Positive electrode material and preparation method thereof, battery and electric device

Abstract

The invention discloses a positive electrode material and a preparation method thereof, a battery and a power utilization device, the positive electrode material is a lithium manganese iron phosphate positive electrode material, the positive electrode material comprises secondary particles, the secondary particles are spherical and comprise a plurality of primary crystal grains, the primary crystal grains are arranged along the radial direction of the secondary particles, and the primary crystal grains are arranged along the radial direction of the secondary particles. The positive electrode material is formed by stacking and agglomerating primary crystal grains, so that a lithium ion diffusion channel is widened, the diffusion path of lithium ions is effectively shortened, the migration rate of the lithium ions is improved, the concentration polarization of the lithium ions in the charge-discharge process is effectively reduced, and the rate discharge performance and the low-temperature discharge performance are improved. In addition, the concentration gradient distribution design of manganese can effectively relieve stress accumulation and dissolution phenomena generated in the charge-discharge process, and the structural stability is improved, so that the cycle performance of the positive electrode material is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a positive electrode material and a preparation method thereof, a battery, and an electrical device. Background Art

[0002] Olivine-structured lithium iron phosphate cathode materials have the advantages of long cycle life, low cost and high safety performance. Due to its excellent comprehensive performance, they are widely used in the fields of electric vehicles and energy storage. However, the low discharge capacity and discharge platform of lithium iron phosphate make it unable to adapt to the growing requirements of high energy density. Lithium manganese iron phosphate, which also has an olivine structure, has a high voltage platform and high energy density, which can be compatible with high safety and high energy density requirements. It is an upgraded product of lithium iron phosphate and a very promising new cathode material. However, lithium manganese iron phosphate itself has poor lithium ion migration and diffusion capabilities, and lithium ion concentration polarization is prone to occur during the charge and discharge process, resulting in poor rate discharge performance and low-temperature discharge performance. Summary of the invention

[0003] In view of the problems existing in the background technology, the present invention provides a positive electrode material, which can increase the lithium ion migration rate of the positive electrode material, effectively reduce the concentration polarization of lithium ions during charging and discharging, and thus improve the rate discharge performance and low temperature discharge performance.

[0004] The first aspect of the present invention provides a positive electrode material, the positive electrode material comprises secondary particles, the secondary particles are spherical and comprise a plurality of primary crystal grains, the primary crystal grains are arranged along the radial direction of the secondary particles, and the positive electrode material is a lithium manganese iron phosphate positive electrode material. .

[0005] The lithium manganese iron phosphate positive electrode material of the present invention is a secondary particle, which is formed by the accumulation and agglomeration of primary crystal grains. Since the primary crystal grains are arranged radially along the secondary particles, the lithium ion diffusion channel is widened, the diffusion path of lithium ions is effectively shortened, the lithium ion migration rate is increased, and the concentration polarization of lithium ions during the charge and discharge process is effectively reduced, thereby improving the rate discharge performance and low-temperature discharge performance.

[0006] In some embodiments, the positive electrode material contains manganese, and the content of manganese decreases gradually from the center to the surface of the secondary particles. The manganese concentration gradient distribution design can effectively alleviate the stress accumulation generated during the charge and discharge process and improve the structural stability; at the same time, the low manganese content on the surface and the high manganese content in the bulk can effectively alleviate the dissolution of manganese during the cycle process, so that the positive electrode material has better structural stability, thereby improving the cycle performance of the positive electrode material.

[0007] In some embodiments, the chemical formula of the positive electrode material is LiMn x Fe 1-x M y N zPO4, where M and N each contain a metal element, 0 < x < 1, 0 ≤ y ≤ 0.05, 0 ≤ z ≤ 0.05, and y + z ≤ 0.06. Doping with metal elements can increase the lattice defects of lithium iron manganese phosphate, which is beneficial to improving the diffusion rate of Li + and the internal conductivity of the particles; it is beneficial to expand the one-dimensional diffusion channel of Li + along the b-axis and improve the intercalation / deintercalation of Li + inside the material and the lithium ion transport rate; at the same time, it is also beneficial to reduce the band gap widths of the lithium iron manganese phosphate and lithium iron manganese phosphate phases, and improve the electronic conductivity. Therefore, doping with metal elements can improve the rate discharge performance and low-temperature discharge performance of the cathode material.

[0008] In some embodiments, both y and z are not equal to 0, and the valence state of M is less than that of N. The surface binding energy of high-valence metal ions is higher than that of low-valence metal ions, which is beneficial to the radial orientation growth arrangement of primary grains, improving the lithium ion migration rate, effectively reducing the concentration polarization of lithium ions during the charge and discharge process, and improving the rate discharge performance and low-temperature discharge performance.

[0009] In some embodiments, the valence state of M is below 4; the valence state of N is above 5; M includes Mg +2 , Ti +4 , Cu +2 , Zn +2 , Y +3 or one or more of them; N includes Nb +5 , Mo +6 , V +5 , W +6 or one or more of them.

[0010] In some embodiments, the secondary particles are polycrystalline secondary particles, and the median particle size D50 of the secondary particles is 2 μm - 10 μm. Optimizing the particle size of the polycrystalline secondary particles is beneficial to improving the rate discharge performance, low-temperature discharge performance, and cycling performance of the cathode material.

