Multilayer coated doped lithium manganese iron phosphate positive electrode material and preparation method thereof and lithium battery

Abstract

The present invention belongs to the technical field of lithium-ion batteries and provides a multi-layer coated doped lithium iron manganese phosphate positive electrode material, a preparation method thereof, and a lithium battery. The positive electrode material is composed of a lithium iron manganese phosphate core layer, a composite silicon carbide intermediate layer, and a carbon conductive outer layer; the composite silicon carbide intermediate layer is composed of nitrogen-doped nano-silicon carbide composite particles; the nitrogen-doped nano-silicon carbide composite particles contain 20-30% lithium silicate. The multi-layer coated structure not only ensures that the obtained lithium iron manganese phosphate positive electrode material has good electrical conductivity and lithium ion diffusion properties, but also effectively avoids the problem of manganese dissolution, thereby improving the cycle life of the material. The nitrogen-doped nano-silicon carbide composite particles can significantly improve the conductivity of the intermediate layer, and the composite lithium silicate particles can provide more transmission paths for lithium ions, accelerating the insertion and extraction process of lithium ions, providing an additional lithium source for the positive electrode material, and improving the energy density of the positive electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a multi-layer coated doped lithium manganese iron phosphate positive electrode material, a preparation method thereof, and a lithium battery. Background Art

[0002] The rapid development of new energy technologies, particularly lithium-ion battery technology, is becoming a core force driving the transformation of the global energy structure. Lithium-ion batteries, with their high energy density and long cycle life, are widely used in electric vehicles, portable electronic devices, and energy storage systems, becoming an indispensable energy solution for modern society. The cathode material is a key component in determining lithium-ion battery performance, and phosphate cathode materials, with their unique structural characteristics and excellent performance, have become a research focus.

[0003] As an emerging olivine-type positive electrode material, lithium manganese iron phosphate positive electrode material combines the high safety of lithium iron phosphate and the high voltage platform of lithium manganese phosphate. However, the lithium ion diffusion channel in the lithium manganese iron phosphate positive electrode material is still a one-dimensional channel, resulting in poor electronic conductivity and lithium ion migration rate. In addition, when the manganese ion content in the material increases, the Jahn-Teller effect caused by trivalent manganese ions will cause the lattice to distort and manganese to dissolve, resulting in a large volume change during the charge and discharge process. In addition, the dissolved manganese will be deposited on the negative electrode surface, further reducing the cycle stability of the material. Summary of the Invention

[0004] The purpose of the present invention is to provide a multi-layer coated doped lithium manganese iron phosphate positive electrode material, a preparation method thereof and a lithium battery, so as to solve the problems of poor conductivity, slow lithium ion migration rate and manganese dissolution of the lithium manganese iron phosphate positive electrode material.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] In the first aspect, the present invention provides a multi-layer coated doped lithium manganese iron phosphate positive electrode material, which consists of a lithium manganese iron phosphate core layer, a composite silicon carbide intermediate layer and a carbon conductive outer layer; the composite silicon carbide intermediate layer is composed of nitrogen-doped nano-silicon carbide composite particles; the nitrogen-doped nano-silicon carbide composite particles include 20-30% lithium silicate.

[0007] Preferably, the positive electrode material includes a lithium manganese iron phosphate core layer with a mass fraction of 65-85%, a composite silicon carbide intermediate layer with a mass fraction of 10-30%, and a carbon conductive outer layer with a mass fraction of 2-5%.

[0008] By adopting the above technical solution, the lithium iron manganese phosphate positive electrode material of the present invention has a multi-layer coating structure, which is conducive to constructing a three-dimensional lithium ion diffusion channel and a three-dimensional conductive network, and can enhance the conductivity of the positive electrode material and the lithium ion deintercalation rate; on the other hand, the multi-layer coating structure can effectively prevent the lithium iron manganese phosphate core layer from directly contacting the electrolyte, thereby solving the problem of valence disproportionation and dissolution of manganese during the charge and discharge cycle, thereby reducing the probability of lattice distortion and improving the cycle stability of the positive electrode material.

