Lithium manganese iron phosphate positive electrode material, and preparation method therefor and use thereof

Abstract

A lithium manganese iron phosphate positive electrode material, which comprises first lithium manganese iron phosphate particles and second lithium manganese iron phosphate particles, wherein in the first lithium manganese iron phosphate particles, the molar ratio of Mn to Fe elements is greater than or equal to 1; and in the second lithium manganese iron phosphate particles, the molar ratio of Mn to Fe elements is less than or equal to that in the first lithium manganese iron phosphate particles. The particle size of the first lithium manganese iron phosphate particles is less than or equal to that of the second lithium manganese iron phosphate particles. A preparation method for the lithium manganese iron phosphate positive electrode material and the use thereof. A first precursor having a manganese content larger than or equal to the iron content thereof inhibits crystal growth during sintering, and has a small particle size; and a second precursor having a manganese-iron ratio less than or equal to the ratio in the first precursor is beneficial to crystal growth during sintering, and has a large particle size, thereby forming particle size grading, improving the space utilization rate of particle accumulation, and improving the compaction density and the volume specific capacity of the lithium manganese iron phosphate positive electrode material.

Description

Lithium manganese iron phosphate positive electrode material and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the Patent Office of China on May 31, 2023, with application number 202310637557.5 and application name “Lithium manganese iron phosphate positive electrode material, preparation method and application thereof”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the field of lithium battery technology, and specifically relates to lithium manganese iron phosphate positive electrode materials and preparation methods and applications thereof. Background Art

[0003] Lithium-ion battery cathode materials, especially cathode active materials, are crucial components of lithium-ion batteries and play a decisive role in their performance. For example, in lithium-ion batteries used in electric vehicles, the energy density of the cathode material determines the range of the electric vehicle, and energy density is determined by the specific capacity and voltage of the cathode material. Existing cathode materials struggle to achieve both high energy density and low cost. For example, ternary cathode materials offer a much higher energy density than lithium iron phosphate cathode materials, but they are expensive. While lithium iron phosphate is inexpensive, its theoretical capacity and voltage are relatively low (170 mAh / g, 3.4 V), making it difficult to meet the requirements for high range. As a product of the pursuit of a high-voltage platform, lithium manganese iron phosphate has a theoretical voltage of 4.1 V, and its theoretical energy density is much higher than that of lithium iron phosphate.

[0004] However, lithium manganese iron phosphate has a relatively obvious disadvantage. The lithium manganese iron phosphate positive electrode material prepared by conventional preparation methods has a low compaction density, resulting in a low volumetric capacity of the positive electrode material and the battery made from the positive electrode material, which hinders the practical application of the material. Technical issues

[0005] The purpose of this application is to overcome the above-mentioned deficiencies of the prior art and to provide a lithium iron manganese phosphate positive electrode material and a preparation method thereof, so as to solve the technical problems of low compaction density and low volume specific capacity of the existing lithium iron manganese phosphate positive electrode material. Technical Solutions

[0006] To achieve the above-mentioned application objectives, the first aspect of the present application provides a lithium iron manganese phosphate positive electrode material. The lithium iron manganese phosphate positive electrode material of the present application comprises first lithium iron manganese phosphate particles and second lithium iron manganese phosphate particles; in the first lithium iron manganese phosphate particles, the molar ratio of Mn to Fe is greater than or equal to 1; in the second lithium iron manganese phosphate particles, the molar ratio of Mn to Fe is less than or equal to the molar ratio of Mn to Fe in the first lithium iron manganese phosphate particles.

[0007] Optionally, in the lithium manganese iron phosphate positive electrode material, the Mn element accounts for 30% to 70% of the total molar amount of the Mn element and the Fe element.

[0008] Optionally, the chemical formula of the first lithium manganese iron phosphate particle is Li a1 Fe b1 Mn c1 (PO4) d1 , among which, 0.9≤a1≤1.1, 0<b1≤0.5, 0.5≤c1<1, 0.9≤d1≤1.1.

[0009] Optionally, in the first lithium manganese iron phosphate particles, 0.2≤b1≤0.4, and 0.6≤c1≤0.8.

[0010] Optionally, a1:(b1+c1):d1=1~1.08:0.95~1.05:1.

[0011] The particle size of the first lithium iron manganese phosphate particles is smaller than the particle size of the second lithium iron manganese phosphate particles.

[0012] Optionally, the particle size of the first lithium iron manganese phosphate particles is 20 nm to 350 nm.

[0013] Optionally, in the first lithium manganese iron phosphate particles, a molar ratio of the doping element M to the phosphorus element contained in the first lithium manganese iron phosphate particles is greater than 0 and less than or equal to 0.4:1.

[0014] Optionally, in the second lithium manganese iron phosphate particles, a molar ratio of Mn to Fe elements is less than or equal to 1.

[0015] Optionally, the chemical formula of the second lithium manganese iron phosphate particle is Li a2 Fe b2 Mn c2 (PO4) d2 , among which, 0.9≤a2≤1.1, 0.4≤b2<1, 0<c2≤0.6, 1≤d2≤1.1.

[0016] Optionally, in the second lithium manganese iron phosphate particles, 0.5≤b2≤0.8, 0.2≤c2≤0.5.

[0017] Optionally, a2:(b2+c2):d2=1~1.08:0.95~1.05:1.

[0018] Optionally, the particle size of the second lithium manganese iron phosphate particles is 300 nm to 800 nm.

[0019] Optionally, the mass ratio of the first lithium iron manganese phosphate particles to the second lithium iron manganese phosphate particles is 10:90 to 70:30.

[0020] Optionally, at least one of the first lithium iron manganese phosphate particles and the second lithium iron manganese phosphate particles further includes a doping element M.

[0021] Optionally, in the second lithium manganese iron phosphate particles, a molar ratio of the doping element M to the phosphorus element contained in the second lithium manganese iron phosphate particles is greater than 0 and less than or equal to 0.4:1.

[0022] Optionally, the doping element M includes at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn.

[0023] Optionally, the method further includes a first coating layer and a second coating layer, wherein the first coating layer coats the first lithium iron manganese phosphate particles, and the second coating layer coats the second lithium iron manganese phosphate particles.

[0024] Optionally, the thickness of the first cladding layer and the second cladding layer are the same or different and are between 1 nm and 10 nm.

[0025] Optionally, the material of at least one of the first coating layer and the second coating layer includes carbon.

[0026] Optionally, the mass percentage of carbon to the mass percentage of the lithium manganese iron phosphate positive electrode material is 1% to 3%.

[0027] Optionally, the hybridization of carbon includes at least one of SP2 and SP3 hybridization.

[0028] Optionally, in the first coating layer or the second coating layer, the hybridization mode of carbon includes SP2 and SP3 hybridization, and the molar ratio of the SP2 hybridized carbon element to the SP3 hybridized carbon element is greater than 0 and less than or equal to 5:1.

[0029] Optionally, the material of at least one of the first cladding layer and the second cladding layer further includes a doping element N.

[0030] Optionally, the doping element N includes at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn.

[0031] Optionally, the content of the doping element N in the first cladding layer or the second cladding layer is 0.01% to 4%.

[0032] The second aspect of the present application provides a method for preparing the lithium manganese iron phosphate positive electrode material of the present application. The method for preparing the lithium manganese iron phosphate positive electrode material of the present application comprises the following steps:

[0033] Performing a first mixing process on the first lithium iron manganese phosphate precursor particles and the second lithium iron manganese phosphate precursor particles to obtain a first mixture precursor;

[0034] Performing a first sintering treatment on the first mixture precursor in a first protective gas to obtain a first sintered product, wherein the first sintered product includes first lithium iron manganese phosphate particles and second lithium iron manganese phosphate particles;

[0035] Among them, the molar ratio of Mn to Fe elements in the first lithium manganese iron phosphate precursor particles is greater than or equal to 1, the molar ratio of Mn to Fe elements in the second lithium manganese iron phosphate precursor particles is less than or equal to the molar ratio of Mn to Fe elements in the first lithium manganese iron phosphate precursor particles, and the particle size of the first lithium manganese iron phosphate particles is less than or equal to the particle size of the second lithium manganese iron phosphate particles.

[0036] Optionally, the preparation method of the first lithium manganese iron phosphate precursor particles includes the following steps:

[0037] A first mixed solution including a first lithium source, a first manganese source, a first iron source, and a first phosphate source is prepared, and a first carbon source is added for precipitation to obtain first manganese iron lithium phosphate precursor particles.

[0038] Optionally, in the step of preparing the first mixed solution, the first lithium source, the first manganese source, the first iron source, and the first phosphate source are prepared according to the molar ratio of Li element contained in the first lithium source: the sum of Mn element contained in the first manganese source and Fe element contained in the first iron source: P element contained in the first phosphate source, that is, Li: (Mn+Fe): P of 1~1.08:0.95~1.05:1.

[0039] Optionally, the first carbon source is added so that the first carbon source accounts for 1% to 12% of the theoretical mass of the first lithium manganese iron phosphate particles.

[0040] Optionally, the first carbon source includes at least one of polyethylene glycol, cetyltrimethylammonium bromide, ethylene glycol, polyvinyl pyrrolidone, triethanolamine, salicylic acid, ethanol, methanol, citric acid, glucose, sucrose, soluble starch, oxalic acid, tartaric acid, fatty acid glyceride, and amino acid.

