Method for preparing boron-doped ferromanganese oxide precursor and lithium ferromanganese phosphate

Abstract

The invention provides a method for preparing a boron-doped ferromanganese oxide precursor by a liquid phase coprecipitation method, which comprises the following steps: S1, adding a manganese source and an iron source into deionized water according to a certain proportion to obtain a mixed solution A; s2, adding a complexing agent or a surfactant into the mixed solution A to obtain a mixed solution B; s3, preparing an alkaline precipitant solution C with a certain concentration; s4, the mixed solution B and the alkaline precipitant solution C are jointly dropwise added into a reaction kettle for a co-precipitation reaction, solid-liquid separation is conducted after the reaction is completed, a filter cake is obtained, and the boron-doped ferromanganese oxide precursor is obtained after calcination; wherein the reaction base solution in the step S4 is a boric acid composite salt buffer solution.

Description

Technical Field

[0001] The invention relates to the technical field of lithium batteries, and in particular to a method for preparing a boron-doped manganese iron oxide precursor by a liquid phase co-precipitation method and lithium manganese iron phosphate. Background Art

[0002] Under the background of the "dual carbon" policy, new energy vehicles and green energy storage battery technology have ushered in rapid development in recent years. As one of the core materials of lithium-ion secondary batteries, the current mainstream positive electrode materials include lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide and ternary materials. Due to its advantages of low cost, high safety and long service life, lithium iron phosphate has been widely used in new energy storage industries and pure electric vehicles or hybrid electric vehicles. However, according to the latest data released by the Ministry of Industry and Information Technology, the energy density of lithium iron phosphate positive electrode materials has approached the upper limit, and there is still a gap compared with ternary positive electrode materials. Lithium iron manganese phosphate has the same olivine structure as lithium iron phosphate, and the theoretical gram capacity is 170 mAh g-1, but the voltage platform characteristics of manganese make the energy density of lithium iron manganese phosphate 10-20% higher than that of lithium iron phosphate. Therefore, the research and development of low-cost and high-performance lithium iron manganese phosphate materials is the trend of lithium-ion battery positive electrode materials.

[0003] There are many methods for preparing lithium iron manganese phosphate positive electrode materials, and the preparation process is also divided into two categories: solid phase method and liquid phase method. Among them, the traditional high-temperature solid phase method uses grinding to mix raw materials to prepare a precursor, and then coats the carbon source on the surface of the material by high-temperature sintering, and finally grinds and crushes to obtain the finished product. Patent CN105702954A discloses a method for preparing lithium iron manganese phosphate by a solid phase method, that is, first mixing the manganese source, iron source and phosphorus source according to a molar ratio, drying after ball milling, and then heat treating to obtain a pyrophosphate manganese iron precursor, and then grinding and mixing the pyrophosphate manganese iron precursor with a lithium source and a carbon source, and then calcining at high temperature to obtain lithium iron manganese phosphate. In addition, patent CN111933915A also uses a solid phase method to grind a lithium source, a manganese source, an iron source and a doping element, and then microwave activation to obtain a precursor, and then the above-mentioned precursor and a carbon source are ground twice to obtain a carbon-coated intermediate, and finally calcined at high temperature to obtain lithium iron manganese phosphate. Although the solid phase method can be prepared on a large scale, the metal elements cannot be evenly distributed during the grinding process, especially the doping elements, which will lead to poor batch experiment repeatability. The liquid phase coprecipitation method has many advantages over the solid phase method, including low raw material requirements, low energy consumption, uniform distribution of metal elements, and low overall production cost. At present, the process of preparing lithium iron manganese phosphate by liquid phase coprecipitation method is to first prepare a precursor with uniform distribution of manganese iron, then grind / ball mill with lithium source and carbon source, and finally calcine the obtained lithium iron manganese phosphate. For example, CN110980682A discloses a method of preparing a precursor of manganese iron phosphate or manganese iron oxalate by liquid phase coprecipitation reaction in a supergravity rotating bed, and then the precursor is mixed with a lithium source and dried, and finally calcined in a nitrogen atmosphere to obtain lithium iron manganese phosphate. The method is fast in preparing the precursor, small in primary particle size, and more uniform in carbon coating, but requires expensive and special equipment. At present, the precursors prepared by the liquid phase coprecipitation method mainly include ammonium manganese iron phosphate, manganese iron oxalate, and manganese iron carbonate. For lithium iron manganese phosphate prepared from ferromanganese oxalate and ferromanganese carbonate precursors, it is difficult for them to show low compaction density and energy density. For the preparation of ammonium ferromanganese phosphate precursor, it is necessary to strictly control the conditions of liquid phase co-precipitation, because the pH of the reaction system is high and it is easy to generate impurities; and ammonia will be generated during the calcination process, polluting the environment. In view of this, it is of great significance to develop a new type of nano-scale precursor with uniform distribution of manganese iron through a liquid phase method for the preparation of high-performance lithium iron manganese phosphate.

