Lithium manganese iron phosphate material prepared by taking lithium iron phosphate as template agent as well as preparation method and application of lithium manganese iron phosphate material

Abstract

The invention discloses a lithium manganese iron phosphate material prepared by taking lithium iron phosphate as a template agent as well as a preparation method and application of the lithium manganese iron phosphate material. Specifically, lithium iron phosphate is taken as a template agent, other iron sources, manganese sources, lithium sources, phosphorus sources and carbon sources are added, high-energy ball milling mixing is carried out in water or an organic solvent, rheological phase reaction is carried out, and the surface of the lithium iron phosphate is uniformly coated with the raw materials. And then drying, pre-calcining, secondary carbon coating and high-temperature calcining are carried out, and bulk phase diffusion and epitaxial growth of the raw materials are synchronously carried out on the surface of the lithium iron phosphate by utilizing a solid phase diffusion-epitaxial growth coupling mechanism, so that the lithium iron manganese phosphate material is finally obtained. The lithium iron manganese phosphate prepared by taking lithium iron phosphate as a template agent and matching with other iron sources has the characteristic of manganese element concentration gradient distribution, and meanwhile, nanoscale particle refinement is realized. The secondary battery prepared by taking the lithium manganese iron phosphate as the positive electrode material has excellent electrochemical performance and cycling stability.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to lithium manganese iron phosphate cathode materials, particularly to a lithium manganese iron phosphate material prepared using lithium iron phosphate as a template agent, its method, and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4) with an olivine structure has attracted widespread attention from researchers due to its high safety and stability. However, the Fe content of LiFePO4... 3+ / Fe 2+ The voltage plateau for the redox reaction is only 3.2V (vs. Li). + The presence of manganese in lithium iron phosphate (LiFePO4) results in a low theoretical energy density, making it difficult to meet the requirements for long-range operation. Replacing some of the iron in lithium iron phosphate with manganese yields lithium manganese iron phosphate (LiMnFePO4). x Fe 1-x While maintaining the olivine structure of lithium iron phosphate, PO4) utilizes Mn 2+ / Mn 3+ Higher redox pair (4.1V vs. Li) + / Li) increases the theoretical energy density of lithium iron phosphate batteries by about 20% to 25%, thus extending the driving range.

[0003] While lithium manganese iron phosphate (LFP) has improved some of the performance of existing lithium-ion battery cathode materials, it has also introduced new problems such as low electronic conductivity and lithium-ion diffusion rate, and manganese dissolution, limiting its theoretical capacity, rate performance, and stability. Therefore, it is necessary to modify LFP based on mature industrial production to optimize battery performance and promote further industrial development. Controlling the concentration distribution of metal elements, particle size, and morphology are common methods for optimizing the electrochemical performance of LFP. Chinese patent application CN104852037A discloses a precursor with an iron-rich surface and a manganese-rich core, and a method for preparing carbon-coated LFP materials using this precursor. Although this method can use an iron-rich surface precursor as a raw material to prepare carbon-coated LFP materials with a "core" (manganese-rich) - "shell" (iron-rich) structure, thereby reducing manganese dissolution and improving conductivity and electrochemical performance, its synthesis process is complex, and the raw materials used are expensive, making it unsuitable for industrial applications. Liu J, Wu Y, Zhang B, et al., A promising solid-state synthesis of LiMn 1-y Fe yLi4MnPO4 cathode for lithium-ion batteries[J]Small 2024,20,2309629 Lithium iron phosphate with large size is used as a morphology framework, and lithium manganese iron phosphate precursor is fused at high temperature to prepare lithium manganese iron phosphate, which overcomes the trade-off between specific surface area and electrochemical performance, but the prepared lithium manganese iron iron phosphate is in a two-phase structure, which cannot avoid the formation of manganese aggregates and manganese dissolution, and has poor cycle performance. Therefore, it is a technical problem to be solved at present to develop a preparation method of lithium manganese iron phosphate positive electrode material with high production efficiency, low cost and excellent electrochemical performance. SUMMARY

[0004] In view of the problems of low lithium ion diffusion rate and manganese dissolution of lithium manganese iron phosphate material, the application provides a lithium manganese iron phosphate material prepared by using lithium iron phosphate as a template agent and a method and application thereof. The lithium manganese iron phosphate material prepared by using lithium iron phosphate as a template agent and cooperating with other iron sources has the characteristics of manganese element concentration gradient distribution by using the solid phase diffusion-epitaxial growth coupling mechanism, and realizes nanoscale particle refinement. The secondary battery prepared by using lithium manganese iron phosphate as a positive electrode material has excellent electrochemical performance and cycle stability. The method provided by the application has low cost, is environment-friendly, and has high economic benefits and social benefits.

