Lithium manganese iron phosphate composite positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention provides a lithium manganese iron phosphate composite positive electrode material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of lithium ion battery positive electrode materials. The chemical general formula of the material is Li < 1 + x > Fe < 0.4-y-z > Mg < y > Ti < z > Mn < 0.6 > PO4 / C, x is less than or equal to 0.08, 0lt; y is less than or equal to 0.03, 0lt; z < = 0.02. The preparation method comprises the following steps: by taking ferric pyrophosphate as an iron source, preparing a precursor through liquid-phase coprecipitation; a core material is formed through two-step sintering by adopting a non-equal stoichiometric ratio lithium proportion and introducing multiple elements such as magnesium and titanium for synergistic doping; and finally, constructing a conductive carbon layer through graded carbon coating. The material has the characteristics of uniform element distribution, stable structure and excellent conductivity, the specific discharge capacity at 0.1 C is greater than or equal to 160 mAh / g, the capacity retention rate after 1000 cycles at 1C is greater than or equal to 85%, the capacity retention rate at 5C rate is greater than or equal to 85%, and the material is suitable for high-performance lithium ion batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery cathode material, and particularly relates to a lithium manganese iron phosphate composite cathode material, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] Lithium manganese iron phosphate (LiMn x Fe 1-x PO4) is considered as a strong candidate for the next generation of high-performance lithium ion battery cathode materials due to its high voltage platform (about 4.1V) and high theoretical specific capacity (about 170mAh / g). However, its commercial application still faces challenges: first, manganese (Mn) and iron (Fe) ions are difficult to achieve atomic-scale uniform mixing in traditional solid-phase reactions, which easily leads to phase separation during charging and discharging; second, the material itself has low electronic conductivity and ion diffusion rate, affecting the rate performance; third, the Jahn-Teller effect of Mn easily causes structural distortion and capacity decay during the cycle process. The prior art such as CN202310263085.1 uses iron pyrophosphate as an iron source to improve uniformity by a liquid phase method, but the comprehensive performance of the material still has room for improvement; CN119929772A improves performance through a complex two-step sintering and doping process, but has the problems of long process flow and high cost. Therefore, it is of great significance to develop a preparation method that is relatively simple and can simultaneously solve the problems of manganese-iron uniformity, conductivity and structural stability. SUMMARY

[0003] The present application aims to provide a lithium manganese iron phosphate composite cathode material, a preparation method thereof and a lithium ion battery, so as to solve the above technical problems.

[0004] In order to achieve the above application purposes, the present application provides the following technical solutions: The present application provides a lithium manganese iron phosphate composite cathode material, the chemical general formula of which is: Li 1+x Fe 0.4-y-z Mg y Ti z Mn 0.6 PO4 / C, wherein 0 < x ≤ 0.08, 0 < y ≤ 0.03, and 0 < z ≤ 0.02.

[0005] The present application also provides a preparation method of the above lithium manganese iron phosphate composite cathode material, which comprises the following steps: 1) mixing an iron source, a manganese source and a phosphorus source in an acidic aqueous solution, obtaining a lithium manganese iron phosphate precursor through a co-precipitation reaction in an inert atmosphere in the presence of a reducing agent; 2) Mix the lithium manganese iron phosphate precursor, lithium source, magnesium source and titanium source in a molar ratio. The resulting mixture is then ball-milled, dried and pressed into tablets in sequence. Finally, it is sintered in an inert atmosphere in two steps to obtain the doped and modified lithium manganese iron phosphate core material. 3) The doped and modified lithium manganese iron phosphate core material is mixed with a carbon source and then sintered to form a conductive carbon layer on the surface of the material, thereby obtaining the lithium manganese iron phosphate composite cathode material.

[0006] Furthermore, the iron source comprises ferric pyrophosphate, the reducing agent comprises ascorbic acid, and the amount of reducing agent added is 1 to 5% of the total molar mass of the iron source and the manganese source.

[0007] Furthermore, the coprecipitation reaction is carried out at a temperature of 60-70°C for 10-14 hours, and the pH of the reaction system is 6.0-7.0.

[0008] Furthermore, the molar ratio of the lithium source, magnesium source, titanium source and phosphorus source is 1.05~1.08:0.01~0.03:0.005~0.02:1.05~1.08; the molar ratio of the total molar mass of the iron source and manganese source to the molar mass of the lithium source is 0.5~1:1.05~1.08.