[0011] The second aspect of the present invention provides a method for preparing the cathode material of the first aspect of the present invention, including the following steps:

[0012] Mix a manganese source, an iron source, a phosphorus source, an acid, a complexing agent, and a solvent, and the pH value of the reaction system is 2.0 - 5.0. After the reaction, a first slurry is obtained;

[0013] Add a lithium source to the first slurry and continue the reaction to obtain a second slurry;

[0014] Spray-dry and sinter the second slurry to obtain the cathode material.

[0015] By controlling the pH of the reaction system to a lower value, it is beneficial to induce the primary grains to grow and arrange in a radial orientation, thereby increasing the lithium ion migration rate, effectively reducing the concentration polarization of lithium ions during the charge and discharge process, and improving the rate discharge performance and low-temperature discharge performance.

[0016] In addition, the present invention adopts a liquid phase synthesis method to prepare lithium manganese iron phosphate positive electrode material, which can achieve uniform mixing of various elements at the molecular or atomic level, the precursor tends to be more nano-sized, and the doping elements are easy to enter the interior of the particles, effectively avoiding the enrichment or segregation of the doping elements, and the batch stability is good, and the product performance is excellent. Lithium manganese iron phosphate precursor and positive electrode material. In addition, by adopting liquid phase synthesis, the raw material ratio is accurate and adjustable, the mixing is more uniform, and the particle size distribution is uniform and controllable, and the product consistency and quality control are good.

[0017] In some embodiments, the concentration of the complexing agent is 0.1-0.4 mol / L.

[0018] The use of relatively high complexing agent concentration and relatively low pH can form a more suitable crystal nucleation environment, thereby promoting the radial arrangement and growth of primary grains along the secondary particles and dominating them, reducing the formation of a large number of new nuclei and new grains between the grains.

[0019] In some embodiments, in the step of preparing the first slurry, the amount of manganese source added is controlled to decrease in a gradient from the start of the reaction to the end of the reaction. By controlling the amount of manganese source added to change in a gradient, the content of manganese in the positive electrode material can be reduced in a gradient from the center to the surface of the polycrystalline secondary particles, thereby effectively alleviating the stress accumulation and dissolution phenomenon generated during the charge and discharge process, so that the positive electrode material has better structural stability and improves the cycle performance of the positive electrode material.

[0020] In some embodiments, preparing the second slurry includes: adding the lithium source and the doping metal source to the first slurry, and continuing the reaction to obtain the second slurry. The rate discharge performance and low temperature discharge performance of the positive electrode material can be improved by doping with metal elements.

[0021] In some embodiments, the doping metal source includes an M source and an N source, and the valence of M is less than that of N; preferably, the valence of M is less than 4; the valence of N is greater than 5; preferably, the M source includes one or more of a Mg source, a Ti source, a Cu source, a Zn source, and a Y source; preferably, the N source includes one or more of a W source, a V source, a Mo source, and a Nb source.

[0022] M source and N source are co-doped metal sources. High-valent metal ions have higher surface binding energy than low-valent metal ions, which is conducive to the radial orientation of primary grains, increasing the migration rate of lithium ions, effectively reducing the concentration polarization of lithium ions during charging and discharging, and improving rate discharge performance and low-temperature discharge performance.

[0023] In some embodiments, after obtaining the second slurry and before spray drying, the method further comprises: removing impurities from the second slurry, centrifuging, and drying to obtain a lithium iron manganese phosphate precursor; grinding and mixing the lithium iron manganese phosphate precursor with a carbon source. Preferably, the carbon source includes one or more of glucose, sucrose, starch, and phenolic resin. The use of a carbon source during the preparation process can form a coating layer on the particle surface, thereby improving the conductivity of the lithium iron manganese phosphate.

[0024] In some embodiments, the acid comprises one or more of acetic acid and carbonic acid; preferably, the chelating agent comprises one or more of oxalic acid, citric acid, and ethylenediaminetetraacetic acid; preferably, the manganese source comprises one or more of manganese oxalate, manganese sulfate, manganese nitrate, and manganese phosphate; preferably, the iron source comprises one or more of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous oxalate; preferably, the phosphorus source comprises one or more of phosphoric acid, diammonium hydrogen phosphate, and diammonium dihydrogen phosphate; preferably, the lithium source comprises one or more of lithium carbonate, lithium phosphate, and lithium hydroxide; preferably, the solvent comprises one or more of water, ethylene glycol, polyethylene glycol, propylene glycol, and hexylene glycol.

[0025] In some embodiments, the sintering includes primary sintering and secondary sintering; the primary sintering temperature is 400°C-550°C, and the time is 3h-5h; the secondary sintering temperature is 650°C-800°C, and the time is 6h-15h. The two-step sintering and the control of the sintering conditions are beneficial to improving the electrochemical properties of the positive electrode material.

[0026] The third aspect of the present invention provides a battery, comprising the positive electrode material of the first aspect of the present invention or the positive electrode material obtained by the method of the second aspect of the present invention.

[0027] A fourth aspect of the present invention provides an electrical device comprising the battery of the third aspect of the present invention.

[0028] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic structural diagram of the lithium manganese iron phosphate positive electrode material of the present invention.

[0030] Figure 2This is an X-ray diffraction (XRD) structure diagram of the lithium manganese iron phosphate positive electrode material prepared in Example 1.

[0031] Figure 3 This is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate positive electrode material prepared in Example 1.

[0032] Figure 4 This is the first charge and discharge curve of the button cell corresponding to the lithium manganese iron phosphate positive electrode material prepared in Example 1.

[0033] Figure 5 This is a 25°C rate discharge performance diagram of the soft-pack battery corresponding to the lithium manganese iron phosphate positive electrode material prepared in Example 1.

