A positive electrode active material, a method for preparing the same, an electrode, and a lithium ion battery

Abstract

To overcome the problems of low energy density, poor high-temperature storage and low-temperature rate discharge performance of the positive active material in the prior art, the application provides a positive active material, a preparation method thereof, an electrode and a lithium ion battery. The positive active material comprises phosphate first particles and phosphate second particles, the shape of the phosphate first particles comprises a smooth polyhedral long strip block shape, the shape of the phosphate second particles comprises a spherical shape, and the particle size of the phosphate first particles is larger than that of the phosphate second particles. The positive active material has a hierarchical structure, the specific surface area of the long strip block-shaped first particles is smaller, the occurrence of material surface side reactions can be reduced, the high-temperature storage and cycle performance are favorable, lithium ions can migrate along the short side of the long strip block-shaped first particles, the lithium ion migration path is effectively shortened like the spherical second particles, the ion migration activity is ensured, and the charge and discharge performance is improved; and the first particles have a unique smooth polyhedral long strip shape, the unique cross section of the first particles can make the contact between the first particles more compact, and the hierarchical structure makes the material have the accumulation of large and small particles, and the overall compaction density is greatly improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of energy storage, and particularly relates to a positive electrode active material, a preparation method thereof, an electrode and a lithium ion battery. BACKGROUND

[0002] Olivine phosphate type materials have the advantages of good safety performance, long cycle life and low raw material cost, and are widely used in the field of power batteries. However, the energy density is low, and the low-temperature rate discharge performance is poor, which limits the actual application. At present, the energy density per unit volume of the material is improved by increasing the material compaction density, and the low-temperature rate performance is improved by controlling the particle size and doping modification, but the two cannot be considered at the same time. High compaction of the material requires large particles, but large particles will significantly reduce the low-temperature rate performance of the material, and too many small particles will reduce the compaction density of the material. SUMMARY

[0003] In view of the problems in the prior art, the application provides a positive electrode active material, a preparation method thereof, an electrode and a lithium ion battery.

[0004] To solve the above problems, the application provides the following technical solutions:

[0005] In a first aspect, the application provides a positive electrode active material, which comprises a phosphate type positive electrode active material, the phosphate type positive electrode active material comprises phosphate first particles and phosphate second particles, the shape of the phosphate first particles comprises a smooth polyhedral long strip block, and the shape of the phosphate second particles comprises a spherical shape; the particle size of the phosphate first particles is greater than that of the phosphate second particles.

[0006] In some embodiments, the particle size of the phosphate first particles is 300 nm to 1800 nm, and the particle size of the phosphate second particles is 30 nm to 150 nm.

[0007] In some embodiments, the volume ratio of the phosphate first particles to the phosphate second particles is (30% to 60%):(40% to 70%).

[0008] In some embodiments, the volume ratio of the phosphate first particles to the phosphate second particles is (40% to 50%):(50% to 60%).

[0009] In some embodiments, the length-width-height range ratio of the phosphate first particles is 1 to 4:0.8 to 1:0.6 to 1, and the length-diameter ratio of the phosphate second particles is 1:1.5.

[0010] In some embodiments, the phosphate type cathode active material comprises one or more of lithium iron phosphate, lithium manganese iron phosphate.

[0011] In a second aspect, the present application provides a preparation method of a cathode active material, comprising the following steps:

[0012] S1: mixing and adding an iron source A, a phosphorus source, a lithium source and a carbon source into a dispersion medium for grinding treatment to obtain a primary mixed solution;

[0013] S2: adding an iron source B into the primary mixed solution for further grinding treatment to obtain a precursor slurry;

[0014] S3: adding an additive into the precursor slurry and then performing drying treatment to obtain a precursor dry powder;

[0015] S4: sintering the precursor dry powder to obtain a phosphate type cathode active material, wherein the phosphate type cathode active material obtained comprises smooth polyhedral long strip blocks and spherical-like shapes.

[0016] In some embodiments, in step S1, the grinding treatment is sand grinding, which comprises coarse grinding and fine grinding, the time for the coarse grinding treatment is 0.5-2h, and the time for the fine grinding treatment is 0.5h-8h, and the linear speed for the sand grinding treatment is 5m / s-15m / s.

[0017] In some embodiments, in step S2, the grinding treatment is sand grinding, the time for the sand grinding treatment is 0.5h-5h, and the linear speed for the grinding treatment is 5m / s-15m / s.

[0018] In some embodiments, in step S3, the time for the drying treatment is 1h-8h.