[0041] Optionally, the preparation method of the second lithium manganese iron phosphate precursor particles includes the following steps:

[0042] A second mixed solution including a second lithium source, a second manganese source, a second iron source, and a second phosphate source is prepared, and a second carbon source is added for precipitation to obtain second manganese iron lithium phosphate precursor particles.

[0043] Optionally, the second carbon source is added so that the second carbon source accounts for 1% to 12% of the theoretical mass of the second lithium manganese iron phosphate particles.

[0044] Optionally, in at least one of the steps of preparing the first mixed solution and preparing the second mixed solution, at least one of a source of a doping element M and an additive is added.

[0045] Optionally, the additive includes at least one of polyethylene glycol, cetyltrimethylammonium bromide, ethylene glycol, and ethylenediaminetetraacetic acid.

[0046] Optionally, a doping element M source is added in the step of preparing the first mixed solution, and the doping element M source is added at a molar ratio of the doping element M to the element P contained in the first phosphate source greater than 0 and less than or equal to 0.4:1.

[0047] Optionally, additives are added in the step of preparing the first mixed solution, and the additives are added so that the additives account for 1% to 20% of the theoretical mass of the first lithium manganese iron phosphate particles.

[0048] Optionally, a doping element M source is added in the step of preparing the second mixed solution, and the doping element M source is added according to a molar ratio of the doping element M to the element P contained in the second phosphate source of greater than 0 and less than or equal to 0.4:1.

[0049] Optionally, additives are added in the step of preparing the second mixed solution, and the additives are added so that the additives account for 1% to 20% of the theoretical mass of the second lithium manganese iron phosphate particles.

[0050] Optionally, the first protective gas includes at least one of N2, Ar, methane, and ethanol.

[0051] Optionally, the first sintering process includes at least one of the following conditions (1) to (3):

[0052] (1) Temperature is 200℃~750℃;

[0053] (2) heating to the temperature of the first sintering treatment at a heating rate of 1°C / min to 10°C / min;

[0054] (3) The holding time of the first sintering treatment is 10h to 25h.

[0055] Optionally, after the first sintering process, the following steps are further included:

[0056] performing a second mixing process on the first sintered product and the third carbon source to obtain a second mixture precursor;

[0057] The second mixture precursor is subjected to a second sintering process in a second protective gas.

[0058] Optionally, the third carbon source includes at least one of the first carbon source including polyethylene glycol, hexadecyltrimethylammonium bromide, ethylene glycol, polyvinyl pyrrolidone, triethanolamine, salicylic acid, ethanol, methanol, citric acid, glucose, sucrose, soluble starch, oxalic acid, tartaric acid, fatty acid glyceride, and amino acid.

[0059] Optionally, the third carbon source is subjected to a second mixing treatment according to a mass ratio of the third carbon source to the first sintered product of 1:99 to 15:85.

[0060] Optionally, the second protective gas includes at least one of N2, Ar, methane, and ethanol.

[0061] Optionally, in the second mixing step, a doping element N source is further added to the first sintered product and the third carbon source.

[0062] Optionally, the second sintering process includes at least one of the following conditions (1) to (3):

[0063] (1) Temperature is 500℃~800℃;

[0064] (2) heating to the temperature of the second sintering treatment at a heating rate of 1°C / min to 10°C / min;

[0065] (3) The second sintering treatment holding time is 10h to 25h.

[0066] Optionally, the doping element N source is added according to a mass ratio of the doping element N source to the first sintered product of 0.01% to 5%.

[0067] In a third aspect, the present application provides a positive electrode sheet. The positive electrode sheet includes a current collector and a positive electrode active layer bonded to the current collector, wherein the positive electrode active material contained in the positive electrode active layer includes the lithium iron manganese phosphate positive electrode material of the present application or the lithium iron manganese phosphate positive electrode material prepared by the preparation method of the present application.

[0068] In a fourth aspect of the present application, a secondary battery is provided. The secondary battery of the present application comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises the positive electrode sheet of the present application.

[0069] Compared with the existing technology, this application has the following technical effects:

[0070] The lithium iron manganese phosphate positive electrode material provided by the present application includes first lithium iron manganese phosphate particles and second lithium iron manganese phosphate particles with different manganese and iron ratios. The particle size of the first lithium iron manganese phosphate particles is less than or equal to the particle size of the second lithium iron manganese phosphate particles. The manganese content in the first lithium iron manganese phosphate particles is greater than or equal to the iron content, and the particle size of the particles is relatively small. The ratio of the manganese content to the iron content in the second lithium iron manganese phosphate particles is less than or equal to the ratio in the first lithium iron manganese phosphate particles, and the particle size of the particles is relatively large. In this way, a gradation of large and small particles is formed in the lithium iron manganese phosphate positive electrode material including the first lithium iron manganese phosphate particles and the second lithium iron manganese phosphate particles, which improves the spatial utilization of particle stacking, increases the compaction density of the lithium iron manganese phosphate positive electrode material of the present application, and increases the volumetric capacity.

[0071] The present application provides a method for preparing a lithium iron phosphate (LFP) cathode material. First LFP precursor particles and second LFP precursor particles with different manganese and iron ratios are mixed, and then subjected to a first sintering process to produce LFP cathode materials having two particle sizes. During the first sintering process, the first LFP precursor particles, because the molar content of manganese is greater than or equal to the molar content of iron, inhibit crystal growth, resulting in first LFP particles with a relatively smaller particle size. The second LFP precursor particles, because the ratio of the molar content of manganese to the molar content of iron is less than or equal to the ratio in the first LFP precursor particles, facilitate crystal growth, resulting in second LFP particles with a relatively larger particle size. This creates a large-small particle gradation, which is beneficial for improving the compacted density and volumetric capacity of the LFP cathode material. This preparation method is process-controllable, and the resulting LFP cathode material has stable properties.

[0072] Since the active material of the positive electrode sheet of the present application includes lithium iron manganese phosphate positive electrode material, the capacity is higher under the same volume compared with the positive electrode sheet made of lithium iron manganese phosphate positive electrode material in the prior art.

[0073] Since the secondary battery of the present application includes the positive electrode sheet of the above-mentioned application, the capacity is higher under the same volume compared with the secondary battery made of the positive electrode sheet made of lithium manganese iron phosphate positive electrode material in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0075] FIG1 is a schematic diagram of the structures of the first lithium iron manganese phosphate particle and the second lithium iron manganese phosphate particle of the present application;

[0076] FIG2 is a schematic structural diagram of the first lithium iron manganese phosphate particle and the second lithium iron manganese phosphate particle of the present application, both of which are coated with a coating layer;

[0077] FIG3 is an SEM image and particle size distribution diagram of particles contained in the lithium manganese iron phosphate positive electrode material provided in Example A10 of the present application;

[0078] Figure (a) is an SEM image of the first lithium iron manganese phosphate particle in Example A10 of the present application;

[0079] Figure (b) is a particle size distribution diagram of the first lithium iron manganese phosphate particles in Example A10 of the present application;

[0080] Figure (c) is an SEM image of the second lithium iron manganese phosphate particle in Example A10 of the present application;

[0081] Figure (d) is a particle size distribution diagram of the second lithium iron manganese phosphate particles in Example A10 of the present application;

[0082] FIG4 is an SEM image of the lithium manganese iron phosphate positive electrode material provided in Example A1 of the present application;

[0083] Reference numerals: 1 - first lithium iron manganese phosphate particle; 2 - first coating layer; 3 - second lithium iron manganese phosphate particle; 4 - second coating layer. Modes for Carrying Out the Invention

[0084] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present 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.

[0085] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0086] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0087] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0088] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0089] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the mass in the examples of this application may be μg, mg, g, kg, etc., which are mass units known in the chemical industry.

[0090] The terms "first" and "second" are used solely for descriptive purposes to distinguish objects, such as substances, from one another and should not be understood to indicate or imply relative importance or to implicitly specify the quantity of the technical features being referred to. For example, without departing from the scope of the embodiments of this application, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of such features.

[0091] In a first aspect, an embodiment of the present application provides a lithium iron manganese phosphate positive electrode material. The lithium iron manganese phosphate positive electrode material of the embodiment of the present application includes a first lithium iron manganese phosphate particle 1 and a second lithium iron manganese phosphate particle 3.

[0092] Among them, in the first lithium manganese iron phosphate particle 1, the molar ratio of Mn to Fe elements is greater than or equal to 1; in the second lithium manganese iron phosphate particle 3, the molar ratio of Mn to Fe elements is less than or equal to the molar ratio of Mn to Fe elements in the first lithium manganese iron phosphate particle 1; the particle size of the first lithium manganese iron phosphate particle 1 is less than or equal to the particle size of the second lithium manganese iron phosphate particle 3.

