High-compaction lithium manganese iron phosphate as well as preparation method and application thereof

Abstract

The invention provides high-compaction lithium manganese iron phosphate as well as a preparation method and application thereof. The high-compaction lithium manganese iron phosphate has excellent physical and chemical properties of high compaction, high capacity, high magnification and long circulation. The preparation method comprises the following steps: firstly, preparing lithium manganese phosphate and lithium iron phosphate from iron phosphate and manganese phosphate and lithium carbonate, then mixing the lithium manganese phosphate and the lithium iron phosphate with a fluxing agent, an organic lithium salt and a phosphorus source, carrying out secondary firing to prepare lithium ferric manganese phosphate doubly coated with lithium metaborate and lithium pyrophosphate, and finally, crushing the fired material to obtain the lithium ferric manganese phosphate coated with lithium metaborate and lithium pyrophosphate. The high-compaction lithium manganese iron phosphate is obtained. The preparation method is simple to operate, the material performance is excellent, and the ratio of lithium manganese phosphate to lithium iron phosphate can be randomly adjusted according to requirements.

Description

Technical Field

[0001] This application belongs to the field of cathode materials for lithium-ion batteries, and specifically relates to a high-compact lithium iron manganese phosphate and a preparation method thereof. Background Art

[0002] Lithium iron manganese phosphate is a product of the mixed doping of lithium iron phosphate and lithium manganese phosphate, and has the same structure as lithium iron phosphate, which is an ordered and regular olivine structure. Lithium iron manganese phosphate and lithium iron phosphate have the same advantages of low cost, high safety performance, high thermal stability, no spontaneous combustion during针刺 (it should be "pinprick" or "penetration" in English, but the original text seems incorrect here), overcharging, long life, and no explosion risk. It can be said to combine the advantages of lithium iron phosphate and lithium manganese phosphate, and at the same time can弥补 (should be "make up for" in English) the shortcoming of low energy density of lithium iron phosphate. Therefore, it is also known as the "upgraded version of lithium iron phosphate".

[0003] However, lithium iron manganese phosphate also has its weaknesses. First, the cycle life of lithium iron manganese phosphate is relatively short, and its charge and discharge ability is poor. The addition of manganese element in lithium iron manganese phosphate will cause the dissolution of manganese, resulting in a shorter cycle life and poorer charge and discharge ability and life. Secondly, the low conductivity of lithium iron manganese phosphate makes it difficult to exert its capacity, and the side reaction with the electrolyte also makes it difficult to exert the capacity of the material. Therefore, means such as nanosizing, carbon coating, and ion doping are usually used to improve the comprehensive electrochemical performance of the material. However, one of the disadvantages brought by this is that the tap density of the material is greatly reduced, which not only reduces the processing performance of the material but also affects the final energy density of the battery cell, thus limiting the further development of lithium iron manganese phosphate materials to a certain extent.

[0004] CN117923458B discloses a method for preparing lithium iron manganese phosphate using a solvothermal method precursor. The LiMnPO4 precursor and LiFePO4 precursor synthesized by the solvothermal method are mixed and calcined to prepare LiMn 1-x Fe x PO4, where 0 < x < 1. Specifically, the following steps are included: adding a carbon source and a dispersant to the precursor mixture, transferring the mixture to a ball mill jar, and ball milling at 1500 r / min for 30 min for充分混合 (should be "fully mixing" in English); transferring the uniformly mixed powder to a tube furnace, heating to 700 °C at a rate of 5 °C / min under an argon atmosphere, and holding for 8 h; after sintering, naturally cooling to obtain the product. However, the disadvantage of this method is that when lithium iron phosphate and lithium manganese phosphate are calcined under conventional conditions, it is difficult for Mn and Fe to diffuse and mix evenly in a large lattice system, and there are still separate lithium iron phosphate phases and lithium manganese phosphate phases in the calcined material, resulting in poor cycle performance of the final material.

[0005] In addition, compared with ternary layered materials, the intrinsic lithium ion conductivity of lithium iron manganese phosphate results in poor rate performance of the material, which further limits the application of lithium iron manganese phosphate cathode materials in the new energy field. Summary of the Invention

[0006] The purpose of this application is to provide a lithium manganese iron phosphate with high compaction, high capacity, high rate and long cycle time. To this end, this application provides the following technical solutions:

[0007] In a first aspect, the present application provides a method for preparing high-density lithium manganese iron phosphate, comprising: mixing lithium manganese phosphate, lithium iron phosphate, an organic lithium salt, a flux and a phosphorus source, spray drying the mixture, and then calcining the mixture at a high temperature to obtain the high-density lithium manganese iron phosphate;

[0008] Wherein, the organic lithium salt is selected from at least one of lithium oxalate, lithium acetate, lithium formate, lithium phenoxide, lithium p-phenoxide, lithium m-phenoxide, and lithium trisphenoxide;

[0009] The flux includes lithium metaborate.

[0010] Wherein, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, phosphoric acid, and lithium dihydrogen phosphate; in order to further reduce costs, the phosphorus source is preferably ammonium dihydrogen phosphate.

