Boron-doped ferrous manganese phosphate, carbon-coated boron-doped lithium manganese iron phosphate and preparation method thereof

Abstract

The invention provides boron-doped ferromanganese phosphate, the content of boron in the boron-doped ferromanganese phosphate is 0.005-0.05 wt%, and the ratio of manganese to iron in the boron-doped ferromanganese phosphate meets the chemical formula ((Mn < x > Fe < y >) < 3 > (P < 1-z > B < z > O < 4 >) < 2 >, wherein 0 lt; xlt; 1, 0lt; yt; Yt; 1, 0.001 < = z < = 0.05. According to the method provided by the invention, manganese and iron elements are uniformly distributed in the ferromanganese phosphate by controlling the charging sequence and the reaction process conditions, and the batch stability and the cycle performance of the carbon-coated boron-doped lithium ferromanganese phosphate can be well improved by using the ferromanganese phosphate as the precursor to prepare the carbon-coated boron-doped lithium ferromanganese phosphate; the compaction density of the carbon-coated boron-doped lithium manganese iron phosphate can also be improved; the method provided by the invention is simple and easy to operate, green and environment-friendly in preparation process, suitable for large-scale production and good in application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron manganese phosphate precursor and lithium iron manganese phosphate preparation, and in particular to boron-doped ferromanganese phosphate, carbon-coated boron-doped lithium iron manganese phosphate and preparation methods thereof. Background Art

[0002] Lithium iron phosphate has been widely used in new energy storage industries and pure electric vehicles or hybrid electric vehicles due to its advantages such as low cost, high safety and long service life. As an upgraded version of lithium iron phosphate, lithium iron manganese phosphate has an energy density that is 10-20% higher than that of lithium iron phosphate. Therefore, the development of high-quality lithium iron manganese phosphate materials is the mainstream trend in the future power battery and energy storage market. Compared with the mature lithium iron phosphate preparation process, lithium iron manganese phosphate still lacks suitable precursors for the development of high-quality lithium iron manganese phosphate.

[0003] At present, there are many methods for preparing lithium iron manganese phosphate positive electrode materials, and the mainstream process is still the traditional high-temperature solid-phase method, that is, lithium source, manganese source, iron source, phosphorus source, and carbon source are mixed in a certain proportion, and then sintered at high temperature to obtain carbon-coated lithium iron manganese phosphate. Although this process continues the mainstream process route of lithium iron phosphate, the quality of the prepared lithium iron manganese phosphate is poor. The main reason is that it is difficult for manganese and iron elements to form a uniform solid solution, the batch stability of the material is poor, and the final lithium iron manganese phosphate will show poor cycle performance. Therefore, it is necessary to develop a precursor with uniform distribution of manganese and iron for the development of high-quality lithium iron manganese phosphate.

[0004] Patent application CN115043387A discloses a method for preparing an ammonium ferromanganese phosphate precursor by a coprecipitation method, that is, a metal salt solution is obtained by mixing a ferrous source and a manganese source, and then a reaction is regulated with a phosphorus source in an organic phase by ammonia water, and finally a solid-liquid separation is performed to obtain an ammonium ferromanganese phosphate precursor, and then the above precursor is mixed with a carbon source and a lithium source and calcined at high temperature to obtain lithium ferromanganese phosphate. Patent application CN11110433A discloses a manganese ferrocarbonate precursor, that is, ferrous sulfate and manganese sulfate are first dissolved in water in proportion to obtain a mixed solution, and then stirred and reacted with sodium carbonate to obtain manganese ferrocarbonate, and finally the manganese ferrocarbonate precursor is ball-milled and mixed with a lithium source, a carbon source, and a phosphorus source in proportion, and calcined to obtain lithium ferromanganese phosphate. Patent application CN110980682A discloses a method for preparing a manganese ferrooxalate precursor in a supergravity rotating bed, and then the precursor is mixed with a lithium source and dried, and finally the lithium ferromanganese phosphate is sintered; this method is fast in preparing the precursor, but requires expensive and special equipment. The precursor prepared by the above method will also be accompanied by the generation of a large amount of water vapor, ammonia and carbon dioxide during the calcination process, which puts great pressure on environmental protection; at the same time, the large gas production will also cause the final lithium iron manganese phosphate to show a lower compaction density. Compared with the oxalate method, the precursor prepared by the phosphate method has a higher compaction density; however, the solubility product values ​​of ferrous phosphate and manganese phosphate are quite different, and it is easy to generate impurities under the conventional liquid phase reaction system, and it is difficult to co-precipitate to form a high-purity iron manganese phosphate precursor. In addition, the ionic conductivity and electronic conductivity of pure phase lithium iron manganese phosphate are 3-4 orders of magnitude different from those of lithium iron phosphate. In view of this, it is of great significance to develop a high-purity and high-conductivity ferrous manganese phosphate precursor for the preparation of high-quality lithium iron manganese phosphate. Summary of the invention

[0005] In view of the above problems existing in the existing preparation process of lithium manganese iron phosphate, the purpose of the present invention is to provide a boron-doped ferromanganese phosphate, a carbon-coated boron-doped lithium manganese iron phosphate and a preparation method thereof. To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, a method for preparing high-purity boron-doped ferromanganese phosphate at low cost is provided, comprising the following steps:

[0007] (1) preparing an iron-phosphorus mixed solution and a manganese-phosphorus mixed solution by taking a manganese source, an iron source, a phosphorus source, an antioxidant and deionized water, and adding an acid solution to adjust the pH of the iron-phosphorus mixed solution and the manganese-phosphorus mixed solution to ≤ 2;

[0008] (2) adding an iron-phosphorus mixed solution and a precipitant to the base liquid, reacting under an inert atmosphere, continuously stirring and controlling the pH of the reaction system to be between 5.0 and 6.5 throughout the process, obtaining a precipitate after the reaction is completed, and then aging, filtering and washing the precipitate to obtain a filter cake 1; the obtained filter cake 1 is ferrous phosphate Fe3(PO4)2·nH2O;

[0009] (3) adding a manganese-phosphorus mixed solution and a precipitant to the base liquid, continuously stirring and controlling the pH of the reaction system to be between 7.0 and 8.5 throughout the process, obtaining a precipitate after the reaction is completed, and then aging, filtering and washing the precipitate to obtain a filter cake 2; the obtained filter cake 2 is manganous phosphate Mn3(PO4)2·nH2O;

[0010] (4) The filter cake 1, the filter cake 2 and the boron source are mixed uniformly, and then dried, and then calcined under an inert atmosphere to obtain high-purity boron-doped ferromanganese phosphate.

[0011] In any embodiment, the antioxidant in step 1 is selected from at least one of ascorbic acid, isoascorbic acid and its salts, disodium edetate, citric acid, tea polyphenols and PVP;

[0012] In any embodiment, the molar ratio of the phosphorus source to the iron source added to the iron-phosphorus mixed solution in step (1) is (1.4-1.5):1 in terms of Fe and P; the concentration of the iron source is 0.1-2 mol / L.

[0013] In any embodiment, the molar ratio of the manganese source and the phosphorus source added to the manganese-phosphorus mixed solution in step (1) is (1.4-1.5):1 in terms of Mn and P; the concentration of the manganese source is 0.1-2 mol / L.

[0014] In any embodiment, the manganese source in step (1) comprises at least one of manganese sulfate, manganous chloride, manganese acetate, manganese oxalate, manganese nitrate, manganese citrate and manganese carbonate;

[0015] Furthermore, the manganese source may contain crystalline water or may not contain crystalline water.

[0016] In any embodiment, the iron source in step (1) is a soluble iron source;

[0017] Furthermore, the soluble iron source includes at least one of ferrous sulfate, ferrous chloride, ferric sulfate, ferric chloride, ferric nitrate, ferric citrate and ferrous oxalate;

[0018] Furthermore, the iron source may contain or may not contain crystal water.

[0019] In any embodiment, the phosphorus source in step (1) is at least one of phosphoric acid, ammonium phosphate, diammonium phosphate, diammonium hydrogen phosphate and phosphoric acid.

[0020] In any embodiment, the base liquid in step (1) is a solution of a complexing agent or a surfactant, and the amount added is 0.1-5% of the total molar amount of the manganese source and the iron source;

[0021] Furthermore, the complexing agent is at least one of ethylenediaminetetraacetic acid (EDTA), disodium ethylenediaminetetraacetate (EDTA-2Na), citric acid (CA) and tartaric acid (TA);

[0022] Furthermore, the surfactant is at least one of polyvinyl pyrrolidone (PVP), PEG-2000, sodium dodecyl sulfate (SDS), sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and cetyltrimethylammonium bromide (CTAB).

[0023] In any embodiment, the acid solution in step (1) is at least one of hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid.

[0024] In any embodiment, the precipitant in step (2) is an alkaline solution; the alkaline solution is at least one of aqueous ammonia, sodium hydroxide solution and urea solution; the concentration of the alkaline solution is 0.1-1 mol / L.

[0025] In any embodiment, the aging time in step (2) and step (3) is greater than 0.5 h, and the aging temperature is between 30-95° C.

[0026] In any embodiment, the washing in step (2) and step (3) can be performed using a filter press or a centrifuge until the ionic conductivity of the filtrate is less than 80 μs / cm;

[0027] In any embodiment, the boron source in step (4) is at least one of boron oxide, boric acid, lithium borate, trimethyl borate and triethyl borate.

[0028] In any embodiment, the calcination temperature in step (4) is 350-550° C. and the calcination time is 4-10 h;

[0029] In any embodiment, in step (4), filter cake 1, filter cake 2 and the boron source are uniformly mixed by grinding, and the particle size D50 of the mixed material obtained after mixing is 0.3-1.5 μm;

[0030] Furthermore, the grinding includes but is not limited to ball milling and sand milling.

[0031] In a second aspect, a boron-doped ferromanganese phosphate is provided, wherein the boron doping amount of the boron-doped ferromanganese phosphate is 0.005-0.05wt%, and the manganese to iron ratio in the boron-doped ferromanganese phosphate satisfies the chemical formula ((Mn x Fe y )3(P 1-z B z O4)2,0 <x<1,0<y<1,0.001≤z≤0.05。

[0032] In a third aspect, a method for preparing carbon-coated boron-doped lithium manganese iron phosphate is provided, comprising:

[0033] The boron-doped manganese iron phosphate obtained above is mixed with a lithium source, a phosphorus source, a carbon source and a doping element M, and then ground, spray-dried, and finally calcined under an inert atmosphere to obtain the carbon-coated boron-doped manganese iron lithium phosphate.