[0093] The lithium iron manganese phosphate positive electrode material provided in the embodiment of the present application includes a first lithium iron manganese phosphate particle 1 (hereinafter referred to as the first particle 1) and a second lithium iron manganese phosphate particle 3 (hereinafter referred to as the second particle 3) with different manganese and iron ratios. At the same time, as shown in Figure 1, the particle size of the first particle 1 is smaller than the particle size of the second particle 3. The particle size here refers to the actual particle size. Comparing the first particle 1 with the second particle 3, the manganese content in the first particle 1 is greater than or equal to the iron content, and the particle size of the particle is relatively small. The manganese-iron molar ratio in the second particle 3 is less than or equal to the manganese-iron molar ratio in the first particle 1, and the particle size of the particle is relatively large. In this way, a large and small particle gradation is formed in the lithium iron manganese phosphate positive electrode material including the first particle 1 and the second particle 3, which improves the space utilization rate of the particle stacking, improves the compaction density of the lithium iron manganese phosphate positive electrode material in the embodiment of the present application, and improves the volumetric capacity.

[0094] In some embodiments, the chemical formula of the first particle 1 can be Li a1 Fe b1 Mn c1 (PO4) d1, wherein 0.9≤a1≤1.1, 0<b1≤0.5, 0.5≤c1<1, 1≤d1≤1.1. In some embodiments, a1:(b1+c1):d1=1-1.08:0.95-1.05:1. The molar content of manganese in the first particles 1 is greater than or equal to the molar content of iron, and the molar contents of iron and manganese are further controlled within the above range. This can further reduce the particle size of the first particles 1 relative to the particle size of the second particles 3, thereby better forming a large-small particle gradation with the second particles 3, thereby increasing the compaction density of the lithium manganese iron phosphate positive electrode material of the present application embodiment and improving the volumetric capacity. In the exemplary example, the value of b1 may include but is not limited to 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5; the value of c1 may include but is not limited to 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95; in a further exemplary example, the value range of b1 may be 0.2≤b1≤0.4, and the value range of c1 may be 0.6≤c1≤0.8, so as to further improve the compaction density and volumetric capacity.

[0095] In some embodiments, the molar ratio of Mn to Fe elements in the second particles 3 can be less than or equal to 1, so that the molar ratio of Mn to Fe elements in the second particles 3 is always less than or equal to that in the first particles 1, thereby further making the particle size of the second particles 3 larger than the particle size of the first particles 1, forming a better gradation, and improving the compaction density and volumetric capacity of the material.

[0096] In some embodiments, the chemical formula of the second particle 3 can be Li a2 Fe b2 Mn c2 (PO4) d2 , wherein 0.9≤a2≤1.1, 0.4≤b2<1, 0<c2≤0.6, 1≤d2≤1.1. In some embodiments, a2:(b2+c2):d2=1-1.08:0.95-1.05:1. The molar ratio of manganese to iron in the second particles 3 is less than or equal to the molar ratio of manganese to iron in the first particles 1, and the molar contents of iron and manganese are further controlled within the above ranges. The particle size of the second particles 3 is larger than that of the first particles 1, thereby better forming a large-small particle gradation with the first particles 1, thereby increasing the compaction density of the lithium manganese iron phosphate positive electrode material of the present embodiment and improving the volumetric capacity. In the exemplary example, the value of b2 may include but is not limited to 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95; the value of c2 may include but is not limited to 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6; in a further exemplary example, the value range of b2 may be 0.5≤b2≤0.8, and the value range of c2 may be 0.2≤c2≤0.5, so as to further improve the compaction density and volumetric capacity.

[0097] In the lithium manganese iron phosphate positive electrode material of the embodiment of the present application, the Mn element can account for 30% to 70% of the total molar sum of the Mn element and the Fe element, that is, (c1+c2) / (b1+c1+b2+c2) is 0.3 to 0.7. By controlling the molar proportion of the manganese element in the sum of the manganese element and the iron element in the entire lithium manganese iron phosphate positive electrode material to be within this range, it is beneficial to form a better particle size grading of the first particles 1 and the second particles 3, and improve the compaction density and volume specific capacity of the material. In the exemplary embodiment, the proportion can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, and 70%.

[0098] It should be noted that a1, b1, c1, d1, a2, b2, c2, and d2 in the above chemical formula should be understood as the molar ratios of the corresponding elements in the material, and should not be understood from the perspective of chemical formula balancing.

[0099] In some embodiments, the mass ratio of the first particles 1 to the second particles 3 is between 10:90 and 70:30. Controlling the mass ratio of the first particles 1 to the second particles 3 within this range can better form a large-small particle size distribution between the first particles 1 with a relatively small particle size and the second particles 3 with a relatively large particle size, thereby improving the spatial utilization of the particle stacking, thereby further increasing the compaction density of the lithium manganese iron phosphate positive electrode material of the present application embodiment and increasing the volumetric capacity. In exemplary embodiments, this mass ratio may include, but is not limited to, 10:90, 20:80, 40:60, 60:40, and 70:30.

[0100] In some embodiments, the particle size of the first particle 1 can be 20nm to 350nm. In some embodiments, the particle size of the second particle 3 can be 300nm to 800nm, and the particle size here refers to the actual particle size. Controlling the particle size of the first particle 1 or the second particle 3 within these ranges can make the particle size of the first particle 1 and the second particle 3 more compatible, improve the grading effect of large and small particles, further improve the compaction density of the lithium manganese iron phosphate positive electrode material of the embodiment of the present application, and improve the volumetric capacity. In the exemplary embodiment, the particle size of the first particle 1 can include but is not limited to 20nm, 80nm, 120nm, 200nm, 300nm, 350nm; the particle size of the second particle 3 can include but is not limited to 300nm, 400nm, 500nm, 600nm, 700nm, 800nm.

[0101] In some embodiments, at least one of the first particles 1 and the second particles 3 further includes a doping element M; and in the first particles 1, the molar ratio of the doping element M to the phosphorus contained in the first particles 1 is greater than 0 and less than or equal to 0.4:1; in the second particles 3, the molar ratio of the doping element M to the phosphorus contained in the second particles 3 is greater than 0 and less than or equal to 0.4:1. By doping the doping element M into the lithium iron manganese phosphate positive electrode material, it can further modify the lithium iron manganese phosphate positive electrode material, improve the structural stability of the first particles 1 and the second particles 3, and improve the cycling stability of the material, which is conducive to the lithium iron manganese phosphate positive electrode material of the present application embodiment to exhibit the excellent performance of compacted density and volumetric capacity described above. In addition, the doping element M can also improve the electrochemical performance of the lithium iron manganese phosphate positive electrode material of the present application embodiment, making it better suitable for use in positive electrode materials and lithium batteries. The doping element M can be doped into the first particle 1 and / or the second particle 3. The doping amount of the doping element M in the first particle 1 or the second particle 3 can be independently doped with the phosphorus element contained in the first particle 1 or the second particle 3 in a certain molar ratio. In an exemplary embodiment, the molar ratio may include but is not limited to 0.05:1, 0.1:1, 0.2:1, 0.3:1, and 0.4:1.

[0102] In some embodiments, the doping element M may include at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn. These doping elements M have a better modification effect on the lithium manganese iron phosphate cathode material, further improving the structural stability, cycle stability, and electrochemical performance of the lithium manganese iron phosphate cathode material.

[0103] As shown in FIG2 , in some embodiments, the lithium manganese iron phosphate positive electrode material of the present application embodiment may further include a first coating layer 2 and a second coating layer 4 , wherein the first coating layer 2 coats the first particles 1 , and the second coating layer 4 coats the second particles 3 . By setting a coating layer, the coating layer coats the lithium manganese iron phosphate particles to form a core-shell structure (for the sake of convenience, the first coating layer 2 coating the first particle 1 as a whole is referred to as the first core-shell, and the second coating layer 4 coating the second particle 3 as a whole is referred to as the second core-shell). Such a core-shell structure can coat and modify the first particle 1 and the second particle 3, thereby improving the structural stability of the first particle 1 and the second particle 3, and the first coating layer 2 or the second coating layer 4 can modify the surface of the first particle 1 or the second particle 3, improve the surface morphology of the lithium manganese iron phosphate positive electrode material, so that the gaps between large particles can better accommodate small particles, which is more conducive to the formation of grading of particles of different particle sizes, thereby further improving the compaction density and volumetric specific capacity of the lithium manganese iron phosphate positive electrode material of the embodiment of the present application; in addition, according to the materials of the first coating layer 2 and the second coating layer 4, the electronic conductivity, ionic conductivity and other properties of the lithium manganese iron phosphate positive electrode material of the embodiment of the present application can also be improved, so as to be better applied to positive electrode materials and lithium batteries.

[0104] In some embodiments, the thickness of the first coating layer 2 and the second coating layer 4 can be the same or different, ranging from 1 nm to 10 nm. Controlling the thickness of the first coating layer 2 and the second coating layer 4 can improve the stability of the first particle 1 or the second particle 3, modify the surface of the first particle 1 or the second particle 3, and further improve the compaction density and volumetric capacity of the lithium manganese iron phosphate positive electrode material of the embodiment of the present application; it can also improve the coating modification effect of the coating layer, such as electronic conductivity, ionic conductivity and other properties. In exemplary embodiments, the thickness of the first coating layer 2 or the second coating layer 4 can include, but is not limited to, 1 nm, 3 nm, 5 nm, 7 nm, and 10 nm.