[0019] In some embodiments, the additive comprises one or more of magnesium chloride, magnesium nitrate, aluminum nitrate, zirconium nitrate, zirconium oxide, tetraethyl titanate, ethyl titanate, cobalt acetate, cobalt nitrate, vanadium pentoxide, ammonium metavanadate, manganese nitrate, manganese chloride, manganese sulfate, tungsten disulfide, tin chloride, tin oxide, molybdenum sulfide, niobium pentachloride, molybdenum oxide.

[0020] In some embodiments, in step S4, the time for the sintering is 5h-15h, and the sintering temperature is 650℃-750℃.

[0021] In a third aspect, the present application further provides an electrode comprising the cathode active material according to any one of the above-mentioned embodiments or the cathode active material prepared by the preparation method according to any one of the above-mentioned embodiments.

[0022] In a fourth aspect, the present application further provides a lithium ion battery comprising the electrode.

[0023] The positive electrode active material provided by the application has the following effects: (1) the positive electrode active material has a hierarchical structure, including polyhedral long strip block-shaped phosphate first particles and spherical-like phosphate second particles, the specific surface area of the long strip block-shaped first particles is smaller, which can reduce the occurrence of material surface side reactions, is conducive to high-temperature storage and cycle performance; (2) at the same time, lithium ions can migrate along the short side of the long strip block-shaped first particles, as the spherical-like second particles, effectively shortening the lithium ion migration path, ensuring ion migration activity, and improving the charge and discharge performance; (3) the first particles have a unique smooth polyhedral long strip block shape, the unique section of which can make the contact between the first particles more close, and the hierarchical structure makes the material exist in the accumulation of large and small particles, so that the overall compaction density is greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is the SEM image of lithium iron phosphate of Example 1 of the application;

[0025] Figure 2 is the SEM image of lithium iron phosphate of Example 2 of the application;

[0026] Figure 3 is the SEM image of lithium iron phosphate of Example 3 of the application;

[0027] Figure 4 is the SEM image of lithium iron phosphate of Example 4 of the application;

[0028] Figure 5 is the SEM image of lithium iron phosphate of Example 5 of the application;

[0029] Figure 6 is the SEM image of lithium iron phosphate of Example 6 of the application;

[0030] Figure 7 is the SEM image of lithium iron phosphate of Comparative Example 1 of the application; DETAILED DESCRIPTION

[0031] In order to make the technical problems solved by the application, the technical solutions and beneficial effects clearer and more apparent, the application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the application and do not limit the application.

[0032] The application provides a positive electrode active material, including a phosphate-type positive electrode active material, the phosphate-type positive electrode active material including phosphate first particles and phosphate second particles, the shape of the phosphate first particles including a smooth polyhedral long strip block shape, and the shape of the phosphate second particles including a spherical-like shape; the particle size of the phosphate first particles is greater than the particle size of the phosphate second particles.

[0033] Specifically, the polyhedral long strip block-shaped particle refers to a long strip-shaped particle surrounded by four or more polygons; and the spheroid-shaped particle refers to a particle with a shape similar to a sphere.

[0034] Specifically, the surface of the polyhedral long strip block-shaped phosphate first particle is smooth, without a rough surface, which is different from the particles of the phosphate positive active material in the prior art.

[0035] The positive active material provided in the application has the following effects: (1) the positive active material has a hierarchical structure, including polyhedral long strip block-shaped phosphate first particles and spheroid-shaped phosphate second particles, the specific surface area of the long strip block-shaped first particles is smaller, which can reduce the occurrence of material surface side reactions, and is beneficial to high-temperature storage and cycle performance; (2) at the same time, lithium ions can migrate along the short side of the long strip block-shaped first particles, and the migration path of lithium ions is effectively shortened as the spheroid-shaped second particles, which ensures ion migration activity and improves charge and discharge performance; (3) the first particles have a unique smooth polyhedral long strip block shape, the unique cross section of which can make the contact between the first particles more compact, and the hierarchical structure makes the material exist in the accumulation of large and small particles, so that the overall compaction density is greatly improved.

[0036] In some embodiments, the particle size of the phosphate first particles is 300 nm to 1800 nm; and the particle size of the phosphate second particles is 30 nm to 150 nm.

[0037] Specifically, the particle size of the phosphate first particles refers to the edge length of the phosphate first particles, and the particle size of the phosphate first particles is 300 nm to 1800 nm, that is, the edge length of the phosphate first particles ranges from 300 nm to 1800 nm; and the particle size of the phosphate second particles refers to the particle size of the phosphate second particles, that is, the particle size of the phosphate second particles is 30 nm to 150 nm, and the average particle size is 60 nm to 70 nm.