[0004] Lithium iron manganese phosphate is an upgraded version of lithium iron phosphate, but due to the Jahn-Teller effect of manganese, the crystal of lithium iron manganese phosphate will be deformed, the resistance to electron transport will increase, and the electronic conductivity of lithium iron manganese phosphate will be lower than that of lithium iron phosphate. In addition, the dissolution of manganese in lithium iron manganese phosphate during the charging and discharging process will cause structural collapse, affecting its cycle stability and battery life. Studies have shown that ion doping can slow down the dissolution of manganese and improve the structural stability of the material. The ionic radius of boron ions is much smaller than that of manganese, and the valence state is similar to that of manganese ions. Therefore, in theory, it is easy to dope into the crystal gap of manganese iron oxide precursor. The traditional boron doping strategy of lithium iron manganese phosphate is usually a sol-gel method or a mixture of precursors and lithium salts. These two methods are either not suitable for industrial production, or the doping elements are often enriched on the surface instead of being evenly distributed. The patent of the present invention provides a method for uniformly doping boron elements into the lattice of manganese iron oxide precursors during coprecipitation, which has the advantages of low cost, good uniformity, and suitability for industrial production. Summary of the invention

[0005] In view of the problems existing in the existing lithium iron manganese phosphate precursor technology, the purpose of the present invention is to provide a method for preparing a boron-doped nano-scale lithium iron manganese phosphate precursor (manganese iron oxide) and to prepare high-performance lithium iron manganese phosphate using the precursor.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is: In one aspect, the present invention provides a method for preparing a boron-doped manganese iron oxide precursor by a liquid phase co-precipitation method, comprising the following steps: S1: adding a manganese source and an iron source into deionized water in a certain proportion to obtain a mixed solution A; S2: adding a complexing agent or a surfactant to the mixed solution A to obtain a mixed solution B; S3: prepare alkaline precipitant solution C of a certain concentration; S4: adding the mixed solution B and the alkaline precipitant solution C dropwise into a reaction kettle for co-precipitation reaction, performing solid-liquid separation after the reaction is completed to obtain a filter cake, and calcining to obtain the boron-doped manganese iron oxide precursor; Wherein, the reaction base solution in step S4 is a borate complex salt buffer.

[0007] Conventional liquid phase coprecipitation method, when preparing boron-doped lithium manganese iron phosphate, will first prepare a precursor with uniform distribution of manganese iron, such as: ammonium manganese iron phosphate, manganese iron oxalate and manganese iron carbonate, and then grind / ball mill with lithium source and boron source, and finally calcine to obtain lithium manganese iron phosphate. This method has the problem of low compaction density and energy density of lithium manganese iron phosphate, or the system pH is high during the coprecipitation reaction, and it is easy to generate heterogeneous phases. In this regard, the inventor thought of first using liquid phase coprecipitation method to synthesize boron-doped manganese iron oxide precursor with uniform distribution of manganese iron, and, in the process of liquid phase coprecipitation, using boric acid complex salt buffer as coprecipitation bottom liquid, it is possible to prevent the pH value from changing dramatically during the whole reaction process, thereby ensuring the stability and repeatability of the experimental conditions, and is suitable for large-scale amplification production. By adjusting the concentration of boric acid complex salt, the doping amount of boron can be accurately controlled, and the boron-doped nano-scale lithium manganese iron phosphate precursor can be quantitatively designed, effectively improving the electrical properties and compaction density of lithium manganese iron phosphate.

[0008] In the present invention, in the boron-doped manganese iron oxide precursor, boron can exist in the form of surface coating, that is, forming a coating layer on the surface of the oxide precursor, or in the form of lattice doping, that is, boron atoms can occupy the sites of manganese or iron atoms.

[0009] In a specific embodiment of the present invention, the boric acid complex salt buffer comprises boric acid and at least one of ammonia water, sodium borate, ammonium borate, sodium borate, potassium borate, sodium hydroxide and potassium hydroxide.

[0010] Preferably, the boric acid complex salt buffer is a boric acid-ammonia buffer or a boric acid-sodium hydroxide buffer.

[0011] In a specific embodiment of the present invention, the boron content in the borate complex salt buffer is in the range of 1000-10000 ppm; preferably, the boron content in the borate complex salt buffer is in the range of 5000-10000 ppm.

[0012] In a specific embodiment of the present invention, the pH value of the borate complex salt buffer is 10-13.

[0013] In a specific embodiment of the present invention, the manganese source is selected from at least one of manganese sulfate, manganous chloride, manganese acetate, manganese oxalate, manganese nitrate, manganese citrate and manganese carbonate.

[0014] In a specific embodiment of the present invention, the iron source is selected from at least one of ferrous sulfate, ferrous chloride, ferric sulfate, ferric chloride, ferric nitrate, ferrous citrate, ferrous carbonate and ferrous oxalate, wherein the ferrous sulfate includes ferrous sulfate purified from titanium dioxide by-product.

[0015] In the present invention, the manganese source and the iron source are both soluble salts, which may contain crystal water or may not contain crystal water.