[0105] In some embodiments, at least one of the first coating layer 2 and the second coating layer 4 may include carbon. Carbon as a coating material can improve the structural stability and cycling stability of the lithium iron manganese phosphate cathode material of the present invention, modify the surface of the first particles 1 or the second particles 3, further increasing the compacted density and volumetric capacity of the lithium iron manganese phosphate cathode material of the present invention, and also improve the electronic conductivity of the material. In some embodiments, the mass percentage of carbon in the lithium iron manganese phosphate cathode material can be 1% to 3%. This can be understood as the total mass of the carbon in the first coating layer 2 and the second coating layer 4 accounting for 1% to 3% of the total mass of the lithium iron manganese phosphate cathode material. Alternatively, the carbon in the first coating layer 2 can account for 1% to 3% of the total mass of the first coating layer 2 and the first particles 1, or the carbon in the second coating layer 4 can account for 1% to 3% of the total mass of the second coating layer 4 and the second particles 3. Controlling the carbon mass ratio can further enhance these beneficial effects. In exemplary embodiments, this mass percentage may include, but is not limited to, 1%, 1.5%, 2%, 2.5%, and 3%.

[0106] In some embodiments, the carbon hybridization includes at least one of SP2 and SP3 hybridization. The hybridization pattern of interconnected carbon atoms in the coating layer is closely related to the material's compaction density, volumetric capacity, and other properties. SP2-hybridized carbon is beneficial for improving the coating effect, enhancing structural stability, increasing conductivity, and reducing resistivity. However, excessive content of SP2-hybridized carbon in the coating layer can slightly hinder lithium ion insertion and extraction, reducing the capacity. Conversely, SP3-hybridized carbon is beneficial for improving the material's capacity, but excessive content in the coating layer can affect the material's structural stability.

[0107] In some embodiments, the carbon hybridization in the first coating layer 2 or the second coating layer 4 includes SP2 and SP3 hybridization, and the molar ratio of the SP2-hybridized carbon element to the SP3-hybridized carbon element is greater than 0 and less than or equal to 5:1. Controlling the ratio of SP2 and SP3 hybridization of interconnected carbon atoms can further improve the structural stability of the material, while increasing the compacted density and volumetric capacity, and balancing these properties. In exemplary embodiments, the molar ratio may include, but is not limited to, 0.5:1, 1:1, 2:1, 3:1, 4:1, and 5:1.

[0108] In some embodiments, the material of at least one of the first coating layer 2 and the second coating layer 4 may further include the doping element N. Doping the doping element N into the coating layer can further improve the coating layer performance, enhance the stability of the lithium manganese iron phosphate cathode material of the present application embodiment, modify the particle surface morphology, increase the compaction density and volumetric capacity, and further improve the electronic conductivity of the lithium manganese iron phosphate cathode material of the present application embodiment. When the coating layer material contains carbon, the N element may exist in the form of a carbide in the carbon coating layer. In some embodiments, the doping element N may include at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn. The content of the doping element N in the first coating layer 2 or the second coating layer 4 may be 0.01% to 4%. In exemplary embodiments, its content may include, but is not limited to, 0.01%, 0.1%, 0.5%, 1%, and 4%. These doping elements N or their content impart to the coating layer the effect of further improving the performance of the lithium manganese iron phosphate cathode material of the present application embodiment.

[0109] In a second aspect, the present invention also provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0110] S10: performing a first mixing process on the first lithium iron manganese phosphate precursor particles (hereinafter referred to as the first precursor) and the second lithium iron manganese phosphate precursor particles (hereinafter referred to as the second precursor) to obtain a first mixture precursor;

[0111] S20: performing a first sintering treatment on the first mixture precursor in a first protective gas to obtain a first sintered product, the first sintered product including first particles 1 and second particles 3;

[0112] Among them, the molar content of manganese element in the first precursor is greater than or equal to the molar content of iron element, the molar ratio of manganese to iron in the second precursor is less than or equal to the molar ratio of manganese to iron in the first precursor, and the particle size of the first particle 1 is less than or equal to the particle size of the second particle 3 (referring to the actual particle size).

[0113] The preparation method of the embodiment of the present application comprises mixing a first precursor and a second precursor with different manganese-iron ratios, and then performing a first sintering process to produce a lithium iron manganese phosphate positive electrode material comprising first particles 1 and second particles 3. Thus, during the first sintering process, the first precursor, because the molar content of manganese is greater than or equal to the molar content of iron, inhibits crystal growth, resulting in first particles 1 with a relatively small particle size. The second precursor, because the molar ratio of manganese to iron is less than or equal to that of the first precursor, results in larger crystal growth, resulting in second particles 3 with a relatively larger particle size. This creates a gradation of large and small particles, which is beneficial for improving the compacted density and volumetric capacity of the resulting lithium iron manganese phosphate positive electrode material. This preparation method is process-controllable, and the resulting lithium iron manganese phosphate positive electrode material has stable properties.

[0114] Step S10:

[0115] The preparation of the first precursor or the preparation of the second precursor in step S10 may be a solid phase method, a liquid phase method, or the like. Taking the liquid phase method as an example, in some embodiments, step S10 may include the following steps:

[0116] Step S11: preparing a first mixed solution including a first lithium source, a first manganese source, a first iron source, and a first phosphate source, adding a first carbon source for precipitation to obtain a first precursor.

[0117] Step S12: preparing a second mixed solution including a second lithium source, a second manganese source, a second iron source, and a second phosphate source, adding a second carbon source for precipitation to obtain a second precursor.

[0118] The liquid phase method prepares the first precursor and the second precursor separately, so that the raw materials can be mixed evenly in the mixed solution to form a stable and uniform solution, and then the solid precursor particles are prepared by the first carbon source and the second carbon source, thereby improving the consistency of the prepared precursor particle materials.

[0119] Step S11:

[0120] The first lithium source may include but is not limited to at least one of lithium carbonate, lithium oxalate, lithium hydroxide, and lithium acetate; the first manganese source may include but is not limited to at least one of manganese chloride, manganese nitrate, and manganese sulfate; the first iron source may include but is not limited to at least one of ferrous chloride, ferrous sulfate, and ferrous nitrate; the first phosphate source may include but is not limited to at least one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0121] In some embodiments, in the step of preparing the first mixed solution, the first lithium source, the first manganese source, the first iron source, and the first phosphate source can be prepared according to the molar ratio of Li contained in the first lithium source: the sum of the Mn element contained in the first manganese source and the Fe element contained in the first iron source: the P element contained in the first phosphate source, that is, Li: (Mn + Fe): P of 1-1.08:0.95-1.05:1. Of course, the Mn element content should also be greater than or equal to the Fe element content, and the mixing ratio of the first manganese source and the first iron source can be further controlled with reference to the range of 0 < b1 ≤ 0.5, 0.5 ≤ c1 < 1 in the lithium manganese iron phosphate positive electrode material of the above embodiment. The control ratio relationship can be within the above range to obtain the desired first precursor, and the content of each element in the first precursor is stable. In an exemplary embodiment, the molar ratio of Li:(Mn+Fe):P may include, but is not limited to, (1 or 1.02 or 1.04 or 1.06 or 1.08):(0.95 or 0.98 or 1 or 1.02 or 1.05):1.

[0122] In addition, in the step of preparing the first mixed solution in step S11, at least one of a doping element M source and an additive may be added.

[0123] ① Adding a doping element M source can cause the first precursor to contain the doping element M, thereby allowing the first particles 1 produced after the first sintering process to contain the doping element M. This improves the stability of the first particles 1, embodying the advantages of high compaction density and high volumetric capacity, and further enhancing electrochemical performance. The doping element M in the doping element M source may include, but is not limited to, at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn. The doping element M source can be added in a molar ratio of the doping element M to the element P contained in the first phosphate source that is greater than 0 and less than or equal to 0.4:1. These doping elements and the amount of the doping element M source added can further enhance the aforementioned properties of the subsequently produced first particles 1. In exemplary embodiments, the molar ratio of M to P can include, but is not limited to, 0.05:1, 0.1:1, 0.2:1, 0.3:1, and 0.4:1.

[0124] ② Adding additives can reduce particle agglomeration during the liquid phase preparation of the first precursor and improve particle sphericity. After a subsequent first sintering treatment, first particles 1 with uniform particle size distribution and improved surface morphology can be formed, thereby further improving the compaction density and volumetric capacity of the resulting lithium manganese iron phosphate cathode material. Furthermore, the additives can be organic and can further form a carbon layer to enhance the coating modification effect. The additives can include at least one of PEG (polyethylene glycol), CATB (cetyltrimethylammonium bromide), EG (ethylene glycol), and EDTA (ethylenediaminetetraacetic acid). The additives can be added in an amount such that the additives account for 1% to 20% of the theoretical mass of the first particles 1. The theoretical mass refers to the calculated mass of the target product (e.g., the first particles 1 in this case). The additives are added based on this mass. These additives or the amounts added can further improve the performance of the additives during the preparation of the first precursor, thereby improving the compaction density, volumetric capacity, and electrochemical properties of the resulting lithium manganese iron phosphate cathode material. In exemplary embodiments, the percentage can be 1%, 5%, 10%, 15%, or 20%.

