Preparation of lithium iron manganese phosphate precursor and method for preparing lithium iron manganese phosphate using the same

Abstract

The present invention provides a method for preparing a lithium iron manganese phosphate precursor and using the same to prepare lithium iron manganese phosphate. The method uses pyrophosphate iron as the iron source for preparing lithium iron manganese phosphate by a liquid phase method, dissolves the pyrophosphate iron by adjusting the pH value, and then evenly mixes it with a soluble manganese source, a soluble phosphorus source and an organic reducing agent, controls the pH value of the mixed solution to make it clear; uses a strong base such as hydrazine hydrate or ethylenediamine to quickly control the pH value of the mixed solution to near neutrality to form a lithium iron manganese phosphate precursor by coprecipitation. Finally, the precursor, a lithium source and a water-soluble carbon source are mixed and calcined at a high temperature to prepare a lithium iron manganese phosphate material. The method of the present invention breaks through the conventional operation of using only pyrophosphate iron as the iron source of the solid phase method, solves the problem of uneven distribution of manganese iron in the preparation of lithium iron manganese phosphate, and the manganese iron in the prepared lithium iron manganese phosphate is evenly distributed and has good electrical properties.

Description

Technical Field

[0001] The present invention belongs to the field of lithium ion batteries and energy materials, and specifically relates to a preparation method of a lithium iron manganese phosphate precursor and a method for preparing lithium iron manganese phosphate using the precursor. Background Art

[0002] Among phosphate materials, the most widely studied one is LiFePO4, which has a relatively simple synthesis and has been mass-produced. However, due to the low lithium deintercalation potential platform (about 3.4V), the LiFePO4 material reduces the overall energy density of the battery and limits its development in electric vehicles. The working voltage of LiMnPO4 to Li is 4.1V. If LiMnPO4 can obtain a specific capacity equivalent to that of LiFePO4, it means that the energy density will be 35% higher than that of LiFePO4. At the same time, low raw material cost and environmental friendliness are also the advantages of LiMnPO4. However, the conductivity of LiMnPO4 is very low, almost an insulator, only one thousandth of LiFePO4; at the same time, the Jahn-Teller effect will exist during the redox reaction, resulting in poor material rate performance and low discharge specific capacity. In order to increase the electrochemical performance of olivine-structured LiMnPO4, the two methods of accelerating electron and ion transport are mainly used. The improvement of ion transport can be achieved by reducing the particle size and ensuring a uniform size distribution, and the improvement of electron transport can be achieved by using carbon coating and doping. From the current research status, it can be seen that LiMn x Fe (1-x) PO4 positive electrode material has high energy density, which can compensate for the shortcomings of LiFePO4 positive electrode material in this regard, while improving the problem of low rate and discharge specific capacity of LiMnPO4 positive electrode material, and increasing the possibility of phosphoric acid positive electrode material becoming power lithium-ion battery material.

[0003] At present, a single high-temperature solid-phase method is used to prepare lithium manganese iron phosphate materials. When preparing the precursor, it is difficult for Fe and Mn to be evenly distributed in the main structure of the material, which will lead to Mn 3+The Jahn-Teller effect is serious, which affects the cycle and rate performance of the battery. In view of this point, CN106887586B provides a carbon aerogel network lithium iron manganese phosphate battery electrode material and preparation method to improve the problem of uneven distribution of iron and manganese. It uses the segmentation reaction of aerogel to obtain lithium iron manganese phosphate with uniform particles, but the method adds low-density aerogel to reduce the compaction density of lithium iron manganese phosphate. For example, CN106340620A provides a preparation method of lithium iron manganese phosphate / carbon composite positive electrode material for lithium batteries, which uses ammonium oxalate as a precipitant to obtain an oxalic acid iron manganese precursor, and then synthesizes lithium iron manganese phosphate with a lithium source, a phosphorus source and a carbon source. This method uses oxalate as a raw material, and anhydrous ethanol needs to be used as a solvent when synthesizing lithium iron manganese phosphate, resulting in increased costs. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a method for preparing lithium iron manganese phosphate, a positive electrode material for a lithium battery, and its precursor, and the specific technical scheme is as follows:

[0005] The first aspect of the present invention provides a method for preparing lithium manganese iron phosphate, comprising the following steps:

[0006] (1) mixing ferric pyrophosphate with pure water to dissolve the ferric pyrophosphate to obtain a mixed solution A, and continuously stirring;

[0007] (2) adding a manganese source and a phosphorus source to the mixed solution A in sequence and mixing them thoroughly to obtain a mixed solution B;

[0008] (3) preparing a fixing solution C by mixing an alkaline solution and pure water, and then rapidly spraying the fixing solution C into the continuously stirred mixed solution B, and continuing to spray until the pH value of the mixed solution B stabilizes at 6 to 6.5;

[0009] (4) Filter the mixed slurry in step (3) to obtain a filter cake, wash the filter cake repeatedly with pure water, and then vacuum dry to obtain a lithium manganese iron phosphate precursor.

