Lithium manganese iron phosphate material, preparation method thereof, positive pole piece and lithium ion battery

Abstract

The invention provides a lithium manganese iron phosphate material, a preparation method thereof, a positive pole piece and a lithium ion battery. The preparation method of the lithium manganese iron phosphate material comprises the following steps: sequentially mixing and sintering a manganese iron phosphate precursor, a manganese iron pyrophosphate precursor, a lithium source, a carbon source and an organic additive to obtain the lithium manganese iron phosphate material, the ferromanganese phosphate precursor is in an amorphous state; the crystal structure of the ferromanganese pyrophosphate precursor is a monoclinic system; the organic additive is a polymer of which the molecular structure carries an anionic functional group; sintering comprises first-stage sintering, second-stage sintering, third-stage sintering, fourth-stage sintering and fifth-stage sintering which are performed in sequence. According to the invention, the unique properties of the amorphous ferromanganese phosphate precursor and the monoclinic system ferromanganese pyrophosphate precursor are utilized, and the special organic additive and the five-stage sintering process are combined, so that the particle filling effect in the sintering process is enhanced, and the purpose of optimizing the particle gradation and the element distribution uniformity is achieved.

Description

TECHNICAL FIELD

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

[0002] Lithium manganese iron phosphate (LMFP) is a new type of positive electrode material obtained by adding manganese element to lithium iron phosphate. The basic process route is similar to that of lithium iron phosphate. In the past, due to its lower conductivity and rate performance, the commercialization process was slow. With the progress of modification technologies such as carbon coating and nanocrystallization, the advantages of lithium manganese iron phosphate battery in endurance and safety are more and more obvious, and the industrialization process has begun to accelerate.

[0003] The production process of LMFP is different from that of LFP. In order to produce high-quality manganese iron lithium, manganese iron needs to form a uniform solid solution. Therefore, it is highly probable that liquid phase method will be used to produce high-quality manganese iron lithium. Liquid phase method can dissolve all raw materials. According to the principle of solution "uniformity", molecular level combination can be achieved, and the precursor obtained is more uniform, which can effectively prevent the aggregation of manganese-rich phase and improve the electrochemical performance of the material. Compared with liquid phase method, the compaction of the material synthesized by solid phase method is relatively high.

[0004] The compaction density is an important parameter in the manufacture of lithium ion batteries. During the production of lithium batteries, the compaction density has a great influence on the performance of the battery. The compaction density of the current mainstream lithium manganese iron phosphate is 2.3 g / cm 3 , and some reach 2.4 g / cm 3 . The compaction density of lithium iron phosphate can reach 2.6 g / cm 3 . The traditional method to improve the compaction density is to increase the sintering temperature and prolong the holding time. However, there is a difference in the sintering resistance between large and small particles. The sintering resistance of large particles is relatively strong, while the sintering resistance of small particles is relatively weak. Under higher temperature and longer holding time, some side reactions will occur, which seriously affects the safety and electrical performance of the battery. Therefore, it is necessary to find a balance point between improving the energy density and maintaining good electrochemical performance.

[0005] Therefore, how to optimize the preparation method of lithium manganese iron phosphate material so as to prepare a phosphate positive electrode material with higher compaction density and more superior electrochemical performance is one of the important technical problems to be solved in the field. SUMMARY

[0006] The main purpose of the present application is to provide a lithium manganese iron phosphate material, a preparation method thereof, a positive electrode sheet and a lithium ion battery, so as to solve the problems of low compaction density and poor electrochemical performance of the lithium manganese iron phosphate material in the prior art.

[0007] In order to achieve the above object, the first aspect of the present application provides a preparation method of a lithium manganese iron phosphate material, wherein the molar ratio of Mn element to Fe element in the lithium manganese iron phosphate material is (0.5-0.9):(0.1-0.5), and the preparation method comprises: sequentially mixing and sintering a manganese iron phosphate precursor, a manganese iron pyrophosphate precursor, a lithium source, a carbon source and an organic additive to obtain the lithium manganese iron phosphate material; the manganese iron phosphate precursor is amorphous, and the micro-morphology of the manganese iron phosphate precursor is nano-sized particles; the crystal structure of the manganese iron pyrophosphate precursor is monoclinic, and the micro-morphology of the manganese iron pyrophosphate precursor is micron-sized two-dimensional flaky; the organic additive is a polymer carrying an anionic functional group in the molecular structure; and the sintering comprises sequentially performing one-stage sintering, two-stage sintering, three-stage sintering, four-stage sintering and five-stage sintering.

[0008] In the synthesis of the lithium manganese iron phosphate material, the present application selects two precursors of manganese iron phosphate and manganese iron pyrophosphate for use in combination, and through the use of the two precursors with large differences in crystal structure and composition, and the use of the difference in grain growth in the multi-stage sintering process, a particle grading is formed. Meanwhile, the present application adopts a special sintering schedule and a special organic additive to reduce the free interlayer carbon, thereby synthesizing a high-compaction high-homogeneous lithium manganese iron phosphate positive electrode material. Compared with the conventional lithium manganese iron phosphate material, the obtained lithium manganese iron phosphate material has a higher particle grading degree, better electrical performance, lower polarization and higher compaction performance.

[0009] Further, the chemical formula of the manganese iron phosphate precursor is Mn a Fe b PO4, wherein a is 0.5-0.8 and b is 0.2-0.5; and / or the unit cell of the manganese iron pyrophosphate precursor belongs to the C2 / m(12) space group, and the chemical formula of the manganese iron pyrophosphate precursor is Mn c Fe d P2O7, wherein c is 0.5-0.8 and d is 0.2-0.5; and / or the ratio of the total molar amount of Mn element and Fe element to the molar amount of P element in the manganese iron phosphate precursor and the manganese iron pyrophosphate precursor is independently (0.965-1):1. The specific chemical formula of the above two preferred precursors and the element ratio relationship therein can promote the formation of a more ideal solid solution, and more effectively inhibit the generation of impurities during the sintering process, thereby further optimizing the redox properties of the obtained lithium manganese iron phosphate material and improving the electrochemical activity thereof.