[0009] The lithium iron phosphate core layer in the lithium iron phosphate positive electrode material provides the main active substance for the positive electrode material. The composite silicon carbide intermediate layer can improve the temperature resistance of the positive electrode material by virtue of the properties of silicon carbide itself. It can also protect the active components of the lithium iron phosphate from structural damage due to volume changes during the charge and discharge process, thereby increasing the structural stability of the positive electrode material. The carbon conductive outer layer provides good electronic conductivity for the positive electrode material.

[0010] The composite silicon carbide intermediate layer is composed of nitrogen-doped nano-silicon carbide composite particles. The nano-silicon carbide particles have high hardness, high thermal conductivity and wide bandgap. The nano-silicon carbide particles modified by nitrogen doping have good electrical conductivity and energy storage performance. They can also retain the excellent physical and chemical properties of silicon carbide itself to the maximum extent, improve the conductivity and structural stability of the positive electrode material, and broaden the application range of lithium-ion batteries.

[0011] At the same time, due to the poor ion transport performance of nitrogen-doped nano-silicon carbide particles, the nitrogen-doped nano-silicon carbide composite particles also include lithium silicate. Lithium silicate can provide more transmission paths for lithium ions, accelerate the insertion and extraction process of lithium ions, and effectively improve the rate performance of the positive electrode material; the composite of lithium silicate particles also provides an additional lithium source for the positive electrode material, providing more lithium ions during the first charge and discharge process, thereby improving the energy density of the positive electrode material, and can also replenish the loss of lithium ions during the charge and discharge cycle, thereby extending the cycle life of the lithium-ion battery.

[0012] Preferably, the nitrogen-doped nano-silicon carbide composite particles are prepared according to the following method:

[0013] The nano-silicon carbide particles are mixed with a nitrogen source, then ground and sieved, and pre-sintered at 800-1000°C for 1-2 hours to obtain a pre-reaction powder; the pre-reaction powder is dispersed in deionized water to form a suspension, a granulation aid is added, stirred and mixed for 15-30 minutes, and then lithium silicate particles are added, the pH value of the solution is adjusted to 8-9, ball milled for 3-5 hours, dried, and sintered at 1400-1600°C for 5-6 hours to obtain nitrogen-doped nano-silicon carbide composite particles.

[0014] Preferably, the nitrogen source includes a combination of one or more of urea, dicyandiamide and melamine; the added amount of the nitrogen source is 0.5-1% of the mass of the nano-silicon carbide particles.

[0015] Preferably, the granulation aid includes a combination of one or more of polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, sodium carboxymethyl cellulose and sodium alginate; the added amount of the granulation aid is 4-6% of the mass of the nano-silicon carbide particles.

[0016] Preferably, the lithium silicate particles are prepared according to the following method:

[0017] The lithium source and nano-silica are mixed and added to a solvent. After adding a dispersant and mixing evenly, the temperature is raised to 65-85°C, and the solvent is removed by stirring. Subsequently, the temperature is raised to 700-800°C under an inert gas atmosphere for heat treatment, and finally silicate particles are obtained by grinding.

[0018] More preferably, the lithium source includes a combination of one or more of lithium acetate, lithium carbonate, lithium hydroxide and lithium nitrate.

[0019] More preferably, the molar ratio of lithium source to nano-silicon dioxide is (2-3):1.

[0020] More preferably, the solvent includes a combination of one or more of anhydrous ethanol, isopropyl alcohol, acetone and methanol.

[0021] More preferably, the dispersant includes a combination of one or more of polyvinyl pyrrolidone, sodium lauryl sulfate, sodium dodecylbenzenesulfonate and cetyltrimethylammonium bromide.

