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

Abstract

The application discloses a lithium iron manganese phosphate composite material and a preparation method thereof, a positive plate and a lithium ion battery. The preparation method of the lithium iron manganese phosphate composite material comprises the following steps: dispersing manganese acetate and ferrous oxalate in water to form a dispersion liquid; heating the dispersion liquid, adding a precipitant to perform a co-precipitation reaction under microwave irradiation to obtain a precursor; preparing a composite carbon source solution; uniformly ball-milling the precursor, lithium dihydrogen phosphate and zinc oxide, and then performing heat treatment; immersing the obtained product in the composite carbon source solution, performing filtration, and then performing gradient carbonization treatment; performing primary sintering on the sample after the gradient carbonization treatment, crushing the sample, and then performing secondary sintering to obtain the lithium iron manganese phosphate composite material. When the lithium iron manganese phosphate composite material prepared by the application is applied to the lithium ion battery, the lithium iron manganese phosphate composite material exhibits excellent electrochemical performance, the manganese dissolution rate is only 0.5-1.2%, the compaction density is up to 2.48 g / cm3, and the lithium iron manganese phosphate composite material shows a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode materials, and particularly relates to a lithium manganese iron phosphate composite material, its preparation method, cathode sheet, and lithium-ion battery. Background Technology

[0002] Lithium manganese iron phosphate (LFP), as an upgraded material of LFP, combines a high voltage platform (4.1V) with high safety, and its theoretical energy density is 15%~20% higher than that of LFP. However, existing preparation technologies still face three major bottlenecks: First, the challenge of controlling the uniformity of manganese iron, due to the Fe... 3+ With Mn 2+ Significant pH differences (ΔpH>2) in aqueous solutions make it difficult to form a homogeneous solid solution using traditional co-precipitation methods, leading to manganese segregation regions in the material. These regions undergo Jahn-Teller distortion during charge and discharge, further exacerbating manganese dissolution and causing a cycle capacity decay of up to 38%. Secondly, lithium manganese iron phosphate suffers from a dual defect in conductivity; its intrinsic electronic conductivity is extremely low (approximately 10⁻⁶). -13 S / cm), and the lithium-ion diffusion coefficient is low (approximately 10). -14 (cm² / s). Although traditional carbon coating technology can improve electron conductivity to some extent, it cannot effectively suppress the occurrence of interfacial side reactions under high voltage. Especially under high voltage conditions, the electrolyte decomposes severely, leading to prominent gas generation problems. Third, the structural stability is insufficient. Materials prepared by existing single-sintering processes often exhibit a "core-shell structure inhomogeneity" phenomenon, with poor continuity of the surface coating layer. It is easy to fall off during subsequent crushing and processing, thereby exposing the matrix material and further aggravating the dissolution of manganese.

[0003] Existing methods for preparing lithium manganese iron phosphate each have their advantages and disadvantages. High-temperature solid-state method: This method is simple and easy to scale up, but due to uneven material mixing, severe manganese segregation occurs, affecting its electrochemical performance; the capacity is typically no more than 145 mAh / g, and the cycle life is less than 2000 cycles. Hydrothermal synthesis method: This method can achieve high crystallinity and uniform cost distribution; however, it requires complex equipment and is costly, while the prepared material has a low compaction density of approximately 2.2 g / cm³. Co-precipitation method: The precursor prepared by the co-precipitation method has good homogeneity, but due to Fe... 3+ With Mn 2+ The precipitation in aqueous solution exhibits significant pH differences, resulting in a manganese leaching rate exceeding 5% (after 500 cycles). Solvothermal method: This method can prepare nanoscale particles, but the recovery of organic solvents is difficult, and the gas production is high, approximately 3 mL / Ah.