[0011] The present application provides a method for preparing high-density lithium manganese iron phosphate, wherein lithium manganese phosphate and lithium iron phosphate are mixed and then mixed with a flux, an organic lithium salt, and a phosphorus source and subjected to a second calcination to prepare lithium manganese iron phosphate doubly coated with lithium pyrophosphate and lithium metaborate. This new preparation method is not only simple to operate, has low raw material prices, and excellent material properties, but also allows the ratio of lithium manganese phosphate to lithium iron phosphate to be arbitrarily adjusted according to needs.

[0012] The conventional solid-phase method of mixing lithium manganese phosphate and lithium iron phosphate and calcining them into lithium manganese iron phosphate is difficult because Mn and Fe are difficult to mix evenly, so it is difficult to form pure phase lithium manganese iron phosphate. Through a lot of research, this application found that by using lithium metaborate (LiBO2) as a flux, the flux reaction crystallization is carried out at a lower temperature, and finally the atomic-level mixing of Mn and Fe is achieved, which greatly suppresses the Jan-Taylor effect of the material during the cycle and improves the cycle performance of the material.

[0013] Using lithium metaborate as a flux offers two advantages: First, the introduction of LiBO2 can increase the material's compaction density. This is because LiBO2 forms a liquid phase during the sintering process, promoting sintering. During cooling, it partially adheres to the sample surface, forming an amorphous film layer. Therefore, the addition of LiBO2 can fill the gaps between particles, reducing the material's porosity and improving its density. Second, the introduction of LiBO2 can form a layer of fast ion conductor for lithium ions at the interface of the lithium manganese iron phosphate material, significantly reducing the material's grain boundary resistance and thereby improving its ionic conductivity.

[0014] In the present application, the organic lithium salt provides a carbon source for the system during the heating process. Secondly, at an appropriate temperature, it reacts with a phosphorus source to generate a fast ion conductor, lithium pyrophosphate (Li4P2O7). The formation temperature of pyrophosphate is very strict, and the optimal crystallization range is approximately between 600-700°C. At this lower temperature, lithium metaborate as a flux can better allow lithium iron phosphate and lithium manganese phosphate to react at a relatively low temperature to generate lithium manganese iron phosphate (the conventional formation temperature is above 700°C). Furthermore, lithium pyrophosphate, as a fast ion conductor, can greatly improve the rate performance of the material, and coating lithium manganese iron phosphate can also effectively inhibit manganese dissolution, thereby improving the cycle performance of the material.

[0015] In a specific embodiment of the present application, the number of Li, P, Mn, and Fe atoms in the lithium manganese phosphate and lithium iron phosphate satisfies Li:Fe / Mn:P=(1.0-1.5):1:(1.0-1.5).

[0016] In a specific embodiment of the present application, the number of Li, P, Mn, and Fe atoms in the lithium manganese phosphate and lithium iron phosphate satisfies Li:Fe / Mn:P=1:1:1, 1.1:1:1.1, 1.2:1:1.2, 1.3:1:1.3, 1.4:1:1.4, or 1.5:1:1.5, etc.

[0017] Preferably, the number of Li, P, Mn, and Fe atoms in the lithium manganese phosphate and lithium iron phosphate satisfies Li:Fe / Mn:P=(1.0-1.1):1:(1.0-1.1).

[0018] In a specific embodiment of the present application, the molar ratio of the organic lithium salt to (Mn+Fe) is (0.001-0.05): 1. Preferably, the molar ratio of the organic lithium salt to (Mn+Fe) is (0.01-0.03):1.

[0019] In a specific embodiment of the present application, the molar ratio of the organic lithium salt to (Mn+Fe) is 0.01:1, 0.015:1, 0.02:1, 0.025:1, 0.03:1, etc.

[0020] More preferably, the molar ratio of the organic lithium salt to (Mn+Fe) is (0.025-0.03):1.

[0021] In a specific embodiment of the present application, the molar ratio of the flux to (Mn+Fe) is (0.001-0.1): 1. Preferably, the molar ratio of the flux to (Mn+Fe) is (0.02-0.06): 1.

[0022] In a specific embodiment of the present application, the molar ratio of the flux to (Mn+Fe) is 0.02:1, 0.025:1, 0.03:1, 0.035:1, 0.04:1, 0.045:1, 0.05:1, 0.055:1, 0.06:1, etc.

[0023] More preferably, the molar ratio of the flux to (Mn+Fe) is (0.05-0.06):1.

[0024] In a specific embodiment of the present application, the molar ratio of the phosphorus source to the organic lithium salt is 1:(0.95-1.02).

[0025] In a specific embodiment of the present application, the method for preparing the lithium manganese phosphate includes:

[0026] S1-1, manganese phosphate and lithium carbonate are dispersed in water to form a uniform suspension I;

[0027] S1-2, sand-grinding the suspension I to a certain degree and then spray-drying to obtain a yellow material sample I;

[0028] S1-3. The yellow material sample I is subjected to a first calcination to obtain the lithium manganese phosphate.

[0029] Furthermore, the method for preparing lithium manganese phosphate also includes at least one of the following technical features:

[0030] (1) Step S1-1 further includes a carbon source I and / or a dopant I;

[0031] (2) the carbon source I is selected from at least one of glucose, sucrose, PEG, PVA, asphalt, cellulose, and starch;

[0032] (3) The dopant I is selected from at least one of titanium oxide, magnesium oxide, niobium oxide, aluminum oxide, zirconium oxide, copper oxide, zinc oxide, sodium carbonate, and potassium carbonate;

[0033] (4) The temperature of the first calcination is 650-1000°C, and the calcination time is 5-10 hours.

