CN113072049B - Preparation method of high-compaction-density lithium manganese iron phosphate/carbon composite positive electrode material - Google Patents

Abstract

The invention disclosesA preparation method of a high-compaction-density lithium manganese iron phosphate/carbon composite positive electrode material has a chemical general formula as follows: liMn _x Fe _y PO ₄ X is more than or equal to 0.5 and less than or equal to 0.9, and x + y =1; the preparation method comprises the following steps: diluting phosphoric acid for later use, adding a manganese source, an iron source, a dispersing agent and deionized water into a sealable container, and introducing protective gas; adding diluted phosphoric acid into a sealed container, aging to ensure that the pH value is between 2.0 and 5.0, and uniformly nucleating; cleaning the formed precipitate, vacuum drying, and sintering at high temperature for the first time to form spherical particles A; preparing a solution by using lithium hydroxide monohydrate, adding the spherical particles A into the solution, performing spray drying, and performing high-temperature secondary sintering to obtain spherical particles B; and preparing a glucose solution, adding the spherical particles B into the glucose solution, spray-drying, and sintering at a high temperature for the third time to obtain the nano spherical lithium manganese phosphate/carbon composite material. The invention greatly improves the electrochemical performance and the material compaction density of the material.

Description

A kind of preparation method of lithium manganese iron phosphate/carbon composite cathode material with high compacted density

technical field

The invention belongs to the technical field of energy storage materials, and in particular relates to a preparation method of a high compacted density lithium manganese iron phosphate/carbon composite cathode material.

Background technique

The application of lithium-ion batteries (LIBs) in hybrid and electric vehicles has attracted considerable academic and industrial research interest. Since the pioneering work of Goodenough's group in 1997, olivine-structured polyanionic phosphate materials (LiMPO ₄ , M = Fe, Mn, Co, and Ni) have been used as cathodes due to their high theoretical capacity, thermal stability, and environmental friendliness. Materials are extensively studied. Based on the safety requirement, LiFePO ₄ (LFP) has been successfully used in LIBs, as an alternative LiMnPO ₄ (LMP) provides the same theoretical specific capacity (170mAh g ^-1 ), and compared with Fe ²⁺ /Fe ³⁺ , The redox potential of Mn ²⁺ /Mn ³⁺ is higher, thus enabling the theoretical energy density of LMP to be about 20% higher than that of LFP. Despite these advantages of LMPs, it is still difficult to obtain LMPs with significantly improved power density, high specific capacity, and long-term cycling stability. The extremely low lithium ion diffusion coefficient (D _Li , 10 ^-16 ~ 10 ^-14 cm ² /s) and electronic conductivity (<10 ^-10 s cm ^-1 ) also reduces its electrochemical activity. In addition, the volume change of LMP is larger during the charge-discharge process, and the unstable crystal structure during the charge-discharge cycle will lead to a lower capacity retention. The Jahn-Teller effect and the dissolution of divalent Mn will lead to the deterioration of the electrochemical performance of LMP. The ex-situ carbon coating method can only improve the conductivity between secondary particles, while the in-situ carbon coating can form a three-dimensional continuous conductive system with the primary particles of LiMnPO ₄ cathode material, improving the electronic conductance between primary particles. Rate. In battery materials at the micron scale, the kinetics of charge storage are usually controlled by the diffusion rate of Li ⁺ in the microparticles. Therefore, the reaction kinetics can be enhanced by nanosizing LMP materials. Fe doping can alleviate the lattice distortion caused by the Jahn-Teller effect, thereby stabilizing the crystal structure and enhancing charge transfer. Therefore, replacing Mn with an appropriate amount of Fe is a method to improve the performance of LiMnPO ₄ batteries, and it is necessary to synthesize lithium manganese iron phosphate (LiMn _x Fe _1-x PO ₄ ) cathode materials with excellent electrochemical performance.

In addition, the synthesis method and preparation process determine the particle size, particle size distribution, morphology, specific surface area, crystallinity and lattice defects of LiMn _x Fe _1-x PO ₄ , etc., and the intrinsic structural characteristics will directly affect the lithium ion desorption process. The intercalation process determines the electrochemical performance of the material such as charge-discharge capacity and cycle life. The traditional synthesis method - solid phase method, although the synthesis process is simple, the material is uniform particles, but the space between particles is large. The hydrothermal method has obvious advantages in controlling the chemical composition and crystallite size, but its synthetic materials are lamellar and cannot be densified.