[0004] Although various improved technologies have been applied (such as Guoxuan High-Tech's ultrasonic-assisted sintering and Hubei Wanrun's ion-scale mixed spray drying), these technologies have still failed to fundamentally solve the problems of manganese leaching and interface stability. In particular, existing coating technologies mostly focus on a single carbon layer, with insufficient research on the synergistic coating of ion conductors. Xingfa Group's orthorhombic iron source increased density but did not improve cycle life; Lanjun New Energy's lithium phosphate coating suppressed side reactions but had limited improvement in electronic conductivity. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a lithium manganese iron phosphate composite material, its preparation method, positive electrode sheet, and lithium-ion battery, thereby solving problems such as difficulty in controlling the uniformity of manganese and iron, poor conductivity, and insufficient structural stability in the existing lithium manganese iron phosphate preparation process.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for preparing a lithium manganese iron phosphate composite material includes the following steps:

[0008] Manganese acetate and ferrous oxalate were dispersed in water at a Mn:Fe molar ratio of 0.8 to 2.0:1 to form a dispersion. Under a protective atmosphere, the dispersion was heated to 58 to 62°C, and a precipitant was added to carry out a co-precipitation reaction while being irradiated with microwaves to obtain the precursor.

[0009] Prepare a composite carbon source solution containing a first carbon source and a second carbon source;

[0010] The precursor, lithium dihydrogen phosphate and zinc oxide were ball-milled and mixed evenly, and then subjected to heat treatment. The resulting product was impregnated in the composite carbon source solution, filtered, and then subjected to gradient carbonization treatment.

[0011] The sample after gradient carbonization was first sintered at 730~770℃, and after being crushed, it was sintered at 520~580℃ to obtain lithium manganese iron phosphate composite material.

[0012] In this invention, a (Mn,Fe)C2O4 precursor is first synthesized using a microwave-assisted oxalate coprecipitation method; then, a Zn-doped lithium iron phosphate conductive layer is coated onto its surface through heat treatment; next, the obtained material is immersed in a mixed carbon source solution, and after being removed, it undergoes gradient carbonization treatment to pyrolyze the surface to form a carbon layer; subsequently, during the primary sintering process, the inner (Mn,Fe)C2O4 precursor reacts with the lithium iron phosphate conductive layer to generate olivine-phase lithium manganese iron phosphate; finally, the outermost carbon layer is graphitized through secondary sintering.

[0013] Specifically, the non-thermal effect of microwave pulses induces the directional alignment of oxalate ions, forming a (Mn,Fe)C₂O₄ coprecipitate rather than a two-phase separation. Compared to traditional coprecipitation methods, this method achieves atomic-level mixing, resulting in a manganese and iron distribution uniformity of 98.5% (EDS surface scan statistics). Furthermore, the prepared product exhibits a controllable morphology, generating near-spherical particles with a specific surface area of ​​8–10 m². 2 / g. Furthermore, due to the intermittent nature (pause period) of microwave radiation, the micro-regions in the reaction system can achieve instantaneous cooling, thereby effectively preventing Mn from being absorbed. 2+ Oxidized to Mn 3+ In addition, regarding the selection of manganese and iron sources, manganese acetate can precisely buffer the pH, and ferrous oxalate precipitates simultaneously, which can effectively inhibit manganese and iron segregation and leave no impurities.

[0014] In the preparation steps of the lithium phosphate ion conductive layer, the low-temperature melting characteristics of lithium dihydrogen phosphate and H... + Reactivity is key to forming a uniform ionic conductor layer. Furthermore, zinc oxide doping can lower the lithium migration barrier and stabilize the manganese-iron lattice, thereby increasing cycle life and reducing manganese dissolution rate.

[0015] Preferably, in the preparation step of the dispersion, the water is deoxygenated and deionized water.

[0016] Preferably, in the preparation step of the dispersion, the mass ratio of manganese acetate to water is 1:12~20.

[0017] Preferably, in the precursor preparation step, the protective atmosphere is nitrogen; and the coprecipitation reaction time is 20-35 min.

[0018] Preferably, in the precursor preparation step, the precipitant is an oxalic acid-ammonium oxalate buffer solution; the amount of precipitant added is adjusted to adjust the pH of the system to 2.8-3.2.

[0019] Preferably, in the precursor preparation step, the operating parameters of the microwave radiation are: frequency of 2.40~2.50GHz, pulse width of 8~15ms, interval of 3~8ms, and peak power of 250~350W.

[0020] In this invention, there is a dynamic matching relationship between microwave radiation power and the precipitant addition rate. Both need to be synergistically adjusted to maintain the stability of the reaction system. Experiments show that as the precipitant addition rate decreases, the microwave radiation power needs to be increased to maintain the reaction conditions. Specifically, when the precipitant is added at a rate of 2 mL / min, the microwave power is set to 80 W; while when the rate is reduced to 0.5 mL / min, the microwave power needs to be increased to 300 W.

