A porous manganese iron lithium phosphate / carbon composite material and a preparation method thereof

Abstract

The application belongs to the field of battery material preparation, and particularly relates to a porous lithium manganese iron phosphate / carbon composite material and a preparation method thereof. The preparation method comprises the following steps: dissolving an iron source, a manganese source and a lithium source in deionized water, and then adding polyethylene oxide, polyvinylpyrrolidone and a phosphorus source; then, a gel promoter is added, ultrasonic degassing is performed, and then the mixture is left to stand at room temperature to obtain a wet gel; the wet gel is aged and dried to obtain a dry gel; the dry gel is calcined at 400-600 DEG C under inert gas protection for 2+ / -0.1 h, and then cooled to room temperature to obtain the lithium manganese iron phosphate / carbon composite material. The method can optimize the electrochemical performance of the battery by regulating the porous morphology and carbon coating of the lithium manganese iron phosphate / carbon composite material.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of battery material preparation, and particularly relates to a preparation method of a porous lithium manganese iron phosphate / carbon composite material. BACKGROUND

[0002] Lithium manganese iron phosphate, like lithium iron phosphate, is an olivine structure and belongs to the Pnma space group, in which Li, Fe / Mn atoms each occupy the 4a, 4c sites of an octahedron, and P occupies the 4c site of a tetrahedron. The FeO6 / MnO6 octahedron and the PO4 tetrahedron are alternately connected, and have excellent stability; however, since there is no continuous FeO6 octahedron network, electron conduction can only be carried out through Fe-O-Fe, so that the electronic conductivity is low; in addition, the stable structure of the FeO6 / MnO6 octahedron and the PO4 tetrahedron also limits the embedding and extraction of Li ions in the charging and discharging process. Therefore, methods such as carbon coating and special structure design of lithium manganese iron phosphate are the main research directions for improving the ion diffusion rate and conductivity.

[0003] At present, the preparation methods of lithium manganese iron phosphate can be divided into solid phase method and liquid phase method. The solid phase method has limited mixing uniformity and cannot guarantee the stability of the product. The liquid phase method can uniformly mix the reactants at the atomic level, including hydrothermal / solvothermal method, coprecipitation method, sol-gel method and the like. The raw material consumption of the hydrothermal method is large, and the equipment is expensive. The coprecipitation method has the problems of uneven precipitation and environmental pollution. The sol-gel method has low process requirements, but the production cycle is long.

[0004] CN101734927A improves the conductivity of lithium manganese iron phosphate material by adding carbon nanotubes; CN101752561A uses a graphene coating method to improve the electrical properties of lithium manganese iron phosphate material, which shows that carbon coating can effectively improve the conductivity. Patent CN116443839A uses a combination of spray drying and carbon thermal reduction to prepare in-situ carbon-coated lithium manganese iron phosphate, but the preparation process is relatively complex and is not conducive to large-scale production; patent CN116344762A prepares a spherical porous carbon thin layer coated lithium manganese iron phosphate by MOF derivation, but the pore structure is not obvious. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a porous lithium manganese iron phosphate / carbon composite material and a preparation method thereof.

[0006] To solve the above technical problems, the present application provides a preparation method of a porous lithium manganese iron phosphate / carbon composite material, comprising the following steps:

[0007] Step one: 7.2 mmol of iron source, 0.8±0.08 mmol of manganese source, 8±0.1 mmol of lithium source were dissolved in 5-10 mL of deionized water to obtain solution A;

[0008] Step two: 0.2±0.02 g of polyethylene oxide, 0.4-1.2 g of polyvinylpyrrolidone, 8±0.1 mmol of phosphorus source were added to solution A respectively, and then mixed uniformly under vigorous stirring (until completely dissolved) to obtain sol B;

[0009] Step three: 1.1±0.1 mL of gel promoter was slowly added (the addition time was about 50-70 s) to the stirring sol B under low temperature (0-5℃) and stirring condition, and then the wet gel was obtained after ultrasonic degassing and standing at room temperature (the standing time was 5-10 min);

[0010] Step four: the wet gel obtained in step three was aged at 60±5℃ for 24±1 h, and then dried (60±10℃ air drying for 48±4 h) to obtain a dry gel;

[0011] Step five: the dry gel obtained in step four was calcined at 400-600℃ for 2±0.1 h under the protection of inert gas (such as N2), and then cooled to room temperature to obtain a lithium manganese iron phosphate / carbon composite material.