[0016] In a specific embodiment of the present invention, the molar ratio of manganese to iron in the mixed solution A is x:y, wherein 0.1﹤x﹤1, 0.1﹤y﹤1; that is, the molar ratio of manganese to iron is (0.1-10): 1. Preferably, the molar ratio of manganese to iron is (6:4)-(8:2).

[0017] In a specific embodiment of the present invention, the total concentration of manganese ions and iron ions in the mixed solution A is controlled at 0.5-2 mol / L. If the concentration is too low, the solid content of the product will be low and the production efficiency will be low; if the concentration is too high, it is easy to generate impurities.

[0018] In a specific embodiment of the present invention, step S1 specifically includes: adding a manganese source and an iron source into deionized water in a certain proportion, stirring, ultrasonicating or adding an appropriate amount of an acidic reagent to make them completely dissolved in water, to obtain a mixed solution A.

[0019] The acidic reagent may be an organic acid or an inorganic acid, such as citric acid, oxalic acid, hydrochloric acid, sulfuric acid, nitric acid, and the like.

[0020] In a specific embodiment of the present invention, the complexing agent or surfactant in step S2 is selected from at least one of ethylenediaminetetraacetic acid (EDTA), disodium ethylenediaminetetraacetate (EDTA-2Na), ascorbic acid (VC), citric acid (CA), tartaric acid (TA), PEG-2000, PEG-4000, PVP-K30, sodium dodecyl sulfate (SDS), sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, and cetyltrimethylammonium bromide (CTAB).

[0021] In a specific embodiment of the present invention, the amount of the complexing agent or surfactant is 0.01-2% of the total mass of the manganese source and the iron source, thereby better optimizing the morphology of the material. Preferably, the amount of the complexing agent or surfactant is 0.1-0.5% of the total mass of the manganese source and the iron source.

[0022] In a specific embodiment of the present invention, the alkaline precipitant is selected from at least one of sodium hydroxide, ammonia water and potassium hydroxide.

[0023] In a specific embodiment of the present invention, the concentration of the alkaline precipitant solution C is 1 to 15 mol / L. Preferably, the concentration of the alkaline precipitant solution C is 10 to 15 mol / L.

[0024] In a specific embodiment of the present invention, in step S4, the pH value of the coprecipitation reaction is 10-13, and the reaction temperature is 20-60°C.

[0025] In a specific embodiment of the present invention, in step S4, the calcination temperature is 500-700° C., and the calcination time is between 5 and 7 hours.

[0026] The hydroxide of the precursor is obtained through a coprecipitation reaction, and then the oxide precursor is obtained through high-temperature calcination.

[0027] In the present invention, step S4 specifically includes: S4-1: adding the mixed solution B and the alkaline precipitant solution C to the reactor at a suitable pumping speed through a peristaltic pump, wherein a certain amount of boric acid complex salt buffer is added to the reactor as a reaction base liquid; after the mixed solution B is completely added, the addition of the alkaline precipitant solution C is stopped; S4-2: subjecting the solution obtained after the reaction in step S4-1 to an aging reaction; S4-3: solid-liquid separation is performed on the solution obtained after the reaction in step S4-2 is completed to obtain a first filter cake, which is then washed multiple times to obtain a second filter cake, which is dried and calcined to obtain the boron-doped manganese iron oxide precursor.

[0028] In a specific embodiment of the present invention, the step S4-1 further has at least one of the following technical characteristics: (1) During the dropwise addition process, the pH value of the solution in the reactor was controlled to be between 10 and 13; (2) During the dropwise addition process, the temperature of the solution in the reaction vessel is controlled to be 20-60°C, preferably 20-30°C; (3) The pumping speed of the peristaltic pump is 1~60 mL / min; (4) The stirring speed of the reactor is 20~1200 rpm; preferably 500~1000 rpm; (5) The reactor can be protected by inert gas or not.

[0029] In a specific embodiment of the present invention, the aging time in step S4-2 is 0.5 to 24 h, preferably 0.5 to 1 h.

[0030] In a specific embodiment of the present invention, the step S4-3 further has at least one of the following technical characteristics: Water and / or ethanol can be used for washing, and the ionic conductivity of the filtrate after the second filter cake washing is <0.1 ms / cm.

[0031] In a second aspect of the present invention, a boron-doped manganese iron oxide precursor obtained by the above-mentioned preparation method is provided, wherein the structural formula of the boron-doped manganese iron oxide precursor is (Mn m B 1-m-n Fe n ) 2 O 3 , among which, 0.5﹤m﹤0.8, 0.2﹤n﹤0.4, 0.005﹤1-mn﹤0.01.

[0032] In a specific embodiment of the present invention, the primary particle size of the precursor is 30-1000 nm, preferably 30-500 nm.

[0033] In the third aspect of the present invention, a method for preparing boron-doped lithium manganese iron phosphate is provided, the preparation method comprising: grinding and mixing a precursor, a lithium source, and a phosphorus source in proportion, spray drying, and then calcining for a second time to obtain the boron-doped lithium manganese iron phosphate, wherein the precursor is the boron-doped manganese iron oxide precursor described in the first aspect or the second aspect.