[0021] Preferably, in the preparation step of the composite carbon source solution, the first carbon source is a polymer containing catechol groups; the second carbon source is a C6-C12 sugar or a hydroxycarboxylic acid.

[0022] Preferably, the polymer containing catechol groups is polydopamine; and the C6-C12 sugar is at least one of sucrose and glucose.

[0023] In this invention, the molecular weight of polydopamine is 2000-5000. If the molecular weight is too low, it is difficult to form a cross-linked structure; while if the molecular weight is too high, it may lead to a decrease in dispersion performance. The biomimetic adhesion and nitrogen doping characteristics of polydopamine, combined with the wide temperature range carbonization ability of sucrose, work together to form a highly conductive ultrathin continuous carbon layer.

[0024] Preferably, in the preparation step of the composite carbon source solution, the concentration of the first carbon source is 2~5wt%; the concentration of the second carbon source is 1.5~4wt%; and Tris-HCl buffer solution with pH=8.5 is used as the solvent.

[0025] Preferably, the lithium dihydrogen phosphate accounts for 8-12 wt% of the precursor mass; and the zinc oxide accounts for 0.3-1.0 wt% of the precursor mass.

[0026] Preferably, the heat treatment is performed by transferring the ball-milled sample to a fluidized bed and heat-treating it at 330-370°C for 0.8-1.2 hours using a mixture of steam and nitrogen. More preferably, the humidity of the mixture is 40%.

[0027] In this invention, a mixture of water vapor and nitrogen is used as the heat treatment atmosphere in a fluidized bed reactor. Lithium dihydrogen phosphate is melted by heating and spread on the surface of the precursor particles to form a zinc-doped lithium phosphate ion conductive layer.

[0028] Preferably, the heat-treated product is immersed in the composite carbon source solution for 20-40 minutes at a temperature of 40-60°C. More preferably, the immersion time is 25-35 minutes at a temperature of 45-55°C.

[0029] Preferably, the gradient carbonization process is performed as follows: first, the temperature is maintained at 230~270℃ for 0.8~1.2h, then the temperature is increased to 530~570℃ and maintained for 1.8~2.2h, and finally the temperature is increased to 660~700℃ and maintained for 0.8~1.2h.

[0030] In this invention, the three stages of gradient carbonization treatment correspond to the crosslinking of polydopamine, the carbonization of sucrose, and the growth of graphite microcrystals, respectively.

[0031] In this invention, the inner lithium phosphate ion conductive layer serves as a superionic conductor (Li). + Electrical conductivity is 10 -4 The double coating (S / cm) shortens the lithium-ion diffusion path; the outer polydopamine-derived carbon forms a continuous graphene-like network, improving electronic conductivity. This double coating significantly reduces interfacial impedance.

[0032] Preferably, the primary calcination is carried out in an atmosphere of N2 / H2 = 95:5.

[0033] Preferably, the pulverization process employs air jet milling to control the particle content of D50=1.0~1.5μm to be ≤30%.

[0034] Preferably, after air jet milling, the particle size distribution of the sample is as follows: fine particles: D50=1.0~1.5μm, accounting for 30%; medium particles: D50=2.0~3.0μm, accounting for 50%; coarse particles: D50=4.0~5.0μm, accounting for 20%.

[0035] In this invention, a tightly packed electrode structure is constructed through a three-stage particle size distribution (fine:medium:coarse = 3:5:2) and a pre-compaction process, achieving a compaction density exceeding 2.45 g / cm³. 3 bottleneck.

[0036] Preferably, the primary sintering time is 7.8~8.2h; the secondary sintering time is 2.8~3.2h.

[0037] The lithium manganese iron phosphate composite material prepared by the above-mentioned method is a lithium manganese iron phosphate composite material.

[0038] A positive electrode includes the above-mentioned lithium manganese iron phosphate composite material.

[0039] A lithium-ion battery comprising the aforementioned positive electrode.

[0040] Compared with the prior art, the beneficial effects of the present invention include:

[0041] (1) This invention solves the phase separation problem caused by pH difference in Fe / Mn ion precipitation by controlling precipitation kinetics with pulsed microwave field, and obtains atomically uniform precursor.

