Polystyrene microsphere-assisted high-energy-density lithium manganese iron phosphate composite positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a polystyrene microsphere-assisted high-energy-density lithium manganese iron phosphate composite positive electrode material as well as a preparation method and application of the polystyrene microsphere-assisted high-energy-density lithium manganese iron phosphate composite positive electrode material. The preparation method comprises the following steps: firstly, weighing a manganese source, an iron source and a phosphate radical source according to a stoichiometric ratio of 4.8: 1.2: 6, adding urea and ethylene glycol, carrying out magnetic stirring, then adding polystyrene microspheres, and carrying out ultrasonic dispersion and water bath reaction to obtain precursor slurry; quenching, filtering, washing and drying the slurry to obtain precursor powder; finally, the precursor powder and lithium carbonate are mixed and subjected to ball milling, after high-temperature calcination, the mixture and cane sugar are dispersed in an aqueous solution, drying and annealing are conducted, and a final product is obtained. The composite positive electrode material disclosed by the invention not only keeps the advantages of high energy density and low cost, but also remarkably improves the ion and electron transmission efficiency, weakens electrochemical polarization, improves the rate capability, and is suitable for a high-power lithium ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and particularly relates to a lithium iron manganese phosphate composite cathode material with high energy density assisted by polystyrene microspheres, and a preparation method and application thereof. Background Art

[0002] With the continuous growth of global energy demand and the increasing prominence of environmental problems, the development of efficient, low-cost, and environmentally friendly energy storage technologies has become the focus of current research. Lithium-ion batteries, due to their high energy density, long cycle life, and environmental friendliness, have become the preferred energy storage devices for electric vehicles, portable electronic devices, and renewable energy storage systems. However, traditional lithium-ion battery cathode materials such as lithium cobalt oxide (LiCoO 2 ) and lithium iron phosphate (LiFePO 4 ) have problems such as high cost, scarce resources, or insufficient energy density, which limit their large-scale application. Therefore, the development of new cathode materials to replace traditional materials has become a research hotspot in the field of lithium-ion batteries.

[0003] Lithium iron manganese phosphate (LiMn 1-x Fe x PO 4 , 0 < x < 1, hereinafter referred to as LMFP), as a new type of cathode material, is gradually becoming a research hotspot in the field of lithium-ion batteries due to its high energy density, low cost, environmental friendliness, and excellent thermal stability. However, although the LMFP material has significant advantages in theoretical performance, its practical application still faces two key challenges: low electronic conductivity and slow lithium-ion diffusion rate. These two problems severely limit the application of the LMFP material in high-power and high-energy density batteries. Therefore, how to solve these problems through the optimization of material design and preparation processes has become the focus of current research.

[0004] The LMFP material combines the advantages of lithium iron phosphate (LiFePO 4 ) and lithium manganese phosphate (LiMnPO 4 ). Lithium iron phosphate is known for its excellent thermal stability and cycling performance, but its energy density is relatively low; while lithium manganese phosphate has a relatively high voltage platform (about 4.1V vs. Li / Li +), thus providing a higher energy density. However, its electronic conductivity and ion diffusion rate are relatively low, resulting in poor rate performance. By combining iron and manganese in a certain proportion, the LMFP material can improve its electrochemical performance while maintaining a high energy density. However, the LMFP material still has the following problems: (1) Low electronic conductivity: The intrinsic electronic conductivity of the LMFP material is relatively low, resulting in a large polarization phenomenon during high-rate charge and discharge processes, thus limiting its power performance. (2) Slow lithium-ion diffusion rate: The lithium-ion diffusion rate in the LMFP material is relatively slow. Especially under high-rate charge and discharge conditions, the transport efficiency of lithium ions becomes a key factor restricting its performance.