[0034] In a specific embodiment of the present invention, the lithium source is at least one of lithium carbonate, lithium acetate, lithium hydroxide, lithium dihydrogen phosphate, and lithium phosphate.

[0035] In a specific embodiment of the present invention, the phosphorus source is at least one of lithium dihydrogen phosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and lithium phosphate.

[0036] In a specific embodiment of the present invention, the raw materials of the grinding mixture further include a carbon source, and / or a doping compound.

[0037] In a specific embodiment of the present invention, the carbon source is selected from one or more of sucrose, glucose, citric acid, phenolic resin, polyvinyl alcohol, polyethylene glycol, water-soluble starch, carbon black, acetylene black, graphite, graphene, and conductive carbon tubes.

[0038] In a specific embodiment of the present invention, the carbon source accounts for 10-25% of the mass of the target product (boron-doped lithium manganese iron phosphate).

[0039] In a specific embodiment of the present invention, the doping element M in the doping compound is at least one of titanium, magnesium, nickel and niobium.

[0040] In a specific embodiment of the present invention, the temperature of the second calcination is 500-770° C., and the atmosphere of the second calcination is at least one of nitrogen and argon.

[0041] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention provides a liquid phase co-precipitation method for preparing a boron-doped manganese iron oxide precursor with a nanometer size, which has a simple process, no ammonia emission, high product purity, and the manganese iron element of the precursor prepared by this method is evenly distributed at the atomic level, which can meet the preparation requirements of nano-scale lithium manganese iron phosphate materials.

[0042] (2) The lithium manganese iron phosphate cathode material prepared by the precursor provided by the present invention has excellent electrochemical performance, with a 0.1C discharge capacity greater than 154 mAh / g and a powder compaction density of 2.27 g / cm 3 The above mentioned, low cost of raw materials is conducive to large-scale industrial production of lithium iron manganese phosphate.

[0043] (3) The present invention uses a boric acid complex salt buffer as the coprecipitation bottom liquid, which can prevent the pH value from changing dramatically during the entire reaction process, thereby ensuring the stability and repeatability of the experimental conditions, and is suitable for large-scale production. By adjusting the concentration of the borate, the amount of boron doping can be accurately controlled, and the boron-doped nano-scale manganese iron oxide precursor can be quantitatively designed to effectively improve the electrical properties and compaction density of lithium manganese iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 is an XRD pattern of the nano-sized manganese iron oxide precursor prepared in Example 1 of the present invention; Figure 2 is a TEM image of the nano-sized manganese iron oxide precursor prepared in Example 1 of the present invention; Figure 3 This is a TEM-Mapping image of the nano-sized manganese iron oxide precursor prepared in Example 1 of the present invention; Figure 4 is an XRD diagram of lithium manganese iron phosphate prepared using the nano-scale manganese iron oxide precursor of Example 1 of the present invention; Figure 5 This is a SEM image of lithium manganese iron phosphate prepared using the nano-scale manganese iron oxide precursor of Example 1 of the present invention. DETAILED DESCRIPTION

[0045] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the 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.

[0046] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0047] In the description of the present invention, the terms “first” and “second” are used for descriptive purposes only and should not be understood as indicating or implying relative importance.

[0048] In the description of the present invention, the terms "comprises", "comprising" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of more restrictions, an element defined by the sentence "comprising a ..." does not exclude the presence of other identical elements in the process, method, article or device comprising the element.

[0049] The method for preparing the boron-doped manganese iron oxide precursor and lithium manganese iron phosphate provided by the present invention comprises the following steps: (1) A manganese source and an iron source are added to deionized water in a certain proportion, and the two are completely dissolved in water by stirring, ultrasonication or adding an appropriate amount of an acidic reagent to obtain a mixed solution A of a certain concentration. The total concentration of metal ions (manganese ions and iron ions) in the mixed solution is controlled at 0.5-2 mol / L, and the acidic reagent includes at least one of an organic acid and an inorganic acid.

[0050] (2) Adding a complexing agent or a surfactant to the mixed solution A of step (1), and then stirring or ultrasonicating it to completely dissolve it, to obtain a mixed solution B. The amount of the complexing agent or the surfactant is 0.01-2% of the total mass of the manganese source and the iron source.

[0051] (3) Add sodium hydroxide and / or ammonia water into deionized water to prepare an alkaline precipitant solution C, wherein the concentration of the alkaline precipitant solution C is 1-15 mol / L.

[0052] (4) Add a certain amount of boric acid complex salt buffer into the reactor as the base liquid.

[0053] (5) The mixed solution B in step (2) and the alkaline precipitant solution C in step (3) are added dropwise to the reactor in step (4) by a peristaltic pump at a suitable pumping speed, respectively, until the mixed solution B is completely added, and then the addition of the alkaline precipitant solution C is stopped. During the addition process, ensure that the pH of the solution in the reactor is between 10 and 13, the solution temperature is between 20 and 60° C., the pump speed is between 1 and 60 mL / min, and the stirring speed of the reactor is between 20 and 1200 rpm. The reactor may be protected by inert gas or not.

