Lithium iron manganese phosphate positive electrode material, preparation method thereof, positive electrode sheet and lithium ion battery

Abstract

The application discloses a lithium iron manganese phosphate positive electrode material and a preparation method and positive electrode sheet and lithium ion battery, and relates to the technical field of lithium ion batteries.The lithium iron manganese phosphate positive electrode material comprises a lithium iron manganese phosphate inner core and a coating shell layer; the coating shell layer is coated on the surface of the lithium iron manganese phosphate inner core; and the coating shell layer comprises lithium iron phosphate. The positive electrode material can solve the problem of poor high-temperature storage performance of the lithium iron manganese phosphate material, and enables the lithium iron manganese phosphate material to have high capacity and high-temperature storage stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a lithium manganese iron phosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] Lithium manganese iron phosphate has the advantages of relatively high voltage of LiMnPO4 and structural stability of LiFePO4. x Mn 1-x PO4, FeO6 and MnO6 octahedrons are connected through PO4 tetrahedrons, which cannot form a continuous conductive network, thus making the material have poor conductivity. Meanwhile, the structural difference between MnPO4 and FePO4 makes the material have poor thermal stability. How to ensure that the lithium manganese iron phosphate material has high capacity and high-temperature storage performance at the same time is one of the technical difficulties restricting the application of lithium manganese iron phosphate as an energy storage material.

[0003] In order to ensure that the lithium manganese iron phosphate material has high capacity and high-temperature storage performance at the same time, the lithium manganese iron phosphate particles are usually nano-sized first to improve the Li + diffusion rate. Then, a carbon coating method is used to improve the electronic conductivity of the lithium manganese iron phosphate material. However, after nano-sizing, the particle size of the material is small, and after being prepared into a secondary battery, the surface area of the material in contact with the electrolyte increases, which leads to the intensified corrosion of the electrolyte on the lithium manganese iron phosphate material, accelerates the disproportionation reaction of Mn 3+ , and promotes the dissolution of Mn 2+ and Mn 4+ in the electrolyte and the migration of Mn 2+ and Mn 4+ to the negative electrode through the separator, and the reduction reaction of Mn 2+ and Mn 4+ at the negative electrode, which consumes active lithium ions and leads to the decline of high-temperature storage performance. SUMMARY

[0004] The present application aims to provide a lithium manganese iron phosphate positive electrode material and a preparation method thereof, which can solve the problem of poor high-temperature storage performance of the lithium manganese iron phosphate material and enable the lithium manganese iron phosphate material to have high capacity and high-temperature storage stability at the same time.

[0005] In addition, the present application also provides a positive electrode sheet and a lithium ion battery made of the above-mentioned lithium manganese iron phosphate positive electrode material.

[0006] In one aspect, the lithium manganese iron phosphate positive electrode material provided by the present application comprises a lithium manganese iron phosphate inner core and a coating shell layer; the coating shell layer is coated on the surface of the lithium manganese iron phosphate inner core; and the coating shell layer comprises lithium iron phosphate.

[0007] On the one hand, the lithium iron phosphate material of the coating shell layer can also participate in the electrochemical reaction of the lithium manganese iron phosphate, which makes the capacity performance of the lithium manganese iron phosphate material coated with the lithium iron phosphate material not affected.

[0008] On the other hand, the lithium iron phosphate material as the coating shell can be uniformly coated on the surface and the gap of the lithium manganese iron phosphate core, the lithium iron phosphate as the coating shell greatly reduces the direct contact between the electrolyte and the lithium manganese iron phosphate core, reduces the corrosion of the lithium manganese iron phosphate core, and reduces the Mn 3+ The disproportionation reaction occurs, thereby improving the high-temperature cycle performance and high-temperature storage performance of the lithium manganese iron phosphate positive electrode material.

[0009] In some embodiments, the lithium manganese iron phosphate core is formed by aggregation of lithium manganese iron phosphate primary particles, and the particle size of the lithium manganese iron phosphate core is 1-10 μm. The spherical lithium manganese iron phosphate core can greatly reduce the specific surface area in contact with the electrolyte.

[0010] The aforementioned lithium manganese iron phosphate core is formed by aggregation of a certain amount of lithium manganese iron phosphate primary particles into a spherical particle. The spherical particle formed by the lithium manganese iron phosphate core has a size much larger than that of the generated lithium iron phosphate particle, so that the lithium iron phosphate coating shell can be more uniformly coated on the surface of the lithium manganese iron phosphate core.

[0011] Moreover, for the particle size of the spherical lithium manganese iron phosphate core, if the particle size is too small, the size difference between the spherical particle and the lithium iron phosphate particle is reduced, resulting in a non-tight coating of the lithium iron phosphate coating shell; and if the particle size is too large, the spherical particle is broken during subsequent electrode rolling, causing processing problems such as separation from the current collector.

[0012] In some embodiments, the lithium manganese iron phosphate is formed by aggregation of lithium manganese iron phosphate primary particles coated with a first carbon layer, the particle size of the lithium manganese iron phosphate primary particle is 50-500 nm, and the average thickness of the first carbon layer is 1-20 nm.

[0013] In this way, the spherical lithium manganese iron phosphate core particles formed by aggregation of 50-500 nm primary particles have gaps, and the coating shell grows preferentially in the gap structure of the spherical lithium manganese iron phosphate core, so that the lithium iron phosphate coating shell is more densely coated on the surface of the lithium manganese iron phosphate core.

