A core-shell structure manganese iron lithium phosphate material, a preparation method and application thereof

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to a core-shell structure manganese iron lithium phosphate material and a preparation method and application thereof. The core-shell structure manganese iron lithium phosphate material provided by the application has a core of Li7TaO6, has high lithium ion conductivity, can effectively improve the ion conductivity of the positive electrode material, further improves the rate performance, has a lithium supplement effect, can improve the capacity performance and cycle performance of the positive electrode material, and the carbon coating layer improves the electronic conductivity of the material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a core-shell structured lithium manganese iron phosphate material, its preparation method, and its application. Background Technology

[0002] In recent years, with the gradual depletion of traditional fossil fuels and the increasingly serious environmental degradation caused by their use, the research and development of new renewable, green, and clean energy sources has become particularly important. However, renewable energy sources such as wind, solar, and tidal power are discontinuous, requiring the development of supporting energy storage devices for optimal utilization. Lithium-ion batteries, with their advantages of high discharge voltage, high specific energy density, long cycle life, and wide operating temperature range, have become a promising secondary battery for energy storage.

[0003] Currently, olivine-type lithium iron phosphate (LiFePO4) materials have been commercialized as cathode materials for lithium-ion batteries. However, this material has a low discharge platform (operating voltage 3.2V), resulting in low specific energy. Lithium manganese iron phosphate (LiMn2FePO4) is another option. x Fe 1-x LMFP (LiFePO4) has a relatively high operating voltage (3.8V) and the potential for high energy density, making it a valuable material for research. However, LMFP also has some drawbacks, such as lower electronic and ionic conductivity compared to lithium iron phosphate, and poorer capacity utilization.

[0004] To address the low electronic and lithium-ion conductivity of lithium manganese iron phosphate (LFP) materials, surface coating is commonly used to improve electronic conductivity, while ion doping is employed to enhance ionic conductivity. Carbon coating is the most common surface coating method for LFP. Besides improving conductivity, carbon coating also prevents particle agglomeration, reduces particle size, and prevents Fe²⁺ ions from forming. 2+ For oxidation, commonly used carbon sources are divided into two main categories: inorganic carbon sources and organic carbon sources. Among them, organic carbon sources such as glucose, sucrose, starch, and polyethylene glycol have better coating effects. Currently, there is also a lot of research on ion doping of LMFP, mainly using elements such as Ti, Mg, and V for doping, which can improve the ionic conductivity of LMFP and enhance the rate performance of the material.

[0005] Although the above methods can improve the electronic conductivity of LMFP and improve the ionic conductivity to some extent, they cannot solve the problem of poor capacity utilization of lithium manganese iron phosphate. How to improve the capacity utilization of LMFP and further improve the ionic conductivity and electronic conductivity of the material remains the direction for improvement of lithium manganese iron phosphate materials. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the defects of the existing lithium manganese iron phosphate materials, such as poor capacity utilization and the need to further improve ionic conductivity and electronic conductivity, so as to provide a core-shell structure lithium manganese iron phosphate material, its preparation method and application.

[0007] Therefore, the present invention provides the following technical solution:

[0008] This invention provides a core-shell structured lithium manganese iron phosphate material, comprising a Li7TaO6 core, a lithium manganese iron phosphate layer and a carbon coating layer that encapsulate the core from the inside out.

[0009] Optionally, the Li7TaO6 core accounts for 10wt%-30wt% of the total mass of the core-shell structured lithium manganese iron phosphate material;

[0010] Preferably, the Li7TaO6 core comprises 15wt%-20wt%.

[0011] Optionally, the carbon content is 1.0 wt% to 3.5 wt% based on the total mass of the core-shell structured lithium manganese iron phosphate material.

[0012] Optionally, the lithium manganese iron phosphate accounts for 69wt%-89wt% of the total mass of the core-shell structured lithium manganese iron phosphate material;

[0013] Optionally, the chemical composition of the lithium manganese iron phosphate is LiMn. x Fe 1-x PO4, wherein x = 0.5 to 0.8; preferably, x = 0.7.

