Lithium manganese iron phosphate positive electrode active material and preparation method thereof, positive electrode plate, lithium secondary battery and electric equipment

Abstract

The invention provides a lithium manganese iron phosphate type positive electrode active material and a preparation method thereof, a positive pole piece, a lithium secondary battery and electric equipment, the lithium manganese iron phosphate type positive electrode active material is secondary particles formed by gathering primary particles, the secondary particles are internally provided with pores, the porosity of the secondary particles is 20%-55%, and the porosity of the secondary particles is 20%-55%. The ratio of the molar ratio of the element P to the metal element other than Li on the surface of the primary particles is m1, the ratio of the molar ratio of the element P to the metal element other than Li in the primary particle phase is m2, and 1.01 < = m1 / m2 < = 1.07 is satisfied. Therefore, the infiltration effect of the electrolyte on the positive electrode active material can be improved, the corrosion of a small amount of HF in the electrolyte on the bulk phase of the positive electrode active material is reduced, the side reaction between Mn < 3 + > and the electrolyte is reduced, a stable solid-liquid interface is constructed, and the structural stability and capacity exertion of the positive electrode active material under long-circulation and high-temperature conditions are ensured; and the battery with relatively high energy density, excellent cycle performance and quick charge performance is obtained.

Description

Technical Field

[0001] The present application relates to the field of batteries, and specifically, to lithium iron manganese phosphate-based positive electrode active materials and preparation methods thereof, positive electrode sheets, lithium secondary batteries, and electrical equipment. Background Art

[0002] Olivine-type phosphate positive electrode active material LiMPO 4 (M=Fe, Mn) has gradually increased its share in the lithium-ion battery positive electrode active material market due to its excellent structural stability, safety and low cost. Currently, the main commercial materials are LiFePO 4 However, its low platform voltage results in an energy density of only about 580Wh / kg, which limits the application of LiFePO 4 Development and application of materials. LiMnPO 4 The material has a higher platform voltage and theoretical energy density (701Wh / kg), but due to its low conductivity (<10 -9 S / cm), low lithium ion diffusion coefficient (<10 -13 S / cm) and the unstable crystal structure caused by the Jahn-Teller effect of trivalent manganese, which leads to a significant deterioration of the actual electrochemical performance. 4 With LiMnPO 4 Composite preparation of LiMn 1- x Fe x PO 4 The material can take into account the advantages of both materials, but its cycle performance and fast charging performance still need to be improved. Summary of the invention

[0003] The present application aims to solve one of the technical problems in the related art at least to some extent.

[0004] The first aspect of the present application provides a lithium iron manganese phosphate positive electrode active material, wherein the lithium iron manganese phosphate positive electrode active material is a secondary particle formed by agglomerating primary particles, the secondary particles have pores inside, the porosity of the secondary particles is 20%-55%, and the molar ratio of the P element on the surface of the primary particles to the metal element other than the Li element is m 1 The molar ratio of the P element to the metal element other than the Li element in the primary particle phase is m 2 , and satisfy 1.01≤m 1 / m 2 ≤1.07, preferably 1.02≤m 1 / m 2≤1.05. Therefore, the present invention controls the secondary particle porosity and primary particle surface micro-phosphorus enrichment (m 1 / m 2 ) can improve the electrolyte's wetting effect on the positive electrode active material while reducing the corrosion of the positive electrode active material by a small amount of HF in the electrolyte, reducing the Mn 3+ The side reaction between the electrolyte and the cathode material builds a stable solid-liquid interface, ensuring the structural stability and capacity of the cathode active material under long-term cycle and high temperature conditions, thus obtaining a battery with high energy density, excellent cycle performance and fast charging performance.

[0005] According to some embodiments of the present application, the average particle size of the primary particles is 50nm-150nm, and can be 70nm-120nm, thereby shortening the transmission path of lithium ions, increasing the solid-liquid ion exchange interface, and improving the kinetic performance of the positive electrode active material.

[0006] According to some embodiments of the present application, the pores include a first pore and a second pore, the pore size of the second pore is smaller than the pore size of the first pore, the pore size of the first pore is 100nm-300nm, the total pore volume of the first pore and the second pore in the secondary particle is V, and the total pore volume of the first pore in the secondary particle is V 1 , and meets: 20% ≤ V 1 / V≤50%, optionally, 30%≤V 1 / V≤50%. Thus, by controlling the volume proportion of the macropores, the electrolyte can fully and quickly infiltrate the positive electrode active material while ensuring full contact between the primary particles, thereby improving the spherical strength of the secondary particles and improving the electron transmission efficiency.

[0007] According to some embodiments of the present application, 0.02 cm 3 / g≤V 1 ≤0.1cm 3 / g, optional 0.02cm 3 / g≤V 1 ≤0.05cm 3 / g, the pore structure of this part is relatively large, which is convenient for accommodating the rapid entry of electrolyte and then diffusing into the pore structure with smaller pore size, thereby ensuring that the electrolyte fully enters the active material and the stable accumulation between the primary particles, thereby improving the spherical strength of the secondary particles.

[0008] According to some embodiments of the present application, the pore size of the second pore is 1.7nm-100nm. Therefore, the smaller second pores are mostly composed of micropores generated by the decomposition of the carbon source and mesopores formed by the dense packing of the primary particles, which can accommodate the electrolyte, form sufficient contact with the active material inside the secondary particles, and provide a stable solid-liquid interface for lithium ion exchange.

[0009] According to some embodiments of the present application, the specific surface area of ​​the lithium manganese iron phosphate positive electrode active material is 10m 2 / g-30m 2 / g, optional 15m 2 / g-25m 2 / g. Thus, the lithium ion exchange rate is increased, the risk of moisture absorption of the material is reduced, and the processing capacity of the slurry, coating, rolling and other processes in the production of the electrode is improved.