[0005] Currently, the main methods for preparing LMFP materials include high-temperature solid-state method, hydrothermal method, sol-gel method, etc. Each method has its unique advantages and limitations: (1) High-temperature solid-state method: This is the most commonly used method in industrial production at present. Its process is simple and suitable for large-scale production. However, the particle size obtained by the high-temperature solid-state method is large and the distribution is uneven, resulting in poor electrochemical performance of the material. In addition, due to the low diffusion rate of carbon, it is difficult to form a uniform carbon coating on the surface and inside of the particles, thus affecting the electron conductivity and lithium-ion diffusion rate. (2) Hydrothermal method: The hydrothermal method can prepare LMFP materials with smaller particle size and uniform distribution at a lower temperature. However, its reaction conditions need to be carried out under high temperature and high pressure, which requires high equipment requirements and increases the difficulty and cost of industrial production. (3) Sol-gel method: This method can prepare LMFP materials with uniform size. However, the drying process of its precursor is complex and the final annealing time is long, resulting in low production efficiency and difficult to meet the needs of large-scale production. Therefore, there is an urgent need to find a simple, reliable and less chemically intensive technology to finally achieve high-quality LMFP materials required for large-scale production of battery applications. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the present invention provides a polystyrene microsphere-assisted lithium iron manganese phosphate composite cathode material with high energy density, its preparation method and application. The primary particles of the flexible structure of this material have less agglomeration and the secondary particles are more densely distributed. It not only has the advantages of uniform carbon coating and increased electrode density, but also can reduce the diffusion distance of Li + and increase the number of active centers, which will greatly improve the weight / volume reversible capacity, weaken the electrochemical polarization and improve the charge and discharge efficiency.

[0007] The present invention is realized by the following technical solutions:

[0008] A preparation method of a polystyrene microsphere-assisted lithium iron manganese phosphate composite cathode material with high energy density, comprising the following steps:

[0009] Step 1) Weigh the manganese source, iron source, and phosphate source according to a stoichiometric ratio of 4.8:1.2:6, and add deionized water to obtain solution A;

[0010] Step 2) Add urea to solution A, and after magnetic stirring, obtain solution B;

[0011] Step 3) Add ethylene glycol to solution B, after magnetic stirring, then add polystyrene microsphere powder, and finally ultrasonicate the mixed solution to obtain solution C;

[0012] Step 4) React the solution C under magnetic stirring in a water bath at 50 - 100 °C in a nitrogen atmosphere to obtain an in-situ carbon-coated precursor slurry D;

[0013] Step 5) Quench, filter, wash the precursor slurry D, and dry it in a vacuum drying oven to obtain a precursor powder;

[0014] Step 6) Mix and ball-mill the precursor powder with lithium carbonate, and then perform high-temperature calcination treatment in an argon-hydrogen mixed gas atmosphere to obtain an in-situ carbon-coated lithium iron manganese phosphate cathode material;

[0015] Step 7) Disperse the in-situ carbon-coated lithium iron manganese phosphate cathode material and sucrose in an aqueous solution according to a ratio, dry, and anneal to finally obtain a uniformly carbon-coated nano lithium iron manganese phosphate cathode material, thus obtaining the product.

[0016] Preferably, in step 1), the manganese source is manganese sulfate, the iron source is ferrous sulfate, and the phosphate source is ammonium dihydrogen phosphate.

[0017] Preferably, in step 2), the dosage of solution A is 20 mL, the dosage of urea is 6.0 g, and the magnetic stirring time is 5 min; in step 3), the magnetic stirring time is 5 min, and the ultrasonic time is 15 min; in step 4), the water bath reaction time is 1.5 - 2 h.

[0018] Preferably, the preparation process of the polystyrene microspheres in step 3) is as follows:

[0019] First, dissolve polyvinylpyrrolidone in deionized water under stirring, after passing nitrogen, add styrene containing 4-tert-butylcatechol, and heat to 65 - 80 °C to form a white emulsion; then add an aqueous solution containing an azobisisobutyramidine hydrochloride initiator to the white emulsion, and perform complete polymerization in a nitrogen atmosphere at 65 - 80 °C, wash and filter the precipitate until neutral; finally, wash the polystyrene nanospheres with ethanol, vacuum dry overnight at 60 °C, and grind for standby.

[0020] Preferably, the washing in step 5) is specifically: washing 3 - 5 times with deionized water and absolute ethanol respectively; the drying temperature is 50 - 60 °C, and the time is 8 - 10 h.

[0021] Preferably, in the argon - hydrogen mixed gas atmosphere in step 6), the volume ratio of hydrogen is 5%; the ball - milling time is 2 h; the high - temperature calcination temperature is 650 - 750 °C, and the time is 8 h; preferably, the high - temperature calcination temperature is 700 °C.

[0022] Preferably, in step 7), the mass ratio of the in - situ carbon - coated lithium iron phosphate manganese cathode material to sucrose is 100:(5 - 25); the annealing atmosphere is an argon - hydrogen mixed gas atmosphere, where the volume ratio of hydrogen is 5%; the annealing temperature is 650 - 750 °C, and the time is 2 - 3 h.

