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

Abstract

The invention discloses a lithium manganese iron phosphate positive electrode material and a preparation method thereof, a positive electrode plate and a lithium ion battery, and relates to the technical field of lithium ion batteries. The lithium manganese iron phosphate positive electrode material comprises a lithium manganese iron phosphate inner core and a coating shell layer, the surface of the lithium manganese iron phosphate inner core is coated with the coating shell layer; 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 manganese iron phosphate material, so that the lithium manganese iron phosphate material has high capacity and high-temperature storage stability.

Description

Technical Field

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

[0002] Lithium manganese iron phosphate combines the advantages of the relatively high voltage of LiMnPO4 and the structural stability of LiFePO4. However, LiFe... x Mn 1-x In lithium manganese iron phosphate (LFP) octahedra, FeO6 and MnO6 are connected by PO4 tetrahedra, preventing the formation of a continuous conductive network and resulting in poor conductivity. Furthermore, the structural differences between MnPO4 and FePO4 also lead to poor thermal stability. Ensuring that LFP materials achieve both high capacity and effective high-temperature storage performance is one of the key technical challenges hindering their application as energy storage materials.

[0003] To ensure that lithium manganese iron phosphate materials possess both high capacity and high-temperature storage performance, the lithium manganese iron phosphate particles are typically nano-sized to improve the Li- content of the material. + Diffusion rate. Further carbon coating is used to improve the electronic conductivity of lithium manganese iron phosphate (MnFeP) materials. However, the small particle size after nano-sizing increases the surface area in contact with the electrolyte when fabricated into a secondary battery, leading to increased electrolyte erosion of the MnFeP material and accelerating the diffusion rate of MnFeP. 3+ Disproportionation reaction promotes Mn 2+ and Mn 4+ Dissolved in the electrolyte, it migrates to the negative electrode through the membrane, where it undergoes a reduction reaction and precipitates, consuming active lithium ions and causing a decrease in high-temperature storage performance. Summary of the Invention

[0004] The purpose of this invention is to provide a lithium manganese iron phosphate cathode material and its preparation method, which can solve the problem of poor high-temperature storage performance of lithium manganese iron phosphate material, and enable lithium manganese iron phosphate material to have both high capacity and high-temperature storage stability.

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

[0006] Firstly, the lithium manganese iron phosphate cathode material provided in this application includes a lithium manganese iron phosphate core and a coating layer; the coating layer covers the surface of the lithium manganese iron phosphate core; the coating layer includes lithium iron phosphate.

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

[0008] On the other hand, lithium iron phosphate material, acting as a coating layer, can uniformly coat the surface and gaps of lithium manganese iron phosphate core. This coating layer significantly reduces direct contact between the electrolyte and the core lithium manganese iron phosphate, minimizing erosion of the core and reducing Mn content in high-temperature environments. 3+ The disproportionation reaction occurs, thereby improving the high-temperature cycling performance and high-temperature storage performance of lithium manganese iron phosphate cathode materials.

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

[0010] In this way, the aforementioned lithium manganese iron phosphate core is formed by the aggregation of a certain number of primary lithium manganese iron phosphate particles into spherical particles. The size of the spherical particles formed by the lithium manganese iron phosphate core is much larger than the size of the generated lithium iron phosphate particles, which allows the lithium iron phosphate coating layer to be more uniformly coated on the surface of the lithium manganese iron phosphate core.

[0011] Furthermore, for the spherical lithium manganese iron phosphate core, if the particle size is too small, it will reduce the size difference between the spherical particles and the lithium iron phosphate particles, resulting in a loose coating of the lithium iron phosphate shell; while if the particle size is too large, it will cause the spherical particles to break during subsequent electrode rolling, resulting in processing problems such as detachment from the current collector.

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

[0013] In this way, there are gaps between the spherical lithium manganese iron phosphate core particles formed by the aggregation of 50~500nm particles. When the coating shell grows, it preferentially grows in the gap structure of the spherical lithium manganese iron phosphate core, making the lithium iron phosphate coating shell more densely covering the surface of the lithium manganese iron phosphate core.

[0014] In some embodiments, the aforementioned coating layer preferably uses ferrous oxalate as the iron source. This allows the ferrous oxalate to precipitate in situ from the solution onto the surface of the spherical lithium manganese iron phosphate core and within the aforementioned void structure, thereby forming iron-site anchors for lithium iron phosphate growth. This ensures that the lithium iron phosphate stably and uniformly coats the spherical lithium manganese iron phosphate core during in-situ growth. Alternatively, the iron source for the coating layer can also be selected from ferrous phosphate, iron phosphate, and iron hydroxyl oxide.

[0015] Secondly, this application also provides a method for preparing lithium manganese iron phosphate cathode material, in which an iron source is deposited in situ on the surface of the lithium manganese iron phosphate core as an anchoring shell layer.