[0010] According to some embodiments of the present application, the D 10 , D 50 , D = satisfies: k 90 =(D 90 -D 10 ) / D 50 , 1.5≤k 90 ≤2.5. As a result, the secondary particles have a narrow and uniform particle size distribution, good consistency and are easy to process.

[0011] According to some embodiments of the present application, the D 50 Satisfy: 5μm≤D 50 ≤25μm, optionally, 7μm≤D 50 ≤15μm. This can reduce the processing difficulty of the electrode preparation process.

[0012] According to some embodiments of the present application, at least part of the surface of the lithium iron manganese phosphate positive electrode active material has a carbon coating layer, and the lithium iron manganese phosphate positive electrode active material includes a compound shown in Formula I:

[0013] Li 1+a Mn x Fe y M' z (PO 4 ) 1+b / C

[0014] Among them, -0.1≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0<z≤0.05, and x+y+z=1, 0≤b≤0.2, wherein M' includes at least one of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

[0015] The second aspect of the present application provides a method for preparing the lithium manganese iron phosphate positive electrode active material provided in the first aspect of the present application, the method comprising: mixing an iron source, a first phosphorus source, a manganese source, a lithium source, a first M' source, a first carbon source, and a first solvent to form a first dispersion, grinding, drying, and then performing a first sintering to obtain an intermediate material; mixing the intermediate material, a second phosphorus source, a second M' source, a second carbon source, and a second solvent to form a second dispersion, wherein the sum of the molar amounts of the metal elements other than the lithium element in the intermediate material and the second M' source is n 1 The molar amount of P element in the second phosphorus source is n 2 , and satisfy 0<n 2 / n 1 ≤0.025, after grinding and drying, a second sintering is performed, and the temperature of the second sintering is greater than the temperature of the first sintering to obtain the lithium manganese iron phosphate positive electrode active material. Therefore, the method for preparing lithium manganese iron phosphate positive electrode active materials proposed in this application can control the micro-phosphorus richness of the primary particle body and surface, and control the porosity of the secondary particles by adding a phosphorus source twice during the first sintering and the second sintering process, while improving the infiltration effect of the electrolyte on the positive electrode active material, reducing the corrosion of the positive electrode active material body by a small amount of HF in the electrolyte, and reducing Mn 3+ The side reaction between the electrolyte and the cathode material builds a stable solid-liquid interface, ensuring the structural stability and capacity of the cathode active material under long-term cycle and high temperature conditions, thus obtaining a battery with high energy density, excellent cycle performance and fast charging performance.

[0016] According to some embodiments of the present application, the solid content of the second dispersion is 20%-50%. Thus, by controlling the solid content of the second dispersion before secondary granulation, optimizing the porous structure inside the secondary particles, controlling the volume proportion of the macropores, while allowing the electrolyte to fully and quickly infiltrate the positive electrode active material, ensuring full contact between the primary particles, thereby improving the spherical strength of the secondary particles and improving the electron transmission efficiency.

[0017] According to some embodiments of the present application, the method satisfies at least one of the following conditions: the temperature of the first sintering is 400°C-700°C, and the time of the first sintering is 3h-10h; the temperature of the second sintering is 600°C-800°C, and the time of the second sintering is 6h-12h. Thus, the first sintering and the second sintering can achieve efficient capacity matching.

[0018] According to some embodiments of the present application, the method satisfies at least one of the following conditions: the iron source includes at least one of an oxide containing iron, a carbonate containing iron, a hydroxide containing iron, and a phosphate containing iron; the manganese source includes at least one of an oxide containing manganese, a carbonate containing manganese, a hydroxide containing manganese, and a phosphate containing manganese; the lithium source includes at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium carbonate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, and lithium citrate; the first The phosphorus source and the second phosphorus source independently include at least one of phosphoric acid and phosphates; the first M' source and the second M' source independently include at least one of oxides containing M', hydroxides containing M', hydroxy oxides containing M', carbonates containing M', oxalates containing M', sulfates containing M', acetates containing M', and nitrates containing M'; the first carbon source and the second carbon source independently include at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol, and dopamine.

[0019] The third aspect of the present application provides a positive electrode plate, which includes the lithium iron manganese phosphate positive electrode active material provided in the first aspect of the present application or the lithium iron manganese phosphate positive electrode active material prepared by the method provided in the second aspect of the present application.

[0020] The fourth aspect of the present application provides a lithium secondary battery, comprising the positive electrode plate provided in the third aspect of the present application.

[0021] The fifth aspect of the present application provides an electrical device, including the lithium secondary battery provided by the fourth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0023] Figure 1 A schematic flow chart of a method for preparing lithium manganese iron phosphate positive electrode active material according to an embodiment of the present application is shown.

[0024] Figure 2 The SEM image of the secondary particles of the lithium manganese iron phosphate positive electrode active material prepared in Example 1 is shown.

[0025] Figure 3 The cross-sectional SEM image of the secondary particles of the lithium manganese iron phosphate positive electrode active material prepared in Example 1 is shown. DETAILED DESCRIPTION

[0026] The embodiments of the present application are described in detail below. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific technology or conditions are indicated in the embodiments, the technology or conditions described in the document in this area or the product specification are used. The reagents used or the instruments that do not indicate the manufacturer are all conventional products that can be obtained commercially.