[0034] Preferably, the temperature of the first calcination is 700-800° C., and the calcination time is 8-10 h.

[0035] In a specific embodiment of the present application, in step S1-2, the sand grinding to a certain degree means that the slurry particle size D50 is 100-500 nm and the slurry solid content is 25-35%.

[0036] In a specific embodiment of the present application, in step S1-2, the specific conditions of the sanding are: first sanding at a rotation speed of 200-1000 r / min for 10-100 min, and then sanding at a rotation speed of 1000-3000 r / min for 5-120 min.

[0037] In a specific embodiment of the present application, in step S1-2, a spray dryer is used for spray drying, the inlet temperature is controlled to 150-250°C, the outlet temperature is controlled to 70-100°C, the feed rate is controlled to 3-12 mL / min, and the outlet temperature is controlled to be stable according to the viscosity and feed rate of the slurry. Finally, the spray-dried yellow material sample I is collected in a sample collection bottle.

[0038] In a specific embodiment of the present application, in step S1-3, the first calcination is carried out under an inert atmosphere, the flow rate of the inert gas is 2-10 mL / min, and the heating rate is 1-10° C. / min.

[0039] In a specific embodiment of the present application, the method for preparing lithium iron phosphate includes:

[0040] S2-1, dispersing iron phosphate and lithium carbonate in water to form a uniform suspension II;

[0041] S2-2, grinding the suspension II to a certain degree and then spray drying to obtain a yellow material sample II;

[0042] S2-3. The yellow material sample II is subjected to a second calcination to obtain the lithium iron phosphate.

[0043] Furthermore, the method for preparing lithium iron phosphate also includes at least one of the following technical features:

[0044] (1) Step S2-1 further includes a carbon source II and / or a dopant II;

[0045] (2) the carbon source II is selected from at least one of glucose, sucrose, PEG, PVA, asphalt, cellulose, and starch;

[0046] (3) The dopant II is at least one selected from titanium oxide, magnesium oxide, niobium oxide, aluminum oxide, zirconium oxide, copper oxide, zinc oxide, sodium carbonate, and potassium carbonate;

[0047] (4) The temperature of the second calcination is 650-1000°C, and the calcination time is 5-10 hours.

[0048] Preferably, the temperature of the second calcination is 750-850° C., and the calcination time is 8-10 h.

[0049] In a specific embodiment of the present application, in step S2-2, the sand grinding to a certain degree means that the slurry particle size D50 is 100-500 nm and the slurry solid content is 25-35%.

[0050] In a specific embodiment of the present application, in step S2-2, the specific conditions of the sanding are: first sanding at a rotation speed of 200-1000 r / min for 10-100 min, and then sanding at a rotation speed of 1000-3000 r / min for 5-120 min.

[0051] In a specific embodiment of the present application, in step S2-2, a spray dryer is used for spray drying, the inlet temperature is controlled to 150-250°C, the outlet temperature is controlled to 70-100°C, the feed rate is controlled to 3-12 mL / min, and the outlet temperature is controlled to be stable according to the viscosity and feed rate of the slurry. Finally, the spray-dried yellow material sample II is collected in a sample collection bottle.

[0052] In a specific embodiment of the present application, in step S2-3, the second calcination is carried out under an inert atmosphere, the flow rate of the inert gas is 2-10 mL / min, and the heating rate is 1-10° C. / min.

[0053] In a specific embodiment of the present application, the method for preparing the high-density lithium manganese iron phosphate specifically includes:

[0054] S1. Add lithium manganese phosphate and lithium iron phosphate to deionized water, then add an organic lithium salt, a flux and a phosphorus source, and stir to obtain a suspension;

[0055] S2, grinding the suspension to a certain degree and then spray drying it to obtain a black material sample I;

[0056] S3, calcining the black material sample I at high temperature to obtain black material sample II;

[0057] S4. Crushing the black material sample II to obtain the high-density lithium manganese iron phosphate.

[0058] In a specific embodiment of the present application, in step S1, the suspension further includes a carbon source III;

[0059] Preferably, the carbon source III is selected from at least one of glucose, sucrose, PEG, PVA, asphalt, cellulose, and starch.

[0060] In a specific embodiment of the present application, in step S2, the sand grinding to a certain degree means that the slurry particle size D50 is 100-500 nm and the slurry solid content is 35-45%.

[0061] In a specific embodiment of the present application, in step S2, the specific conditions of the sanding are: first sanding at a rotation speed of 200-1000 r / min for 10-100 min, and then sanding at a rotation speed of 1000-3000 r / min for 5-120 min.

[0062] In a specific embodiment of the present application, in step S2, a spray dryer is used for spray drying, the inlet temperature is controlled to 150-250°C, the outlet temperature is controlled to 70-100°C, the feed rate is controlled to 3-12 mL / min, and the outlet temperature is controlled to be stable according to the viscosity and feed rate of the slurry. Finally, the spray-dried black material sample I is collected in a sample collection bottle.