[0014] In some embodiments, the aforementioned coating shell preferably uses ferrous oxalate as the iron source. In this way, the aforementioned ferrous oxalate can be precipitated on the surface of the spherical lithium manganese iron phosphate core and in the aforementioned gap structure in the form of in-situ precipitation of a solution, thereby forming an iron site anchor point for growth of the lithium iron phosphate, so that the lithium iron phosphate stably and uniformly coats the spherical lithium manganese iron phosphate core during in-situ growth. In addition, the iron source of the coating shell can also be one of ferrous phosphate, ferric phosphate, and hydroxyl ferric oxide.

[0015] Both, the application also provides a preparation method of the lithium iron manganese phosphate positive electrode material, in which an iron source is precipitated in situ on the surface of the lithium iron manganese phosphate core as an anchoring shell layer;

[0016] Then, the lithium iron phosphate is generated in situ with the anchoring shell layer as an anchor point and coated on the surface of the lithium iron manganese phosphate core as a coating shell layer.

[0017] In this way, the iron source is precipitated in situ on the surface and in the gaps of the lithium iron manganese phosphate core, and the generated lithium iron phosphate uniformly and densely coats the surface and gaps of the lithium iron manganese phosphate. The coating shell layer formed by the lithium iron phosphate can effectively prevent the electrolyte from eroding the lithium iron manganese phosphate core and reduce the Mn 3+ disproportionation reaction, thereby improving the high-temperature storage performance of the lithium iron manganese phosphate material.

[0018] In some embodiments, the lithium iron manganese phosphate core is prepared by the following method:

[0019] The sand-milled lithium iron manganese phosphate raw slurry is subjected to two-fluid spray drying at a solid content of 30% to 40%, and the obtained first spray-dried material is sintered at a temperature of 500 to 700°C to obtain the lithium iron manganese phosphate core.

[0020] In this step, if the spray solid content is too low when the two-fluid spray drying is performed, the first spray-dried material formed will be loose inside, and the sphericity of the first spray-dried material will be reduced, which is not conducive to the formation of the lithium iron manganese phosphate core in a spherical shape. If the spray solid content is too high, the water content difference between the inner and outer layers of the spray ball will be too large, and annular spray balls will be formed, which will affect the formation of the lithium iron manganese phosphate core in a spherical shape. Moreover, the particle size range of the first spray-dried material needs to be controlled at 1 to 10 μm by adjusting the gas flow during spraying.

[0021] Moreover, when the first spray-dried material is sintered, lithium iron manganese phosphate crystal nuclei begin to form and grow at 400°C. At this time, the carbon source in the lithium iron manganese phosphate raw slurry melts and breaks, and is included in the surface and gaps of the lithium iron manganese phosphate spray ball. As the temperature rises, the carbon layer carbonizes at high temperature and adheres to the surface of the lithium iron manganese phosphate particles, thereby limiting the growth of the primary particles of the lithium iron manganese phosphate. In the subsequent sintering process, the primary particles of the lithium iron manganese phosphate stably aggregate during the sintering process, and the lithium iron manganese phosphate core in a spherical shape is formed by the aggregation of the primary particles of the lithium iron manganese phosphate in a size of 50 to 500 nm.

[0022] In addition, in the sintering process, as the gaseous substances escape, the components begin to diffuse and react to generate lithium iron manganese phosphate grains. As the crystal nuclei continuously generate, the grain spacing decreases, and the structure of the lithium iron manganese phosphate core in a spherical shape is significantly enhanced. In this way, the spherical structure will not be broken when the iron source is precipitated subsequently.

[0023] In some embodiments, the anchoring shell layer is prepared by:

[0024] After dispersing the lithium manganese iron phosphate core, the hydrated ferric sulfate salt is added, and then the co-precipitation solution is added dropwise while controlling the pH. After the reaction is completed, the mixed slurry is dehydrated and dried to obtain the lithium manganese iron phosphate core coated with the anchoring shell layer precursor.

[0025] In the process of adding the co-precipitation solution, it is slowly added dropwise under stirring, and at the same time, the pH of the environment is adjusted with sodium hydroxide solution.

[0026] In this way, the hydrated ferric sulfate salt completely dissolved in water is adjusted in pH, and most of the in-situ formed iron source is precipitated on the surface of the lithium manganese iron phosphate core and a small amount is precipitated in the gap of the lithium manganese iron phosphate core during the slow dropwise addition of the co-precipitation solution, thereby forming the lithium manganese iron phosphate core coated with the anchoring shell layer precursor. Because of the in-situ precipitation, the addition of the iron source for coating in the system can effectively avoid the formation of large particles directly precipitated by a large amount of agglomeration in the solution system. After in-situ precipitation, the iron source precipitated on the surface and in the gap of the lithium manganese iron phosphate will act as an anchor point for the generation of lithium iron phosphate, so that the generated lithium iron phosphate is uniformly and stably coated on the surface of the lithium manganese iron phosphate.

[0027] In some embodiments, the hydrated ferric sulfate salt includes one of ferrous sulfate heptahydrate and ferric sulfate nonahydrate, and the co-precipitation solution includes one of phosphoric acid, sodium hydroxide, and oxalic acid.

[0028] When the co-precipitation is performed with ferrous sulfate heptahydrate and oxalic acid, the iron source of the coating shell layer is ferrous oxalate; when the co-precipitation is performed with ferrous sulfate heptahydrate and phosphoric acid, the iron source of the coating shell layer is ferrous phosphate; when the co-precipitation is performed with ferric sulfate nonahydrate and phosphoric acid, the iron source of the coating shell layer is ferric phosphate; and when the co-precipitation is performed with ferrous sulfate heptahydrate and sodium hydroxide, the iron source of the coating shell layer is hydroxyl ferric oxide.

