A lithium manganese iron phosphate positive electrode material and its preparation method and application

Abstract

The present invention discloses a lithium manganese iron phosphate positive electrode material and its preparation method and application, belonging to the technical field of lithium ion battery positive electrode materials. The preparation method comprises the following steps: ball milling a lithium source, an iron source, a manganese source, a phosphorus source, a carbon source, and an emulsifier; drying, grinding, calcining, and ball milling the obtained slurry A to obtain D 50 The large particle size precursor of 0.8-1.5μm and D 50 A small-particle precursor with a diameter of 0.1-0.4 μm is prepared. The small-particle precursor is then added to a starch suspension, ultrasonically dispersed, and a crosslinker is added. The mixture is heated and stirred for reaction. The resulting colloidal mixture is dried and ball-milled to obtain Material C. Finally, the large-particle precursor and Material C are mixed, and the resulting mixture is calcined under a protective gas atmosphere to obtain a lithium iron manganese phosphate cathode material. This preparation method reduces the agglomeration of the lithium iron manganese phosphate cathode material and increases its dispersibility and conductivity, thereby improving the overall performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery positive electrode materials, and in particular to a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] Phosphate-based cathode materials have an olivine-type structure, offering superior safety and cost advantages. Currently, lithium iron phosphate (LiFePO4) has been widely used in some power battery applications, but its low voltage platform and theoretical energy density make it difficult to meet higher demands. Lithium iron manganese phosphate (LiMnFePO4) inherits the advantages of LiFePO4 and leverages the synergistic effects of manganese and iron to improve its voltage platform and theoretical energy density. However, LiMnFePO4 suffers from low electronic conductivity, slow lithium ion diffusion rate, and low compaction density, which compromise the battery's electrochemical performance and energy density. To improve the electrochemical performance of LiMnFePO4 cathode materials, methods are often employed, including reducing crystal size to the nanoscale, controlling particle morphology, coating with conductive carbon, and doping with cations. However, as particle size decreases, the specific surface area and surface energy increase, and the tendency for particles to aggregate increases, making dispersion difficult and affecting uniformity. Furthermore, smaller particles increase the number of side reactions and reduce cycling performance. Summary of the Invention

[0003] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a lithium iron manganese phosphate positive electrode material and its preparation method and application. The preparation method can reduce the agglomeration of the lithium iron manganese phosphate positive electrode material during the preparation process and increase the dispersibility and conductivity of the lithium iron manganese phosphate positive electrode material, thereby improving the overall performance of the battery.

[0004] To achieve the above object, the technical solution adopted by the present invention is: a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0005] S1: A lithium source, an iron source, a manganese source, a phosphorus source, and a carbon source are mixed and dry-milled, and then an emulsifier is added and wet-milled. The resulting slurry A is dried, ground, and calcined to obtain material B;

[0006] S2: Ball mill material B to obtain D 50 Large particle size precursors of 0.8-1.5 μm and D 50 It is a small particle size precursor of 0.1-0.4μm;

[0007] S3: adding the small particle size precursor to the starch suspension, ultrasonically dispersing it, adding a crosslinking agent, heating and stirring the mixture to react, and drying and ball milling the resulting colloidal mixture to obtain material C;

[0008] S4: mixing the large-particle precursor and material C, and calcining the resulting mixture under protective gas to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to material C is 0.5-5:1.

[0009] The method provided by the present invention adds a small-particle precursor to a starch suspension, temporarily separates the agglomerated small-particle precursor through ultrasonic dispersion, heats and stirs to cause a cross-linking reaction between a cross-linking agent and starch to form a colloid, and gradually fixes the small-particle precursor that is temporarily maintained in a dispersed state in the three-dimensional network formed by the starch colloid, thereby preventing the small-particle precursor from agglomerating again after the ultrasonic dispersion is removed.