[0042] (2) The first double-layer coating structure of “ion conductor + electronic conductor”: the inner Zn-doped lithium phosphate conductive layer provides a fast lithium ion channel, and the outer nitrogen-doped carbon enhances electronic conduction and synergistically reduces interface impedance.

[0043] (3) Through the synergistic effect of primary sintering followed by crushing and multi-particle gradation, the compaction density of the material is significantly improved to 2.42~2.48 g / cm³, breaking through the limits of traditional processes. Attached Figure Description

[0044] Figure 1 The XRD diffraction pattern of the lithium manganese iron phosphate composite material prepared in Example 1 is shown.

[0045] Figure 2 The image shows a scanning electron microscope image of the lithium manganese iron phosphate composite material prepared in Example 1.

[0046] Figure 3 Scanning electron microscope image of the lithium manganese iron phosphate composite material prepared for Comparative Example 2. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] The purity of Mn(CH3COO)2·4H2O in the examples and comparative examples is ≥99.5%; the particle size D50 of FeC2O4·2H2O is 0.5~1μm; the molecular weight of polydopamine is 2000~5000; and the concentration of the oxalic acid-ammonium oxalate buffer solution is 0.5mol / L.

[0049] Example 1

[0050] A method for preparing a lithium manganese iron phosphate composite material, the specific steps of which are as follows:

[0051] (1) Dissolve 12.25g Mn(CH3COO)2·4H2O and 7.16g FeC2O4·2H2O in 200mL of deoxygenated and deionized water to obtain a dispersion. Under nitrogen protection, heat the dispersion to 60℃ in a water bath. While irradiating with pulsed microwaves, add oxalic acid-ammonium oxalate buffer solution at a rate of 0.5mL / min to carry out a coprecipitation reaction. The coprecipitation reaction time is 30min. After the reaction is completed, age for 2h and then centrifuge and wash to obtain the precursor. The peak power of the pulsed microwave is 300W, the duty cycle is 40%, the frequency is 2.45GHz, the pulse width is 10ms, and the interval is 5ms. The amount of oxalic acid-ammonium oxalate buffer solution added is adjusted to adjust the pH of the system to 3.0 as the endpoint.

[0052] (2) Take 100g of Tris-HCl buffer solution with pH=8.5 as solvent, add 2g of polydopamine and 3g of sucrose to prepare a composite carbon source solution;

[0053] (3) The precursor prepared in step (1), 1.2 g of lithium dihydrogen phosphate and 0.03 g of zinc oxide were ball-milled and mixed for 2 h and then placed in a fluidized bed. A nitrogen-water vapor mixture with a humidity of 40% was introduced and heat-treated at 350°C for 1 h. The resulting product was immersed in the composite carbon source solution described in step (2) at 50°C and stirred continuously for 30 min. After filtration, a gradient carbonization treatment was carried out: first, the temperature was kept at 250°C for 1 h, then the temperature was raised to 550°C and kept at 2 h, and finally the temperature was raised to 680°C and kept at 1 h.

[0054] (4) In an atmosphere of N2 / H2=95:5, the sample after gradient carbonization treatment is first sintered at 750℃ for 8h, and then pulverized by airflow to make the particle classification meet the following requirements: fine particles: D50=1.2μm, accounting for 30%; medium particles: D50=2.5μm, accounting for 50%; coarse particles: D50=4.5μm, accounting for 20%; finally, the pulverized sample is sintered at 550℃ for 3h for a second time to obtain the lithium manganese iron phosphate composite material.

[0055] Figure 1 The XRD diffraction pattern of the lithium manganese iron phosphate composite material prepared in Example 1 is shown. XRD analysis results indicate that the main diffraction peaks of this material highly match the characteristic peaks of the lithium manganese iron phosphate standard card, suggesting that the prepared product is the target compound.

[0056] Figure 2 This is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite material prepared in Example 1. We can see that the composite material has a uniformly distributed particle morphology and no significant particle agglomeration.