[0125] The first carbon source may include, but is not limited to, at least one of polyethylene glycol, hexadecyltrimethylammonium bromide, ethylene glycol, polyvinylpyrrolidone, triethanolamine, salicylic acid, ethanol, methanol, citric acid, glucose, sucrose, soluble starch, oxalic acid, tartaric acid, fatty acid glycerides, and amino acids. The first carbon source plays the role of dispersion, precipitation, and reduction. It can be used in a liquid phase method to precipitate the relevant solutes in the first mixed solution to obtain a solid first precursor for subsequent sintering. The first carbon source, as an organic matter, can form a carbon coating layer during the subsequent sintering to further improve the above-mentioned structural stability, cycle stability, compaction density, volumetric capacity, electrochemical performance, etc. of the prepared lithium manganese iron phosphate positive electrode material. The addition amount of the first carbon source can account for 1% to 12% of the theoretical mass of the first particle 1. In exemplary embodiments, the addition amount can be 1%, 5%, 8%, or 12%.

[0126] Finally, in the step of preparing the first mixed solution, the order of adding the doping element M source, the additive, and the first carbon source can be: after all other raw materials are added to form a stable mixed solution, the first carbon source is added thereto. The above is only an example of adding the first carbon source. If it is desired to ultimately produce a lithium manganese iron phosphate positive electrode material with particles having no coating layer, especially no carbon coating layer, other precipitation methods (such as heat precipitation) can be used, or the first precursor can be subjected to impurity removal treatment.

[0127] Step S12:

[0128] The preparation of the second precursor in step S12 can refer to the step of preparing the first precursor in the above-mentioned S11, with the only difference being that when preparing the second mixed solution, the ratio of the second manganese source to the second iron source should be less than or equal to the manganese-iron ratio in the first precursor according to the molar ratio of the Mn element contained in the second manganese source to the Fe element contained in the second iron source, so that the desired second precursor can be obtained. The ratio of the second manganese source to the second iron source can be further referred to in the above embodiment of the lithium manganese iron phosphate positive electrode material, 0.4≤b2<1, 0<c2≤0.6, and can be further controlled by referring to the range of 0.5≤b2≤0.8, 0.2≤c2≤0.5. As for the types and addition ratios of other raw materials, the types and addition ratios of the doping element M source, the types and addition ratios of the first carbon source, and the types and addition ratios of the additives, they can all refer to but are not limited to step S11, and the beneficial effects achieved are also the same.

[0129] Of course, the first precursor prepared in step S11 and the second precursor prepared in step S12 can be further crushed to control the particle size so as to be better used for the subsequent first sintering treatment. The particle size control can be performed on the two precursors independently before the first mixing treatment, or on the first mixture precursor after the first mixing treatment.

[0130] Step S20:

[0131] Step S20 is a step of subjecting the first mixture precursor to a first sintering treatment to obtain a first sintered product. In the first sintering treatment, the first precursor will inhibit crystal growth because the molar content of manganese is greater than or equal to the molar content of iron, and produce first particles 1 with relatively small particle size; the second precursor will be beneficial to crystal growth because the molar ratio of manganese to iron is less than or equal to the ratio in the first precursor, and the growth rate is higher than that of the first precursor, which will form a gradation of large and small particles, which is beneficial to improving the compaction density and volumetric capacity of the obtained lithium manganese iron phosphate positive electrode material.

[0132] In some embodiments, the first protective gas may include at least one of N2, Ar, methane, and ethanol. If a lithium iron manganese phosphate cathode material without a coating layer is to be prepared, the first protective gas may not include methane or ethanol (both in gaseous form). If a lithium iron manganese phosphate cathode material with a coating layer is to be prepared, methane and ethanol may be further included. By controlling parameters such as the gas flow rate and gas content, the thickness of the coating layer can be further regulated to further improve the coating modification effect of the coating layer and to help adjust the ratio of carbon SP2 hybridization to SP3 hybridization in the coating layer.

[0133] In some embodiments, the temperature of the first sintering treatment can be 200°C to 750°C; and / or, the holding time can be 10h to 25h; and / or, the temperature can be raised to the temperature of the first sintering treatment at a heating rate of 1°C / min to 10°C / min. By controlling the parameters of the first sintering treatment within these ranges, the rate and degree of crystal growth of the material in the first precursor and the second precursor during the first sintering treatment can be increased, thereby producing first particles 1 with smaller particle size and second particles 3 with larger particle size, forming a large and small particle gradation, thereby increasing the compaction density and volumetric specific capacity of the obtained lithium manganese iron phosphate positive electrode material. In addition, if the first precursor or the second precursor also includes a first carbon source and an organic matter as an additive, a carbon coating layer can also be formed through the first sintering treatment to further improve the stability and electrochemical properties of the obtained material. In an exemplary embodiment, the temperature may include, but is not limited to, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, and 750°C; the holding time may include, but is not limited to, 10 hours, 13 hours, 16 hours, 19 hours, 22 hours, and 25 hours; and the heating rate may include, but is not limited to, 1°C / min, 3°C / min, 5°C / min, 7°C / min, 9°C / min, and 10°C / min. Furthermore, if agglomerates are present after cooling after the first sintering process, the first sintered product may be pulverized by jet milling to obtain a first sintered product.

[0134] In some embodiments, after the first sintering process, the following steps may be further included:

[0135] S30: performing a second mixing process on the first sintered product and the third carbon source to obtain a second mixture precursor;

[0136] S40: performing a second sintering process on the second mixture precursor in a second protective gas.

[0137] Steps S30 and S40 can produce a lithium manganese iron phosphate cathode material containing a coating layer. Furthermore, the secondary sintering process can further improve the crystallinity and stability of the first sintered product, forming a core-shell structure and improving sphericity. This further enables the first particles 1 and the second particles 3 to form a more appropriate particle size distribution, thereby increasing the compacted density and volumetric capacity of the lithium manganese iron phosphate cathode material.

[0138] Step S30:

[0139] In step S30, the first sintered product and the third carbon source are subjected to a second mixing treatment, which can be performed by ball milling to fully and evenly mix the first sintered product and the third carbon source. During the ball milling process, the ball size is 0.3 mm to 5 mm, which can include but is not limited to 0.3 mm, 0.6 mm, 0.8 mm, 1 mm, 3 mm, 4 mm, and 5 mm. The ball milling can be performed to a particle size D50 (statistical particle size) of 0.2 μm to 0.6 μm. In the exemplary embodiment, the ball milling can be performed to include but is not limited to 0.2 μm to 0.5 μm and 0.5 μm to 0.6 μm.

[0140] In some embodiments, the third carbon source can include at least one of PEG (polyethylene glycol), CTAB (cetyltrimethylammonium bromide), EG (ethylene glycol), PVP (polyvinyl pyrrolidone), TEOA (triethanolamine), YS (salicylic acid), ethanol, methanol, citric acid, glucose, sucrose, soluble starch, oxalic acid, tartaric acid, fatty acid glycerides, and amino acids. The second mixing process can be performed at a mass ratio of the third carbon source to the first sintered product of 1:99 to 15:85. Controlling the third carbon source to these substances and the proportion of the third carbon source added can further improve the performance of the coating formed during the second sintering process, thereby further enhancing the stability, compaction density, volumetric capacity, and electrochemical properties of the resulting lithium manganese iron phosphate cathode material. In exemplary embodiments, the mass ratio can include, but is not limited to, 1:99, 5:95, 10:90, and 15:85. Furthermore, the type and amount of the third carbon source added can also adjust the ratio of SP2- to SP3-hybridized carbon in the coating layer of the resulting material.

[0141] In some embodiments, during the second mixing step, a doping element N source may be added to the first sintered product and the third carbon source. The doping element N source can cause the carbon coating layer formed in the subsequent second sintering process to include the doping element N, thereby further improving the structural stability, cycle stability, compaction density and volumetric capacity, and electronic conductivity of the material obtained above. The doping element N may include at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn. The doping element N source may be added in a mass ratio of 0.01% to 4% of the doping element N contained in the first sintered product.

[0142] Step S40:

[0143] The second sintering process in step S40 can sinter the third carbon source into a coating layer, further improve the crystallinity and stability of the first sintered product, form a core-shell structure, and improve sphericity, thereby further achieving a more appropriate particle size distribution between the first core-shell and the second core-shell, thereby increasing the compaction density and volumetric capacity of the resulting lithium manganese iron phosphate cathode material. To further enhance these effects, the second sintering process can be performed at a temperature of 500°C to 800°C; and / or the temperature can be increased to the second sintering process temperature at a heating rate of 1°C / min to 10°C / min; and / or the holding time can be 10 hours to 25 hours. In exemplary embodiments, the temperature may include, but is not limited to, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, and 800°C; the holding time may include, but is not limited to, 10 hours, 13 hours, 16 hours, 19 hours, 22 hours, and 25 hours; and the heating rate may include, but is not limited to, 1°C / min, 3°C / min, 5°C / min, 7°C / min, 9°C / min, and 10°C / min. The second protective gas may include at least one of N2, Ar, methane, and ethanol. Gaseous methane and ethanol have similar effects to those in the first protective gas. By controlling parameters such as gas flow rate and content, the thickness of the coating layer can be further controlled to further enhance the coating modification effect of the coating layer and assist in adjusting the ratio of carbon SP2 hybridization to SP3 hybridization in the coating layer. If the target product does not include a coating layer, or if control of the coating layer thickness is not required, the second protective gas may include at least one of N2 and Ar.