[0010] The method provided by the present invention uniformly mixes the large-particle precursor and material C, and the gaps formed by the closely arranged large-particle precursors are filled with the small-particle material C, thereby greatly improving the compaction density and morphology of the lithium iron manganese phosphate positive electrode material; the material C is coated with starch, and when the large-particle precursor and material C are mixed, the barrier effect of the starch prevents the small-particle precursor and the large-particle precursor from agglomerating, and after uniform mixing, sintering is performed to obtain a lithium iron manganese phosphate positive electrode material with a uniform particle size distribution, thereby improving the cycle performance of the lithium iron manganese phosphate positive electrode material; in addition, the carbon source forms a carbon coating layer on the surface of the large-particle precursor and the small-particle precursor, and the starch is converted into carbon during the sintering process in step S4, further forming a carbon coating layer on the surface of the large-particle precursor and the small-particle precursor. The carbon coating layer can not only prevent the lithium iron manganese phosphate positive electrode material from agglomerating, but also the rich carbon coating layer on the surface of the lithium iron manganese phosphate positive electrode material provides more paths for electron transmission, thereby improving the electrochemical performance of the lithium iron manganese phosphate positive electrode material.

[0011] In step S1, in order to further improve the compaction density and morphology of material B, the dried product is crushed and ground before calcination to obtain a powder with a relatively uniform particle size.

[0012] Preferably, in step S1, the amount of lithium source, iron source, manganese source and phosphorus source added is as follows: x Mn 1-x The stoichiometric ratio of PO4 is calculated, wherein 0.1≤x≤0.5; the mass of the carbon source is 1-5% of the total mass of the lithium source, iron source, manganese source, phosphorus source and carbon source.

[0013] Preferably, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium dihydrogen phosphate;

[0014] Preferably, the iron source is at least one of ferrous nitrate, ferrous sulfate, ferrous chloride, ferrous oxalate, and ferric phosphate;

[0015] Preferably, the manganese source is at least one of manganese carbonate, manganese oxalate, manganese phosphate, manganese sulfate, manganese dioxide, and manganese tetraoxide;

[0016] Preferably, the phosphorus source is at least one of iron phosphate, manganese phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate;

[0017] Preferably, the carbon source is at least one of sucrose, fructose, glucose, starch, maltose, and citric acid;

[0018] Preferably, the emulsifier is one of polyethylene glycol, glycerol fatty acid ester, sucrose fatty acid ester, and polyoxyethylene ether.

[0019] Preferably, in step S1, the rotation speed of dry ball milling and wet ball milling is 300-600 r / min, for example, 300 r / min, 350 r / min, 400 r / min, 450 r / min, 500 r / min, 550 r / min or 600 r / min; the total time of dry ball milling and wet ball milling is 3-8 h, for example, 3 h, 4 h, 5 h, 6 h, 7 h or 8 h.

[0020] The present invention uniformly disperses the lithium source, iron source, manganese source, phosphorus source and carbon source in slurry A through dry ball milling and wet ball milling, thereby improving the dispersibility of each component in the lithium iron manganese phosphate positive electrode material and thus improving the electrochemical performance of the lithium iron manganese phosphate positive electrode material.

[0021] Preferably, in step S1, the specific steps of calcining are: placing the ground product in an inert gas atmosphere furnace for sintering, the sintering heating rate is 5-10°C / min, the sintering temperature is 400-500°C, and the sintering time is 3-5h.

[0022] Preferably, the inert gas is one of nitrogen, argon and helium.

[0023] Those skilled in the art can select appropriate drying temperature and drying time to dry the slurry A according to actual conditions, for example, the drying temperature is 80-100° C. and the drying time is 3-5 hours.

[0024] Those skilled in the art can obtain D by controlling the ball milling speed and ball milling time in step S2 according to actual needs. 50 Large particle size precursors of 0.8-1.5 μm and D 50 For example, the ball milling speed of the large particle size precursor is 300-600 r / min, and the time is 2-5 h; the ball milling speed of the small particle size precursor is 300-600 r / min, and the time is 10-13 h.

[0025] Preferably, in step S3, the method for preparing the starch suspension comprises: adding starch to deionized water and stirring uniformly to obtain a starch suspension;

[0026] Preferably, the mass of the cross-linking agent is 0.1-1% of the mass of the starch; the cross-linking agent is sodium trimetaphosphate;

[0027] Preferably, in step S3, the heating temperature is 40-65°C, the temperature rising rate is 1-10°C / min, and the stirring reaction time is 1-3h.