[0038] In some embodiments, the volume ratio of the phosphate first particles to the phosphate second particles is (30% to 60%):(40% to 70%).

[0039] Specifically, the volume ratio of the phosphate first particles to the phosphate second particles is (30%~60%):(40%~70%), which can be but is not limited to 30%:70%, 40%:60%, 50%:50%, 60%:40%. The larger the volume ratio of the phosphate first particles to the phosphate second particles, the larger the volume proportion of the phosphate first particles, the smaller the specific surface area of the positive active material particles, the fewer the side reactions on the surface of the material, the closer the contact between the first particles, and the higher the compaction density and cycle performance of the positive active material. Therefore, when the volume ratio of the phosphate first particles to the phosphate second particles is not less than 30%:70%, the battery has better electrochemical performance. The smaller the volume ratio of the phosphate first particles to the phosphate second particles, the larger the volume proportion of the phosphate second particles, the faster the diffusion speed of lithium ions, and the easier the second particles to fill into the gaps between the first particles, thereby improving the compaction density of the material. Therefore, when the volume ratio of the phosphate first particles to the phosphate second particles is not greater than 60%:40%, the positive active material has better compaction density.

[0040] In some preferred embodiments, the volume ratio of the phosphate first particles to the phosphate second particles is (40%~50%):(50%~60%).

[0041] Specifically, the volume ratio of the phosphate first particles to the phosphate second particles is (40%~50%):(50%~60%), which can be but is not limited to 40%:60%, 43%:67%, 45%:55%, 48%:52%, 50%:60%. When the volume ratio of the phosphate first particles to the phosphate second particles is within this range, the diffusion speed of lithium ions can be improved, the occurrence of side reactions can be reduced, and the cycle performance of the battery can be improved while ensuring the compaction density.

[0042] In some embodiments, the length-width-height range ratio of the phosphate first particles is 1~4:0.8~1:0.6~1, and the length-diameter ratio of the phosphate second particles is 1:1.5.

[0043] Specifically, the length, width and height of the phosphate first particles are defined based on the morphology of the phosphate first particles. Since the phosphate first particles are smooth polyhedral long strip-shaped, the long side of the long strip-shaped first particles is defined as the length of the phosphate first particles, the short side in the left-right direction of the long side of the phosphate first particles is defined as the height of the first particles, and the long side in the left-right direction of the long side of the phosphate first particles is defined as the width of the first particles. The length of the phosphate first particles is in the range of 600nm~1800nm, and the width and height of the phosphate first particles are in the range of 300nm~600nm. The length-diameter ratio of the phosphate second particles refers to the ratio of the longest diameter through the particle interior to the longest diameter perpendicular to it.

[0044] In some embodiments, the phosphate type cathode active material comprises one or more of lithium iron phosphate, lithium manganese iron phosphate.

[0045] In a second aspect, the application provides a preparation method of the above-mentioned cathode active material, comprising the following steps:

[0046] S1: mixing and adding iron source A, phosphorus source, lithium source and carbon source into a dispersion medium for grinding treatment to obtain a primary mixed solution;

[0047] S2: adding iron source B into the primary mixed solution for further grinding treatment to obtain a precursor slurry;

[0048] S3: adding an additive into the precursor slurry and then performing drying treatment to obtain a precursor dry powder;

[0049] S4: sintering the precursor dry powder to obtain a phosphate type cathode active material, wherein the obtained phosphate type cathode active material comprises smooth polyhedral long strip blocks and spherical-like shapes.

[0050] Specifically, after the sintering process of S4 step, phosphate particles of different shapes and particle sizes are formed, including smooth polyhedral long strip block-shaped phosphate first particles and spherical-like phosphate second particles.

[0051] In some embodiments, the iron source is one or more of anhydrous iron phosphate, dihydrate iron phosphate, diiron trioxide, iron chloride, iron nitrate;

[0052] The phosphorus source is one or more of phosphoric acid, iron phosphate, dihydrate iron phosphate, lithium phosphate, lithium dihydrogen phosphate, lithium monohydrogen phosphate, ammonium dihydrogen phosphate, and hydrogen diammonium phosphate;

[0053] The lithium source is one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, lithium hydroxide, and lithium nitrate;

[0054] The carbon source is one or more of sucrose, water-soluble phenolic resin, glucose, polyethylene glycol, hydroxymethyl cellulose, polyacrylamide, starch, polyvinyl alcohol, high-conductivity carbon tube, and single-layer or multi-layer graphene;

[0055] The dispersion medium is one or more of deionized water, methanol, ethanol, or NMP;

[0056] Specifically, the dispersion medium acts as a dispersant during the material grinding process, which disperses the material particles and makes the grinding more sufficient.