[0009] Furthermore, the ball mill operates at a speed of 300-500 rpm, with a ball-to-material ratio of 10-20:1, and the dispersion medium is polyethylene glycol and ethanol.

[0010] Furthermore, the two-step sintering process is as follows: first, the temperature is raised to 350-400℃ at a rate of 2-5℃ / min and held for 2-3 hours, then the temperature is raised to 650-750℃ at a rate of 3-5℃ / min and sintered for 10-15 hours.

[0011] Furthermore, the carbon source comprises one or more of glucose, sucrose, and polyethylene glycol, and the amount of carbon source added is 3 to 8% of the mass of the doped and modified lithium manganese iron phosphate core material.

[0012] Furthermore, in step 3), the sintering process is carried out under an inert atmosphere at a temperature of 500-600°C for 5-8 hours.

[0013] The present invention also provides a lithium-ion battery, wherein the positive electrode active material of the lithium-ion battery comprises a lithium manganese iron phosphate composite positive electrode material.

[0014] The beneficial effects of this invention are: (1) This invention combines liquid-phase co-precipitation of iron pyrophosphate with non-equisional lithium ratio, multi-element synergistic doping and stepwise carbon coating. The process is highly innovative and effectively solves the core problems of uneven distribution of manganese and iron, poor conductivity and poor cycle stability of lithium manganese iron phosphate materials.

[0015] (2) The lithium manganese iron phosphate composite cathode material prepared by the present invention has the characteristics of high capacity, high rate, long cycle life and excellent voltage stability. Its performance indicators are significantly better than those reported in the prior art and comparative products.

[0016] (3) The preparation method described in this invention has a clear process route, precise parameter control, good repeatability, and is easy to achieve large-scale production, and has important industrial application prospects. Attached Figure Description

[0017] Figure 1 The image shows the X-ray diffraction (XRD) pattern of the lithium manganese iron phosphate composite cathode material prepared in Example 1 of this invention.

[0018] Figure 2 This is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite cathode material prepared in Example 1 of the present invention.

[0019] Figure 3 This is a comparison chart of the discharge specific capacity of the cathode materials prepared in Example 1 and Comparative Example 1 of the present invention at different rates from 0.1C to 5C.

[0020] Figure 4 The figures show the cycling performance curves of the cathode materials prepared in Example 1 and Comparative Example 1 of this invention at a 1C rate. Detailed Implementation

[0021] This invention provides a lithium iron phosphate composite cathode material, the chemical formula of which is: Li 1+x Fe 0.4-y-z Mg y Ti z Mn 0.6 PO4 / C, where 0 <x≤0.08,0< y≤0.03,0< z≤0.02。

[0022] In this invention, x is preferably 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, or 0.08; y is preferably 0.01, 0.02, or 0.03; and z is preferably 0.01 or 0.02.

[0023] In this invention, the lithium manganese iron phosphate composite cathode material has a discharge specific capacity of ≥160mAh / g at 0.1C rate, a capacity retention rate of ≥85% after 1000 cycles at 1C rate, and a capacity retention rate of ≥85% at 5C rate.

[0024] This invention also provides a method for preparing the above-mentioned lithium manganese iron phosphate composite cathode material, comprising the following steps: 1) Iron, manganese and phosphorus sources are mixed in an acidic aqueous solution, and lithium manganese iron phosphate precursor is obtained by co-precipitation reaction in the presence of a reducing agent and in an inert atmosphere; 2) Mix the lithium manganese iron phosphate precursor, lithium source, magnesium source and titanium source in a molar ratio. The resulting mixture is then ball-milled, dried and pressed into tablets in sequence. Finally, it is sintered in an inert atmosphere in two steps to obtain the doped and modified lithium manganese iron phosphate core material. 3) The doped and modified lithium manganese iron phosphate core material is mixed with a carbon source and then sintered to form a conductive carbon layer on the surface of the material, thereby obtaining the lithium manganese iron phosphate composite cathode material.

[0025] In this invention, the iron source is preferably iron pyrophosphate (Fe4(P2O7)3), the manganese source is preferably manganese carbonate (MnCO3) or manganese sulfate (MnSO4·H2O), and the phosphorus source is preferably ammonium dihydrogen phosphate (NH4H2PO4).