[0010] Further, the weight ratio of the manganese iron phosphate precursor to the manganese iron pyrophosphate precursor is (2-4):1. The above weight ratio can more significantly optimize the particle size distribution, achieve more effective complementarity of the nano and micro particles, and better balance the electrochemical performance of the obtained lithium manganese iron phosphate material, inhibit the performance deviation caused by a single precursor, and promote more balanced performance of the obtained lithium manganese iron phosphate material in high capacity, high stability, and low polarization.

[0011] Further, the preparation process of the manganese iron phosphate precursor includes: mixing a first manganese source, a first iron source, a first phosphorus source, and an oxidizing agent, and then performing a first calcination at a holding temperature of 400-480°C to obtain the manganese iron phosphate precursor. Preferably, the above condition parameters for synthesizing the manganese iron phosphate precursor can more effectively remove impurities in the synthesis system, and obtain an amorphous particulate manganese iron phosphate precursor with higher purity and more uniform particle size distribution.

[0012] Further, the preparation process of the manganese iron iron pyrophosphate precursor includes: mixing a second manganese source, a second iron source, a second phosphorus source, and a reducing agent, and then performing a second calcination at a holding temperature of 600-800°C to obtain the manganese iron pyrophosphate precursor. Preferably, the above condition parameters for synthesizing the manganese iron pyrophosphate precursor can further improve the crystal structure of the manganese iron pyrophosphate precursor, promote its crystal growth, form a product with higher crystallinity, and ultimately form a lithium manganese iron phosphate material with more superior particle size distribution and higher tap density.

[0013] Further, the organic additive is selected from one or more of sodium polystyrene sulfonate, polyacrylic acid, poly-4-styrene sulfonic acid, and ammonium polyacrylate; the carbon source is selected from one or more of glucose, starch, sucrose, polyethylene glycol, cyclodextrin, and fructose; the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium powder; the addition amount of the organic additive is 1-3% and the addition amount of the carbon source is 10-15%, based on 100% of the total weight of the manganese iron phosphate precursor and the manganese iron pyrophosphate precursor; and / or, the total molar amount of Mn and Fe elements in the manganese iron phosphate precursor is A, the total molar amount of Mn and Fe elements in the manganese iron pyrophosphate precursor is B, and the molar amount of Li element in the lithium source is C; A, B, and C satisfy: C / (A+B)=1.02-1.03. Preferably, the above types and amounts of the organic additive, the carbon source, and the lithium source can further enhance the interface properties of the obtained lithium manganese iron phosphate material, stabilize its crystal structure, construct a more efficient conductive network, and ultimately more significantly improve the electrical conductivity of the obtained lithium manganese iron phosphate positive electrode material.

[0014] Further, the sintering comprises sequentially performing: a first-stage sintering at a holding temperature of 100-200°C, a second-stage sintering at a holding temperature of 400-500°C, a third-stage sintering at a holding temperature of 650-730°C, a fourth-stage sintering at a holding temperature of 750-780°C, and a fifth-stage sintering at a holding temperature of 650-730°C; and the holding time of the first-stage sintering, the second-stage sintering, the third-stage sintering, the fourth-stage sintering, and the fifth-stage sintering is independently 10 min-6 h. The above-mentioned preferred sintering process can further finely control the internal transformation and external morphology of the obtained lithium manganese iron phosphate material, and more effectively reduce the side reactions and lattice distortion caused by one-time high-temperature sintering, so as to finally obtain a lithium manganese iron phosphate material with higher tap density and better electrochemical activity.

[0015] The second aspect of the present application provides a lithium manganese iron phosphate material prepared by the above-mentioned preparation method of lithium manganese iron phosphate material. Since the above-mentioned preparation method is based on two different crystal structures and precursors with different morphologies, combined with special organic additives and a five-stage sintering process, the purpose of optimizing the particle size distribution and element distribution uniformity is achieved. Therefore, the obtained lithium manganese iron iron phosphate material has excellent particle size distribution, thereby showing the advantages of high tap density and excellent electrochemical performance.

[0016] It needs to be particularly pointed out that due to the complex structure formation and composition change during the preparation process, and due to the limitations of the special nature of the material field and the existing test characterization means, it is difficult to comprehensively quantitatively characterize the complex microstructure of the above-mentioned obtained lithium manganese iron phosphate material. However, the performance test results show that the above-mentioned lithium manganese iron phosphate material obtained by the present application has a higher tap density and exhibits more superior electrochemical activity in application.

[0017] In several preferred embodiments, the tap density of the lithium manganese iron phosphate material is 2.41 g / cc-2.70 g / cc.

[0018] The third aspect of the present application provides a positive electrode sheet comprising a positive electrode active material, which is the above-mentioned lithium manganese iron phosphate material; or the positive electrode active material is prepared by the above-mentioned preparation method of lithium manganese iron phosphate material. Since the lithium manganese iron phosphate material synthesized by using the above-mentioned preparation method provided by the present application has more uniform particle size, more excellent microstructure, higher tap density, and higher electrochemical activity, the obtained electrode sheet has higher energy density and more stable cycle performance.

[0019] The fourth aspect of the present application provides a lithium ion battery comprising the above positive electrode sheet. Since the above lithium manganese iron phosphate material provided by the present application has superior particle size characteristics, higher compaction density and excellent electrochemical performance, the lithium ion battery in which the material is used exhibits higher energy density and charge-discharge specific capacity.