[0014] The present invention also provides a method for preparing the above-mentioned core-shell structured lithium manganese iron phosphate material, comprising the following steps:

[0015] S1, Li7TaO6 is mixed with phosphorus source, iron source and manganese source, pH is adjusted, and reaction is carried out in a closed environment to obtain core-shell structured precursor;

[0016] S2, the obtained core-shell structure precursor is mixed with a lithium source and a carbon source, and sintered under a protective atmosphere to obtain the core-shell structure lithium manganese iron phosphate material.

[0017] Optionally, in step S1, the pH is adjusted to 2-4;

[0018] And / or, the reaction temperature is 150-300℃, and the reaction time is 2-6h;

[0019] And / or, first dissolve the phosphorus source, iron source and manganese source in water, and then mix with Li7TaO6;

[0020] Optionally, the mass ratio of water to (phosphorus source + iron source + manganese source) is 2.5-3.5:1.

[0021] And / or, the Li7TaO6 is prepared by a solid-state method.

[0022] The solid-state method used in this invention to prepare Li7TaO6 is a conventional method in the field, which is typical but not limiting. The specific preparation method is as follows: weigh appropriate amounts of Li2CO3 and Ta2O5 according to the molar ratio, mix them, and sinter them at 700-1000℃ for 8-12 hours in an oxygen atmosphere.

[0023] Optionally, the sintering process is repeated twice.

[0024] Optionally, the primary particle size of the Li7TaO6 is 100nm-300nm. By limiting the primary particle size, this invention can further improve rate performance. This is because a large primary particle size leads to a long lithium-ion transport channel, slow ion diffusion, and poor rate performance.

[0025] Optionally, in step S2, the sintering temperature is 700-800℃ and the sintering time is 2-5h;

[0026] And / or, the protective atmosphere is at least one of nitrogen or an inert gas.

[0027] Optionally, the preparation method of the core-shell structured lithium manganese iron phosphate material satisfies at least one of the following (1)-(9):

[0028] (1) The iron source is selected from at least one of the soluble salts of iron;

[0029] (2) The iron source is selected from one or more of ferric nitrate, ferric chloride, ferric acetate, and ferric sulfate;

[0030] (3) The manganese source is selected from at least one of the soluble salts of manganese;

[0031] (4) The manganese source is one or more of manganese nitrate, manganese chloride, manganese acetate, and manganese sulfate;

[0032] (5) The phosphorus source is selected from at least one of soluble compounds containing phosphate;

[0033] (6) The phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate;

[0034] (7) The lithium source is selected from at least one of lithium carbonate or lithium hydroxide;

[0035] (8) The carbon source is selected from at least one organic carbon source; preferably, the carbon source is selected from at least two organic carbon sources;

[0036] (9) The carbon source is selected from glucose, sucrose, starch and polyethylene glycol.

[0037] The present invention also provides a positive electrode comprising the core-shell structured lithium manganese iron phosphate material described above or the core-shell structured lithium manganese iron phosphate material prepared by the above preparation method.

[0038] The present invention also provides a lithium battery, including the above-mentioned positive electrode, and further including a negative electrode, electrolyte, separator, and casing.

[0039] In this invention, the positive electrode and other components and preparation methods of the lithium-ion battery are all conventional in the field.

[0040] Typically, and not specifically, the composition and preparation method of the positive electrode are as follows: all positive electrode materials are baked in a vacuum oven at 80-120°C for 6-12 hours, then homogenized according to the ratio of positive electrode main material:SP:PVDF = 96:2:2 to 90:5:5, and then the slurry is coated on aluminum foil with an areal density of 6-15 mg / cm³. 2 After drying, the electrodes are rolled to achieve a compaction density of 1.8-2.5 g / cm³. 3 After cutting, the positive electrode sheet is obtained.

[0041] The coin cell is a CR2032 battery, and the preparation method is as follows: the coin cell lower shell, pad, lithium negative electrode, separator, and positive electrode pad are placed in sequence from bottom to top, and then 100-250μL of electrolyte is dripped in to wet it. Finally, the upper shell is installed.