[0034] In any embodiment, the calcination temperature is between 500-770°C;

[0035] In any embodiment, the lithium source is at least one of lithium carbonate, lithium acetate, lithium hydroxide, lithium dihydrogen phosphate and lithium phosphate.

[0036] In any embodiment, the carbon source is at least one of sucrose, glucose, citric acid, phenolic resin, polyvinyl alcohol, polyethylene glycol, water-soluble starch, carbon black, acetylene black, graphite, graphene and conductive carbon tubes;

[0037] In any embodiment, the carbon source accounts for 10-25% of the mass of the carbon-coated boron-doped lithium manganese iron phosphate, and the carbon source added during the preparation process will all become the carbon coating layer.

[0038] In any embodiment, the added molar amount of the doping element is 0.1-2% of the total molar amount of the manganese source and the iron source.

[0039] In any embodiment, the doping element M is at least one of titanium, magnesium and niobium.

[0040] In any embodiment, the doping element M is added in an amount of 0-2% of the total molar amount of iron and manganese;

[0041] In any embodiment, in the technical solution of the present invention, the inert atmosphere includes at least one of nitrogen and argon.

[0042] In a fourth aspect, a carbon-coated boron-doped lithium manganese iron phosphate prepared by the method described above is provided, wherein the carbon-coated boron-doped lithium manganese iron phosphate satisfies the structural formula LiMn x Fe y M 1-x-y P 1-z B z O4@C, where 0 <x<1,0<y<1,0.001≤z≤0.05。

[0043] In a fifth aspect, a carbon-coated boron-doped lithium manganese iron phosphate is provided for use in the field of battery positive electrode materials.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] 1. The present invention controls the preparation process conditions so that the prepared manganese ferrophosphate has no crystal water, and no water gas and ammonia are generated during the calcination process, which is beneficial to improve the uniformity and compaction density of the carbon coating layer on the surface of the lithium manganese iron phosphate prepared using the method, and finally makes the prepared carbon-coated boron-doped lithium manganese iron phosphate material have a higher capacity retention rate and better cycle performance;

[0046] 2. The method provided by the present invention controls the reaction conditions such as the order of adding materials and pH, so that the manganese iron element is evenly distributed in the ferromanganese phosphate precursor, which can better improve the batch stability and cycle performance of the carbon-coated boron-doped lithium manganese iron phosphate prepared by using the method, and also improve the compaction density of the prepared carbon-coated boron-doped lithium manganese iron phosphate;

[0047] 3. The present invention provides a method for effectively alleviating the Mn 2+ The John-Teller effect improves the cycle performance of lithium manganese iron phosphate prepared using it;

[0048] 4. The method provided by the present invention is also simple and easy to operate, the preparation process is green and environmentally friendly, suitable for large-scale production, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a SEM image of the carbon-coated boron-doped lithium manganese iron phosphate prepared in Example 1. DETAILED DESCRIPTION

[0050] The technical solution of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

[0051] Example 1

[0052] 1. Preparation of boron-doped ferromanganese phosphate precursor:

[0053] (1) 0.3 mol of ferrous sulfate and 0.2 mol of ammonium dihydrogen phosphate are dissolved in 300 ml of deionized water, and the mixture is stirred continuously. Appropriate concentrated sulfuric acid is added until the pH value of the mixed solution is 2, and then 2% ascorbic acid is added to the total molar amount of the iron source to obtain an iron-phosphorus mixed solution; 0.3 mol of manganese sulfate and 0.2 mol of ammonium dihydrogen phosphate are dissolved in 300 ml of deionized water, and the mixture is stirred continuously. Appropriate concentrated sulfuric acid is added until the pH value of the mixed solution is 1.5 to obtain a manganese-phosphorus mixed solution;

[0054] (2) 200 ml of deionized water and 0.02 mol of ascorbic acid were added to the reactor as the base liquid. The iron-phosphorus mixed solution and 0.5 mol / L ammonia water were added dropwise to the reactor through a peristaltic pump under stirring (800 rpm). The pH was controlled to be 5.5 by adjusting the feed rate. After the reaction was completed, a slurry was obtained, which was kept at 90° C. for 2 h. After the slurry was cooled, it was transferred to a centrifuge and washed with deionized water for multiple times until the filtrate was less than 80 μs / cm, thereby obtaining a filter cake 1.

[0055] (3) Add 200 ml of deionized water and 0.02 mol EDTA into the reactor to obtain a mixed solution as the base solution. Under stirring (800 rpm), the manganese-phosphorus mixed solution and 0.5 mol / L ammonia water are added dropwise to the reactor through a peristaltic pump. The pH is controlled to be 8.0 by adjusting the feed rate throughout the process. After the reaction is completed, a slurry is obtained, which is kept at 90° C. for 2 h. After the slurry is cooled, it is transferred to a centrifuge and washed with deionized water for multiple times until the filtrate is less than 80 μs / cm, thereby obtaining a filter cake 2.