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

[0017] In this way, the iron source is deposited in situ on the surface and pores of the lithium manganese iron phosphate core, allowing the generated lithium iron phosphate to uniformly and densely coat the surface and pores of the lithium manganese iron phosphate. This coating layer effectively prevents the electrolyte from eroding the lithium manganese iron phosphate core and reduces Mn content. 3+ The disproportionation reaction improves the high-temperature storage performance of lithium manganese iron phosphate materials.

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

[0019] The milled lithium manganese iron phosphate slurry was subjected to two-fluid spray drying at a solid content of 30%~40%. The first spray-dried material was then sintered at a temperature of 500~700℃ to obtain the lithium manganese iron phosphate core.

[0020] In this step, during two-fluid spray drying, if the spray solids content is too low, the resulting first spray-dried material will be loose internally, reducing its sphericity and hindering the formation of a spherical lithium manganese iron phosphate core. Conversely, if the spray solids content is too high, the large difference in moisture content between the inner and outer layers of the spray sphere will result in the formation of annular spray spheres, affecting the spherical formation of the lithium manganese iron phosphate core. Furthermore, the particle size range of the first spray-dried material must be controlled within 1~10μm by adjusting the gas flow rate during spraying.

[0021] Furthermore, during the sintering of the first spray-dried material, lithium manganese iron phosphate (LMP) nuclei begin to form and grow at 400°C. At this time, the carbon source in the LMP slurry melts and fractures, including on the surface and in the gaps of the LMP spray spheres. As the temperature rises, the carbon layer carbonizes at high temperatures and adheres to the surface of the LMP particles, thus limiting the growth of the primary LMP particles. Subsequently, during the continuous sintering process, the primary LMP particles stably aggregate, forming a spherical LMP core composed of primary LMP particles with a size of 50-500 nm.

[0022] In addition, during the sintering process, as gaseous substances escape, the components begin to diffuse and react, generating lithium manganese iron phosphate grains. As crystal nuclei are continuously generated, the grain spacing decreases, which significantly enhances the spherical lithium manganese iron phosphate core structure. This prevents the spherical structure from cracking during subsequent iron source precipitation.

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

[0024] After dispersing the lithium manganese iron phosphate core, hydrated ferric sulfate is added, and then a co-precipitation solution is added dropwise. The pH is controlled. After the reaction is completed, the mixed slurry is dehydrated and dried to obtain the precursor of the lithium manganese iron phosphate core with anchored shell coating.

[0025] The process of adding the coprecipitation solution involves slow, dropwise addition while stirring; and simultaneously, sodium hydroxide solution is used to adjust the pH of the environment.

[0026] In this way, after pH adjustment, the hydrated ferric sulfate, completely soluble in water, allows most of the iron source formed in situ to precipitate on the surface of the lithium manganese iron phosphate (LFP) core during the slow addition of the co-precipitation solution. A small amount precipitates in the gaps within the LFP core, thus forming an anchoring shell to coat the LFP core precursor. Because of this in-situ precipitation, the agglomeration of the iron source added to the system for coating is effectively avoided, preventing the formation of large particles that directly precipitate. After in-situ precipitation, the iron source precipitated on the surface and in the gaps of the LFP acts as an anchor point for LFP formation, resulting in a uniform and stable coating of LFP on the LFP surface.

[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 ferrous sulfate heptahydrate is co-precipitated with oxalic acid, the iron source of the aforementioned coating layer is ferrous oxalate; when ferrous sulfate heptahydrate is co-precipitated with phosphoric acid, the iron source of the aforementioned coating layer is ferrous phosphate; when ferrous sulfate nonahydrate is co-precipitated with phosphoric acid, the iron source of the aforementioned coating layer is ferric phosphate; and when ferrous sulfate heptahydrate is co-precipitated with sodium hydroxide, the iron source of the aforementioned coating layer is ferric hydroxide.

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

[0030] Because the size of the lithium iron phosphate coating 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 1~10μm. This is mainly to ensure the maintenance of the particle size of the aforementioned first spray-dried material.

[0031] The lithium manganese iron phosphate core covered with the anchoring shell is mixed with a soluble lithium source, a soluble phosphorus source and a soluble carbon source in a dispersed phase. The slurry is then subjected to two-fluid spray drying. The resulting second spray-dried material is sintered at a temperature of 600~800℃ to obtain lithium manganese iron phosphate cathode material coated with lithium iron phosphate.

[0032] During the second spray drying process, the soluble lithium source, soluble phosphorus source, and soluble carbon source are sprayed in a two-fluid environment. As the hot nitrogen at the bottom comes into contact with the spray slurry, the spray slurry is rapidly vaporized and carried away by the nitrogen. The lithium source, phosphorus source, and carbon source will adhere to the surface of the lithium manganese iron phosphate core precursor due to evaporation and crystallization. After subsequent sintering, a high-capacity lithium manganese iron phosphate material with high-temperature storage performance is obtained.