[0027] The first aspect of the present application provides a lithium iron manganese phosphate positive electrode active material, wherein the lithium iron manganese phosphate positive electrode active material is a secondary particle formed by aggregation of primary particles, the secondary particle has pores inside, the porosity of the secondary particle is 20%-55%, and the molar ratio of the P element on the surface of the primary particle to the metal element other than the Li element is m 1 The molar ratio of the P element to the metal element other than the Li element in the primary particle phase is m 2 , and satisfy 1.01≤m 1 / m 2 ≤1.07.

[0028] The lithium manganese iron phosphate positive electrode active material (hereinafter referred to as the positive electrode active material) proposed in the present application can improve the infiltration effect of the electrolyte on the positive electrode active material by controlling the porosity of the secondary particles, increase the diffusion rate of lithium ions, and thus improve the rate performance of the battery. At the same time, under long cycle and high temperature conditions, by controlling the micro-phosphorus enrichment on the surface of the primary particles, a chemically stable phosphate phase thin layer with a certain lithium ion conductivity is generated on the surface of the primary particles, blocking the direct contact between the electrolyte and the positive electrode active material, reducing the corrosion of the positive electrode active material by a small amount of HF in the electrolyte, and reducing the Mn 3+ The side reaction between the electrolyte and the cathode material builds a stable solid-liquid interface, ensuring the structural stability and capacity of the cathode active material under long-term cycle and high temperature conditions, thus obtaining a battery with high energy density, excellent cycle performance and fast charging performance.

[0029] If the porosity of the secondary particles is too small, that is, the primary particles are too densely packed, the electrolyte can only enter the secondary particles in small amounts, and the lithium ion exchange between the electrolyte and the positive electrode active material occurs more on the surface of the secondary particles. Due to the low bulk ion diffusion capacity of the lithium iron manganese phosphate material itself, it takes a long time for lithium ions to diffuse from the inside of the secondary particles to the surface of the secondary particles. Overcoming the high energy barrier will result in a lower capacity and higher electrochemical polarization in the battery test. If the porosity of the secondary particles is too large, the primary particles are too loosely packed. Although the electrolyte can fully infiltrate, the contact positions between the primary particles are reduced, and the conduction routes of electrons in the active material become fewer, which will also lead to an increase in the ohmic impedance of the material, an increase in the polarization of the battery test, and a weakening of the structural strength of the secondary particles, making them prone to breakage and pulverization. Some primary particles lose their activity due to loss of contact.

[0030] If m 1 / m 2 <1.01, there is no phosphorus-rich phase protective layer on the surface of the primary particles, the positive electrode active material is easily corroded by substances such as HF in the electrolyte, and the metal elements such as Mn and Fe in the surface crystal structure are easily corroded and dissolved into the electrolyte. Electrochemically inert byproducts are formed on the surface of the positive electrode active material, which hinders ion diffusion and increases the interface impedance. The dissolved metal ions diffuse to the negative electrode, which will destroy the negative electrode SEI film and aggravate the loss of active lithium, thereby damaging the battery cycle performance; if m 1 / m 2 >1.07, too much phosphate and interfacial phosphate-rich phase as non-electrochemically active substances will significantly affect the lithium ion interface exchange and inhibit the capacity of the positive electrode active material, especially the lithium deintercalation rate of the positive electrode active material at high rate.

[0031] The porosity of the lithium iron phosphate positive electrode active material proposed in the present application can be obtained by testing with a specific surface area and pore size analyzer. The sample is degassed at 300°C before testing, and then an adsorption and desorption test is performed with nitrogen. The porosity of the material is obtained based on the BJH adsorption model.

[0032] In the present application, when testing the micro-phosphorus enrichment of the positive electrode active material, the secondary particles are cut by ion beam, at least n primary particles (n≥10) are randomly selected from the cross section, and the molar ratio of the P element on the surface of the primary particles to the content of the metal element other than the Li element is measured by an energy dispersive spectrometer (EDS). n1 , and the molar ratio of P element to metal elements other than Li element in the primary particle phase m n2 , calculate m n1 / m n2 , m of n primary particles n1 / m n2The average value of the sum can represent the micro-phosphorus enrichment of the secondary particles. n1 When 5 points are randomly selected on the surface of the primary particle for measurement, the average value is taken as m n1 ; Test m n2 When , 5 points can be randomly selected in the primary particle phase for measurement, and the average value is taken as m n2 .

[0033] Specifically, the surface of a primary particle refers to an area perpendicular to the tangent line of the primary particle surface and within a range of ≤10 nm from the inside of the primary particle, and the bulk of a primary particle refers to an area located at the center of the primary particle and with a minimum distance from the surface of the primary particle greater than 10 nm.

[0034] As an example, the porosity of the secondary particles may be 20%, 30%, 40%, 50%, 55%, etc., or may be in a range consisting of any of the above values.

[0035] As an example, m 1 / m 2 It can be 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, etc., or can be a range consisting of any of the above values. According to some specific embodiments of the present application, 1.02≤m 1 / m 2 ≤1.05.

[0036] According to some embodiments of the present application, the average particle size of the primary particles may be 50nm-150nm, for example, 50nm, 70nm, 90nm, 110nm, 130nm, 150nm, etc., or may be a range composed of any of the above numerical values. Thus, the transmission path of lithium ions in the solid phase is shortened, the solid-liquid ion exchange interface is increased, the kinetic properties of the positive electrode active material are improved, and the fast charging performance of the battery is improved. According to some specific embodiments of the present application, the average particle size of the primary particles may be 70nm-120nm.

[0037] In the present application, the average particle size of the primary particles can be tested by scanning electron microscopy (SEM). Specifically, the image magnification is 30K, 100 primary particles in the image are randomly selected, the length of the longest diagonal and the shortest diagonal of each primary particle are measured respectively, and the average value is taken to obtain the primary particle size. The SEM test requires random sampling of primary particles and random selection of areas. The SEM image obtained by the test can represent the average particle size of the primary particles of the positive electrode active material.