[0023] The high - energy - density lithium iron phosphate manganese composite cathode material assisted by polystyrene microspheres prepared by the above preparation method.

[0024] The application of the above - mentioned high - energy - density lithium iron phosphate manganese composite cathode material assisted by polystyrene microspheres in the preparation of lithium - ion batteries.

[0025] Preferably, the high - energy - density lithium iron phosphate manganese composite cathode material assisted by polystyrene microspheres is mixed with conductive carbon black and binder polyvinylidene fluoride in a mass ratio of 95:3:2, using N - methylpyrrolidone as a solvent, made into a slurry and uniformly coated on the surface of aluminum foil, and dried at 120 °C for 12 h to make a positive electrode plate.

[0026] A lithium - ion battery, comprising a positive electrode plate, a separator, an electrolyte and a negative electrode plate; the positive electrode plate is made of the above - mentioned high - energy - density lithium iron phosphate manganese composite cathode material assisted by polystyrene microspheres; the separator is a Celgard 2400 polypropylene porous membrane; the electrolyte is LiPF 6 / (EC + DEC + DMC); the negative electrode plate is a metal lithium sheet.

[0027] The beneficial effects of the present invention are as follows:

[0028] (1) Starting from saving costs and reducing energy consumption, the present invention provides a simple polystyrene template - assisted coprecipitation method for synthesizing high - performance P - LiMn 0.8 Fe 0.2 PO 4 / C composite material. This method not only retains the advantages of high energy density and low cost of the lithium iron phosphate manganese cathode material, but also significantly improves its ion and electron transport efficiency, effectively weakens the electrochemical polarization phenomenon, and thus greatly improves the rate performance of the material.

[0029] (2) During the preparation process of the present invention, the polystyrene nanospheres play a dual role: on the one hand, as a template, they inhibit particle aggregation and reduce particle size, thereby accelerating the lithium-ion conduction rate, increasing the diffusion coefficient of lithium ions, and enhancing the transport efficiency; on the other hand, as a carbon source, they jointly construct a uniform conductive carbon layer with sucrose, optimize the electron transport path, improve the electron conductivity, inhibit particle aggregation, ensure the stability of the material structure, and further enhance the energy density.

[0030] (3) The preparation process of the present invention is simple and efficient. Under the conditions of normal pressure and 80 °C, the polystyrene-coated precursor can be directly obtained, and then through annealing treatment with lithium carbonate and sucrose, the uniformly carbon-coated P-LiMn 0.8 Fe 0.2 PO 4 / C nanocomposite can be finally obtained. This method has simple operation and mild reaction conditions, and is suitable for large-scale industrial production.

[0031] (4) By adjusting the size and concentration of the polystyrene nanospheres, the present invention can precisely control the particle size of the composite material within the range of 100 - 300 nm, thereby optimizing the electrochemical performance of the material. This particle size control technology provides a guarantee for the high performance of the material.

[0032] (5) The results of electrochemical performance tests show that the P-LiMn 0.8 Fe 0.2 PO 4 / C composite material prepared by the present invention has an initial discharge specific capacity of more than 140 mAh / g at a rate of 0.1C, approaching its theoretical capacity (170 mAh / g). After 300 cycles at a rate of 0.1C, the capacity retention rate exceeds 95%, showing excellent cycle stability. In addition, the material also shows outstanding discharge performance at high rates, indicating its suitability for high-power lithium-ion batteries.

[0033] In summary, the lithium iron phosphate manganese composite cathode material with high energy density assisted by polystyrene microspheres and its preparation method provided by the present invention not only have simple process and low cost, but also can significantly improve the electrochemical performance of the P-LiMn 0.8 Fe 0.2 PO 4 / C composite material, including high specific capacity, excellent cycle stability and good rate performance. This process provides reliable technical support for the large-scale industrial production of lithium iron phosphate manganese cathode materials and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 The electron micrographs of the materials prepared in Examples 1 - 3 and Comparative Example 1 are as follows: a is Comparative Example 1, b is Example 1, c is Example 2, and d is Example 3;

[0035] Figure 2 Charge-discharge data graphs at 0.1C for the materials prepared in Examples 1-3 and Comparative Example 1. Detailed implementation manners

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] Unless otherwise specified, the technical means used in the following examples are all conventional means well known to those skilled in the art. The experimental methods without specific conditions mentioned are all conventional methods in the art.