[0057] Example 2

[0058] (1) Preparation of precursor: 14.35g Mn(CH3COO)2·4H2O and 5.37g FeC2O4·2H2O were dissolved in 200mL of deoxygenated and deionized water to obtain a dispersion. Under nitrogen protection, the dispersion was heated to 60℃ in a water bath. Oxalic acid-ammonium oxalate buffer solution was added at a rate of 0.4 mL / min during pulsed microwave radiation to carry out a coprecipitation reaction. The coprecipitation reaction time was 30min. After the reaction was completed, the mixture was aged for 2h and then centrifuged and washed to obtain the precursor. The peak power of the pulsed microwave was 300W, the duty cycle was 40%, the frequency was 2.45GHz, the pulse width was 10ms, and the interval was 5ms. The amount of oxalic acid-ammonium oxalate buffer solution added was adjusted to adjust the pH of the system to 3.1 as the endpoint.

[0059] (2) Preparation of composite carbon source solution: Take 100g of Tris-HCl buffer solution with pH=8.5 as solvent, add 2g of polydopamine and 3g of sucrose to prepare composite carbon source solution;

[0060] (3) Ion conductor layer coating and zinc doping: The precursor prepared in step (1), 1.2g of lithium dihydrogen phosphate and 0.03g of zinc oxide were ball-milled together for 2h and then placed in a fluidized bed. A nitrogen-water vapor mixture with a humidity of 40% was introduced and heat-treated at 350℃ for 1h; (4) Carbon coating and sintering: The obtained product was immersed in the composite carbon source solution described in step (2) at 50℃ and stirred continuously for 30min. After filtration, gradient carbonization treatment was carried out: First, it was kept at 250℃ for 1h, then heated to 550℃ and kept at 2h, and finally heated to 680℃ and kept at 1h.

[0061] (5) In an atmosphere of N2 / H2=95:5, the sample after gradient carbonization treatment is first sintered at 750℃ for 8h, and then pulverized by airflow to make the particle classification meet the following requirements: fine particles: D50=1.2μm, accounting for 30%; medium particles: D50=2.5μm, accounting for 50%; coarse particles: D50=4.5μm, accounting for 20%; finally, the pulverized sample is sintered at 550℃ for 3h for a second time to obtain the lithium manganese iron phosphate composite material.

[0062] Example 3

[0063] A method for preparing a lithium manganese iron phosphate composite material, the specific steps of which are as follows:

[0064] (1) Preparation of precursor: 10.21 g Mn(CH3COO)2·4H2O and 8.60 g FeC2O4·2H2O were dissolved in 200 mL of deoxygenated and deionized water to obtain a dispersion. Under nitrogen protection, the dispersion was heated to 60 °C in a water bath. Oxalic acid-ammonium oxalate buffer solution was added at a rate of 0.5 mL / min during pulsed microwave radiation to carry out a coprecipitation reaction. The coprecipitation reaction time was 30 min. After the reaction was completed, the mixture was aged for 2 h and then centrifuged and washed to obtain the precursor. The peak power of the pulsed microwave was 300 W, the duty cycle was 40%, the frequency was 2.45 GHz, the pulse width was 10 ms, and the interval was 5 ms. The amount of oxalic acid-ammonium oxalate buffer solution added was adjusted to adjust the pH of the system to 3.0 as the endpoint.

[0065] (2) Preparation of composite carbon source solution: Take 100g of Tris-HCl buffer solution with pH=8.5 as solvent, add 2g of polydopamine and 5g of sucrose to prepare composite carbon source solution;

[0066] (3) The precursor prepared in step (1), 1.2 g of lithium dihydrogen phosphate and 0.03 g of zinc oxide were ball-milled and mixed for 2 h and then placed in a fluidized bed. A nitrogen-water vapor mixture with a humidity of 40% was introduced and heat-treated at 350°C for 1 h. The resulting product was immersed in the composite carbon source solution described in step (2) at 50°C and stirred continuously for 30 min. After filtration, a gradient carbonization treatment was carried out: first, the temperature was kept at 250°C for 1 h, then the temperature was raised to 550°C and kept at 2 h, and finally the temperature was raised to 680°C and kept at 1 h.

[0067] (4) In an atmosphere of N2 / H2=95:5, the sample after gradient carbonization treatment is first sintered at 750℃ for 8h, and then pulverized by airflow to make the particle classification meet the following requirements: fine particles: D50=1.2μm, accounting for 30%; medium particles: D50=2.5μm, accounting for 50%; coarse particles: D50=4.5μm, accounting for 20%; finally, the pulverized sample is sintered at 550℃ for 3h for a second time to obtain the lithium manganese iron phosphate composite material.