[0034] Figure 6 This is the 45°C cycle capacity retention curve of the soft-pack battery corresponding to the lithium manganese iron phosphate positive electrode material prepared in Example 1.

[0035] Figure 7 This is a three-dimensional spatial diagram of olivine-structured lithium manganese iron phosphate. DETAILED DESCRIPTION

[0036] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0037] Olivine-structured lithium iron phosphate cathode materials have the advantages of long cycle life, low cost and high safety performance. Due to its excellent comprehensive performance, they are widely used in the fields of electric vehicles and energy storage. However, the low discharge capacity and discharge platform of lithium iron phosphate make it unable to adapt to the growing requirements of high energy density. Lithium manganese iron phosphate, which also has an olivine structure, has a high voltage platform and high energy density, which can be compatible with high safety and high energy density requirements. It is an upgraded product of lithium iron phosphate and a very promising new cathode material. However, lithium manganese iron phosphate itself has poor lithium ion migration and diffusion capabilities, and lithium ion concentration polarization is prone to occur during the charge and discharge process, resulting in poor rate discharge performance and low-temperature discharge performance.

[0038] In order to improve the rate performance and low-temperature performance of lithium iron manganese phosphate, the present invention proposes a positive electrode material, in which the primary grains are arranged radially along the secondary particles, and the secondary particles formed by agglomeration widen the lithium ion diffusion channel, effectively shorten the diffusion path of lithium ions, and increase the lithium ion migration rate, effectively reducing the concentration polarization of lithium ions during the charge and discharge process, thereby improving the rate discharge performance and low-temperature discharge performance.

[0039] Specifically, a first aspect of the present invention provides a cathode material, which is a secondary particle. The secondary particle is spherical and comprises a plurality of primary grains, and the primary grains are arranged radially along the secondary particle. The cathode material is a lithium iron manganese phosphate cathode material.

[0040] In the present invention, the radial direction refers to the direction along the radius. "The primary grains are arranged radially along the secondary particle" can be understood as that from the center to the surface of the secondary particle, a plurality of primary grains are arranged along the same radial direction. The structural schematic diagram of the cathode material of the present invention is as Figure 1 shown, and from the center to the surface, the cathode material shows a radially growing structure in a radial direction.

[0041] In some embodiments, the cathode material contains manganese element, and the content of manganese element decreases in a gradient from the center to the surface of the secondary particle. The design of the concentration gradient distribution of manganese can effectively relieve the stress accumulation generated during charge and discharge, and improve the structural stability; at the same time, the low manganese content on the surface and the high manganese content in the bulk phase can effectively relieve the dissolution phenomenon of manganese element during the cycling process, making the cathode material have better structural stability, thereby improving the cycling performance of the cathode material.

[0042] In some embodiments, the chemical formula of the cathode material is LiMn x Fe 1-x M y N z PO4, where M and N each include a metal element, 0 < x < 1, 0 ≤ y ≤ 0.05, 0 ≤ z ≤ 0.05, and y + z ≤ 0.06. Doping with metal elements can increase the lattice defects of lithium iron manganese phosphate, which is beneficial to improving the diffusion rate of Li + and the internal conductivity of the particles; it is beneficial to expand the one-dimensional diffusion channel of Li + along the b-axis and improve the intercalation / deintercalation of Li + inside the material and the lithium ion transport rate; at the same time, it is also beneficial to reduce the band gap widths of the lithium iron manganese phosphate and lithium iron manganese phosphate two phases and improve the electronic conductivity. Therefore, by doping with metal elements, the rate discharge performance and low-temperature discharge performance of the cathode material can be improved.

[0043] The three-dimensional space diagram of lithium iron manganese phosphate is as Figure 7As shown in the figure, in the crystal structure of lithium manganese iron phosphate, Fe, Mn atoms and Li atoms all occupy the center of the octahedron, forming FeO octahedron, MnO octahedron and LiO octahedron, and P atoms occupy the center of the tetrahedron, forming PO tetrahedron. In the a-axis direction: the alternating FeO octahedron, MnO octahedron, LiO octahedron and PO tetrahedron form a layered structure, and there is no channel for lithium ion transmission. In the c-axis direction: the parallel planes formed by each FeO and MnO octahedron are connected by PO tetrahedron. Each PO has a common point with one FeO layer and MnO layer, and a common edge and a common point with another FeO layer and MnO layer. There is no connection between PO tetrahedrons, and lithium ion transmission is extremely difficult. The olivine structure causes the lithium ion transmission in the lithium manganese iron phosphate lattice to diffuse in one dimension along the b-axis. At the same time, it has been found that the migration rate of lithium ions along the b-axis is much faster than that in other directions.

[0044] In some embodiments, both y and z are not equal to 0, and the valence of M is less than that of N. High-valence metal ions have higher surface binding energy than low-valence metal ions, which is beneficial for oriented growth and arrangement of primary grains in radial direction, improving the migration rate of lithium ions, effectively reducing the concentration polarization of lithium ions during charging and discharging, and improving rate discharge performance and low-temperature discharge performance.

[0045] In some specific embodiments, the valence of M is 4 or less, such as 4, 3 or 2. The valence of N is 5 or more, such as 5 or 6.

[0046] In some specific embodiments, M includes Mg +2 、Ti +4 , Cu +2 、Zn +2 , Y +3 Preferably, N includes Nb +5 、Mo +6 、V +5 , W +6 M and N are usually different metal elements.