[0057] The additive is one or more of magnesium chloride, magnesium nitrate, aluminum nitrate, zirconium nitrate, zirconium oxide, tetraethyl titanate, ethyl titanate, cobalt acetate, cobalt nitrate, vanadium pentoxide, ammonium metavanadate, manganese nitrate, manganese chloride, manganese sulfate, tungsten disulfide, tin chloride, tin oxide, molybdenum sulfide, niobium pentachloride, and molybdenum oxide.

[0058] Specifically, the additive can play a role of fluxing or crystal inhibition in the sintering process, which helps the particle growth or inhibits the particle growth.

[0059] Specifically, the type and amount of the additive can be adjusted to control the particle growth or inhibit the particle growth to achieve the desired technical effect.

[0060] In some embodiments, the molar ratio of the iron source, the phosphorus source and the lithium source is 1:1:(1-1.05).

[0061] In some embodiments, the molar ratio of the iron source, the phosphorus source and the lithium source is 1:1:(1-1.03).

[0062] In some embodiments, the mass of the carbon source is 9%-16% of the mass of the iron source, and can be but is not limited to 9%, 10%, 11%, 12%, 13%, 14%, 15% or 16%.

[0063] In some embodiments, in step S1, the grinding method is sand grinding, and the grinding process includes coarse grinding and fine grinding. The coarse grinding time is 0.5-2h, and the fine grinding time is 0.5h-8h. The linear speed of the grinding process is 5m / s-15m / s.

[0064] Specifically, the coarse grinding time can be but is not limited to 0.5h, 1h, 1.5h or 2h. The fine grinding time can be but is not limited to 0.5h, 1h, 2h, 5h or 8h. The linear speed of the grinding process can be but is not limited to 5m / s, 6m / s, 7m / s, 8m / s, 9m / s, 10m / s, 12m / s or 15m / s. The difference in the grinding time and the linear speed will affect the particle size of the ground raw material. If the grinding time is too long or the linear speed is too large, the particle size of the slurry will be too small. If the grinding time is too short or the linear speed is too small, the slurry will not be ground sufficiently, and the particle size of the slurry will be too large, which will affect the size of the lithium iron phosphate primary particles in the subsequent sintering process.

[0065] In some embodiments, the fine grinding time is 0.5h-3h, and the linear speed of the grinding process is 7m / s-12m / s.

[0066] Specifically, the grinding method is sand grinding, and the grinding process includes coarse grinding and fine grinding. The size of the zirconium balls used in the coarse grinding is 0.5mm-0.6mm, and the size of the zirconium balls used in the fine grinding is 0.1mm-0.3mm. The sand grinding is continuous sand grinding for a certain number of hours under a certain linear speed.

[0067] In some embodiments, in step S2, the grinding process includes sand grinding. The sand grinding time is 0.5h-5h, and the linear speed of the sand grinding is 5m / s-15m / s.

[0068] Specifically, the grinding time can be but is not limited to 0.5h, 1h, 2h, 5h, and the linear speed of grinding can be but is not limited to 5m / s, 6m / s, 7m / s, 8m / s, 9m / s, 10m / s, 12m / s, 15m / s. The size difference of grinding time and linear speed will affect the particle size of the raw material after grinding, and then affect the formation of lithium iron phosphate particles in the subsequent sintering process.

[0069] In some embodiments, the grinding time is 0.5h-3h, and the linear speed of grinding is 7m / s-12m / s.

[0070] In some embodiments, after adding the additive in the precursor slurry in step S3, a drying process is further included, and the drying time is 1h-8h.

[0071] Specifically, the drying time can be but is not limited to 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h. The dispersant added in the grinding process can be removed through the drying process. The longer the drying time is, the more sufficient the drying is, and the less the residual dispersant is. Therefore, the drying time is required to be greater than 1h. The shorter the drying time is, the less likely the precursor is to deteriorate, and the less likely the precursor is to introduce impurities. Therefore, the drying time is required to be less than 8h. The drying time in this range can ensure the drying degree of the precursor and avoid introducing impurities in the precursor.

[0072] Specifically, the drying process includes spray drying and vacuum drying, and the drying time used according to different drying methods is also different.