[0038] The materials, reagents, etc. used in the following examples can be obtained from commercial sources unless otherwise specified.

[0039] Example 1

[0040] A preparation method of a lithium iron phosphate manganese composite cathode material with high energy density assisted by polystyrene microspheres is as follows:

[0041] (1) Preparation of polystyrene microspheres:

[0042] First, 0.15 - 0.2 g of polyvinylpyrrolidone (PVP) is stirred and dissolved in 80 - 100 mL of deionized water. After purging with nitrogen for 30 min, 10 mL of styrene (containing 4-tert-butylcatechol as a stabilizer) is added, and the mixture is heated to 70 °C and kept warm for 30 min to form a white emulsion. Then, 1.2 - 1.4 mL of an aqueous solution containing 0.1 - 0.4 g of azobisisobutyramidine hydrochloride (V-50) initiator is added to the white emulsion, and the reaction is carried out for 24 h under a nitrogen atmosphere at 70 °C for complete polymerization. The filtered precipitate is washed with a large amount of water until neutral. Finally, the polystyrene nanospheres are washed with ethanol and vacuum dried overnight at 60 °C. The dried powder is ground and collected in a 50 mL centrifuge tube for standby.

[0043] (2) Weigh the manganese source, iron source, and phosphate source according to a stoichiometric ratio of 4.8:1.2:6, specifically 4.8 mmol of manganese sulfate (MnSO 4 ·H 2 O), 1.2 mmol of ferrous sulfate (FeSO 4 ·7H 2 O), and 6.0 mmol of ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ), pour them into a 250 mL three-necked flask, and add 20 mL of deionized water to obtain solution A.

[0044] (3) Add 6.0 g of urea (CO(NH 2 )2 ) After magnetic stirring for 5 min, solution B was obtained.

[0045] (4) 30 mL of ethylene glycol was added to solution B. After magnetic stirring for 5 min, a certain amount of the prepared polystyrene microsphere powder was added. Finally, the mixed solution was ultrasonically treated for 15 min to obtain solution C.

[0046] (5) Solution C was subjected to a water bath reaction under magnetic stirring at 80 °C in a nitrogen atmosphere for 2 h to obtain an in-situ carbon-coated precursor slurry D.

[0047] (6) The precursor slurry D was quenched to room temperature with cooling water, filtered, and the filter cake was washed thoroughly 3 times with deionized water and absolute ethanol respectively. The washed filter cake was placed in a vacuum drying oven and dried at 60 °C for 8 h to obtain a white precursor powder.

[0048] (7) 20 mmol of the white powder was mixed and ball-milled with 55 mmol of lithium carbonate (Li 2 CO 3 ) for 2 h, and then transferred to a tube furnace. It was calcined at 650 °C for 8 h under an argon-hydrogen mixed gas atmosphere (the volume ratio of hydrogen was 5%) to obtain a black powder of in-situ carbon-coated lithium iron manganese phosphate cathode material.

[0049] (8) The black powder and sucrose were dispersed in an aqueous solution at a mass ratio of 100:(5 - 25), dried, transferred to a tube furnace, and annealed at 650 °C for 2 h under an argon-hydrogen mixed gas atmosphere (the volume ratio of hydrogen was 5%) to finally obtain a uniformly carbon-coated nano-lithium iron manganese phosphate cathode material P-LiMn 0.8 Fe 0.2 PO 4 / C (P-LMFP / C), thus obtained.

[0050] Example 2

[0051] A preparation method of a polystyrene microsphere-assisted high-energy density lithium iron manganese phosphate composite cathode material, the specific steps are as follows:

[0052] (1) Preparation of polystyrene microspheres:

[0053] First, dissolve 0.15 - 0.2 g of polyvinylpyrrolidone (PVP) in 80 - 100 mL of deionized water by stirring. After purging with nitrogen for 30 min, add 10 mL of styrene (containing 4-tert-butylcatechol as a stabilizer), heat to 70 °C, and keep the temperature for 30 min to form a white emulsion. Then, add 1.2 - 1.4 mL of an aqueous solution containing 0.1 - 0.4 g of azobisisobutyramidine hydrochloride (V-50) initiator to the white emulsion, and react for 24 h under a nitrogen atmosphere at 70 °C for complete polymerization. Wash the filtered precipitate with a large amount of water until neutral. Finally, wash the polystyrene nanospheres with ethanol and dry them overnight in a vacuum at 60 °C. The dried powder is ground and collected in a 50 mL centrifuge tube for standby.