[0026] In this invention, the lithium source is lithium carbonate (Li2CO3) or lithium hydroxide (LiOH·H2O), the magnesium source is magnesium oxide (MgO) or magnesium acetate (Mg(CH3COO)2), and the titanium source is titanium dioxide (TiO2, anatase type) or tetrabutyl titanate (C 16 H 36 O4Ti).

[0027] In this invention, the iron source comprises ferric pyrophosphate, the reducing agent comprises ascorbic acid, and the amount of reducing agent added is 1 to 5% of the total molar mass of the iron source and manganese source, preferably 2 to 4%, and more preferably 3%.

[0028] In this invention, the temperature of the coprecipitation reaction is 60~70℃, preferably 62~68℃, more preferably 65℃; the time is 10~14h, preferably 12h; and the pH of the reaction system is 6.0~7.0, preferably 6.5.

[0029] In this invention, the molar ratio of the lithium source, magnesium source, titanium source and phosphorus source is 1.05~1.08:0.01~0.03:0.005~0.02:1.05~1.08, preferably 1.06~1.07:0.02:0.01~0.02:1.05~1.07; the molar ratio of the total molar mass of the iron source and manganese source to the molar ratio of the lithium source is 0.5~1:1.05~1.08, preferably 0.7~1:1.06~1.07.

[0030] In this invention, the ball mill rotation speed is 300~500 rpm, preferably 350~450 rpm; the ball-to-material ratio is 10~20:1, preferably 15:1; and the dispersion medium is polyethylene glycol and ethanol.

[0031] In this invention, the two-step sintering is as follows: first, the temperature is raised to 350-400℃ at a rate of 2-5℃ / min and held for 2-3 hours, then the temperature is raised to 650-750℃ at a rate of 3-5℃ / min and sintered for 10-15 hours; preferably, the temperature is raised to 360-380℃ at a rate of 3-4℃ / min and held for 2-3 hours, then the temperature is raised to 680-720℃ at a rate of 3-4℃ / min and sintered for 10-14 hours.

[0032] In this invention, the carbon source comprises one or more of glucose, sucrose and polyethylene glycol, and the amount of carbon source added is 3 to 8% of the mass of the doped and modified lithium manganese iron phosphate core material, preferably 4 to 7%, and more preferably 5 to 6%.

[0033] In this invention, in step 3), the sintering process is carried out under an inert atmosphere at a temperature of 500~600℃, preferably 520~580℃, and more preferably 550℃; the time is 5~8h, preferably 6~7h.

[0034] This invention utilizes iron pyrophosphate as the iron source for liquid-phase co-precipitation, achieving uniform mixing of iron and manganese ions at the molecular level. A non-isostoichiometric lithium ratio (Li / (Fe+Mn) = 1.05-1.08) creates an appropriate number of lithium vacancies, promoting rapid lithium ion migration. The introduction of magnesium (Mg), titanium (Ti), and other elements for synergistic doping stabilizes the structure and improves conductivity. Mg It mainly occupies Fe sites, due to Mg The smaller ionic radius of T helps stabilize the structure; Occupying Fe sites, due to their high valence state, allows for the introduction of electrons to improve conductivity or suppress phase separation. Finally, a uniform and moderately conductive carbon layer is formed through stepwise carbon coating technology, significantly improving the electronic conductivity of the material without affecting lithium-ion diffusion.

[0035] The present invention also provides a lithium-ion battery, wherein the positive electrode active material of the lithium-ion battery comprises a lithium manganese iron phosphate composite positive electrode material.

[0036] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0037] Example 1

[0038] This embodiment provides a method for preparing a lithium manganese iron phosphate composite cathode material, including the following steps: Step 1: Add 0.1 mol (approximately 74.5 g) of ferric pyrophosphate (F... ( Dissolve the compound in 1.2 L of deionized water and adjust the pH to 2.8 with dilute phosphoric acid. Add 0.6 mol (approximately 69.0 g) of manganese carbonate (MnC). 1.05 mol (approximately 120.8 g) of ammonium dihydrogen phosphate (N P 0.02 mol (approximately 3.5 g) of ascorbic acid was stirred at 65°C for 2 hours until completely dissolved. Under nitrogen protection, a 10% ammonia solution was slowly added dropwise, controlling the pH to 6.5. After precipitation, the solution was aged for 12 hours. The solution was filtered, washed three times with deionized water, and dried under vacuum at 100°C for 10 hours to obtain the lithium manganese iron phosphate precursor.