[0054] (6) Aging the solution obtained after the reaction in step (5) for 0.5 to 24 h.

[0055] (7) The precipitate obtained after the reaction in step (6) is collected to obtain a first filter cake, which is then washed multiple times to obtain a second filter cake, which is dried and calcined to obtain the boron-doped manganese iron oxide precursor, wherein the calcination temperature is between 500 and 700° C. and the calcination time is between 5 and 7 h.

[0056] (8) Grinding and mixing the boron-doped manganese iron oxide precursor obtained in step (7) with a lithium source, a phosphorus source, a carbon source, and a doping compound in proportion, spray drying, and finally calcining under an inert atmosphere at a calcination temperature between 500 and 770° C. to obtain the target product of lithium manganese iron phosphate.

[0057] The present invention is further described in detail below through specific embodiments. Example 1

[0058] This embodiment prepares a boron-doped nano-sized manganese iron oxide precursor, and the specific process is as follows: Step 1, dissolving manganese sulfate and ferrous sulfate heptahydrate in deionized water at a manganese to iron molar ratio of 6:4, mechanically stirring for 10 min, and then adding an appropriate amount of concentrated sulfuric acid until manganese sulfate and ferrous sulfate are completely dissolved to obtain solution A, wherein the total concentration of manganese and iron metal ions is 1.0 mol / L.

[0059] Step 2: Add 0.2% ascorbic acid to the total mass of manganese sulfate and ferrous sulfate heptahydrate in step 1 to the mixed solution A, and continue mechanical stirring for 10 min until the ascorbic acid is completely dissolved to obtain a mixed solution B.

[0060] Step 3: prepare a mixture of 10 mol / L NaOH and 1.5 mol / L ammonia water as alkaline precipitant solution C.

[0061] Step 4: Add boric acid-ammonia buffer solution with a boron content of 5000 ppm and a pH of 12 into the reactor as a base solution.

[0062] Step 5, the mixed solution B in step 2 and the alkaline precipitant solution C in step 3 are added to the reactor in step 4 through two peristaltic pumps at a suitable pumping speed, until the solution B is added dropwise, and the addition of solution C is stopped. The stirring speed of the reactor is 800 rmp, the solution in the reactor maintains a constant pH of 12, and the temperature of the solution in the reactor is maintained at 25°C by circulating cooling water.

[0063] Step 6, the solution obtained after the reaction in step 5 is allowed to stand and age for 30 min.

[0064] Step 7, collecting the precipitate obtained in step 6, and then washing it with deionized water until the conductivity of the second filter cake filtrate is less than 0.1 ms / cm.

[0065] Step 8, drying the filter cake obtained in step 7 at 95°C for 12 h, and then calcining it in a muffle furnace at 600°C for 6 h to obtain a boron-doped nano-sized manganese iron oxide precursor (Mn 0.595 B 0.005 Fe 0.4 ) 2 O 3 .

[0066] Figure 1 It is a precursor of manganese iron oxide (Mn 0.595 B 0.005 Fe 0.4 ) 2 O 3 According to the test results, the precursor synthesized by this technical solution is consistent with the standard card PDF#24-0507 ((Mn 0.983 Fe 0.017 ) 2 O 3 ) all diffraction peaks corresponded perfectly, which indicated that the manganese iron oxide precursor prepared by this method was not composed of Mn 2 O 3 and Fe 2 O 3 A two-phase mixture.

[0067] Figure 2 It is a precursor of manganese iron oxide (Mn 0.595 B 0.005 Fe 0.4 ) 2 O 3 From the TEM image, we can see that the manganese iron oxide precursor (Mn 0.595 B 0.005 Fe 0.4 ) 2 O 3 It is composed of nanoparticles with a size of 30-200 nm and good particle uniformity.

[0068] Figure 3 It is a precursor of manganese iron oxide (Mn 0.595 B 0.005 Fe 0.4 ) 2 O 3 From the TEM-mapping diagram, we can see that the manganese iron oxide precursor (Mn 0.595 B 0.005 Fe 0.4 ) 2 O 3 The distribution of manganese and iron in the precursor is very uniform, which further proves that the precursor is not composed of Mn 2 O 3 and Fe 2 O 3 This is of great significance for the development of high-purity and high-performance lithium manganese iron phosphate. Example 2

[0069] This embodiment prepares a boron-doped nano-sized manganese iron oxide precursor, and the specific process is as follows: Step 1, dissolving manganese sulfate and ferrous sulfate heptahydrate in deionized water at a manganese to iron molar ratio of 6:4, mechanically stirring for 10 min, and then adding an appropriate amount of concentrated sulfuric acid until manganese sulfate and ferrous sulfate are completely dissolved to obtain solution A, wherein the total concentration of manganese and iron metal ions is 1.0 mol / L.