[0054] (2) Weigh the manganese source, iron source, and phosphate source according to a stoichiometric ratio of 4.8:1.2:6, specifically 4.8 mmol of manganese sulfate (MnSO 4 ·H 2 O), 1.2 mmol of ferrous sulfate (FeSO 4 ·7H 2 O), and 6.0 mmol of ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ), pour them into a 250 mL three-necked flask, and add 20 mL of deionized water to obtain solution A.

[0055] (3) Add 6.0 g of urea (CO(NH 2 ) 2 ) to solution A, and after magnetic stirring for 5 min, obtain solution B.

[0056] (4) Add 30 mL of ethylene glycol to solution B, magnetic stir for 5 min, then add a certain amount of the polystyrene microsphere powder prepared above, and finally ultrasonicate the mixed solution for 15 min to obtain solution C.

[0057] (5) React solution C in a water bath under a nitrogen atmosphere and magnetic stirring at 80 °C for 2 h to obtain an in-situ carbon-coated precursor slurry D.

[0058] (6) Quench the precursor slurry D to room temperature with cooling water, filter it, and wash the filter cake with deionized water and absolute ethanol three times respectively. Place the washed filter cake in a vacuum drying oven and dry it at 60 °C for 8 h to obtain a white precursor powder.

[0059] (7) Mix the white powder with lithium carbonate (Li 2 CO 3)Mix and ball-mill for 2 h in a molar ratio of 20:55, then transfer to a tubular furnace and calcine at 700 °C for 8 h under an argon-hydrogen mixed atmosphere (the volume ratio of hydrogen is 5%) to obtain a black powder of in-situ carbon-coated lithium iron manganese phosphate cathode material.

[0060] (8)Disperse the black powder and sucrose in an aqueous solution in a mass ratio of 100:(5 - 25), dry, transfer to a tubular furnace, and anneal at 700 °C for 2 h under an argon-hydrogen mixed atmosphere (the volume ratio of hydrogen is 5%) to finally obtain a uniformly carbon-coated nano lithium iron manganese phosphate cathode material P-LiMn 0.8 Fe 0.2 PO 4 / C (P-LMFP / C), thus obtained.

[0061] Example 3

[0062] A preparation method of a polystyrene microsphere-assisted lithium iron manganese phosphate composite cathode material with high energy density, the specific steps are as follows:

[0063] (1) Prepare polystyrene microspheres:

[0064] First, dissolve 0.15 - 0.2 g of polyvinylpyrrolidone (PVP) in 80 - 100 mL of deionized water by stirring. After purging with nitrogen for 30 min, add 10 mL of styrene (containing 4-tert-butylcatechol as a stabilizer), heat to 70 °C, and keep warm for 30 min to form a white emulsion; then add 1.2 - 1.4 mL of an aqueous solution containing 0.1 - 0.4 g of azodiisobutyramidine hydrochloride (V-50) initiator to the white emulsion, react for 24 h under a nitrogen atmosphere at 70 °C for complete polymerization, and wash the filtered precipitate with a large amount of water until neutral; finally, wash the polystyrene nanospheres with ethanol and vacuum dry overnight at 60 °C. The dried powder is ground and collected in a 50 mL centrifuge tube for standby.

[0065] (2) Weigh the manganese source, iron source, and phosphate source according to a stoichiometric ratio of 4.8:1.2:6, specifically 4.8 mmol of manganese sulfate (MnSO 4 ·H 2 O), 1.2 mmol of ferrous sulfate (FeSO 4 ·7H 2 O), 6.0 mmol of ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ), pour into a 250 mL three-necked flask, and add 20 mL of deionized water to obtain solution A.

[0066] (3) Add 6.0 g of urea (CO(NH 2 )) 2After magnetic stirring for 5 min, solution B was obtained.

[0067] (4) 30 mL of ethylene glycol was added to solution B. After magnetic stirring for 5 min, a certain amount of the above-prepared polystyrene microsphere powder was added. Finally, the mixed solution was ultrasonicated for 15 min to obtain solution C.

[0068] (5) Solution C was subjected to a water bath reaction at 80 °C under a nitrogen atmosphere with magnetic stirring for 2 h to obtain an in-situ carbon-coated precursor slurry D.