[0020] By using the unique properties of the amorphous manganese iron phosphate precursor and the monoclinic manganese iron pyrophosphate precursor, combined with special organic additives and a five-stage sintering process, the particle packing effect during sintering is strengthened, the particle size distribution and element distribution uniformity are optimized, and the compaction density and electrochemical performance of the obtained lithium manganese iron phosphate positive electrode material are significantly improved, and the technical effect of reducing polarization is also achieved. BRIEF DESCRIPTION OF DRAWINGS

[0021] The accompanying drawings, which form a part of the present application, are included to provide a further understanding of the application, and are incorporated herein for purposes of illustrating the illustrative embodiments of the present application and the explanations provided herein, and are not intended as a limitation of the present application. In the drawings:

[0022] Figure 1 XRD pattern of the manganese iron phosphate precursor and the manganese iron pyrophosphate precursor obtained in Example 1 of the present application;

[0023] Figure 2 XRD pattern of the lithium manganese iron phosphate material obtained in Example 1 of the present application;

[0024] Figure 3 SEM characterization results of the manganese iron phosphate precursor obtained in Example 1 of the present application;

[0025] Figure 4 SEM characterization results of the manganese iron pyrophosphate precursor obtained in Example 1 of the present application;

[0026] Figure 5 Pressure-compaction density change curve of the lithium manganese iron phosphate material obtained in Example 1 of the present application and the lithium manganese iron phosphate material obtained in Comparative Example 1. DETAILED DESCRIPTION

[0027] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the embodiments.

[0028] As described in the background section, existing lithium manganese iron phosphate materials suffer from low compaction density and poor electrochemical performance. To address these issues, the first aspect of this invention provides a method for preparing lithium manganese iron phosphate materials, wherein the molar ratio of Mn to Fe is (0.5–0.9):(0.1–0.5), and the method comprises: sequentially mixing and sintering a manganese iron phosphate precursor, a manganese iron pyrophosphate precursor, a lithium source, a carbon source, and an organic additive to obtain the lithium manganese iron phosphate material; the manganese iron phosphate precursor is amorphous and has a nanoscale particle morphology; the manganese iron pyrophosphate precursor has a monoclinic crystal structure and a micrometer-scale two-dimensional sheet-like morphology; the organic additive is a polymer carrying anionic functional groups in its molecular structure; the sintering includes sequentially performing one-stage sintering, two-stage sintering, three-stage sintering, four-stage sintering, and five-stage sintering.

[0029] This invention utilizes a combination of specific manganese iron phosphate precursors and manganese iron pyrophosphate precursors, along with lithium sources, carbon sources, and specific organic additives, to prepare a high-compact, homogeneous lithium manganese iron phosphate cathode material through a multi-stage sintering process. Specifically, amorphous materials, lacking periodic atomic arrangement, exhibit high internal disorder, leading to greater activity during sintering and easier reaction with other components to form a long-range ordered structure. The monoclinic manganese iron pyrophosphate precursor, with its highly ordered structure, helps maintain material stability. The combination of both promotes uniform and dense crystal growth within the material while maintaining structural stability. The differences in structure, size, and morphology between the two precursors create a particle gradation effect during sintering. Nanoscale manganese iron phosphate precursors fill the voids between micron-sized manganese iron pyrophosphate precursors, significantly increasing the final material's compaction density and thus enhancing the battery's energy density. Simultaneously, the gradation effect also helps reduce polarization, improve conductivity, and enhance cycle stability.

[0030] Building upon this foundation, polymer additives carrying anionic functional groups can form stable complexes with the material surface, improving the material's dispersibility and surface activity. During sintering, the pyrolysis products of the organic additives can also synergistically interact with the carbon source to form a thin and uniform carbon coating layer, improving the material's conductivity, reducing surface defects, and further optimizing electrochemical performance. Furthermore, the synergistic use of these specific organic additives can promote the good binding of manganese, iron, and phosphorus elements in the precursor, ensuring the formation of a uniform solid solution during sintering. This not only increases the material's compaction density but also ensures the uniformity of elemental distribution, avoiding inconsistencies in electrochemical performance and enhancing the material's cycle stability and rate performance.

[0031] Equally important, this invention divides the sintering process into five stages to more significantly control the particle size distribution, specific surface area, and compaction density of the final product, thereby optimizing the overall performance of the resulting lithium manganese iron phosphate material.

[0032] In summary, this invention utilizes a combination of two precursors, ferromanganese phosphate and ferromanganese pyrophosphate, in the synthesis of lithium manganese iron phosphate (LFP) materials. By employing these two precursors with significantly different crystal structures and compositions, and through a multi-stage sintering process, the differences in grain growth are utilized to achieve a specific particle size distribution. Simultaneously, this invention employs a special sintering regime, combined with special organic additives, to reduce free interlayer carbon, thereby synthesizing a high-compact, highly homogeneous LFP cathode material. Compared to conventional LFP materials, the resulting LFP material exhibits a higher degree of particle size distribution, superior electrical performance, lower polarization, and better compaction performance.

[0033] In practical applications, the preferred primary particle size of the manganese iron phosphate precursor is 10 nm to 100 nm to further improve the compaction density of the resulting cathode material. To avoid ambiguity, the term "primary particle size" in this invention refers to the particle size of the unaggregated spherical or near-spherical particles that form the lithium manganese iron phosphate material.

[0034] In several typical implementations, to obtain lithium manganese iron phosphate materials with higher electrochemical activity, the preferred chemical formula of the manganese iron phosphate precursor is Mn. a Fe b PO4, wherein a is 0.5–0.8 and b is 0.2–0.5; and / or, preferably, the unit cell of the manganese ferrophosphate pyrophosphate precursor belongs to the C2 / m(12) space group, and the chemical formula of the manganese ferrophosphate pyrophosphate precursor is Mn c Fe d P2O7, wherein c is 0.5–0.8 and d is 0.2–0.5. Furthermore, in order to form a more ideal solid solution and more effectively suppress the formation of impurity phases during sintering, thereby further optimizing the redox properties of the obtained lithium manganese iron phosphate material and improving its electrochemical activity, the ratio of the total molar amount of Mn and Fe to the molar amount of P in both the manganese iron phosphate precursor and the manganese iron pyrophosphate precursor is preferably (0.965–1):1.

[0035] In particular, when the preferred pyrophosphate iron manganese precursor has a sheet diameter of 1 μm to 20 μm and a thickness of 0.05 μm to 0.5 μm, a more stable substrate can be formed during sintering. This also enables the nanoscale pyrophosphate iron manganese precursor to be more effectively embedded between these two-dimensional sheet structures, thereby achieving more significant optimization of particle size distribution and further improving the compaction density of the obtained lithium manganese iron phosphate material.