[0005] The technical scheme adopted by the application is as follows:

[0006] A method for preparing lithium manganese iron phosphate by using lithium iron phosphate material as a template agent, dispersing manganese source, iron source, lithium source, phosphorus source, carbon source and lithium iron phosphate in water or an organic solvent for high-efficiency ball milling to form a rheological body, and then transferring the rheological body to a hydrothermal reactor for rheological phase reaction. The formation of the rheological body forms a nanoscale liquid film between the dispersed agents in the gap between the solid particles, and a close contact interface is formed between the solid particles, thereby significantly increasing the effective specific surface area and promoting the diffusion between the solid molecules, and further uniformly coating the raw materials on the surface of the lithium iron phosphate. The lithium iron phosphate material as a hard particle can enhance the transmission of mechanical energy, promote the refinement and uniform mixing of the raw materials, thereby reducing the particle size of the product. If the lithium iron phosphate recovered from the positive electrode material of the waste lithium iron phosphate battery is used as a template agent, since many microcracks are often generated on the surface during long-time charge and discharge cycles and the pretreatment recovery process, the manganese source at the crack can enter the interior of the lithium iron phosphate particle more quickly, and the gradient distribution of manganese concentration is realized.

[0007] The mixture after the above rheological phase reaction is dried and pre-calcined to decompose the raw material into a carbon-coated lithium manganese iron phosphate precursor. The precursor is then mixed with the carbon source through secondary ball milling and calcined under high temperature conditions. Taking advantage of the fact that lithium iron phosphate and lithium manganese iron phosphate have the same olivine structure, lithium manganese iron phosphate is promoted to grow epitaxially on the surface of lithium iron phosphate. At the same time, the template agent effect of lithium iron phosphate is used to limit the growth of lithium manganese iron phosphate particles, thus preparing lithium manganese iron phosphate material with small particle size and good rate performance.

[0008] The specific steps are as follows:

[0009] (1) Using lithium iron phosphate as a template agent, based on lithium manganese iron phosphate Li y Mn x Fe 1-x The proportions of each element in PO4 are as follows: lithium iron phosphate, other iron sources, manganese sources, lithium sources, and phosphorus sources are weighed and dispersed in water or an organic solvent to obtain a raw material dispersion, wherein 0.4 < x ≤ 0.7, 1 ≤ y ≤ 1.3, and the amount of lithium iron phosphate added is equal to that of lithium manganese iron phosphate (Li). y Mn x Fe 1-x PO4 comprises 10–30 wt% of the total mass of all raw materials;

[0010] (2) Add the carbon source to the raw material dispersion, mix and ball mill to form a rheotype, transfer it to a hydrothermal reactor for heating to carry out the rheotype reaction, and dry to obtain a solid mixture;

[0011] (3) The solid mixture is pre-calcined in a protective gas atmosphere to obtain carbon-coated lithium manganese iron phosphate precursor.

[0012] (4) The carbon-coated lithium manganese iron phosphate precursor is mixed with the carbon source again and ball-milled to obtain a secondary carbon-coated lithium manganese iron phosphate precursor.

[0013] (5) The secondary carbon-coated lithium manganese iron phosphate precursor is calcined at high temperature in a protective gas atmosphere to obtain lithium manganese iron phosphate material.

[0014] The present application uses lithium iron phosphate as a template agent, one or both of iron phosphate and iron oxalate as other iron sources, simultaneously adds manganese source, lithium source, phosphorus source and carbon source, carries out high-energy ball milling mixing in water or organic solvent, and modulates into rheological body. Subsequently, it is transferred to a hydrothermal reactor for heating to carry out rheological phase reaction, so that the raw materials are uniformly coated on the surface of lithium iron phosphate. Then, 250-400℃ pre-calcination treatment is carried out to decompose the raw materials coated on the surface of lithium iron phosphate and make them transform into specific phases, forming lithium manganese iron phosphate precursor; on the other hand, 250-400℃ pre-calcination treatment can make the precursor gradually release internal stress, avoiding material cracking or deformation due to stress concentration. Subsequently, secondary carbon coating and 500-800℃ high-temperature calcination are carried out, which can supplement the uncovered area of the first carbon coating, so that the carbon layer coated on the surface of lithium manganese iron phosphate has higher integrity and compactness. The 500-800℃ high-temperature treatment process makes the surface part of Mn 2+ The manganese element diffuses into the lithium iron phosphate crystal lattice by solid phase diffusion to replace the iron element to form lithium manganese iron phosphate, and the Mn 2+ The surface Fe + , Li 3- , PO4 2+ reacts under the induction of epitaxial growth of lithium iron phosphate to form a shell layer of lithium manganese iron phosphate with the same olivine structure as lithium iron phosphate. The lithium manganese iron phosphate material prepared by the present application using lithium iron phosphate as a template agent and using solid phase diffusion-epitaxial growth coupling mechanism has the characteristics of manganese element concentration gradient distribution, and at the same time realizes nanoscale particle refinement, thereby significantly improving its electrochemical performance. The lithium iron phosphate template agent used in the present application has a dual source characteristic, that is, commercial lithium iron phosphate can be used, or lithium iron phosphate recovered from waste batteries can be used. The recovered lithium iron phosphate is obtained by high-temperature calcination, screening, washing and drying of waste positive electrode materials of retired lithium iron phosphate batteries, without any chemical treatment.