[0069] (6) The precursor slurry D was quenched to room temperature with cooling water and filtered. The filter cake was washed thoroughly 3 times with deionized water and absolute ethanol respectively. The washed filter cake was placed in a vacuum drying oven and dried at 60 °C for 8 h to obtain a white precursor powder.

[0070] (7) The white powder was mixed with lithium carbonate (Li 2 CO 3 ) in a molar ratio of 20:55 and ball-milled for 2 h. Then it was transferred to a tube furnace and calcined at 750 °C for 8 h under an argon-hydrogen mixed atmosphere (the volume ratio of hydrogen was 5%) to obtain a black powder of in-situ carbon-coated lithium iron manganese phosphate cathode material.

[0071] (8) The black powder and sucrose were dispersed in an aqueous solution in a mass ratio of 100:(5 - 25), dried, transferred to a tube furnace, and annealed at 750 °C for 2 h under an argon-hydrogen mixed atmosphere (the volume ratio of hydrogen was 5%) to finally obtain a uniformly carbon-coated nano lithium iron manganese phosphate cathode material P-LiMn 0.8 Fe 0.2 PO 4 / C (P-LMFP / C), thus obtained.

[0072] Comparative Example 1

[0073] A preparation method of a lithium iron manganese phosphate cathode material, the specific steps are as follows:

[0074] (1) The manganese source, iron source, and phosphate source were weighed according to a stoichiometric ratio of 4.8:1.2:6, specifically 4.8 mmol of manganese sulfate (MnSO 4 ·H 2 O), 1.2 mmol of ferrous sulfate (FeSO 4 ·7H 2 O), and 6.0 mmol of ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ). They were poured into a 250 mL three-necked flask, and 20 mL of deionized water was added to obtain solution A.

[0075] (2) 6.0 g of urea (CO(NH2 ) 2 ) After magnetic stirring for 5 min, solution B was obtained.

[0076] (3) 30 mL of ethylene glycol was added to solution B. After magnetic stirring for 5 min, the mixture was subjected to a water bath reaction at 80 °C under a nitrogen atmosphere with magnetic stirring for 2 h, and then quenched to room temperature, filtered. The filter cake was washed thoroughly three times with deionized water and absolute ethanol respectively. The washed filter cake was placed in a vacuum drying oven and dried at 60 °C for 8 h to obtain a precursor white powder.

[0077] (4) The white powder was mixed with lithium carbonate (Li 2 CO 3 ) in a molar ratio of 20:55 and ball-milled for 2 h. Then it was transferred to a tube furnace and calcined at 550 °C for 8 h under an argon-hydrogen mixed gas atmosphere (the volume ratio of hydrogen was 5%) to obtain the lithium iron manganese phosphate cathode material, thus obtained.

[0078] Test Example 1

[0079] (1) Electrode preparation

[0080] The P-LMFP / C cathode composite materials prepared in Examples 1-3 and the lithium iron manganese phosphate cathode material prepared in Comparative Example 1 were respectively mixed with conductive carbon black and binder polyvinylidene fluoride (PVDF) in a mass ratio of 95:3:2. Using N-methylpyrrolidone (NMP) as a solvent, the mixture was made into a uniform slurry. The slurry was uniformly coated on the surface of aluminum foil and then dried at 120 °C for 12 h. The dried electrode sheet was pressed to the required thickness by a roll press to make a positive electrode sheet.

[0081] (2) Battery assembly

[0082] Celgard 2400 polypropylene porous membrane was used as the separator, and 1 mol / L LiPF 6 / (EC + DEC + DMC) (volume ratio 1:1:1) was used as the electrolyte. In a glove box filled with argon, the prepared positive electrode sheet and a lithium metal sheet (negative electrode sheet) were assembled into a CR 2025 type coin-shaped half-cell.

[0083] (3) Electrochemical performance test

[0084] The assembled coin-shaped half-cell was subjected to charge-discharge tests at room temperature. The test conditions were: the charge-discharge voltage range was 2.5 V to 4.3 V (vs. Li / Li + ), and the charge-discharge rate was 0.1C. Electrochemical performance parameters such as the first discharge specific capacity, first discharge Coulombic efficiency and capacity retention rate after cycling of the battery were recorded through the tests.

[0085] (4) Test results

[0086] The electron microscope images of the materials prepared in Examples 1-3 and Comparative Example 1 at different temperatures are as follows Figure 1 shown. It was found by observation that the particle size of the materials was distributed between 100 and 300 nm.