[0012] As an improvement of the preparation method of the lithium manganese iron phosphate / carbon composite material of the present application, in step one:

[0013] The iron source is any one of ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferric sulfate, ferrous sulfate, and ferric acetate;

[0014] The manganese source is any one of manganese nitrate, manganous nitrate, manganese chloride, manganous chloride, manganese sulfate, manganous sulfate, and manganese acetate;

[0015] The lithium source is any one of lithium nitrate, lithium hydroxide, lithium carbonate, lithium acetate, lithium hydrogen phosphate, and lithium dihydrogen phosphate;

[0016] The phosphorus source is any one of phosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, lithium hydrogen phosphate, and lithium dihydrogen phosphate;

[0017] The gel promoter is any one of ammonia, ammonium phosphate, ammonium hydrogen phosphate, and propylene oxide.

[0018] As a further improvement of the preparation method of the lithium manganese iron phosphate / carbon composite material of the present application, in step two, the average molecular weight of the polyethylene oxide is 100000, and the average molecular weight of the polyvinylpyrrolidone is 10000.

[0019] As a further improvement of the preparation method of the lithium iron manganese phosphate / carbon composite material of the present application, the inert gas used in the pyrolysis in step five is any one of helium, nitrogen and argon;

[0020] Thus, the carbon base in the material can reduce the high-valence Fe ions to low-valence Fe ions and maintain the low-valence state, and in-situ carbon coating is performed, while the three-dimensional co-continuous macroporous structure is reserved, and the porous lithium iron manganese phosphate / carbon composite material is prepared.

[0021] As a further improvement of the preparation method of the lithium iron manganese phosphate / carbon composite material of the present application, the general stirring speed is 800-1000 r / min, and the vigorous stirring is 1600-2000 r / min.

[0022] All the stirring is magnetic stirring.

[0023] As a further improvement of the preparation method of the lithium iron manganese phosphate / carbon composite material of the present application, the following steps are included:

[0024] Step one, 7.2 mmol of iron chloride hexahydrate, 0.8 mmol of manganese chloride tetrahydrate and 8 mmol of lithium carbonate are added to 8±0.5 mL of deionized water, and stirred until the iron chloride hexahydrate, manganese chloride tetrahydrate and lithium carbonate are completely dissolved (the stirring time is about 10-30 min), to obtain solution A;

[0025] Step two, 0.2 g of polyethylene oxide and 0.8 g of polyvinylpyrrolidone are added to solution A, and 8 mmol of phosphoric acid is added, and stirred vigorously for 10-90 min, to obtain sol B;

[0026] Step three, under stirring, 1.1 mL of propylene oxide as a gel promoter is slowly added (the adding time is about 50-70 s) to sol B at a low temperature of 0-2℃;

[0027] Then, the gel is placed (the standing time is 5-10 min) after ultrasonic degassing (about 20-40 s) to obtain a wet gel;

[0028] Step four, the wet gel obtained in step three is placed in an oven and aged at 60±5℃ for 24±1 h, and then dried (60±10℃ air drying for 48±4 hours) to obtain a dry gel;

[0029] Step five: the dry gel obtained in step four is calcined at 400±10℃ for 2±0.1 h under N2 protection, and then cooled to room temperature to obtain the lithium iron manganese phosphate / carbon composite material.

[0030] The present application also simultaneously provides the lithium iron manganese phosphate / carbon composite material prepared by any one of the above methods.

[0031] The obtained lithium manganese iron phosphate / carbon composite material has a three-dimensional co-continuous macroporous structure, and the crystal structure is olivine type,

[0032] The sol-gel method with phase separation is used to prepare the porous lithium manganese iron phosphate / carbon composite material, in-situ carbon coating is realized, and more pore structures are reserved.

[0033] The method of the present application regulates the porous morphology and carbon coating of the lithium manganese iron phosphate / carbon composite material, so as to improve the electronic conductivity and lithium ion diffusion rate, and optimize the battery electrochemical performance.

[0034] In the present application:

[0035] In step one, polyethylene oxide and polyvinylpyrrolidone are used to regulate the phase separation process together, so as to form a uniform macroporous skeleton structure, and then propylene oxide is used as a gel promoter to increase the pH of the solution to gel, so as to reserve the skeleton structure.

[0036] In step five, the organic matter can be reduced to carbon to obtain lithium manganese iron phosphate under inert gas such as N2, while the macroporous skeleton and rich mesoporous structure are reserved.

[0037] Compared with the prior art, the present application has the following beneficial effects:

[0038] 1) The sol-gel method is used to prepare the porous lithium manganese iron phosphate / carbon composite material, which has a unique three-dimensional co-continuous macroporous skeleton structure.

[0039] 2) The organic matter can be reduced to carbon to obtain lithium manganese iron phosphate under inert atmosphere, while the unique macroporous skeleton and rich mesoporous structure are reserved.