[0144] The product of the second sintering treatment in step S40 can be further processed by cooling and crushing, and the degree of crushing can make the particle size D50 less than 2μm, thereby obtaining a lithium manganese iron phosphate positive electrode material with high compaction density and high volumetric capacity.

[0145] Finally, in order to improve the preparation efficiency, the above embodiment is to mix the first precursor and the second precursor through steps S10 to S40, and then undergo a first sintering treatment, and further undergo a second mixing treatment and a second sintering treatment to obtain a lithium iron manganese phosphate positive electrode material. If the preparation efficiency is not considered, the first precursor and the second precursor can also be subjected to a first sintering treatment respectively, and each can be further subjected to a second mixing treatment and a second sintering treatment to obtain a first core-shell and a second core-shell, respectively, and then the first core-shell and the second core-shell are mixed to obtain a lithium iron manganese phosphate positive electrode material. In addition, the first precursor and the second precursor can also be subjected to a first sintering treatment respectively, and the first particles 1 and the second particles 3 of the first sintered products can be obtained respectively, and the first sintered products can be mixed, and then sintered according to S30 to S40 to obtain a lithium iron manganese phosphate positive electrode material containing a first core-shell and a second core-shell. These two preparation methods also belong to the contents recorded in this application.

[0146] In a third aspect, embodiments of the present application further provide a positive electrode sheet. The positive electrode sheet of the embodiments of the present application includes a current collector and a positive electrode active layer bonded to the current collector, wherein the positive electrode active material contained in the positive electrode active layer includes the lithium manganese iron phosphate positive electrode material of the above embodiments or the lithium manganese iron phosphate positive electrode material prepared by the preparation method of the above embodiments.

[0147] Since the active material of the positive electrode sheet of the embodiment of the present application includes lithium iron manganese phosphate positive electrode material, the capacity is higher at the same volume compared with the positive electrode sheet made of lithium iron manganese phosphate positive electrode material in the prior art.

[0148] The mass content of the lithium iron manganese phosphate positive electrode material of the above-mentioned embodiment of the present application in the positive electrode active layer can be designed based on the battery performance requirements. For example, in one embodiment, the content of the lithium iron manganese phosphate positive electrode material of the above-mentioned embodiment of the present application in the positive electrode active layer can be controlled to be 85% to 95%. The lithium iron manganese phosphate positive electrode material within this content range provides the positive electrode sheet with a higher capacity within the same volume while ensuring various performance characteristics.

[0149] In one embodiment, the positive electrode active layer includes, in addition to the lithium iron manganese phosphate positive electrode material, a binder and a conductive agent. In the embodiment, the binder can be a commonly used electrode binder, such as one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives. In the embodiment, the conductive agent can be a commonly used conductive agent, such as one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60 and carbon nanotubes. Of course, the positive electrode active layer can also contain other electrode active materials, such as one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium vanadium phosphate fluorophosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0150] In the embodiment, the positive electrode sheet preparation process can be: mixing the positive electrode active material, the conductive agent and the binder to obtain an electrode slurry, coating the electrode slurry on the positive electrode collector, and preparing the positive electrode sheet through steps such as drying, rolling, and die cutting.

[0151] In a fourth aspect, the embodiments of the present application further provide a secondary battery. The secondary battery of the embodiments of the present application includes a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet includes the positive electrode sheet of the embodiments above.

[0152] Since the secondary battery of the embodiment of the present application includes the positive electrode sheet of the embodiment above, it has a higher capacity at the same volume compared to the secondary battery made of the positive electrode sheet made of lithium manganese iron phosphate positive electrode material in the prior art.

[0153] The following describes the details in conjunction with specific embodiments.

[0154] 1. Lithium manganese iron phosphate positive electrode material and preparation method thereof

[0155] Example A1

[0156] This embodiment provides a lithium manganese iron phosphate positive electrode material, including first particles and second particles, both of which contain the doping element M of Ti; the Li:Fe:Mn:Ti:PO4 in the first particles is 1:0.3:0.695:0.005:1, and the particle size is 80nm~140nm (actual particle size); the Li:Fe:Mn:Ti:PO4 in the second particles is 1:0.595:0.4:0.005:1, and the particle size is 400nm~600nm (actual particle size); the mass ratio of the first particles to the second particles is 50:50.

[0157] It also includes a first coating layer and a second coating layer, the first coating layer coats the first particles, and the second coating layer coats the second particles. The thickness of the first coating layer and the second coating layer are both 5nm, and the materials are the same, both are carbon substrates, and the doping element N is Zr; the molar ratio of SP2 hybridized carbon atoms to SP3 hybridized carbon atoms is 3:1.

[0158] The preparation method of the lithium manganese iron phosphate positive electrode material in this embodiment includes the following steps:

[0159] S1: lithium hydroxide, ferric nitrate, manganese nitrate, titanium dioxide (doped element M is Ti), and ammonium dihydrogen phosphate are fully mixed in a solvent according to the molar ratio of Li:Fe:Mn:Ti:PO4 of 1:0.3:0.695:0.005:1 to form a stable first mixed solution, PEG (additive) at 2% of the theoretical mass of lithium manganese iron phosphate is added, and then glucose (first carbon source) at 7% of the theoretical mass of lithium manganese iron phosphate is added, and a solid first precursor is obtained by heating and precipitation;

[0160] Lithium hydroxide, ferric nitrate, manganese nitrate, titanium dioxide (doped element M is Ti), and ammonium dihydrogen phosphate are fully mixed in a solvent according to the molar ratio of Li:Fe:Mn:Ti:PO4 of 1:0.595:0.4:0.005:1 to form a stable second mixed solution, PEG (additive) of 2% of the theoretical mass of lithium manganese iron phosphate is added, and then glucose (second carbon source) of 7% of the theoretical mass of lithium manganese iron phosphate is added, and a solid second precursor is obtained by heating and precipitation;

[0161] The first precursor and the second precursor were crushed and samples were taken out from each other in a mass ratio of 50:50, and mixed to obtain a first mixture precursor;

[0162] S2: performing a first sintering treatment on the first mixture precursor in a N2 atmosphere at a sintering temperature of 600° C. and a holding time of 15 h to obtain a first sintered product;

[0163] S3: The first sintered product is fully mixed with 8% starch (third carbon source) and 0.3% zirconium oxide (doping element N source) by ball milling to form a second mixture precursor, and the mixture is ball milled to a particle size D50 of 0.2 μm to 0.5 μm;

[0164] S4: subjecting the second mixture precursor to a second sintering treatment in N2 at a sintering temperature of 700° C. and a holding time of 15 h to obtain a lithium manganese iron phosphate positive electrode material.

[0165] Example A2

[0166] This embodiment provides a lithium manganese iron phosphate positive electrode material, which differs from Example A1 in that the Li:Fe:Mn:Ti:PO4 in the first particle is 1:0.4:0.595:0.005:1, and the Li:Fe:Mn:Ti:PO4 in the second particle is 1:0.495:0.5:0.005:1.

[0167] The difference between the preparation method and Example A1 is that the addition amounts of manganese and iron elements are adjusted accordingly in step S1.

[0168] Example A3

[0169] This embodiment provides a lithium manganese iron phosphate positive electrode material, which differs from embodiment A1 in that the particle size of the first particles is 200 nm to 300 nm, and the particle size of the second particles is 800 nm to 1000 nm.

[0170] The difference between the preparation method and Example A1 is that the ball milling treatment in step S3 controls the particle size D50 of the second mixture precursor to be between 0.5μm and 0.6μm, so that the particle size (real particle size) of the first particles and the second particles finally obtained is within the above range.

[0171] Example A4

[0172] This embodiment provides a lithium manganese iron phosphate positive electrode material, which differs from Embodiment A1 in that the mass ratio of the first particles to the second particles is 30:70.

[0173] The difference between the preparation method and Example A1 is that in step S1, the first precursor and the second precursor are crushed and then adjusted to a mass ratio of 50:50 to 30:70, and samples are taken out and mixed.

[0174] Example A5

[0175] This embodiment provides a lithium manganese iron phosphate positive electrode material, which differs from embodiment A1 in that the first particles and the second particles do not contain the doping element Ti, the Fe:Mn ratio in the first particles is changed from 0.3:0.695 to 0.3:0.7, and the Fe:Mn ratio in the second particles is changed from 0.595:0.4 to 0.6:0.4.

[0176] The difference between the preparation method and Example A1 is that titanium dioxide is not added in step S1. Instead, corresponding manganese nitrate is added to the first mixed solution, and corresponding ferric nitrate is added to the second mixed solution.

[0177] Example A6

[0178] This embodiment provides a lithium manganese iron phosphate positive electrode material, which differs from Embodiment A1 in that the first coating layer and the second coating layer do not include the doping element Zr.

[0179] The preparation method differs from Example A1 in that zirconium oxide is not added in step S3.

[0180] Example A7

[0181] This embodiment provides a lithium manganese iron phosphate positive electrode material, which differs from Embodiment A1 in that the molar ratio between the SP2 hybridized carbon atoms and the SP3 hybridized carbon atoms is 8:1.

[0182] The difference between the preparation method and Example A1 is that 8% starch is added in step S3 to adjust the content to 3% starch and 5% polyethylene glycol.

