A lithium iron manganese phosphate positive electrode material and a preparation method thereof

Abstract

The application relates to the technical field of battery material preparation, and discloses a lithium manganese iron phosphate positive electrode material and a preparation method thereof. The method comprises the following steps: S1, adding a ferrous salt and a manganese salt into a phosphorus salt solution, and then adding a reducing agent to obtain a mixed solution; S2, adjusting the pH of the mixed solution to 6-6.5, adding a metal ion dopant, and then performing a first stirring reaction, so as to obtain a precursor doped with metal ions after separation; S3, adding the precursor doped with metal ions, a lithium source, an organic acid and a crosslinking agent into deionized water, adjusting the pH to 6-7, and then performing a second stirring reaction, so as to obtain a gel substance after separation; S4, performing first calcination on the gel substance in an inert atmosphere, so as to obtain an intermediate product; S5, adding the intermediate product and an organic carbon source into deionized water to obtain a slurry; and S6, performing ball milling and drying on the slurry, and then performing second calcination, so as to obtain the lithium manganese iron phosphate positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of battery material preparation technology, specifically, it relates to a lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] LMFP batteries are a type of lithium-ion battery based on lithium iron phosphate (LFP) batteries, manufactured by replacing some of the iron used as the positive electrode material with manganese. They offer the advantage of achieving higher energy density than LFP while maintaining the same cost and safety levels. The high voltage platform of manganese-iron phosphate batteries results in higher energy density, with cycle life and safety comparable to LFP, and better low-temperature performance. In terms of low-temperature performance, ternary > LMFP > LFP. Ternary materials generally retain over 80% of their capacity at -20℃. LFP and LMFP, due to their lower conductivity, have worse low-temperature performance than ternary materials. In terms of cycle life, LFP > LMFP > ternary.

[0003] While lithium manganese iron phosphate (LFP) theoretically possesses the safety and cycle advantages of lithium iron phosphate (LFP) and has a higher energy density, it suffers from low conductivity, a dual voltage plateau, and manganese leaching, leading to difficulties in industrial production and relatively poor material performance. Therefore, improving the specific capacity, rate performance, and stability of LFP is a pressing issue that needs to be addressed. Summary of the Invention

[0004] This application aims to address the technical problems of poor specific capacity, rate performance, and stability of lithium manganese iron phosphate in existing technologies.

[0005] Therefore, the primary objective of this invention is to provide a method for preparing lithium manganese iron phosphate cathode material, which improves the specific capacity, rate performance, and stability of lithium manganese iron phosphate by increasing its electronic conductivity and reducing the migration impedance of lithium ions.

[0006] Specifically, a method for preparing a lithium manganese iron phosphate cathode material includes the following steps:

[0007] S1 adds ferrous and manganese salts to a phosphate salt solution, followed by the addition of a reducing agent to obtain a mixed solution;

[0008] S2 adjusts the pH of the mixture to 6-6.5, adds a metal ion dopant, and then performs a first stirring reaction. After separation, a precursor doped with metal ions is obtained.

[0009] S3 The precursor doped with metal ions, along with a lithium source, organic acid, and crosslinking agent, is added to deionized water, and the pH is adjusted to 6-7. Then, a second stirring reaction is carried out, and a gel substance is obtained after separation.

[0010] S4 involves calcining the gel material in an inert atmosphere to obtain an intermediate product;

[0011] S5. The intermediate product and organic carbon source are added to deionized water to obtain a slurry;

[0012] S6 ball mills and dries the slurry, followed by a second calcination to obtain lithium manganese iron phosphate cathode material.

[0013] Furthermore, the temperature of the first stirring reaction is 30-40°C, the stirring speed is 200-350 r / min, and the reaction time is 1.5-3 h; the temperature of the second stirring reaction is 55-65°C; and the pH is adjusted using an ammonia solution with a mass concentration of 2-5%.

[0014] Furthermore, the molar ratio of the lithium source, the organic acid, the metal ion additive, and the crosslinking agent is 1:(0.004~0.01):(0.004~0.02):(0.01~0.05); the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium oxalate, and lithium acetate; the organic acid is citric acid; the metal ion dopant is a magnesium ion dopant, which includes one or more of magnesium oxide, magnesium sulfate, and magnesium nitrate; the crosslinking agent includes one or more of starch and polyvinylpyrrolidone.