[0028] Heating at a slow heating rate allows the cross-linking agent to react with starch to form a colloid, which can better disperse the small-particle precursor evenly.

[0029] Preferably, in step S3, the ball milling speed is 300-600 r / min, and the ball milling time is 3-8 h.

[0030] Preferably, in step S4, the mass ratio of the large-particle precursor to the material C is 1-3:1.

[0031] In the present invention, the mass ratio of the large-particle precursor to the material C within the above-mentioned preferred ratio range can significantly improve the compaction density and electrochemical performance of the lithium manganese iron phosphate positive electrode material.

[0032] Preferably, in step S4, the calcination temperature is 650-800° C., and the calcination time is 8-12 hours.

[0033] Preferably, in step S4, the mixture needs to be pre-sintered before calcination, the pre-sintering temperature is 250-350° C., and the pre-sintering time is 3-4 hours.

[0034] By combining pre-sintering and calcination, bonding, densification, organizational structure changes and rearrangement between lithium manganese iron phosphate precursors can be achieved within different temperature ranges, thereby improving the compaction density and electrochemical properties of lithium manganese iron phosphate positive electrode materials.

[0035] Preferably, in step S4, both the pre-sintering and the calcination are performed under the protection of an inert gas, and the inert gas is one of nitrogen, argon and helium.

[0036] Preferably, in step S4, the calcined product may be post-processed to further improve the morphology of the lithium manganese iron phosphate cathode material. The post-processing steps include crushing, screening, etc.

[0037] In a second aspect, the present invention provides a lithium iron manganese phosphate positive electrode material, which is prepared by the preparation method of the lithium iron manganese phosphate positive electrode material.

[0038] In a third aspect, the present invention provides a positive electrode sheet, which includes the lithium manganese iron phosphate positive electrode material.

[0039] In a fourth aspect, the present invention provides a lithium-ion battery, comprising the positive electrode sheet.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. The present invention disperses the small-particle precursor with a starch suspension and then dries it to prevent the small-particle precursor from agglomerating. At the same time, the surface of the dried small-particle precursor is coated with starch. During the mixing process with the large-particle precursor, the barrier effect of the starch prevents the small-particle precursor and the large-particle precursor from agglomerating. After uniform mixing, sintering is performed to obtain a lithium manganese iron phosphate positive electrode material with a uniform particle size distribution, thereby improving the cycle performance of the lithium manganese iron phosphate positive electrode material.

[0042] 2. In the present invention, the carbon source forms a carbon coating layer on the surface of the large-particle precursor and the small-particle precursor. The starch coated on the surface of the small-particle precursor will be converted into carbon during the subsequent calcination process, and further form a carbon coating layer on the surface of the large-particle precursor and the small-particle precursor. The carbon coating layer can not only prevent the agglomeration of the lithium iron manganese phosphate positive electrode material, but also the rich carbon coating layer on the surface of the lithium iron manganese phosphate positive electrode material provides more paths for electron transmission, thereby improving the electrochemical performance of the lithium iron manganese phosphate positive electrode material. DETAILED DESCRIPTION

[0043] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments and comparative examples. Its purpose is to understand the content of the present invention in detail, rather than to limit the present invention. All other embodiments obtained by those of ordinary skill in the art without making creative work premise all fall within the protection scope of the present invention. The experimental reagents and instruments involved in the implementation of the present invention are all conventional common reagents and instruments unless otherwise specified.

[0044] Example 1

[0045] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0046] S1: 9.05 g of ferric phosphate, 3.48 g of manganese dioxide, 3.75 g of lithium carbonate, 4.60 g of ammonium dihydrogen phosphate, and 1.55 g of sucrose were weighed and added to a ball mill and dry-milled for 1 h at a speed of 400 r / min. Then, 120 g of water and 0.2 g of polyethylene glycol were added and wet-milled for 3 h at a speed of 600 r / min to obtain slurry A.

[0047] The slurry A was placed in a blast furnace for drying, and then crushed and ground. The resulting powder was placed in an inert gas atmosphere furnace and sintered under inert gas protection. The sintering temperature was 400°C, the heating rate was 5°C / min, and the sintering time was 3h to obtain material B.