[0022] By adopting the above technical solution, nano-silicon carbide has excellent thermodynamic and mechanical properties, but compared with the carbon phase, silicon carbide has poor conductivity, and after being compounded with positive-valent materials, it cannot significantly improve the conductivity of the positive electrode material. When a nitrogen source is mixed with nano-silicon carbide particles and pre-sintered, nitrogen atoms will occupy the position of carbon atoms in the nano-silicon carbide grains, causing the ionization of donor impurities, forming positively charged ions, and also generating free electrons as carriers, making nano-silicon carbide have excellent conductive properties. In addition, the doping of nitrogen atoms will cause lattice distortion of silicon carbide, providing a path for the transmission of free electrons. After being compounded with lithium manganese iron phosphate positive electrode materials, the conductivity of the positive electrode material can be improved.

[0023] At the same time, lithium silicate is added for compounding during the preparation process. In addition to providing lithium ion transmission channels and additional lithium sources for the composite silicon carbide intermediate layer, thereby improving the lithium ion diffusion rate of the positive electrode material and enhancing the rate performance of the positive electrode material, there is also a secondary interface between the lithium silicate particles and the nitrogen-doped nano-silicon carbide. On the one hand, it can improve the bonding force between the two, and on the other hand, it is beneficial to reduce the porosity of nano-silicon carbide and improve the density of the composite silicon carbide intermediate layer, which can not only enhance the structural stability, but also balance the electron transmission and lithium ion migration rate, thereby enhancing the overall performance of the positive electrode material.

[0024] Preferably, the raw material of the carbon conductive outer layer is a high molecular organic carbon source; the high molecular organic carbon source includes one or more combinations of glucose, sucrose, citric acid, polyethylene glycol, cellulose derivatives, chitosan, phenolic resin and polyacrylic acid.

[0025] By adopting the above technical solution, the raw material of the carbon conductive outer layer of the present invention adopts a high molecular organic carbon source, which can well disperse the particles and coat them on the surface of the positive electrode material, and can form a three-dimensional porous conductive network, which can increase and shorten the transmission path of free electrons and lithium ions, improve the transmission efficiency of electrons and lithium ions, and thus improve the rate performance of the positive electrode material.

[0026] In a second aspect, the present invention provides a method for preparing a multilayer coated doped lithium manganese iron phosphate positive electrode material, comprising the following process steps:

[0027] S1. The phosphorus source, iron source, manganese source and lithium source are mixed in proportion, dried and then sintered to obtain a lithium manganese iron phosphate active component;

[0028] S2. The lithium manganese iron phosphate active component is used as the core layer, which is mixed with nitrogen-doped nano-silicon carbide composite particles and ball-milled for 1 to 3 hours, followed by a second sintering to obtain an intermediate product;

[0029] S3. The intermediate product is dispersed in deionized water, a high molecular organic carbon source is added and mixed evenly, and after drying, a third sintering is performed to obtain a multilayer coated doped lithium manganese iron phosphate positive electrode material.

[0030] Preferably, the first sintering temperature is 600-800° C., and the time is 5-7 hours; the second sintering temperature is 350-550° C., and the time is 2-4 hours; and the third sintering temperature is 700-800° C., and the time is 3-5 hours.

[0031] Preferably, the molar ratio of the phosphorus source, the iron source, the manganese source and the lithium source is 1: (0.3-0.8): (0.2-0.7): (0.95-1.1).

[0032] Preferably, the phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium hydrogen phosphate, lithium dihydrogen phosphate, sodium phosphate and potassium phosphate.

[0033] Preferably, the iron source includes one or more combinations of iron powder, ferric nitrate, ferric sulfate, ferric carbonate, ferric chloride, ferrous nitrate and ferric oxide.

[0034] Preferably, the manganese source includes one or more of manganese dioxide, manganese tetraoxide, manganese carbonate, manganese oxalate, manganese phosphate, manganese dichloride and manganese sulfate.

[0035] Preferably, the lithium source includes one or a combination of lithium hydroxide, lithium acetate, lithium carbonate, lithium oxalate, lithium chloride and lithium acetate.