[0063] In a specific embodiment of the present application, in step S3, the high-temperature calcination temperature is 500-800° C., and the calcination time is 5-10 h.

[0064] Preferably, in step S3, the high-temperature calcination temperature is 600-700° C., and the calcination time is 5-7 hours.

[0065] In a specific embodiment of the present application, in step S3, the temperature of the high-temperature calcination is 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, etc.

[0066] More preferably, in step S3, the temperature of the high-temperature calcination is 630-670°C.

[0067] Further preferably, in step S3, the temperature of the high-temperature calcination is 640-660°C.

[0068] In a specific embodiment of the present application, in step S3, the high-temperature calcination time is 5h, 5.5h, 6h, 6.5h, 7h, etc.

[0069] More preferably, in step S3, the high-temperature calcination time is 5-6 hours.

[0070] Further preferably, in step S3, the high-temperature calcination time is 5-5.5 hours.

[0071] In a specific embodiment of the present application, in step S3, the high-temperature calcination is carried out under an inert atmosphere, the flow rate of the inert gas is 2-10 mL / min, and the heating rate is 1-10°C / min.

[0072] In a specific embodiment of the present application, in step S4, the crushing refers to multiple crushings until the particle size D50 reaches 1-5 μm.

[0073] On the other hand, the present application provides a high-density lithium manganese iron phosphate obtained by the above preparation method, wherein the structural formula of the lithium manganese iron phosphate is LiMn 1-x Fe x PO4, wherein 0<x≤0.5, and the lithium manganese iron phosphate is doubly coated with lithium metaborate and lithium pyrophosphate.

[0074] The high-density lithium manganese iron phosphate provided in this application has the characteristics of high density, high capacity, low specific surface area, high rate and long cycle time.

[0075] On the other hand, the present application provides the use of the high-density lithium manganese iron phosphate as a positive electrode material for lithium-ion batteries.

[0076] Beneficial effects:

[0077] (1) The preparation method provided in this application first reacts iron phosphate and manganese phosphate with lithium carbonate to prepare lithium manganese phosphate and lithium iron phosphate, respectively. The two are then mixed with a flux, an organic lithium salt, and a phosphorus source and calcined to prepare lithium manganese iron phosphate doubly coated with lithium metaborate and lithium pyrophosphate. Finally, the calcined material is crushed to obtain high-performance lithium manganese iron phosphate. This new preparation method is not only simple to operate, has low raw material prices, and excellent performance, but also allows the ratio of lithium manganese phosphate to lithium iron phosphate to be adjusted as needed.

[0078] (2) The preparation method provided by the present application introduces lithium metaborate flux, which can form a liquid phase during the sintering process to promote sintering, and partially adhere to the sample surface during the cooling process to form an amorphous film layer, thereby improving the compactness of lithium manganese iron phosphate and increasing the compaction density of the material; at the same time, lithium metaborate can form a layer of lithium ion fast ion conductor at the interface of the lithium manganese iron phosphate material, greatly reducing the grain boundary impedance of the material, thereby improving the ionic conductivity of the material. Therefore, the high-density lithium manganese iron phosphate obtained by the preparation method of the present application has both excellent electrical properties and high compaction density.

[0079] (3) The present application introduces an organic lithium salt and a phosphorus source during the secondary calcination process. After calcination at a specific temperature, on the one hand, the organic lithium salt can provide a coating carbon source for the lithium manganese iron phosphate, thereby improving the electronic conductivity of the material; on the other hand, the organic lithium salt reacts with the phosphorus source when heated to form lithium manganese iron phosphate coated with lithium pyrophosphate. As an ultrafast conductor of lithium ions, lithium pyrophosphate can greatly improve the lithium ion conductivity of the material, thereby improving the rate performance of the material. In addition, lithium pyrophosphate on the surface of the material can also effectively inhibit the dissolution of manganese in the material during the cycle, thereby improving the cycle performance of the material.

[0080] (4) Double coating of lithium metaborate and lithium pyrophosphate works synergistically to significantly improve the compaction density, rate capability and long cycle performance of lithium manganese iron phosphate materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Figure 1 This is a flow chart of the preparation process of high-density lithium manganese iron phosphate provided in Example 1 of the present application;

[0082] Figure 2 The XRD pattern of the high-density lithium manganese iron phosphate provided in Example 1 of the present application;

[0083] Figure 3 This is a scanning electron microscope image of the high-density lithium manganese iron phosphate provided in Example 1 of the present application;

[0084] Figure 4 The charge and discharge curves of the high-density lithium manganese iron phosphate provided in Example 1 of the present application at different rates;

[0085] Figure 5 This is the cycling performance of the high-density lithium manganese iron phosphate provided in Example 1 of the present application at a rate of 0.33C. DETAILED DESCRIPTION

[0086] In order to make the technical problems, technical solutions and beneficial effects solved by this application more clearly understood, this application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0087] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0088] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply the existence of any such actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or device comprising the element.