[0048] S2: Place the material B obtained in step S1 in a ball mill, and control the ball milling time at a ball milling speed of 400 r / min to obtain precursors with different particle sizes; the small particle size precursor D 50 The D of the large particle size precursor is 0.2 μm and the ball milling time is 12 h. 50 The particle size was 0.8 μm, and the ball milling time was 5 h;

[0049] S3: adding 100 mL of deionized water and 3.2 g of starch to a reactor equipped with an ultrasonic device, and stirring the mixture evenly with ultrasonic stirring to obtain a starch suspension; adding the small-particle precursor obtained in step S1 to the starch suspension, and simultaneously performing ultrasonic dispersion during the addition process. After uniform mixing, 0.05 g of sodium trimetaphosphate was added, and the obtained mixture 1 was heated to 50° C. at a heating rate of 3° C. / min under stirring to completely convert the mixture 1 into a colloidal mixture. Stirring was continued for 1 h to uniformly disperse the small-particle precursor in the starch colloid to obtain a mixture 2;

[0050] The obtained mixed material 2 was placed in a blast furnace for drying, and the obtained dry product was placed in a ball mill jar and ball milled at a ball mill speed of 400 r / min for 1 h to obtain material C;

[0051] S4: The large-particle precursor obtained in step S2 and the material C obtained in step S3 are evenly mixed, and the resulting mixture is sintered under a nitrogen atmosphere. The sintering is divided into two stages, first calcined at 300°C for 3 hours, and then heated to 650°C for 12 hours; the sintered material is crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to the material C is 2:1.

[0052] This embodiment also provides a method for preparing a lithium ion battery, comprising the following steps:

[0053] The lithium iron manganese phosphate cathode material obtained in this example was added to N-methylpyrrolidone with the conductive agents SP and PVDF in a mass ratio of 80:10:10. The resulting mixture was ball-milled to obtain a cathode slurry. The cathode slurry was coated onto aluminum foil and vacuum-dried to produce a cathode electrode sheet. A lithium-ion battery was assembled using 1 mol / L LiPF6 as the electrolyte, Celgard polypropylene film as the separator, and a metal lithium sheet as the negative electrode. The electrochemical performance of the battery was tested in the voltage range of 2.5V-4.5V and at a rate of 0.1C. The test results are shown in Table 1.

[0054] Example 2

[0055] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0056] S1: 9.05 g of ferric phosphate, 3.48 g of manganese dioxide, 3.75 g of lithium carbonate, 4.60 g of ammonium dihydrogen phosphate, and 1.55 g of sucrose were weighed and added to a ball mill and dry-milled for 1 h at a speed of 400 r / min. Then, 120 g of water and 0.2 g of polyethylene glycol were added and wet-milled for 3 h at a speed of 600 r / min to obtain slurry A.

[0057] The slurry A was placed in a blast furnace for drying, and then crushed and ground. The resulting powder was placed in an inert gas atmosphere furnace and sintered under inert gas protection. The sintering temperature was 400°C, the heating rate was 5°C / min, and the sintering time was 3h to obtain material B.

[0058] S2: Place the material B obtained in step S1 in a ball mill, and control the ball milling time at a ball milling speed of 400 r / min to obtain precursors with different particle sizes; the small particle size precursor D 50 The D of the large particle size precursor is 0.3 μm and the ball milling time is 11 h. 50 The particle size was 1.0 μm, and the ball milling time was 4 h;

[0059] S3: adding 100 mL of deionized water and 3.2 g of starch to a reactor equipped with an ultrasonic device, and stirring the mixture evenly with ultrasonic stirring to obtain a starch suspension; adding the small-particle precursor obtained in step S1 to the starch suspension, and simultaneously performing ultrasonic dispersion during the addition process. After uniform mixing, 0.05 g of sodium trimetaphosphate was added, and the obtained mixture 1 was heated to 60° C. at a heating rate of 3° C. / min under stirring to completely convert the mixture 1 into a colloidal mixture. Stirring was continued for 0.8 h to uniformly disperse the small-particle precursor in the starch colloid to obtain a mixture 2;

[0060] The obtained mixed material 2 was placed in a blast furnace for drying, and the obtained dry product was placed in a ball mill jar and ball milled at a ball mill speed of 400 r / min for 1 h to obtain material C;

[0061] S4: The large-particle precursor obtained in step S2 and the material C obtained in step S3 are mixed evenly, and the resulting mixture is sintered under nitrogen atmosphere protection. The sintering is divided into two stages, first calcined at 300°C for 3 hours, and then heated to 650°C for 12 hours; the sintered material is crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to material C is 3:1.