[0056] (4) According to the molar ratio of manganese, iron and boron of 9:6:0.2, filter cake 1, filter cake 2 and 2000 ppm boric acid were re-dispersed in deionized water by mechanical stirring to obtain a slurry, and the slurry was mixed by sand milling to control the particle size D50≈1.0 μm, and then flash dried to obtain a precursor mixture, which was calcined at 550°C for 6 h in a nitrogen atmosphere to obtain a boron-doped manganese ferrophosphate precursor (Mn 0.6 Fe 0.4 )3(P 0.98 B 0.02 O4)2).

[0057] 2. Preparation of Carbon-Coated Boron-Doped Lithium Manganese Iron Phosphate

[0058] The boron-doped manganese ferrophosphate (Mn) prepared above was prepared in a molar ratio of 1:1:1.03 to the molar ratio of the sum of the molar numbers of Mn and Fe elements to the molar numbers of P and Li elements. 0.6 Fe 0.4 )3(P 0.98 B 0.02 O4)2) with lithium carbonate and diammonium phosphate, and then adding glucose and PEG-2000 to mix and sand grind to obtain a mixture, wherein the mass of glucose accounts for 5% of the mixture, and the mass of PEG-2000 accounts for 5% of the mixture. The particle size of the mixture after sand grinding is D50=0.3μm, and then spray drying is performed with an inlet air temperature of 220°C and an outlet air temperature of 90°C to obtain a lithium iron manganese phosphate precursor, and finally calcined at 740°C for 8h to obtain a carbon-coated boron-doped lithium iron manganese phosphate, and then air flow pulverization is performed to control the particle size D50≈0.8μm of the carbon-coated boron-doped lithium iron manganese phosphate. The chemical formula of the obtained carbon-coated boron-doped lithium iron manganese phosphate is LiMn 0.6 Fe 0.4P 0.98 B 0.02 O4@C.

[0059] Example 2

[0060] 1. Preparation of boron-doped ferromanganese phosphate:

[0061] (1) dissolving 0.3 mol of ferrous chloride and 0.2 mol of phosphoric acid in 300 ml of deionized water, stirring continuously, adding appropriate concentrated hydrochloric acid until the pH value of the mixed solution is 1.5, and then adding 1.5% ascorbic acid of the total molar amount of the iron source to obtain an iron-phosphorus mixed solution; dissolving 0.3 mol of manganese chloride and 0.2 mol of phosphoric acid in 300 ml of deionized water, stirring continuously, and adding appropriate concentrated hydrochloric acid until the pH value of the mixed solution is 2.0 to obtain a manganese-phosphorus mixed solution;

[0062] (2) Add 200 ml of deionized water, 200 ml of ethanol and 0.02 mol of ascorbic acid into a reaction kettle to obtain a mixed solution as a base solution. The iron-phosphorus mixed solution and 0.1 mol / L sodium hydroxide solution are added dropwise to the reaction kettle through a peristaltic pump under stirring (600 rpm). The pH is controlled to be 6.0 by adjusting the feed rate throughout the process. After the reaction is completed, a slurry is obtained, which is kept at 90° C. for 2 h. After the slurry is cooled, it is transferred to a centrifuge and washed with deionized water for multiple times until the filtrate is less than 80 μs / cm, thereby obtaining a filter cake 1.

[0063] (3) 200 ml of deionized water, 200 ml of ethanol and 0.02 mol SDS were added to the reactor, and the manganese-phosphorus mixed solution and 0.1 mol / L sodium hydroxide solution were added dropwise to the reactor through a peristaltic pump under stirring (600 rpm). The pH was controlled to be 7.8 by adjusting the feed rate during the whole process. After the reaction, a slurry was obtained, which was kept at 90° C. for 2 h. After the slurry was cooled, it was transferred to a centrifuge and washed with deionized water for multiple times until the filtrate was less than 80 μs / cm, to obtain filter cake 2;

[0064] (4) According to the molar ratio of manganese, iron and boron of 9:6:0.04, filter cake 1, filter cake 2 and 4000 ppm boron oxide were re-dispersed in deionized water by mechanical stirring to obtain a slurry, and the slurry was mixed by sand milling to control the particle size D50≈1.0 μm, and then flash dried to obtain a precursor mixture, which was calcined at 550°C for 6 h in a nitrogen atmosphere to obtain boron-doped manganese ferrophosphate (Mn 0.6 Fe 0.4 )3(P 0.996 B 0.004 O4)2).

[0065] 2. Preparation of Carbon-Coated Boron-Doped Lithium Manganese Iron Phosphate

[0066] The boron-doped manganese phosphate (Mn) was prepared by taking the molar ratio of the sum of the molar numbers of Mn and Fe to the molar ratio of P and Li as 1:1:1.03. 0.6 Fe 0.4 )3(P 0.996 B 0.004 O4)2) and lithium phosphate, and then add sucrose and PEG-2000 to mix and sand grind to obtain a mixture, wherein sucrose accounts for 6% of the mass of the obtained mixture, PEG-2000 accounts for 6% of the mass of the mixture, and the particle size of the mixture after sand grinding is D50=0.3μm, and then spray drying is carried out, the inlet air temperature is 220℃, and the outlet air temperature is 90℃, to obtain a lithium iron manganese phosphate precursor, and finally calcined at 750℃ for 6h to obtain carbon-coated boron-doped lithium iron manganese phosphate, and finally air flow pulverization is carried out to control the particle size D50≈0.8μm of the carbon-coated boron-doped lithium iron manganese phosphate, and the chemical formula of the obtained carbon-coated boron-doped lithium iron manganese phosphate is LiMn 0.6 Fe 0.4 P 0.996 B 0.004 O4@C.