[0073] In some embodiments, in step S4, the sintering time is 5h-15h, and the sintering temperature is 650℃-750℃.

[0074] Specifically, the sintering time is 5h, 6h, 7h, 8h, 9h, 10h, 15h, and the sintering temperature is 650℃, 680℃, 700℃, 730℃, 750℃. Through high-temperature sintering, the raw material is subjected to a multi-phase reaction at a certain temperature to form the target active positive electrode material. The sintering time and temperature have a great influence on the formation and crystallinity of the material. If the sintering temperature is too high and the sintering time is too long, the primary particles will be too large, but it will help to improve the crystallinity of the material. If the sintering temperature is too low and the sintering time is too short, the primary particles will be too small, and the crystallinity of the material will be low, which is not conducive to the charge and discharge performance of the material.

[0075] In one embodiment, the sintering time is 6h-10h, and the sintering temperature is 680℃-740℃.

[0076] Compared with the prior art, the preparation method of the positive electrode active material provided in the present application can obtain phosphate positive electrode active material by sintering at a lower temperature (650°C to 750°C), saving energy consumption and reducing production costs. The method is simple, easy to operate, and easy to mass produce. After sintering under these conditions, smooth polyhedral long block phosphate first particles and spherical phosphate second particles can be obtained.

[0077] In a third aspect, the present application provides an electrode, comprising the positive electrode active material described above, or the positive electrode active material prepared by the method for preparing the positive electrode active material described above.

[0078] It should be noted that the electrode provided in this application contains the above-mentioned positive electrode active material, including a positive electrode and a negative electrode. The positive electrode includes a positive electrode active material, a conductive agent, and a binder. The negative electrode includes a negative electrode active material, a conductive agent, and a binder. The negative electrode active material, conductive agent, and binder are all existing technologies and will not be repeated here.

[0079] In a fourth aspect, the present application provides a lithium-ion battery comprising the electrode described above.

[0080] The present invention is further described in detail below by way of examples.

[0081] Example 1

[0082] This embodiment is used to illustrate the positive electrode active material and its preparation method, electrode, and lithium ion battery disclosed in the present invention, and includes the following steps:

[0083] (1) Preparation of positive electrode active materials

[0084] S1: 0.93 kg of lithium carbonate, 2.25 kg of iron phosphate A, and 70 g of sucrose were mixed and added to 10 kg of deionized water. After stirring, the mixture was uniformly mixed to obtain a primary mixed solution. The solution was added to a coarse grinding device through a diaphragm pump for coarse grinding. After coarse grinding for 0.5 h, the slurry was put into a fine grinding mill and sand-milled at a linear speed of 10 m / s for 1 h.

[0085] S2: Add 1.5 kg of iron phosphate B to the above solution and continue sand milling for 0.5 h to obtain a particle size-classified lithium iron phosphate precursor slurry;

[0086] S3: Add 40 g of vanadium pentoxide to the lithium iron phosphate precursor slurry, stir evenly, and then spray dry for 3 hours to obtain a lithium iron phosphate precursor dry powder;

[0087] S4: Sintering the lithium iron phosphate precursor dry powder at 730° C. for 8 h to obtain a lithium iron phosphate positive electrode active material.

[0088] (2) Preparation of the positive electrode sheet: the positive electrode material, conductive agent, and binder, etc. are uniformly mixed by a mixer, then the mixed slurry is uniformly coated on the aluminum foil by a film coating machine, and is placed in an oven for drying, rolled and cut into a sheet to obtain the positive electrode sheet.

[0089] (3) Preparation of the negative electrode sheet: the negative electrode sheet is purchased lithium sheet.

[0090] (4) Preparation of the lithium ion battery: the positive and negative electrode battery shell, positive and negative electrode sheet, separator, foamed nickel, electrolyte, etc. are placed in a glove box. ① The foamed nickel is placed in the negative electrode shell, and the lithium sheet is placed on the foamed nickel; ② the positive electrode sheet is placed in the positive electrode shell, and the electrolyte is added dropwise to soak, then the separator soaked with electrolyte is covered on the positive electrode sheet; ③ the positive and negative electrodes are placed in the sealing machine mold slot, and the lithium ion battery is obtained by pressing and sealing.