[0015] Furthermore, the heating rate of the first calcination is 2-10℃ / min, the heating endpoint temperature is 400-500℃, and the holding time is 7-9h;

[0016] Furthermore, the second calcination includes a first heating section and a second heating section; wherein, the heating endpoint temperature of the first heating section is 400-500℃, and the holding time is 5-7h; the heating rate of the second heating section is 4-8℃ / min, the heating endpoint temperature is 700-800℃, and the holding time is 5-7h.

[0017] Furthermore, in the slurry, the intermediate product has a mass fraction of 40-50%, and the organic carbon source has a mass fraction of 4-5%; the organic carbon source includes one or more of starch, sucrose, glucose, polyvinyl alcohol, and polyethylene glycol.

[0018] Furthermore, in the mixed solution, the molar concentration of phosphate is 0.1–4 mol / L, and the molar concentrations of ferrous and manganese salts are 0.1–2 mol / L; the reducing agent is ascorbic acid; the ferrous salt includes one or more of ferrous chloride, ferrous oxalate, ferrous carbonate, and ferrous nitrate; the manganese salt includes one or more of manganese chloride, manganese sulfate, and manganese nitrate; and the phosphate salt includes one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, and ammonium phosphate.

[0019] Furthermore, the reaction time of the ball milling is 4 to 6 hours, and the particle size D50 of the solid particles after ball milling is 0.4 μm to 0.5 μm.

[0020] The technical mechanism is as follows:

[0021] (1) The oxidation of metal ions is prevented by adding a reducing agent, and a mixed precipitate of manganese iron phosphate is formed by ferrous salt, manganese salt and phosphate.

[0022] (2) The dopant elements are evenly distributed and the doping effect is good. Predoping is carried out in the solution phase; during this reaction, the dopant metal ions react with phosphate ions to form a mixed precipitate of manganese iron phosphate containing the dopant elements. This can achieve the premixing of the dopant elements with the precursor, thereby avoiding local agglomeration of the dopant elements and improving the doping effect.

[0023] (3) The crosslinking agents or thickeners used are starch and PVP. These enable the substance to become a gel, locking in more water molecules, thus forming a porous structure during the calcination process. At the same time, starch and PVP can also act as organic carbon sources to coat the surface of lithium manganese iron phosphate, increasing the electronic conductivity of the product.

[0024] (4) Using magnesium ion dopants such as magnesium oxide and magnesium sulfate as metal ion additives, lithium manganese iron phosphate prepared with dopants has certain lattice defects, Mg 2+ Doping helps reduce the impedance during the charge transfer process in LMFP, overcoming the kinetic limitations of this process. A small amount of Mg... 2+ This can reduce the cell volume, and the degree of shrinkage increases with increasing doping concentration. It improves the cycle performance and specific capacity of LMFPs.

[0025] (5) Adding organic acids, such as citric acid, during the lithium formulation process neutralizes excess hydroxide ions and provides the organic carbon source required for carbon coating. The carbon coating layer formed by the organic carbon source can improve the rate performance and structural stability of lithium manganese iron phosphate while suppressing the doping of Mn. 2+ The problem of dissolution.

[0026] The second objective of this invention is to provide a lithium manganese iron phosphate cathode material obtained by the preparation method of the lithium manganese iron phosphate cathode material described in any one of the embodiments of the first objective.

[0027] A third objective of the present invention is to provide a lithium-ion battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises the lithium manganese iron phosphate positive electrode material described in the embodiments of the second objective.

[0028] The beneficial effects of this application are as follows:

[0029] This application provides a method for preparing lithium manganese iron phosphate cathode material. By optimizing the preparation method and doping with metal ions, the electronic conductivity of lithium manganese iron phosphate can be effectively improved and the migration impedance of lithium ions can be reduced, thereby improving the high-rate performance and stability of lithium manganese iron phosphate. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0031] First, this invention provides a method for preparing lithium manganese iron phosphate cathode material, the method comprising the following steps:

[0032] S1 adds ferrous and manganese salts to a phosphate salt solution, followed by the addition of a reducing agent to obtain a mixed solution;

[0033] In this invention, the mixed solution contains phosphate with a molar concentration of 0.1–4 mol / L and ferrous and manganese salts with a molar concentration of 0.1–2 mol / L; the reducing agent is ascorbic acid; the ferrous salt includes one or more of ferrous chloride, ferrous oxalate, ferrous carbonate, and ferrous nitrate; the manganese salt includes one or more of manganese chloride, manganese sulfate, and manganese nitrate; and the phosphate salt includes one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, and ammonium phosphate.