[0067] Example 3

[0068] 1. Preparation of boron-doped ferromanganese phosphate:

[0069] (1) dissolving 0.3 mol of ferrous sulfate and 0.2 mol of phosphoric acid in 300 ml of deionized water, stirring continuously, adding appropriate amount of sulfuric acid until the pH value of the mixed solution is 1.5, and then adding 2% of the total molar amount of iron source isoascorbic acid to obtain an iron-phosphorus mixed solution; dissolving 0.3 mol of manganese chloride and 0.2 mol of phosphoric acid in 300 ml of deionized water, stirring continuously, adding appropriate amount of sulfuric acid until the pH value of the mixed solution is 2.0, to obtain a manganese-phosphorus mixed solution;

[0070] (2) Add 400 ml of deionized water and 0.02 mol of PVP into the reactor to obtain a mixed solution as the base liquid. The iron-phosphorus mixed solution and 1 mol / L ammonia water are added dropwise to the reactor through a peristaltic pump under stirring (400 rpm). The pH is controlled to be 6.2 by adjusting the feed rate during the whole process. After the reaction is completed, a slurry is obtained, which is kept at 90° C. for 2 h. After the slurry is cooled, it is transferred to a centrifuge and washed with deionized water for multiple times until the filtrate is less than 80 μs / cm to obtain a filter cake 1.

[0071] (3) Add 400 ml of deionized water and 0.02 mol of PVP into the reactor to obtain a mixed solution as the base liquid. Under stirring (500 rpm), the manganese-phosphorus mixed solution and 1 mol / L ammonia water are added dropwise to the reactor through a peristaltic pump. The pH is controlled to be 7.5 by adjusting the feed rate during the whole process. After the reaction is completed, a slurry is obtained, which is kept at 90° C. for 2 h. After the slurry is cooled, it is transferred to a centrifuge and washed with deionized water for multiple times until the filtrate is less than 80 μs / cm to obtain filter cake 2;

[0072] (4) According to the molar ratio of manganese, iron and boron of 2.1:0.9:0.05, filter cake 1, filter cake 2 and 5000 ppm lithium borate were re-dispersed in deionized water by mechanical stirring to obtain a slurry, and the slurry was mixed by sand milling to control the particle size D50≈1.0 μm, and then flash dried to obtain a precursor mixture, which was calcined at 550°C for 6 h in a nitrogen atmosphere to obtain boron-doped manganese ferrophosphate (Mn 0.7 Fe 0.3 )3(P 0.995 B 0.005 O4)2).

[0073] 2. Preparation of Carbon-Coated Boron-Doped Lithium Manganese Iron Phosphate

[0074] The boron-doped manganese phosphate precursor (Mn) obtained above was prepared according to the molar ratio of the sum of the molar numbers of Mn and Fe elements to the molar ratio of P and Li elements being 1:1:1.03. 0.7 Fe 0.3 )3(P 0.995 B 0.005 O4)2) and lithium phosphate, and then add citric acid and mix them, and then sand grind them to obtain a mixed slurry, wherein the mass of citric acid accounts for 25% of the mass of the obtained mixture, and the particle size of the sand-ground mixture is D50=0.3μm, and then spray drying is performed, with an inlet air temperature of 220°C and an outlet air temperature of 90°C to obtain a lithium iron manganese phosphate precursor, and finally calcined at 750°C for 6h to obtain a carbon-coated boron-doped lithium iron manganese phosphate, and finally air flow pulverization is performed to control the particle size D50 of the carbon-coated boron-doped lithium iron manganese phosphate to be ≈1.0μm, and the chemical formula of the obtained carbon-coated boron-doped lithium iron manganese phosphate is LiMn 0.7 Fe 0.3 P 0.995 B 0.005 O4@C.

[0075] Example 4

[0076] 1. Preparation of boron-doped ferromanganese phosphate precursor:

[0077] (1) 0.3 mol of ferrous sulfate and 0.2 mol of ammonium dihydrogen phosphate were dissolved in 300 ml of deionized water, and the mixture was stirred continuously. Appropriate concentrated sulfuric acid was added until the pH value of the mixed solution was 1.5, and then 1% ascorbic acid of the total molar amount of the iron source was added to obtain an iron-phosphorus mixed solution; 0.3 mol of manganese sulfate and 0.2 mol of ammonium dihydrogen phosphate were dissolved in 300 ml of deionized water, and the mixture was stirred continuously. Appropriate concentrated sulfuric acid was added until the pH value of the mixed solution was 1.5 to obtain a manganese-phosphorus mixed solution;

[0078] (2) 200 ml of deionized water and 0.02 mol of ascorbic acid were added to the reactor as the base liquid. The iron-phosphorus mixed solution and 0.3 mol / L urea solution were added dropwise to the reactor through a peristaltic pump under stirring (500 rpm). The pH was controlled to be 6.5 by adjusting the feed rate. After the reaction was completed, a slurry was obtained, which was kept at 90° C. for 2 h. After the slurry was cooled, it was transferred to a centrifuge and washed with deionized water for multiple times until the filtrate was less than 80 μs / cm, thereby obtaining a filter cake 1.