[0038] According to some embodiments of the present application, the pores include a first pore and a second pore, the pore size of the second pore is smaller than the pore size of the first pore, the pore size of the first pore is 100nm-300nm, the total pore volume of the first pore and the second pore in the secondary particle is V, and the total pore volume of the first pore in the secondary particle is V 1 , and meets: 20% ≤ V 1 / V≤50%.

[0039] Specifically, the pore size of the second pore is smaller than that of the first pore. The smaller second pore is formed by the accumulation of primary particles, can accommodate electrolyte, and provide a stable solid-liquid interface to provide ion exchange, while the larger first pore only serves as a channel for the electrolyte to enter the interior of the secondary particles. Too many first pores are not conducive to the contact between primary particles and are a break in the electron transmission path.

[0040] Therefore, the present application controls the volume proportion of the first pores, and ensures sufficient contact between the primary particles while allowing the electrolyte to fully and quickly infiltrate the positive electrode active material, thereby improving the electron transmission efficiency, improving the spherical strength of the secondary particles, and further increasing the electrode load.

[0041] In the present application, the total pore volume V of the first pores in the secondary particles 1 , and the total pore volume of the first pore and the second pore in the secondary particle is V as follows: the positive electrode active material is placed in a nitrogen purge environment, dehydrated at 300°C for 30 minutes, and then placed in a surface area test tube for nitrogen adsorption and desorption test. The pore volumes corresponding to the pore structures with different pore sizes in the range of 1.7nm to 300nm are analyzed based on the BJH adsorption model. The total pore volume in the pore size range of 100nm to 300nm is the total pore volume V of the first pore in the secondary particle. 1 The total pore volume within the pore size range of 1.7 nm to 300 nm is the total pore volume V of the first pores and the second pores in the secondary particles.

[0042] As an example, V 1 / V can be 20%, 30%, 40%, 50%, etc., or can be a range consisting of any of the above values. According to some specific embodiments of the present application, 30%≤V 1 / V≤50%.

[0043] According to some embodiments of the present application, 0.02 cm 3 / g≤V 1 ≤0.10cm 3 / g, for example, can be 0.02cm 3 / g, 0.05cm 3 / g, 0.07cm3 / g, 0.09cm 3 / g, 0.1cm 3 / g, etc., or can be a range composed of any of the above values. Thus, by controlling the pore volume of the larger first pores within the above range, sufficient contact between the primary particles is ensured, thereby improving the spherical strength of the secondary particles and improving the electron transfer efficiency. According to some specific embodiments of the present application, 0.02cm 3 / g≤V 1 ≤0.10cm 3 / g.

[0044] According to some embodiments of the present application, the pore size of the second pore may be 1.7 nm-100 nm, for example, 1.7 nm, 10 nm, 30 nm, 50 nm, 70 nm, 90 nm, 100 nm, etc., or may be a range of any of the above values. Thus, the smaller second pore can accommodate the electrolyte and provide a stable solid-liquid interface for lithium ion exchange.

[0045] According to some embodiments of the present application, the specific surface area of ​​the lithium manganese iron phosphate positive electrode active material can be 10m 2 / g-30m 2 / g, for example, it can be 10m 2 / g, 15m 2 / g, 20m 2 / g, 25m 2 / g, 30m 2 / g, etc., or can be a range of any of the above values. In this way, the lithium ion exchange rate of the material can be increased, the risk of moisture absorption of the material can be reduced, and the processing capabilities of the slurry, coating, rolling and other processes in the electrode manufacturing process can be improved. According to some specific embodiments of the present application, the specific surface area of ​​the lithium manganese iron phosphate positive electrode active material can be 15m 2 / g-25m 2 / g.

[0046] In the present application, the specific surface area of ​​the positive electrode active material can be obtained by testing with a specific surface area and pore size analyzer. Specifically, the sample is degassed at 300°C before testing, and then subjected to a nitrogen adsorption and desorption test, and the specific surface area of ​​the material is obtained based on the BJH adsorption model.

[0047] According to some embodiments of the present application, the D 10 , D 50 , D 90 Satisfy: k 90 =(D 90 -D 10 ) / D 50 , 1.5≤k 90≤2.5. As a result, the secondary particles have a narrow and uniform particle size distribution, good consistency and are easy to process.

[0048] According to some embodiments of the present application, the D 50 Satisfy: 5μm≤D 50 ≤25μm, for example, it can be 5μm, 10μm, 15μm, 20μm, 25μm, etc., or it can be a range of any of the above values. Thus, the difficulty of processing the material electrode preparation process can be reduced. According to some specific embodiments of the present application, 7μm≤D 50 ≤15μm.

[0049] According to some embodiments of the present application, at least part of the surface of the lithium iron manganese phosphate positive electrode active material has a carbon coating layer, and the lithium iron manganese phosphate positive electrode active material includes a compound shown in Formula I:

[0050] Li 1+a Mn x Fe y M' z (PO 4 ) 1+b / C

[0051] Among them, -0.1≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0≤z≤0.05, and x+y+z=1, 0≤b≤0.2, wherein M' includes at least one of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

[0052] As an example, a may be -0.1, 0, 0.1, 0.2, etc., or may be a range consisting of any of the above values.

[0053] It should be noted that, as the battery undergoes formation and circulation processes, lithium ions will be consumed, so the measured lithium content 1+a in the positive electrode active material may be less than 1. At the same time, if the positive electrode and the negative electrode use a lithium supplement, the battery will undergo formation and circulation processes, and the measured lithium content 1+a in the positive electrode active material may be greater than 1.