[0091] Example 2

[0092] Example 2 differs from Example 1 in that the preparation of the positive electrode active material is different, as follows, and the rest is the same as Example 1:

[0093] S1: 0.93 kg of lithium carbonate, 2.25 kg of iron phosphate A, and 70 g of sucrose are mixed and added to 10 kg of deionized water, stirred uniformly to obtain a primary mixed solution, and then added to a coarse grinding device for coarse grinding. After coarse grinding for 0.5 h, the slurry is poured into a fine grinding sand mill, and sand grinding is carried out at a grinding speed of 10 m / s for 2 h;

[0094] S2: 1.5 kg of iron phosphate C is added to the above solution, and sand grinding is continued for 1 h to obtain a particle size graded lithium iron phosphate precursor slurry;

[0095] S3: 40 g of vanadium pentoxide is added to the lithium iron phosphate precursor slurry, stirred uniformly, and then spray dried for 3 h to obtain a lithium iron phosphate precursor dry powder;

[0096] S4: The lithium iron phosphate precursor dry powder is sintered at 690°C for 10 h to obtain a lithium iron phosphate positive electrode active material.

[0097] Example 3

[0098] Example 3 differs from Example 1 in that the preparation of the positive electrode active material is different, as follows, and the rest is the same as Example 1:

[0099] (1) Preparation of the positive electrode active material

[0100] S1: 0.5 kg of lithium phosphate, 0.96 kg of phosphoric acid, 0.6 kg of ferric oxide and 200 g of glucose were mixed and added to 10 kg of deionized water, and after stirring uniformly, an initial mixed solution was obtained, which was added to a coarse grinding device for coarse grinding. After coarse grinding for 0.5 h, the slurry was poured into a fine grinding sand mill, and sand grinding was performed at a linear speed of 12 m / s for 3 h;

[0101] S2: 0.28 kg of iron chloride was added to the above solution, and sand grinding was continued for 1 h to obtain a particle size graded lithium iron phosphate precursor slurry;

[0102] S3: 53 g of tin chloride was added to the lithium iron phosphate precursor slurry, and after stirring uniformly, the lithium iron phosphate precursor dry powder was obtained after spray drying for 3 h;

[0103] S4: The lithium iron phosphate precursor dry powder was sintered at 730°C for 6 h to obtain a lithium iron phosphate positive electrode active material.

[0104] Example 4

[0105] Example 4 differs from Example 1 in that the preparation of the positive electrode active material is different, as follows, and the rest is the same as Example 1:

[0106] (1) Preparation of the positive electrode active material

[0107] S1: 0.93 kg of lithium carbonate, 2.25 kg of iron phosphate A and 70 g of sucrose were mixed and added to 10 kg of ethanol, and after stirring uniformly, an initial mixed solution was obtained. The solution was added to a coarse grinding device for coarse grinding, and after coarse grinding for 0.5 h, the slurry was poured into a fine grinding sand mill, and sand grinding was performed at a linear speed of 10 m / s for 2 h;

[0108] S2: 1.5 kg of iron phosphate B was added to the above solution, and sand grinding was continued for 1 h to obtain a particle size graded lithium iron phosphate precursor slurry;

[0109] S3: 50 g of ethyl titanate was added to the lithium iron phosphate precursor slurry, and after stirring uniformly, the lithium iron phosphate precursor dry powder was obtained after vacuum drying for 6 h;

[0110] S4: The lithium iron phosphate precursor dry powder was sintered at 730°C for 8 h to obtain a lithium iron phosphate positive electrode active material.

[0111] Example 5

[0112] Example 5 differs from Example 1 in that the preparation of the positive electrode active material is different, as follows, and the rest is the same as Example 1:

[0113] (1) Preparation of the positive electrode active material

[0114] S1: 0.5 kg of lithium phosphate, 0.96 kg of phosphoric acid, 0.6 kg of ferric oxide and 200 g of glucose were mixed and added to 10 kg of methanol, and after stirring uniformly, an initial mixed solution was obtained, which was added to a coarse grinding device by a diaphragm pump for coarse grinding. After coarse grinding for 0.5 h, the slurry was poured into a fine grinding sand mill, and sand grinding was performed at a linear speed of 10 m / s for 3 h;

[0115] S2: 0.28 kg of iron chloride was added to the above solution, and sand grinding was continued for 1.5 h to obtain a particle size graded lithium iron phosphate precursor slurry;

[0116] S3: 53 g of niobium pentachloride was added to the lithium iron phosphate precursor slurry, and after stirring uniformly, vacuum drying was performed for 8 h to obtain a lithium iron phosphate precursor dry powder;

[0117] S4: The lithium iron phosphate precursor dry powder was sintered at 700°C for 10 h to obtain a lithium iron phosphate positive electrode active material.