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

[0034] In particular, when lithium iron phosphate coating is too light, its protective effect on the lithium manganese iron phosphate core will be reduced. Conversely, when lithium iron phosphate coating is too heavy, the Mn content in the lithium manganese iron phosphate core will decrease, resulting in a drop in average voltage and thus a decrease in the material's energy density.

[0035] Therefore, the lithium manganese iron phosphate cathode material provided in this application, when applied in batteries, can not only isolate the electrolyte from Mn but also... 3+ This process prevents corrosion and disproportionation reactions, thereby improving the high-temperature storage performance of the lithium manganese iron phosphate core. Furthermore, the lithium iron phosphate material used as the coating layer can also participate in the electrochemical reactions of lithium manganese iron phosphate, ensuring that the capacity performance of the lithium iron phosphate material coating the lithium manganese iron phosphate material remains unaffected. Consequently, batteries using the lithium manganese iron phosphate material described in this application possess both high capacity and excellent high-temperature storage performance.

[0036] Thirdly, this application also provides a positive electrode sheet, including the aforementioned lithium manganese iron phosphate positive electrode material, or the lithium manganese iron phosphate positive electrode material prepared by the aforementioned method for preparing lithium manganese iron phosphate positive electrode material.

[0037] Fourth, this application also provides a lithium-ion battery, including the aforementioned positive electrode. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of the lithium manganese iron phosphate cathode material prepared in this invention.

[0039] Among them, 1-lithium manganese iron phosphate, 2-primary carbon coating, 3-lithium iron phosphate, and 4-secondary carbon coating. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0041] Therefore, the following detailed description of embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0042] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0043] Please combine Figure 1 As shown, in some embodiments, this application provides a lithium manganese iron phosphate cathode material, including a lithium manganese iron phosphate core and a coating layer; the coating layer covers the surface of the lithium manganese iron phosphate core; the coating layer includes lithium iron phosphate.

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

[0045] On the other hand, lithium iron phosphate material, as a coating layer, can uniformly coat the surface and gaps of lithium manganese iron phosphate core. Utilizing lithium iron phosphate significantly reduces the direct contact between the electrolyte and the lithium manganese iron phosphate core, thereby reducing the erosion of the lithium manganese iron phosphate core and minimizing the impact of Mn on high-temperature environments. 3+ The disproportionation reaction occurs, thereby improving the high-temperature cycling performance and high-temperature storage performance of lithium manganese iron phosphate cathode materials.

[0046] In some embodiments, the lithium manganese iron phosphate core is formed by the aggregation of primary lithium manganese iron phosphate particles, wherein the particle size of the lithium manganese iron phosphate core is 1~10μm.

[0047] Thus, the aforementioned lithium manganese iron phosphate core is formed by the aggregation of a certain number of primary lithium manganese iron phosphate particles into spherical particles. The size of the spherical particles formed by the lithium manganese iron phosphate core is much larger than the size of the generated lithium iron phosphate particles, allowing the lithium iron phosphate coating layer to be more uniformly coated on the surface of the lithium manganese iron phosphate core.

[0048] Furthermore, for the spherical lithium manganese iron phosphate core, if the particle size is too small, it will reduce the size difference between the spherical particles and the lithium iron phosphate particles, resulting in a loose coating of the lithium iron phosphate shell; while if the particle size is too large, it will cause the spherical particles to break during subsequent electrode rolling, resulting in processing problems such as detachment from the current collector.

[0049] In some embodiments, the lithium manganese iron phosphate is formed by the aggregation of primary lithium manganese iron phosphate particles coated with a first carbon layer, wherein the particle size of the primary lithium manganese iron phosphate particles is 50~500nm and the thickness of the first carbon layer is 1~20nm.

[0050] In this way, there are gaps between the spherical lithium manganese iron phosphate core particles formed by the aggregation of 50~500nm particles. When the coating shell grows, it preferentially grows in the gap structure of the spherical lithium manganese iron phosphate core, making the lithium iron phosphate coating shell more densely covering the surface of the lithium manganese iron phosphate core.

[0051] In some embodiments, the aforementioned coating layer preferably uses ferrous oxalate as the iron source. This allows the ferrous oxalate to precipitate in situ onto the surface of the spherical lithium manganese iron phosphate core and within the aforementioned porous structure, thereby forming iron-site anchors for lithium iron phosphate growth. This ensures that lithium iron phosphate stably and uniformly coats the spherical lithium manganese iron phosphate core during in-situ growth. The in-situ precipitation method also avoids the excessive agglomeration and precipitation of ferrous oxalate in the dispersion system, which would affect its coating of the spherical lithium manganese iron phosphate core. Furthermore, the iron source for the coating layer can also be selected from ferrous phosphate, iron phosphate, and iron hydroxide.