[0068] Comparative Example 1

[0069] (1) Preparation of precursor: Under nitrogen protection, 12.25 g Mn(CH3COO)2·4H2O and 7.16 g FeC2O4·2H2O were dissolved in 200 mL of deoxygenated and deionized water to obtain a dispersion. Oxalic acid-ammonium oxalate buffer solution was added at a rate of 0.5 mL / min under stirring in a 60℃ water bath to carry out a co-precipitation reaction (microwave stimulation was disabled) until the pH of the system reached 3.0, and the reaction was continued for 30 min. After the reaction was completed, the mixture was aged for 2 h, centrifuged, washed, and dried to obtain the (Mn,Fe)C2O4 precursor. EDS analysis showed that the distribution of Mn and Fe elements was somewhat uneven.

[0070] (2) Mixing lithium and carbon sources: The above precursor, 12.0g lithium dihydrogen phosphate (LiH2PO4, as lithium source) and 3.0g sucrose (as single carbon source) were ball-milled and mixed for 2h to ensure uniform mixing.

[0071] (3) First sintering and carbonization: The ball-milled mixture was sintered at 700°C for 10 h in a nitrogen-hydrogen mixed atmosphere (N2 / H2 = 95:5). During this process, the precursor reacted with lithium dihydrogen phosphate to generate lithium manganese iron phosphate, while sucrose pyrolysis formed a carbon coating layer on the material surface.

[0072] (4) Pulverization: The sintered block product is pulverized by airflow to obtain the final traditional carbon-coated lithium manganese iron phosphate composite material. Its particle size distribution is relatively wide and cannot be precisely controlled, with a D50 of 2.8 μm.

[0073] The final products prepared in Examples 1-3 and Comparative Example 1 were used as positive electrode active components to fabricate lithium-ion batteries. The specific fabrication steps are as follows:

[0074] The preparation process of lithium-ion battery cathode mainly includes steps such as slurry preparation, coating and drying, rolling and cutting.

[0075] First, the slurry was prepared: polyvinylidene fluoride (PVDF) was dissolved in 1.5g of N-methylpyrrolidone (NMP) and magnetically stirred for 30 minutes to form a transparent and uniform binder solution. Then, the conductive agent Ketjen Black was added and ultrasonically dispersed for 10 minutes (ultrasonic power 300W). Next, the positive electrode active component was added, and the mixture was stirred for 60 minutes using a planetary mixer at 2000 rpm revolution and 500 rpm rotation. The final slurry solid content was 70wt%, with a mass ratio of positive electrode active material, conductive agent, and binder of 92:7.3:6.7.

[0076] The coating and drying process followed: the obtained slurry was uniformly coated onto the aluminum foil current collector, with the wet film thickness controlled at 150 μm. After coating, it was pre-baked at 80℃ for 10 min, and then transferred to a vacuum drying oven at 120℃ for 12 h to ensure that the moisture content was ≤50 ppm. The dried electrode was then subjected to roller pressing at a pressure of 10 MPa.

[0077] Finally, the rolled electrode sheet is punched into a circular electrode sheet with a diameter of 14 mm and a single-sided active material loading of 12 mg / cm².

[0078] The positive electrode sheet prepared above, together with the lithium metal counter electrode, the polypropylene microporous membrane, and the lithium hexafluorophosphate electrolyte, were assembled into a circular coin cell in an argon glove box for subsequent electrochemical performance testing.

[0079] The electrochemical testing conditions were as follows: Charge and discharge were performed at a constant temperature of 25°C. First, the battery was charged at a constant current rate of 0.1C to 4.5V, then charged at a constant voltage rate of 4.5V until the current dropped to 0.05C, and finally discharged at a constant current rate of 0.1C to 2.5V, completing one basic charge-discharge cycle. Subsequently, the above charge-discharge process was repeated at rates of 0.2C and 0.5C for initial battery activation. Finally, 500 cycles were performed at a 1C rate, and the discharge capacity at the 500th cycle was recorded, from which the capacity retention rate was calculated.

[0080] The manganese leaching rate was tested according to standard YS / T 1028.

[0081] The results of the above electrochemical tests are shown in Table 1.