[0089] The preparation method of high-density lithium manganese iron phosphate provided in this application comprises the following steps:

[0090] (1) Preparation of lithium manganese phosphate:

[0091] 1-50 g of carbon source I was added to 500-1000 mL of deionized water and stirred at a rate of 300-1000 r / min until dissolved. 50-200 g of manganese phosphate powder and 10-80 g of lithium carbonate powder were then added in sequence. After stirring evenly, 0.2-2 g of dopant I was added and stirred at a speed of 300-1000 r / min for 2-24 h to obtain a uniformly dispersed suspension I.

[0092] The above-mentioned uniformly dispersed suspension I is added to a sand mill for sand milling. The sand milling is first performed at a rotation speed of 200-1000 r / min for 10-100 min, and then at a rotation speed of 1000-3000 r / min for 5-120 min. The slurry particle size D50 is monitored during the sand milling process. The sand milling is terminated when D50 is in the range of 100-500 nm. At this time, the slurry solid content is between 25-35%.

[0093] The above slurry was spray dried using a spray dryer, the inlet temperature was controlled at 150-250°C, the outlet temperature was controlled at 70-100°C, the feed rate was controlled at 3-12 mL / min, and the outlet temperature was controlled to be stable according to the viscosity of the slurry and the feed rate. Finally, the spray-dried yellow material sample I was collected in a sample collection bottle.

[0094] The yellow material sample I was calcined under an inert atmosphere, with the inert gas flow rate controlled at 2-10 mL / min, the heating rate at 1-10°C / min, the calcination temperature at 650-1000°C, the calcination time at 5-10 h, and then naturally cooled to room temperature to obtain lithium manganese phosphate.

[0095] (2) Preparation of lithium iron phosphate:

[0096] Add 1-50g of carbon source II to 500-1000mL of deionized water and stir until dissolved at a stirring rate of 300-1000r / min. Then add 50-200g of iron phosphate powder and 10-80g of lithium carbonate powder in sequence. After stirring evenly, add 0.2-2g of dopant II and stir at a speed of 300-1000r / min for 2-24h to obtain a uniformly dispersed suspension II.

[0097] The above-mentioned evenly dispersed suspension II is added to a sand mill for sand milling. First, sand milling is carried out at a rotation speed of 200-1000 r / min for 10-100 min, and then sand milling is carried out at a rotation speed of 1000-3000 r / min for 5-120 min. During the sand milling process, the slurry particle size D50 is monitored until D50 is in the range of 100-500 nm. The sand milling is terminated, and the solid content of the slurry is between 25-35%.

[0098] The above slurry was spray dried using a spray dryer, the inlet temperature was controlled at 150-250°C, the outlet temperature was controlled at 70-100°C, the feed rate was controlled at 3-12 mL / min, and the outlet temperature was controlled to be stable according to the viscosity of the slurry and the feed rate. Finally, the spray-dried yellow material sample II was collected in a sample collection bottle.

[0099] The yellow material sample II is calcined under an inert atmosphere, with the inert gas flow rate controlled at 2-10 mL / min, the heating rate at 1-10°C / min, the calcination temperature at 650-1000°C, the calcination time at 5-10 h, and then naturally cooled to room temperature to obtain lithium iron phosphate.

[0100] (3) Preparation of high-density lithium manganese iron phosphate:

[0101] Lithium manganese phosphate sample and lithium iron phosphate sample were weighed separately so that the number of Li, P, Mn, and Fe atoms satisfied Li:Fe / Mn:P=(1.0-1.5):1:(1.0-1.5), and added to 200-500 mL of deionized water. Then, carbon source III, organic lithium salt, lithium metaborate, and phosphorus source were added so that the molar ratio of organic lithium salt to (Mn+Fe) was (0.001-0.01):1, the molar ratio of flux to (Mn+Fe) was (0.001-0.05):1, and the molar ratio of phosphorus source to organic lithium salt was 1:(0.95-1.02). After stirring evenly, a suspension is obtained, which is added to a sand mill for sand milling. First, sand milling is performed at a rotation speed of 200-1000 r / min for 10-100 min, and then sand milling is performed at a rotation speed of 1000-3000 r / min for 5-120 min. During the sand milling process, the slurry particle size D50 is monitored until D50 is in the range of 100-500 nm. The sand milling is terminated, and the slurry solid content is between 35-45%.

[0102] The above slurry was spray dried using a spray dryer, the inlet temperature was controlled at 150-250°C, the outlet temperature was controlled at 70-100°C, the feed rate was controlled at 3-12 mL / min, and the outlet temperature was controlled to be stable according to the viscosity of the slurry and the feed rate. Finally, the spray-dried black material sample I was collected in a sample collection bottle.

[0103] The above-mentioned black material sample I was calcined under an inert atmosphere, with the flow rate of the inert gas controlled at 2-10 mL / min, the heating rate at 1-10°C / min, the calcination temperature at 500-800°C, the calcination time at 5-10 h, and then naturally cooled to room temperature to obtain black material sample II.

[0104] The black material sample II was crushed multiple times until its D50 particle size reached 1-5 μm, thereby obtaining high-density lithium manganese iron phosphate.

[0105] The present application is further described in detail below through specific embodiments.