[0047] In some specific embodiments, M includes Ti and N includes Nb. The valence of Ti (4) is smaller than that of Nb (5). 4+ With Nb 5+ The molar ratio is 1:(1.8-2.2), for example 1:1.8, 1:1.9, 1:2.0, 1:2.1 or 1:2.2.

[0048] In some embodiments, the secondary particles are polycrystalline secondary particles, and the median particle size D50 of the secondary particles is 2 μm-10 μm. Optimizing the particle size of polycrystalline secondary particles is beneficial to improving the rate discharge performance, low temperature discharge performance and cycle performance of the positive electrode material. The median particle size D50 refers to the particle size corresponding to when the cumulative volume percentage of the material reaches 50%.

[0049] In some specific embodiments, the median particle size D50 of the polycrystalline secondary particles is 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4.0 μm, 5.0 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0050] The second aspect of the present invention provides a method for preparing the positive electrode material of the first aspect of the present invention, comprising the following steps:

[0051] A manganese source, an iron source, a phosphorus source, an acid, a complexing agent, and a solvent are mixed, and the pH value of the reaction system is 2.0-5.0, and a first slurry is obtained after the reaction;

[0052] adding a lithium source to the first slurry and continuing the reaction to obtain a second slurry;

[0053] The second slurry is spray-dried and sintered to obtain a positive electrode material.

[0054] By controlling the pH of the reaction system to a lower value, it is beneficial to induce the primary grains to grow and arrange in a radial orientation, thereby increasing the lithium ion migration rate, effectively reducing the concentration polarization of lithium ions during the charge and discharge process, and improving the rate discharge performance and low-temperature discharge performance.

[0055] In some embodiments, the pH of the reaction system is 2.8, 3.0, 3.2, 3.5, 3.8 or 4.0.

[0056] In some embodiments, the concentration of the complexing agent is 0.1-0.4 mol / L.

[0057] The use of relatively high complexing agent concentration and relatively low pH can form a more suitable crystal nucleation environment, thereby promoting the radial arrangement and growth of primary grains along the secondary particles and dominating them, reducing the formation of a large number of new nuclei and new grains between the grains.

[0058] In some embodiments, in the step of preparing the first slurry, the amount of manganese source added is controlled to decrease in a gradient from the start of the reaction to the end of the reaction. By controlling the amount of manganese source added to change in a gradient, the content of manganese in the positive electrode material can be reduced in a gradient from the center to the surface of the polycrystalline secondary particles, thereby effectively alleviating the stress accumulation and dissolution phenomenon generated during the charge and discharge process, so that the positive electrode material has better structural stability and improves the cycle performance of the positive electrode material.

[0059] In some specific embodiments, mixing the manganese source, iron source, phosphorus source, acid, chelating agent and solvent comprises: after mixing the acid, chelating agent and solvent, adding the iron source and phosphorus source at a fixed rate, and adding the manganese source at a gradient decreasing rate.

[0060] In some embodiments, preparing the second slurry includes: adding the lithium source and the doping metal source to the first slurry, and continuing the reaction to obtain the second slurry. The rate discharge performance and low temperature discharge performance of the positive electrode material can be improved by doping with metal elements.

[0061] In some embodiments, the doping metal source includes an M source and an N source, and the valence state of M is smaller than the valence state of N.

[0062] M source and N source are co-doped metal sources. High-valent metal ions have higher surface binding energy than low-valent metal ions, which is conducive to the radial orientation of primary grains, increasing the migration rate of lithium ions, effectively reducing the concentration polarization of lithium ions during charging and discharging, and improving rate discharge performance and low-temperature discharge performance.

[0063] In some specific embodiments, the valence of M is 4 or less, such as 4, 3 or 2. The valence of N is 5 or more, such as 5 or 6.

[0064] In some specific embodiments, the M source includes one or more of a Mg source, a Ti source, a Cu source, a Zn source, and a Y source. The N source includes one or more of a W source, a V source, a Mo source, and a Nb source.

[0065] In some specific embodiments, in the step of preparing the second slurry, Li + Molar amount and Fe 2+ and Mn 2+ The total molar ratio of is (2.85-3.15):1, for example, 2.85:1, 2.90:1, 2.95:1, 3.00:1 or 3.15:1.

[0066] In some embodiments, in the step of preparing the second slurry, (Fe 2+ +Mn 2+ ) to the total molar amount of the doping element is (94-100):(6-0), for example, 94:6, 95:5, 96:4, 97:3 or 98:2.

[0067] In some embodiments, after obtaining the second slurry and before spray drying, the method further includes: removing impurities from the second slurry, centrifuging, and drying to obtain a lithium iron manganese phosphate precursor; grinding and mixing the lithium iron manganese phosphate precursor with a carbon source. The use of a carbon source during the preparation process can form a coating layer on the surface of the particles, thereby improving the conductivity of the lithium iron manganese phosphate. Adding a dispersant is conducive to uniformly dispersing the lithium iron manganese phosphate precursor and the carbon source in the solvent, thereby improving the uniformity of the lithium iron manganese phosphate positive electrode material particles.

[0068] In some specific embodiments, the mass of the carbon source accounts for 5%-8%, for example, 5%, 6%, 7% or 8%, of the total mass of the lithium manganese iron phosphate precursor and the carbon source.

[0069] In some embodiments, the carbon source includes one or more of glucose, sucrose, starch, and phenolic resin.

[0070] In some embodiments, the acid includes one or more of acetic acid and carbonic acid; preferably, the complexing agent includes one or more of oxalic acid, citric acid, and ethylenediaminetetraacetic acid.

