form a mixed powder; Step S3: Heat-treat the mixed powder in an inert atmosphere to obtain the lithium iron phosphate cathode material.

5. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, In Step S1, the molar ratio of the Li element in the lithium source, the Fe element in the iron source, the Pd element in the palladium source, the P element in the phosphorus source, the F element in the fluorine source, and the C element in the carbon source is (1 + y): (1 - x): x: 1: y: (3 - 6); where 0 < x ≤ 0.3, 0 < y ≤ 0.

5.

6. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, 0.01 ≤ x ≤ 0.2, 0.01 ≤ y ≤ 0.

3.

7. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, 0.05 ≤ x ≤ 0.2, 0.05 ≤ y ≤ 0.

3.

8. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, 0.05 ≤ x ≤ 0.15, 0.15 ≤ y ≤ 0.

25.

9. The method for preparing the lithium iron phosphate cathode material according to any one of claims 4 to 8, characterized in that, In Step S1, the lithium source is selected from one or more of lithium hydroxide monohydrate, lithium oxide, lithium peroxide, lithium oxalate, lithium hydroxide, lithium carbonate, lithium phosphate, lithium nitrate, and lithium chloride; and / or, the iron source is selected from one or more of iron powder, iron phosphate, ferrous oxalate, ferrous acetate, and iron nitrate; and / or, the palladium source is selected from one or more of palladium dichloride, sodium tetrachloropalladate, and dichlorotetraamminepalladium; and / or, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and ammonium phosphate; and / or, the fluorine source is selected from one or more of lithium fluoride, iron fluoride, and sodium fluoride; and / or, the carbon source is selected from one or more of citric acid, glucose, glycine, sucrose, ascorbic acid, and polyvinyl alcohol.

10. The method for preparing the lithium iron phosphate cathode material according to any one of claims 4 to 8, characterized in that, In Step S2, first mix the lithium source, the iron source, the palladium source, the phosphorus source, the fluorine source, the carbon source, and an organic solvent to form a gel; then dry the gel to form the mixed powder.

11. The method for preparing the lithium iron phosphate cathode material according to claim 10, characterized in that, The organic solvent includes an alcohol solvent; the alcohol solvent is ethylene glycol; and / or, in Step S2, dissolve the carbon source and the iron source in water, stir until completely dissolved, add the phosphorus source, the lithium source, the palladium source, and the fluorine source, stir to form a mixed solution; add ethylene glycol to the mixed solution, stir until a gel is formed; dry the gel under vacuum and then grind it to obtain the mixed powder.

12. The method for preparing the lithium iron phosphate cathode material according to claim 11, characterized in that, In step S2, the carbon source and the iron source are dissolved in water and stirred at 60°C to 100°C until completely dissolved; the phosphorus source and the lithium source are added and magnetically stirred at 70°C to 100°C for 1 to 4 hours to form a mixed solution; the palladium source, the fluorine source and the ethylene glycol are added to the mixed solution and magnetically stirred at 100°C to 140°C for 1 to 5 hours until a gel is formed.

13. The method for preparing the lithium iron phosphate cathode material according to any one of claims 4 to 8, characterized in that, In step S3, the mixed powder is first subjected to initial calcination in an inert atmosphere to obtain an initial calcination product. The initial calcination product is then ground to obtain a ground product. The ground product is then calcined in an inert atmosphere to obtain the lithium iron phosphate cathode material.

14. The method for preparing the lithium iron phosphate cathode material according to claim 13, characterized in that, The conditions for the initial firing treatment include: a heating rate of 1~5℃ / min, a temperature of 350℃~400℃, and a time of 3.5~4.5h; And / or, the calcination conditions include: a heating rate of 1~2℃ / min, a temperature of 650℃~800℃, and a time of 6~12h; And / or, the inert atmosphere is an argon atmosphere or a nitrogen atmosphere.

15. A positive electrode material, characterized in that, The cathode material includes the lithium iron phosphate cathode material according to any one of claims 1 to 3 or the lithium iron phosphate cathode material prepared by the method according to any one of claims 4 to 14.

16. A lithium-ion battery, comprising a positive electrode material; characterized in that, The cathode material is the cathode material as described in claim 15.

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