[0106] Example 1

[0107] (1) Preparation of lithium manganese phosphate:

[0108] 384mL deionized water is added to a 2000mL beaker, 21.36g of glucose is added, magnetic stirring is carried out at a stirring speed of 800r / min, until dissolved, 121.14g of manganese phosphate powder is added, stirring is continued at a stirring speed of 800r / min, to a suspension liquid which is relatively uniformly dispersed, and at the same stirring speed, 29.70g of lithium carbonate powder is gradually added. At this time, the solution has a small amount of bubbles to produce, and is stirred to a light yellow suspension liquid which is uniformly dispersed under the same stirring speed. Finally, 0.32g of nano titanium dioxide and 0.24g of nano magnesium dioxide are added and stirred at a rotating speed of 1000r / min for 2h to obtain a uniformly dispersed suspension I.

[0109] The above-mentioned uniformly dispersed suspension I was added to a clean sand mill for sand grinding at a speed of 800 r / min for 10 min and 2500 r / min for 60 min. After the sand grinding was completed, the particle size was measured using Malvern 3000 until D50 was about 300 nm. At this time, the solid content of the slurry was 31%.

[0110] The above slurry was spray dried using a spray dryer, the inlet temperature was adjusted to 220°C, the outlet temperature was controlled to 80°C, the feed rate was controlled to 8 mL / min, and the outlet temperature was controlled to be stable according to the viscosity of the slurry and the feed rate. Finally, the spray-dried yellow material sample I was collected in a sample collection bottle.

[0111] 100 g of the above-mentioned yellow material sample I was weighed, added to a graphite crucible, vibrated, and placed in a box furnace for calcination. Before heating, it was necessary to evacuate and introduce high-purity nitrogen. After repeating 3-5 times, the vent valve was reduced and N2 was slowly introduced. The flow rate of N2 was adjusted to 3 mL / min, the heating rate was 3 ° C / min, the calcination temperature was 750 ° C, the calcination time was 10 h, and the cooling was natural cooling. After cooling to room temperature, lithium manganese phosphate black material was obtained.

[0112] (2) Preparation of lithium iron phosphate:

[0113] Add 384 mL of deionized water to a 2000 mL beaker, add 21.36 g of glucose, and stir magnetically at 800 r / min until dissolved. Then add 121.87 g of iron phosphate powder and continue stirring at 800 r / min until a relatively uniform suspension is obtained. At the same stirring rate, gradually add 29.70 g of lithium carbonate powder. At this point, the solution will produce trace bubbles. Stir at the same stirring rate until the light yellow suspension is uniformly dispersed. Finally, add 0.32 g of nano-titanium dioxide and 0.24 g of nano-magnesium dioxide and stir at 1000 r / min for 2 h to obtain a uniformly dispersed suspension II.

[0114] The above-mentioned evenly dispersed suspension II was added to a clean sand mill for sand grinding at a speed of 800 r / min for 10 min and 2500 r / min for 60 min. After the sand grinding was completed, the particle size was measured using Malvern 3000 until D50 was around 300 nm. At this time, the solid content of the slurry was 31%.

[0115] The above slurry was spray dried using a spray dryer, the inlet temperature was adjusted to 220°C, the outlet temperature was controlled to 80°C, the feed rate was controlled to 8 mL / min, and the outlet temperature was controlled to be stable according to the viscosity of the slurry and the feed rate. Finally, the spray-dried yellow material sample II was collected in a sample collection bottle.

[0116] Weigh 100 g of the above-mentioned yellow material sample II, add it to a graphite crucible, vibrate it, and place it in a box furnace for calcination. Before heating, it is necessary to evacuate and introduce high-purity nitrogen. After repeating 3-5 times, reduce the vent valve and slowly introduce N2. The flow rate of N2 is adjusted to 3 mL / min, the heating rate is 3°C / min, the calcination temperature is 800°C, the calcination time is 10 h, and the cooling is natural cooling. After cooling to room temperature, lithium iron phosphate black material is obtained.

[0117] (3) Preparation of lithium manganese iron phosphate:

[0118] Weigh 95.06 g of lithium manganese phosphate black material and 63.74 g of lithium iron phosphate black material, add them to a 1000 mL beaker, add 365.37 mL of deionized water, then add 5.35 g of glucose, 3.1 g of lithium oxalate, 2.6 g of lithium metaborate, and 3.45 g of ammonium dihydrogen phosphate. After stirring for half an hour, a black suspension is obtained. The evenly dispersed suspension is added to a clean sand mill for sand milling. The sand milling speed is 800 r / min for 10 min and 2500 r / min for 60 min. After the sand milling is completed, the particle size is measured using Malvern 3000 until D50 is about 300 nm. At this time, the slurry solid content is 40%.

[0119] The above slurry was spray dried using a spray dryer, the inlet temperature was adjusted to 220°C, the outlet temperature was controlled to 80°C, the feed rate was controlled to 10 mL / min, and the outlet temperature was controlled to be stable according to the viscosity of the slurry and the feed rate. Finally, the spray-dried lithium manganese iron phosphate black material I was collected in a sample collection bottle.

[0120] Weigh 80 g of the above-mentioned lithium iron manganese phosphate black material, add it to a graphite crucible, vibrate it, and place it in a box furnace for calcination. Before heating, it is necessary to evacuate and introduce high-purity nitrogen. After repeating 3-5 times, reduce the vent valve and slowly introduce N2. The flow rate of N2 is adjusted to 3 mL / min, the heating rate is 3 ° C / min, the calcination temperature is 650 ° C, the calcination time is 5 h, and the cooling is natural cooling. After cooling to room temperature, lithium iron manganese phosphate black material II is obtained.