[0071] In some embodiments, the manganese source includes one or more of manganese oxalate, manganese sulfate, manganese nitrate, and manganese phosphate.

[0072] In some embodiments, the iron source includes one or more of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous oxalate.

[0073] In some embodiments, the phosphorus source includes one or more of phosphoric acid, diammonium phosphate, and monoammonium phosphate.

[0074] In some embodiments, the lithium source includes one or more of lithium carbonate, lithium phosphate, and lithium hydroxide.

[0075] In some embodiments, the solvent includes one or more of water, ethylene glycol, polyethylene glycol, propylene glycol, and hexylene glycol.

[0076] In some embodiments, the sintering includes primary sintering and secondary sintering. The two-step sintering and the control of sintering conditions are beneficial to improving the electrochemical performance of the positive electrode material.

[0077] In some specific embodiments, the primary sintering temperature is 400° C.-550° C., and the time is 3 h-5 h. The secondary sintering temperature is 650° C.-800° C., and the time is 6 h-12 h.

[0078] In some specific embodiments, the heating rate of the primary sintering is 1.5°C / min-2.5°C / min, for example, 1.5°C / min, 1.6°C / min, 1.7°C / min, 1.8°C / min, 1.9°C / min, 2.0°C / min, 2.1°C / min, 2.2°C / min, 2.3°C / min, 2.4°C / min or 2.5°C / min.

[0079] In some specific embodiments, the heating rate of the secondary sintering is 3° C. / min-5° C. / min, for example, 3° C. / min, 3.5° C. / min, 4° C. / min, 4.5° C. / min or 5° C. / min.

[0080] In some embodiments, in the step of preparing the first slurry, the reaction temperature is 50° C.-65° C., for example, 50° C., 55° C., 60° C. or 65° C. The reaction time is 4 h-8 h, for example, 4 h, 5 h, 6 h, 7 h or 8 h. During the reaction, an inert gas is introduced to remove oxygen from the reaction system.

[0081] The scheme of the present invention will be explained below in conjunction with the embodiments. It will be appreciated by those skilled in the art that the following embodiments are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Where specific techniques or conditions are not indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used are not indicated by the manufacturer and are all conventional products that can be obtained commercially.

[0082] Example 1

[0083] (1) A mixed solvent of ethylene glycol and deionized water is prepared as a reaction base solution, wherein the volume ratio of deionized water to ethylene glycol is 2:1; an aqueous solution of MnSO4, an aqueous solution of FeSO4, and an aqueous solution of H3PO4 are prepared as reaction solutions, and the concentration of each solution is 0.2 mol / L; an aqueous solution of oxalic acid is used as a complexing agent solution, and its molar concentration is 0.25 mol / L; an aqueous solution of acetic acid is used as an acid solution, and its molar concentration is 1 mol / L; an aqueous solution of Li2CO3 is prepared as a lithium source solution, and its molar concentration is 0.25 mol / L; and the mixed solution is a mixed aqueous solution of TiCl4 and NbCl5, wherein Ti 4+ With Nb 5+ The molar ratio is 1:1.

[0084] (2) The reaction base solution, complexing agent solution and acid solution were added to the reactor in sequence through metering pumps. The reaction temperature was controlled at 55°C and the stirring speed was 420 rpm. High-purity N2 was continuously introduced to remove oxygen from the solution. MnSO4, FeSO4 and H3PO4 reaction solutions were gradually added. The addition rate of H3PO4 reaction solution was kept constant (40 L / h). The addition rates of FeSO4 and MnSO4 were controlled. The addition rate of FeSO4 was controlled during the particle growth process. 2+ :Mn 2+ Addition rate, where Fe 2+ The addition rate was adjusted from 12L / h to 50L / h, Mn 2+ The addition rate was adjusted from 48L / h to 10L / h, and the total addition amount of FeSO4 solution and MnSO4 solution was ensured to be 60L / h during the regulation process. 2+ :Mn 2+ When the molar ratio is 40:60, stop adding MnSO4, FeSO4 and H3PO4 reaction solution. 2+ +Mn 2+ ) and PO4 3- The molar ratio of is 3:2, and the pH value of the reaction system is controlled at 3.2. The reaction time is 8h.

[0085] (3) Gradually add Li2CO3 solution, diammonium hydrogen phosphate solution and mixed solution into the reactor, and the lithium compound containing Li + The amount of substance and (Fe 2+ +Mn 2+ )The total molar ratio is 1.06:1, (Fe 2+ +Mn 2+ ) and (Ti 4+ +Nb 5+ ) was 99.4:0.6 in total molar amount, and the reaction was continued until the median particle size D50 of the particles reached 4.5 μm and then the reaction was stopped.

[0086] (4) The slurry after the reaction is subjected to impurity removal, centrifugation, and drying to obtain a lithium iron manganese phosphate precursor, and the lithium iron manganese phosphate precursor is ground and mixed with glucose, wherein the amount of glucose added is 6.2% of the total mass of the two.

[0087] (5) The ball-milled lithium iron manganese phosphate precursor and glucose were mixed and spray-dried to obtain a precursor powder. The obtained precursor powder was calcined at high temperature in a nitrogen atmosphere, and the temperature was raised to 500°C at a rate of 3°C / min for a primary sintering, and the sintering time was 4 hours. Then, the temperature was raised to 760°C at a rate of 3°C / min for a secondary sintering, and the sintering time was 10 hours. After natural cooling, the lithium iron manganese phosphate positive electrode material was collected, and the chemical formula was Li(Mn 0.6 Fe0.4 ) 0.994 Ti 0.003 Nb 0.003 PO4@C, the particle size test results show that the median particle size D50 is 7.2μm.