[0036] Furthermore, through extensive experimentation, the inventors optimized the weight ratio of the manganese iron phosphate precursor to the manganese iron pyrophosphate precursor to (2-4):1, thereby significantly improving the particle size distribution and achieving more effective complementarity between nano- and micro-sized particles. Simultaneously, this optimized weight ratio also better balances the electrochemical performance of the resulting lithium manganese iron phosphate material, suppressing performance bias caused by a single precursor dominance, and promoting a more balanced performance in terms of high capacity, high stability, and low polarization.

[0037] In several typical embodiments, the preparation process of the ferromanganese phosphate precursor includes: mixing a first manganese source, a first iron source, a first phosphorus source, and an oxidant, followed by a first calcination at a holding temperature of 400℃ to 480℃ to obtain the ferromanganese phosphate precursor; and / or, the preparation process of the ferromanganese pyrophosphate precursor includes: mixing a second manganese source, a second iron source, a second phosphorus source, and a reducing agent, followed by a second calcination at a holding temperature of 600℃ to 800℃ to obtain the ferromanganese pyrophosphate precursor. In this preferred embodiment, both the ferromanganese phosphate precursor and the ferromanganese pyrophosphate precursor are synthesized by a liquid-phase method to further improve the uniformity of elemental distribution and optimize the electrochemical performance of the final lithium manganese iron phosphate material. In particular, the first calcination temperature for synthesizing the ferromanganese phosphate precursor is preferably 400℃ to 480℃ to more effectively remove impurities in the synthesis system and obtain amorphous granular ferromanganese phosphate precursor with higher purity and more uniform particle size distribution. In the process of synthesizing the precursor of manganese iron pyrophosphate, the temperature range of 600℃ to 800℃ helps to further improve the crystal structure of the precursor, promote its crystal growth, form a product with higher crystallinity, and ultimately form lithium manganese iron phosphate material with better particle size distribution and higher compaction density.

[0038] Furthermore, the preferred heating rate for the first calcination of the synthesized manganese ferrophosphate precursor is 5 ± 0.5 °C / min, which helps to more effectively reduce structural defects caused by local temperature gradients, thereby forming a more uniform amorphous manganese ferrophosphate precursor material. Additionally, the preferred heating rate for the second calcination is 5 ± 0.5 °C / min to improve the uniformity of crystal growth, reduce crystal defects or structural discontinuities caused by abrupt temperature changes, and thus more effectively improve the crystal purity and long-range order of the obtained manganese ferrophosphate pyrophosphate precursor.

[0039] In practical applications, during the liquid-phase synthesis of manganese ferric phosphate precursors, the first manganese source is selected from one or more of elemental manganese, manganese oxide, manganese dioxide, manganese trioxide, manganese tetroxide, manganese nitrate, manganese sulfate, and manganese chloride; and / or, the first iron source is selected from one or more of elemental iron, ferric hydroxide, ferric oxide, ferric tetroxide, ferrous sulfate, ferric sulfate, ferrous nitrate, ferric nitrate, ferrous chloride, and ferric chloride; and / or, the first phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ammonium phosphate, and sodium phosphate; and / or, the oxidant is selected from one or more of hydrogen peroxide, concentrated sulfuric acid, nitric acid, hypochlorous acid, and ozone. In the process of synthesizing manganese ferric pyrophosphate precursor by liquid phase, the second manganese source is selected from one or more of elemental manganese, manganese oxide, manganese dioxide, manganese trioxide, manganese tetroxide, manganese nitrate, manganese sulfate, and manganese chloride; and / or, the second iron source is selected from one or more of elemental iron, ferric hydroxide, ferric oxide, ferric tetroxide, ferrous sulfate, ferric sulfate, ferrous nitrate, ferric nitrate, ferrous chloride, and ferric chloride; and / or, the second phosphorus source is pyrophosphate and / or sodium pyrophosphate; and / or, the reducing agent is selected from one or more of ascorbic acid, catechol, and tea polyphenols.

[0040] In several typical embodiments, the organic additives are selected from one or more of sodium polystyrene sulfonate, polyacrylic acid, poly-4-styrene sulfonic acid, and ammonium polyacrylate. These preferred organic additives can form a more uniform modified layer on the surface of the lithium manganese iron phosphate cathode material, thereby enhancing its interfacial properties and improving conductivity. In particular, the anionic functional groups in the structures of these organic additives can form stable complexes with metal ions such as manganese and iron, further improving the material's dispersibility and stability, significantly promoting the homogenization of the crystal structure, and enhancing its electrochemical performance. In practical applications, the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium powder.

[0041] Furthermore, in order to promote more effective decomposition of the carbon source during sintering, form a more uniform carbon coating layer, and construct a more efficient conductive network, thereby significantly improving the conductivity of the obtained lithium manganese iron phosphate cathode material, preferably: the carbon source is selected from one or more of glucose, starch, sucrose, polyethylene glycol, cyclodextrin, and fructose; and / or, based on the total weight of the manganese iron phosphate precursor and the manganese iron pyrophosphate precursor as 100%, the amount of organic additives added is 1% to 3%, and the amount of carbon source added is 10% to 15%.

[0042] Regarding the amount of lithium source used: Let A be the total molar amount of Mn and Fe in the manganese iron phosphate precursor; let B be the total molar amount of Mn and Fe in the manganese iron pyrophosphate precursor; let C be the molar amount of Li in the lithium source; A, B, and C satisfy: C / (A+B)=1.02~1.03.