[0042] The technical solution of this invention has the following advantages:

[0043] The core-shell structured lithium manganese iron phosphate material provided by this invention has a core of Li7TaO6, which has high lithium-ion conductivity, effectively improving the ionic conductivity of the cathode material, thereby improving rate performance, and also has a lithium replenishment effect, which can improve the capacity utilization of the cathode material (the initial charge capacity can reach 160mAh / g, and the initial discharge efficiency can reach more than 155mAh / g) and cycle performance; the carbon coating layer improves the electronic conductivity of the material. Among them, using Li7TaO6 as the core, compared with using it as the coating layer, the cathode material of this structure can not only ensure the material has excellent electrical properties, but also have good processing performance. This is because Li7TaO6 is strongly alkaline. If it is used as the coating layer, it will react with PVDF during the cathode homogenization stage, causing the slurry to gel, resulting in poor flowability and inability to be coated. In addition to improving electronic conductivity, the carbon coating layer can prevent the cathode particles from growing during high-temperature processing, and can also act as a reducing agent to ensure that the Mn and Fe elements in the generated LMFP are both in the +2 oxidation state. Therefore, using Li7TaO6 as the core and carbon as the outermost coating layer is the optimal structure.

[0044] The core-shell structured lithium manganese iron phosphate material provided by this invention further improves the initial charge and discharge capacity by further limiting the core content. Within the preferred range, the initial charge capacity can reach 170 mAh / g, and the initial discharge efficiency can reach more than 164 mAh / g. Attached Figure Description

[0045] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the structure of the lithium manganese iron phosphate material provided by the present invention;

[0047] Figure 2 The graph shows the cycle performance test results of Embodiments 1 and 2 of the present invention.

[0048] Figure label:

[0049] 1. Core; 2. Lithium manganese iron phosphate layer; 3. Carbon coating layer. Detailed Implementation

[0050] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0051] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0052] Example 1

[0053] This embodiment provides a core-shell structured lithium manganese iron phosphate material, the structural schematic of which is shown below. Figure 1 As shown, from the inside out, it includes a core 1, a lithium manganese iron phosphate layer 2, and a carbon coating layer 3, with Li7TaO6 as the core. The specific preparation method and operating parameters are as follows:

[0054] (1) Accurately weigh 12.88g LiOH and 16.97g Ta2O5 and place them in a ball mill jar. Use anhydrous ethanol as the medium for ball milling. After mixing thoroughly for 10h, dry at 70℃. Transfer the dried raw material to a corundum crucible and sinter at 900℃ for 5h in a high-temperature box-type muffle furnace. Grind the preliminarily sintered white Li7TaO6 (LTO) powder and sinter it again at 1000℃ for 8h to obtain the final product LTO. The sample after the second sintering has good compositional uniformity and a primary particle size of 200nm.

[0055] (2) Take 5g of LTO powder prepared in (1), 21.81g of Mn(NO3)2·4H2O, 15.04g of Fe(NO3)3·9H2O, and 12.16g of H3PO4 and place them in a reaction vessel. Add 150g of deionized water to the reaction vessel, stir to dissolve, and adjust the pH to 3 with nitric acid. Place the sealed reaction vessel in an oven at 200℃ for 4h, and after filtration, washing, and drying, obtain a core of LTO and a shell of Mn. 0.7 Fe 0.3 The core-shell precursor of PO4.

[0056] (3) After thoroughly mixing 24.08g of the precursor obtained in (2) with 4.59g of Li2CO3, 0.625g of glucose, and 0.459g of PEG (manufacturer: Sinopharm model polyethylene glycol 200), the mixture was transferred to a corundum crucible and then placed in a tube furnace. Under the protection of N2, the mixture was sintered at 800℃ for 4h to obtain the final core-shell structured lithium manganese iron phosphate composite material LTO / LiMn with a carbon coating. 0.7 Fe 0.3 PO4 / C, wherein the content of LTO is 20wt%, and LiMn 0.7 Fe 0.3 The PO4 content is 78 wt%, and the carbon content is 2 wt%.

[0057] Example 2

[0058] This embodiment provides a core-shell structured lithium manganese iron phosphate material, the specific preparation method and operating parameters of which are as follows:

[0059] (1) Accurately weigh 12.88g LiOH and 16.97g Ta2O5 and place them in a ball mill jar. Use anhydrous ethanol as the medium for ball milling. After mixing thoroughly for 10h, dry at 70℃. Transfer the dried raw material to a corundum crucible and sinter at 900℃ for 5h in a high-temperature box-type muffle furnace. Grind the preliminarily sintered white Li7TaO6 (LTO) powder and sinter it again at 1000℃ for 8h to obtain the final product LTO. The sample after the second sintering has good compositional uniformity and a primary particle size of 200nm.