[0015] Further, the lithium iron phosphate in step (1) is preferably analytical pure lithium iron phosphate or lithium iron phosphate recovered from waste batteries. The recovered lithium iron phosphate is mainly obtained by crushing the positive electrode sheet of waste lithium iron phosphate batteries, calcining at 400-700℃ for 1-3h in a nitrogen or argon atmosphere, sieving through a 100-300 mesh sieve, washing with water and drying. The recovered lithium iron phosphate can be determined for its phase and structure by an X-ray diffractometer, and its morphology can be observed by a scanning electron microscope.

[0016] The manganese source in step (1) is one or more of manganese carbonate, manganese dioxide and trimanganese tetroxide, and further, the manganese source is preferably trimanganese tetroxide or manganese carbonate; the other iron source is preferably iron phosphate or iron oxalate; the lithium source is one or more of lithium carbonate, lithium dihydrogen phosphate and lithium hydroxide, and further, the lithium source is preferably lithium carbonate, lithium dihydrogen phosphate or lithium hydroxide; and the phosphorus source is one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate and monoammonium phosphate.

[0017] The carbon source in step (2) is preferably glucose, sucrose, citric acid or ascorbic acid, and the addition amount of the carbon source is preferably 5-20wt% of the total mass of the solid raw material in the raw material dispersion liquid; the ball milling time is 4-12h, the ball milling rate is 400-600rpm / min, and the heating temperature is 100-150℃, and the heating time is 12-48h.

[0018] The protective gas in step (3) is preferably nitrogen or argon, the pre-calcination temperature is 250-400℃, and the calcination time is 1-4h.

[0019] The carbon source in step (4) is preferably glucose, sucrose, citric acid or ascorbic acid, and the addition amount of the carbon source is preferably 2-10wt% of the total mass of the solid raw material in the raw material dispersion liquid; the ball milling time is 1-4h, and the ball milling rate is 400-600rpm / min.

[0020] The protective gas in step (5) is preferably nitrogen or argon, the calcination temperature is 500-800℃, and the calcination time is 4-10h.

[0021] The application also provides a lithium manganese iron phosphate material prepared by the method.

[0022] The application also provides application of the lithium manganese iron phosphate material as a positive electrode material in preparation of a lithium secondary battery.

[0023] Compared with the prior art, the application has the following technical effects:

[0024] (1) The application uses a rheological phase reaction method to pretreat the solid raw material, so that the solid particles and the dispersant are in a rheological phase state, the surface area of the solid microparticles and the nanoscale liquid film formed on the surface are effectively utilized, the diffusion between the solid molecules is promoted under high temperature conditions, and the iron source, the manganese source, the lithium source, the phosphorus source and the carbon source are uniformly coated on the surface of the lithium iron phosphate.

[0025] (2) The application introduces lithium iron phosphate as a template agent, which can enhance the transmission of mechanical energy and promote the refinement and uniform mixing of the raw materials, thereby reducing the particle size of the product; on the other hand, based on the same olivine structure of lithium iron phosphate and lithium manganese iron phosphate and the similar radius of Mn 2+ and Fe 2+ , the use of the epitaxial growth induction effect, heterogeneous lattice mismatch and interface stress effect of lithium iron phosphate makes the nucleation rate of lithium manganese iron phosphate much higher than the grain growth rate, hinders the complete growth of the grains, and obtains nanoscale lithium manganese iron phosphate particles with uniform size distribution.

[0026] (3) The lithium manganese iron phosphate prepared by using the lithium iron phosphate as a template agent has a large specific surface area, reduces the diffusion path of lithium ions, and improves the migration rate of lithium ions, so that the lithium manganese iron phosphate material with excellent electrochemical performance is obtained.

[0027] (4) The lithium manganese iron phosphate material prepared by using the recycled lithium iron phosphate as a template agent and combining with other iron sources through a solid-phase diffusion-epitaxial growth coupling mechanism utilizes the micro-cracks on the surface of the lithium iron phosphate, realizes the gradient distribution of the Mn element concentration in the structure of the lithium manganese iron phosphate, inhibits the dissolution of Mn ions in the lithium manganese iron phosphate material during the charge-discharge cycle process, and thus more excellent electrochemical performance is obtained.

[0028] (5) The application provides a method for recycling lithium iron phosphate as a positive material of a waste lithium iron phosphate battery.

[0029] (6) The lithium manganese iron phosphate material is prepared by using the lithium iron phosphate material as a template agent and adopting a high-temperature solid-phase method, and has low cost, environmental friendliness, high economy and high practicability. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The X-ray diffraction pattern of the lithium manganese iron phosphate prepared in Example 1.

[0031] Figure 2 The charge-discharge curve of the lithium manganese iron phosphate positive material prepared in Example 1.