[0036] By adopting the above technical solution, the lithium manganese iron phosphate active component is used as the core layer, and solid-phase composited with nitrogen-doped nano-silicon carbide composite particles is carried out, and a layer of composite silicon carbide intermediate layer is coated on the surface of the core layer. This can improve the structural stability and heat resistance of the positive-electrode material, and can also improve the conductivity and lithium ion diffusion rate of the positive electrode material. Finally, it is composited with a polymer organic carbon source and sintered to obtain a carbon conductive outer layer, which further coats the lithium manganese iron phosphate active component. The multi-layer coated doped lithium manganese iron phosphate positive electrode material obtained in this way not only has good conductivity and lithium ion diffusion performance, but also can effectively avoid the problem of manganese dissolution, thereby improving the cycle life of the material.

[0037] In a third aspect, the present invention provides a lithium battery, the positive electrode material of which is the multi-layer coated doped lithium manganese iron phosphate positive electrode material obtained above.

[0038] Beneficial effects of the present invention:

[0039] 1. The multi-layer coated doped lithium iron manganese phosphate positive electrode material of the present invention is composed of a lithium iron manganese phosphate core layer, a composite silicon carbide intermediate layer and a carbon conductive outer layer. The lithium iron manganese phosphate core layer provides the main active substance for the positive electrode material. The composite silicon carbide intermediate layer can improve the temperature resistance and structural stability of the positive electrode material and can improve the conductivity and lithium ion transmission efficiency of the positive material, thereby broadening the application range of lithium batteries. The carbon conductive outer layer provides good electronic conductivity and protection for the positive electrode material, so that the obtained lithium iron manganese phosphate positive electrode material not only has good conductivity and lithium ion diffusion performance, but also can effectively avoid the problem of manganese dissolution, thereby improving the cycle life of the material.

[0040] 2. The composite silicon carbide intermediate layer of the present invention is composed of nitrogen-doped nano-silicon carbide composite particles. Nitrogen-doped nano-silicon carbide can significantly improve the conductivity of the intermediate layer. At the same time, the composite particles also contain lithium silicate, which can provide more transmission paths for lithium ions, accelerate the insertion and extraction process of lithium ions, and provide an additional lithium source for the positive electrode material, thereby improving the energy density of the positive electrode material. DETAILED DESCRIPTION

[0041] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0042] Preparation Example

[0043] Preparation Example 1: A nitrogen-doped nano-silicon carbide composite particle was prepared according to the following method:

[0044] 66 g of lithium acetate and 30 g of nano-silica (average particle size of 50 nm) were mixed and added to 250 mL of anhydrous ethanol. 8 g of polyvinyl pyrrolidone was added and mixed evenly. The temperature was raised to 75 ° C. The anhydrous ethanol was evaporated by stirring. The temperature was then raised to 750 ° C under a nitrogen atmosphere for heat treatment. Finally, lithium silicate particles were obtained by grinding.

[0045] 7.5 g of nano-silicon carbide particles (average particle size of 500 nm) were mixed with 0.06 g of urea, ground and sieved, and pre-sintered at 850°C for 1.5 h to obtain a pre-reacted powder. The pre-reacted powder was dispersed in deionized water to form a suspension, 0.4 g of polyvinyl alcohol (weight-average molecular weight of 13,000-23,000) was added and stirred for 20 min, and then 2.5 g of the lithium silicate particles obtained above were added. The pH value of the solution was adjusted to 9, ball milled for 4 h, dried, and sintered at 1,500°C for 5 h to obtain nitrogen-doped nano-silicon carbide composite particles.