[0070] Step 2: Add 0.2% ascorbic acid to the total mass of manganese sulfate and ferrous sulfate heptahydrate in step 1 to the mixed solution A, and continue mechanical stirring for 10 min until the ascorbic acid is completely dissolved to obtain a mixed solution B.

[0071] Step 3: prepare a mixed solution of 10 mol / L NaOH and 1.5 mol / L ammonia water as alkaline precipitant solution C.

[0072] Step 4: Add boric acid-sodium borate buffer with a boron content of 10,000 ppm and a pH of 10 into the reactor as a base solution.

[0073] Step 5, the mixed solution B in step 2 and the mixed solution C in step 3 are added to the reactor in step 4 by two peristaltic pumps at a suitable pumping speed, until the solution B is added dropwise, and the addition of solution C is stopped. The stirring speed of the reactor is 800 rmp, the solution in the reactor maintains a constant pH of 10, and the temperature of the solution in the reactor is maintained at 25°C by circulating cooling water.

[0074] Step 6, the solution obtained after the reaction in step 5 is allowed to stand and age for 30 min.

[0075] Step 7, collecting the precipitate obtained in step 6, and then washing it with deionized water until the conductivity of the second filter cake filtrate is less than 0.1 ms / cm.

[0076] Step 8, drying the filter cake obtained in step 7 at 95°C for 12 h, and then calcining it in a muffle furnace at 600°C for 6 h to obtain a boron-doped nano-sized manganese iron oxide precursor (Mn 0.59 B 0.01 Fe 0.4 ) 2 O 3 . Example 3

[0077] This embodiment prepares a boron-doped nano-sized manganese iron oxide precursor, and the specific process is as follows: Step 1, dissolving manganese chloride and ferrous chloride in deionized water at a molar ratio of manganese to iron of 7:3, mechanically stirring for 10 min, and then adding an appropriate amount of concentrated sulfuric acid to adjust until manganese sulfate and ferrous sulfate are completely dissolved to obtain solution A, wherein the total concentration of manganese and iron metal ions is 1.0 mol / L.

[0078] Step 2: Add 0.2% of the total mass of manganous chloride and ferrous chloride in step 1 to the mixed solution A, and continue mechanical stirring for 10 min until the ethylenediaminetetraacetic acid (EDTA) is completely dissolved to obtain a mixed solution B.

[0079] Step 3: prepare a mixed solution of 8 mol / L NaOH and 2 mol / L ammonia water as alkaline precipitant solution C.

[0080] Step 4: Add boric acid-sodium hydroxide buffer solution with a boron content of 8000 ppm and a pH of 11 into the reactor as a base solution.

[0081] Step 5, the mixed solution B in step 2 and the mixed solution C in step 3 are added to the reactor in step 4 by two peristaltic pumps at a suitable pumping speed, until the solution B is added dropwise, and the addition of solution C is stopped. The stirring speed of the reactor is 600 rmp, the solution in the reactor maintains a constant pH of 11, and the temperature of the solution in the reactor is maintained at 30°C by circulating cooling water.

[0082] Step 6, the solution obtained after the reaction in step 5 is allowed to stand and age for 30 min.

[0083] Step 7, collecting the precipitate obtained in step 6, and then washing it with deionized water until the conductivity of the second filter cake filtrate is less than 0.1 ms / cm.

[0084] Step 8: Dry the filter cake obtained in step 7 at 95°C for 12 h, and then calcine it in a muffle furnace at 650°C for 6 h to obtain a boron-doped nano-sized manganese iron oxide precursor (Mn 0.692 B 0.008 Fe 0.3 ) 2 O 3 . Example 4

[0085] This embodiment prepares a boron-doped nano-sized manganese iron oxide precursor, and the specific process is as follows: Step 1, dissolving manganese sulfate and ferrous sulfate heptahydrate in deionized water at a manganese to iron molar ratio of 8:2, mechanically stirring for 10 min, and then adding an appropriate amount of concentrated sulfuric acid until manganese sulfate and ferrous sulfate are completely dissolved to obtain a mixed solution A, wherein the total concentration of manganese and iron metal ions is 1.0 mol / L.

[0086] Step 2: Add 0.2% ascorbic acid to the total mass of manganese sulfate and ferrous sulfate heptahydrate in step 1 to the mixed solution A, and continue mechanical stirring for 10 min until the ascorbic acid is completely dissolved to obtain a mixed solution B.

[0087] Step 3: prepare a mixture of 10 mol / L NaOH and 1.5 mol / L ammonia water as alkaline precipitant solution C.

[0088] Step 4: Add boric acid-ammonia buffer solution with a boron content of 5000 ppm and a pH of 12 into the reactor as a base solution.

[0089] Step 5, the mixed solution B in step 2 and the alkaline precipitant solution C in step 3 are added to the reactor in step 4 through two peristaltic pumps at a suitable pumping speed, until the solution B is added dropwise, and the addition of solution C is stopped. The stirring speed of the reactor is 800 rmp, the solution in the reactor maintains a constant pH of 12, and the temperature of the solution in the reactor is maintained at 25°C by circulating cooling water.