[0052] Secondly, this application also provides a method for preparing lithium manganese iron phosphate cathode material.

[0053] Iron source is deposited in situ onto the surface of lithium manganese iron phosphate core as an anchoring shell layer;

[0054] Then, lithium iron phosphate is generated in situ using the anchoring shell as the anchor point, and coated on the surface of the lithium manganese iron phosphate core as the coating shell.

[0055] In this way, the iron source of the coating layer is deposited in situ on the surface and pores of the lithium manganese iron phosphate core, resulting in a uniform and dense coating of the lithium manganese iron phosphate surface and pores. This coating layer effectively prevents the electrolyte from eroding the lithium manganese iron phosphate core and reduces Mn content. 3+ The disproportionation reaction reduces the consumption of active lithium ions, thereby improving the high-temperature storage performance of lithium manganese iron phosphate materials.

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

[0057] The milled lithium manganese iron phosphate slurry was subjected to two-fluid spray drying at a solid content of 30%~40%. The first spray-dried material was then sintered at a temperature of 500~700℃ to obtain the lithium manganese iron phosphate core.

[0058] Furthermore, the manganese source of the aforementioned raw slurry can be manganese tetroxide, the iron source is preferably ferrous oxalate, the lithium and phosphorus sources can be lithium dihydrogen phosphate, the carbon source can be sucrose, and titanium dioxide can be used as a dopant, with methanol as the dispersed phase.

[0059] After the raw slurry is ground in a sand mill, it is subjected to two-fluid spray drying to prepare the first spray-dried material.

[0060] In this step, during two-fluid spray drying, if the spray solids content is too low, the resulting first spray-dried material will be loose internally, reducing its sphericity and hindering the formation of a spherical lithium manganese iron phosphate core. Conversely, if the spray solids content is too high, the large difference in moisture content between the inner and outer layers of the spray sphere will result in the formation of annular spray spheres, affecting the spherical formation of the lithium manganese iron phosphate core. Furthermore, the particle size range of the first spray-dried material must be controlled within 1~10μm by adjusting the gas flow rate during spraying.

[0061] Furthermore, during the sintering of the first spray-dried material, lithium manganese iron phosphate (LMP) nuclei begin to form and grow at 400°C. At this time, the carbon source in the LMP slurry melts and fractures, including on the surface and in the gaps of the LMP spray spheres. As the temperature rises, the carbon layer carbonizes at high temperatures and adheres to the surface of the LMP particles, thus limiting the growth of the primary LMP particles. Subsequently, during the continuous sintering process, the primary LMP particles stably aggregate, forming a spherical LMP core composed of primary LMP particles with a size of 50-500 nm.

[0062] In addition, during the sintering process, as gaseous substances escape, the components begin to diffuse and react, generating lithium manganese iron phosphate grains. As crystal nuclei are continuously generated, the grain spacing decreases, which significantly enhances the spherical lithium manganese iron phosphate core structure. This prevents the spherical structure from cracking during subsequent iron source precipitation.

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

[0064] After dispersing the lithium manganese iron phosphate core, hydrated ferric sulfate is added, and then a co-precipitation solution is added dropwise. The pH is controlled. After the reaction is completed, the mixed slurry is dehydrated and dried to obtain the precursor of the lithium manganese iron phosphate core with anchored shell coating.

[0065] Furthermore, after adding the co-precipitation solution dropwise, the reaction was allowed to continue until the supernatant was collected for Fe testing. 2+ Once the concentration drops below 5 ppm, the reaction is complete, and the mixed slurry can be dehydrated and dried. The precipitation reaction temperature is controlled between 20℃ and 60℃, and the reaction system is deionized water.

[0066] The process of adding the coprecipitation solution involves slow, dropwise addition while stirring; and simultaneously, sodium hydroxide solution is used to adjust the pH of the environment.

[0067] In this way, after pH adjustment, the hydrated ferric sulfate, completely soluble in water, allows most of the iron source formed in situ to precipitate on the surface of the lithium manganese iron phosphate (LFP) core during the slow addition of the co-precipitation solution. A small amount precipitates in the gaps within the LFP core, thus forming an anchoring shell that coats the LFP core precursor. This in-situ precipitation effectively avoids the large-particle aggregation and direct precipitation that can occur when iron sources are directly added to the solution. The iron source precipitated on and within the LFP acts as an anchor point for LFP formation, resulting in a uniform and stable coating of LFP on the LFP surface.