[0046] Preparation Examples 2 to 7, as well as Preparation Examples 9 and 10, are nitrogen-doped nano-silicon carbide composite particles. The only difference from Preparation Example 1 is that the raw material ratios in the preparation process of the nitrogen-doped nano-silicon carbide composite particles are adjusted, as shown in Table 1:

[0047] Table 1 Formulations of Preparation Examples 1 to 7, 9 and 10

[0048]

[0049] Preparation Example 8: A nano-silicon carbide composite particle was prepared according to the following method:

[0050] 7.5 g of nano-silicon carbide particles were dispersed in deionized water to form a suspension, 0.4 g of polyvinyl alcohol (weight-average molecular weight of 13,000-23,000) was added, and the mixture was stirred for 20 min. Then, 2.5 g of the lithium silicate particles prepared in Preparation Example 1 were added. The pH value of the solution was adjusted to 9, and the mixture was ball-milled for 4 h. After drying, the mixture was sintered at 1500°C for 5 h to obtain nano-silicon carbide composite particles.

[0051] Preparation Example 11: A nitrogen-doped nano-silicon carbide composite particle was prepared according to the following method:

[0052] 7.5 g of nano-silicon carbide particles (average particle size of 500 nm) were mixed with 0.06 g of urea, ground and sieved, and pre-sintered at 850°C for 1.5 h to obtain a pre-reacted powder; the pre-reacted powder was dispersed in a 5% by mass aqueous solution of polyvinyl alcohol (weight-average molecular weight of 13,000-23,000), stirred and mixed for granulation, dried, and sintered at 1,500°C for 5 h to obtain nitrogen-doped nano-silicon carbide composite particles.

[0053] Example

[0054] Example 1: A multi-layer coated doped lithium manganese iron phosphate positive electrode material is prepared according to the following method:

[0055] S1. 1 mol of phosphoric acid, 0.6 mol of ferric sulfate, 0.4 mol of manganese nitrate and 1 mol of lithium hydroxide were mixed, dried and then sintered at 650 ° C for 6 hours to obtain a lithium manganese iron phosphate active component;

[0056] S2. The lithium manganese iron phosphate active component was used as the core layer, which was mixed with the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 and ball-milled for 2 h, and then subjected to a second sintering at 400 ° C for 3 h to obtain an intermediate product;

[0057] S3. The intermediate product was dispersed in deionized water with a solid content of 40%, sucrose was added and mixed evenly, and after drying, a third sintering was performed at 800°C for 4 hours to obtain a multilayer coated doped lithium manganese iron phosphate positive electrode material.

[0058] The multi-layer coated doped lithium manganese iron phosphate positive electrode material consists of a 75% lithium manganese iron phosphate core layer, a 22% composite silicon carbide intermediate layer and a 3% carbon conductive outer layer.

[0059] Example 2, a multi-layer coated doped lithium iron manganese phosphate positive electrode material, differs from Example 1 only in that the multi-layer coated doped lithium iron manganese phosphate positive electrode material consists of a 65% lithium iron manganese phosphate core layer, a 30% composite silicon carbide intermediate layer and a 5% carbon conductive outer layer.

[0060] Example 3, a multi-layer coated doped lithium iron manganese phosphate positive electrode material, the only difference from Example 1 is that the multi-layer coated doped lithium iron manganese phosphate positive electrode material consists of 85% lithium iron manganese phosphate core layer, 10% composite silicon carbide intermediate layer and 5% carbon conductive outer layer.

[0061] Example 4, a multi-layer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 2 are used to replace the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1.

[0062] Example 5, a multi-layer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 are replaced by an equal amount of nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 3.

[0063] Example 6, a multilayer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 4 are used to replace the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1.

[0064] Example 7, a multi-layer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 are replaced by an equal amount of nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 5.

[0065] Example 8, a multi-layer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 are replaced by an equal amount of nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 6.

[0066] Example 9, a multi-layer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 7 are used to replace the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1.

[0067] Example 10, a multi-layer coated doped lithium iron manganese phosphate positive electrode material, the only difference from Example 1 is that the multi-layer coated doped lithium iron manganese phosphate positive electrode material consists of 90% lithium iron manganese phosphate core layer, 5% composite silicon carbide intermediate layer and 5% carbon conductive outer layer.