[0060] (2) Take 5g of LTO powder prepared in (1), 21.81g of Mn(NO3)2·4H2O, 15.04g of Fe(NO3)3·9H2O, and 12.16g of H3PO4 and place them in a reaction vessel. Add 150g of deionized water to the reaction vessel, stir to dissolve, and adjust the pH to 3 with nitric acid. Place the sealed reaction vessel in an oven at 200℃ for 4h, and after filtration, washing, and drying, obtain a core of LTO and a shell of Mn. 0.7 Fe 0.3 The core-shell precursor of PO4.

[0061] (3) 24.08 g of the precursor obtained in (2) was mixed with 4.59 g of Li2CO3 and 1.25 g of glucose, and then transferred to a corundum crucible. The crucible was then placed in a tube furnace and sintered at 800 °C for 4 h under N2 protection to obtain the final core-shell structure lithium manganese iron phosphate composite material LTO / LiMn with a carbon coating. 0.7 Fe 0.3 PO4 / C, wherein the content of LTO is 20wt%, and LiMn 0.7 Fe 0.3 The PO4 content is 78 wt%, and the carbon content is 2 wt%.

[0062] Example 3

[0063] This embodiment provides a core-shell structured lithium manganese iron phosphate material, the specific preparation method and operating parameters of which are as follows:

[0064] (1) Accurately weigh 12.88g LiOH and 16.97g Ta2O5 and place them in a ball mill jar. Use anhydrous ethanol as the medium for ball milling. After mixing thoroughly for 10h, dry at 70℃. Transfer the dried raw material to a corundum crucible and sinter at 900℃ for 5h in a high-temperature box-type muffle furnace. Grind the preliminarily sintered white Li7TaO6 (LTO) powder and sinter it again at 1000℃ for 8h to obtain the final product LTO. The sample after the second sintering has good compositional uniformity and a primary particle size of 200nm.

[0065] (2) Take 3.75g of LTO powder prepared in (1), 23.20g of Mn(NO3)2·4H2O, 16.01g of Fe(NO3)3·9H2O, and 12.94g of H3PO4 and place them in a reaction vessel. Add 150g of deionized water to the reaction vessel, stir to dissolve, and adjust the pH to 3 with nitric acid. Place the sealed reaction vessel in an oven at 200℃ for 4h, and after filtration, washing, and drying, obtain a core of LTO and a shell of Mn. 0.7 Fe 0.3 The core-shell precursor of PO4.

[0066] (3) The 23.59g precursor obtained in (2) was mixed evenly with 4.88g Li2CO3, 0.625g glucose, and 0.459g PEG (manufacturer: Sinopharm model polyethylene glycol 200), and then transferred to a corundum crucible. The crucible was then placed in a tube furnace and sintered at 800℃ for 4h under N2 protection to obtain the final core-shell structure lithium manganese iron phosphate composite material LTO / LiMn with carbon coating. 0.7 Fe 0.3 PO4 / C, of ​​which LTO content is 15wt%, LiMn 0.7 Fe 0.3 The PO4 content is 83 wt%, and the carbon content is 2 wt%.

[0067] Example 4

[0068] This embodiment provides a core-shell structured lithium manganese iron phosphate material, the specific preparation method and operating parameters of which are as follows:

[0069] (1) Accurately weigh 12.88g LiOH and 16.97g Ta2O5 and place them in a ball mill jar. Use anhydrous ethanol as the medium for ball milling. After mixing thoroughly for 10h, dry at 70℃. Transfer the dried raw material to a corundum crucible and sinter at 900℃ for 5h in a high-temperature box-type muffle furnace. Grind the preliminarily sintered white Li7TaO6 (LTO) powder and sinter it again at 1000℃ for 8h to obtain the final product LTO. The sample after the second sintering has good compositional uniformity and a primary particle size of 200nm.