[0118] Example 6

[0119] Example 6 differs from Example 1 in that the preparation of the positive electrode active material is different, as follows, and the rest is the same as Example 1:

[0120] (1) Preparation of the positive electrode active material

[0121] S1: 0.93 kg of lithium carbonate, 0.96 kg of phosphoric acid, 0.6 kg of ferric oxide and 200 g of glucose were mixed and added to 10 kg of deionized water, and after stirring uniformly, an initial mixed solution was obtained, which was added to a coarse grinding device by a diaphragm pump for coarse grinding. After coarse grinding for 0.5 h, the slurry was poured into a fine grinding sand mill, and sand grinding was performed at a linear speed of 12 m / s for 2 h;

[0122] S2: 0.28 kg of iron chloride was added to the above solution, and sand grinding was continued for 1 h to obtain a particle size graded lithium iron phosphate precursor slurry;

[0123] S3: 65 g of manganese sulfate was added to the lithium iron phosphate precursor slurry, and after stirring uniformly, spray drying was performed for 3 h to obtain a lithium iron phosphate precursor dry powder;

[0124] S4: The lithium iron phosphate precursor dry powder was sintered at 700°C for 10 h to obtain a lithium iron phosphate positive electrode active material.

[0125] Comparative Example 1

[0126] Comparative Example 1 differs from Example 1 in that the preparation of the positive electrode active material is different, as follows, and the rest is the same as Example 1:

[0127] (1) Preparation of the positive electrode active material

[0128] S1: 0.93 kg of lithium carbonate, 3.72 kg of iron phosphate and 70 g of sucrose were mixed and added to 10 kg of deionized water, and after stirring uniformly, an initial mixed solution was obtained, which was added to a coarse grinding device by a diaphragm pump for coarse grinding. After coarse grinding for 0.5 h, the slurry was poured into a fine grinding sand mill, and sand grinding was carried out at a linear speed of 10 m / s for 1 h;

[0129] S2: 40 g of vanadium pentoxide was added to the lithium iron phosphate precursor slurry, and after stirring uniformly, the lithium iron phosphate precursor dry powder was obtained by spray drying for 3 h;

[0130] S3: The lithium iron phosphate precursor dry powder was sintered at 730℃ for 8h to obtain the lithium iron phosphate positive electrode active material.

[0131] Electrochemical performance test

[0132] 1. Capacity retention rate test at -20℃:

[0133] The prepared battery was charged at room temperature at 0.1C rate to 3.8V, then constant voltage was applied at 3.8V until the current was 0.1C, then discharged at 0.1C to 2.5V, then cycled again, then charged at 0.5C rate to 3.8V, then constant voltage was applied at 3.8V until the current was 0.1C, and then the battery was placed in a refrigerator at -20℃, and discharged at 1C rate to 2.0V. The ratio of the discharge capacity at -20℃ to the charge capacity at 0.5C at room temperature is the low temperature capacity retention rate of the battery.

[0134] 2. Discharge capacity retention rate test at 15C rate:

[0135] CCCV charged to 3.8V at 0.2C rate, and the cutoff current was 0.02C, then discharged at 15C rate to 2.5V, and the ratio of the discharge capacity at 15C rate to the discharge capacity at 0.2C rate was the discharge rate efficiency at 15C rate.

[0136] 3. Powder compaction density test:

[0137] First, calibrate the mold with air pressure, then weigh 1 g of powder and place it in the mold, set the pressure value to 3T, and start the test program.

[0138] 4. Particle volume ratio test:

[0139] Currently, the particle size test results are mainly used for estimation: 0.2 g of powder material is ultrasonically treated in 10 mL of ethanol for 8 min, and the particle size of the powder material is tested on a Malvern particle size tester. The test results of the present material are bimodal sharp shape, and the peak area ratio of the two peaks is regarded as the volume ratio of large and small particles.

[0140] Table 1 is a summary table of parameter settings of each example and comparative example.

[0141]

[0142] Table 2 is the performance test results of the above examples and comparative examples.

[0143] Table 1

[0144]

[0145] In summary, from the data of examples 1-6 and comparative example 1 and SEM images, it can be seen that by changing the preparation method of the positive active material, the shape of the first particles in the prepared lithium iron phosphate positive active material can be controlled, the shape of the first particles of the phosphate is a smooth polyhedral long strip, the shape of the second particles of the phosphate is a spherical shape, the capacity retention rate and the compaction density of the battery are obviously improved, thereby improving the capacity retention rate and the compaction density of the battery at low temperature and high rate; controlling the volume ratio of the large lithium iron phosphate particles and the small lithium iron phosphate particles in the range of (30%-60%):(40%-70%) can further improve the capacity retention rate and the compaction density of the battery.