[0062] The preparation method of the battery in this embodiment differs from that in Example 1 only in that the lithium manganese iron phosphate positive electrode material is the lithium manganese iron phosphate positive electrode material obtained in this embodiment. The battery testing method in this embodiment is consistent with that in Example 1.

[0063] Example 3

[0064] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0065] S1: 9.05 g of ferric phosphate, 3.48 g of manganese dioxide, 3.75 g of lithium carbonate, 4.60 g of ammonium dihydrogen phosphate, and 1.55 g of sucrose were weighed and added to a ball mill and dry-milled for 1 h at a speed of 400 r / min. Then, 120 g of water and 0.2 g of polyethylene glycol were added and wet-milled for 3 h at a speed of 600 r / min to obtain slurry A.

[0066] The slurry A was placed in a blast furnace for drying, and then crushed and ground. The resulting powder was placed in an inert gas atmosphere furnace and sintered under inert gas protection. The sintering temperature was 400°C, the heating rate was 5°C / min, and the sintering time was 3h to obtain material B.

[0067] S2: Place the material B obtained in step S1 in a ball mill, and control the ball milling time at a ball milling speed of 400 r / min to obtain precursors with different particle sizes; the small particle size precursor D 50 The ball milling time is 13h; the D 50 The particle size was 1.2 μm, and the ball milling time was 3 h;

[0068] S3: adding 100 mL of deionized water and 3.2 g of starch to a reactor equipped with an ultrasonic device, and stirring the mixture evenly with ultrasonic stirring to obtain a starch suspension; adding the small-particle precursor obtained in step S1 to the starch suspension, and simultaneously performing ultrasonic dispersion during the addition process. After uniform mixing, adding 0.05 g of sodium trimetaphosphate, and heating the obtained mixture 1 to 45° C. at a heating rate of 3° C. / min under stirring to completely convert the mixture 1 into a colloidal mixture, and continuing stirring for 1 h to uniformly disperse the small-particle precursor in the starch colloid: to obtain a mixture 2;

[0069] The obtained mixed material 2 was placed in a blast furnace for drying, and the obtained dry product was placed in a ball mill jar and ball milled at a ball mill speed of 400 r / min for 1 h to obtain material C;

[0070] S4: The large-particle precursor obtained in step S2 and the material C obtained in step S3 are mixed evenly, and the resulting mixture is sintered under nitrogen atmosphere protection. The sintering is divided into two stages, first calcined at 300°C for 3 hours, and then heated to 650°C for calcination for 12 hours; the sintered material is crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to the material C is 1:1.

[0071] The preparation method of the battery in this embodiment differs from that in Example 1 only in that the lithium manganese iron phosphate positive electrode material is the lithium manganese iron phosphate positive electrode material obtained in this embodiment. The battery testing method in this embodiment is consistent with that in Example 1.

[0072] Example 4

[0073] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0074] S1: 9.05 g of ferric phosphate, 3.48 g of manganese dioxide, 3.75 g of lithium carbonate, 4.60 g of ammonium dihydrogen phosphate, and 1.55 g of sucrose were weighed and added to a ball mill and dry-milled for 2 h at a speed of 300 r / min. Then, 120 g of water and 0.2 g of polyethylene glycol were added and wet-milled for 6 h at a speed of 400 r / min to obtain slurry A.

[0075] The slurry A was placed in a blast furnace for drying, and then crushed and ground. The resulting powder was placed in an inert gas atmosphere furnace and sintered under inert gas protection. The sintering temperature was 450°C, the heating rate was 8°C / min, and the sintering time was 4h to obtain material B.