[0040] 3) The sol-gel method realizes in-situ carbon coating, while the co-continuous structure is reserved.

[0041] In summary, the organic polymer (polyethylene oxide, polyvinylpyrrolidone) is introduced for compounding, the organic polymer is cracked, and finally the porous lithium manganese iron phosphate / carbon composite material is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0042] The specific embodiments of the present application will be described in further detail below with reference to the accompanying drawings.

[0043] Figure 1 SEM photograph of the prepared porous lithium manganese iron phosphate / carbon composite material obtained from step 5) of Example 1.

[0044] Figure 2 SEM photograph of the prepared porous lithium manganese iron phosphate / carbon composite material obtained from step 5) of Example 1.

[0045] Figure 3 XRD pattern of the prepared porous lithium manganese iron phosphate / carbon composite material obtained from step 5) of Example 1.

[0046] Figure 4 SEM pattern of the prepared porous lithium manganese iron phosphate / carbon composite material obtained from Example 2-1.

[0047] Figure 5 SEM pattern of the prepared porous lithium manganese iron phosphate / carbon composite material obtained from Example 2-2.

[0048] Figure 6 SEM pattern of the prepared porous lithium manganese iron phosphate / carbon composite material obtained from Example 2-3.

[0049] Figure 7 XRD pattern of the prepared porous lithium manganese iron phosphate / carbon composite material obtained from Example 2-4.

[0050] Figure 8 SEM pattern of the prepared lithium manganese iron phosphate xerogel obtained from Example 3-1.

[0051] Figure 9 SEM pattern of the prepared lithium manganese iron phosphate xerogel obtained from Example 3-2.

[0052] Figure 10 SEM pattern of the prepared lithium manganese iron phosphate xerogel obtained from Example 4-2. DETAILED DESCRIPTION

[0053] The present application will be further described below with reference to specific examples, but the scope of protection of the present application is not limited to the following:

[0054] The stirring speed is 800-1000 r / min, and the intense stirring speed is 1600-2000 r / min; both are magnetic stirring.

[0055] The average molecular weight of the polyethylene oxide is 100000, and the average molecular weight of the polyvinylpyrrolidone is 10000.

[0056] Example 1, a method for preparing a porous lithium manganese iron phosphate / carbon composite material, comprising the following steps in sequence:

[0057] 1) 7.2 mmol of iron chloride hexahydrate, 0.8 mmol of manganese chloride tetrahydrate, and 8 mmol of lithium carbonate were added into 8 mL of deionized water, and stirred until the iron chloride hexahydrate, manganese chloride tetrahydrate, and lithium carbonate were completely dissolved (the stirring time was about 10-30 min), to obtain solution A;

[0058] 2) 0.2 g of polyethylene oxide and 0.8 g of polyvinylpyrrolidone were added into solution A, and 8 mmol of phosphoric acid was added, and stirred vigorously for 70-90 min, to form a uniform transparent sol B;

[0059] 3) 1.1 mL of propylene oxide as a gelation accelerator was slowly added (the adding time was about 60 s) into sol B under stirring at 0°C, at this time, the color of sol B changed from transparent to turbid;

[0060] Then, the gel was placed for standing (the standing time was 5-10 min) after ultrasonic degassing for 30 s, to obtain a wet gel;

[0061] 4) The wet gel was placed in an oven for aging at 60°C for 24 h, and then evaporated and dried at 60°C for 48 h, to obtain a macroporous gel (dry gel).

[0062] The SEM photograph of the dry gel is shown in Figure 1 According to Figure 1 , it can be known that the dry gel has a clear three-dimensional co-continuous macroporous skeleton structure. The porosity of the obtained dry gel was 61.4%.

[0063] The porosity was detected according to the conventional GB / T 21650.1-2008.

[0064] 5) The macroporous gel (dry gel) obtained in step 4) was placed in a N2 atmosphere for heat treatment at a calcination temperature of 400°C for 2 h; after cooling to room temperature, a lithium manganese iron phosphate / carbon composite material was obtained. The SEM photograph of the lithium manganese iron phosphate / carbon composite material is shown in Figure 2 According to Figure 2 , it can be known that the porous lithium manganese iron phosphate / carbon composite material retained the original three-dimensional co-continuous macroporous skeleton structure, that is, the obtained lithium manganese iron phosphate / carbon composite material still had an intact three-dimensional co-continuous skeleton structure. The specific surface area was 12.4 m 2 / g, detected by BET.

[0065] The specific surface area was detected according to the conventional GB / T 21650.1-2008.