For example, Chinese Patent Publication No. CN104009234A discloses a method for synthesizing LiMn _0.7 Fe _0.3 PO ₄ , the anode material for lithium-ion batteries, by co-precipitation-microwave method, in which Li ₂ CO ₃ , Fe ₂ O ₃ , and phosphoric acid After specific metering, phosphoric acid is made into phosphoric acid solution, and citric acid is added to the phosphoric acid solution to prepare an aqueous solution of citric acid and phosphoric acid; Li ₂ CO ₃ , Fe ₂ O ₃ are added, stirred evenly to obtain a paste mixture; after aging 1. Precursor A is obtained after microwave heat treatment; Precursor B is prepared in the same way by using Li ₂ CO ₃ , MnCO ₃ or MnO ₂ , and phosphoric acid as raw materials, mixing Precursor A and B, adding glucose solution, and obtaining a paste precursor; Finally, the positive electrode material was obtained by microwave sintering, and the discharge capacity of the prepared lithium manganese iron phosphate positive electrode material was 138.6mAh g ^-1 .

Chinese Patent Publication No. CN103515599A discloses a preparation method of lithium manganese phosphate and carbon nanotube nanocomposite material. The method adopts dropping phosphoric acid into lithium hydroxide solution to obtain lithium phosphate; then adding manganese salt and iron salt to lithium phosphate Stir in the dispersion liquid to obtain a precursor solution, and then perform high-temperature sintering to obtain lithium manganese phosphate or iron-doped lithium manganese phosphate, then deposit a catalyst on its surface, and finally sinter in an atmosphere of carbon source gas and protective gas to obtain lithium manganese phosphate and lithium manganese phosphate. Carbon nanotube nanocomposite material or iron-doped lithium manganese phosphate and carbon nanotube nanocomposite material.

Chinese Patent Publication No. CN109524644A discloses a preparation method of LiMn _1-x Mg _x PO ₄ /C cathode material, the method includes the following steps: (1) Slowly drop phosphoric acid or phosphate solution B into Lithium salt solution A and continuously stirred to obtain suspension C; (2) Slowly pour manganese salt and magnesium salt mixed solution D into the suspension C, stir and disperse evenly, microwave reaction after cooling, centrifuge, wash, Vacuum drying and grinding to obtain lithium manganese magnesium phosphate precursor powder; (3) mixing the lithium manganese magnesium phosphate precursor powder and carbon source ball mill evenly, roasting under an inert gas atmosphere, and then grinding to obtain LiMn _1-x Mg _x PO ₄ /C cathode material, wherein x=0.01-0.15.

Chinese Patent Publication No. CN103413940A discloses a method for synthesizing nano-manganese lithium phosphate as a positive electrode material for lithium-ion batteries. First, phosphoric acid is dissolved in deionized water to obtain a phosphoric acid solution, and PEG400 is added under stirring to obtain a mixed solution of phosphoric acid PEG400, and then the hydrogen oxidized Lithium is dissolved in deionized water to obtain a lithium hydroxide aqueous solution, which is added to the phosphoric acid PEG400 mixed solution under stirring to obtain a white emulsion, and manganese sulfate is dissolved in deionized water to form a manganese sulfate solution. Add it to the obtained white emulsion to obtain a precursor emulsion, put it into a microwave reactor at a controlled temperature of 140-180°C for microwave reaction for 5-20 minutes, then centrifuge, wash, and dry to obtain a lithium-ion battery cathode material nanometer Lithium manganese phosphate.

Chinese Patent Publication No. CN102956887A discloses a method for preparing a nanoscale lithium manganese phosphate positive electrode material. The molar ratio of lithium source, manganese source, phosphorus source and doping element compound is (0.9-1): (0.9-1): (0.9～1): (0～0.1) make a solution, then add a complexing agent, and add a carbon source to the solution to obtain a uniform colloid; the colloid is dried to obtain a precursor powder, and the precursor powder is heated in a protective atmosphere A nanometerized powder is obtained, and the nanometerized powder is sintered in a protective atmosphere to obtain a nanoscale lithium manganese phosphate cathode material.

The above published patents use different methods to synthesize lithium manganese phosphate or iron-doped lithium manganese phosphate materials, but the compaction density of the materials is not improved.