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

[0069] When ferrous sulfate heptahydrate is co-precipitated with oxalic acid, the iron source of the aforementioned coating layer is ferrous oxalate; when ferrous sulfate heptahydrate is co-precipitated with phosphoric acid, the iron source of the aforementioned coating layer is ferrous phosphate; when ferrous sulfate nonahydrate is co-precipitated with phosphoric acid, the iron source of the aforementioned coating layer is ferric phosphate; and when ferrous sulfate heptahydrate is co-precipitated with sodium hydroxide, the iron source of the aforementioned coating layer is ferric hydroxide.

[0070] Moreover, regarding the selection of reaction conditions, when the iron source for the coating layer is ferrous oxalate, the pH range during the co-precipitation process is controlled between 2.5 and 3.

[0071] When the iron source for the coating layer is ferrous phosphate, the pH range is controlled between 6 and 8 during the co-precipitation process.

[0072] When the iron source for the coating layer is iron phosphate, the pH range is controlled between 5 and 9 during the co-precipitation process.

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

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

[0075] The lithium manganese iron phosphate core covered with the anchoring shell is mixed with a soluble lithium source, a soluble phosphorus source and a soluble carbon source in a dispersed phase. The slurry is then subjected to two-fluid spray drying. The resulting second spray-dried material is sintered at a temperature of 600~800℃ to obtain lithium manganese iron phosphate cathode material coated with lithium iron phosphate.

[0076] Furthermore, the aforementioned soluble carbon source can be a water-soluble organic carbon source, such as glucose or sucrose. The aforementioned soluble phosphoric acid and soluble lithium source can both be lithium dihydrogen phosphate.

[0077] Because the size of the lithium iron phosphate coating 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 1~10μm. This is mainly to ensure the maintenance of the particle size of the aforementioned first spray-dried material.

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

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

[0080] In particular, when lithium iron phosphate coating is too light, its protective effect on the lithium manganese iron phosphate core will be reduced. Conversely, when lithium iron phosphate coating the lithium manganese iron phosphate core is too heavy, the Mn content in the core lithium manganese iron phosphate will decrease, resulting in a drop in average voltage and thus a decrease in the material's energy density.

[0081] Therefore, the lithium manganese iron phosphate cathode material provided in this application, when applied in batteries, can not only isolate the electrolyte from Mn but also... 3+ This process prevents corrosion and disproportionation reactions, thereby improving the high-temperature storage performance of the lithium manganese iron phosphate core. Furthermore, the lithium iron phosphate material used as the coating layer can also participate in the electrochemical reactions of lithium manganese iron phosphate, ensuring that the capacity performance of the lithium iron phosphate material coating the lithium manganese iron phosphate material remains unaffected. Consequently, batteries using the lithium manganese iron phosphate material described in this application possess both high capacity and excellent high-temperature storage performance.

[0082] The technical solution of this application will be described in detail below using different specific embodiments.

[0083] Example 1

[0084] This embodiment provides a lithium manganese iron phosphate cathode material, comprising a spherical core formed by the primary aggregation of lithium manganese iron phosphate particles, with lithium iron phosphate as the coating layer, and ferrous oxalate as the iron source for the coating layer. This allows the prepared lithium manganese iron phosphate cathode material to possess both high capacity and good high-temperature storage characteristics.

[0085] Specifically, in this embodiment, the amount of lithium iron phosphate coating is 1% of that of lithium manganese iron phosphate, and the overall Mn:Fe molar ratio of the cathode material is controlled to be 6:4; the specific steps are as follows:

[0086] Step 1: Take manganese tetroxide, ferrous oxalate, LiH2PO4, titanium dioxide, and sucrose into a ball mill jar, with amounts of 2.61 kg, 4 kg, 6.08 kg, 0.08 kg, and 0.5 kg respectively. Add anhydrous methanol to form a slurry. Pour the slurry into a ball mill and mix thoroughly. Then transfer it to a sand mill at 800 rpm. When the sand milled particle size reaches 0.5 ± 0.05 μm, transfer it to a two-fluid spray tower for spraying to obtain the first spray-dried material. Adjust the nitrogen flow rate of the spray tower to control the spray ball particle size between 1 and 10 μm. Sinter the first spray-dried material at 500℃ to 700℃ for 10 hours to obtain a first-burnt sample.

[0087] Step 2: Take 9.22 kg of the calcined sample obtained in Step 1, pour it into a stainless steel reactor, add 15 kg of ultrapure water to disperse it, and after complete dispersion, add 0.17 kg of ferrous sulfate heptahydrate and stir to mix evenly. Slowly add the prepared 42% oxalic acid solution into the reactor, and control the pH of the solution within the range of 2.5-3 by slowly adding 8% sodium hydroxide solution. After reacting for 120 min, take the supernatant to test for Fe. 2+ The content is less than 5 ppm. After the reaction, the slurry is centrifuged and dehydrated in a centrifuge. It is then washed three times with pure water at 45℃ at high and low speeds. The centrifuged material is then placed in an 80℃ oven and dried for 20 hours to obtain the anchored shell-coated lithium manganese iron phosphate core precursor.