[0090] Step 6, the solution obtained after the reaction in step 5 is allowed to stand and age for 30 min.

[0091] Step 7, collecting the precipitate obtained in step 6, and then washing it with deionized water until the conductivity of the second filter cake filtrate is less than 0.1 ms / cm.

[0092] Step 8, drying the filter cake obtained in step 7 at 95°C for 12 h, and then calcining it in a muffle furnace at 600°C for 6 h to obtain a nano-sized manganese iron oxide precursor (Mn 0.795 B 0.005 Fe 0.2 ) 2 O3 . Comparative Example 1

[0093] Referring to Example 1, the difference between this comparative example and Example 1 is that the base liquid adopts a pure aqueous solution. Comparative Example 2

[0094] Referring to Example 1, the difference between this comparative example and Example 1 is that the base liquid adopts an ammonia solution with a pH value of 10. Preparation of lithium manganese iron phosphate 1:

[0095] The boron-doped manganese iron oxide precursor obtained in Examples 1 to 4 above was used as a raw material to prepare lithium manganese iron phosphate positive electrode material. The specific process is as follows: According to the reaction equation for preparing lithium manganese iron phosphate from nano manganese iron oxide precursor, feed the materials. (Mn 0.6-z B z Fe 0.4 ) 2 O 3 +Li 2 CO 3 +2NH 4 H 2 PO 4 +C (carbon source) → 2LiMn 0.6-z B z Fe 0.4 PO 4 +2NH 3 ↑+CO 2 ↑+3H 2 O+CO↑; Step 1: sand-mill and mix the boron-doped manganese iron oxide precursor, lithium carbonate, ammonium dihydrogen phosphate, glucose and deionized water obtained in the embodiment or comparative example according to the amount of the reaction equation until the primary particle size D50 of the sand-milled slurry is 150 nm.

[0096] Step 2, spray drying the slurry obtained in step 1, setting the air inlet temperature to 150° C. and the air outlet temperature to 75° C., and collecting the lithium manganese iron phosphate precursor material after spray drying.

[0097] Step 3: calcining the material collected in step 2 at 700°C for 8 h in a nitrogen atmosphere to obtain black lithium manganese iron phosphate LiMn 0.6-z B z Fe 0.4 PO 4 @C powder.

[0098] Step 4, the lithium manganese iron phosphate and polytetrafluoroethylene (PVDF), acetylene black, N-methylpyrrolidone for homogenization, then coated on aluminum foil, dried at 110°C in vacuum, and rolled and punched to obtain the positive electrode sheet.

[0099] Step 5, assembling button cells with the positive electrode sheets obtained in step 4, and then conducting electrochemical performance tests.

[0100] Figure 4 This is the XRD diagram of lithium manganese iron phosphate prepared by using the nano-scale manganese iron oxide precursor of Example 1 of the present invention. It can be seen from the figure that the prepared lithium manganese iron phosphate is completely consistent with the standard card.

[0101] Figure 5 This is a SEM picture of lithium manganese iron phosphate prepared by using the nano-sized manganese iron oxide precursor of Example 1 of the present invention. It can be seen from the picture that the prepared lithium manganese iron phosphate is a micron-sized spherical material composed of nano-sized particles with high sphericity. Preparation of lithium manganese iron phosphate 2:

[0102] The lithium iron manganese phosphate precursor obtained in the above comparative examples 1-2 was used as a raw material to prepare the lithium iron manganese phosphate positive electrode material. The specific process is as follows: Step 1: sand-mill the lithium iron manganese phosphate precursor, lithium carbonate, ammonium dihydrogen phosphate, glucose, 5000 ppm boric acid and deionized water in proportion until the primary particle size D50 of the sand-milled slurry is 150 nm.

[0103] Step 2, spray drying the slurry obtained in step 1, setting the air inlet temperature to 150° C. and the air outlet temperature to 75° C., and collecting the lithium manganese iron phosphate precursor material after spray drying.

[0104] Step 3: calcining the material collected in step 2 at 700°C for 8 h in a nitrogen atmosphere to obtain black lithium manganese iron phosphate LiMn 0.6-z B z Fe 0.4 PO 4 @C powder.

[0105] Step 4, the lithium manganese iron phosphate and polytetrafluoroethylene (PVDF), acetylene black, N -methylpyrrolidone for homogenization, then coated on aluminum foil, dried at 110°C in vacuum, and rolled and punched to obtain the positive electrode sheet.

[0106] Step 5, assembling button cells with the positive electrode sheets obtained in step 4, and then conducting electrochemical performance tests.