[0039] Step 2: Mix the precursor obtained in Step 1 with 1.06 mol (approximately 78.3 g) of lithium carbonate (L... C 0.02 mol (approximately 0.81 g) magnesium oxide (MgO), 0.01 mol (approximately 0.8 g) titanium dioxide (TiO) The anatase-type lithium iron phosphate core material was placed in a ball mill jar, and an appropriate amount of anhydrous ethanol and polyethylene glycol (PEG-400) dispersant were added. Zirconia balls were used as the milling media, with a ball-to-material ratio of 15:1. The mixture was milled at 350 rpm for 6 hours. The resulting slurry was removed, dried at 80°C for 12 hours, and then pressed into sheets. The sheets were placed in a tube furnace and, under argon atmosphere protection, heated to 380°C at a rate of 5°C / min and held for 2 hours. Then, the temperature was increased to 700°C at a rate of 3°C / min and sintered for 12 hours. After furnace cooling, the material was pulverized to obtain the doped and modified lithium manganese iron phosphate core material.

[0040] Step 3: The core material obtained in Step 2 is dry-mixed with 5% glucose by mass. After thorough mixing, the mixture is placed in a tube furnace and sintered at 550°C for 6 hours under an argon atmosphere. After natural cooling, it is pulverized and passed through a 400-mesh sieve to obtain the final product L. F M T M P / C, which is the lithium manganese iron phosphate composite cathode material.

[0041] Example 2

[0042] This embodiment provides a method for preparing a lithium manganese iron phosphate composite cathode material, including the following steps: Step 1: Same as Step 1 in Example 1.

[0043] Step 2: The precursor obtained in Step 1 is reacted with 1.07 mol (approximately 44.9 g) of lithium hydroxide (LiOH· 0.03 mol (approximately 1.2 g) magnesium oxide (MgO), 0.008 mol (approximately 0.6 g) titanium dioxide (TiO) The mixture was placed in a ball mill jar, and an appropriate amount of anhydrous ethanol was added. The ball milling media was zirconia balls, with a ball-to-material ratio of 15:1. The mixture was ball milled at 350 rpm for 6 hours. The mixed slurry was removed and dried at 80°C for 12 hours, then pressed into sheets. The sheets were placed in a tube furnace and, under an argon atmosphere, heated to 380°C at a rate of 5°C / min and held for 2 hours. Then, the temperature was increased to 650°C at a rate of 3°C / min and sintered for 12 hours. After cooling in the furnace, the material was pulverized to obtain the doped and modified lithium manganese iron phosphate core material.

[0044] Step 3: The core material obtained in Step 2 is dry-mixed with 4% (by weight) of sucrose. After thorough mixing, the mixture is placed in a tube furnace and sintered at 520°C for 6 hours under an argon atmosphere. After natural cooling, it is pulverized and passed through a 400-mesh sieve to obtain the final product L. F Mg 0.03 T M P / C.

[0045] Comparative Example 1

[0046] This comparative example provides a method for preparing a comparative material to illustrate the effect of not performing elemental doping. It includes the following steps: Step 1: Same as Step 1 in Example 1.

[0047] Step 2: Mix the precursor obtained in Step 1 with 1.00 mol (approximately 73.89 g) of lithium carbonate (L... C The lithium was placed in a ball mill jar (i.e., lithium was not in excess and was in stoichiometric ratio), and no doping elements (magnesium source, titanium source) were added. The remaining ball milling, drying, pressing and sintering operations were the same as step 2 of Example 1, to obtain undoped lithium manganese iron phosphate material with lithium in stoichiometric ratio.

[0048] Step 3: Same as step 3 in Example 1, obtain the comparative material LiF. M P / C.

[0049] Comparative Example 2

[0050] This comparative example provides a method for preparing a comparative material to illustrate the effect of only performing "lithium excess" modification without elemental doping. The method includes the following steps: Step 1: Same as Step 1 in Example 1.

[0051] Step 2: The precursor obtained in Step 1 is mixed with 1.06 mol (approximately 78.32 g) of lithium carbonate (L... C The lithium iron phosphate material (with a 6% lithium excess) was placed in a ball mill jar without adding any doping elements (magnesium source, titanium source). The remaining operations were the same as step 2 of Example 1, to obtain lithium iron phosphate material with only lithium excess but no doping.

[0052] Step 3: Same as step 3 in Example 1, obtain comparative material L. F M P / C.