Contents of the invention

Aiming at the problems existing in the prior art, the present invention provides a method for preparing a high compacted density lithium manganese iron phosphate/carbon composite positive electrode material. The method prepares nano-spherical particles through a three-step sintering synthesis method, which can realize electrochemical discharge capacity and the like. The performance is greatly improved, and the shape of the product particles is regular, the size distribution is reasonable, the particle gap is smaller, and the compaction density is greatly improved (up to 2.8g cm ^-3 or more).

The present invention is achieved in this way, a method for preparing a high compacted density lithium manganese iron phosphate/carbon composite positive electrode material, the chemical formula of the lithium manganese iron phosphate/carbon composite positive electrode material is: LiMn _x Fe _y PO ₄ /C , where, 0.5≤x≤0.9, x+y=1;

Concrete preparation method comprises the following steps:

1) Dilute phosphoric acid to a mass concentration of 2-55% for later use, add manganese source, iron source, dispersant and deionized water in a sealable container, and pass protective gas into the container;

2) Slowly add the diluted phosphoric acid solution into a sealed container to produce ferromanganous phosphate precipitation, age for 10-20 days, and adjust the pH value at any time to stabilize it at 2.0-5.0, so that it can nucleate uniformly;

3) Wash the formed precipitate with 5-50 times the mass of deionized water for 3-5 times, then dry it in vacuum at 60-80°C, and then sinter at high temperature in a protective gas atmosphere for the first time to form spherical particles of 1-10 μm A;

4) Take lithium hydroxide monohydrate LiOH·H ₂ O to prepare a solution with a mass concentration of 1-15%, add spherical particles A into it, spray dry, and sinter at high temperature for the second time in a protective gas atmosphere to obtain spherical particles B ;

5) Glucose is prepared into a solution with a mass concentration of 30% to 70%, and spherical particles B are added therein. After spray drying, they are sintered for the third time at high temperature in a protective gas atmosphere to obtain nano-spherical lithium manganese iron phosphate/carbon (LiMn _x Fe _{yPO 4} /C) material.

In the above technical scheme, preferably, the manganese source is manganese sulfate (MnSO ₄ ), manganese chloride (MnCl ₂ ), manganese oxalate (MnC ₂ O ₄ ) and manganese acetate (Mn(CH ₃ COO) ₂ ). one or more of.

In the above technical scheme, preferably, the iron source is ferrous sulfate (FeSO ₄ ), ferrous chloride (FeCl ₂ ), ferrous oxalate (FeC ₂ O ₄ ) and ferrous acetate (Fe(CH ₃ COO ) One or more of ₂ ).

In the above technical solution, preferably, the dispersant is one or more of polyethylene glycol 400, polyethylene wax, stearamide and monoglyceride stearate.

In the above technical solution, preferably, the mass ratio of the dispersant to deionized water is (1-5): (70-150).

In the above technical solution, preferably, in the steps 1), 3), 4), and 5), the protective gas is argon or nitrogen.

In the above technical solution, preferably, in the steps 4) and 5), the precursor material is spray-dried by using a spray dryer or a centrifugal spray dryer or a pressure spray dryer, and the spray drying inlet temperature is 200-350°C , The spray drying outlet temperature is 60-120°C.

In the above technical solution, preferably, in the steps 3), 4), and 5), the sintering temperature is 700-800°C and the time is 4h-12h.

The advantages and positive effects that the present invention has are:

(1) The present invention adopts the co-precipitation method to synthesize the ferromanganese source precursor, the reaction conditions are easy to control in the reaction process, and the product performance is good in repeatability.

(2) The prepared lithium manganese iron phosphate/carbon composite positive electrode material is nanoparticles, and the size distribution is uniform, which can effectively reduce the extraction and insertion paths of lithium ions during charging and discharging; and the positive electrode material has high specific capacity, working It has the advantages of high voltage, good rate performance and good cycle stability.

(3) The present invention adopts co-precipitation-three-step sintering synthesis to prepare lithium manganese iron phosphate/carbon composite positive electrode material, each step can control the particle shape and size, and can improve the material compaction density, and the material compaction density reaches 2.8 g cm ^-3 or more.

(4) The main raw materials used in the present invention are rich in sources and low in price, and the invention has good application prospects.

Description of drawings

Fig. 1 is the SEM picture of the high compacted density lithium manganese iron phosphate/carbon composite positive electrode material prepared in Example 1 of the present invention;

Fig. 2 is the XRD pattern of the lithium manganese iron phosphate/carbon composite positive electrode material of high compacted density prepared in embodiment 1 of the present invention;

Fig. 3 is a graph showing charge and discharge performance curves of the lithium manganese iron phosphate/carbon composite cathode material with high compacted density prepared in Example 1 of the present invention in the range of 2.5-4.5V and at a rate of 0.2C.