[0034] S2 adjusts the pH of the mixture to 6-6.5, adds a metal ion dopant, and then performs a first stirring reaction. After separation, a precursor doped with metal ions is obtained.

[0035] In this invention, the temperature of the first stirring reaction is 30-40°C, the stirring speed is 200-350 r / min, and the reaction time is 1.5-3 h; the pH adjustment is achieved using an ammonia solution with a mass concentration of 2-5%.

[0036] In view of this, the applicant has discovered that by adding a reducing agent to prevent the oxidation of metal ions, a mixed precipitate of manganese iron phosphate is formed by ferrous salt, manganese salt and phosphate.

[0037] The main concept behind the doping in this invention is as follows: The basic preparation approach for existing lithium manganese iron phosphate (LFP) involves directly mixing lithium, manganese, iron, phosphorus, and carbon sources with metal ion dopants, followed by grinding, drying, and sintering processes to obtain LFP material. A problem with this method is that it's difficult to obtain a uniformly mixed material due to the simultaneous mixing of multiple raw materials. The mixing process is generally a solid-liquid mixing process where multiple solids are simultaneously dispersed in a liquid phase. Especially with the addition of low-component dopant compounds, these compounds are prone to adsorption or reaction on the surfaces of other solid raw materials, leading to uneven distribution of the dopant elements. Furthermore, the resulting composite material exhibits low electrochemical performance. In this invention, the dopant elements are evenly distributed, resulting in a better doping effect. Pre-doping is performed in the solution phase; during this reaction, the dopant metal ions react with phosphate ions to form a mixed precipitate of LFP containing the dopant elements. This allows for pre-mixing of the dopant elements with the precursor, thereby avoiding local agglomeration of the dopant elements and improving the doping effect.

[0038] S3 The precursor doped with metal ions, along with a lithium source, organic acid, and crosslinking agent, is added to deionized water, and the pH is adjusted to 6-7. Then, a second stirring reaction is carried out, and a gel substance is obtained after separation.

[0039] In this invention, the temperature of the second stirring reaction is 55-65°C; the pH adjustment is achieved using an ammonia solution with a mass concentration of 2-5%.

[0040] In this invention, the molar ratio of the lithium source, the organic acid, the metal ion additive, and the crosslinking agent is 1:(0.004~0.01):(0.004~0.02):(0.01~0.05); the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium oxalate, and lithium acetate; the organic acid is citric acid; the metal ion dopant is a magnesium ion dopant, which includes one or more of magnesium oxide, magnesium sulfate, and magnesium nitrate; the crosslinking agent includes one or more of starch and polyvinylpyrrolidone.

[0041] The crosslinking agents or thickeners used are starch and PVP. These enable the material to form a gel, locking in more water molecules and thus creating a porous structure during calcination. Simultaneously, starch and PVP act as organic carbon sources, coating the surface of lithium manganese iron phosphate to increase the product's electronic conductivity. Magnesium ion dopants such as magnesium oxide and magnesium sulfate are used as metal ion additives. The preparation of lithium manganese iron phosphate with dopants exhibits certain lattice defects, and Mg... 2+ Doping helps reduce the impedance during the charge transfer process in LMFP, overcoming the kinetic limitations of this process. A small amount of Mg... 2+This can reduce the cell volume, with the degree of shrinkage increasing with increasing doping concentration. It improves the cycle performance and specific capacity of LMFPs. Adding organic acids, such as citric acid, during lithium formulation neutralizes excess hydroxide ions and provides the organic carbon source needed for carbon coating. The carbon coating layer formed by the organic carbon source can improve the rate performance and structural stability of lithium manganese iron phosphate while suppressing the doping of Mn. 2+ The problem of dissolution.

[0042] S4. The gel material is subjected to a first calcination in an inert atmosphere to obtain an intermediate product;

[0043] In this invention, the heating rate of the first calcination is 2-10℃ / min, the heating endpoint temperature is 400-500℃, and the holding time is 7-9h;

[0044] S5. Add the intermediate product and organic carbon source to deionized water to obtain a slurry;

[0045] In this invention, the intermediate product in the slurry has a mass fraction of 40-50%, and the organic carbon source has a mass fraction of 4-5%; the organic carbon source includes one or more of starch, sucrose, glucose, polyvinyl alcohol, and polyethylene glycol.

[0046] S6 ball mills and dries the slurry, followed by a second calcination to obtain lithium manganese iron phosphate cathode material.