[0070] (2) Take 2.5g of LTO powder prepared in (1), 24.60g of Mn(NO3)2·4H2O, 16.97g of Fe(NO3)3·9H2O, and 13.72g of H3PO4 and place them in a reaction vessel. Add 150g of deionized water to the reaction vessel, stir to dissolve, and adjust the pH to 3 with nitric acid. Place the sealed reaction vessel in an oven at 200℃ for 4h, and after filtration, washing, and drying, obtain a product with an LTO core and a Mn shell. 0.7 Fe 0.3 The core-shell precursor of PO4.

[0071] (3) The 23.53g precursor obtained in (2) was mixed evenly with 5.17g Li2CO3, 0.625g glucose, and 0.459g PEG (manufacturer: Sinopharm model polyethylene glycol 200), and then transferred to a corundum crucible. The crucible was then placed in a tube furnace and sintered at 800℃ for 4h under N2 protection to obtain the final core-shell structure lithium manganese iron phosphate composite material LTO / LiMn with carbon coating. 0.7 Fe 0.3PO4 / C, wherein the content of LTO is 10wt%, and LiMn 0.7 Fe 0.3 The PO4 content is 88 wt%, and the carbon content is 2 wt%.

[0072] Example 5

[0073] This embodiment provides a core-shell structured lithium manganese iron phosphate material, the specific preparation method and operating parameters of which are as follows:

[0074] (1) Accurately weigh 12.88g LiOH and 16.97g Ta2O5 and place them in a ball mill jar. Use anhydrous ethanol as the medium for ball milling. After mixing thoroughly for 10h, dry at 70℃. Transfer the dried raw material to a corundum crucible and sinter at 900℃ for 5h in a high-temperature box-type muffle furnace. Grind the preliminarily sintered white Li7TaO6 (LTO) powder and sinter it again at 1000℃ for 8h to obtain the final product LTO. The sample after the second sintering has good compositional uniformity and a primary particle size of 200nm.

[0075] (2) Take 5g of LTO powder prepared in (1), 21.53g of Mn(NO3)2·4H2O, 14.85g of Fe(NO3)3·9H2O, and 12.01g of H3PO4 and place them in a reaction vessel. Add 150g of deionized water to the reaction vessel, stir to dissolve, and adjust the pH to 3 with nitric acid. Place the sealed reaction vessel in an oven at 200℃ for 4h, and after filtration, washing, and drying, obtain a core of LTO and a shell of Mn. 0.7 Fe 0.3 The core-shell precursor of PO4.

[0076] (3) The 23.40g precursor obtained in (2) was mixed evenly with 4.53g Li2CO3, 0.94g glucose, and 0.69g PEG (manufacturer: Sinopharm model polyethylene glycol 200), and then transferred to a corundum crucible. The crucible was then placed in a tube furnace and sintered at 800℃ for 4h under N2 protection to obtain the final core-shell structure lithium manganese iron phosphate composite material LTO / LiMn with carbon coating. 0.7 Fe 0.3 PO4 / C, of ​​which LTO content is 20wt%, LiMn 0.7 Fe 0.3 The PO4 content is 77 wt%, and the carbon content is 3 wt%.

[0077] Comparative Example 1

[0078] This comparative example provides a lithium iron phosphate manganese phosphate material, the specific preparation method and operating parameters of which are as follows:

[0079] 21.53 g of Mn(NO3)2·4H2O, 14.85 g of Fe(NO3)3·9H2O, and 12.01 g of H3PO4 were placed in a reaction vessel. 150 g of deionized water was added, and the mixture was stirred until dissolved. The pH was adjusted to 3 with nitric acid. The sealed reaction vessel was placed in an oven at 200°C for 4 hours. After filtration, washing, and drying, Mn was obtained. 0.7 Fe 0.3 PO4 precursor. The obtained 18.41g precursor and 4.53g lithium source Li2CO3 were then mixed evenly and transferred to an alumina crucible, which was then placed in a tube furnace and sintered at 800℃ for 4h under N2 protection to obtain pure LiMn. 0.7 Fe 0.3 PO4 powder.