[0088] Step 3: Take 8 kg of the anchored shell-coated lithium manganese iron phosphate core precursor, 0.07 kg of LiH2PO4, and 0.006 kg of sucrose prepared in Step 2, add 27 kg of pure water, and disperse thoroughly in a stirring tank. Then, control the spray ball particle size to 1-10 μm using a two-fluid sprayer. Sinter the sprayed sample at 600-800℃ for 10 h to obtain lithium iron phosphate-coated lithium manganese iron phosphate cathode material.

[0089] The Mn:Fe molar ratio of the cathode material prepared in this embodiment is 6:4; the cathode material, based on a mass of 100%, contains 1% lithium iron phosphate layer, 1.6% primary carbon coating layer, and 1.1% secondary carbon coating layer.

[0090] Example 2

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

[0092] Example 3

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

[0094] Example 4

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

[0096] Example 5

[0097] This embodiment provides a lithium manganese iron phosphate cathode material, which has a multilayer structure consisting of lithium iron phosphate as the coating layer, a spherical core formed by the primary aggregation of lithium manganese iron phosphate particles, and an iron source with ferrous oxalate as the coating layer. This allows the prepared lithium manganese iron phosphate cathode material to possess both high capacity and good high-temperature storage characteristics.

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

[0099] Step 1: Take manganese tetroxide, ferrous oxalate, LiH2PO4, titanium dioxide, and sucrose into a ball mill jar, with amounts of 2.54 kg, 4 kg, 6.00 kg, 0.080 kg, and 0.50 kg respectively. Add anhydrous methanol to form a slurry. Pour the slurry into a ball mill and mix thoroughly. Then transfer it to a sand mill at 800 rpm. When the sand milled particle size reaches 0.5 ± 0.05 μm, transfer it to a two-fluid spray tower for spraying to obtain the first spray-dried material. Adjust the nitrogen flow rate of the spray tower to control the spray ball particle size between 1 and 10 μm. Sinter the first spray-dried material at 500℃ to 700℃ for 10 hours to obtain a first-burnt sample.

[0100] Step 2: Take 9.3 kg of the sample obtained in Step 1, pour it into a stainless steel reactor, 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 the prepared 42% oxalic acid solution into the reactor, react for 120 min, and then centrifuge the slurry after the reaction to remove water. Wash the slurry three times with 45℃ pure water at high and low speeds, and then dry the centrifuged material in an 80℃ oven for 20 h to obtain the anchored shell-coated lithium manganese iron phosphate core precursor.

[0101] Step 3: Take 8 kg of the anchored shell-coated lithium manganese iron phosphate core precursor, 0.196 kg of LiH2PO4, and 0.017 kg of sucrose prepared in Step 2, add 27 kg of pure water, and disperse thoroughly in a stirring tank. Then, control the spray ball particle size to 1~10 μm using a two-fluid sprayer. Sinter the sprayed sample at 600~800℃ for 10 h to obtain lithium iron phosphate-coated lithium manganese iron phosphate cathode material.

[0102] In this embodiment, the positive electrode material has a lithium iron phosphate layer content of 3%, a primary carbon coating layer content of 1.6%, and a secondary carbon coating layer content of 1.1%.

[0103] Example 6

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

[0105] Example 7

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

[0107] Example 8

[0108] In this embodiment, the difference from Embodiment 2 is that step 2 is replaced. In this embodiment, step 2 is as follows: Take 9.66 kg of the calcined sample obtained in step 1, pour it into a stainless steel reactor, add 15.8 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 the prepared 30% phosphoric acid solution into the reactor, and slowly add 30% sodium hydroxide solution dropwise, controlling the pH of the solution within the range of 6-8. After reacting for 120 min, take the supernatant to test Fe. 2+ The content is below 5 ppm. After the reaction, the slurry is centrifuged and dehydrated, then washed three times with 45°C pure water at high and low speeds. The centrifuged material is then dried in an 80°C oven for 20 hours. Therefore, in this embodiment, the iron source for the lithium iron phosphate coating is ferrous phosphate.

[0109] Meanwhile, in this embodiment, lithium carbonate is selected as the lithium source in step 3.

[0110] Other reaction conditions and procedures are the same as in Example 2.

[0111] Example 9

[0112] In this embodiment, the difference from Embodiment 2 is that step 2 is replaced. In this embodiment, step 2 is as follows: Take 9.66 kg of the calcined sample obtained in step 1, pour it into a stainless steel reactor, add 15.8 kg of ultrapure water for dispersion, and after complete dispersion, add 0.52 kg of ferric sulfate nonahydrate and stir to mix evenly. Slowly add the prepared 30% phosphoric acid solution to the reactor, and slowly add 30% sodium hydroxide solution dropwise, controlling the pH of the solution within the range of 5-9. After reacting for 120 min, take the supernatant to test Fe. 3+ The content is below 5 ppm. After the reaction, the slurry is centrifuged and dehydrated, then washed three times with 45°C pure water at high and low speeds. Finally, the centrifuged material is dried in an 80°C oven for 20 hours. Therefore, in this embodiment, the iron source for the lithium iron phosphate coating is iron phosphate.