[0079] (3) Add 200 ml of deionized water and 0.02 mol CA into the reactor to obtain a mixed solution as the base solution. Under stirring (500 rpm), the manganese-phosphorus mixed solution and 0.3 mol / L urea solution are added dropwise to the reactor through a peristaltic pump. The pH is controlled to be 7.0 by adjusting the feed rate during the whole process. After the reaction is completed, a slurry is obtained, which is kept at 90° C. for 2 h. After the slurry is cooled, it is transferred to a centrifuge and washed with deionized water for multiple times until the filtrate is less than 80 μs / cm to obtain a filter cake 2.

[0080] (4) According to the molar ratio of manganese, iron and boron of 9:6:0.4, filter cake 1, filter cake 2 and 3000 ppm boric acid were re-dispersed in deionized water by mechanical stirring to obtain a slurry, and the slurry was mixed by sand milling to control the particle size D50≈1.0 μm, and then flash dried to obtain a precursor mixture, which was calcined at 550°C for 6 h in a nitrogen atmosphere to obtain a boron-doped manganese ferrophosphate precursor (Mn 0.6 Fe 0.4 )3(P 0.98 B 0.04 O4)2).

[0081] 2. Preparation of Carbon-Coated Boron-Doped Lithium Manganese Iron Phosphate

[0082] The boron-doped manganese ferrophosphate (Mn) prepared above was prepared in a molar ratio of 1:1:1.03 to the molar ratio of the sum of the molar numbers of Mn and Fe elements to the molar numbers of P and Li elements. 0.6 Fe 0.4 )3(P 0.98 B 0.04O4)2) with lithium carbonate and diammonium phosphate, then add glucose, PEG-2000 and titanium oxide with a total molar amount of 1% of iron and manganese, mix and grind to obtain a mixture, wherein the mass of glucose accounts for 10% of the mixture, and the mass of PEG-2000 accounts for 5% of the mixture. The particle size D50 of the mixture after sand grinding is 0.3μm, and then spray drying is performed with an inlet air temperature of 220°C and an outlet air temperature of 90°C to obtain a lithium iron manganese phosphate precursor, and finally calcined at 740°C for 8h to obtain a carbon-coated boron-doped lithium iron manganese phosphate, and then air flow pulverization is performed to control the particle size D50 of the carbon-coated boron-doped lithium iron manganese phosphate to be 0.8μm. The chemical formula of the obtained carbon-coated boron-doped lithium iron manganese phosphate is LiMn 0.6 Fe 0.39 Ti 0.01 P 0.98 B 0.04 O4@C or LiMn 0.59 Fe 0.4 Ti 0.01 P 0.98 B 0.04 O4@C.

[0083] Comparative Example 1

[0084] 1. Preparation of ferromanganese phosphate precursor:

[0085] (1) dissolving 0.3 mol of ferrous sulfate and 0.2 mol of ammonium dihydrogen phosphate in 300 ml of deionized water, stirring continuously, adding appropriate concentrated sulfuric acid until the pH of the mixed solution is 2, and then adding 2% ascorbic acid of the total molar amount of the iron source to obtain an iron-phosphorus mixed solution; dissolving 0.3 mol of manganese sulfate and 0.2 mol of ammonium dihydrogen phosphate in 300 ml of deionized water, stirring continuously, and adding appropriate concentrated sulfuric acid until the pH of the mixed solution is 1.5 to obtain a manganese-phosphorus mixed solution;

[0086] (2) 200 ml of deionized water and 0.02 mol of ascorbic acid were added to the reaction kettle as the base liquid. The iron-phosphorus mixed solution and 0.5 mol / L ammonia water were added dropwise to the reaction kettle through a peristaltic pump under stirring (800 rpm). The pH was controlled to be 5.5 by adjusting the feed rate throughout the process. After the reaction was completed, a slurry was obtained, which was kept at 90° C. for 2 h. After the slurry was cooled, it was transferred to a centrifuge and washed with deionized water for multiple times until the filtrate was less than 80 μs / cm, thereby obtaining a filter cake 1.

[0087] (3) 200 ml of deionized water and 0.02 mol EDTA were added to the reactor as the base liquid. The manganese-phosphorus mixed solution and 0.5 mol / L ammonia water were added dropwise to the reactor through a peristaltic pump under stirring (800 rpm). The pH was controlled to be 8.0 by adjusting the feed rate during the whole process. After the reaction, a slurry was obtained, which was kept at 90° C. for 2 h. After the slurry was cooled, it was transferred to a centrifuge and washed with deionized water for multiple times until the filtrate was <80 μs / cm, to obtain filter cake 2;

[0088] (4) Filter cake 1 and filter cake 2 were re-dispersed in deionized water at a manganese-iron ratio of 6:4 by mechanical stirring to obtain a slurry, and then mixed by sand milling to control the particle size D50≈1.0 μm, and then flash dried to obtain a precursor mixture, which was calcined at 550° C. in a nitrogen atmosphere for 6 h to obtain ferromanganese phosphate (Mn 0.6 Fe 0.4 )3(PO4)2.

[0089] 2. Preparation of Carbon-Coated Lithium Manganese Iron Phosphate

[0090] The preparation process is the same as that in Example 1. The chemical formula of the obtained carbon-coated lithium manganese iron phosphate is LiMn 0.6 Fe 0.4 PO4@C.