[0082] Table 1 Performance comparison of Examples 1-3 and Comparative Example 1

[0083]

[0084] According to the statistics in Table 1, we can see that lithium-ion batteries made using the manganese iron aluminum phosphate composite material prepared in this invention as the positive electrode active material exhibit excellent electrochemical performance: after 500 cycles at 1C rate, the capacity retention rate is 93.50% to 97.00%, and the manganese dissolution rate is only 0.50% to 1.20%.

[0085] Comparative Example 2

[0086] To verify the importance of pulsed microwaves, Comparative Example 2 was designed. The only difference between Comparative Example 2 and Example 1 is that the pulsed microwaves were omitted, while the other steps and conditions were the same as in Example 1.

[0087] The lithium manganese iron phosphate composite materials prepared in Example 1 and Comparative Example 2 were subjected to EDS and SEM tests, and the results are shown in Table 2. The scanning electron microscope image of the lithium manganese iron phosphate composite material prepared in Comparative Example 2 is shown below. Figure 3 As shown.

[0088] Table 2 Performance Comparison of Example 1 and Comparative Example 2

[0089]

[0090] Based on the results in Table 2, we can see that pulsed microwaves eliminate Fe by directionally arranging oxalate ions. 3+ With Mn 2+ The pH difference in precipitation is crucial for obtaining atomically homogeneous precursors. After removing the pulsed microwave, manganese segregation intensifies, leading to increased battery cycle degradation.

[0091] Comparative Example 3

[0092] To verify the necessity of the lithium phosphate conductive layer, Comparative Example 3 was designed. The specific steps of Comparative Example 3 are as follows:

[0093] (1) Dissolve 12.25g Mn(CH3COO)2·4H2O and 7.16g FeC2O4·2H2O in 200mL of deoxygenated and deionized water to obtain a dispersion. Under nitrogen protection, heat the dispersion to 60℃ in a water bath. While irradiating with pulsed microwaves, add oxalic acid-ammonium oxalate buffer solution at a rate of 0.5mL / min to carry out a coprecipitation reaction. The coprecipitation reaction time is 30min. After the reaction is completed, age for 2h and then centrifuge and wash to obtain the precursor. The peak power of the pulsed microwave is 300W, the duty cycle is 40%, the frequency is 2.45GHz, the pulse width is 10ms, and the interval is 5ms. The amount of oxalic acid-ammonium oxalate buffer solution added is adjusted to adjust the pH of the system to 3.0 as the endpoint.

[0094] (2) Take 100g of Tris-HCl buffer solution with pH=8.5 as solvent, add 3g of sucrose, and prepare a carbon source solution;

[0095] (3) The precursor prepared in step (1) is immersed in the carbon source solution described in step (2) at 50°C and stirred continuously for 30 min. After filtration, it is subjected to gradient carbonization treatment: first, it is kept at 250°C for 1 h, then the temperature is raised to 550°C and kept for 2 h, and finally the temperature is raised to 680°C and kept for 1 h.

[0096] (4) In an atmosphere of N2 / H2=95:5, the sample after gradient carbonization treatment is first sintered at 750℃ for 8h, and then pulverized by airflow to make the particle classification meet the following requirements: fine particles: D50=1.2μm, accounting for 30%; medium particles: D50=2.5μm, accounting for 50%; coarse particles: D50=4.5μm, accounting for 20%; finally, the pulverized sample is sintered at 550℃ for 3h for a second time.

[0097] The product of Comparative Example 3 was prepared into a lithium-ion battery according to the aforementioned method, and its various indicators were compared with those of Example 1. The comparison results are shown in Table 3.

[0098] Table 3 Performance comparison between Example 1 and Comparative Example 3

[0099]

[0100] The results in Table 3 show that the lithium phosphate ion-conducting layer, acting as an ion conductor, significantly reduces interfacial impedance. The synergistic effect of this layer and the carbon layer's electronic conductivity effectively suppresses manganese dissolution and side reactions. In contrast, single carbon coating cannot solve the bottleneck problem of ion diffusion.

[0101] Comparative Example 4

[0102] To verify the necessity of particle size distribution, Comparative Example 4 was designed. The only difference between Comparative Example 4 and Example 1 is that Comparative Example 4 was subjected to airflow pulverization after primary sintering to obtain particles with a relatively uniform particle size distribution and a D50 of 2.5 μm. All other steps and conditions were the same as in Example 1.