[0032] Figure 3 The charge-discharge curve of the lithium manganese iron phosphate positive material prepared in Example 2.

[0033] Figure 4 The charge-discharge curve of the lithium manganese iron phosphate positive material prepared in Example 3.

[0034] Figure 5 The X-ray diffraction pattern of the recycled lithium iron phosphate in Example 4.

[0035] Figure 6 The scanning electron microscope (SEM) image of the recycled lithium iron phosphate in Example 4.

[0036] Figure 7 The X-ray diffraction pattern of the lithium manganese iron phosphate prepared in Example 4.

[0037] Figure 8 The scanning electron microscope (SEM) image of the lithium manganese iron phosphate prepared in Example 4.

[0038] Figure 9 The charge-discharge curve of the lithium manganese iron phosphate positive material prepared in Example 4.

[0039] Figure 10Mn elemental line profile of lithium manganese iron phosphate prepared in Example 5

[0040] Figure 11 Charge-discharge curve of lithium manganese iron phosphate cathode material prepared in Example 5.

[0041] Figure 12 Impedance plot of lithium manganese iron phosphate cathode material prepared in Example 6.

[0042] Figure 13 Charge-discharge curve of lithium manganese iron phosphate cathode material prepared in Example 6.

[0043] Figure 14 Rate capability test plot of lithium manganese iron phosphate cathode material prepared in Example 6.

[0044] Figure 15 Scanning electron micrograph of lithium manganese iron phosphate prepared in Comparative Example 1.

[0045] Figure 16 Charge-discharge curve of lithium manganese iron phosphate cathode material prepared in Comparative Example 1.

[0046] Figure 17 Scanning electron micrograph of lithium manganese iron phosphate prepared in Comparative Example 2.

[0047] Figure 18 Charge-discharge curve of lithium manganese iron phosphate cathode material prepared in Comparative Example 2.

[0048] Figure 19 Charge-discharge curve of lithium manganese iron phosphate cathode material prepared in Comparative Example 3.

[0049] Figure 20 Charge-discharge cycle plot of lithium manganese iron phosphate cathode material prepared in Comparative Example 4. DETAILED DESCRIPTION

[0050] The following examples will aid in the understanding of the present application but the scope of the application is not limited to them.

[0051] Example 1: Preparation of LiMnPO4 with Mn / Fe ratio of 6:4 using analytical grade lithium iron phosphate as a template agent 0.6 Fe 0.4 PO4

[0052] Mn304(1.43 g), FeP04(0.86 g), LiH2P04(1.95 g), Li2C03(0.55 g) and analytical reagent lithium iron phosphate (1.06 g) were weighed out, dispersed in ethanol (10 mL), and then glucose (0.88 g, 15 wt%) was added, and ball-milling was performed at 500 r / min for 8 h. Subsequently, the mixture was transferred to a hydrothermal reactor and heated at 100 °C for 36 h, and then dried. The dried mixture was calcined at 300 °C for 2 h under an Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with glucose (0.59 g, 10 wt%), ball-milled for 2 h, and then calcined at 600 °C for 10 h under an Ar atmosphere to obtain a LiMn 0.6 Fe 0.4 P04material with a Mn / Fe ratio of 6:4, designated as LMFP-1st-1. The corresponding X-ray diffraction pattern is shown in FIG. 1. Figure 1 The diffraction peaks thereof were consistent with the standard card of olivine-type Li(Mn, Fe)P04(JCPDS 74-0375).

[0053] The positive electrode sheet was obtained by uniformly mixing and drying LMFP-1st-1, conductive carbon black and PVDF (mass ratio of 8:1:1). A CR2025 type button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode, a LiPF6solution (1 mol / L) as the electrolyte, and a celgard 2500 type polypropylene material as the separator. The button cell was subjected to charge-discharge cycle testing in a constant-temperature cell test system at 25 °C. The test results showed that the initial specific discharge capacity of LMFP-1st-1 was 154.6 mAh / g at a current density of 0.1 C, and the capacity retention rate was 99.7% after 500 cycles of charge-discharge at a current density of 0.5 C.

[0054] Example 2: Preparation of a LiMn 0.5 Fe 0.5 P04

[0055] Mn3O4(1.19 g), Fe2(C2O4)3(1.12 g), LiH2PO4(1.62 g), LiOH (116 mg) and analytical reagent lithium iron phosphate (>99%, 1.70 g) were weighed out, dispersed in water (10 mL), and then ascorbic acid (0.86 g, 15 wt%) was added, and ball-milled at 500 r / min for 12 h. Subsequently, it was transferred to a hydrothermal reactor and heated at 150 °C for 12 h, and dried. The dried mixture was calcined at 400 °C for 1 h under an Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with ascorbic acid (0.12 g, 2 wt%), ball-milled at 500 r / min for 2 h, and then calcined at 800 °C for 6 h under an Ar atmosphere to obtain a LiMn 0.5 Fe 0.5 PO4 material with a Mn / Fe ratio of 5:5, designated as LMFP-1st-2.