Detailed ways

In order to make the object, technical solution and advantages of the present invention more clear, the present invention will be further described in detail below in conjunction with the examples. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

An embodiment of the present invention provides a method for preparing a high compacted density lithium manganese iron phosphate/carbon composite positive electrode material. The general chemical formula of the lithium manganese iron phosphate/carbon composite positive electrode material is: LiMn _x Fe _y PO ₄ /C, Wherein, 0.5≤x≤0.9, x+y=1;

Concrete preparation method comprises the following steps:

1) Dilute phosphoric acid to a mass concentration of 2-55% for later use, add manganese source, iron source, dispersant and deionized water in a sealable container, and pass protective gas into the container;

2) Slowly add the diluted phosphoric acid solution into a sealed container to produce ferromanganous phosphate precipitation, age for 10-20 days, and adjust the pH value at any time to stabilize it at 2.0-5.0, so that it can nucleate uniformly;

3) Wash the formed precipitate with 5-50 times the mass of deionized water for 3-5 times, then dry it in vacuum at 60-80°C, and then sinter at high temperature in a protective gas atmosphere for the first time to form spherical particles of 1-10 μm A;

4) Take lithium hydroxide monohydrate LiOH·H ₂ O to prepare a solution with a mass concentration of 1-15%, add spherical particles A into it, spray dry, and sinter at high temperature for the second time in a protective gas atmosphere to obtain spherical particles B ;

5) Glucose is prepared into a solution with a mass concentration of 30% to 70%, and spherical particles B are added therein. After spray drying, they are sintered for the third time at high temperature in a protective gas atmosphere to obtain nano-spherical lithium manganese iron phosphate/carbon (LiMn _x Fe _{yPO 4} /C) material.

The manganese source is one or more of manganese sulfate (MnSO ₄ ), manganese chloride (MnCl ₂ ), manganese oxalate (MnC ₂ O ₄ ) and manganese acetate (Mn(CH ₃ COO) ₂ ).

The iron source is one or more of ferrous sulfate (FeSO ₄ ), ferrous chloride (FeCl ₂ ), ferrous oxalate (FeC ₂ O ₄ ) and ferrous acetate (Fe(CH ₃ COO) ₂ ). kind.

The dispersant is one or more of polyethylene glycol 400, polyethylene wax, stearamide and monoglyceride stearate.

The mass ratio of the dispersant to deionized water is (1-5): (70-150).

In the steps 1), 3), 4), and 5), the protective gas is argon or nitrogen.

In the steps 4) and 5), use a spray dryer, a centrifugal spray dryer or a pressure spray dryer to spray-dry the precursor material, the temperature of the spray-drying inlet is 200-350°C, and the temperature of the spray-drying outlet is 60-120°C. ℃.

In the steps 3), 4), and 5), the sintering temperature is 700-800°C and the time is 4h-12h.

Example 1

1) Weigh 28.7ml of 85% phosphoric acid and slowly add 200ml of deionized water to dilute and prepare a solution for later use. Add 45.30g of manganese sulfate MnSO ₄ , 55.61g of ferrous sulfate heptahydrate FeSO ₄ 7H ₂ O, 4g polyethylene glycol 400 and 100ml deionized water, pass into nitrogen protection gas in this container.

2) Slowly add diluted phosphoric acid into a sealed container to produce ferromanganous phosphate precipitation, age for 15 days, and adjust the pH value to 4.0 at any time to make it evenly nucleate.

3) The formed precipitate was washed 4 times with 10 ml of deionized water, and then dried under vacuum at 60°C. Then sinter for the first time at a high temperature of 750° C. for 5 hours in a nitrogen protective gas atmosphere to form spherical particles A of 1-10 μm.

4) Take 20.98g of lithium hydroxide monohydrate LiOH·H ₂ O to prepare a solution with a mass concentration of 10%, add spherical particles A into it, and spray dry. The temperature at the inlet of the spray drying is 200-350° C. 60-120°C, followed by sintering at a high temperature of 750°C for 5 hours in a nitrogen atmosphere for the second time to obtain spherical particles B.

5) Take 17.09g of glucose and make it into a 50% mass concentration solution, add spherical particles B therein, and spray dry. The third sintering at a high temperature of 750° C. for 5 hours in the atmosphere obtained a nano-spherical lithium manganese iron phosphate/carbon (LiMn _0.6 Fe _0.4 PO ₄ /C) composite positive electrode material.