[0088] The XRD structure of the lithium manganese iron phosphate positive electrode material prepared in Example 1 is as follows: Figure 2 As shown in the SEM images Figure 3 shown.

[0089] Example 2

[0090] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that in step 2, the amount of the acid solution was different so that the pH value of the reaction system was controlled to be 3.6. The chemical formula of the lithium iron manganese phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Ti 0.003 Nb 0.003 PO4@C. The particle size test results show that the median particle size D50 is 8.0 μm.

[0091] Example 3

[0092] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that the amount of the complexing agent was different, and the citric acid aqueous solution was used as the complexing agent solution, and its molar concentration was 0.3 mol / L. The chemical formula of the lithium iron manganese phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Ti 0.003 Nb 0.003 PO4@C. The particle size test results show that the median particle size D50 is 6.8μm.

[0093] Example 4

[0094] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that the amount of the complexing agent was different, and the citric acid aqueous solution was used as the complexing agent solution, and its molar concentration was 0.2 mol / L. The chemical formula of the lithium iron manganese phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Ti 0.003 Nb 0.003 PO4@C. The particle size test results show that the median particle size D50 is 7.6μm.

[0095] Example 5

[0096] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that the amount of the complexing agent was different, and the citric acid aqueous solution was used as the complexing agent solution, and its molar concentration was 0.01 mol / L. The chemical formula of the lithium iron manganese phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Ti 0.003 Nb 0.003 PO4@C. The particle size test results show that the median particle size D50 is 8.6μm.

[0097] Example 6

[0098] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that the amount of the complexing agent was different, and the citric acid aqueous solution was used as the complexing agent solution, and its molar concentration was 0.5 mol / L. The chemical formula of the lithium iron manganese phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Ti 0.003 Nb 0.003 PO4@C. The particle size test results show that the median particle size D50 is 5.6μm.

[0099] Example 7

[0100] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that the reaction process in step 2 was regulated by 2+ :Mn 2+ The addition rate is constant, and the addition rate is 30L / h and 30L / h, but the total amount of FeSO4 and MnSO4 is the same as that in step 2 of Example 1. The chemical formula of the lithium manganese iron phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Ti 0.003 Nb 0.003 PO4@C. The particle size test results show that the median particle size D50 is 7.4μm.

[0101] Example 8

[0102] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that the mixed solution in step 1 was an aqueous solution of TiCl4, (Fe 2+ +Mn 2+ ) and the total molar amount of Ti 4+ The total molar ratio of lithium manganese iron phosphate is 99.4:0.6. The chemical formula of lithium manganese iron phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Ti 0.006PO4@C. The particle size test results show that the median particle size D50 is 8.0 μm.

[0103] Example 9

[0104] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that the mixed solution in step 1 was an aqueous solution of NbCl5, (Fe 2+ +Mn 2+ ) and Nb 5+ The total molar ratio of lithium manganese iron phosphate is 99.4:0.6. The chemical formula of lithium manganese iron phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Nb 0.006 PO4@C. The particle size test results show that the median particle size D50 is 7.0 μm.

[0105] Comparative Example 1

[0106] The lithium iron manganese phosphate positive electrode material was prepared according to the method of Example 1, except that in step 2, the amount of the acid solution was different so that the pH value of the reaction system was controlled to be 1.8. The chemical formula of the lithium iron manganese phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Nb 0.006 PO4@C. The particle size test results show that the median particle size D50 is 7.0 μm.

[0107] Comparative Example 2

[0108] The lithium manganese iron phosphate positive electrode material was prepared according to the method of Example 1, except that in step 2, the amount of acid solution was different so that the pH value of the reaction system was controlled to be 5.5. The chemical formula of the lithium manganese iron phosphate positive electrode material is Li(Mn 0.6 Fe 0.4 ) 0.994 Nb 0.006 PO4@C. The particle size test results show that the median particle size D50 is 7.7μm.

[0109] According to the following general method, button batteries were prepared using the lithium manganese iron phosphate positive electrode materials prepared in the above examples and comparative examples.

[0110] Preparation of button cells:

[0111] The lithium iron manganese phosphate material prepared in the above embodiment and comparative example is used as the positive electrode, and the positive electrode material, acetylene black and polyvinylidene fluoride PVDF are weighed and mixed according to a mass ratio of 90:5:5, and the slurry is adjusted to 50% solid content by N-methylpyrrolidone (NMP) solvent and coated on a 12+2μm composite carbon-coated aluminum foil, placed in a vacuum oven at 80℃ for drying, and the positive electrode sheet is punched to obtain a circular electrode sheet with a diameter of 12mm, and then the positive electrode sheet is placed in a vacuum drying oven and dried at 120℃. The negative electrode uses a lithium metal sheet with a diameter of 14mm and a thickness of 1mm; the diaphragm uses a polyethylene porous membrane with a thickness of 15μm and is cut into a 19mm diameter disc, and the electrolyte uses an equal amount of a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) + ethylene carbonate (EMC) with 1mol / L LiPF6 as the electrolyte. The positive electrode sheet, separator, negative electrode sheet and electrolyte were assembled into a CR2025 button cell in an argon glove box with a water content and an oxygen content of <1ppm according to the assembly order.

[0112] According to the following general method, soft-pack batteries were prepared using the lithium manganese iron phosphate positive electrode materials prepared in Examples 1 to 8 and Comparative Example 1.