[0047] In this invention, the reaction time of ball milling is 4 to 6 hours, and the particle size D50 of the solid particles after ball milling is 0.4 μm to 0.5 μm.

[0048] In this invention, the second calcination includes a first heating section and a second heating section; wherein, the heating endpoint temperature of the first heating section is 400-500℃, and the holding time is 5-7h; the heating rate of the second heating section is 4-8℃ / min, the heating endpoint temperature is 700-800℃, and the holding time is 5-7h.

[0049] The carbon source forms a carbon coating layer on the surface of the intermediate product. During calcination, it transforms into carbon, further forming a carbon coating layer on the surface of the intermediate product. This carbon coating layer not only prevents the agglomeration of lithium manganese iron phosphate cathode material, but also provides more pathways for electron transport on the abundant carbon coating layer on the surface of the lithium manganese iron phosphate cathode material, thereby improving the electrochemical performance of the lithium manganese iron phosphate cathode material. Two-stage heating promotes the formation of the layered phase.

[0050] Second, the present invention provides a lithium manganese iron phosphate cathode material obtained by the preparation method of the lithium manganese iron phosphate cathode material described in any one of the embodiments of the first objective.

[0051] Third, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises the lithium manganese iron phosphate positive electrode material described in the second embodiment.

[0052] <Example>

[0053] Example 1

[0054] The preparation method of the lithium manganese iron phosphate cathode material in Example 1 includes the following steps:

[0055] S11. Add 135g of ferrous oxalate and 125g of manganese chloride to 1000ml of 2mol / L diammonium hydrogen phosphate solution, then add 10g of ascorbic acid to obtain a mixed solution.

[0056] S21. The pH of the mixture was adjusted to 6 by using a 5% ammonia solution, 2.0g of magnesium chloride was added, and then the mixture was stirred at 350r / min for 2h at 40℃. After separation, a green precipitate of doped metal ions was obtained.

[0057] S31. The precipitate, 105g lithium oxalate, 1.5g citric acid and 2.6g starch were added to 2000ml deionized water. The pH was adjusted to 7 with a 5% ammonia solution. The mixture was then stirred at 350r / min for 4h at 60℃. After separation, a gel substance was obtained.

[0058] S41. In a nitrogen atmosphere, the gel material is heated to 450°C in a tube furnace at a heating rate of 2°C / min, and held at 450°C for 8 hours to obtain an intermediate product.

[0059] S51. Add the intermediate product and 8g of starch to 100ml of deionized water to obtain a slurry;

[0060] S61. The slurry was ball-milled for 5 hours, and the average particle size D50 of the solid particles after ball milling was 0.45 μm. It was then dried in an oven at 60°C for 24 hours, and then in a tube furnace under nitrogen atmosphere to obtain lithium manganese iron phosphate cathode material.

[0061] Example 2

[0062] The difference between this embodiment and Embodiment 1 is that ammonium phosphate is used as the phosphate salt.

[0063] Example 3

[0064] The difference between this embodiment and Embodiment 1 is that the crosslinking agent used is polyvinylpyrrolidone.

[0065] Example 4

[0066] The difference between this embodiment and Embodiment 1 is that lithium carbonate is used as the lithium source.

[0067] Example 5

[0068] The difference between this embodiment and Embodiment 4 is that magnesium nitrate is used as the magnesium ion dopant.

[0069] Example 6

[0070] The difference between this embodiment and Embodiment 4 is that the pH of the mixture is adjusted to 6.5.

[0071] <Comparative Example>

[0072] Comparative Example 1

[0073] The difference between this comparative example and Example 1 is that magnesium chloride, a metal ion dopant, is not added.

[0074] Comparative Example 2

[0075] The difference between this comparative example and Example 1 is that the organic acid citric acid is not added.

[0076] Comparative Example 3

[0077] The difference between this comparative example and Example 1 is that no cross-linking agent, starch, is added.

[0078] <Experimental Example>

[0079] Electrochemical performance tests were conducted using Examples 1-6 and Comparative Examples 1-3 as samples.

[0080] (1) Preparation of positive electrode sheet

[0081] Weigh 0.1500 g of polyvinylidene fluoride binder (PVDF) into 5.4 g of N-methylpyrrolidone (NMP) using an analytical balance, stir until completely dissolved; then add 1.2000 g of the above-mentioned positive electrode material (from Examples 1-6 and Comparative Examples 1-3, respectively) and 0.1500 g of carbon black conductive agent (SP), and stir until homogeneous to obtain a paste. The mass ratio of the composite material, polyvinylidene fluoride binder (PVDF), and carbon black conductive agent (SP) is 8:1:1.