[0054] As an example, x may be 0.3, 0.5, 0.7, 0.9, 0.99, etc., or may be a range consisting of any of the above values.

[0055] As an example, y may be 0.1, 0.3, 0.5, 0.7, etc., or may be a range consisting of any of the above values.

[0056] As an example, z may be 0, 0.01, 0.03, 0.05, etc., or may be a range consisting of any of the above values.

[0057] As an example, b may be 0, 0.05, 0.1, 0.2, etc., or may be a range consisting of any of the above values.

[0058] The second aspect of the present application provides a method for preparing the lithium manganese iron phosphate positive electrode active material provided in the first aspect of the present application, the method comprising: mixing an iron source, a first phosphorus source, a manganese source, a lithium source, a first M' source, a first carbon source, and a first solvent to form a first dispersion, grinding, drying, and then performing a first sintering to obtain an intermediate material; mixing the intermediate material, a second phosphorus source, a second carbon source, a second M' source, and a second solvent to form a second dispersion, wherein the sum of the molar amounts of metal elements other than lithium in the intermediate material and the second M' source is n 1 The molar amount of P element in the second phosphorus source is n 2 , and satisfy 0<n 2 / n 1 ≤0.025, after grinding and drying, a second sintering is performed, and the temperature of the second sintering is greater than the temperature of the first sintering to obtain the lithium manganese iron phosphate positive electrode active material. Therefore, the method for preparing lithium manganese iron phosphate positive electrode active materials proposed in this application can control the micro-phosphorus richness of the primary particle body and surface, and control the porosity of the secondary particles by adding a phosphorus source twice during the first sintering and the second sintering process, while improving the infiltration effect of the electrolyte on the positive electrode active material, reducing the corrosion of the positive electrode active material body by a small amount of HF in the electrolyte, and reducing Mn 3+ The side reaction between the electrolyte and the cathode material builds a stable solid-liquid interface, ensuring the structural stability and capacity of the cathode active material under long-term cycle and high temperature conditions, thus obtaining a battery with high energy density, excellent cycle performance and fast charging performance.

[0059] The method for preparing lithium manganese iron phosphate positive electrode active material proposed in this application is described in detail below. Figure 1 , the method comprising:

[0060] S10: Mix the iron source, the first phosphorus source, the manganese source, the lithium source, the first M' source, the first carbon source, and the first solvent to form a first dispersion, grind, dry, and perform a first sintering to obtain an intermediate material.

[0061] According to some embodiments of the present application, the iron source, the first phosphorus source, the manganese source, the lithium source, the first M' source, and the first carbon source are weighed in a certain metering ratio and dispersed in the first solvent, and then ground and atomized and dried, and then transferred to a protective atmosphere furnace for the first sintering to obtain an intermediate material. In this way, the amount of the raw materials added can be accurately controlled, and they can be fully dispersed and mixed evenly in the first solvent.

[0062] According to some embodiments of the present application, the temperature of the first sintering may be 400° C.-700° C., and the time of the first sintering may be 3 h-10 h.

[0063] As an example, the temperature of the first sintering may be 400° C., 500° C., 600° C., 700° C., etc., or may be a range consisting of any of the above values.

[0064] As an example, the first sintering time may be 3 h, 5 h, 8 h, 10 h, etc., or may be within a range consisting of any of the above values.

[0065] Therefore, by setting the temperature and time of the first sintering within the above ranges, the raw materials can react sufficiently to form lithium iron manganese phosphate crystals, and the primary particle size can be controlled so as not to grow excessively.

[0066] According to some embodiments of the present application, the iron source includes at least one of an iron-containing oxide, an iron-containing carbonate, an iron-containing hydroxide, and an iron-containing phosphate.

[0067] According to some embodiments of the present application, the manganese source includes at least one of an oxide containing manganese, a carbonate containing manganese, a hydroxide containing manganese, and a phosphate containing manganese.

[0068] According to some embodiments of the present application, the lithium source includes at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium carbonate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, and lithium citrate.

[0069] According to some embodiments of the present application, the first phosphorus source includes at least one of phosphoric acid and phosphate.

[0070] According to some embodiments of the present application, the first M' source includes at least one of an oxide containing M', a hydroxide containing M', a hydroxyl oxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'.

[0071] According to some embodiments of the present application, the first carbon source includes at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol and dopamine.

[0072] S20: Mix the intermediate material, the second phosphorus source, the second carbon source, the second M' source, and the second solvent to form a second dispersion, grind and dry, and then perform a second sintering, wherein the temperature of the second sintering is higher than the temperature of the first sintering, to obtain the lithium manganese iron phosphate positive electrode active material

[0073] Therefore, by selecting and using the second carbon source, the second phosphorus source, and the second M' source, the internal pore structure of the secondary particles and the micro-phosphorus enrichment on the surface of the primary particles can be further regulated within a larger range, thereby improving the compatibility of the material of this application with different application scenarios and usage requirements.

[0074] According to some embodiments of the present application, the solid content of the second dispersion is 20%-50%. For example, it can be 20%, 30%, 40%, 50%, etc., or it can be a range composed of any of the above values. Therefore, by controlling the solid content of the second dispersion before the secondary granulation, the degree of spherical shrinkage, the distribution of carbon sources and the pore structure of the residual pores after the evaporation of water during the secondary granulation process can be adjusted, the porous structure inside the secondary particles can be optimized, and the volume proportion of the macropores can be controlled. While the electrolyte fully and quickly infiltrates the positive electrode active material, it is ensured that the primary particles are fully in contact, thereby improving the spherical strength of the secondary particles and improving the electron transmission efficiency.