[0043] Generally speaking, compared with lithium iron phosphate, lithium manganese iron phosphate has a relatively lower calcination temperature and a faster particle growth rate. The requirements for the carbon source in the entire sintering process are significantly different from those of lithium iron phosphate. It requires a carbon source with a lower pyrolysis temperature, a wider pyrolysis temperature range, a higher degree of graphitization, and a better coating effect. Based on this, the sintering process in this invention preferably includes the following sequential steps: a first-stage sintering at a holding temperature of 100℃~200℃, a second-stage sintering at a holding temperature of 400℃~500℃, a third-stage sintering at a holding temperature of 650℃~730℃, a fourth-stage sintering at a holding temperature of 750℃~780℃, and a fifth-stage sintering at a holding temperature of 650℃~730℃; the holding times for each of the first, second, third, fourth, and fifth stages of sintering are independently 10 min to 6 h. In the aforementioned sintering process, the low-temperature stage (100–200℃) is used to remove moisture and volatile substances, stabilizing the material structure; the medium-temperature stage (400–500℃) promotes the initial solid solution formation, more effectively improving the crystal order; the high-temperature stages (650–730℃ and 750–780℃) further accelerate the crystallization and densification of the material, forming a more mature crystal structure; the cooling stage (again 650–730℃) reduces the internal stress of the material, minimizing structural damage caused by thermal shock. This optimized, more specific segmented sintering strategy, through gradually increasing the temperature and flexibly adjusting the holding time (10 min to 6 h), allows for more precise control over the internal transformation and external morphology of the resulting lithium manganese iron phosphate material. It also more effectively reduces side reactions and lattice distortions that may result from a single high-temperature sintering process, ultimately yielding lithium manganese iron phosphate material with higher compaction density and better electrochemical activity.

[0044] In several typical implementations, to achieve a more ideal grain size distribution, further increase compaction density, better maintain the structural stability of the material, reduce potential excessive grain growth or structural damage, and ultimately result in a material with superior electrochemical performance, the preferred temperatures for each sintering stage are: a holding temperature of 1h–2h for the first stage of sintering; and / or, a holding temperature of 2h–3h for the second stage of sintering; and / or, a holding temperature of 3h–4h for the third stage of sintering; and / or, a holding temperature of 10min–30min for the fourth stage of sintering; and / or, a holding temperature of 4h–6h for the fifth stage of sintering. Furthermore, to further purify the material structure, reduce impurities, and thus improve its structural stability, sintering is preferably carried out in a protective atmosphere, preferably nitrogen and / or argon.

[0045] A second aspect of this invention provides a lithium manganese iron phosphate material, which is prepared by the aforementioned method for preparing lithium manganese iron phosphate. Because the aforementioned preparation method is based on two precursors with different crystal structures and morphologies, combined with special organic additives and a five-stage sintering process, it achieves optimized particle size distribution and elemental uniformity. Therefore, the resulting lithium manganese iron phosphate material possesses excellent particle size distribution, thus exhibiting high compaction density and superior electrochemical performance.

[0046] It should be noted that due to the complex structural formation and compositional changes during the preparation process, and the limitations of the material field and existing testing and characterization methods, it is difficult to perform a comprehensive quantitative characterization of the complex microstructure of the obtained lithium manganese iron phosphate material. However, performance test results show that the lithium manganese iron phosphate material obtained in this invention has a higher particle size distribution, superior electrical properties, lower polarization, and higher compaction performance compared to conventional lithium manganese iron phosphate materials.

[0047] In several preferred embodiments, the D50 of the lithium manganese iron phosphate material is 0.7 μm to 1.2 μm; and / or, the specific surface area of ​​the lithium manganese iron phosphate material is 14 m². 2 / g~17m 2 / g; and / or, the compaction density of lithium manganese iron phosphate material is 2.41 g / cc to 2.70 g / cc.

[0048] A third aspect of the present invention provides a positive electrode sheet comprising a positive electrode active material, wherein the positive electrode active material is the aforementioned lithium manganese iron phosphate material; or, the positive electrode active material is prepared by the aforementioned method for preparing lithium manganese iron phosphate material. Because the lithium manganese iron phosphate material synthesized using the aforementioned preparation method provided by the present invention has a more uniform particle size, a better microstructure, a higher compaction density, and higher electrochemical activity, the resulting electrode sheet thus exhibits higher energy density and more stable cycle performance.

[0049] A fourth aspect of the present invention provides a lithium-ion battery comprising the aforementioned positive electrode sheet. Because the lithium manganese iron phosphate material provided by the present invention possesses superior particle size characteristics, higher compaction density, and better electrochemical performance, the lithium-ion battery containing it exhibits higher energy density and charge / discharge specific capacity.

[0050] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0051] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0052] Example 1

[0053] A method for preparing lithium manganese iron phosphate material:

[0054] (1) Manganese powder and iron powder were weighed according to the molar ratio Mn:Fe = 6:4. 85% phosphoric acid was diluted to 30% by mass ratio (Mn+Fe):P = 0.975 and mixed with the metal powder. The mixture was heated to 90℃ and reacted for 12 hours. The temperature was then lowered to 60℃, and hydrogen peroxide was introduced for oxidation. The precipitate was washed and dried, and then heated to 450℃ at a rate of 5℃ / min and held for 5 hours to obtain a brownish-yellow anhydrous ferromanganese phosphate product with a primary particle size ≈30nm, i.e., the ferromanganese phosphate precursor. The obtained ferromanganese phosphate precursor was amorphous, and its chemical formula was Mn. 0.6 Fe 0.4 PO4.

[0055] (2) According to the molar ratio Mn:Fe = 6:4, ferrous chloride and manganese chloride were weighed, and sodium pyrophosphate was weighed with a molar ratio (Mn+Fe):P = 0.975. After mixing with pure water, a 1.5 mol / L Mn / Fe salt mixed solution was obtained. Ascorbic acid was added to prevent oxidation. After stirring for 120 min, the solution was washed with pure water, separated from solids and liquids and dried under vacuum. The temperature was raised to 700℃ at a rate of 5℃ / min and kept at that temperature for 5 h to obtain an anhydrous manganese ferrophosphate product with a grayish-white flake diameter of ≈15 μm and a thickness of ≈0.2 μm, which is the precursor of manganese ferrophosphate. The obtained manganese ferrophosphate precursor has a monoclinic crystal system, belongs to the C2 / m(12) space group, and its chemical formula is Mn 0.6 Fe 0.4 P2O7.