[0091] Comparative Example 2

[0092] 1. Preparation of ferromanganese phosphate precursor:

[0093] (1) dissolving 0.3 mol of ferrous sulfate and 0.2 mol of ammonium dihydrogen phosphate in 300 ml of deionized water, stirring continuously, adding appropriate concentrated sulfuric acid until the pH of the mixed solution is 2, and then adding 2% ascorbic acid of the total molar amount of the iron source to obtain an iron-phosphorus mixed solution; dissolving 0.3 mol of manganese sulfate and 0.2 mol of ammonium dihydrogen phosphate in 300 ml of deionized water, stirring continuously, and adding appropriate concentrated sulfuric acid until the pH of the mixed solution is 1.5 to obtain a manganese-phosphorus mixed solution;

[0094] (2) 200 ml of deionized water and 0.02 mol of ascorbic acid were added to the reactor as the base liquid. Under stirring (800 rpm), the iron-phosphorus mixed solution and 0.5 mol / L ammonia water were added dropwise to the reactor through a peristaltic pump. The pH was controlled to be 5.5 by adjusting the feed rate during the whole process. After the reaction was completed, a slurry was obtained, which was kept at 90° C. for 2 h. After the slurry was cooled, it was transferred to a centrifuge and washed with deionized water for multiple times until the filtrate was less than 80 μs / cm, thereby obtaining a filter cake 1.

[0095] (3) 200 ml of deionized water and 0.02 mol EDTA were added to the reaction kettle as the base liquid. The manganese-phosphorus mixed solution and 0.5 mol / L ammonia water were added dropwise to the reaction kettle through a peristaltic pump under stirring (800 rpm). The pH was controlled to be 8.0 by adjusting the feed rate during the whole process. After the reaction was completed, a slurry was obtained, which was kept at 90° C. for 2 h. After the slurry was cooled, it was transferred to a centrifuge and washed with deionized water for multiple times until the filtrate was less than 80 μs / cm to obtain filter cake 2;

[0096] (4) According to the molar ratio of manganese to iron of 6:4, filter cake 1 and filter cake 2 were re-dispersed in deionized water by mechanical stirring to obtain a slurry, and the slurry was mixed by sand milling to control the particle size D50≈1.0 μm, and then flash dried to obtain a precursor mixture, which was calcined at 550°C for 6 h in a nitrogen atmosphere to obtain ferromanganese phosphate (Mn 0.6 Fe 0.4 )3(PO4)2).

[0097] 2. Preparation of Carbon-Coated Boron-Doped Lithium Manganese Iron Phosphate

[0098] The manganese ferrophosphate (Mn 0.6 Fe 0.4 )3(PO4)2 is mixed with 20000ppm boric acid, and then lithium carbonate and ammonium dihydrogen phosphate are added according to the molar ratio of the sum of the molar numbers of Mn and Fe elements to the molar ratio of P and Li elements of 1:1:1.03, and then glucose and PEG-2000 are added and mixed and sand-milled to obtain a mixture, wherein the mass of glucose accounts for 5% of the mass of the mixture, and the mass of PEG-2000 accounts for 5% of the mass of the mixture. The particle size D50 is controlled to be 0.3μm by sand-milling, and then spray-dried with an inlet air temperature of 220℃ and an outlet air temperature of 90℃ to obtain a lithium manganese iron phosphate precursor, and finally calcined at 740℃ for 8h to obtain carbon-coated boron-doped lithium manganese iron phosphate, and then air flow pulverized to control the particle size D50≈0.8μm of the carbon-coated boron-doped lithium manganese iron phosphate. The chemical formula of the obtained carbon-coated boron-doped lithium manganese iron phosphate is LiMn 0.6 Fe 0.4 P 0.98 B 0.02 O4@C.

[0099] Comparative Example 3

[0100] This embodiment prepares a lithium manganese iron phosphate positive electrode material, and the specific process is as follows:

[0101] 1) Add 30.15g of ferric phosphate, 22.9g of manganese tetraoxide, 2% of the total molar amount of iron and manganese in boric acid (calculated as B), 34.5g of ammonium dihydrogen phosphate, 18.95g of lithium carbonate and 10.34g of sucrose to 360.5ml of pure water, mix well and pour into a sand mill, sand grind to a slurry particle size D50 = 300nm.

[0102] 2) The slurry obtained in step 1) is spray dried, the air inlet temperature is set to 220° C., and the air outlet temperature is set to 90° C. After the spray drying is completed, the lithium manganese iron phosphate precursor material is collected.

[0103] 3) The material collected in step 2) was calcined at 740°C for 8 h in a nitrogen atmosphere to obtain black lithium manganese iron phosphate LiMn 0.6 Fe 0.4 P 0.98 B 0.02 O4@C powder.

[0104] 4) The lithium manganese iron phosphate obtained in step 3) is subjected to air flow pulverization to control the particle size D50≈0.8 μm.

[0105] Performance Testing

[0106] The performance test results of the embodiments of the present invention and the comparative examples are shown in Table 1.