[0113] Meanwhile, in this embodiment, the lithium source in step 3 is lithium carbonate.

[0114] Other reaction conditions and procedures are the same as in Example 2.

[0115] In this embodiment, iron phosphate is selected as the iron source for in-situ generation of lithium iron phosphate. During the generation of lithium iron phosphate, the process of iron reduction and participation in the generation of lithium iron phosphate can be carried out simultaneously with the carbon coating process. This not only promotes the uniform and tight coating of lithium iron phosphate on the surface of lithium manganese iron phosphate, but also makes the carbon coating on the surface of lithium iron phosphate tighter.

[0116] Example 10

[0117] In this embodiment, the difference from Embodiment 2 is that step 2 is replaced. In this embodiment, step 2 is as follows: Take 9.66 kg of the calcined sample obtained in step 1, pour it into a stainless steel reactor, add 15.8 kg of ultrapure water for dispersion, and after complete dispersion, add 0.51 kg of ferrous sulfate heptahydrate, then add 0.183 g of NaOH and stir until evenly mixed. Control the reaction temperature at approximately 40°C, and introduce air at 60 L / min to the bottom of the reactor for 20 min, then switch to introducing air at 10 L / min. When the pH of the system remains constant, stop introducing oxygen. Then add 30% sodium hydroxide solution to adjust the pH of the product to between 7 and 9. Stir and age for 120 min. Take the supernatant to test for Fe. 2+ The content is below 5 ppm. After the reaction, the slurry is centrifuged and dehydrated, then washed three times with pure water at 45°C at alternating high and low speeds. Finally, the centrifuged material is dried in an oven at 80°C for 20 hours. Therefore, in this embodiment, the iron source for the in-situ generation of the lithium iron phosphate coating is iron hydroxyl oxide.

[0118] Other reaction conditions and procedures are the same as in Example 2.

[0119] Comparative Example 1

[0120] In this comparative example, the difference from Example 2 is that the spherical lithium manganese iron phosphate coated with lithium iron phosphate obtained in Example 2 is crushed, while the other conditions and processes are the same as in Example 2.

[0121] Comparative Example 2

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

[0123] LiH₂PO₄, ferrous oxalate, and sucrose were weighed at 0.07 kg, 0.037 kg, and 0.006 kg respectively, and 0.15 kg of pure water were added. The mixture was milled in a sand mill with the particle size controlled at 0.5 ± 0.05 μm to obtain a lithium iron phosphate precursor. In a stirred tank, 9.22 kg of the sample obtained in step 1 was added, along with 27 kg of pure water. After milling, the mixture was transferred to the stirred tank and thoroughly dispersed. Then, a two-fluid spray was applied, controlling the spray ball particle size to be 1–10 μm. The sprayed sample was sintered at 600–800 °C for 10 h to obtain a lithium iron phosphate-coated lithium manganese iron phosphate cathode material.

[0124] Comparative Example 3

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

[0126] Preparation Example

[0127] Lithium-ion batteries were prepared using the cathode materials of Examples 1 to 7 and Comparative Examples 1 to 4, specifically including:

[0128] Positive electrode preparation

[0129] The positive electrode material, conductive carbon black, and binder (PVDF) of Examples 1-7 and Comparative Examples 1-4 were weighed out according to a mass ratio of 8:1:1, thoroughly ground and mixed, and collected in a small beaker. An appropriate amount of N-methylpyrrolidone was added to the beaker to prepare a slurry of suitable viscosity. After magnetic stirring for more than 10 hours, the black slurry was evenly coated onto aluminum foil using a slurry coating machine. The aluminum foil coated with slurry was placed in a vacuum drying oven at 120°C and dried for 12 hours. After removal, it was compacted by an electric roller press and then cut into circular electrode sheets with a diameter of 13 mm using a slicing machine. The electrode sheets were vacuum dried at 120°C for 12 hours. After the moisture was fully dried, the mass of the electrode sheets was weighed using a precision balance and numbered for later use.

[0130] Assembly of half-cell

[0131] The half-cell uses an organic electrolyte. The lithium metal sheet is sensitive to oxygen and moisture. Assembly of the half-cell is completed in an argon-filled glove box. The half-cell uses 2032 button cell contacts. Before use, the battery casing is cleaned sequentially with detergent, ultrapure water, and anhydrous ethanol. A polypropylene membrane is used as the separator, and before use, it is cut into 18mm diameter circular separators using a slicer. A lithium metal sheet (Φ15.6*0.45) is used as the negative electrode of the button cell.