[0121] The calcined lithium iron manganese phosphate black material II sample was placed in an air flow powder machine and crushed several times until its D50 particle size reached 1 μm to obtain high-density lithium iron manganese phosphate LiMn 0.6 Fe 0.4 PO4.

[0122] The chemical reaction that occurs in this embodiment is:

[0123] 4FePO4+2Li2CO3+C=4LiFePO4+3CO2↑

[0124] 4MnPO4+2Li2CO3+C=4LiMnPO4+3CO2↑

[0125] 2LiFePO4+3LiMnPO4=5LiMn 0.6 Fe 0.4 PO4

[0126] 4NH4H2PO4+4Li2C2O4=2Li4P2O7+4NH3↑+4H2O↑+7CO2↑+C

[0127] Figure 1 This is a flow chart of the preparation process of high-density lithium manganese iron phosphate provided in Example 1 of the present application.

[0128] Figure 2 The XRD pattern of the high-density lithium manganese iron phosphate provided in Example 1 of this application is as follows: Figure 2 It can be seen that the prepared lithium manganese iron phosphate is relatively pure and has no other impurity peaks.

[0129] Figure 3 The scanning electron microscope image of the high-density lithium manganese iron phosphate provided in Example 1 of this application is as follows: Figure 3It can be seen that the microstructure of lithium manganese iron phosphate material is composed of the accumulation of primary particles of about 100-800nm. This mixture of large and small particles helps to improve the compaction density of the material.

[0130] Figure 4 The charge and discharge curves of the high-density lithium manganese iron phosphate provided in Example 1 of this application at different rates are as follows: Figure 4 As can be seen from the figure, thanks to the double coating, the charge and discharge curves of the material at 0.1C, 0.33C and 0.5C rates are basically the same, showing very excellent rate performance.

[0131] Figure 5 The cycle performance of the high-density lithium manganese iron phosphate provided in Example 1 of this application at a rate of 0.33C is as follows: Figure 5 It can be seen that after 200 cycles, the capacity remains basically stable without any attenuation.

[0132] Example 2

[0133] The difference from Example 1 is that in the preparation step, lithium oxalate is replaced by lithium acetate with an equal molar amount of lithium ions.

[0134] Example 3

[0135] The difference from Example 1 is that in the preparation step, the masses of lithium oxalate and ammonium dihydrogen phosphate are halved, that is, 1.55 g of lithium oxalate and 1.73 g of ammonium dihydrogen phosphate.

[0136] Example 4

[0137] The difference from Example 1 is that in the preparation step, the amount of lithium metaborate is halved, that is, 1.3 g of lithium metaborate.

[0138] Example 5

[0139] The difference from Example 1 is that in the preparation step, the calcination temperature is changed to 700°C.

[0140] Example 6

[0141] The difference from Example 1 is that in the preparation step, the calcination temperature is changed to 600°C.

[0142] Example 7

[0143] The difference from Example 1 is that in the preparation step, 78.43g of lithium manganese phosphate black material and 78.88g of lithium iron phosphate black material are weighed to obtain high-density lithium manganese iron phosphate LiMn 0.5 Fe 0.5 PO4.

[0144] Comparative Example 1

[0145] The difference from Example 1 is that lithium oxalate and ammonium dihydrogen phosphate are not added in the preparation step.

[0146] Comparative Example 2

[0147] The difference from Example 1 is that in the preparation step, lithium oxalate is replaced by lithium carbonate.

[0148] Comparative Example 3

[0149] The difference from Example 1 is that in the preparation step, lithium oxalate is replaced by oxalic acid.

[0150] Comparative Example 4

[0151] The difference from Example 1 is that in the preparation step, lithium metaborate is replaced by strontium carbonate.

[0152] Comparative Example 5

[0153] The difference from Example 1 is that in the preparation step, lithium metaborate is replaced by boric acid.

[0154] Comparative Example 6

[0155] The difference from Example 1 is that, in the preparation step, lithium oxalate, ammonium dihydrogen phosphate and lithium metaborate are not added.

[0156] The performance of the high-density lithium manganese iron phosphate obtained in the above example and the lithium manganese iron phosphate obtained in the comparative example was tested using the group standard "Lithium manganese iron phosphate positive electrode material for lithium-ion batteries" T / CIAPS0029-2023. The test results are shown in Table 1:

[0157] Table 1

[0158]

[0159]

[0160] It can be seen from the data in Table 1 that the high-density lithium manganese iron phosphate provided in the present application has high compaction density, low specific surface area, and excellent rate performance, and its comprehensive performance is better than that of the comparative example.

[0161] In Comparative Example 1, no organic lithium salt and phosphorus source were added, and a lithium pyrophosphate coating layer could not be formed on the surface of the lithium manganese iron phosphate, resulting in a decrease in conductivity and a significant decrease in rate performance. It also could not effectively inhibit manganese dissolution during the cycle, resulting in a decrease in the 200-cycle retention rate.