[0056] The positive electrode sheet was obtained by uniformly mixing and drying LMFP-1st-2, conductive carbon black and PVDF (mass ratio of 8:1:1). A CR2025 type button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode and a LiPF6 solution (1 mol / L) as the electrolyte, and a celgard 2500 type polypropylene material as the separator. The button cell was tested for charge and discharge cycling at 25 °C in a constant temperature cell test system. The test results showed that the initial specific discharge capacity of LMFP-1st-2 was 153.96 mAh / g at a current density of 0.1 C.

[0057] Example 3: Preparation of a LiMn 0.7 Fe 0.3 PO4 material with a Mn / Fe ratio of 7:3 using analytical reagent lithium iron phosphate as a template agent.

[0058] Mn3O4(1.67 g), FePO4(0.90 g), LiH2PO4(1.92 g), Li2CO3(0.35 g) and analytical reagent lithium iron phosphate (>99%, 0.54 g) were weighed out, dispersed in ethanol (10 mL), and then sucrose (0.54 g, 10 wt%) was added, and ball-milled at 500 r / min for 8 h. Subsequently, it was transferred to a hydrothermal reactor and heated at 120 °C for 24 h, and dried. The dried mixture was calcined at 300 °C for 2 h under an Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with sucrose (0.27 g, 5 wt%), ball-milled for 2 h, and then calcined at 500 °C for 10 h under an Ar atmosphere to obtain a LiMn 0.7 Fe 0.3 PO4 material with a Mn / Fe ratio of 7:3, designated as LMFP-1st-3.

[0059] The positive electrode sheet was obtained by mixing LMFP-1st-3, conductive carbon black and PVDF (mass ratio of 8:1:1) uniformly and drying. The CR2025 button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode, LiPF6 solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The button cell was tested for charge and discharge cycling at 25°C in a constant temperature cell test system. The test results showed that the initial specific discharge capacity of LMFP-1st-3 was 152.35 mAh / g at a current density of 0.1C.

[0060] Example 4: Preparation of LiMn 0.6 Fe 0.4 PO4

[0061] The positive electrode sheet of the waste lithium iron phosphate battery was crushed, sieved through a 100 mesh screen after calcination at 700°C for 1h in an Ar atmosphere, washed with water and dried to obtain a recovered lithium iron phosphate material. The X-ray diffraction pattern of the recovered lithium iron phosphate material showed that it had diffraction peaks consistent with the standard card (JCPDS 81-1173). Figure 5 The scanning electron microscope image of the recovered lithium iron phosphate showed that the surface of the recovered lithium iron phosphate had microcracks. Figure 6

[0062] Mn3O4 (1.43g), FePO4 (1.21g), LiH2PO4 (1.95g), Li2CO3 (0.30g) and recovered lithium iron phosphate (0.70g) were dispersed in ethanol (10mL), and then glucose (0.84g, 15wt%) was added, and ball-milling was carried out at 400r / min for 8h. Subsequently, it was transferred to a hydrothermal reactor and heated at 120°C for 24h, and then dried. The dried solid mixture was calcined at 350°C for 2h under Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with glucose (0.30g, 5wt%) and ball-milled at a rotation speed of 600r / min for 2h, and then calcined at 700°C for 4h under Ar atmosphere to obtain a LiMn 0.6 Fe 0.4 PO4 material with a Mn / Fe ratio of 6:4, designated as LMFP-1st-4. The corresponding X-ray diffraction pattern is shown in Figure 7 , which showed that the diffraction peaks of the prepared lithium manganese iron phosphate were consistent with the standard card of olivine-type Li(Mn,Fe)PO4 (JCPDS 74-0375) and had sharp peak shapes, indicating high purity and good crystallinity. Figure 8 ​The scanning electron microscope image of LMFP-1st-4 shows that the particles of LMFP-1st-4 are spherical, and the particle size distribution is about 200 nm.

[0063] The positive electrode sheet was obtained by uniformly mixing and drying LMFP-1st-4, conductive carbon black and PVDF (mass ratio of 8:1:1). A CR2025 type button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode, LiPF6 solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The button cell was tested for charge and discharge cycling at 25°C in a constant temperature cell test system. The test results show that the specific capacity of LMFP-1st-4 is 163.84 mAh / g at a current density of 0.1C, and the specific capacity retention rate is 97.1% after 500 cycles at a current density of 0.5C.