[0029] In some embodiments, the coating shell layer is prepared by:

[0030] Because the size of the lithium iron phosphate coating shell layer formed by sintering is small, the particle size of the second spray-dried material obtained after two-fluid spray drying is still controlled within the range of 1-10 μm, which is mainly to ensure the maintenance of the particle size of the first spray-dried material.

[0031] The lithium manganese iron phosphate core coated with the anchoring shell layer is mixed with a soluble lithium source, a soluble phosphorus source, and a soluble carbon source in a dispersion phase, and then the slurry is subjected to two-fluid spray drying. The obtained second spray-dried material is sintered at a temperature of 600-800°C to obtain a lithium iron phosphate coated lithium manganese iron phosphate positive electrode material.

[0032] In the second spray drying, the soluble lithium source, the soluble phosphorus source and the soluble carbon source are brought into contact with the hot nitrogen gas at the bottom during the two-fluid spraying, so that the spray slurry is rapidly vaporized and taken away by the nitrogen gas, and the lithium source, the phosphorus source and the carbon source in the spray slurry are attached to the surface of the lithium manganese iron phosphate core precursor due to evaporation crystallization, and after subsequent sintering, the lithium manganese iron phosphate material with high capacity and high-temperature storage performance is obtained.

[0033] In some embodiments, the coating amount of the coating shell layer is 1% to 7% of the lithium manganese iron phosphate core.

[0034] In the lithium iron phosphate, when the coating amount of the lithium iron phosphate is too small, the protection of the lithium manganese iron phosphate core is reduced, and when the coating amount of the lithium iron phosphate is too large, the Mn content in the lithium manganese iron phosphate core is reduced, the average voltage is reduced, and the energy density of the material is reduced.

[0035] Therefore, the lithium manganese iron phosphate positive electrode material provided by the application can not only isolate the electrolyte from the Mn 3+ , avoid disproportionation reaction, and improve the high-temperature storage performance of the lithium manganese iron phosphate core; and the lithium iron phosphate material as the coating shell layer can also participate in the electrochemical reaction of the lithium manganese iron phosphate, so that the capacity performance of the lithium iron phosphate material coated with the lithium manganese iron phosphate material is not affected. Further, the battery using the lithium manganese iron phosphate material provided by the application can have the advantages of high capacity and good high-temperature storage performance.

[0036] In addition, the application also provides a positive electrode sheet comprising the aforementioned lithium manganese iron phosphate positive electrode material or the lithium manganese iron phosphate positive electrode material prepared by the aforementioned preparation method.

[0037] In addition, the application also provides a lithium ion battery comprising the aforementioned positive electrode sheet. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 Structure diagram of the lithium manganese iron phosphate positive electrode material prepared by the application.

[0039] 1-lithium manganese iron phosphate, 2-primary carbon coating layer, 3-lithium iron phosphate, 4-secondary carbon coating layer. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the embodiments of the application more clear, the technical scheme in the embodiments of the application will be described in detail below. Obviously, the described embodiments are part of the embodiments of the application, but not all the embodiments of the application.

[0041] Therefore, the following detailed description of the embodiments of the present application is not intended to limit the scope of the application as claimed, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of the present application.

[0042] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0043] Please refer to Figure 1 In some embodiments, the present application provides a lithium manganese iron phosphate positive electrode material, which comprises a lithium manganese iron phosphate core and a coating shell; the coating shell is coated on the surface of the lithium manganese iron phosphate core; and the coating shell comprises lithium iron phosphate.

[0044] On the one hand, the lithium iron phosphate material of the coating shell can also participate in the electrochemical reaction of the lithium manganese iron phosphate material, so that the capacity performance of the lithium manganese iron phosphate material coated with the lithium iron phosphate material is not affected.

[0045] On the other hand, the lithium iron phosphate material as the coating shell can be uniformly coated on the surface and gaps of the lithium manganese iron phosphate core, and the direct contact of the electrolyte with the lithium manganese iron phosphate core is greatly reduced by using the lithium iron phosphate, the corrosion of the lithium manganese iron phosphate core is reduced, and the occurrence of the disproportionation reaction of Mn 3+ in a high-temperature environment is reduced, so as to improve the high-temperature cycle performance and high-temperature storage performance of the lithium manganese iron phosphate positive electrode material.

[0046] In some embodiments, the lithium manganese iron phosphate core is aggregated by lithium manganese iron phosphate primary particles, and the particle size of the lithium manganese iron phosphate core is 1-10 μm.

[0047] The aforementioned lithium manganese iron phosphate core is aggregated by a certain number of lithium manganese iron phosphate primary particles to form a spherical-like particle. The size of the spherical-like particle formed by the spherical-like lithium manganese iron phosphate core is much larger than the size of the generated lithium iron phosphate particle, so that the lithium iron phosphate coating shell can be more uniformly coated on the surface of the lithium manganese iron phosphate core.

[0048] Moreover, for the particle size of the spherical-like lithium manganese iron phosphate core, if the particle size is too small, the size difference between the spherical-like particle and the lithium iron phosphate particle will be reduced, resulting in that the coating of the lithium iron phosphate coating shell is not tight; and if the particle size is too large, the spherical-like particle will be broken during subsequent electrode sheet rolling, causing processing problems such as separation from the current collector.

[0049] In some embodiments, the lithium manganese iron phosphate is aggregated from lithium manganese iron phosphate primary particles coated with a first carbon layer, the lithium manganese iron phosphate primary particles have a particle size of 50-500 nm, and the first carbon layer has a thickness of 1-20 nm.

[0050] In this way, the 50-500 nm primary particles are aggregated to form a spherical lithium manganese iron phosphate core particle with a gap between the particles, and the shell layer grows preferentially in the gap structure of the spherical lithium manganese iron phosphate core, so that the lithium iron phosphate shell layer is more dense on the surface of the lithium manganese iron phosphate core.