[0075] By controlling the solid content of the second dispersion before drying, the porosity of the positive electrode active material can be adjusted. The selection and dosage of the second solvent determine the redispersion of the soluble phosphorus-containing compounds remaining in the intermediate material after sintering, and enrichment on the surface of the primary particles during the second sintering process to form a slightly rich phosphorus content, thereby improving the wetting effect of the electrolyte on the positive electrode active material while reducing the corrosion of the small amount of HF in the electrolyte on the body phase of the positive electrode active material, reducing the Mn 3+ The side reaction between the electrolyte and the cathode material builds a stable solid-liquid interface, ensuring the structural stability and capacity of the cathode active material under long-term cycle and high temperature conditions, thus obtaining a battery with high energy density, excellent cycle performance and fast charging performance.

[0076] According to some embodiments of the present application, the sum of the molar amounts of the metal elements other than lithium in the intermediate material and the second M' source is n 1 The molar amount of P element in the second phosphorus source is n 2 , and satisfy 0<n 2 / n 1 ≤0.025. For example, it can be 0.001, 0.005, 0.01, 0.015, 0.02, 0.025, etc., or can be a range of any of the above values. In this way, the micro-phosphorus enrichment on the surface of the primary particles can be adjusted, and a chemically stable phosphate phase thin layer with a certain lithium ion conductivity can be generated on the surface of the primary particles to block the direct contact between the electrolyte and the positive electrode active material, reduce the corrosion of the positive electrode active material by a small amount of HF in the electrolyte, and reduce the Mn 3+The side reaction between the electrolyte and the cathode material builds a stable solid-liquid interface, ensuring the structural stability and capacity of the cathode active material under long-term cycle and high temperature conditions, thus obtaining a battery with high energy density, excellent cycle performance and fast charging performance.

[0077] According to some embodiments of the present application, the temperature of the second sintering is 600° C.-800° C., and the time of the second sintering is 6 h-12 h.

[0078] As an example, the temperature of the second sintering may be 600° C., 650° C., 700° C., 750° C., 800° C., etc., or may be a range consisting of any of the above values.

[0079] As an example, the second sintering time may be 6 h, 8 h, 10 h, 12 h, etc., or may be within a range consisting of any of the above values.

[0080] Therefore, by making the temperature and time of the second sintering within the above range, it is possible to ensure that the carbon source is fully coated and carbonized on the surface of the lithium manganese iron phosphate material, thereby improving the conductivity of the material, controlling the generation of a micro-phosphorus-rich phase on the surface of the primary particles, and preventing the micro-phosphorus-rich phase from diffusing into the interior of the primary particles.

[0081] According to some embodiments of the present application, the second phosphorus source includes at least one of phosphoric acid and phosphate.

[0082] According to some embodiments of the present application, the second M' source includes at least one of an oxide containing M', a hydroxide containing M', a hydroxyl oxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'.

[0083] According to some embodiments of the present application, the second carbon source includes at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol and dopamine.

[0084] The third aspect of the present application provides a positive electrode plate, which includes the lithium iron manganese phosphate positive electrode active material provided in the first aspect of the present application or the lithium iron manganese phosphate positive electrode active material prepared by the method provided in the second aspect of the present application.

[0085] The fourth aspect of the present application provides a lithium secondary battery, comprising the positive electrode plate provided in the third aspect of the present application.

[0086] The fifth aspect of the present application provides an electrical device, including the lithium secondary battery provided by the fourth aspect of the present application.

[0087] The embodiments of the present application are described in detail below. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific technology or conditions are indicated in the embodiments, the technology or conditions described in the document in this area or the product specification are used. The reagents used or the instruments that do not indicate the manufacturer are all conventional products that can be obtained commercially.

[0088] Example 1

[0089] 1. Preparation of lithium manganese iron phosphate positive electrode active materials

[0090] (1) Lithium carbonate, ferromanganese phosphate and titanium dioxide were weighed and dispersed in pure water in a molar ratio of Li:Mn:Fe:Ti:P=1.04:0.74:0.25:0.01:1.01, and the first carbon source was selected as glucose, the mass of which was 5% of the mass of ferromanganese phosphate, followed by a first grinding, and then transferred to a nitrogen atmosphere furnace after spray drying, and sintered for the first time at 600°C for 8 hours to obtain a first lithium ferromanganese phosphate material;

[0091] (2) The first lithium manganese iron phosphate material and the second carbon source starch were weighed in a weight ratio of 100:10, dispersed in pure water, and 1% of diammonium phosphate based on the molar amount of non-lithium metal elements in the first lithium manganese iron phosphate material was added. The solid content of the second dispersion was controlled to be 40% for a second grinding, and then the dispersion was spray-dried and transferred to a nitrogen atmosphere furnace for a second sintering at 650° C. for 10 hours. After crushing and screening, the finished lithium manganese iron phosphate material Li 1.04 Mn 0.74 Fe 0.25 Ti 0.01 (PO 4 ) 1.02 / C,Li 1.04 Mn 0.74 Fe 0.25 Ti 0.01 (PO 4 ) 1.02 / C SEM image and cross-sectional SEM image are attached Figure 2 and attached Figure 3 .

[0092] 2. Preparation of positive electrode sheet

[0093] The positive electrode active material Li 1.04 Mn 0.74 Fe 0.25 Ti 0.01 (PO 4 ) 1.02 / C, conductive carbon black and binder polyvinylidene fluoride (PVDF) are placed in a slurry mixing tank in a mass ratio of 90:5:5, and N-methylpyrrolidone (NMP) is added dropwise to adjust the mixture. NMP is added and stirred to form a uniform slurry, which is then coated on an aluminum foil, scraped flat, and rolled flat after drying. A positive electrode sheet with a diameter of 12 mm and a thickness of 120 μm is punched out at a pressure of 100 MPa, and then placed in a vacuum oven at 120°C for 12 h.