[0066] The lithium manganese iron phosphate / carbon composite material was prepared by the method as Figure 3The XRD pattern shows that the lithium manganese iron phosphate / carbon composite material has good crystallinity (good crystallinity) and has formed lithium manganese iron phosphate crystals.

[0067] Example 2-1: The calcination temperature in Example 1 was changed to 600℃, otherwise it remained the same as in Example 1. Figure 4 The SEM images shown reveal that the prepared lithium manganese iron phosphate / carbon composite material still possesses a relatively clear three-dimensional co-continuous structure, but large grains precipitate within the loose framework. The specific surface area measured by BET is 13.4 m². 2 / g.

[0068] Example 2-2: The calcination temperature in Example 1 was changed to 800℃, otherwise it remained the same as in Example 1. Figure 5 The SEM images shown indicate that the three-dimensional co-continuous structure of the prepared lithium manganese iron phosphate / carbon composite material underwent local collapse, with localized grain growth. The specific surface area measured by BET was 27.1 m². 2 / g.

[0069] Examples 2-3: The calcination temperature in Example 1 was changed to 1000℃, otherwise it remained the same as in Example 1. Figure 6 The SEM images shown reveal significant collapse of the three-dimensional framework structure of the prepared lithium manganese iron phosphate / carbon composite material. The specific surface area measured by BET is 57.9 m². 2 / g.

[0070] Examples 2-4: The calcination temperature in Example 1 was changed to 200℃, otherwise it remained the same as in Example 1. Figure 7 The XRD pattern shows that the lithium manganese iron phosphate / carbon composite material was not formed due to the low calcination temperature.

[0071] Example 3-1: The polyvinylpyrrolidone in Example 1 was replaced with 0.4g, and the rest was the same as steps 1-4 in Example 1. SEM images of the prepared lithium manganese iron phosphate dry gel are shown below. Figure 8 As shown, by Figure 8 It is evident that its pore structure is relatively small, which is detrimental to maintaining the pore structure after calcination. The porosity of the prepared lithium manganese iron phosphate dry gel is only 42.5%.

[0072] Example 3-2: The polyvinylpyrrolidone in Example 1 was replaced with 1.2g, and the rest of the steps were the same as in Steps 1-4 of Example 1. SEM images of the prepared lithium manganese iron phosphate dry gel are shown below. Figure 9 As shown in Figure 9, the pore structure is relatively small, which is not conducive to maintaining the pore structure after calcination. The porosity of the prepared lithium manganese iron phosphate dry gel is 48.1%.

[0073] Example 4-1: Change the propylene oxide in Example 1 to 0 mL, and the rest is the same as steps 1-4 in Example 1. It is found that the reaction solution cannot undergo sol-gel transition. This is because the sol-gel method used in the present application is through the ring-opening reaction of propylene oxide with chloride ions, which consumes hydrogen ions to cause the pH to rise, thereby promoting hydrolytic polymerization to complete the sol-gel transition. When the amount of propylene oxide added is 0 g, the pH does not change to initiate hydrolytic polymerization, so it cannot gel.

[0074] Example 4-2: Change the propylene oxide in Example 1 to 1.5 mL, and the rest is the same as steps 1-4 in Example 1. It is found that the dry gel prepared also exhibits a three-dimensional co-continuous skeleton, but the pore structure is greatly reduced, as shown in Figure 10 Therefore, it is not recommended to use.

[0075] Example 5-1: Change the iron source in Example 1 to ferrous chloride, and the rest is the same as Example 1. It is found that the reaction solution can undergo sol-gel transition, but a three-dimensional co-continuous skeleton structure cannot be obtained. This is because the sol-gel method used in the present application is through the hydrolytic polymerization of metal cations to form a gel network, and the hydrolysis ability of divalent iron ions is weaker than that of trivalent iron ions, which cannot undergo high hydrolytic polymerization like trivalent iron ions, so it is difficult to form a three-dimensional co-continuous skeleton structure.

[0076] Example 5-2: Change the iron source and manganese source in Example 1 to iron nitrate and manganese nitrate, and the rest is the same as Example 1. It is found that the reaction solution cannot undergo sol-gel transition. This is because in the sol-gel principle used in the present application, propylene oxide added can undergo ring-opening reaction with chloride ions, H2O, and nitrate ions (reaction sequence: chloride ions > H2O > nitrate ions). When chloride salt is used, propylene oxide preferentially undergoes ring-opening reaction with chloride ions while consuming hydrogen ions, causing the solution pH to rise, promoting hydrolytic polymerization to complete the sol-gel transition process. When changed to nitrate salt, PO will preferentially react with water in the solution to generate 1,2-propanediol, but it does not consume hydrogen ions, so the pH does not rise, and therefore the sol-gel process cannot proceed.