[0051] In some embodiments, the shell layer preferably uses ferrous oxalate as the iron source, so that the ferrous oxalate is precipitated in situ on the surface of the spherical lithium manganese iron phosphate core and in the gap structure, thereby forming iron sites for the growth of lithium iron phosphate, and enabling the lithium iron phosphate to uniformly and stably coat the spherical lithium manganese iron phosphate core during in-situ growth. The in-situ precipitation method can also avoid the agglomeration of ferrous oxalate in the dispersion system, which affects the coating of the spherical lithium manganese iron phosphate core. In addition, the iron source of the shell layer can also be one of ferrous phosphate, iron phosphate, and hydroxyl iron oxide.

[0052] In addition, the application also provides a preparation method of a lithium manganese iron phosphate cathode material,

[0053] The iron source is precipitated in situ on the surface of the lithium manganese iron phosphate core as an anchoring shell layer.

[0054] The lithium iron phosphate is then generated in situ using the anchoring shell layer as an anchor point to coat the surface of the lithium manganese iron phosphate core as a shell layer.

[0055] In this way, the iron source of the shell layer is precipitated in situ on the surface of the lithium manganese iron phosphate core and in the gap, and the generated lithium iron phosphate uniformly and densely coats the surface of the lithium manganese iron phosphate core and the gap. The shell layer formed by the lithium iron phosphate can effectively prevent the electrolyte from eroding the lithium manganese iron phosphate core, reduce the consumption of active lithium ions, and improve the high-temperature storage performance of the lithium manganese iron phosphate material. 3+ disproportionation reaction, and reduce the consumption of active lithium ions, thereby improving the high-temperature storage performance of the lithium manganese iron phosphate material.

[0056] In some embodiments, the lithium manganese iron phosphate core is prepared by the following method:

[0057] The sand-milled lithium manganese iron phosphate raw slurry is subjected to two-fluid spray drying at a solid content of 30%-40%, and the obtained first spray-dried material is sintered at a temperature of 500-700°C to obtain the lithium manganese iron phosphate core.

[0058] Further, the aforementioned raw slurry manganese source can be selected as trimanganese tetroxide, the iron source is preferably ferrous oxalate, the lithium source and the phosphorus source can be selected as lithium dihydrogen phosphate, the carbon source can be selected as sucrose, and titanium dioxide can be selected as a dopant, and methanol is selected as a dispersion phase.

[0059] After the raw slurry is ground, two-fluid spray drying is performed to prepare the first spray-dried material.

[0060] In this step, if the spray solid content is too low when the two-fluid spray drying is performed, the first spray-dried material formed will be loose inside, and the sphericity of the first spray-dried material will be reduced, which is not conducive to the formation of the spherical lithium manganese iron phosphate core. If the spray solid content is too high, the water content difference between the inside and outside of the spray ball will be too large, and annular spray balls will be formed, which will affect the formation of the spherical lithium manganese iron phosphate core. Moreover, the particle size range of the first spray-dried material is controlled to be 1-10 μm by adjusting the gas flow during spraying.

[0061] Moreover, when the first spray-dried material is sintered, lithium manganese iron phosphate crystal nuclei begin to form and grow at 400°C. At this time, the carbon source in the lithium manganese iron phosphate raw slurry melts and breaks, and is included in the surface and gaps of the lithium manganese iron phosphate spray ball. As the temperature rises, the carbon layer carbonizes at high temperature and adheres to the surface of the lithium manganese iron phosphate particles, thereby limiting the growth of the lithium manganese iron phosphate primary particles. Further, in the continuous sintering process, the lithium manganese iron phosphate primary particles stably aggregate during sintering to form a spherical lithium manganese iron phosphate core composed of primary particles of 50-500 nm.

[0062] In addition, during the sintering process, as the gas phase material escapes, the components begin to diffuse and react to form lithium manganese iron phosphate grains. As the crystal nuclei continue to form, the grain spacing decreases, significantly enhancing the structure of the spherical lithium manganese iron phosphate core. Thus, when the iron source is subsequently precipitated, the spherical structure will not be broken.

[0063] In some embodiments, the anchoring shell layer is prepared by:

[0064] After the lithium manganese iron phosphate core is dispersed, the hydrated iron sulfate salt is added, and the co-precipitation solution is added dropwise. The pH is controlled, and after the reaction is complete, the mixed slurry is dehydrated and dried to obtain the lithium manganese iron phosphate core precursor coated with the anchoring shell layer.

[0065] Further, after the co-precipitation solution is added, the reaction is carried out until the Fe 2+ content is less than 5 ppm, the reaction is complete, and the mixed slurry can be dehydrated and dried. The precipitation reaction temperature is controlled at 20-60°C, and the reaction system is deionized water.

[0066] The process of adding the co-precipitation solution is slow dropwise under stirring, and the pH of the environment is adjusted by the sodium hydroxide solution at the same time.

[0067] In this way, the iron source formed in situ during the slow dropwise addition of the co-precipitation solution can be mostly precipitated on the surface of the lithium manganese iron phosphate core and a small amount of the iron source can be precipitated in the gaps of the lithium manganese iron phosphate core, thereby forming the lithium manganese iron phosphate core precursor coated with the anchoring shell layer. Because of the in-situ precipitation, the direct addition of the iron source can be effectively avoided to prevent the iron source from forming large particles by agglomeration in the solution system. After the in-situ precipitation, the iron source precipitated on the surface and inside of the lithium manganese iron phosphate can serve as an anchor point for the generation of lithium iron phosphate, so that the generated lithium iron phosphate is uniformly and stably coated on the surface of the lithium manganese iron phosphate.