[0053] Comparative Example 3

[0054] This comparative example provides a method for preparing a comparative material to illustrate the effect of single magnesium doping. It includes the following steps: Step 1: Same as Step 1 in Example 1.

[0055] Step 2: The precursor obtained in Step 1 is mixed with 1.06 mol (approximately 78.32 g) of lithium carbonate (L... C 0.02 mol (approximately 0.81 g) of magnesium oxide (MgO) was placed in a ball mill jar (i.e., only magnesium doping was performed, without adding a titanium source). The remaining operations were the same as step 2 of Example 1, to obtain magnesium-doped lithium manganese iron phosphate material.

[0056] Step 3: Same as step 3 in Example 1, obtain comparative material L. F M M P / C.

[0057] Comparative Example 4

[0058] This comparative example provides a method for preparing a comparative material to illustrate the effect of single titanium doping. It includes the following steps: Step 1: Same as Step 1 in Example 1.

[0059] Step 2: The precursor obtained in Step 1 is mixed with 1.06 mol (approximately 78.32 g) of lithium carbonate (L... C 0.01 mol (approximately 0.80 g) of titanium dioxide (Ti The titanium is placed in a ball mill jar (i.e., only titanium doping is performed, without adding a magnesium source). The remaining operations are the same as step 2 of Example 1 to obtain titanium-doped lithium manganese iron phosphate material.

[0060] Step 3: Same as step 3 in Example 1, obtain comparative material L. F T M P / C.

[0061] Comparative Example 5

[0062] This comparative example provides a method for preparing a comparative material using a traditional solid-phase method, to illustrate the superiority of the liquid-phase coprecipitation method of the present invention.

[0063] Step 1: Add 0.4 mol (approximately 60.33 g) of ferric phosphate (FeP) 0.2 mol (approximately 70.95 g) of manganese phosphate (Mn3(P) 2) 1.06 mol (approximately 78.32 g) of lithium carbonate (L C 0.2 mol (approximately 23.01 g) of ammonium dihydrogen phosphate (N P 0.02 mol (approximately 0.81 g) magnesium oxide (MgO), 0.01 mol (approximately 0.80 g) titanium dioxide (TiO) Mix the mixture with a certain amount of glucose (5% of the total mass) and ball mill for 8 hours.

[0064] Step 2: After drying and pressing the mixture into tablets, sinter it at 700℃ for 12 hours under argon protection. After cooling, pulverize and sieve to obtain the comparative material L. F M T M P / C.

[0065] Battery assembly and performance testing

[0066] The positive electrode materials obtained in Examples 1-2 and Comparative Examples 1-5 were applied to the preparation of lithium-ion batteries. The method for preparing the battery positive electrode sheet includes: The positive electrode material, conductive carbon black (Super P), and binder (PVDF) of the examples or comparative examples were weighed out in a mass ratio of 90:5:5, and thoroughly ground and mixed. An appropriate amount of N-methylpyrrolidone (NMP) was then added to prepare a slurry of suitable viscosity. After magnetic stirring for 10 hours, the slurry was evenly coated onto an aluminum foil current collector using a coating machine. The aluminum foil coated with the slurry was placed in a vacuum drying oven at 120°C and dried for 12 hours. After being removed, it was compacted by a roller press and then cut into circular electrode sheets with a diameter of 12 mm using a slicing machine. The electrode sheets were then vacuum dried at 120°C for 12 hours until the moisture was completely removed, and then accurately weighed for use.

[0067] The assembly of the half-cell (CR2032 button cell) was completed in an argon-filled glove box. A lithium metal sheet was used as the negative electrode, a Celgard 2400 polypropylene membrane as the separator, and a 1 mol / L LiP electrolyte was used. The solution used was EC / DEC / EMC (volume ratio 1:1:1). The battery assembly process was as follows: A lithium sheet, a separator with electrolyte added, a positive electrode sheet, a stainless steel gasket, a spring contact, and the positive electrode shell were placed sequentially in the center of the negative electrode shell, ensuring the separator was fully wetted by the electrolyte. Then, the battery was sealed using a button cell battery packaging machine. After sealing, the battery was allowed to stand for 12 hours before electrochemical performance testing. The test results are shown in the table below.