[0162] In Comparative Example 2, lithium carbonate is used instead of lithium oxalate, resulting in a decrease in the carbon content of the material. In addition, the carbon dioxide generated by the high temperature of lithium carbonate causes an increase in the pores at the material interface, a significant decrease in the compaction density, a decrease in the rate, and a significant decrease in the cycle performance.

[0163] In Comparative Example 3, oxalic acid is used instead of lithium oxalate, which makes it difficult to form lithium pyrophosphate, and the rate, cycle and compaction density are significantly reduced.

[0164] In Comparative Example 4, strontium carbonate is used as a flux, which cannot work synergistically with lithium pyrophosphate. Although the compaction density and cycle performance are not significantly reduced, the rate performance is significantly reduced.

[0165] In Comparative Example 5, boric acid is used as a flux, the lithium ions in the system are relatively reduced, and the overall performance is inferior to that of Example 1.

[0166] Comparative Example 6 does not add any additives, which is the most commonly used preparation method, has no gain and the worst overall performance.

[0167] The above describes in detail the specific implementation methods of the present application. However, the present application is not limited to the specific details in the above implementation methods. Within the technical concept of the present application, various simple modifications can be made to the technical solution of the present application, and these simple modifications all fall within the scope of protection of the present application.

[0168] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner unless there is any contradiction. In order to avoid unnecessary repetition, this application will not further describe various possible combinations.

[0169] In addition, the various implementation methods of the present application may be arbitrarily combined, and as long as they do not violate the concept of the present application, they should also be regarded as the contents disclosed in the present application.

Claims

1. A method for preparing high-density lithium manganese iron phosphate, characterized in that: include: The high-density lithium manganese iron phosphate is obtained by mixing lithium manganese phosphate, lithium iron phosphate, an organic lithium salt, a flux and a phosphorus source, spray drying the mixture and then calcining the mixture at a high temperature; Wherein, the organic lithium salt is selected from at least one of lithium oxalate, lithium acetate, lithium formate, lithium phenoxide, lithium p-phenoxide, lithium m-phenoxide, and lithium trisphenoxide; The flux includes lithium metaborate.

2. The preparation method according to claim 1, characterized in that The phosphorus source is selected from at least one of ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, phosphoric acid, and lithium dihydrogen phosphate; The number of atoms of Li, P, Mn and Fe in the lithium manganese phosphate and lithium iron phosphate satisfies Li:Fe / Mn:P=(1.0-1.5):1:(1.0-1.5).

3. The preparation method according to claim 1, characterized in that The molar ratio of the organic lithium salt to (Mn+Fe) is (0.001-0.05):1; Preferably, the molar ratio of the flux to (Mn+Fe) is (0.001-0.1):1; Preferably, the molar ratio of the phosphorus source to the organic lithium salt is 1:(0.95-1.02).

4. The preparation method according to claim 1, characterized in that The preparation method of the lithium manganese phosphate comprises: S1-1, manganese phosphate and lithium carbonate are dispersed in water to form a uniform suspension I; S1-2, sand-grinding the suspension I and spray-drying it to obtain a yellow material sample I; S1-3. The yellow material sample I is subjected to a first calcination to obtain the lithium manganese phosphate.

5. The preparation method according to claim 4, characterized in that The step S1-1 further includes a carbon source I and / or a dopant I; Preferably, the carbon source I is selected from at least one of glucose, sucrose, PEG, PVA, asphalt, cellulose, and starch; Preferably, the dopant I is selected from at least one of titanium oxide, magnesium oxide, niobium oxide, aluminum oxide, zirconium oxide, copper oxide, zinc oxide, sodium carbonate, and potassium carbonate; Preferably, the temperature of the first calcination is 650-1000° C., and the calcination time is 5-10 h.

6. The preparation method according to claim 1, characterized in that The preparation method of the lithium iron phosphate comprises: S2-1, dispersing iron phosphate and lithium carbonate in water to form a uniform suspension II; S2-2, sand-grinding the suspension II to a certain degree and then spray-drying it to obtain a yellow material sample II; S2-3. The yellow material sample II is subjected to a second calcination to obtain the lithium iron phosphate.

7. The preparation method according to claim 6, characterized in that The step S2-1 further includes a carbon source II and / or a dopant II; Preferably, the carbon source II is selected from at least one of glucose, sucrose, PEG, PVA, asphalt, cellulose, and starch; Preferably, the dopant II is selected from at least one of titanium oxide, magnesium oxide, niobium oxide, aluminum oxide, zirconium oxide, copper oxide, zinc oxide, sodium carbonate, and potassium carbonate; Preferably, the second calcination temperature is 650-1000° C., and the calcination time is 5-10 h.

8. The preparation method according to claim 1, characterized in that The mixed and spray-dried raw materials further include carbon source III; Preferably, the carbon source III is selected from at least one of glucose, sucrose, PEG, PVA, asphalt, cellulose, and starch; Preferably, the high-temperature calcination temperature is 500-800° C., and the calcination time is 5-10 h.

9. A high-density lithium iron manganese phosphate doubly coated with lithium metaborate and lithium pyrophosphate obtained by the preparation method according to any one of claims 1 to 8.

10. Use of the high-density lithium manganese iron phosphate according to claim 9 as a positive electrode material for lithium-ion batteries.

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