[0132] The battery assembly process is as follows: The lithium sheet, separator, positive electrode sheet, gasket, spring sheet and positive electrode shell are pressed flat in sequence at the center of the negative electrode shell. An appropriate amount of electrolyte is dripped onto both sides of the separator to wet it. After assembly, the battery is packaged using a button battery packaging machine. After standing for 12 hours, the corresponding electrochemical performance test is carried out.

[0133] Charge and discharge test:

[0134] The discharge specific capacity data of the simulated battery was tested under the condition of 23℃±2℃ according to the operating procedure of the battery charge and discharge tester.

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

[0136]

[0137] The table above shows the charge-discharge performance of the samples under 0.1C and 1C conditions, respectively. Among them, Examples 1-4 show significant improvements in charge-discharge performance and high-temperature storage performance compared to Comparative Examples 1-3.

[0138] Comparing Example 2 with Comparative Example 1, it was found that their capacity performance was similar. However, after air jet milling, the high-temperature storage performance of Comparative Example 1 decreased. This is because after air jet milling, the outer lithium iron phosphate coating layer was destroyed, which significantly reduced its high-temperature storage performance.

[0139] Comparing Example 2 with Comparative Example 2, it was found that the high-temperature storage performance of Comparative Example 2 was significantly lower than that of Example 2. This is because the unanchored ferrous oxalate could not be uniformly coated on the surface and in the gaps of the lithium manganese iron phosphate core, resulting in an insufficiently dense coating layer and a significant decrease in high-temperature storage performance. Comparing Example 2 with Comparative Example 3, it was found that the capacity was significantly reduced. This is because during the electrode processing, the particle size of Comparative Example 3 was larger, resulting in uneven protrusions on the electrode surface after coating. After rolling, the particles broke and easily detached from the current collector, causing a capacity difference.

[0140] A comparison of Examples 5-7 reveals that while the average voltage of the material increases significantly with increasing Mn content, the number of days of high-temperature storage decreases significantly. This is because the electronic conductivity and lithium-ion conductivity of the material system decrease significantly with increasing Mn content, making Li+ insertion / extraction more difficult during cycling and resulting in a significant decline in high-temperature storage performance. Therefore, the optimal Mn:Fe ratio is 6:4.

[0141] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lithium manganese iron phosphate cathode material, characterized in that, It includes a lithium manganese iron phosphate core and a coating shell; the coating shell covers the surface of the lithium manganese iron phosphate core; the coating shell includes lithium iron phosphate.

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

3. The lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, The lithium manganese iron phosphate core is formed by the aggregation of primary lithium manganese iron phosphate particles, and the particle size of the primary lithium manganese iron phosphate particles is 50~500nm.

4. A method for preparing lithium manganese iron phosphate cathode material, characterized in that, Iron source is deposited in situ onto the surface of lithium manganese iron phosphate core as an anchoring shell layer; Then, lithium iron phosphate is generated in situ using the anchoring shell as the anchor point, and coated on the surface of the lithium manganese iron phosphate core as the coating shell.

5. The method for preparing lithium manganese iron phosphate cathode material according to claim 4, characterized in that, The lithium manganese iron phosphate core is prepared by the following method: The ball-milled lithium manganese iron phosphate slurry was subjected to two-fluid spray drying at a solid content of 30%~40%. The resulting first spray-dried material was sintered at a temperature of 500~700℃ to obtain the lithium manganese iron phosphate core.

6. The method for preparing lithium manganese iron phosphate cathode material according to claim 4, characterized in that, The anchoring shell is prepared in the following manner: After dispersing the lithium manganese iron phosphate core, hydrated ferric sulfate is added, and then a co-precipitation solution is added dropwise. The pH is controlled. After the reaction is completed, the mixed slurry is dehydrated and dried to obtain the precursor of the lithium manganese iron phosphate core with anchored shell coating.

7. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, 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.

8. The method for preparing lithium manganese iron phosphate cathode material according to any one of claims 5 to 7, characterized in that, The coating shell is prepared in the following manner: The lithium manganese iron phosphate core covered with the anchoring shell is mixed with a soluble lithium source, a soluble phosphorus source and a soluble carbon source in a dispersed phase. The slurry is then spray-dried, and the resulting second spray-dried material is sintered at a temperature of 600~800℃ to obtain lithium manganese iron phosphate cathode material coated with lithium iron phosphate.

9. A positive electrode sheet, characterized in that, The lithium iron phosphate cathode material includes the lithium manganese phosphate cathode material according to any one of claims 1 to 3, or the lithium manganese phosphate cathode material prepared by the method for preparing lithium manganese phosphate cathode material according to any one of claims 4 to 8.

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

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