[0077] In summary, the formula corresponding to Example 1, i.e., 0.2 g of polyethylene oxide and 0.8 g of polyvinylpyrrolidone are added at the same time, and the porous lithium iron manganese phosphate / carbon composite material prepared has good porous structure and stability (i.e., the three-dimensional skeleton structure before calcination is still maintained after calcination, without collapse).

[0078] Finally, it should be noted that the above enumeration is only several specific embodiments of the present application. Obviously, the present application is not limited to the above embodiments, and there are many variations. All variations that can be directly derived or inferred by those of ordinary skill in the art from the disclosure of the present application should be considered within the scope of the present application.

Claims

1. A method for preparing a porous lithium manganese iron phosphate / carbon composite material, characterized in that... Includes the following steps: Step 1: Dissolve 7.2 mmol of iron source, 0.8 ± 0.08 mmol of manganese source, and 8 ± 0.1 mmol of lithium source in 5–10 mL of deionized water and mix thoroughly to obtain solution A; Step 2: Add 0.2±0.02g polyethylene oxide, 0.4~1.2g polyvinylpyrrolidone, and 8±0.1mmol phosphorus source to solution A respectively, and mix vigorously until homogeneous to obtain sol B; Step 3: Slowly add 1.1±0.1 mL of gelation accelerator to the stirred sol B at low temperature, degas by sonication, and let stand at room temperature to obtain wet gel; Step 4: The wet gel obtained in Step 3 is aged at 60±5℃ for 24±1h, and then dried to obtain a dry gel; Step 5: Calcine the dry gel obtained in Step 4 at 400-600℃ for 2±0.1h under inert gas protection, and then cool it to room temperature to obtain lithium manganese iron phosphate / carbon composite material.

2. The preparation method of the lithium manganese iron phosphate / carbon composite material according to claim 1, characterized in that... In step one: The iron source is any one of ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferrous sulfate, ferrous sulfate, and ferric acetate; The manganese source is any one of manganese nitrate, manganese nitrate, manganese chloride, manganese chloride, manganese sulfate, manganese sulfate, and manganese acetate; The lithium source is any one of lithium nitrate, lithium hydroxide, lithium carbonate, lithium acetate, lithium dihydrogen phosphate, and lithium dihydrogen phosphate; The phosphorus source is any one of phosphoric acid, ammonium phosphate, ammonium hydrogen phosphate, dilithium hydrogen phosphate, and lithium dihydrogen phosphate; The gelation accelerator is any one of ammonia, ammonium phosphate, ammonium hydrogen phosphate, and propylene oxide.

3. The method for preparing the lithium manganese iron phosphate / carbon composite material according to claim 2, characterized in that: In step two, the average molecular weight of polyethylene oxide is 100,000, and the average molecular weight of polyvinylpyrrolidone is 10,000.

4. The method for preparing the lithium manganese iron phosphate / carbon composite material according to claim 3, characterized in that: The inert gas used for pyrolysis in step five is any one of helium, nitrogen, or argon.

5. The method for preparing the lithium manganese iron phosphate / carbon composite material according to claim 4, characterized in that: The stirring speed is 800-1000 r / min; vigorous stirring is 1600-2000 r / min.

6. The method for preparing lithium manganese iron phosphate / carbon composite material according to any one of claims 1 to 5, characterized in that... Includes the following steps: Step 1: Add 7.2 mmol ferric chloride hexahydrate, 0.8 mmol manganese chloride tetrahydrate, and 8 mmol lithium carbonate to 8 ± 0.5 mL of deionized water and stir until ferric chloride hexahydrate, manganese chloride tetrahydrate, and lithium carbonate are completely dissolved to obtain solution A; Step 2: Add 0.2g of polyethylene oxide and 0.8g of polyvinylpyrrolidone to solution A, and add 8mmol of phosphoric acid. Stir vigorously for 10-90 minutes to obtain sol B. Step 3: Under stirring conditions, at a low temperature of 0-2℃, slowly add 1.1 mL of propylene oxide as a gelation promoter to sol B; Then, after ultrasonic degassing, the gel was allowed to stand to obtain a wet gel. Step 4: Place the wet gel obtained in Step 3 in an oven and age it at 60±5℃ for 24±1h, then dry it to obtain a dry gel. Step 5: The dry gel obtained in Step 4 is calcined at 400±10℃ for 2±0.1h under N2 protection and then cooled to room temperature to obtain lithium manganese iron phosphate / carbon composite material.

7. The lithium manganese iron phosphate / carbon composite material prepared by any one of claims 1 to 6.

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