[0064] Example 5: Using recycled lithium iron phosphate as a template agent, changing the manganese source to prepare LiMn 0.6 Fe 0.4 PO4

[0065] The positive electrode sheet of the waste lithium iron phosphate battery was crushed, calcined at 600°C for 2h in an Ar atmosphere, sieved through a 200 mesh screen, washed with water and dried to obtain a recycled lithium iron phosphate material. MnCO3 (2.156g), FePO4 (1.21g), LiH2PO4 (1.95g), Li2CO3 (0.30g) and recycled lithium iron phosphate (0.70g) were weighed, dispersed in ethanol (10mL), and then glucose (1.26g, 20wt%) was added. Ball milling was carried out at 600r / min for 4h. Subsequently, it was transferred to a hydrothermal reactor and heated at 150°C for 12h, and then dried. The dried solid mixture was calcined at 250°C for 4h under N2 atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with glucose (0.32g, 5wt%) and ball milled at a speed of 500r / min for 2h, and then calcined at 700°C for 4h under N2 atmosphere to obtain a LiMn 0.6 Fe 0.4 PO4 material with a Mn / Fe ratio of 6:4, named LMFP-1st-5. Figure 10 The Mn element line scan spectrum of LMFP-1st-5 shows the concentration gradient distribution of Mn element. LMFP-1st-5 has an excellent specific surface area (42m 2 / g), which increases its contact area with the electrolyte and the transmission rate of Li + .

[0066] The positive electrode sheet was obtained by uniformly mixing and drying LMFP-1st-5, conductive carbon black and PVDF (mass ratio of 8:1:1). A CR2025 type button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, negative electrode shell, with lithium metal as the negative electrode, LiPF6 solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The button cell was tested for charge and discharge cycling at 25°C in a constant temperature cell test system. The test results showed that the initial specific discharge capacity of LMFP-1st-5 was 158.55 mAh / g at a current density of 0.1C.

[0067] Example 6: Using recycled lithium iron phosphate as a template agent, changing manganese source and phosphorus source to prepare LiMn 0.6 Fe 0.4 PO4

[0068] The positive electrode sheet of the waste lithium iron phosphate battery was crushed, sieved through a 100 mesh screen after calcination at 500°C for 3h in an Ar atmosphere, washed with water and dried to obtain a recycled lithium iron phosphate material. MnO2 (1.63g), FePO4 (1.21g), NH4H2PO4 (2.16g), Li2CO3 (0.99g) and recycled lithium iron phosphate (0.70g) were weighed and dispersed in ethanol (10mL), then citric acid (0.34g, 5wt%) was added, and ball milling was carried out at 400r / min for 12h. Subsequently, it was transferred to a hydrothermal reactor and heated at 150°C for 15h, and then dried. The dried solid mixture was calcined at 400°C for 2h under Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with citric acid (0.34g, 5wt%) and ball milled for 2h, and then calcined at 700°C for 6h under Ar atmosphere to obtain a LiMn 0.6 Fe 0.4 PO4 material with a Mn / Fe ratio of 6:4, named LMFP-1st-6.

[0069] The positive electrode sheet was obtained by uniformly mixing and drying LMFP-1st-6, conductive carbon black and PVDF (mass ratio of 8:1:1). A CR2025 type button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, negative electrode shell, with lithium metal as the negative electrode, LiPF6 solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The button cell was tested for charge and discharge cycling at 25°C in a constant temperature cell test system. The test results showed that the initial specific discharge capacity of LMFP-1st-6 was 158.55 mAh / g at a current density of 0.1C. Figure 12The LMFP-1st-6 has lower charge transfer impedance and lithium ion diffusion impedance. The coin cell was tested for charge-discharge cycling test and rate capability test in a constant temperature cell test system at 25°C. The test results show that the LMFP-1st-6 has a first discharge specific capacity of 151.18 mAh / g at a current density of 0.1C, and has excellent capacity recovery capability.

[0070] Comparative Example 1: Preparation of lithium manganese iron phosphate without adding template agent lithium iron phosphate

[0071] Mn3O4(1.43 g), FePO4(1.88 g), LiH2PO4(1.95 g), Li2CO3(0.46 g) were weighed and dispersed in ethanol (10 mL), and then glucose (0.86 g, 15 wt%) was added, and ball-milled at 400 r / min for 8 h. Subsequently, it was transferred to a hydrothermal reactor and heated at 120°C for 24 h, and then dried. The dried solid mixture was calcined at 350°C for 2 h under Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with glucose (0.29 g, 5 wt%), ball-milled at 600 r / min for 2 h, and then calcined at 700°C for 4 h under Ar atmosphere to obtain a LiMn 0.6 Fe 0.4 PO4material, named as LMFP-std. Figure 15 The scanning electron microscope image of the LMFP-std shows that the particle size distribution of the LMPF-std is uneven, and the particle size distribution is in the range of 100 nm-1 μm. The specific surface area of the LMFP-std is 32 m 2 / g.

[0072] The positive electrode sheet was obtained by uniformly mixing and drying the LMFP-std, conductive carbon black and PVDF (mass ratio of 8:1:1). A CR2025 type coin cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode, LiPF6solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The coin cell was tested for charge-discharge cycling test in a constant temperature cell test system at 25°C. The test results show that the LMFP-std has a discharge specific capacity of 138.43 mAh / g at a current density of 0.1C.