[0076] S2: Place the material B obtained in step S1 in a ball mill, and control the ball milling time at a ball milling speed of 400 r / min to obtain precursors with different particle sizes; the small particle size precursor D 50 The D of the large particle size precursor is 0.2 μm and the ball milling time is 12 h. 50 The particle size was 0.8 μm, and the ball milling time was 5 h;

[0077] S3: adding 100 mL of deionized water and 3.2 g of starch to a reactor equipped with an ultrasonic device, and stirring the mixture evenly with ultrasonic stirring to obtain a starch suspension; adding the small-particle precursor obtained in step S1 to the starch suspension, and simultaneously performing ultrasonic dispersion during the addition process. After uniform mixing, 0.05 g of sodium trimetaphosphate was added, and the obtained mixture 1 was heated to 50° C. at a heating rate of 3° C. / min under stirring to completely convert the mixture 1 into a colloidal mixture. Stirring was continued for 1 h to uniformly disperse the small-particle precursor in the starch colloid to obtain a mixture 2;

[0078] The obtained mixed material 2 was placed in a blast furnace for drying, and the obtained dry product was placed in a ball mill jar and ball milled at a ball mill speed of 400 r / min for 1 h to obtain material C;

[0079] S4: The large-particle precursor obtained in step S2 and the material C obtained in step S3 are evenly mixed, and the resulting mixture is sintered under a nitrogen atmosphere. The sintering is divided into two stages, first calcined at 300°C for 3 hours, and then heated to 700°C for 10 hours; the sintered material is crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to the material C is 0.5:1.

[0080] The preparation method of the battery in this embodiment differs from that in Example 1 only in that the lithium manganese iron phosphate positive electrode material is the lithium manganese iron phosphate positive electrode material obtained in this embodiment. The battery testing method in this embodiment is consistent with that in Example 1.

[0081] Example 5

[0082] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0083] S1: 9.05 g of ferric phosphate, 3.48 g of manganese dioxide, 3.75 g of lithium carbonate, 4.60 g of ammonium dihydrogen phosphate, and 1.55 g of sucrose were weighed and added to a ball mill and dry-milled for 1 h at a speed of 600 r / min. Then, 120 g of water and 0.2 g of polyethylene glycol were added and wet-milled for 5 h at a speed of 300 r / min to obtain slurry A.

[0084] The slurry A was placed in a blast furnace for drying, and then crushed and ground. The resulting powder was placed in an inert gas atmosphere furnace and sintered under inert gas protection. The sintering temperature was 500°C, the heating rate was 10°C / min, and the sintering time was 5h to obtain material B.

[0085] S2: Place the material B obtained in step S1 in a ball mill, and control the ball milling time at a ball milling speed of 400 r / min to obtain precursors with different particle sizes; the small particle size precursor D 50 The D of the large particle size precursor is 0.2 μm and the ball milling time is 12 h. 50 The particle size was 0.8 μm, and the ball milling time was 5 h;

[0086] S3: adding 100 mL of deionized water and 3.2 g of starch to a reactor equipped with an ultrasonic device, and stirring the mixture evenly with ultrasonic stirring to obtain a starch suspension; adding the small-particle precursor obtained in step S1 to the starch suspension, and simultaneously performing ultrasonic dispersion during the addition process. After uniform mixing, 0.05 g of sodium trimetaphosphate was added, and the obtained mixture 1 was heated to 50° C. at a heating rate of 3° C. / min under stirring to completely convert the mixture 1 into a colloidal mixture. Stirring was continued for 1 h to uniformly disperse the small-particle precursor in the starch colloid to obtain a mixture 2;

[0087] The obtained mixed material 2 was placed in a blast furnace for drying, and the obtained dry product was placed in a ball mill jar and ball milled at a ball mill speed of 400 r / min for 1 h to obtain material C;

[0088] S4: The large-particle precursor obtained in step S2 and the material C obtained in step S3 are evenly mixed, and the resulting mixture is sintered under nitrogen atmosphere protection. The sintering is divided into two stages, first calcined at 300°C for 3 hours, and then heated to 800°C for calcination for 8 hours; the sintered material is crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to material C is 5:1.

[0089] The preparation method of the battery in this embodiment differs from that in Example 1 only in that the lithium manganese iron phosphate positive electrode material is the lithium manganese iron phosphate positive electrode material obtained in this embodiment. The battery testing method in this embodiment is consistent with that in Example 1.