[0068] In some embodiments, the aforementioned hydrated ferric sulfate includes one of ferrous sulfate heptahydrate or ferric sulfate nonahydrate, and the co-precipitation solution includes one of phosphoric acid, sodium hydroxide, and oxalic acid.

[0069] When the co-precipitation is performed using ferrous sulfate heptahydrate and oxalic acid, the aforementioned iron source of the coating shell layer is ferrous oxalate; when the co-precipitation is performed using ferrous sulfate heptahydrate and phosphoric acid, the aforementioned iron source of the coating shell layer is ferrous phosphate; when the co-precipitation is performed using ferric sulfate nonahydrate and phosphoric acid, the aforementioned iron source of the coating shell layer is ferric phosphate; and when the co-precipitation is performed using ferrous sulfate heptahydrate and sodium hydroxide, the aforementioned iron source of the coating shell layer is hydroxyl ferric oxide.

[0070] Furthermore, for the selection of the reaction conditions, when the iron source of the coating shell layer is ferrous oxalate, the pH range during the co-precipitation process is controlled to be 2.5-3;

[0071] When the iron source of the coating shell layer is ferrous phosphate, the pH range during the co-precipitation process is controlled to be 6-8;

[0072] When the iron source of the coating shell layer is ferric phosphate, the pH range during the co-precipitation process is controlled to be 5-9;

[0073] When the iron source of the coating shell layer is hydroxyl ferric oxide, in addition to the adjustment of the pH to be in the range of 7-9 by using sodium hydroxide for aging, air needs to be continuously introduced during the co-precipitation process until the pH of the system no longer changes.

[0074] In some embodiments, the coating shell layer is prepared by:

[0075] The lithium manganese iron phosphate core covered with the anchoring shell layer is mixed with a soluble lithium source, a soluble phosphorus source and a soluble carbon source in a dispersion phase, and then the slurry is subjected to two-fluid spray drying, and the obtained second spray-dried material is sintered at a temperature of 600-800°C to obtain a lithium iron phosphate-coated lithium manganese iron phosphate positive electrode material.

[0076] Further, the aforementioned soluble carbon source uses a water-soluble organic carbon source, which can be glucose, sucrose, etc. The aforementioned soluble phosphorus source and soluble lithium source can both use lithium dihydrogen phosphate.

[0077] Since the lithium iron phosphate coating shell layer formed by sintering is small in size, the particle size of the obtained second spray-dried material is still controlled within the range of 1-10 μm after two-fluid spray drying, which is mainly to ensure the maintenance of the particle size of the aforementioned first spray-dried material.

[0078] The soluble lithium source, soluble phosphorus source and soluble carbon source in the obtained second spray-dried material are all crystallized on the surface of the lithium manganese iron phosphate core precursor coated with the anchoring shell layer, and after subsequent sintering, a lithium manganese iron phosphate material with high capacity and good high-temperature storage performance is obtained.

[0079] In some embodiments, the coating amount of the coating shell layer is 1%-7% of the lithium manganese iron phosphate core.

[0080] Since too little lithium iron phosphate coating will reduce the protection of the lithium manganese iron phosphate core, and too much lithium iron phosphate coating on the lithium manganese iron phosphate core will result in a decrease in the Mn content in the lithium manganese iron phosphate core, causing a decrease in the average voltage and a decrease in the energy density of the material.

[0081] Thus, the lithium manganese iron phosphate positive electrode material provided in the present application, when applied in a battery, not only can isolate the electrolyte from corroding Mn 3+ , thereby improving the high-temperature storage performance of the lithium manganese iron phosphate core, but also the lithium iron phosphate material as the coating shell layer can participate in the electrochemical reaction of the lithium manganese iron phosphate, which makes the capacity performance of the lithium iron phosphate material-coated lithium manganese iron phosphate material unaffected. Further, the battery using the lithium manganese iron phosphate material related to the present application can have the advantages of high capacity and good high-temperature storage performance at the same time.

[0082] Next, the technical solutions of the present application will be described in detail using different specific embodiments.

[0083] Example 1

[0084] The embodiment provides a lithium manganese iron phosphate positive electrode material, which has a lithium iron phosphate coating layer, a spherical core formed by gathering lithium manganese iron phosphate primary particles, and an iron source with a ferrous oxalate coating layer. Thus, the prepared lithium manganese iron phosphate positive electrode material can have high capacity and good high-temperature storage characteristics.

[0085] Specifically, in the embodiment, the amount of lithium iron phosphate is 1% of the lithium manganese iron phosphate, and the Mn:Fe molar ratio of the whole positive electrode material is 6:4; the specific steps are as follows:

[0086] Step 1: respectively take three manganese tetroxide, ferrous oxalate, LiH2PO4, titanium dioxide and sucrose in a ball mill tank, the amount is 2.61kg, 4kg, 6.08kg, 0.08kg and 0.5kg respectively, add anhydrous methanol to form the original slurry; pour into a ball mill and mix fully, then transfer into a sand mill, the sand mill speed is 800rpm, when the sand mill particle size is 0.5±0.05μm, transfer into a two-fluid spray tower to spray, get the first spray-dried material; adjust the nitrogen flow of the spray tower to control the spray particle size in 1~10μm. Sinter the first spray-dried material after spraying at 500℃~700℃ for 10h to get a sintered sample.