[0073] Comparative Example 2: Preparation of lithium manganese iron phosphate by first preparing a lithium manganese iron phosphate precursor and then fusing with recovered lithium iron phosphate

[0074] The positive electrode sheet of waste lithium iron phosphate battery was crushed, calcined at 700°C for 1 h in Ar atmosphere, sieved through a 100 mesh screen, washed with water and dried to obtain the recovered lithium iron phosphate material. Mn304(1.43 g), FeP04(1.21 g), LiH2P04(1.95 g), Li2C03(0.30 g) were weighed and dispersed in ethanol (10 mL), then glucose (0.84 g, 17 wt%) was added and ball-milled at 400 r / min for 8 h. Subsequently, it was transferred to a hydrothermal reactor and heated at 120°C for 24 h, and then dried. The dried solid mixture was calcined at 350°C for 2 h in Ar atmosphere to obtain the lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with the recovered lithium iron phosphate (0.70 g) and glucose (0.30 g, 5 wt%), ball-milled at 600 r / min for 2 h, and then calcined at 700°C for 4 h in Ar atmosphere to obtain the LiMn 0.6 Fe 0.4 P04material with a Mn / Fe ratio of 6:4, named as LMFP-2nd-1. The specific surface area of LMFP-2nd-1 was 38 m 2 / g.

[0075] The positive electrode sheet was obtained by uniformly mixing LMFP-2nd-1, conductive carbon black and PVDF (mass ratio of 8:1:1) and drying. A CR2025 type button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode, LiPF6solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The button cell was tested for charge-discharge cycling at 25°C in a constant temperature cell test system. The test results showed that the discharge specific capacity of LMFP-2nd-1 was 147.93 mAh / g at a current density of 0.1C.

[0076] Comparative Example 3: First, the lithium manganese iron phosphate precursor was prepared, and then the analytical pure lithium iron phosphate was fused to prepare lithium manganese iron phosphate

[0077] Mn304(1.43 g), FeP04(1.21 g), LiH2P04(1.95 g), Li2C03(0.30 g) were weighed out, dispersed in ethanol (10 mL), and then glucose (0.84 g, 17 wt%) was added, and ball-milled at 400 r / min for 8 h. Subsequently, it was transferred to a hydrothermal reactor and heated at 120 °C for 24 h, and dried. The dried solid mixture was calcined at 350 °C for 2 h under Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with commercial lithium iron phosphate (>99%, 0.70 g) and glucose (0.30 g, 5 wt%), ball-milled at 600 r / min for 2 h, and then calcined at 700 °C for 4 h under Ar atmosphere to obtain a LiMn 0.6 Fe 0.4 P04 material with a Mn / Fe ratio of 6:4, named as LMFP-2nd-2.

[0078] The positive electrode sheet was obtained by mixing LMFP-2nd-2, conductive carbon black and PVDF (mass ratio of 8:1:1) uniformly and drying. A CR2025 type button cell was assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode and LiPF6 solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The button cell was tested for charge and discharge cycling at 25 °C in a constant temperature cell test system. The test results showed that the discharge specific capacity of LMFP-2nd-2 was 145.75 mAh / g at a current density of 0.1 C.

[0079] Comparative Example 4: Lithium manganese iron phosphate was prepared using lithium iron phosphate as the total iron source without adding other iron sources.

[0080] Mn304(1.91 g), LiH2P04(2.60 g) and analytical pure lithium iron phosphate (2.63 g) were weighed out, dispersed in ethanol (10 mL), and then glucose (0.71 g, 15 wt%) was added, and ball-milled at 400 r / min for 6 h. Subsequently, it was transferred to a hydrothermal reactor and heated at 120 °C for 20 h, and dried. The dried solid mixture was calcined at 350 °C for 2 h under Ar atmosphere to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with glucose (0.355 g, 5 wt%), ball-milled at 600 r / min for 2 h, and then calcined at 700 °C for 4 h under Ar atmosphere to obtain a LiMn 0.6 Fe 0.4 P04 material with a Mn / Fe ratio of 6:4, named as LMFP-Mn@LFP. The specific surface area of LMFP-Mn@LFP was 37 m 2 / g.

[0081] The positive electrode sheet is obtained by mixing LMFP-Mn@LFP, conductive carbon black and PVDF (mass ratio of 8:1:1) uniformly and drying. A CR2025 type button cell is assembled in an argon glove box in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell, with lithium metal as the negative electrode, LiPF6 solution (1 mol / L) as the electrolyte, and celgard 2500 type polypropylene material as the separator. The button cell is subjected to charge-discharge cycle test in a constant temperature cell test system at 25°C. The test results show that the initial discharge specific capacity of LMFP-Mn@LFP is 141.93 mAh / g at a current density of 0.1C. Figure 20 The charge-discharge cycle curve of LMFP-Mn@LFP at a current density of 0.5C is shown in the above table. The first discharge capacity is 133.8 mAh / g, and the capacity retention rate is 96.7% after 500 cycles.