[0068] Table 1. Comparison of electrochemical performance of materials in different embodiments and comparative examples

[0069] The comparison data above clearly shows that: 1. Comparative Example 2 (lithium excess only) outperformed Comparative Example 1 (stoichiometric lithium and no doping) in terms of capacity, rate performance and cycle life, demonstrating the effectiveness of non-stoichiometric lithium ratio in improving the overall performance of the material.

[0070] 2. The performance of Comparative Example 3 (single Mg doping) and Comparative Example 4 (single Ti doping) is better than that of Comparative Example 2, but both are inferior to Example 1 (Mg / Ti co-doping). This strongly proves that there is a synergistic effect between magnesium and titanium. The performance improvement brought about by co-doping is not a simple superposition of the effects of single element doping, which reflects the inventiveness of the present invention.

[0071] 3. Comparative Example 5 (Solid-phase method), even using the same doping elements and ratios as Example 1, exhibited significantly lower performance than Example 1. This highlights the decisive role of the liquid-phase co-precipitation method of iron pyrophosphate in achieving atomic-level uniform mixing of manganese and iron, thereby obtaining superior electrochemical performance. This invention successfully prepared a lithium iron phosphate cathode material with high capacity, excellent rate performance, and ultra-long cycle life through the organic combination of three technologies: liquid-phase co-precipitation, non-stoichiometric lithium ratio, and multi-element synergistic doping. Its comprehensive performance is significantly superior to materials prepared by any single technology or traditional method.

[0072] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate composite cathode material, characterized in that, The general chemical formula of the lithium manganese iron phosphate composite cathode material is: Li 1+x Fe 0.4-y-z Mg y Ti z Mn 0.6 PO4 / C, where 0 <x≤0.08,0< y≤0.03,0< z≤0.02。 2. The preparation method of the lithium manganese iron phosphate composite cathode material according to claim 1, characterized in that, Includes the following steps: 1) Iron, manganese and phosphorus sources are mixed in an acidic aqueous solution, and lithium manganese iron phosphate precursor is obtained by co-precipitation reaction in the presence of a reducing agent and in an inert atmosphere; 2) Mix the lithium manganese iron phosphate precursor, lithium source, magnesium source and titanium source in a molar ratio. The resulting mixture is then ball-milled, dried and pressed into tablets in sequence. Finally, it is sintered in an inert atmosphere in two steps to obtain the doped and modified lithium manganese iron phosphate core material. 3) The doped and modified lithium manganese iron phosphate core material is mixed with a carbon source and then sintered to form a conductive carbon layer on the surface of the material, thereby obtaining the lithium manganese iron phosphate composite cathode material.

3. The preparation method according to claim 2, characterized in that, The iron source contains ferric pyrophosphate, and the reducing agent contains ascorbic acid. The amount of reducing agent added is 1 to 5% of the total molar mass of the iron and manganese sources.

4. The preparation method according to claim 2 or 3, characterized in that, The coprecipitation reaction was carried out at a temperature of 60-70°C for 10-14 hours, and the pH of the reaction system was 6.0-7.

0.

5. The preparation method according to claim 2, characterized in that, The molar ratio of the lithium source, magnesium source, titanium source and phosphorus source is 1.05~1.08:0.01~0.03:0.005~0.02:1.05~1.08; the molar ratio of the total molar mass of the iron source and manganese source to the molar mass of the lithium source is 0.5~1:1.05~1.

08.

6. The preparation method according to claim 2 or 5, characterized in that, The ball mill operates at a speed of 300-500 rpm, with a ball-to-material ratio of 10-20:1, and the dispersion medium is polyethylene glycol and ethanol.

7. The preparation method according to claim 6, characterized in that, The two-step sintering process is as follows: first, the temperature is raised to 350-400℃ at a rate of 2-5℃ / min and held for 2-3 hours, then the temperature is raised to 650-750℃ at a rate of 3-5℃ / min and sintered for 10-15 hours.

8. The preparation method according to claim 7, characterized in that, The carbon source comprises one or more of glucose, sucrose, and polyethylene glycol, and the amount of carbon source added is 3 to 8% of the mass of the doped and modified lithium manganese iron phosphate core material.

9. The preparation method according to claim 2, 7, or 8, characterized in that, In step 3), the sintering process is carried out under an inert atmosphere at a temperature of 500-600℃ for 5-8 hours.

10. A lithium-ion battery, characterized in that, The positive electrode active material of the lithium-ion battery comprises the lithium manganese iron phosphate composite positive electrode material as described in claim 1.

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