[0090] Comparative Example 1

[0091] This comparative example provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0092] S1: 9.05 g of ferric phosphate, 3.48 g of manganese dioxide, 3.75 g of lithium carbonate, 4.60 g of ammonium dihydrogen phosphate, and 1.55 g of sucrose were weighed and added to a ball mill and dry-milled for 1 h at a speed of 400 r / min. Then, 120 g of water and 0.2 g of polyethylene glycol were added and wet-milled for 3 h at a speed of 600 r / min to obtain slurry A.

[0093] The slurry A was placed in a blast furnace for drying, and then crushed and ground. The resulting powder was placed in an inert gas atmosphere furnace and sintered under inert gas protection. The sintering temperature was 400°C, the heating rate was 5°C / min, and the sintering time was 3h to obtain material B.

[0094] S2: Place the material B obtained in step S1 in a ball mill, and control the ball milling time at a ball milling speed of 400 r / min to obtain precursors with different particle sizes; the small particle size precursor D 50The D of the large particle size precursor is 0.2 μm and the ball milling time is 12 h. 50 The particle size was 0.8 μm, and the ball milling time was 5 h;

[0095] S3: adding 100 mL of deionized water and 3.2 g of starch to a reactor equipped with an ultrasonic device, and stirring the mixture by ultrasonic stirring to obtain a starch suspension; adding the small-particle precursor obtained in step S1 to the starch suspension, and ultrasonically dispersing the mixture during the addition process; after uniform mixing, adding the large-particle precursor to the mixture 1, ultrasonically dispersing the mixture until uniformly mixed, and then adding 0.05 g of sodium trimetaphosphate; heating the obtained mixture 2 to 50° C. at a heating rate of 3° C. / min under stirring to completely convert the mixture 2 into a colloidal mixture; continuing stirring for 1 h to uniformly disperse the small-particle precursor in the starch colloid; and obtaining a mixture 3;

[0096] The obtained mixed material 3 was placed in a blast furnace for drying, and the obtained dry product was placed in a ball mill jar and ball milled at a ball mill speed of 400 r / min for 1 h to obtain material C;

[0097] S4: The material C obtained in step S3 is sintered under a nitrogen atmosphere. The sintering is divided into two stages: first, calcined at 300°C for 3 hours, and then heated to 650°C for 12 hours. The sintered material is crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material. The mass ratio of the large-particle precursor to the material C is 2:1.

[0098] The preparation method of the battery in this comparative example differs from that in Example 1 only in that the lithium manganese iron phosphate positive electrode material is the lithium manganese iron phosphate positive electrode material obtained in this comparative example. The battery testing method in this comparative example remains the same as that in Example 1.

[0099] Comparative Example 2

[0100] This comparative example provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0101] S1: 9.05 g of ferric phosphate, 3.48 g of manganese dioxide, 3.75 g of lithium carbonate, 4.60 g of ammonium dihydrogen phosphate, and 1.55 g of sucrose were weighed and added to a ball mill and dry-milled for 1 h at a speed of 400 r / min. Then, 120 g of water and 0.2 g of polyethylene glycol were added and wet-milled for 3 h at a speed of 600 r / min to obtain slurry A.

[0102] The slurry A was placed in a blast furnace for drying, and then crushed and ground. The resulting powder was placed in an inert gas atmosphere furnace and sintered under inert gas protection. The sintering temperature was 400°C, the heating rate was 5°C / min, and the sintering time was 3h to obtain material B.

[0103] S2: Place the material B obtained in step S1 in a ball mill, and control the ball milling time at a ball milling speed of 400 r / min to obtain precursors with different particle sizes; the small particle size precursor D 50 The D of the large particle size precursor is 0.2 μm and the ball milling time is 12 h. 50 The particle size was 0.8 μm, and the ball milling time was 5 h;

[0104] S3: The large-particle precursor and the small-particle precursor of step S2 are mixed evenly, and the resulting mixture is sintered under nitrogen atmosphere protection. The sintering is divided into two stages, first calcined at 300°C for 3 hours, and then heated to 650°C for 12 hours; the sintered material is crushed and sieved to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to material C is 2:1.