[0056] (3) Weigh the two precursors according to the mass ratio of ferromanganese phosphate to ferromanganese pyrophosphate = 3:1, add lithium carbonate (n(Li):n(Mn+Fe) = 1.025), glucose (13.6% of the total mass of precursors), and ammonium polyacrylate (2.0% of the total mass of precursors), and stir with deionized water to obtain a slurry with a solid content of 45%. Liquid-phase milling is stopped when the slurry D50 particle size is 0.45 μm. The slurry is then fed into a spray dryer for granulation. The resulting dried material is sintered in a roller kiln under a high-purity nitrogen atmosphere at 150℃ for 2 h, 500℃ for 3 h, 650℃ for 4 h, 770℃ for 20 min, and 700℃ for 5 h, followed by natural cooling. Finally, the sintered material is subjected to air jet milling, with the D50 of the pulverized material controlled at 1.1 μm, to obtain the finished lithium manganese iron phosphate material. The molar ratio of Mn to Fe is 0.6:0.4.

[0057] In the above preparation process, the XRD patterns of the manganese iron phosphate precursor obtained in step (1) and the manganese iron pyrophosphate precursor obtained in step (2) are shown below. Figure 1 .from Figure 1 It can be seen that the manganese iron phosphate precursor has poor crystallinity, exhibiting a "bun peak" pattern, indicating that the phase is amorphous at this stage; while the manganese iron pyrophosphate precursor has strong diffraction peaks, corresponding to the standard card PDF#29-0891, with no other impurity peaks. The XRD pattern of the final lithium manganese iron phosphate material obtained in this embodiment is shown below. Figure 2 The diffraction peaks all corresponded to the Pnma olivine structure of the LiFePO4 standard card, and no obvious impurity peaks were observed, indicating the successful synthesis of a pure solid solution phase material and confirming the effectiveness of the experimental scheme of this invention. The absence of carbon diffraction peaks indicates that the pyrolytic carbon on the surface of the obtained material has an amorphous structure. Furthermore, compared to LiFePO4, the diffraction peaks of the sample slightly shifted towards lower diffraction angles; this phenomenon is related to the ionic radius.

[0058] Figure 3 and Figure 4 The scanning electron microscope (SEM) images of the manganese iron phosphate precursor obtained in step (1) and the manganese iron pyrophosphate precursor obtained in step (2) show obvious differences. The manganese iron phosphate precursor is a nano-sized particle aggregate, while the manganese iron pyrophosphate precursor exhibits a two-dimensional lamellar structure.

[0059] Example 2

[0060] A method for preparing lithium manganese iron phosphate material:

[0061] The only difference between this embodiment and embodiment 1 is that in step (3), the weight ratio of manganese iron phosphate precursor to manganese iron pyrophosphate precursor is changed to 7:3.

[0062] Example 3

[0063] A method for preparing lithium manganese iron phosphate material:

[0064] The only difference between this embodiment and embodiment 1 is that in step (3), the weight ratio of manganese iron phosphate precursor to manganese iron pyrophosphate precursor is changed to 4:1.

[0065] Example 4

[0066] A method for preparing lithium manganese iron phosphate material:

[0067] The only difference between this embodiment and embodiment 1 is that in step (3), the sintering process is changed to: sintering at 150°C for 2 hours, at 500°C for 3 hours, at 690°C for 4 hours, at 780°C for 10 minutes, at 710°C for 5 hours, and then naturally cooled.

[0068] Example 5

[0069] A method for preparing lithium manganese iron phosphate material:

[0070] The only difference between this embodiment and embodiment 1 is that in steps (1) and (2), the amount of each raw material is changed so that the ratio of (Mn+Fe):P in the obtained manganese ferric phosphate precursor is 0.8:1 and the ratio of (Mn+Fe):P in the obtained manganese ferric pyrophosphate precursor is 1.2:1.

[0071] Example 6

[0072] A method for preparing lithium manganese iron phosphate material:

[0073] The only difference between this embodiment and Embodiment 1 is that in step (1), the calcination holding temperature is changed to 350°C.

[0074] Example 7

[0075] A method for preparing lithium manganese iron phosphate material:

[0076] The only difference between this embodiment and Embodiment 1 is that in step (1), the calcination holding temperature is changed to 500°C.

[0077] Example 8

[0078] A method for preparing lithium manganese iron phosphate material:

[0079] The only difference between this embodiment and embodiment 1 is that in step (2), the calcination holding temperature is changed to 550°C.

[0080] Example 9

[0081] A method for preparing lithium manganese iron phosphate material:

[0082] The only difference between this embodiment and embodiment 1 is that in step (2), the calcination holding temperature is changed to 850°C.

[0083] Example 10

[0084] A method for preparing lithium manganese iron phosphate material:

[0085] The only difference between this embodiment and embodiment 1 is that in step (3), the weight ratio of manganese iron phosphate precursor to manganese iron pyrophosphate precursor is changed to 1:1.

[0086] Example 11

[0087] A method for preparing lithium manganese iron phosphate material:

[0088] The only difference between this embodiment and embodiment 1 is that in step (3), the weight ratio of manganese ferric phosphate precursor to manganese ferric pyrophosphate precursor is changed to 5:1.

[0089] Example 12

[0090] A method for preparing lithium manganese iron phosphate material:

[0091] The only difference between this embodiment and Example 1 is that in step (3), the amount of organic additive is changed to 0.5% of the total mass of the two precursors.

[0092] Example 13

[0093] A method for preparing lithium manganese iron phosphate material:

[0094] The only difference between this embodiment and Embodiment 1 is that, in step (3), the amount of organic additive is changed to 4.0% of the total mass of the two precursors.

[0095] Example 14

[0096] A method for preparing lithium manganese iron phosphate material:

[0097] The only difference between this embodiment and embodiment 1 is that in step (3), the sintering process is changed to: sintering at 80°C for 3 hours, sintering at 350°C for 4 hours, sintering at 750°C for 2 hours, sintering at 740°C for 40 minutes, sintering at 750°C for 3 hours, and then naturally cooling.