[0087] As Figure 1 shown in a, at this temperature (550 °C), the sample state was poor, so the capacity was low and there were huge particles; as Figure 1 shown in b, when the temperature was 650 °C, there were relatively more large particles and a small amount of agglomeration; as Figure 1 shown in c, when the temperature was 700 °C, the particles were relatively uniform and round. For this reason, the data of Example 2 in Table 1 below showed better capacity, initial efficiency and cycling performance of the sample; as Figure 1 shown in d, when the temperature was 750 °C, a large amount of agglomeration occurred during particle sintering, resulting in some larger particles and poor performance.

[0088] The 0.1C first charge-discharge curves of the P-LMFP / C cathode composite materials prepared in Examples 1-3 at different temperatures are as follows Figure 2 shown. The charge-discharge test results are shown in Table 1 below.

[0089] Table 1 Charge-discharge test results

[0090]

[0091] From Table 1 and the test results shown in Figure 1 and Figure 2 it can be seen that the first discharge specific capacities of the P-LMFP / C composite materials prepared in Examples 1-3 at different calcination temperatures all reached above 140 mAh / g at a 0.1C rate, approaching their theoretical capacity (170 mAh / g). After cycling 300 times at a 0.1C rate, the capacity retention rates of Examples 1-3 all exceeded 95%, showing excellent cycling stability. In contrast, the first discharge specific capacity and cycling stability of the conventional lithium iron manganese phosphate cathode material prepared in Comparative Example 1 were significantly lower than those of Examples 1-3 under the same test conditions.

[0092] Specifically, the P-LMFP / C cathode composite material prepared in Example 2 at a calcination temperature of 700 °C showed the best electrochemical performance, and its first discharge specific capacity and cycling stability were both better than those of Example 1 and Example 3. When the calcination temperature was increased to 750 °C (Example 3), the electrochemical performance of the material decreased. This was because particle aggregation occurred at high temperatures, resulting in an increase in the particle size of the material and an extension of the lithium ion diffusion path, thus limiting the fast diffusion kinetics of lithium ions. In contrast, under the calcination conditions of 700 °C in Example 2, the material particles were evenly distributed and the particle size was moderate, effectively optimizing the lithium ion transport path and significantly improving the electrochemical performance of the material.

[0093] Further comparing the performance data of Examples 1-3 with that of Comparative Example 1, it can be seen that the initial discharge specific capacity of the P-LMFP / C cathode composite material prepared in Examples 1-3 at a rate of 0.1C is significantly higher than that of Comparative Example 1, and the difference in the Coulomb efficiency of its initial discharge reaches up to 6.4%. In addition, after 300 cycles, the capacity retention rate of Examples 1-3 is up to 5.8% higher than that of Comparative Example 1. These data fully demonstrate that the P-LMFP / C cathode composite material provided by the present invention is significantly superior to the conventional lithium iron manganese phosphate cathode material in terms of specific capacity, Coulomb efficiency, and cycle stability.

[0094] In summary, the present invention has successfully prepared a high-performance P-LMFP / C cathode composite material by optimizing the preparation process. This material not only has a high specific capacity and excellent cycle stability, but also significantly improves the diffusion kinetics performance of lithium ions, providing an important material basis for the development of high-energy-density lithium-ion batteries.

[0095] The embodiments described above are only a part of the embodiments of the present invention, rather than all of the embodiments. The detailed description of the embodiments of the present invention is not intended to limit the scope of the present invention claimed, but merely represents the selected embodiments of the present invention. The scope of protection of the present invention shall be subject to the scope claimed in the claims. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing a high energy density lithium iron manganese phosphate composite positive electrode material assisted by polystyrene microspheres, characterized in that: The following steps are involved: Step 1) weighing a manganese source, an iron source, and a phosphate source according to a stoichiometric ratio of 4.8:1.2:6, and adding deionized water to obtain a solution A; Step 2) adding urea to solution A and stirring with magnetic force to obtain solution B; Step 3) Ethylene glycol was added to solution B, and after magnetic stirring, polystyrene microsphere powder was added, and finally the mixed solution was sonicated to obtain solution C; Step 4) reacting solution C in a water bath under a nitrogen atmosphere at 50-100° C. with magnetic stirring to obtain an in-situ carbon coating precursor slurry D; Step 5) quenching the precursor slurry D, filtering, washing, and drying in a vacuum drying oven to obtain a precursor powder; Step 6) mixing the precursor powder with lithium carbonate and ball milling, and then calcining at high temperature in an argon-hydrogen mixed atmosphere to obtain an in-situ carbon-coated lithium manganese iron phosphate positive electrode material; Step 7) The in-situ carbon-coated lithium manganese iron phosphate positive electrode material and sucrose are dispersed in an aqueous solution in proportion, dried, and annealed to finally obtain a uniformly carbon-coated nano-lithium manganese iron phosphate positive electrode material.