[0010] After the ferric pyrophosphate is dissolved in step (1), the pH value of the mixed solution A is adjusted to ≤0.8, and the pH value in step (3) is 6.5-7.0.

[0011] The manganese source in the step (2) is one or more manganese sources soluble in water or acid, and the phosphorus source is one or more phosphorus sources soluble in water or acid.

[0012] More specifically, the manganese source is one or more of manganous chloride, manganous sulfate, manganous carbonate, manganous carbonate and manganous nitrate;

[0013] The phosphorus source is one or more of phosphoric acid, diammonium phosphate, monoammonium phosphate and ammonium phosphate.

[0014] The manganese source and phosphorus source in (1) are respectively Mn and PO4 3- The molar ratio of the iron source and the manganese source in the mixed solution B is 1.0:(1.0-1.1); the sum of the concentrations of the iron source and the manganese source in the mixed solution B is calculated by Fe + , Mn is calculated to be 0.5~2mol / L.

[0015] The alkaline solution in (3) is one of a hydrazine hydrate solution and an ethylenediamine solution or a mixture of the two, and the total amount of hydrazine hydrate, ethylenediamine or hydrazine hydrate and ethylenediamine in the fixed solution C in (3) accounts for 1 to 20 wt%.

[0016] The time for adding the fixing solution C in step (3) is 20-40 seconds.

[0017] The washing end condition in step (4) is that the conductivity of the final washed filtrate is ≤0.5 mS / cm.

[0018] A second aspect of the present invention provides a method for producing lithium manganese iron phosphate, comprising the following steps:

[0019] (1) mixing a lithium iron manganese phosphate precursor, a lithium source, a carbon source and pure water and then sand-milling the mixture to obtain a slurry;

[0020] (2) spray drying the sand-milled slurry obtained in (1) to obtain a powder;

[0021] (3) Finally, the powder obtained in (2) is calcined, crushed and sieved under the protection of inert gas to obtain lithium manganese iron phosphate.

[0022] The sand grinding process controls the D50 of the slurry to be ≤250nm.

[0023] The lithium source is one or more of lithium carbonate, lithium hydroxide and lithium nitrate.

[0024] The carbon source is one or more of starch, sucrose, glucose, polyethylene glycol and citric acid.

[0025] The lithium source and the lithium iron manganese phosphate precursor are respectively Li and Fe + The molar ratio of Mn to lithium source is (1.0-1.05):1, and the carbon source accounts for 5-10wt% of the total amount of the lithium source, lithium manganese iron phosphate precursor and the carbon source.

[0026] Ferric pyrophosphate is slightly soluble in water and is generally used as an iron source in the solid phase method for preparing lithium manganese iron phosphate. It cannot be used as an iron source in the liquid phase method. However, the use of ferric pyrophosphate as an iron source in the solid phase method for preparing lithium manganese iron phosphate is prone to uneven distribution of manganese and iron. The method of the present invention uses ferric pyrophosphate as an iron source in the liquid phase method to prepare lithium manganese iron phosphate. Compared with the prior art, the beneficial effects are as follows:

[0027] The present invention adopts ferric pyrophosphate as the iron source of lithium manganese iron phosphate by liquid phase method, adjusts the pH value to dissolve the ferric phosphate, and then evenly mixes it with a soluble manganese source and a soluble phosphorus source, and controls the pH value of the mixed solution to make it clear; then, a uniform mixture of ferric phosphate and ammonium manganese phosphate, which are iron manganese phosphorus precursors, is prepared by coprecipitation method at a suitable pH value, and finally the lithium manganese iron phosphate is prepared by the precursor. The method of the present invention breaks through the conventional operation of only using ferric pyrophosphate as the iron source of solid phase method, and the manganese iron in the prepared lithium manganese iron phosphate is evenly distributed and has good electrical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a TEM image of the lithium manganese iron phosphate precursor prepared in Example 1.