[0098] Example 15

[0099] A method for preparing lithium manganese iron phosphate material:

[0100] The only difference between this embodiment and embodiment 1 is that in step (3), the sintering process is changed to: sintering at 240℃ for 0.5h, sintering at 550℃ for 2h, sintering at 640℃ for 5h, sintering at 800℃ for 5min, sintering at 620℃ for 8h, and then naturally cooling.

[0101] Example 16

[0102] A method for preparing lithium manganese iron phosphate material:

[0103] The only difference between this embodiment and Embodiment 1 is that ferromanganese pyrophosphate with a sheet diameter of 5 μm and a thickness of 0.08 μm is used as the precursor for ferromanganese pyrophosphate.

[0104] Example 17

[0105] A method for preparing lithium manganese iron phosphate material:

[0106] The only difference between this embodiment and Embodiment 1 is that in step (1), ferromanganese phosphate with a primary particle size of 5 nm is used as the precursor of ferromanganese phosphate.

[0107] Example 18

[0108] A method for preparing lithium manganese iron phosphate material:

[0109] The only difference between this embodiment and Embodiment 1 is that, in step (1), ferromanganese phosphate with a primary particle size of 200 nm is used as the precursor of ferromanganese phosphate.

[0110] Comparative Example 1

[0111] This comparative example directly uses commercially available HC-LMFP64 as the material sample obtained.

[0112] Comparative Example 2

[0113] A method for preparing lithium manganese iron phosphate material:

[0114] The only difference between this comparative example and Example 1 is that no organic additives were added in step (3); at the same time, the amount of carbon source was adjusted to 14.5% of the total mass of the precursor so that the carbon content of the final lithium manganese iron phosphate material was consistent with that of Example 1.

[0115] Comparative Example 3

[0116] A method for preparing lithium manganese iron phosphate material:

[0117] The only difference between this comparative example and Example 1 is that the five-stage sintering at 770°C for 20 minutes was not performed in step (3).

[0118] Test methods for material samples

[0119] Primary particle size: Particles in SEM images are statistically analyzed using image processing software.

[0120] Carbon content: obtained from carbon and sulfur analyzer tests.

[0121] Specific surface area: obtained by testing with a specific surface area testing instrument.

[0122] 3T powder compaction density (226MPa): obtained from compaction density tester.

[0123] Furthermore, the pressure-compacted density variation curves of the lithium manganese iron phosphate material obtained in Example 1 and the lithium manganese iron phosphate material obtained in Comparative Example 1 are shown in the figure. Figure 5 As can be seen from the figure, at 226 MPa, the compaction of the samples in Example 1 and Comparative Example 1 were 2.48 g / cc and 2.26 g / cc, respectively, representing an increase of 9.29%. This is because, through the synthesis process of two precursors with different morphologies, the embodiments of the present invention formed a situation where particles of different sizes coexist and complement each other, which can improve the compaction density of lithium manganese iron phosphate cathode material.

[0124] Battery sample preparation and electrical performance testing

[0125] The obtained material sample, conductive carbon black, and polyvinylidene fluoride were weighed at a mass ratio of 90:5:5, mixed uniformly, and then evenly dispersed in N-methylpyrrolidone and stirred to form a slurry. The slurry was evenly coated onto aluminum foil and dried at 80°C for 8 hours. The slurry was then cut into 12cm positive electrode discs using a die-cutting machine. In an argon-atmospheric glove box, CR2032 button batteries were assembled using a stainless steel battery casing, positive electrode disc, separator, electrolyte, nickel mesh, and lithium metal as the counter electrode. Subsequently, they were placed in a constant temperature room at 25°C for 12 hours before use.

[0126] Charge-discharge and cycle tests were performed using the Blue Electric testing system, with a charging termination voltage of 4.35V and a discharging termination voltage of 2.0V. Each coin cell sample was subjected to 0.1C and 1C charge-discharge rate tests at 25℃.

[0127] The material samples obtained from each embodiment and comparative example were subjected to the above tests, and the results are shown in Table 1.

[0128] Table 1

[0129]

[0130]

[0131] As can be seen from the above description, compared to the comparative examples, the embodiments of the present invention utilize two precursors with significantly different crystal structures, sizes, and morphologies. Through a multi-stage sintering process, the differences in grain growth are utilized to form a particle size distribution, resulting in a lithium manganese iron phosphate material with both high capacity and high compaction. Compared to the comparative examples, the lithium manganese iron phosphate materials obtained in the above embodiments exhibit superior electrical and compaction properties. This is mainly because there are voids between the larger primary particles; only by filling and compacting them with smaller particles can a high compaction effect be achieved.

[0132] Specifically, in each embodiment:

[0133] Comparing Example 5 with Example 1, it can be seen that the preferred ratio of (Mn+Fe):P in the manganese iron phosphate precursor and the manganese iron pyrophosphate precursor can promote the formation of a more ideal solid solution and more effectively suppress the generation of impurity phases during sintering, thereby further optimizing the redox properties of the final lithium manganese iron phosphate material and improving its electrochemical activity.

[0134] Comparing Examples 6 and 7 with Example 1, it can be seen that by optimizing the calcination holding temperature during the preparation of the ferromanganese phosphate precursor, impurities in the synthesis system can be removed more effectively, resulting in amorphous granular ferromanganese phosphate precursors with higher purity and more uniform particle size distribution. Comparing Examples 8 and 9 with Example 1, it can be seen that by optimizing the calcination holding temperature during the preparation of the ferromanganese pyrophosphate precursor, the crystal structure of the ferromanganese pyrophosphate precursor can be further improved, promoting crystal growth, forming a product with higher crystallinity, and ultimately forming lithium manganese iron phosphate material with superior particle size distribution and higher compaction density.