[0080] Comparative Example 2

[0081] This comparative example provides a lithium iron phosphate manganese phosphate material, the specific preparation method and operating parameters of which are as follows:

[0082] 21.53 g of Mn(NO3)2·4H2O, 14.85 g of Fe(NO3)3·9H2O, and 12.01 g of H3PO4 were placed in a reaction vessel. 150 g of deionized water was added, and the mixture was stirred until dissolved. The pH was adjusted to 3 with nitric acid. The sealed reaction vessel was placed in an oven at 200°C for 4 hours. After filtration, washing, and drying, Mn was obtained. 0.7 Fe 0.3 PO4 precursor. The obtained 18.41g precursor, 4.53g lithium source Li2CO3, and 0.98g glucose were mixed evenly and transferred to a corundum crucible. The mixture was then placed in a tube furnace and sintered at 800℃ for 4h under N2 protection to obtain lithium manganese iron phosphate (LiMn) with a carbon coating. 0.7 Fe 0.3 PO4 / C material, with a carbon content of 2wt%.

[0083] Test case

[0084] Electrochemical performance testing

[0085] The core-shell composite lithium manganese iron phosphate materials obtained in each embodiment and the lithium manganese iron phosphate samples provided in each comparative example were used to prepare coin cells under the same conditions using conventional methods. The composition and preparation method of the positive electrode were as follows: all positive electrode materials were baked in a vacuum oven at 100°C for 10 hours, and then homogenized according to the ratio of positive electrode main material:SP:PVDF=92:4:4. The slurry was then coated on aluminum foil with an areal density of 10 mg / cm³. 2 After drying, the electrodes are rolled and pressed to a compaction density of 2.1 g / cm³. 3After cutting, the positive electrode sheet is obtained. The coin cell is a CR2032 battery, and the preparation method is as follows: the coin cell lower shell, pad, lithium negative electrode, separator, and positive electrode pad are placed in the order of bottom to top, then 200μL of electrolyte is dripped in to wet it, and finally the upper shell is installed. The electrochemical performance of the prepared coin cell was tested at room temperature, with a voltage range of 2.0-4.3V and a rate of 0.1C. The initial charge and discharge capacity is shown in Table 1.

[0086] The powder resistivity of the core-shell structured composite lithium manganese iron phosphate materials prepared in each embodiment and the lithium manganese iron phosphate samples provided in each comparative example was tested using a four-probe tester at 4 kN. The specific test results are shown in the table below:

[0087] Table 1

[0088]

[0089]

[0090] As shown in the table above, the capacity performance of the embodiments is significantly higher than that of the comparative examples. The initial charge capacity reaches 160 mAh / g, the initial discharge efficiency exceeds 155 mAh / g, and the resistivity is lower than that of the comparative examples. A comparison of the data from Examples 1-4 shows that with the increase of core content, the initial charge and discharge capacity further improves. Within the preferred range, the initial charge capacity can reach 170 mAh / g, and the initial discharge efficiency can exceed 164 mAh / g. This high capacity performance is due to the high lithium-ion conductivity of the core Li7TaO6 material and its lithium replenishment effect, which improves the capacity performance of the cathode material. The preparation of the carbon coating layer further enhances the high electronic conductivity of the material. The combination of these two modification methods results in excellent electrochemical performance of the cathode material.

[0091] Coin cell rate performance test

[0092] The coin cell (with the same composition and preparation method as above) was tested at room temperature with a voltage range of 2.0-4.3V and rates of 1C, 2C and 4C. The test results are shown in Table 2.

[0093] Table 2

[0094] Capacity retention rate (%) 1C 2C 4C Example 1 94.66 90.98 85.87 Example 2 94.03 90.12 84.17 Example 3 91.37 88.27 81.69 Example 4 89.25 85.34 77.39 Example 5 94.00 89.65 84.01 Comparative Example 1 80.12 70.16 58.46 Comparative Example 2 83.64 74.24 64.23

[0095] As shown in the table above, the rate performance of the examples is better than that of the comparative examples. A comparison of the data from Examples 1-4 shows that the rate performance further improves with increasing core content. This is because the core of the material in the examples is Li7TaO6, a fast ion conductor with high lithium-ion conductivity, which effectively improves the ionic conductivity of the cathode material, thereby enhancing its rate performance.