[0113] Preparation of soft pack batteries:

[0114] The lithium iron manganese phosphate material prepared in the above embodiments and comparative examples is used as the positive electrode, and artificial graphite is used as the negative electrode. Commercial electrolyte and separator are used to assemble soft-pack cells on a lithium-ion battery production line. The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order, and a bare cell without liquid injection is obtained through a lamination process, wherein the separator is located between the positive and negative electrode sheets to play the role of isolation and subsequent transmission of lithium ions; the bare cell is heat-sealed in an aluminum-plastic film shell, and then the electrolyte is injected into the bare cell with a moisture value of <200ppm after baking, and a lithium-ion soft-pack battery is obtained through vacuum packaging, hot and cold pressing shaping, formation, shaping, sorting and other processes.

[0115] Battery performance test

[0116] 1. Button battery discharge capacity and initial efficiency test

[0117] The button batteries corresponding to the above embodiments and comparative examples were activated, and charge and discharge tests were performed in the range of 2.2-4.3V. The charge and discharge regime is as follows: 0.1C rate constant current constant voltage charge to 4.3V, constant voltage charge cutoff current is 0.02C, stand for 10 minutes, then 0.1C rate constant current discharge to 2.2V, stand for 10 minutes. Calculate the first discharge gram capacity value and the first charge and discharge efficiency, where the first charge and discharge efficiency = first discharge gram capacity / first charge gram capacity. The first charge and discharge curve of the button battery corresponding to Example 1 is shown in the figure below: Figure 4 shown.

[0118] 2.25 °C Discharge Performance Test

[0119] Place the soft-pack lithium-ion batteries corresponding to the above examples and comparative examples in a constant-temperature oven at 25 ± 1 °C and let them stand for 60 min. Charge them at a constant current of 1C to the upper limit voltage (4.25V), then switch to constant-voltage charging until the cut-off current reaches 0.05C, and let them stand for 10 min. Then discharge them at 1C to the lower limit voltage (2.2V) and let them stand for 10 min. Perform discharge tests at nC rates (nC = 1 / 3C, 1C, 2C, 4C). The retention rate of discharge capacity at nC rate = discharge capacity at nC rate / discharge capacity at 0.33C rate. Statistically analyze the value of 4C discharge capacity / 0.33C discharge capacity as the retention rate of discharge capacity at nC rate. The test results are shown in Table 1. The test results of the soft-pack lithium-ion battery corresponding to Example 1 are as Figure 5 shown

[0120] 3. High and Low Temperature Discharge Performance Test

[0121] Place the soft-pack lithium-ion batteries corresponding to the above examples and comparative examples in a constant-temperature oven at 25 ± 1 °C and let them stand for 60 min. Discharge them at 1C to the lower limit voltage (2.2V) and let them stand for 10 min. Charge them at 1C to the upper limit voltage (4.25V), then switch to constant-voltage charging until the cut-off current reaches 0.05C, and let them stand for 10 min. In a constant-temperature oven environment, after standing for 120 min at different temperatures T °C (T °C = 25 °C, 45 °C, 0 °C, -20 °C), discharge them at 1C to the lower limit voltage; repeat the steps until all temperature discharge tests are completed; set the discharge cut-off voltage: 2.2V, 0 °C ≤ T °C ≤ 45 °C; 2.0V, -20 °C < T °C < 0 °C. The retention rate of high and low temperature discharge capacity = discharge capacity at T °C / discharge capacity at 25 °C. Statistically analyze the value of -20 °C discharge capacity / 25 °C discharge capacity as the retention rate of low temperature discharge capacity. The test results are shown in Table 1

[0122] 4. 25 °C Cycle Life Test

[0123] Place the soft-pack lithium-ion batteries corresponding to the above examples and comparative examples in a constant-temperature oven at 25 ± 1 °C and let them stand for 60 min; discharge them at 1C to the lower limit voltage (2.2V) and let them stand for 30 min; charge them at 1C to the upper limit voltage (4.25V), then switch to constant-voltage charging until the cut-off current reaches 0.05C; perform 2 - 3 charge-discharge cycles to calibrate the rated capacity C0

[0124] Cycle test: Discharge at 1C0 to the lower limit voltage (2.2V) and let it stand for 10 min; charge at 1C0 to the upper limit voltage (4.25V), then switch to constant-voltage charging until the cut-off current reaches 0.05C, and let it stand for 10 min; repeat the cycle test steps n times (where 0 < n < 5000);

[0125] Capacity retention rate formula: the first cycle test capacity is recorded as A1, and the test capacity after 1000 cycles is recorded as A2; capacity retention rate = A2 / A1×100%. The specific test results are shown in Table 1.

[0126] 5.45℃ Cycle Life Test

[0127] The soft-pack cells corresponding to the above embodiments and comparative examples were placed in a 45±1°C constant temperature box and allowed to stand for 60 min; discharged at 1C to the lower limit voltage (2.2V) and allowed to stand for 30 min; charged at 1C to the upper limit voltage (4.25V) and then switched to constant voltage charging, with a cutoff of 0.05C; cycled 2-3 times to calibrate the capacity C0.