[0135] Comparing Examples 10 and 11 with Example 1, it can be seen that by optimizing the weight ratio of the manganese iron phosphate precursor to the manganese iron pyrophosphate precursor, the particle size distribution can be significantly optimized, achieving more effective complementarity between nano- and micro-sized particles. Simultaneously, it can better balance the electrochemical performance of the resulting lithium manganese iron phosphate material, suppressing the performance bias caused by a single precursor dominance, and promoting a more balanced performance in terms of high capacity, high stability, and low polarization.

[0136] Comparing Examples 12 and 13 with Example 1, it can be seen that by optimizing the amount of organic additives, the carbon source can be synergistically decomposed more effectively during sintering, forming a more uniform carbon coating layer and constructing a more efficient conductive network, ultimately significantly improving the conductivity of the obtained lithium manganese iron phosphate cathode material.

[0137] Comparing Examples 14 and 15 with Example 1, it can be seen that by further optimizing the five-stage sintering process, a more ideal grain size distribution can be formed, the compaction density can be further improved, the structural stability of the material can be better maintained, the possible excessive grain growth or structural damage can be reduced, and the final material can have better electrochemical performance.

[0138] Comparing Example 16 with Example 1, it can be seen that by optimizing the size characteristics of the manganese iron pyrophosphate precursor, a more stable substrate can be formed during sintering, and the nano-sized manganese iron phosphate precursor can be more effectively embedded between these two-dimensional sheet-like structures, thereby more significantly optimizing the particle size distribution and further improving the compaction density of the obtained lithium manganese iron phosphate material.

[0139] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0140] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing lithium manganese iron phosphate material, wherein the molar ratio of Mn to Fe in the lithium manganese iron phosphate material is (0.5-0.9):(0.1-0.5), characterized in that, The preparation method of the lithium manganese iron phosphate material includes: mixing and sintering the manganese iron phosphate precursor, the manganese iron pyrophosphate precursor, the lithium source, the carbon source and the organic additive in sequence to obtain the lithium manganese iron phosphate material. The manganese iron phosphate precursor is amorphous, and the microstructure of the manganese iron phosphate precursor is nanoscale particles. The crystal structure of the manganese iron pyrophosphate precursor is monoclinic, and the microstructure of the manganese iron pyrophosphate precursor is a two-dimensional sheet at the micrometer level. The organic additive is a polymer with anionic functional groups in its molecular structure; The sintering process includes sequentially performing first-stage sintering, second-stage sintering, third-stage sintering, fourth-stage sintering, and fifth-stage sintering.

2. The method for preparing lithium manganese iron phosphate material according to claim 1, characterized in that, The chemical formula of the manganese iron phosphate precursor is Mn. a Fe b PO4, where a is 0.5–0.8 and b is 0.2–0.5; and / or, The unit cell of the manganese iron pyrophosphate precursor belongs to the C2 / m(12) space group, and the chemical formula of the manganese iron pyrophosphate precursor is Mn. c Fe d P₂O₇, where c is 0.5–0.8 and d is 0.2–0.5; and / or, In the ferromanganese phosphate precursor and the ferromanganese pyrophosphate precursor, the ratio of the total molar amount of Mn and Fe to the molar amount of P is independently (0.965~1):

1.

3. The method for preparing lithium manganese iron phosphate material according to claim 1 or 2, characterized in that, The weight ratio of the manganese iron phosphate precursor to the manganese iron pyrophosphate precursor is (2-4):

1.

4. The method for preparing lithium manganese iron phosphate material according to any one of claims 1 to 3, characterized in that, The preparation process of the manganese iron phosphate precursor includes: mixing a first manganese source, a first iron source, a first phosphorus source and an oxidant, and then calcining at a holding temperature of 400℃~480℃ to obtain the manganese iron phosphate precursor.

5. The method for preparing lithium manganese iron phosphate material according to any one of claims 1 to 4, characterized in that, The preparation process of the manganese ferropyrophosphate precursor includes: mixing a second manganese source, a second iron source, a second phosphorus source and a reducing agent, and then calcining the mixture at a temperature of 600℃~800℃ to obtain the manganese ferropyrophosphate precursor.

6. The method for preparing lithium manganese iron phosphate material according to any one of claims 1 to 5, characterized in that, The organic additive is selected from one or more of sodium polystyrene sulfonate, polyacrylic acid, poly4-styrene sulfonic acid, and ammonium polyacrylate. The carbon source is selected from one or more of glucose, starch, sucrose, polyethylene glycol, cyclodextrin, and fructose; The lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium powder; Based on the total weight of the ferromanganese phosphate precursor and the ferromanganese pyrophosphate precursor as 100%, the amount of the organic additive added is 1% to 3%, and the amount of the carbon source added is 10% to 15%; and / or, Let A be the total molar amount of Mn and Fe in the manganese iron phosphate precursor; let B be the total molar amount of Mn and Fe in the manganese iron pyrophosphate precursor; let C be the molar amount of Li in the lithium source; A, B, and C satisfy: C / (A+B) = 1.02~1.

03.

7. The method for preparing lithium manganese iron phosphate material according to any one of claims 1 to 6, characterized in that, The sintering process includes, in sequence: a first-stage sintering at a holding temperature of 100℃ to 200℃, a second-stage sintering at a holding temperature of 400℃ to 500℃, a third-stage sintering at a holding temperature of 650℃ to 730℃, a fourth-stage sintering at a holding temperature of 750℃ to 780℃, and a fifth-stage sintering at a holding temperature of 650℃ to 730℃; the holding time for each of the first-stage, second-stage, third-stage, fourth-stage, and fifth-stage sintering processes is independently 10 min to 6 h.

8. A lithium manganese iron phosphate material, characterized in that, The lithium manganese iron phosphate material is prepared by the method for preparing lithium manganese iron phosphate material according to any one of claims 1 to 7; Preferably, the compaction density of the lithium manganese iron phosphate material is 2.41 g / cc to 2.70 g / cc.

9. A positive electrode sheet, comprising a positive electrode active material, characterized in that, The positive electrode active material is the lithium manganese iron phosphate material according to claim 8; or, the positive electrode active material is prepared by the preparation method of lithium manganese iron phosphate material according to any one of claims 1 to 7.

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

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