[0087] Step 2: take 9.22kg of the sintered sample prepared in step 1, pour into a stainless steel reaction kettle, add 15kg of ultrapure water to disperse, after complete dispersion, add 0.17kg of ferrous sulfate heptahydrate, stir to mix evenly. Slowly add 42% oxalic acid solution to the reaction kettle, control the solution pH in the range of 2.5~3 by slowly adding 8% sodium hydroxide solution, after reaction for 120min, take the upper clear liquid to test Fe 2+ content is less than 5ppm. After the reaction is completed, the slurry is centrifuged and dehydrated in a centrifuge, washed 3 times with 45℃ pure water at high and low speed, then the centrifuged material is placed in an 80℃ oven and dried for 20h to obtain an anchor shell coated lithium manganese iron phosphate core precursor.

[0088] Step 3: respectively take the anchor shell coated lithium manganese iron phosphate core precursor prepared in step 2, LiH2PO4 and sucrose, the amount is 8kg, 0.07kg, 0.006kg respectively, add 27kg of pure water, disperse fully in a stirring tank, then control the spray particle size in 1~10μm by two-fluid spraying. Sinter the sample after spraying at 600~800℃ for 10h to obtain a lithium iron phosphate coated lithium manganese iron phosphate positive electrode material.

[0089] The Mn:Fe molar ratio of the positive electrode material prepared in the embodiment is 6:4; the content of the lithium iron phosphate layer is 1%, the content of the primary carbon coating layer is 1.6%, and the content of the secondary carbon coating layer is 1.1% based on the mass of the positive electrode material.

[0090] Example 2

[0091] In this embodiment, the coating amount of lithium iron phosphate is 3% of lithium manganese iron phosphate, and the remaining conditions and processes are the same as those of Example 1.

[0092] Example 3

[0093] In this embodiment, the coating amount of lithium iron phosphate is 5% of lithium manganese iron phosphate, and the remaining conditions and processes are the same as those of Example 1.

[0094] Example 4

[0095] In this embodiment, the coating amount of lithium iron phosphate is 7% of lithium manganese iron phosphate, and the remaining conditions and processes are the same as those of Example 1.

[0096] Example 5

[0097] This embodiment provides a lithium manganese iron phosphate positive electrode material, which has a multi-layer structure with lithium iron phosphate as a coating shell layer, a spherical core formed by aggregation of primary particles of lithium manganese iron phosphate, and a source of iron coating shell layer of ferrous oxalate. Thus, the prepared lithium manganese iron phosphate positive electrode material can have high capacity and good high-temperature storage characteristics.

[0098] Specifically, in this embodiment, the coating amount of lithium iron phosphate is 3% of lithium manganese iron phosphate, and the Mn:Fe molar ratio of the inner layer lithium manganese iron phosphate is controlled to be 6:4; the specific steps are as follows:

[0099] Step 1: Take three manganese tetroxide, ferrous oxalate, LiH2PO4, titanium dioxide, and sucrose in a ball mill jar, respectively, with a dosage of 2.54 kg, 4 kg, 6.00 kg, 0.080 kg, and 0.50 kg, respectively, add anhydrous methanol to form the original slurry; pour into a sand mill and mix thoroughly, then transfer to a sand mill at a speed of 800 rpm, and when the sand mill particle size is 0.5±0.05 μm, transfer to a two-fluid spray tower for spraying to obtain a first spray-dried material; adjust the nitrogen flow of the spray tower to control the spray particle size to be 1~10 μm. Sinter the first spray-dried material obtained after spraying at 500℃~700℃ for 10h to obtain a sintered sample.

[0100] Step 2: Take 9.3 kg of the sintered sample prepared in Step 1 and pour it into a stainless steel reaction kettle, then add 15.21 kg of ultrapure water for dispersion, and after complete dispersion, add 0.51 kg of ferrous sulfate heptahydrate and stir to mix evenly. Slowly add a 42% oxalic acid solution prepared in the reaction kettle, and after 120 min of reaction, the slurry is discharged into a centrifuge for dehydration, washed with 45℃ pure water at high and low speeds for 3 times, and then the centrifuged material is placed in an 80℃ oven for drying for 20h to obtain an anchor shell coated lithium manganese iron phosphate core precursor.

[0101] Step 3: Take the anchor shell coated lithium iron manganese phosphate core precursor, LiH2PO4, sucrose prepared in step 2 respectively, the amount is 8 kg, 0.196 kg, 0.017 kg, add 27 kg pure water, after fully dispersed in the stirring tank, through the two fluid spray control spray particle size in 1~10 μm. The sample after spraying is sintered at 600~800℃ for 10h, and the lithium iron phosphate coated lithium manganese iron phosphate positive electrode material is obtained.

[0102] The content of the positive electrode material lithium iron phosphate layer prepared in this example is 3%, the content of the first carbon coating layer is 1.6%, and the content of the second carbon coating layer is 1.1%.

[0103] Example 6

[0104] In this example, the Mn:Fe molar ratio of the inner layer lithium manganese iron phosphate is controlled to be 7:3, and the rest of the conditions and processes are the same as those of example 5.

[0105] Example 7

[0106] In this example, the Mn:Fe molar ratio of the inner layer lithium manganese iron phosphate is controlled to be 8:2, and the rest of the conditions and processes are the same as those of example 5.

[0107] Example 8

[0108] In this example, the difference from example 2 is that step 2 is replaced, in this example, step 2 is: take 9.66 kg of the sample prepared in step 1, pour into a stainless steel reaction kettle, add 15.8 kg of ultrapure water, disperse, then add 0.51 kg of ferrous sulfate heptahydrate, stir to mix evenly. Slowly add 30% phosphoric acid solution to the reaction kettle, slowly add 30% sodium hydroxide solution, control the pH of the solution in the range of 6~8, after reaction for 120 min, take the upper clear liquid to test Fe 2+ content is less than 5 ppm. After the reaction is completed, the slurry is centrifuged to remove water, washed with 45℃ pure water at high and low speed for 3 times, and then placed in an 80℃ oven for drying for 20h. Thus, in this example, the iron source of the lithium iron phosphate coating shell is ferrous phosphate.