[0082] The above results show that by adding a certain amount of lithium iron phosphate material as a template agent during solid-phase ball milling, the particle size of the product manganese iron lithium phosphate after high-temperature calcination can be reduced, the specific surface area and Li + diffusion rate of the product can be improved, and thus an excellent electrochemical performance of the manganese iron lithium phosphate material can be prepared. The use of recycled lithium iron phosphate material as a template agent and the micro-cracks on the surface thereof can help the concentration gradient distribution of manganese elements, and can further improve the electrochemical performance of the manganese iron lithium phosphate. The manganese iron lithium phosphate as a positive electrode material of a lithium ion battery has excellent discharge specific capacity and cycle stability when lithium sheet is used as the negative electrode.

[0083] The above only describes some embodiments of the present application, and is not a limitation on the embodiments. The obvious changes or modifications derived therefrom are still within the protection scope of the present application.

Claims

1. A method for preparing lithium manganese iron phosphate materials using lithium iron phosphate as a template agent, characterized in that, Includes the following steps: (1) Based on lithium manganese iron phosphate Li y Mn x Fe 1-x The proportions of each element in PO4 are as follows: lithium iron phosphate, other iron sources, manganese sources, lithium sources, and phosphorus sources are weighed out. Lithium iron phosphate is used as a template agent. The other iron sources are one or both of iron phosphate and iron oxalate. Each component is dispersed in water or an organic solvent to obtain a raw material dispersion, wherein 0.4 < x ≤ 0.7, 1 ≤ y ≤ 1.3, and the amount of lithium iron phosphate added accounts for a certain percentage of the total lithium manganese iron phosphate (LiFePO4) content. y Mn x Fe 1-x PO4 comprises 10–30 wt% of the total mass of all raw materials; (2) Add the carbon source to the above raw material dispersion, mix it by high-energy ball milling, prepare it into a rheological substance, and then transfer it to a hydrothermal reactor for rheological reaction. After the reaction, dry it to obtain a solid mixture. (3) The solid mixture was precalcined at 250-400°C in a protective gas atmosphere to obtain lithium manganese iron phosphate precursor; (4) The lithium manganese iron phosphate precursor is mixed with a carbon source for a second time, ball-milled and dried in water or organic solvent medium to obtain a secondary carbon-coated lithium manganese iron phosphate precursor. (5) The secondary carbon-coated lithium manganese iron phosphate precursor is calcined at 500-800°C in a protective gas atmosphere to obtain lithium manganese iron phosphate material.

2. The method according to claim 1, characterized in that, In step (1), the lithium iron phosphate is one or more of commercial lithium iron phosphate and lithium iron phosphate recycled from waste batteries.

3. The method according to claim 2, characterized in that, Lithium iron phosphate recycled from waste batteries is obtained through the following steps: the positive electrode sheet of the waste lithium iron phosphate battery is mechanically crushed, calcined at 400-700℃ for 1-3 hours in a nitrogen or argon atmosphere, then sieved through a 100-300 mesh screen, washed with water and dried to obtain the recycled lithium iron phosphate material.

4. The method according to claim 1, characterized in that, In step (1), the manganese source is one or more of manganese carbonate, manganese dioxide, and manganese tetroxide; The lithium source is one or more of lithium carbonate, lithium dihydrogen phosphate, and lithium hydroxide; The phosphorus source is one or more of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and ammonium monohydrogen phosphate.

5. The method according to claim 1, characterized in that, In step (2), the carbon source is one or more of glucose, sucrose, citric acid, and ascorbic acid; In step (4), the carbon source used is one or more of glucose, sucrose, citric acid, and ascorbic acid.

6. The method according to claim 1, characterized in that, In step (2), the amount of carbon source added is 5 to 20 wt% of the total mass of the solid raw materials in the raw material dispersion; In step (2), high-energy ball milling is carried out at a rate of 400-600 r / min for 4-12 h; In step (2), the rheological reaction is carried out at 100-150℃ for 12-48 hours.

7. The method according to claim 1, characterized in that, In steps (3) and (5), the protective gas is nitrogen or argon; In step (3), the solid mixture is pre-calcined at 250–400°C for 1–4 hours in a protective gas atmosphere; In step (5), the secondary carbon-coated lithium manganese iron phosphate precursor is calcined at 500-800°C for 4-10 hours in a protective gas atmosphere.

8. The method according to claim 1, characterized in that, In step (4), the amount of carbon source added is 2 to 10 wt% of the solid raw material in the raw material dispersion; In step (4), the mixture is ball-milled at a rate of 400–600 r / min for 1–4 h in water or organic solvent medium and then dried.

9. A lithium manganese iron phosphate material prepared by the method according to any one of claims 1 to 8.

10. The application of the lithium manganese iron phosphate material of claim 9 as a cathode material in the preparation of lithium-ion batteries.

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