[0107] The lithium manganese iron phosphate obtained above was subjected to ICP test for B element content and compaction density test, and the assembled button battery was subjected to electrochemical performance test. The test results are shown in Table 1 below: Table 1 Experiment number B content (ppm) 0.1C charging mAh / g 0.1C discharge mAh / g 0.5C discharge mAh / g Powder compaction density (g / cm3) Example 1 4967 160.42 155.03 150.76 2.32 Example 2 9975 160.59 154.89 150.29 2.29 Example 3 8148 159.98 155.21 149.98 2.35 Example 4 4988 159.18 154.21 147.98 2.27 Comparative Example 1 4789 154.34 149.45 137.56 2.20 Comparative Example 2 4767 156.89 150.84 139.11 2.16 ; It can be seen from the above table that the boron-doped lithium manganese iron phosphate cathode material prepared by the method of the present invention has a higher compaction density. This is because the manganese iron oxide precursor has the characteristics of small particle size and good particle uniformity. The lithium manganese iron phosphate cathode material inherits the advantages of the manganese iron oxide precursor and also has the characteristics of good particle uniformity, so the powder compaction density is higher than 2.27 g / cm 3 .

[0108] In Comparative Examples 1 and 2, conventional reaction base liquid was used, and boron was not doped in the precursor. Instead, an external boron source was added for doping during the preparation of the lithium iron manganese phosphate positive electrode material. The results showed that the external boron source would deviate from the actual design amount, mainly because the boron element was difficult to be evenly distributed in the lithium iron manganese phosphate, and the compaction density and electrical properties were lower than those when boron was directly doped in the precursor.

[0109] The specific embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0110] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0111] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for preparing a boron-doped manganese iron oxide precursor by liquid phase coprecipitation, characterized in that: The following steps are involved: S1: adding a manganese source and an iron source into deionized water in a certain proportion to obtain a mixed solution A; S2: adding a complexing agent or a surfactant to the mixed solution A to obtain a mixed solution B; S3: prepare alkaline precipitant solution C of a certain concentration; S4: adding the mixed solution B and the alkaline precipitant solution C dropwise into a reaction kettle for co-precipitation reaction, performing solid-liquid separation after the reaction is completed to obtain a filter cake, and calcining to obtain the boron-doped manganese iron oxide precursor; Wherein, the reaction base solution in step S4 is a borate complex salt buffer.

2. The preparation method according to claim 1, characterized in that: The boric acid complex salt buffer comprises boric acid and at least one of ammonia water, sodium borate, ammonium borate, sodium borate, potassium borate, sodium hydroxide and potassium hydroxide; Preferably, the boric acid complex salt buffer is a boric acid-ammonia buffer or a boric acid-sodium hydroxide buffer; Preferably, the boron content in the borate complex salt buffer is in the range of 1000 to 10000 ppm; Preferably, the pH value of the borate complex salt buffer is 10-13.

3. The preparation method according to claim 1, characterized in that: The manganese source is selected from at least one of manganese sulfate, manganous chloride, manganese acetate, manganese oxalate, manganese nitrate, manganese citrate, and manganese carbonate; The iron source is selected from at least one of ferrous sulfate, ferrous chloride, ferric sulfate, ferric chloride, ferric nitrate, ferrous citrate, ferrous carbonate and ferrous oxalate.

4. The preparation method according to claim 1, characterized in that: The total concentration of manganese ions and iron ions in the mixed solution A is controlled at 0.5-2 mol / L; Preferably, the molar ratio of manganese element to iron element in the mixed solution A is x:y, wherein 0.1﹤x﹤1, 0.1﹤y﹤1.

5. The preparation method according to claim 1, characterized in that: The complexing agent or surfactant in step S2 is selected from at least one of ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetate, ascorbic acid, citric acid, tartaric acid, PEG-2000, PEG-4000, PVP-K30, sodium dodecyl sulfonate, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and hexadecyltrimethylammonium bromide; Preferably, the amount of the complexing agent or surfactant is 0.01-2% of the total mass of the manganese source and the iron source.

6. The preparation method according to claim 1, characterized in that: The alkaline precipitant is selected from at least one of sodium hydroxide, ammonia water and potassium hydroxide; Preferably, the concentration of the alkaline precipitant solution C is 1-15 mol / L.

7. The preparation method according to claim 1, characterized in that: In the step S4, the pH value of the coprecipitation reaction is 10-13, and the reaction temperature is 20-60° C.; Preferably, the calcination temperature is 500-700°C.

8. The boron-doped manganese iron oxide precursor obtained by the preparation method according to any one of claims 1 to 7, characterized in that: The structural formula of the precursor is (Mn x B 1-x-y Fe y )2O3; among them, 0.1﹤x﹤1, 0.1﹤y﹤1.

9. The boron-doped manganese iron oxide precursor according to claim 8, characterized in that: The particle size of the precursor primary particles is 30-1000 nm.

10. A method for preparing boron-doped lithium manganese iron phosphate, characterized in that: include: The precursor, lithium source, and phosphorus source are ground and mixed in proportion, spray-dried, and then calcined for the second time to obtain the boron-doped lithium manganese iron phosphate, wherein the precursor is the boron-doped manganese iron oxide precursor according to any one of claims 1 to 7.

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