[0094] 3. Preparation of negative electrode sheet

[0095] The negative electrode uses a purchased Li metal sheet with a diameter of 17 mm and a thickness of 1 mm.

[0096] 4. Electrolyte

[0097] The electrolyte used was 1 mol / L LiPF 6 As the electrolyte, a carbonate electrolyte with a mixed solution of ethyl oxalate: dicarboximide: acetimide in a volume ratio of 1:1:1 as the solvent.

[0098] 5. Diaphragm

[0099] The thickness of the diaphragm is 25 μm, the manufacturer is Celgard, and the model is 2325.

[0100] 6. Assemble the battery

[0101] The button battery assembly process is carried out in an Ar gas-protected glove box, where the water content and oxygen content are both less than 5ppm. The positive electrode battery shell, positive electrode sheet, electrolyte, isolation membrane, electrolyte, negative electrode lithium sheet, spring gasket, and negative electrode shell are placed in sequence, pressed and sealed, and tested after standing for 6 hours.

[0102] Performance Testing

[0103] 1. First week charge and discharge capacity, first efficiency, and rate test:

[0104] The assembled button cell was placed in a constant temperature box at 25°C and connected to a Xinwei battery test cabinet with a test fixture. It was charged at a constant current of 0.1C (1C = 140mA / g) to a cut-off voltage of 4.3V, and then kept charging at a constant voltage for 30min. Subsequently, it was discharged at a constant current of 0.1C to a cut-off voltage of 2.5V. The charge and discharge capacity of this time was taken as the charge and discharge capacity of the battery in the first week, and the ratio of the discharge capacity to the charge capacity was taken as the first efficiency.

[0105] After the first charge and discharge is completed, the above charge and discharge process is repeated in sequence with currents of 0.2C, 0.33C, 0.5C, 1C, and 2C, and the ratio of 1C discharge capacity to 0.1C discharge capacity is used as the evaluation index of rate performance.

[0106] 2.80 weeks capacity retention rate:

[0107] The button cell assembled above was placed in a 45°C constant temperature box and connected to the Xinwei battery test cabinet with a test fixture. Two charge and discharge cycles were completed with a current of 0.1C according to the above charge and discharge process. Then, 80 weeks of charge and discharge were completed with a current of 1C according to the above charge and discharge process. The ratio of the discharge capacity in the 80th week of the 1C charge and discharge test to the discharge capacity in the 1st week was used as the evaluation index of the 80-week capacity retention rate.

[0108] Examples 2-9 and Comparative Examples 1-2

[0109] The positive electrode active material was prepared according to the method of Example 1, and the material composition and specific process conditions were changed as shown in Table 1. Examples 2-9 and Comparative Examples 1-2 were carried out respectively. The corresponding physical and chemical indicators of the samples were shown in Table 2, and the tested electrical properties were shown in Table 3.

[0110]

[0111]

[0112] Table 3

[0113]

[0114] In the process of preparing lithium manganese iron phosphate positive electrode active materials in Comparative Example 1, a phosphorus source was added only during the first sintering process. The content of P element on the surface of the primary particles and in the bulk of the primary particles of the obtained positive electrode active material was the same, the P element was not enriched on the surface of the primary particles, and the battery cycle performance was poor.

[0115] In Comparative Example 2, a phosphorus source is added twice during the preparation of the lithium manganese iron phosphate positive electrode active material, but the amount of the second phosphorus source added is too much, resulting in excessive P element content enriched on the surface of the primary particles, which will reduce the battery capacity and rate performance.

[0116] It can be seen from the comparison between Examples 1 to 9 and Comparative Examples 1 and 2 that by controlling the porosity and m 1 / m 2 The ratio of can make the battery have higher charge and discharge capacity and cycle capacity retention rate, which means that by controlling the porosity of the secondary particles, the wetting effect of the electrolyte on the positive electrode active material can be guaranteed, thereby increasing the diffusion rate of lithium ions; by controlling the micro-phosphorus enrichment on the surface of the primary particles, a thin layer of phosphate phase can be formed on the surface of the primary particles, blocking the direct contact between the electrolyte and the positive electrode active material, reducing the corrosion of the positive electrode active material by a small amount of HF in the electrolyte, and improving the cycle performance of the battery, thereby obtaining a battery with better rate performance, higher capacity utilization and better cycle performance.

[0117] It can be seen from the comparison between Examples 1 and 4 that the enrichment degree of P element on the surface of primary particles can be adjusted by adjusting the amount of the second phosphorus source added, that is, m 1 / m 2 The ratio of , thereby increasing the charge and discharge capacity while improving the battery's cycle capacity retention rate

[0118] It can be seen from the comparison between Example 1 and Example 5-Example 7 that when the addition amounts of the first phosphorus source and the second phosphorus source are the same, the m of the positive electrode active material is 1 / m 2 The total volume V of the first pores in the secondary particles can be adjusted by changing the solid content of the second dispersion. 1 and the total pore volume V of the first pore and the second pore in the secondary particle, thereby adjusting V 1 / V ratio, which increases the diffusion rate of lithium ions, thereby increasing the battery capacity and improving the battery's rate performance and cycle performance.

[0119] It can be seen from Examples 8 and 9 that by selecting different types of the first carbon source, the second phosphorus source, and the second carbon source, and by adjusting the temperature and time of the first sintering and the second sintering, a lithium manganese iron phosphate positive electrode active material with stable structure, good cycle performance and rate performance can be obtained.