Example 2

1) Weigh 28.7ml of 85% phosphoric acid and slowly add 200ml of deionized water to dilute and prepare a solution for later use. Add 37.77g of manganese dichloride MnCl ₂ , 25.35g of ferrous chloride FeCl ₂ , 4g of polyethylene Glycol 400 and 100ml deionized water, pass into nitrogen protection gas in this container.

2) Slowly add diluted phosphoric acid into a sealed container to produce ferromanganous phosphate precipitation, age for 12 days, and adjust the pH value to 4.0 at any time to make it evenly nucleate.

3) The formed precipitate was washed 4 times with 15 ml of deionized water, and then dried under vacuum at 70°C. Then sinter for the first time at a high temperature of 720° C. for 8 hours in a nitrogen protective gas atmosphere to form spherical particles A of 1-10 μm.

4) Take 20.98g of lithium hydroxide monohydrate LiOH·H ₂ O to prepare a solution with a mass concentration of 10%, add spherical particles A into it, and spray dry. The temperature at the inlet of the spray drying is 200-350° C. 60-120°C, followed by second sintering at a high temperature of 720°C for 8 hours in a nitrogen atmosphere to obtain spherical particles B.

5) Take 13.31g of glucose and make it into a 50% mass concentration solution, add spherical particles B therein, and spray dry. The third sintering at a high temperature of 720° C. for 8 hours in the atmosphere obtained a nano-spherical lithium manganese iron phosphate/carbon (LiMn _0.6 Fe _0.4 PO ₄ /C) composite positive electrode material.

Example 3

1) Weigh 28.7ml of 85% phosphoric acid and slowly add 200ml of deionized water to dilute and prepare a solution for later use. Add 42.89g of manganese oxalate MnSO ₄ , 28.77g of ferrous oxalate FeSO ₄ 7H ₂ O, 4g of polyethylene Glycol 400 and 100ml deionized water, pass into nitrogen protection gas in this container.

2) Slowly add diluted phosphoric acid into a sealed container to produce ferromanganous phosphate precipitation, age for 18 days, and adjust the pH value to 4.0 at any time to make it evenly nucleate.

3) The formed precipitate was washed 5 times with 20 ml of deionized water, and then dried under vacuum at 80°C. Then sinter for the first time at a high temperature of 780° C. for 4 hours in a nitrogen protective gas atmosphere to form spherical particles A of 1-10 μm.

4) Take 20.98g of lithium hydroxide monohydrate LiOH·H ₂ O to prepare a solution with a mass concentration of 10%, add spherical particles A into it, and spray dry. The temperature at the inlet of the spray drying is 200-350° C. 60-120°C, followed by sintering at a high temperature of 780°C for 4 hours in a nitrogen atmosphere for the second time to obtain spherical particles B.

5) Take 14.16g of glucose and make it into a 50% mass concentration solution, add spherical particles B therein, and spray dry. The third sintering at a high temperature of 780° C. for 4 hours in the atmosphere obtained a nano-spherical lithium manganese iron phosphate/carbon (LiMn _0.6 Fe _0.4 PO ₄ /C) composite positive electrode material.

Performance Testing

The nano-spherical lithium manganese iron phosphate/carbon (LiMn _0.6 Fe _0.4 PO ₄ /C) composite cathode material prepared in Example 1 was tested. Figure 1 is a scanning electron microscope image of the nano-spherical lithium manganese iron phosphate/carbon (LiMn _0.6 Fe _0.4 PO ₄ /C) composite positive electrode material prepared in Example 1. It can be seen from Figure 1 that the secondary particles are all spherical, and It is distributed in different particle sizes, and its particle size ranges from 2 to 15 μm. The secondary particle ball is composed of the accumulation of primary particles, and the primary particles are closely connected with almost no gaps.

Figure 2 is the XRD pattern of the nano-spherical lithium manganese iron phosphate/carbon (LiMn _0.6 Fe _0.4 PO ₄ /C) composite positive electrode material prepared in Example 1. As can be seen from Figure 2, the peak position of the spectrum is similar to that of lithium manganese iron phosphate Standard PDF cards dovetail.