[0082] The paste is evenly coated onto carbon cloth using a coater and dried in a vacuum drying oven to remove the solvent NMP. After rolling and punching, a long sheet with a length of 1.6 cm and a width of 0.8 cm is obtained, which serves as the positive electrode sheet.

[0083] (2) Battery assembly

[0084] Using the above-mentioned positive electrode as the positive electrode, lithium metal sheet as the negative electrode, PE-PP composite film as the battery separator, and 1.0 mol / L LiPF6 / (DMC+DMC) as the electrolyte, with EC to DMC volume ratio of 1:1, a CR2032 coin cell was assembled.

[0085] (3) Electrochemical performance testing

[0086] The main purpose of the battery performance test was to determine the discharge specific capacity and capacity retention of the composite material. The above-mentioned coin cells were subjected to cyclic charge-discharge tests at charge-discharge rates of 0.1C, 1.0C, and 5.0C, with a total of 50 cycles. The test temperature was 25.0℃, and the charge-discharge voltage ranged from 2.0 to 4.3V. The results are summarized in Table 1 below.

[0087] Table 1 Electrochemical performance test results

[0088]

[0089] As can be seen from the table, the cathode materials of Examples 1-6 have higher specific capacity and capacity retention at different discharge rates, indicating that the cathode materials of Examples 1-6 are significantly better than those of Comparative Examples 1-3 in terms of actual specific capacity, rate performance, and cycle stability.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1. Ferrous salt and manganese salt are added to a phosphate salt solution, followed by the addition of a reducing agent to obtain a mixed solution; S2 The pH of the mixture is adjusted to 6~6.5, a metal ion dopant is added, followed by a first stirring reaction, and the precursor doped with metal ions is obtained after separation; S3 The precursor doped with metal ions, along with a lithium source, organic acid, and crosslinking agent, is added to deionized water, and the pH is adjusted to 6-7. Then, a second stirring reaction is carried out, and the gel material is obtained after separation. S4 The gel material is subjected to a first calcination in an inert atmosphere to obtain an intermediate product; S5. The intermediate product and organic carbon source are added to deionized water to obtain a slurry; S6 The slurry is ball-milled and dried, and then subjected to a second calcination to obtain lithium manganese iron phosphate cathode material; The molar ratio of the lithium source, the organic acid, the metal ion additive, and the crosslinking agent is 1:(0.004~0.01):(0.004~0.02):(0.01~0.05); the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium oxalate, and lithium acetate; the organic acid is citric acid; the metal ion dopant is a magnesium ion dopant, which includes one or more of magnesium oxide, magnesium sulfate, and magnesium nitrate; the crosslinking agent includes one or more of starch and polyvinylpyrrolidone.

2. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The temperature of the first stirring reaction is 30~40℃, the stirring speed is 200~350r / min, and the reaction time is 1.5~3h; the temperature of the second stirring reaction is 55~65℃; the pH is adjusted using an ammonia solution with a mass concentration of 2~5%.

3. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The heating rate of the first calcination is 2~10℃ / min, the heating endpoint temperature is 400~500℃, and the holding time is 7~9h.

4. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The second calcination includes a first heating section and a second heating section; wherein, the heating endpoint temperature of the first heating section is 400~500℃, and the holding time is 5~7h; the heating rate of the second heating section is 4~8℃ / min, the heating endpoint temperature is 700~800℃, and the holding time is 5~7h.

5. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In the slurry, the intermediate product has a mass fraction of 40-50%, and the organic carbon source has a mass fraction of 4-5%; the organic carbon source includes one or more of starch, sucrose, glucose, polyvinyl alcohol, and polyethylene glycol.

6. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In the mixed solution, the molar concentration of phosphate is 0.1~4 mol / L, and the molar concentrations of ferrous salt and manganese salt are 0.1~2 mol / L; the reducing agent is ascorbic acid; the ferrous salt includes one or more of ferrous chloride, ferrous oxalate, ferrous carbonate, and ferrous nitrate; the manganese salt includes one or more of manganese chloride, manganese sulfate, and manganese nitrate; and the phosphate salt includes one or more of diammonium hydrogen phosphate, diammonium dihydrogen phosphate, and ammonium phosphate.

7. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The ball milling reaction time is 4-6 hours, and the particle size D50 of the solid particles after ball milling is 0.4 μm-0.5 μm.

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