[0146] From the data of examples 1-6 and comparative example 1, it can be seen that the lithium iron phosphate material prepared by the technical scheme of the present application has excellent low temperature, power, and compaction performance. Under-20℃ / 1C discharge condition, the discharge efficiency (discharge capacity / charge capacity) can be as high as 54.1%, which is 34% higher than the capacity retention rate of comparative example 1 under-20℃ / 1C condition, 22.1%; under 15C discharge rate, the discharge capacity retention rate is as high as 65.6% compared with 0.2C discharge rate, which is 12.2% higher than the retention rate of comparative example 1 under 0.2C discharge rate, 53.4%; under 3T pressure condition, the powder compaction density can reach 2.435g / cm 3 .

[0147] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A positive electrode active material, characterized in that The positive electrode active material includes a phosphate-type positive electrode active material, which includes first phosphate particles and second phosphate particles. The shape of the first phosphate particles includes a smooth polyhedral long block, and the shape of the second phosphate particles includes a spherical shape; the particle size of the first phosphate particles is larger than the particle size of the second phosphate particles; the length, width and height ratio of the first phosphate particles is 1 to 4: 0.8 to 1: 0.6 to 1.

2. The positive electrode active material according to claim 1, characterized in that The particle size of the first phosphate particles is 300nm to 1800nm; the particle size of the second phosphate particles is 30nm to 150nm.

3. The positive electrode active material according to claim 1, characterized in that The volume ratio of the first phosphate particles to the second phosphate particles is (30% to 60%): (40% to 70%).

4. The positive electrode active material according to claim 3, characterized in that The volume ratio of the first phosphate particles to the second phosphate particles is (40% to 50%): (50% to 60%).

5. The positive electrode active material according to claim 1, characterized in that The aspect ratio of the second phosphate particles is in the range of 1:1.

5.

6. The positive electrode active material according to claim 1, characterized in that The phosphate-type positive electrode active material includes one or more of lithium iron phosphate and lithium manganese iron phosphate.

7. A method for preparing a positive electrode active material according to any one of claims 1 to 6, characterized in that: The preparation method comprises the following steps: S1: adding the iron source A, phosphorus source, lithium source and carbon source to a dispersion medium and grinding the mixture to obtain a primary mixed solution; S2: adding iron source B to the initial mixed solution and continuing the grinding process to obtain a precursor slurry; S3: adding additives to the precursor slurry and then drying it to obtain a precursor dry powder; S4: Sintering the precursor dry powder to obtain a phosphate-type positive electrode active material, wherein the obtained phosphate-type positive electrode active material includes smooth polyhedral long blocks and spherical shapes.

8. The method for preparing a positive electrode active material according to claim 7, wherein: In step S1, the grinding process is sand grinding, including coarse grinding and fine grinding. The time of the coarse grinding process is 0.5-2 hours; the time of the fine grinding process is 0.5 hours to 8 hours. The linear speed of the sand grinding process is 5m / s to 15m / s.

9. The method for preparing a positive electrode active material according to claim 7, wherein: In step S2, the grinding process is sand grinding, the time of the sand grinding process is 0.5 h to 5 h, and the linear speed of the sand grinding process is 5 m / s to 15 m / s.

10. The method for preparing a positive electrode active material according to claim 7, wherein: In step S3, the drying process takes 1 to 8 hours.

11. The method for preparing a positive electrode active material according to claim 7, wherein: The additives include one or more of magnesium chloride, magnesium nitrate, aluminum nitrate, zirconium nitrate, zirconium oxide, tetraethyl titanate, ethyl titanate, cobalt acetate, cobalt nitrate, vanadium pentoxide, ammonium metavanadate, manganese nitrate, manganese chloride, manganese sulfate, tungsten disulfide, tin chloride, tin oxide, molybdenum sulfide, niobium pentachloride, and molybdenum oxide.

12. The method for preparing a positive electrode active material according to claim 7, wherein: In step S4, the sintering time is 5 hours to 15 hours, and the sintering temperature is 650°C to 750°C.

13. An electrode comprising the positive electrode active material according to any one of claims 1 to 6, or the positive electrode active material prepared by the method for preparing the positive electrode active material according to any one of claims 7 to 12.

14. A lithium ion battery comprising the electrode according to claim 13.

Images