[0068] Example 11, a multi-layer coated doped lithium iron manganese phosphate positive electrode material, the only difference from Example 1 is that the multi-layer coated doped lithium iron manganese phosphate positive electrode material consists of a 60% lithium iron manganese phosphate core layer, a 35% composite silicon carbide intermediate layer and a 5% carbon conductive outer layer.

[0069] Comparative Example

[0070] Comparative Example 1, a multilayer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 are replaced by an equal amount of nano-silicon carbide composite particles prepared in Preparation Example 8.

[0071] Comparative Example 2, a multilayer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 9 are used to replace the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1.

[0072] Comparative Example 3, a multilayer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 are replaced by an equal amount of nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 10.

[0073] Comparative Example 4, a multilayer coated doped lithium manganese iron phosphate positive electrode material, differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 are replaced by an equal amount of nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 11.

[0074] Comparative Example 5 is a multi-layer coated doped lithium manganese iron phosphate positive electrode material, which differs from Example 1 only in that the nitrogen-doped nano-silicon carbide composite particles prepared in Preparation Example 1 are replaced by an equal amount of nano-silicon carbide.

[0075] Comparative Example 6: A lithium manganese iron phosphate positive electrode material was prepared according to the following method:

[0076] S1. 1 mol of phosphoric acid, 0.6 mol of ferric sulfate, 0.4 mol of manganese nitrate and 1 mol of lithium hydroxide were mixed, dried and sintered at 650 ° C for 6 h to obtain lithium manganese iron phosphate active component;

[0077] S2. The lithium manganese iron phosphate active component is dispersed in deionized water, the solid content is controlled to 40%, sucrose is added and mixed evenly, wherein the amount of sucrose added is 5% of the mass of the lithium manganese iron phosphate active component. After drying, the mixture is sintered at 800°C for 4 hours to obtain a lithium manganese iron phosphate positive electrode material.

[0078] Performance testing

[0079] Sample preparation: The lithium manganese iron phosphate positive electrode material obtained in the examples and comparative examples was mixed with a conductive agent (conductive carbon black) and a binder (polyvinylidene fluoride) in a mass ratio of 8:1:1, and a diluted slurry of 0.05 g / mL of N-methyl-2-pyrrolidone was added. The mixture was ground and coated on aluminum foil, and dried at 80°C for 24 h to obtain a positive electrode sheet. Under a nitrogen atmosphere, a 2032-type button battery was prepared using a lithium sheet as the negative electrode material, a polypropylene microporous membrane as the separator, and 1 mol / L LiPF6 as the electrolyte.

[0080] Performance testing:

[0081] 1. Rate performance test: In the voltage range of 2.0~3.8V, at a constant temperature of 25℃, the specific capacity of the sample battery was tested at different rates of 1C, 5C and 10C. The test results are shown in Table 2.

[0082] Table 2 Rate performance test results

[0083]

[0084] 2. Cycle performance test: The sample battery was subjected to charge and discharge cycle test in the voltage range of 2.0-3.8V, 1C rate, and constant temperature of 25°C. The cycle test number was 500 times. The capacity retention rate of the battery was tested and calculated. The test results are shown in Table 3.

[0085] Table 3 Cyclic performance test results

[0086]

[0087] 3. Temperature resistance test: In the voltage range of 2.0 ~ 3.8V, 1C rate, at constant temperature conditions of -20℃, 25℃ and 45℃, the specific capacity of the sample battery was tested for 100 cycles. The test results are shown in Table 4:

[0088] Table 4 Test results of heat resistance performance

[0089]

[0090] According to Tables 2, 3, and 4, combined with Example 1 and Examples 2 to 7, it can be seen that the specific capacity and capacity retention rate of Examples 2 to 7 at different rates and temperatures do not change significantly compared to Example 1, indicating that the rate performance, cycle performance, and temperature resistance of Examples 2 to 7 do not differ significantly from those of Example 1. This is because, compared to Example 1, Examples 2 to 7 only changed the raw materials and raw material ratios, which had little effect on the performance of the resulting positive electrode materials.