[0029] Figure 2 This is the TEM-Mapping image of the lithium manganese iron phosphate precursor prepared in Example 1.

[0030] Figure 3 This is the TEM image of the lithium manganese iron phosphate precursor prepared in Comparative Example 1.

[0031] Figure 4 This is the TEM-Mapping image of the lithium manganese iron phosphate precursor prepared in Comparative Example 1. DETAILED DESCRIPTION

[0032] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0033] Example 1

[0034] 1. Preparation of lithium manganese iron phosphate precursor

[0035] (1) 2.79 g of ferric pyrophosphate was mixed with 37.5 g of pure water, and the pH value was adjusted to 0.5 to obtain a mixed solution A, which was kept stirring;

[0036] (2) adding 3.40 g of manganese sulfate and 2.72 g of ammonium dihydrogen phosphate to the mixed solution A in sequence and mixing thoroughly to obtain a mixed solution B;

[0037] (3) preparing a fixed solution C containing 5 wt% hydrazine hydrate, and then quickly spraying the fixed solution C into the continuously stirred mixed solution B, and stopping when the pH value of the mixed solution B stabilizes at 6. The entire spraying time is 40 s. After the spraying is completed, stirring is continued for a period of time to obtain a mixed slurry;

[0038] (4) filtering the mixed slurry in step (3) to obtain a filter cake, repeatedly washing the filter cake with pure water until the conductivity of the last washing filtrate is ≤0.5 mS / cm, and vacuum drying to obtain a precursor.

[0039] Figure 1 This is a TEM image of the lithium manganese iron phosphate precursor prepared in this embodiment. The primary particle size of the precursor synthesized in this embodiment is about 30 to 80 nm. Figure 2 This is a TEM-Mapping image of the lithium manganese iron phosphate precursor prepared in this embodiment. It can be seen from the image that the oxygen, phosphorus, manganese and iron elements in the precursor material synthesized in this embodiment are uniformly distributed.

[0040] 2. Preparation of lithium manganese iron phosphate

[0041] 1) The lithium manganese iron phosphate precursor obtained in 1 is mixed with 1.46 g of lithium carbonate, 0.73 g of glucose, and 23.57 g of pure water and then sand-milled. The D50 of the slurry after sand milling is ≤250 nm. The slurry after sand milling is spray-dried at an air inlet temperature of 150° C. and an air outlet temperature of 75° C. to obtain a powder after spray drying.

[0042] 2) The powder dried in step 1) is calcined at 600° C. for 8 h under nitrogen protection, crushed and sieved to obtain lithium manganese iron phosphate material.

[0043] Example 2

[0044] The method and steps are the same as those in Example 1, except that during the preparation of the lithium manganese iron phosphate precursor, the pH value is adjusted to 0.8.

[0045] Example 3

[0046] The method and steps are the same as those in Example 1, except that during the preparation of the lithium manganese iron phosphate precursor, the pH value is adjusted to 1.0.

[0047] Example 4

[0048] The method and steps are the same as those in Example 1, except that during the preparation of the lithium manganese iron phosphate precursor, the spraying is stopped after the pH value of the mixed solution B stabilizes at 6.5.

[0049] Example 5

[0050] The method and steps are the same as those in Example 1, except that in the preparation process of the lithium iron manganese phosphate precursor, a fixed solution C containing 20 wt% of hydrazine hydrate is prepared.

[0051] Example 6

[0052] The method and steps are the same as those in Example 1, except that the amount of ferric pyrophosphate added is 2.10 g, the amount of manganese sulfate added is 3.96 g, and the amount of ammonium dihydrogen phosphate added is 3.17 g.

[0053] Comparative Example 1

[0054] (1) 2.79 g of ferric pyrophosphate, 1.72 g of manganese tetraoxide, 2.59 g of ammonium dihydrogen phosphate and 18.25 g of pure water were mixed and sand-milled. The D50 of the slurry after sand-milling was ≤ 250 nm. The slurry after sand-milling was spray-dried at an air inlet temperature of 150 ° C and an air outlet temperature of 75 ° C. After spray drying, a lithium manganese iron phosphate precursor was obtained.

[0055] Figure 3 This is a TEM image of the lithium manganese iron phosphate precursor prepared in this comparative example. The primary particle size of the precursor synthesized in this comparative example is about 50-150 nm.

[0056] Figure 4 This is the TEM-Mapping image of the lithium manganese iron phosphate precursor prepared in this comparative example. It can be seen from the figure that the elements of oxygen, phosphorus, manganese and iron in the precursor material synthesized in this comparative example are non-uniformly distributed.