[0109] At the same time, in this example, lithium carbonate is selected as the lithium source in step 3.

[0110] The other reaction conditions and processes are the same as those of example 2.

[0111] Example 9

[0112] In the present embodiment, the difference from the embodiment 2 is that the step 2 is replaced, in the present embodiment, the step 2 is: taking a sample of 9.66 kg prepared in step 1, pouring into a stainless steel reaction kettle, adding 15.8 kg of ultrapure water for dispersion, after complete dispersion, adding 0.52 kg of iron sulfate heptahydrate, stirring to mix uniformly. Slowly add the prepared 30% phosphoric acid solution into the reaction kettle, slowly drop 30% sodium hydroxide solution, control the solution pH in the range of 5~9, after reaction for 120 min, take the upper clear liquid to test Fe 3+ content is less than 5 ppm. After the reaction is completed, the slurry is punched into a centrifuge for dewatering, after high and low speed cross washing for 3 times using 45 ℃ pure water, the centrifuged material is placed in an 80 ℃ oven for drying for 20 h. Thus, in the present embodiment, the iron source for the lithium iron phosphate coating shell layer is iron phosphate.

[0113] Meanwhile, in the present embodiment, the lithium source in step 3 is selected as lithium carbonate.

[0114] Other reaction conditions and processes are the same as those of the embodiment 2.

[0115] In the present embodiment, iron phosphate is selected as the iron source for in-situ generation of lithium iron phosphate, during the generation of lithium iron phosphate, iron is reduced to participate in the process of lithium iron phosphate generation, and the process of carbon coating can be carried out at the same time, thereby not only promoting the uniform and close coating of lithium iron phosphate on the surface of lithium manganese iron phosphate, but also making the carbon coating on the surface of lithium iron phosphate close.

[0116] Embodiment 10

[0117] In the present embodiment, the difference from the embodiment 2 is that the step 2 is replaced, in the present embodiment, the step 2 is: taking a sample of 9.66 kg prepared in step 1, pouring into a stainless steel reaction kettle, adding 15.8 kg of ultrapure water for dispersion, after complete dispersion, adding 0.51 kg of ferrous sulfate heptahydrate, then adding 0.183 g of NaOH and stirring to mix uniformly. Control the reaction temperature to be about 40 ℃, after 20 min of air input of 60 L / min at the bottom of the reaction kettle, change to air input of 10 min L / min, when the pH of the system is constant, stop the oxygen input. Add 30% sodium hydroxide solution to adjust the pH of the product to be between 7~9. Stir for 120 min. Take the upper clear liquid to test Fe 2+ content is less than 5 ppm. After the reaction is completed, the slurry is punched into a centrifuge for dewatering, after high and low speed cross washing for 3 times using 45 ℃ pure water, the centrifuged material is placed in an 80 ℃ oven for drying for 20 h. Thus, in the present embodiment, the iron source for in-situ generation of lithium iron phosphate coating shell layer is iron hydroxide.

[0118] Other reaction conditions and processes are the same as those of the embodiment 2

[0119] Comparative example 1

[0120] In the present comparative example, the difference from Example 2 is that the lithium iron phosphate-coated spherical-like lithium manganese iron phosphate obtained in Example 2 is subjected to a crushing treatment, and the rest of the conditions and processes are the same as in Example 2.

[0121] Comparative Example 2

[0122] In the present comparative example, the difference from Example 2 is that step 2 is removed, and step 3 is changed to:

[0123] Take LiH2PO4, ferrous oxalate, sucrose 0.07 kg, 0.037 kg, 0.006 kg, respectively, add 0.15 kg of pure water, control the sand grinding particle size in the sand mill to 0.5±0.05 μm, and obtain a lithium iron phosphate precursor; in a stirring tank, add 9.22 kg of a sample burned in step 1, add 27 kg of pure water, and after sand grinding is completed, transfer it into the stirring tank, and after being fully dispersed in the stirring tank, control the spray particle size to 1-10 μm through two-fluid spray. Sinter the sample after spraying at 600-800°C for 10 h to obtain a lithium iron phosphate-coated lithium manganese iron phosphate positive electrode material.

[0124] Comparative Example 3

[0125] In the present comparative example, the difference from Example 2 is that the two-fluid spray in step 1 is replaced by centrifugal spray, and the spray particle size is 5-50 μm. The rest of the conditions and processes are the same as in Example 2.

[0126] Preparation Example

[0127] The positive electrode materials of Examples 1-7 and Comparative Examples 1-4 are used to prepare lithium ion batteries, specifically including:

[0128] Preparation of positive electrode sheet

[0129] According to the mass ratio of 8:1:1, the positive electrode materials of Examples 1-7 and Comparative Examples 1-4, conductive carbon black, and binder (PVDF) are weighed, fully ground and mixed, and then collected in a small beaker. An appropriate amount of N-methyl pyrrolidone is added to the beaker to prepare a slurry with appropriate viscosity. After magnetic stirring for more than 10 h, the black slurry is uniformly coated on an aluminum foil using a slurry coating machine. The aluminum foil coated with the slurry is dried in a vacuum drying oven at 120°C for 12 h, then compacted by an electric roller, and then cut into a circular electrode sheet with a diameter of 13 mm using a slicing machine. The electrode sheet is vacuum dried at 120°C for 12 h, and then weighed using a precision balance after the moisture is fully dried. The electrode sheet is numbered for use.