[0091] Combining Example 1, Example 8, Example 9 and Comparative Example 1, it can be seen that the specific capacity and capacity retention rate of Example 8, Example 9 and Comparative Example 1 at different rates are all lower than those of Example 1, among which the decrease in Comparative Example 1 is more obvious. The reason is that the nitrogen doping content of the nitrogen-doped nano-silicon carbide composite particles used in the composite silicon carbide intermediate layer in Example 8 is reduced, which directly leads to a decrease in the electrical conductivity of the nano-silicon carbide, greatly reducing the number and transmission path of free electrons in the positive electrode material, and the rate performance decreases accordingly; the nano-silicon carbide composite particles used in Comparative Example 1 are not doped with nitrogen, and the conductivity decreases significantly, and the rate performance also decreases significantly. In Example 9, the nitrogen doping content is increased, and the grain distortion content of the nano-silicon carbide increases greatly, which will affect the structural stability of the obtained composite silicon carbide intermediate layer, but is not conducive to the improvement of the material rate performance and temperature resistance.

[0092] In combination with Example 1, Example 10, Example 11 and Comparative Example 6, it can be seen that the specific capacity and capacity retention rate of Example 8, Example 9 and Comparative Example 1 at different rates and different temperatures are all lower than those of Example 1, among which the decrease in Comparative Example 6 is more obvious. The reason is that in Example 10, the proportion of the composite silicon carbide intermediate layer is reduced, which not only reduces the protective effect of the intermediate layer on the lithium manganese iron phosphate core layer, but also reduces the effect of improving the electron transmission and lithium ion migration rate, resulting in a decrease in the rate performance and temperature resistance of the material. In Comparative Example 6, no composite silicon carbide intermediate layer is added, and it is only coated with a carbon conductive outer layer, which not only greatly reduces the diffusion rate of lithium ions, but also increases the occurrence of manganese dissolution, resulting in a decrease in the cycle performance of the material. In Example 11, the proportion of the composite silicon carbide intermediate layer is increased, which is not conducive to the release of the intermediate lithium manganese iron phosphate active component, and the average particle size of the lithium manganese iron phosphate positive electrode material particles formed increases, which is not conducive to the improvement of the material rate performance.

[0093] Combining Example 1 and Comparative Examples 2 to 5, it can be seen that the specific capacity and capacity retention rate of Comparative Examples 2 to 5 at different rates and temperatures are significantly lower than those of Example 1. The reason is that the content of nitrogen-doped nano-silicon carbide composite particles and composite lithium silicate used in the composite silicon carbide intermediate layer in Comparative Example 2 is reduced. On the one hand, it will reduce the diffusion path of lithium ions and the content of additional lithium sources, and the rate of lithium ion deintercalation process is reduced, resulting in a significant decrease in the rate. On the other hand, due to the lack of lithium silicate composite, the density of nitrogen-doped nano-silicon carbide is reduced, the structural stability is reduced, and the transmission rate of electrons and lithium ions cannot reach a balance, resulting in a decrease in the cycle performance and temperature resistance of the material. In Comparative Example 4, no lithium silicate composite is added, and the performance decreases more significantly. In Comparative Example 3, the content of nitrogen-doped nano-silicon carbide composite particles and composite lithium silicate used in the composite silicon carbide intermediate layer is increased, and the decrease in the proportion of nano-silicon carbide will affect the structural stability of the composite silicon carbide intermediate layer, resulting in a decrease in the temperature resistance of the material. In Comparative Example 5, nano-silicon carbide that is not doped with nitrogen and not compounded with lithium silicate is used as the intermediate layer. Not only will the conductivity and diffusivity of lithium ions be significantly reduced, but the porosity of the intermediate layer will be significantly increased, which is not conducive to protecting the lithium manganese iron phosphate core layer, and will increase the probability of manganese dissolution, resulting in a decrease in the cycle performance of the material.