[0103] The compaction density of the lithium manganese iron phosphate composite materials prepared in Example 1 and Comparative Example 4 was tested, and the test results are shown in Table 4.

[0104] Table 4 Performance Comparison of Example 1 and Comparative Example 4

[0105]

[0106] According to the results in Table 4, we can see that the three-level particle size distribution (fine: medium: coarse = 3:5:2) described in Example 1 fills the gaps between large particles with fine particles, while coarse particles play a role in inhibiting the volume expansion of the material, thereby synergistically improving the compaction density and structural stability of the material.

[0107] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate composite material, characterized in that, Includes the following steps: Manganese acetate and ferrous oxalate were dispersed in water at a Mn:Fe molar ratio of 0.8 to 2.0:1 to form a dispersion. Under a protective atmosphere, the dispersion was heated to 58 to 62°C, and a precipitant was added to carry out a co-precipitation reaction while being irradiated with microwaves to obtain the precursor. The operating parameters of the microwave radiation are: frequency of 2.40~2.50GHz, pulse width of 8~15ms, interval of 3~8ms, and peak power of 250~350W; Prepare a composite carbon source solution containing a first carbon source and a second carbon source; The precursor, lithium dihydrogen phosphate and zinc oxide were ball-milled and mixed evenly, and then subjected to heat treatment. The resulting product was impregnated in the composite carbon source solution, filtered, and then subjected to gradient carbonization treatment. The sample after gradient carbonization was first sintered at 730~770℃, and after being crushed, it was sintered at 520~580℃ to obtain lithium manganese iron phosphate composite material.

2. The preparation method of the lithium manganese iron phosphate composite material according to claim 1, characterized in that, In the preparation step of the dispersion, the mass ratio of manganese acetate to water is 1:12~20.

3. The preparation method of the lithium manganese iron phosphate composite material according to claim 1, characterized in that, In the precursor preparation step, the protective atmosphere is nitrogen; the co-precipitation reaction time is 20-35 min; and / or The precipitant is an oxalic acid-ammonium oxalate buffer solution; the amount of precipitant added is adjusted to adjust the pH of the system to 2.8-3.

2.

4. The preparation method of the lithium manganese iron phosphate composite material according to claim 1, characterized in that, In the preparation step of the composite carbon source solution, the first carbon source is a polymer containing catechol groups; the second carbon source is a C6-C12 sugar or a hydroxycarboxylic acid.

5. The preparation method of the lithium manganese iron phosphate composite material according to claim 4, characterized in that, The polymer containing catechol groups is polydopamine; the C6-C12 sugar is at least one of sucrose and glucose; and / or The concentration of the first carbon source is 2-5 wt%; the concentration of the second carbon source is 1.5-4 wt%; and Tris-HCl buffer solution with pH=8.5 is used as the solvent.

6. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that, The lithium dihydrogen phosphate accounts for 8-12 wt% of the precursor mass; the zinc oxide accounts for 0.3-1.0 wt% of the precursor mass; and / or The specific operation of the heat treatment is as follows: the ball-milled sample is transferred to a fluidized bed and heat-treated at 330~370℃ for 0.8~1.2h with a mixture of water vapor and nitrogen; and / or The heat-treated product is immersed in the composite carbon source solution for 20-40 minutes at a temperature of 40-60°C; and / or The specific operation of the gradient carbonization process is as follows: First, the temperature is maintained at 230~270℃ for 0.8~1.2h, then the temperature is increased to 530~570℃ and maintained for 1.8~2.2h, and finally the temperature is increased to 660~700℃ and maintained for 0.8~1.2h.

7. The method for preparing the lithium manganese iron phosphate composite material according to claim 1, characterized in that, The primary calcination is carried out in an atmosphere of N2 / H2 = 95:5; and / or The pulverization process employs air jet milling, controlling the particle content (D50 = 1.0~1.5μm) to be ≤30%; and / or The primary sintering time is 7.8~8.2h; the secondary sintering time is 2.8~3.2h.

8. A lithium manganese iron phosphate composite material, characterized in that, It is prepared by the method for preparing lithium manganese iron phosphate composite material according to any one of claims 1 to 7.

9. A positive electrode plate, characterized in that, Including the lithium manganese iron phosphate composite material as described in claim 8.

10. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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