[0130] Assembly of half-cell

[0131] The half-cell uses an organic electrolyte, the lithium metal sheet is sensitive to oxygen and moisture, the assembly process of the half-cell is completed in an argon-filled glove box, the half-cell adopts a 2032 button cell buckle, and the battery shell is cleaned with a cleaning agent, ultrapure water and anhydrous ethanol in sequence before use. A polypropylene film is used as a separator, which is cut into a circular separator with a diameter of 18 mm using a slicing machine before use. A lithium metal sheet (Φ15.6*0.45) is used as the negative electrode of the button cell.

[0132] The assembly process of the battery is as follows: the lithium sheet, the separator, the positive sheet, the gasket, the spring and the positive shell are pressed in sequence at the center of the negative shell, the separator is wetted by dropping an appropriate amount of electrolyte on both sides, the battery is packaged using a button cell packaging machine after assembly, and the corresponding electrochemical performance test is carried out after standing for 12 h.

[0133] Charge-discharge test:

[0134] The discharge specific capacity data of the simulation battery is tested according to the battery charge-discharge test instrument operation procedure under the condition of 23℃±2℃.

[0135] The test results are shown in Table 1 below:

[0136]

[0137] The above table shows the charge-discharge performance of the samples at 0.1C and 1C, respectively, wherein Examples 1-4 have obvious improvement in charge-discharge performance and high-temperature storage performance compared with Comparative Examples 1-3.

[0138] Comparative Example 2 and Comparative Example 1 show that the capacity is close, but the high-temperature storage performance of Comparative Example 1 decreases after air flow crushing, because the outer coating layer of lithium iron phosphate is destroyed after air flow crushing, which makes the high-temperature storage performance decrease obviously.

[0139] Comparative Example 2 and Comparative Example 2 show that the high-temperature storage performance of Comparative Example 2 decreases obviously compared with Example 2, because the unanchored ferrous oxalate cannot be evenly coated on the surface and gap of the lithium manganese iron phosphate core, which makes the coating shell layer not dense enough, resulting in a significant decrease in high-temperature storage performance. Comparative Example 2 and Comparative Example 3 show that the capacity decreases obviously, because the particle size of Comparative Example 3 is larger during the processing of the sheet, and the surface of the sheet after coating presents uneven protrusions, and after rolling, the particles are easily separated from the current collector, causing capacity difference.

[0140] From the comparison of Examples 5-7, it is found that with the increase of Mn content, the average voltage of the material presents a clear increasing trend, but the number of days of high temperature storage presents a clear decreasing trend, because with the increase of Mn content, the electronic conductivity and the conductivity of lithium ion of the material system obviously decrease, leading to the more difficult deintercalation of Li+ in the cycle process, and the high temperature storage performance obviously decreases. Therefore, the optimal Mn:Fe selection is 6:4.

[0141] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can make modifications to the technical solutions recorded in the foregoing embodiments, or make equivalent replacements to part of the technical features, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A lithium iron manganese phosphate cathode material, characterized in that, The lithium manganese iron phosphate core-in-shell structure includes a lithium manganese iron phosphate core and a coating shell layer; the coating shell layer is coated on the surface of the lithium manganese iron phosphate core; the coating shell layer includes lithium iron phosphate; The lithium manganese iron phosphate core is formed by aggregation of lithium manganese iron phosphate primary particles, and the surface of the lithium manganese iron phosphate core is structured with a void structure; the coating shell layer is grown in situ on the surface of the lithium manganese iron phosphate core after an iron source is deposited in situ on the outer surface of the lithium manganese iron phosphate core; Preparation is performed by the following method: An iron source is deposited in situ on the surface of the lithium manganese iron phosphate core as an anchoring shell layer; Lithium iron phosphate is then generated in situ with the anchoring shell layer as an anchor point, coated on the surface of the lithium manganese iron phosphate core as a coating shell layer; The lithium manganese iron phosphate core is prepared by the following method: After ball milling, the lithium manganese iron phosphate raw slurry is subjected to two-fluid spray drying at a solid content of 30% to 40%, and the obtained first spray-dried material is sintered at a temperature of 500 to 700 DEG C to obtain the lithium manganese iron phosphate core; The anchoring shell layer is prepared by the following method: After dispersion, the lithium manganese iron phosphate core is added with a hydrated iron sulfate salt, and a co-precipitation solution is then added dropwise into the core; pH is controlled, and after the reaction is completed, the mixed slurry is dehydrated and dried to obtain an anchoring shell layer coated lithium manganese iron phosphate core precursor.

2. The lithium iron manganese phosphate cathode material according to claim 1, characterized in that The particle size of the lithium manganese iron phosphate core is 1 to 10 microns.

3. The lithium iron manganese phosphate cathode material of claim 1, wherein, The particle size of the lithium manganese iron phosphate primary particles is 50 to 500 nm.

4. The lithium iron manganese phosphate cathode material of claim 1, wherein, The hydrated iron sulfate salt includes one of ferrous sulfate heptahydrate and iron sulfate nonahydrate, and the co-precipitation solution includes one of phosphoric acid, sodium hydroxide, and oxalic acid.

5. The lithium iron manganese phosphate cathode material according to any one of claims 1 to 4, characterized in that The coating shell layer is prepared by the following method: The lithium manganese iron phosphate core coated with the anchoring shell layer is mixed with a soluble lithium source, a soluble phosphorus source, and a soluble carbon source in a dispersion phase, and then the slurry is subjected to spray drying; the obtained second spray-dried material is sintered at a temperature of 600 to 800 DEG C to obtain a lithium iron phosphate coated lithium manganese iron phosphate positive electrode material.

6. A positive electrode sheet characterized by comprising: The lithium manganese iron phosphate positive electrode material includes any one of claims 1 to 5.

7. A lithium-ion battery, characterized by The positive electrode sheet includes claim 6.

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