[0128] Cycle test: 1C0 is discharged to the lower limit voltage (2.2V), and then left to stand for 10 minutes; 1C0 is charged to the upper limit voltage (4.25V), then switched to constant voltage charging, cut off 0.05C, and left to stand for 10 minutes; repeat the cycle test steps n times (0 <n<5000);

[0129] Capacity retention rate formula: The first cycle test capacity is recorded as A1, and the test capacity after 500 cycles is recorded as A2; Capacity retention rate = A2 / A1×100%. The specific test results are shown in Table 1. The test results of the soft-pack battery cell corresponding to Example 1 are as follows: Figure 6 As shown;

[0130] Table 1

[0131]

[0132] In summary, the lithium manganese iron phosphate positive electrode material of the present invention is formed by the accumulation and agglomeration of primary grains. Since the primary grains are arranged radially along the polycrystalline secondary particles, the lithium ion diffusion channel is widened, the diffusion path of lithium ions is effectively shortened, the lithium ion migration rate is increased, and the concentration polarization of lithium ions during charging and discharging is effectively reduced, thereby improving the rate discharge performance and low-temperature discharge performance.

[0133] In addition, the manganese concentration gradient distribution design can effectively alleviate the stress accumulation and dissolution phenomena generated during the charging and discharging process, improve the structural stability, and thus improve the cycle performance of the positive electrode material.

[0134] In addition, doping with metal elements can improve the rate discharge performance and low-temperature discharge performance of the positive electrode material. In particular, metal elements with specific valence states can promote the radial orientation growth and arrangement of primary grains, increase the migration rate of lithium ions, effectively reduce the concentration polarization of lithium ions during charging and discharging, and improve the rate discharge performance and low-temperature discharge performance.

[0135] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0136] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0137] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A positive electrode material, characterized in that: The positive electrode material includes secondary particles, the secondary particles are spherical and include a plurality of primary crystal grains, the primary crystal grains are arranged along the radial direction of the secondary particles, and the positive electrode material is a lithium manganese iron phosphate positive electrode material.

2. The positive electrode material according to claim 1, characterized in that The positive electrode material contains manganese element, and the content of the manganese element decreases gradually from the center to the surface of the secondary particle.

3. The positive electrode material according to claim 1 or 2, characterized in that The chemical formula of the positive electrode material is LiMn x Fe 1-x M y N z PO4, wherein M and N each include a metal element, 0 <x<1,0≤y≤0.05,0≤z≤0.05,y+z≤0.06。 4. The positive electrode material according to claim 3, characterized in that Both y and z are not equal to 0, and the valence of M is less than the valence of N.

5. The positive electrode material according to claim 4, characterized in that The valence of M is 4 or less; the valence of N is 5 or more; M includes Mg +2 、Ti +4 , Cu +2 、Zn +2 , Y +3 One or more of; N includes Nb +5 、Mo +6 、V +5 , W +6 One or more of .

6. The positive electrode material according to claim 1 or 2, characterized in that: The secondary particles are polycrystalline secondary particles, and the median particle size D50 of the secondary particles is 2 μm-10 μm.

7. A method for preparing the positive electrode material according to any one of claims 1 to 6, characterized in that: The following steps are involved: A manganese source, an iron source, a phosphorus source, an acid, a complexing agent, and a solvent are mixed, the pH value of the reaction system is 2.0-5.0, and a first slurry is obtained after the reaction; adding a lithium source to the first slurry and continuing the reaction to obtain a second slurry; The second slurry is spray-dried and sintered to obtain a positive electrode material.

8. The method according to claim 7, characterized in that The concentration of the complexing agent is 0.1-0.4 mol / L.

9. The method according to claim 7 or 8, characterized in that: In the step of preparing the first slurry, the amount of manganese source added is controlled to decrease gradually throughout the entire process from the start of the reaction to the end of the reaction.

10. The method according to claim 7 or 8, characterized in that: The preparation of the second slurry includes: adding the lithium source and the doping metal source into the first slurry, and continuing the reaction to obtain the second slurry.

11. The method according to claim 10, characterized in that The doping metal source includes an M source and an N source, and the valence state of M is smaller than the valence state of N; The valence of M is 4 or less; the valence of N is 5 or more; The M source includes one or more of a Mg source, a Ti source, a Cu source, a Zn source, and a Y source; the N source includes one or more of a W source, a V source, a Mo source, and a Nb source.

12. The method according to claim 7 or 8, characterized in that: After obtaining the second slurry and before spray drying, the method further comprises: removing impurities from the second slurry, centrifuging and drying to obtain a lithium manganese iron phosphate precursor; grinding and mixing the lithium manganese iron phosphate precursor with a carbon source; The carbon source includes one or more of glucose, sucrose, starch, and phenolic resin.

13. The method according to claim 7 or 8, characterized in that: The acid includes one or more of acetic acid and carbonic acid; The complexing agent includes one or more of oxalic acid, citric acid, and ethylenediaminetetraacetic acid; The manganese source includes one or more of manganese oxalate, manganese sulfate, manganese nitrate, and manganese phosphate; The iron source includes one or more of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous oxalate; The phosphorus source includes one or more of phosphoric acid, diammonium hydrogen phosphate and diammonium dihydrogen phosphate; The lithium source includes one or more of lithium carbonate, lithium phosphate, and lithium hydroxide; The solvent includes one or more of water, ethylene glycol, polyethylene glycol, propylene glycol, and hexylene glycol.

14. The method according to claim 7 or 8, characterized in that The sintering includes primary sintering and secondary sintering; the temperature of the primary sintering is 400°C-550°C and the time is 3h-5h; the temperature of the secondary sintering is 650°C-800°C and the time is 6h-15h.

15. A battery, characterized in that: The invention comprises the positive electrode material according to any one of claims 1 to 6 or the positive electrode material obtained by the method according to any one of claims 7 to 14.

16. An electrical device, characterized in that: Comprising the battery of claim 15.

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