2. The method for preparing a polystyrene microsphere-assisted high energy density lithium manganese iron phosphate composite positive electrode material according to claim 1, characterized in that: In step 1), the manganese source is manganese sulfate, the iron source is ferrous sulfate, and the phosphate source is diammonium phosphate.

3. The method for preparing a polystyrene microsphere-assisted high energy density lithium manganese iron phosphate composite positive electrode material according to claim 1, characterized in that: In step 2), the amount of solution A is 20 mL, the amount of urea is 6.0 g, and the magnetic stirring time is 5 min; in step 3), the magnetic stirring time is 5 min, and the ultrasonic time is 15 min; in step 4), the water bath reaction time is 1.5 to 2 h.

4. The method for preparing a polystyrene microsphere-assisted high energy density lithium manganese iron phosphate composite positive electrode material according to claim 1, characterized in that: Step 3) The preparation process of the polystyrene microspheres is as follows: First, polyvinyl pyrrolidone is stirred and dissolved in deionized water. After nitrogen is passed through, styrene containing 4-tert-butylcatechol is added and heated to 65-80°C to form a white emulsion. Then, an aqueous solution containing azobisisobutylamidine hydrochloride initiator is added to the white emulsion, and complete polymerization is carried out under a nitrogen atmosphere at 65-80°C. The precipitate after washing and filtering is neutral. Finally, the polystyrene nanospheres are washed with ethanol, vacuum dried at 60°C overnight, and ground for use.

5. The method for preparing a polystyrene microsphere-assisted high energy density lithium manganese iron phosphate composite positive electrode material according to claim 1, characterized in that: Step 5) The washing is specifically as follows: washing with deionized water and anhydrous ethanol for 3 to 5 times respectively; the drying temperature is 50 to 60° C. and the drying time is 8 to 10 hours.

6. The method for preparing a polystyrene microsphere-assisted high energy density lithium manganese iron phosphate composite positive electrode material according to claim 1, characterized in that: Step 6) In the argon-hydrogen mixed atmosphere, the volume ratio of hydrogen is 5%; the ball milling time is 2 hours; the high temperature calcination temperature is 650-750°C, and the time is 8 hours; preferably, the high temperature calcination temperature is 700°C.

7. The method for preparing a polystyrene microsphere-assisted high energy density lithium manganese iron phosphate composite positive electrode material according to claim 1, characterized in that: Step 7) The mass ratio of the in-situ carbon-coated lithium manganese iron phosphate positive electrode material to sucrose is 100:(5-25); the annealing atmosphere is an argon-hydrogen mixed atmosphere, in which the volume ratio of hydrogen is 5%; the annealing temperature is 650-750° C., and the time is 2-3 h.

8. A high energy density lithium manganese iron phosphate composite positive electrode material assisted by polystyrene microspheres prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the high energy density lithium iron manganese phosphate composite positive electrode material assisted by polystyrene microspheres as claimed in claim 8 in the preparation of lithium ion batteries.

10. The use according to claim 9, characterized in that: The polystyrene microsphere-assisted high-energy-density lithium manganese iron phosphate composite positive electrode material is mixed with conductive carbon black and binder polyvinylidene fluoride in a mass ratio of 95:3:2, and N-methylpyrrolidone is used as a solvent. After being adjusted into a slurry, it is evenly coated on the surface of an aluminum foil and dried at 120° C. for 12 hours to prepare a positive electrode sheet.

11. A lithium ion battery, characterized in that: It comprises a positive electrode plate, a separator, an electrolyte and a negative electrode plate; the positive electrode plate is made of a high-energy-density lithium iron manganese phosphate composite positive electrode material assisted by polystyrene microspheres as described in claim 8; the separator is a Celgard2400 polypropylene porous membrane; the electrolyte is LiPF6 / (EC+DEC+DMC); and the negative electrode plate is a metal lithium plate.

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