[0120] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A lithium manganese iron phosphate positive electrode active material, characterized in that: The secondary particles are formed by aggregation of primary particles, the secondary particles have pores inside, the porosity of the secondary particles is 20%-55%, the molar ratio of the P element on the surface of the primary particles to the metal element other than the Li element is m1, the molar ratio of the P element to the metal element other than the Li element in the bulk phase of the primary particles is m2, and 1.01≤m1 / m2≤1.07 is satisfied; preferably, 1.02≤m1 / m2≤1.

05.

2. The lithium iron manganese phosphate positive electrode active material according to claim 1, characterized in that: The average particle size of the primary particles is 50nm-150nm, and can be optionally 70nm-120nm.

3. The lithium iron manganese phosphate positive electrode active material according to claim 1 or 2, characterized in that: The pores include first pores and second pores, the pore size of the second pores is smaller than the pore size of the first pores, the pore size of the first pores is 100nm-300nm, the total pore volume of the first pores and the second pores in the secondary particles is V, the total pore volume of the first pores in the secondary particles is V1, and satisfies: 20% V1 / V≤50%, optionally, 30%≤V1 / V≤50%.

4. The lithium iron manganese phosphate positive electrode active material according to claim 3, characterized in that: 0.02cm 3 / g≤V1≤0.1cm 3 / g, optional 0.02cm 3 / g≤V1≤0.05cm 3 / g.

5. The lithium iron manganese phosphate positive electrode active material according to claim 4, characterized in that: The pore diameter of the second pore is 1.7 nm-100 nm.

6. The lithium iron manganese phosphate positive electrode active material according to claim 3, characterized in that: The specific surface area of ​​the lithium manganese iron phosphate positive electrode active material is 10m 2 / g-30m 2 / g, optional 15m 2 / g-25m 2 / g.

7. The lithium iron manganese phosphate positive electrode active material according to claim 3, characterized in that: The D of the secondary particles 10 , D 50 , D 90 Satisfy: k 90 =(D 90 -D 10 ) / D 50 , 1.5≤k 90 ≤2.

5.

8. The lithium iron manganese phosphate positive electrode active material according to claim 7, characterized in that: The D of the secondary particles 50 Satisfy: 5μm≤D 50 ≤25μm, optionally, 7μm≤D 50 ≤15μm.

9. The lithium iron manganese phosphate positive electrode active material according to claim 1, characterized in that: At least part of the surface of the lithium iron manganese phosphate positive electrode active material has a carbon coating layer, and the lithium iron manganese phosphate positive electrode active material includes a compound shown in Formula I: Li 1+a Mn x Fe y M’ z (PO4) 1+b / C Among them, -0.1≤a≤0.2, 0.3≤x<1, 0<y≤0.7, 0<z≤0.05, and x+y+z=1, 0≤b≤0.2, wherein M' includes at least one of B, Mg, Al, Ca, Ti, V, Co, Ni, Sr, Y, Zr, Nb, Mo, and W.

10. A method for preparing the lithium manganese iron phosphate positive electrode active material according to any one of claims 1 to 9, characterized in that: include: An iron source, a first phosphorus source, a manganese source, a lithium source, a first M' source, a first carbon source, and a first solvent are mixed to form a first dispersion, and after grinding and drying, a first sintering is performed to obtain an intermediate material; The intermediate material, a second phosphorus source, a second carbon source, a second M' source, and a second solvent are mixed to form a second dispersion, wherein the sum of the molar amounts of metal elements other than lithium in the intermediate material and the second M' source is n1, the molar amount of P element in the second phosphorus source is n2, and 0<n2 / n1≤0.025 is satisfied. After grinding and drying, a second sintering is performed, and the temperature of the second sintering is greater than the temperature of the first sintering, so as to obtain the lithium manganese iron phosphate positive electrode active material.

11. The method according to claim 10, characterized in that The solid content of the second dispersion is 20%-50%.

12. The method according to claim 10, characterized in that Satisfy at least one of the following conditions: The temperature of the first sintering is 400°C-700°C, and the time of the first sintering is 3h-10h; The temperature of the second sintering is 600° C.-800° C., and the time of the second sintering is 6 h-12 h.

13. The method according to claim 10, characterized in that Satisfy at least one of the following conditions: The iron source includes at least one of an iron-containing oxide, an iron-containing carbonate, an iron-containing hydroxide, and an iron-containing phosphate; The manganese source includes at least one of an oxide containing manganese, a carbonate containing manganese, a hydroxide containing manganese, and a phosphate containing manganese; The lithium source includes at least one of lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium nitrite, lithium formate, lithium acetate, lithium oxalate, lithium carbonate, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, and lithium citrate; The first phosphorus source and the second phosphorus source independently include at least one of phosphoric acid and phosphate; The first M' source and the second M' source independently include at least one of an oxide containing M', a hydroxide containing M', an oxyhydroxide containing M', a carbonate containing M', an oxalate containing M', a sulfate containing M', an acetate containing M', and a nitrate containing M'; The first carbon source and the second carbon source independently include at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol and dopamine.

14. A positive electrode sheet, characterized in that: The invention comprises the lithium iron manganese phosphate positive electrode active material as described in any one of claims 1 to 9 or the lithium iron manganese phosphate positive electrode active material prepared by the method as described in any one of claims 10 to 13.

15. A lithium secondary battery, characterized in that: Including the positive electrode sheet as described in claim 14.

16. An electrical equipment, characterized in that: Includes the lithium secondary battery as claimed in claim 15.

Images