[0096] Cyclic performance test

[0097] Using the core-shell structured composite lithium manganese iron phosphate material prepared in Examples 1 and 2 as the positive electrode active material and conventional graphite material as the negative electrode, a 5Ah all-electric soft-pack battery was prepared under the same conditions according to conventional methods. The room temperature 1C charge / 3C discharge cycle performance of Examples 1 and 2 is as follows: Figure 2 As shown, after 500 cycles, the capacity retention rate of the pouch battery made from the material prepared in Example 1 was 91.79%, while the capacity retention rate of the pouch battery made from the material prepared in Example 2 was 89.12%. This shows that the core-shell structure composite lithium manganese iron phosphate material coated with a composite carbon source has significantly improved cycle performance compared with the material coated with a single carbon source.

[0098] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A core-shell structured lithium manganese iron phosphate material, characterized in that, It includes a Li7TaO6 core, and a lithium manganese iron phosphate layer and a carbon coating layer that encapsulate the core from the inside out; Based on the total mass of the core-shell structured lithium manganese iron phosphate material, the Li7TaO6 core accounts for 10wt%-30wt%; the carbon content accounts for 1.0wt%-3.5wt%; and the lithium manganese iron phosphate accounts for 69wt%-89wt%. The chemical composition of the lithium manganese iron phosphate is LiMn x Fe 1-x PO4, where x = 0.5~0.

8.

2. The core-shell structured lithium manganese iron phosphate material according to claim 1, characterized in that, Based on the total mass of the core-shell structured lithium manganese iron phosphate material, the Li7TaO6 core accounts for 15wt%-20wt%.

3. The core-shell structured lithium manganese iron phosphate material according to claim 1, characterized in that, The chemical composition of the lithium manganese iron phosphate is LiMn. x Fe 1-x PO4, where x = 0.

7.

4. A method for preparing a core-shell structured lithium manganese iron phosphate material according to any one of claims 1-3, characterized in that, Includes the following steps: S1, Li7TaO6 is mixed with phosphorus source, iron source and manganese source, pH is adjusted, and reaction is carried out in a closed environment to obtain core-shell structured precursor; S2, the obtained core-shell structure precursor is mixed with a lithium source and a carbon source, and sintered under a protective atmosphere to obtain the core-shell structure lithium manganese iron phosphate material.

5. The method for preparing core-shell structured lithium manganese iron phosphate material according to claim 4, characterized in that, In step S1, adjust the pH to 2-4; And / or, the reaction temperature is 150-300℃, and the reaction time is 2-6h; And / or, first dissolve the phosphorus source, iron source and manganese source in water, and then mix with Li7TaO6; And / or, the Li7TaO6 is prepared by a solid-state method.

6. The method for preparing core-shell structured lithium manganese iron phosphate material according to claim 4, characterized in that, In step S2, the sintering temperature is 700-800℃ and the sintering time is 2-5h; And / or, the protective atmosphere is at least one of nitrogen or an inert gas.

7. The method for preparing core-shell structured lithium manganese iron phosphate material according to any one of claims 4-6, characterized in that, Satisfy at least one of the following (1)-(5): (1) The iron source is selected from at least one of the soluble salts of iron; (2) The manganese source is selected from at least one of the soluble salts of manganese; (3) The phosphorus source is selected from at least one of soluble compounds containing phosphate ions; (4) The lithium source is selected from at least one of lithium carbonate or lithium hydroxide; (5) The carbon source is selected from at least one of organic carbon sources.

8. The method for preparing core-shell structured lithium manganese iron phosphate material according to claim 7, characterized in that, The iron source is selected from one or more of ferric nitrate, ferric chloride, ferric acetate, and ferric sulfate; And / or, the manganese source is one or more of manganese nitrate, manganese chloride, manganese acetate, and manganese sulfate; Each / or the phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; And / or, the carbon source is selected from at least one of glucose, sucrose, starch, and polyethylene glycol.

9. A positive electrode, characterized in that, The core-shell structured lithium manganese iron phosphate material according to any one of claims 1-3 or the core-shell structured lithium manganese iron phosphate material prepared by the preparation method according to any one of claims 4-8.

10. A lithium battery, characterized in that, It includes the positive electrode as described in claim 9, and also includes a negative electrode.

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