[0183] Example A8

[0184] This embodiment provides a lithium manganese iron phosphate positive electrode material, which differs from Embodiment A1 in that it does not include a first coating layer or a second coating layer.

[0185] The preparation method differs from Example A1 in that the first carbon source, the second carbon source, and the additive PEG are not added in step S1, starch and calcium oxide are not added in step S3, and the ball milling treatment is only performed on the first sintered product.

[0186] Example A9

[0187] This embodiment provides a lithium manganese iron phosphate positive electrode material and a preparation method, which differs from Example A1 in that no PEG additive is added in step S1.

[0188] Example A10

[0189] This embodiment provides a lithium iron manganese phosphate cathode material and preparation method. This embodiment differs from embodiment A1 in that, in step S1, the first and second precursors, after being crushed, are taken out in a 50:50 mass ratio, and are not mixed. Instead, steps S2, S3, and S4 are performed separately. The third carbon source and calcium oxide added in step S3 are added to the first sintered product of the first precursor and the second sintered product of the second precursor, respectively, at 50% and 50% of the amounts added in embodiment A1. Finally, a first core-shell and a second core-shell are obtained, which are then mixed to obtain the lithium iron manganese phosphate cathode material.

[0190] Comparative Example A1

[0191] The lithium iron manganese phosphate positive electrode material provided in this comparative example is different from that in Example A1 in that it does not include the second core-shell but only the first core-shell.

[0192] The difference between the preparation method and Example A1 is that step S1 does not include the step of preparing the second mixed solution, and the addition amounts of the third carbon source and zirconium oxide in step S3 are changed to 50% of that in Example A1.

[0193] Comparative Example A2

[0194] The lithium iron manganese phosphate positive electrode material provided in this comparative example is different from that in Example A1 in that it does not include the first core-shell but only has the second core-shell.

[0195] The difference between the preparation method and Example A1 is that step S1 does not include the step of preparing the first mixed solution, and the addition amounts of the third carbon source and zirconium oxide in step S3 are changed to 50% of that in Example A1.

[0196] 2. Lithium-ion battery:

[0197] 2.1 The various lithium iron manganese phosphate positive electrode materials provided in Examples A1 to A10 and Comparative Examples A1 to A2 were used as the positive electrode active materials for lithium-ion batteries in Examples B1 to B10 and Comparative Examples B1 to B2, respectively, to prepare positive electrode sheets in accordance with the following method:

[0198] (1) Preparation of slurry: 2.33 kg of positive electrode active material, 0.012 kg of superconducting carbon black (SP), and 0.048 kg of binder polyvinylidene fluoride (PVDF) were added simultaneously into an agate ball mill, and then 1.6 kg of solvent N-methylpyrrolidone (NMP) was added. The mixture was ball milled at a speed of 360 r / min for 4 h to prepare a slurry.

[0199] (2) Slurry coating: Adjust the scale of the scraper of the coating machine, evenly apply the milled slurry on the aluminum foil, and place the coated electrode in a vacuum drying oven at 130°C for 3 hours;

[0200] (3) Rolling and punching: Place the aluminum foil coated with slurry flatly in the middle of the roller and roll the electrode; place the front of the rolled electrode close to the punched area and punch the electrode in sequence; the compaction density of the electrode is controlled at 2.0-2.6 g / cm 3 , with a diameter of 14 mm and a thickness of 0.05 to 0.10 mm; place the punched electrode in a vacuum drying oven at 130°C and bake for 3 hours;

[0201] Among them, the lithium iron manganese phosphate positive electrode material in the above Example A1 is used as the positive electrode active material contained in the battery Example B1, the lithium iron manganese phosphate positive electrode material in Example A2 is used as the positive electrode active material contained in the battery Example B2, and so on, and so on. The lithium iron manganese phosphate positive electrode material in the comparative example A1 is used as the positive electrode active material contained in the battery comparative example B1, and so on.

[0202] 2.2 Assemble button batteries. In a glove box, assemble the negative electrode shell, spring, steel sheet, lithium sheet, separator, positive electrode sheet, and positive electrode shell in this order. During the process, inject 10 μL of electrolyte. Then, use a sealing machine to seal the button batteries to obtain battery examples B1 to B10 and battery comparison examples B1 to B2, respectively.

[0203] 3. Related performance tests and result analysis

[0204] 1. Related tests of lithium manganese iron phosphate positive electrode materials

[0205] The lithium iron manganese phosphate cathode materials provided in Examples A1 to A10 and Comparative Examples A1 to A2, and the first core-shell and second core-shell in Example A10 were characterized by SEM, wherein the SEM images of the first core-shell and the first core-shell in Example A10 are shown in Figures (a) and (c) of Figure 3, respectively. It can be seen that the particle size of the first core-shell in Figure (a) is relatively small, and the particle size of the second core-shell in Figure (c) is relatively large. The SME image of the lithium iron manganese phosphate cathode material provided in Example A1 is shown in Figure 4, which shows that the first core-shell and the second core-shell form a gradation of large and small particle sizes, which improves the space utilization of particle stacking, and improves the compaction density and volumetric capacity.

[0206] 2. The particle size of the particles in the scanning electron microscope images of the lithium manganese iron phosphate positive electrode materials provided in Examples A1 to A10 was measured using ImageJ software. The results of Example A10 are shown in Figures (b) and (d) of Figure 3. It can be seen from the figure that the particle size of the first particle is about 113 nm and the particle size of the second particle is about 600 nm.

[0207] 3. The lithium iron manganese phosphate cathode materials provided in Examples A1 to A10 and Comparative Examples A1 to A2 were tested for compaction density using a compaction density meter and recorded in Table 1. The lithium ion batteries provided in Examples B1 to B10 and Comparative Examples B1 to B2 were tested for capacity, and the capacity test results are recorded in Table 1.

[0208] Table 1

[0209] According to Table 1, compared with Comparative Examples A1 and A2, Examples A1 to A10 obtain first particles and second particles with different manganese-iron ratios and different particle sizes through precursors with different manganese-iron ratios, wherein the manganese-iron ratio of the first particles is greater than or equal to 1, the manganese-iron ratio of the second particles is less than or equal to the manganese-iron ratio of the first particles, and the particle size of the first particles is less than or equal to the particle size of the second particles, thereby forming a particle size gradation and improving the compaction density and volumetric capacity.

[0210] The manganese-iron ratio of the first particle and the second particle in Example A1 is different from that in Example A2. The compaction density and capacity of Example A1 are higher. It can be seen that further controlling the manganese-iron ratio of the first particle and the second particle within a certain range is beneficial to improving the compaction density and capacity.

[0211] The particle sizes of the first and second particles in Example A1 are different from those in Example A3. Example A1 has higher compaction density and capacity. This shows that further controlling the particle sizes of the first and second particles within a certain range is beneficial to improving the compaction density and capacity.

[0212] The mass ratio of the first particles to the second particles in Example A1 is different from that in Example A4. Example A1 has higher compaction density and capacity. It can be seen that further controlling the mass ratio of the first particles to the second particles within a certain range is beneficial to improving the compaction density and capacity.

[0213] In Example A1, the first and second particles contain the doping element M, while in Example A5, they do not. Example A1 has higher compaction density and capacity, which shows that doping elements in the first and second particles is beneficial to improving compaction density and capacity.

[0214] The shells in Example A1 all contain the doping element N, but Example A6 does not. Example A1 has higher compaction density and capacity, which shows that doping elements in the shell is beneficial to improving the compaction density and capacity.

[0215] The ratio of SP2-hybridized carbon to SP3-hybridized carbon in the shell in Example A1 is different from that in Example A7. Example A1 has higher compaction density and capacity. It can be seen that adjusting the ratio of carbon hybridization in the shell is beneficial to improving compaction density and capacity.

[0216] Example A1 contains a coating layer, while Example A8 does not. Example A1 has higher compaction density and capacity. It can be seen that coating the particle surface to form a coating layer is beneficial to improving the compaction density and capacity.

[0217] In the preparation method of Example A1, additives are added in step S1, while in Example A9, no additives are added. The compacted density and capacity of Example A1 are higher. It can be seen that adding additives when preparing the mixture solution in step S1 is beneficial to improving the compacted density and capacity.

[0218] The preparation method of Example A10 is to prepare the first core-shell and the second core-shell separately, and finally mix them. Compared with the method of Example A1 for improving the preparation efficiency, the final compaction density and capacity are relatively close. Therefore, both preparation methods can improve the compaction density and capacity.

[0219] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A lithium manganese iron phosphate positive electrode material, characterized in that: The invention comprises a first lithium iron manganese phosphate particle and a second lithium iron manganese phosphate particle; in the first lithium iron manganese phosphate particle, the molar ratio of Mn to Fe element is greater than or equal to 1; in the second lithium iron manganese phosphate particle, the molar ratio of Mn to Fe element is less than or equal to the molar ratio of Mn to Fe element in the first lithium iron manganese phosphate particle; The particle size of the first lithium iron manganese phosphate particles is smaller than or equal to the particle size of the second lithium iron manganese phosphate particles.