[0105] The preparation method of the battery in this comparative example differs from that in Example 1 only in that the lithium manganese iron phosphate positive electrode material is the lithium manganese iron phosphate positive electrode material obtained in this comparative example. The battery testing method in this comparative example remains the same as that in Example 1.

[0106] Table 1 shows the electrochemical properties of the batteries obtained in various examples and comparative examples.

[0107] Table 1

[0108]

[0109]

[0110] The test results in Table 1 show that the capacity of the battery prepared with the lithium iron manganese phosphate cathode material of the present invention is 145-147 mAh / g, and the capacity after 100 cycles is 138-143 mAh / g, indicating that the lithium iron manganese phosphate cathode material of the present invention has excellent electrochemical performance. The capacity of the battery prepared with the lithium iron manganese phosphate cathode material obtained in Comparative Example 2 is 132 mAh / g, and the capacity after 100 cycles is 128 mAh / g. Compared with Example 1, it shows that directly mixing precursors of different particle sizes can reduce the battery capacity and cycle performance.

[0111] The capacity of the battery prepared with the lithium iron manganese phosphate positive electrode material obtained in Comparative Example 1 is 135 mAh / g, and the capacity after 100 cycles is 130 mAh / g; compared with Example 1, it is shown that adding precursors of large and small particle sizes to the starch suspension together will cause the particles of the lithium iron manganese phosphate positive electrode material to aggregate, resulting in a decrease in the capacity and cycle performance of the battery.

[0112] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: S1: A lithium source, an iron source, a manganese source, a phosphorus source, and a carbon source are mixed and dry-milled, and then an emulsifier is added and wet-milled. The resulting slurry A is dried, ground, and calcined to obtain material B; S2: Ball mill material B to obtain D 50 Large particle size precursors of 0.8-1.5 μm and D 50 It is a small particle size precursor of 0.1-0.4μm; S3: adding the small particle size precursor to the starch suspension, ultrasonically dispersing it, adding a crosslinking agent, heating and stirring the mixture to react, and drying and ball milling the resulting colloidal mixture to obtain material C; S4: mixing the large-particle precursor and material C, and calcining the resulting mixture under protective gas to obtain a lithium manganese iron phosphate positive electrode material; the mass ratio of the large-particle precursor to material C is 0.5-5:1; In step S3, the mass of the cross-linking agent is 0.1-1% of the mass of the starch; the cross-linking agent is sodium trimetaphosphate; the heating temperature is 40-65° C., the temperature rising rate is 1-10° C. / min, and the stirring reaction time is 1-3 hours.

2. The preparation method according to claim 1, wherein In step S1, the amount of lithium source, iron source, manganese source and phosphorus source added is based on the amount of LiFe x Mn 1-x The stoichiometric ratio of PO4 is calculated, wherein 0.1≤x≤0.5; the mass of the carbon source is 1-5% of the total mass of the lithium source, iron source, manganese source, phosphorus source and carbon source.

3. The preparation method according to claim 1, wherein In step S1, the rotation speed of dry ball milling and wet ball milling is 300-600 r / min, and the total time of dry ball milling and wet ball milling is 3-8 hours.

4. The preparation method according to claim 1, wherein In step S1, the specific steps of calcination are: placing the ground product in an inert gas atmosphere furnace for sintering, the sintering heating rate is 5-10°C / min, the sintering temperature is 400-500°C, and the sintering time is 3-5h.

5. The preparation method according to claim 1, wherein In step S4, the mass ratio of the large-particle precursor to the material C is 1-3:

1.

6. The preparation method according to claim 1, wherein In step S4, the calcination temperature is 650-800° C., and the calcination time is 8-12 hours.

7. The preparation method according to claim 6, wherein In the step S4, the mixture needs to be pre-sintered before calcination. The pre-sintering temperature is 250-350° C. and the pre-sintering time is 3-4 hours.

8. A lithium manganese iron phosphate positive electrode material, characterized in that: Prepared by the preparation method according to any one of claims 1 to 7.

9. A positive electrode sheet, characterized in that: The positive electrode sheet comprises the lithium manganese iron phosphate positive electrode material as claimed in claim 8.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to claim 9.

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