Figure 3 is a graph showing the charge and discharge performance curves of the nano-spherical lithium manganese iron phosphate/carbon (LiMn _0.6 Fe _0.4 PO ₄ /C) composite positive electrode material prepared in Example 1 in the range of 2.5 to 4.5V and at a rate of 0.2C, from Figure 3 It can be seen that the specific discharge capacity reaches 154mAh/g. Compared with Li ⁺ /Li, this material has two pairs of typical charge and discharge voltage plateaus around 4.1V and 3.5V, corresponding to the redox reactions of Mn ³⁺ /Mn ²⁺ and Fe ³⁺ /Fe ²⁺ respectively .

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it can still be Modifications are made to the technical solutions described in the foregoing embodiments, or some or all of the technical features are equivalently replaced, and these modifications or replacements do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims (6)

1. A preparation method of a high-compaction-density lithium manganese iron phosphate/carbon composite positive electrode material is characterized by comprising the following steps of: the chemical general formula of the lithium iron manganese phosphate/carbon composite anode material is as follows: liMn _x Fe _y PO ₄ /C, wherein x is more than or equal to 0.5 and less than or equal to 0.9, x + y=1;

the preparation method comprises the following steps:

1) Diluting phosphoric acid to a mass concentration of 2-55% for later use, adding a manganese source, an iron source, a dispersing agent and deionized water into a sealable container, and introducing protective gas into the container;

2) Slowly adding the diluted phosphoric acid solution into a sealed container to generate ferromanganese phosphate precipitate, aging for 10 to 20 days, and adjusting the pH value to be stable at 2.0 to 5.0 at any time to ensure that the ferromanganese phosphate precipitate forms cores uniformly;

3) Washing the formed precipitate for 3-5 times by using deionized water with the mass being 5-50 times that of the precipitate, drying at the temperature of 60-80 ℃ in vacuum, and then sintering at high temperature for the first time in a protective gas atmosphere to form spherical particles A with the particle size being 1-10 mu m;

4) Taking monohydrate lithium hydroxide LiOH.H ₂ O, preparing a solution with the mass concentration of 1-15%, adding the spherical particles A into the solution, spray-drying, and sintering at a high temperature for the second time in a protective gas atmosphere to obtain spherical particles B;

5) Taking glucosePreparing a solution with the mass concentration of 30-70%, adding the spherical particles B into the solution, spray-drying, and sintering at high temperature for the third time in a protective gas atmosphere to obtain the nano spherical lithium manganese iron phosphate/carbon (LiMn) _x Fe _y PO ₄ /C) composite positive electrode material of said lithium iron manganese phosphate/carbon (LiMn) _x Fe _y PO ₄ The compacted density of the/C) composite cathode material is 2.8g/cm ³ ；

In the steps 1), 3), 4) and 5), the protective gas is argon or nitrogen;

in the steps 3), 4) and 5), the sintering temperature is 700-800 ℃ and the time is 4-12 h.

2. The method for preparing the high-compaction-density lithium iron manganese phosphate/carbon composite cathode material according to claim 1, wherein the method comprises the following steps: the manganese source is manganese sulfate (MnSO) ₄ ) Manganese chloride (MnCl) ₂ ) Manganese oxalate (MnC) ₂ O ₄ ) And manganese acetate (Mn (CH) ₃ COO) ₂ ) One or more of them.

3. The method for preparing the high-compaction-density lithium iron manganese phosphate/carbon composite cathode material according to claim 1, wherein the method comprises the following steps: the iron source is ferrous sulfate (FeSO) ₄ ) Ferrous chloride (FeCl) ₂ ) Iron oxalate (FeC) ₂ O ₄ ) And ferrous acetate (Fe (CH) ₃ COO) ₂ ) One or more of them.

4. The method for preparing the high-compaction-density lithium iron manganese phosphate/carbon composite cathode material according to claim 1, wherein the method comprises the following steps: the dispersing agent is one or more of polyethylene glycol 400, polyethylene wax, stearamide and stearic acid monoglyceride.

5. The method for preparing the high-compaction-density lithium iron manganese phosphate/carbon composite cathode material according to claim 1, wherein the method comprises the following steps: the mass ratio of the dispersing agent to the deionized water is (1~5): (70 to 150).

6. The method for preparing the high-compaction-density lithium iron manganese phosphate/carbon composite cathode material according to claim 1, wherein the method comprises the following steps: and in the steps 4) and 5), spray drying the precursor material by using a spray dryer or a centrifugal spray dryer or a pressure spray dryer, wherein the inlet temperature of the spray drying is 200 to 350 ℃, and the outlet temperature of the spray drying is 60 to 120 ℃.

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