[0057] (2) The lithium iron manganese phosphate precursor prepared in this example, 1.46 g of lithium carbonate, 0.74 g of glucose and 23.91 g of pure water were mixed and sand-milled. The D50 of the slurry after sand-milling was ≤ 250 nm. The slurry after sand-milling was spray-dried to obtain a powder. The powder was then calcined at 650° C. for 10 h under nitrogen protection, and then crushed and sieved to obtain the lithium iron manganese phosphate material.

[0058] Test Results

[0059]

[0060] In the embodiment of the present invention, a lithium iron manganese phosphate precursor is synthesized by a co-precipitation method using ferric pyrophosphate as an iron source, and manganese and iron are evenly distributed in the synthesized precursor. A lithium source and a carbon source are then added to the precursor to finally synthesize a lithium iron manganese phosphate material. In comparative example 1, a lithium iron manganese phosphate material is synthesized by a solid phase method using ferric pyrophosphate, and the prepared lithium iron manganese phosphate materials are tested respectively. The above test results show that the discharge capacity and rate performance of the lithium iron manganese phosphate prepared by the method of the present invention are improved.

Claims

1. A method for preparing a lithium manganese iron phosphate precursor, characterized in that: The following steps are involved: (1) mixing ferric pyrophosphate with pure water to dissolve the ferric pyrophosphate to obtain a mixed solution A. After the ferric pyrophosphate is dissolved, the pH value of the mixed solution A is adjusted to ≤ 0.8, and stirring is continued; (2) adding a manganese source and a phosphorus source to the mixed solution A in sequence and mixing them thoroughly to obtain a mixed solution B; (3) Mix an alkaline solution and pure water to prepare a fixed solution C, wherein the alkaline solution is a mixture of one or both of a hydrazine hydrate solution and an ethylenediamine solution, and then quickly spray the fixed solution C into the continuously stirred mixed solution B, and keep spraying until the pH value of the mixed solution B stabilizes at 6 to 6.5; (4) The mixed slurry in step (3) is filtered to obtain a filter cake, the filter cake is repeatedly washed with pure water, and then vacuum dried to obtain a lithium iron manganese phosphate precursor, wherein the lithium iron manganese phosphate precursor is a uniform mixture of iron phosphate and ammonium manganese phosphate, which are iron manganese phosphorus precursors.

2. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The manganese source in step (2) is one or more manganese sources soluble in water or acid, and the phosphorus source is one or more phosphorus sources soluble in water or acid.

3. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The manganese source and phosphorus source in (2) are respectively Mn and PO4 3- The molar ratio of the iron source to the manganese source is 1.0: (1.0~1.1); the sum of the concentrations of the iron source and the manganese source in the mixed solution B is 0.5~2 mol / L as Fe+Mn.

4. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: In the fixing solution C in (3), the proportion of hydrazine hydrate, ethylenediamine or the sum of hydrazine hydrate and ethylenediamine added is 0-20wt%.

5. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The time for adding the fixing solution C in step (3) is 20-40 seconds.

6. The method for preparing a lithium iron manganese phosphate precursor according to claim 1, characterized in that: The washing end condition in step (4) is that the conductivity of the final washed filtrate is ≤0.5 mS / cm.

7. A method for preparing lithium manganese iron phosphate using the lithium manganese iron phosphate precursor prepared by the method according to any one of claims 1 to 6, characterized in that: The following steps are involved: (1) mixing a lithium iron manganese phosphate precursor, a lithium source, a carbon source and pure water and then sand-milling the mixture to obtain a slurry; (2) spray drying the sand-milled slurry obtained in (1) to obtain a powder; (3) Finally, the powder obtained in (2) is calcined, crushed and sieved under the protection of inert gas to obtain lithium manganese iron phosphate.

8. The method for preparing lithium manganese iron phosphate according to claim 7, characterized in that: The molar ratio of the lithium source to the lithium iron manganese phosphate precursor is (1.0-1.05):1, calculated as Li and Fe+Mn, respectively.

9. The method for preparing lithium manganese iron phosphate according to claim 7, characterized in that: The carbon source accounts for 5-10wt% of the total amount of the lithium source, the lithium iron manganese phosphate precursor and the carbon source.

10. A lithium manganese iron phosphate prepared by the method according to any one of claims 7 to 9, characterized in that: Used in the field of lithium battery positive electrode materials.

Images