[0107] Test method: Refer to the national standard "Lithium manganese iron phosphate positive electrode material for lithium-ion batteries" group standard T / CIAPS0029-2023 to test the relevant physical and chemical properties and electrochemical properties, including the production of button batteries (the mass ratio of active material, conductive agent and binder is 90:5:5, and the charge and discharge voltage range is 2.5-4.5V).

[0108] Table 1 Performance test table of lithium manganese iron phosphate of embodiment and comparative example

[0109]

[0110] Table 2 Impurity content test table of ferromanganese phosphate in the embodiment and comparative example

[0111]

[0112] According to the test results in Table 1, the carbon-coated boron-doped lithium manganese iron phosphate prepared by the boron-doped ferromanganese phosphate prepared by the method of the present invention has a higher compaction density, and the maximum compaction density can reach 2.39 g / cm 3 This is mainly because the exothermic boron-doped ferromanganese phosphate prepared by the method of the present invention does not contain crystal water, and there is no obvious gas and water production during the entire calcination process. Figure 1It can also be seen from the SEM electron microscope image that the calcined sample has good density, there are obvious large single crystal particles, and the size of the particles is clearly graded. Comparing the conductivity of the obtained carbon-coated boron-doped lithium manganese iron phosphate in Table 1, it can be seen that the lithium manganese iron phosphate prepared by using the boron-doped ferromanganese phosphate precursor has a higher conductivity. This is because boron doping belongs to phosphorus-site lattice doping, which causes lattice distortion, greatly improves the migration rate of lithium ions, and finally shows better electrical properties. It can also be seen from Table 1 that the carbon-coated boron-doped lithium manganese iron phosphate prepared in the embodiment of the present invention has a high discharge specific capacity at a small current density of 0.1C, and also shows a high capacity retention rate in the 1C cycle test. In addition, combined with the impurity element test results of ferromanganese phosphate in Table 2, the impurity content of the boron-doped ferromanganese phosphate prepared in the embodiment is less than 100ppm, and the purity is relatively high, indicating that the ferromanganese phosphate prepared by the present invention can be used to prepare high-quality carbon-coated boron-doped lithium manganese iron phosphate.

Claims

1. A boron-doped ferromanganese phosphate, characterized in that: The boron content of the boron-doped manganese ferrophosphate is 0.005-0.05 wt %, and the manganese-iron ratio satisfies the chemical formula ((Mn x Fe y )3(P 1-z B z O4)2; where 0 <x<1,0<y<1,0.001≤z≤0.05)。 2. A method for preparing boron-doped ferromanganese phosphate, characterized in that: The following steps are involved: (1) taking a manganese source, an iron source, a phosphorus source, an antioxidant and deionized water to prepare an iron-phosphorus mixed solution and a manganese-phosphorus mixed solution respectively, and adding acid solution to adjust the pH of the iron-phosphorus mixed solution and the manganese-phosphorus mixed solution to ≤ 2; (2) adding an iron-phosphorus mixed solution and a precipitant to the base liquid, reacting under an inert atmosphere, continuously stirring and controlling the pH of the reaction system to be between 5.0 and 6.5 throughout the process, obtaining a precipitate after the reaction is completed, and then aging, filtering and washing the precipitate to obtain a filter cake 1; (3) adding a manganese-phosphorus mixed solution and a precipitant to the base liquid, continuously stirring and controlling the pH of the reaction system at 7.0-8.5 throughout the process, obtaining a precipitate after the reaction is completed, and then aging, filtering and washing the precipitate to obtain a filter cake 2; (4) The filter cake 1, the filter cake 2 and the boron source are mixed uniformly, and then dried, and then calcined under an inert atmosphere to obtain high-purity boron-doped ferromanganese phosphate.

3. The method for preparing boron-doped ferromanganese phosphate according to claim 2, characterized in that: The manganese source in step (1) is at least one of manganese sulfate, manganous chloride, manganese acetate, manganese oxalate, manganese nitrate, manganese citrate and manganese carbonate.

4. The method for preparing boron-doped ferromanganese phosphate according to claim 2, characterized in that: The iron source is a soluble iron source.

5. The method for preparing boron-doped ferromanganese phosphate according to claim 2, characterized in that: The phosphorus source in step (1) is at least one of phosphoric acid, ammonium phosphate, diammonium phosphate, diammonium hydrogen phosphate and phosphoric acid.

6. The method for preparing boron-doped ferromanganese phosphate according to claim 2, characterized in that: The base liquid in step (1) is a solution of a complexing agent or a surfactant.

7. The method for preparing boron-doped ferromanganese phosphate according to claim 2, characterized in that: The precipitant in step (2) is an alkaline solution.

8. The method for preparing boron-doped ferromanganese phosphate according to claim 2, characterized in that: The boron source in step (4) is at least one of boron oxide, boric acid, lithium borate, trimethyl borate and triethyl borate.

9. A method for preparing carbon-coated boron-doped lithium manganese iron phosphate, characterized in that: include: The boron-doped manganese iron phosphate prepared by the method according to any one of claims 2 to 8 is mixed with a lithium source, a phosphorus source, a carbon source and a doping element M, and then ground, then spray-dried, and finally calcined under an inert atmosphere to obtain the carbon-coated boron-doped lithium manganese iron phosphate; wherein the addition amount of the doping element M is 0-20000ppm.

10. A carbon-coated boron-doped lithium manganese iron phosphate, characterized in that: Prepared by the method of claim 9.

Images