[0094] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0095] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. Multilayer coated doped lithium manganese iron phosphate positive electrode material, characterized in that: The positive electrode material is composed of a lithium manganese iron phosphate core layer, a composite silicon carbide middle layer and a carbon conductive outer layer; the composite silicon carbide middle layer is composed of nitrogen-doped nano-silicon carbide composite particles; the nitrogen-doped nano-silicon carbide composite particles contain 20-30% lithium silicate; The raw materials of the nitrogen-doped nano-silicon carbide composite particles also include nano-silicon carbide particles and a nitrogen source; the added amount of the nitrogen source is 0.5-1% of the mass of the nano-silicon carbide particles.

2. The multi-layer coated doped lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The positive electrode material comprises a lithium manganese iron phosphate core layer with a mass fraction of 65-85%, a composite silicon carbide intermediate layer with a mass fraction of 10-30%, and a carbon conductive outer layer with a mass fraction of 2-5%.

3. The multi-layer coated doped lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The nitrogen-doped nano-silicon carbide composite particles are prepared according to the following method: The nano-silicon carbide particles are mixed with a nitrogen source, then ground and sieved, and pre-sintered at 800-1000°C for 1-2 hours to obtain a pre-reaction powder; the pre-reaction powder is dispersed in deionized water to form a suspension, a granulation aid is added, stirred and mixed for 15-30 minutes, and then lithium silicate particles are added, the pH value of the solution is adjusted to 8-9, ball milled for 3-5 hours, dried, and sintered at 1400-1600°C for 5-6 hours to obtain nitrogen-doped nano-silicon carbide composite particles.

4. The multi-layer coated doped lithium manganese iron phosphate positive electrode material according to claim 3, characterized in that: The nitrogen source includes one or more of urea, dicyandiamide and melamine.

5. The multi-layer coated doped lithium manganese iron phosphate positive electrode material according to claim 3, characterized in that: The granulation aid comprises a combination of one or more of polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, sodium carboxymethyl cellulose and sodium alginate; and the added amount of the granulation aid is 4-6% of the mass of the nano-silicon carbide particles.

6. The multi-layer coated doped lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The raw material of the carbon conductive outer layer is a high molecular organic carbon source; the high molecular organic carbon source includes one or more combinations of glucose, sucrose, citric acid, polyethylene glycol, cellulose derivatives, chitosan, phenolic resin and polyacrylic acid.

7. A method for preparing a multi-layer coated doped lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: The process steps include: S1. The phosphorus source, iron source, manganese source and lithium source are mixed in proportion, dried and then sintered for the first time to obtain the lithium manganese iron phosphate active component; S2. The lithium manganese iron phosphate active component is used as the core layer, which is mixed with nitrogen-doped nano-silicon carbide composite particles and ball-milled for 1 to 3 hours, followed by a second sintering to obtain an intermediate product; S3. The intermediate product is dispersed in deionized water, a high molecular organic carbon source is added and mixed evenly, and after drying, a third sintering is performed to obtain a multilayer coated doped lithium manganese iron phosphate positive electrode material.

8. The method for preparing a multi-layer coated doped lithium manganese iron phosphate positive electrode material according to claim 7, characterized in that: The sintering temperature of the first sintering is 600-800° C., and the sintering time is 5-7 hours; the sintering temperature of the second sintering is 350-550° C., and the sintering time is 2-4 hours; the sintering temperature of the third sintering is 700-800° C., and the sintering time is 3-5 hours.

9. The method for preparing a multi-layer coated doped lithium manganese iron phosphate positive electrode material according to claim 7, characterized in that: The molar ratio of the phosphorus source, iron source, manganese source and lithium source is 1: (0.3-0.8): (0.2-0.7): (0.95-1.1).

10. A lithium battery, characterized in that: The positive electrode material of the lithium battery is the multi-layer coated doped lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 6.

Images