2. The lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The chemical formula of the first lithium manganese iron phosphate particle is Li a1 Fe b1 Mn c1 (PO4) d1 , wherein 0.9≤a1≤1.1, 0<b1≤0.5, 0.5≤c1<1, 0.9≤d1≤1.1; and / or In the second lithium manganese iron phosphate particles, the molar ratio of Mn to Fe is less than or equal to 1; and / or The chemical formula of the second lithium manganese iron phosphate particle is Li a2 Fe b2 Mn c2 (PO4) d2 , among which, 0.9≤a2≤1.1, 0.4≤b2<1, 0<c2≤0.6, 1≤d2≤1.

1.

3. The lithium iron manganese phosphate positive electrode material according to claim 2, characterized in that: In the first lithium manganese iron phosphate particles, 0.2≤b1≤0.4, 0.6≤c1≤0.8; and / or a1:(b1+c1):d1=1-1.08:0.95-1.05:1; and / or In the second lithium manganese iron phosphate particles, 0.5≤b2≤0.8, 0.2≤c2≤0.5; and / or a2:(b2+c2):d2=1-1.08:0.95-1.05:1; and / or In the lithium manganese iron phosphate positive electrode material, the Mn element accounts for 30% to 70% of the total molar amount of the Mn element and the Fe element.

4. The lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The mass ratio of the first lithium iron manganese phosphate particles to the second lithium iron manganese phosphate particles is 10:90 to 70:30; and / or The particle size of the first lithium iron manganese phosphate particles is 20nm to 350nm; and / or The particle size of the second lithium manganese iron phosphate particles is 300nm to 800nm.

5. The lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: At least one of the first lithium iron manganese phosphate particles and the second lithium iron manganese phosphate particles further includes a doping element M; and In the first lithium manganese iron phosphate particles, a molar ratio of the doping element M to the phosphorus element contained in the first lithium manganese iron phosphate particles is greater than 0 and less than or equal to 0.4:1; and / or In the second lithium manganese iron phosphate particles, a molar ratio of the doping element M to the phosphorus element contained in the second lithium manganese iron phosphate particles is greater than 0 and less than or equal to 0.4:

1.

6. The lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 5, characterized in that: The doping element M includes at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn.

7. The lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: The invention also includes a first coating layer and a second coating layer, wherein the first coating layer coats the first lithium iron manganese phosphate particles, and the second coating layer coats the second lithium iron manganese phosphate particles.

8. The lithium iron manganese phosphate positive electrode material according to claim 7, characterized in that: The thickness of the first coating layer and the second coating layer are the same or different and are 1 nm to 10 nm; and / or The material of at least one of the first coating layer and the second coating layer includes carbon.

9. The lithium iron manganese phosphate positive electrode material according to any one of claims 7 to 8, characterized in that: The material of at least one of the first cladding layer and the second cladding layer further includes a doping element N; and / or The mass percentage of the carbon to the mass percentage of the lithium manganese iron phosphate positive electrode material is 1% to 3%; and / or The hybridization mode of the carbon includes at least one of SP2 and SP3 hybridization.

10. The lithium iron manganese phosphate positive electrode material according to claim 9, characterized in that: The doping element N includes at least one of Ti, V, Cr, Co, Ni, Ca, Nb, Mg, Zr, In, Na, Al, Cd, and Sn; and / or The content of the doping element N in the first cladding layer or the second cladding layer is 0.01% to 4%; and / or In the first coating layer or the second coating layer, the hybridization modes of the carbon include SP2 and SP3 hybridization, and the molar ratio of the SP2 hybridized carbon element to the SP3 hybridized carbon element is greater than 0 and less than or equal to 5:

1.

11. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: The steps include: Performing a first mixing process on the first lithium iron manganese phosphate precursor particles and the second lithium iron manganese phosphate precursor particles to obtain a first mixture precursor; Performing a first sintering treatment on the first mixture precursor in a first protective gas to obtain a first sintered product, wherein the first sintered product includes first lithium iron manganese phosphate particles and second lithium iron manganese phosphate particles; Among them, the molar ratio of Mn to Fe elements in the first lithium iron manganese phosphate precursor particles is greater than or equal to 1, the molar ratio of Mn to Fe elements in the second lithium iron manganese phosphate precursor particles is less than or equal to the molar ratio of Mn to Fe elements in the first lithium iron manganese phosphate precursor particles, and the particle size of the first lithium iron manganese phosphate particles is less than or equal to the particle size of the second lithium iron manganese phosphate particles.

12. The preparation method according to claim 11, characterized in that: The method for preparing the first lithium iron manganese phosphate precursor particles comprises the following steps: A first mixed solution including a first lithium source, a first manganese source, a first iron source, and a first phosphate source is prepared, and a first carbon source is added for precipitation to obtain first lithium iron manganese phosphate precursor particles; and / or The preparation method of the second lithium iron manganese phosphate precursor particles comprises the following steps: A second mixed solution including a second lithium source, a second manganese source, a second iron source and a second phosphate source is prepared, and a second carbon source is added for precipitation to obtain second lithium manganese iron phosphate precursor particles.

13. The preparation method according to claim 12, characterized in that: The first carbon source comprises at least one of polyethylene glycol, hexadecyltrimethylammonium bromide, ethylene glycol, polyvinylpyrrolidone, triethanolamine, salicylic acid, ethanol, methanol, citric acid, glucose, sucrose, soluble starch, oxalic acid, tartaric acid, fatty acid glyceride, and amino acid; and / or The first carbon source is added so that the first carbon source accounts for 1% to 12% of the theoretical mass of the first lithium manganese iron phosphate particles; and / or The second carbon source is added so that the second carbon source accounts for 1% to 12% of the theoretical mass of the second lithium manganese iron phosphate particles; and / or In at least one of the steps of preparing the first mixed solution and preparing the second mixed solution, at least one of a doping element M source and an additive is further added; and / or In the step of preparing the first mixed solution, the first lithium source, the first manganese source, the first iron source, and the first phosphate source are prepared according to the molar ratio of Li element contained in the first lithium source: the sum of Mn element contained in the first manganese source and Fe element contained in the first iron source: P element contained in the first phosphate source, that is, Li: (Mn+Fe): P of 1-1.08:0.95-1.05:

1.

14. The preparation method according to claim 13, characterized in that: In the step of preparing the first mixed solution, the doping element M source is added according to a molar ratio of the doping element M to the element P contained in the first phosphate source that is greater than 0 and less than or equal to 0.4:1; and / or In the step of preparing the second mixed solution, the doping element M source is added in a molar ratio of the doping element M to the element P contained in the second phosphate source of greater than 0 and less than or equal to 0.4:1; and / or The additive comprises at least one of polyethylene glycol, hexadecyltrimethylammonium bromide, ethylene glycol, and ethylenediaminetetraacetic acid; and / or In the step of preparing the first mixed solution, the additive is added so that the additive accounts for 1% to 20% of the theoretical mass of the first lithium manganese iron phosphate particles; and / or The additive is added in the step of preparing the second mixed solution, and the additive is added in such a manner that the additive accounts for 1% to 20% of the theoretical mass of the second lithium manganese iron phosphate particles.

15. The preparation method according to any one of claims 11 to 14, characterized in that: The first protective gas includes at least one of N2, Ar, methane, and ethanol; and / or The first sintering process includes at least one of the following conditions (1) to (3): (1) Temperature is 200℃~750℃; (2) heating the temperature to the temperature of the first sintering treatment at a heating rate of 1° C. / min to 10° C. / min; (3) The first sintering treatment heat preservation time is 10h to 25h.

16. The preparation method according to any one of claims 11 to 15, characterized in that: After the first sintering process, the following steps are also included: Performing a second mixing process on the first sintered product and a third carbon source to obtain a second mixture precursor; The second mixture precursor is subjected to a second sintering process in a second protective gas.

17. The preparation method according to claim 16, characterized in that: The third carbon source includes at least one of the first carbon source including polyethylene glycol, hexadecyltrimethylammonium bromide, ethylene glycol, polyvinylpyrrolidone, triethanolamine, salicylic acid, ethanol, methanol, citric acid, glucose, sucrose, soluble starch, oxalic acid, tartaric acid, fatty acid glyceride, and amino acid; and / or The third carbon source is subjected to a second mixing treatment according to a mass ratio of the third carbon source to the first sintered product of 1:99 to 15:85; and / or The second protective gas includes at least one of N2, Ar, methane, and ethanol; and / or In the second mixing step, a doping element N source is further added to the first sintered product and the third carbon source; and / or The second sintering process includes at least one of the following conditions (1) to (3): (1) Temperature is 500℃~800℃; (2) heating the temperature to the temperature of the second sintering treatment at a heating rate of 1° C. / min to 10° C. / min; (3) The second sintering treatment heat preservation time is 10h to 25h.

18. The preparation method according to claim 17, characterized in that: The doping element N source is added at a mass ratio of 0.01% to 5% of the doping element N source to the first sintered product.

19. A positive electrode sheet, comprising a current collector and a positive electrode active layer bonded to the current collector, characterized in that: The positive electrode active material contained in the positive electrode active layer includes the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 10 or the lithium iron manganese phosphate positive electrode material prepared by the preparation method according to any one of claims 11 to 18.

20. A secondary battery, comprising a positive electrode sheet and a negative electrode sheet, characterized in that: The positive electrode sheet includes the positive electrode sheet according to claim 19.

Images