A lithium manganese iron phosphate composite material, its preparation method and application

Abstract

This invention discloses a lithium manganese iron phosphate composite material, its preparation method, and its application, belonging to the field of lithium battery technology. The lithium manganese iron phosphate composite material includes boron-doped lithium manganese iron phosphate material and a boron-containing coating material coating the surface of the boron-doped lithium manganese iron phosphate material. A method for preparing the lithium manganese iron phosphate composite material is also provided, comprising the following steps: S1. Preparing a mixed solution A containing iron, manganese, phosphorus, and boron elements; S2. Adjusting the pH value of the mixed solution A; S3. Drying the mixed solution A to obtain a dried product B; S4. Mixing the dried product B with lithium elements and then calcining to obtain the lithium manganese iron phosphate composite material. The main purpose of this invention is to provide a lithium manganese iron phosphate composite material with good structural stability; moreover, it can be prepared using a simple sol-heat treatment process, and the one-step sintering preparation method has a simple process flow, suitable for large-scale industrial production.

Description

Technical Field

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

[0002] Currently, commercially available lithium-ion battery cathode materials mainly include lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate. Compared to the layered structure of lithium cobalt oxide and the spinel structure of lithium manganese oxide, the olivine structure of lithium iron phosphate has advantages such as stable operating voltage, higher theoretical capacity, structural stability, and better high-temperature performance. However, lithium iron phosphate has a relatively low voltage plateau (3.4V), resulting in a lower energy density. Lithium manganese iron phosphate has the same structure as lithium iron phosphate, retaining its advantages while also having a higher discharge plateau (4.1V) and higher energy density. Therefore, developing lithium manganese iron phosphate with excellent electrochemical performance as a cathode material for lithium-ion batteries has a very broad market prospect.

[0003] However, the conductivity and ion diffusion coefficient of lithium manganese iron phosphate (LMP) are both 1-2 orders of magnitude lower than those of lithium iron phosphate (LFP), resulting in poor rate performance. Furthermore, the mutual solubility of manganese and iron in LMP leads to crystal structure distortion during charge and discharge, potentially causing manganese leaching during battery cycling and reacting with the electrolyte, thus affecting battery cycle life. Existing technologies have reported methods to improve the electrochemical performance of LMP materials or composite materials through doping and coating, such as doping with metal elements or carbon coating, but unsatisfactory electrochemical performance remains a problem.

[0004] Therefore, there is an urgent need for a lithium manganese iron phosphate composite material with excellent rate performance and structural stability. Summary of the Invention

[0005] 1. The technical problem that the invention aims to solve

[0006] To address the issue of poor structural stability in existing lithium manganese iron phosphate composite materials, a lithium manganese iron phosphate composite material is provided.

[0007] 2. Technical Solution

[0008] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0009] The present invention provides a lithium manganese iron phosphate composite material.

[0010] The lithium manganese iron phosphate composite material includes boron-doped lithium manganese iron phosphate material and a boron-containing coating material covering the surface of the boron-doped lithium manganese iron phosphate material.

[0011] It should be noted that in boron-doped lithium manganese iron phosphate materials and boron-coated materials, boron atoms occupy the octahedral interstitial positions in the transition metal oxide crystal to form doping and bond with lattice oxygen, which improves the structural stability of lithium manganese iron phosphate composite materials and enhances their cycle performance.

[0012] On the other hand, boron-containing coating materials can separate boron-doped lithium manganese iron phosphate materials from the electrolyte, reducing side reactions with the electrolyte, effectively inhibiting manganese dissolution, and improving the stability of lithium manganese iron phosphate composite materials.

[0013] Preferably, the coating material forms a layer on the surface of the boron-doped lithium manganese iron phosphate material.

[0014] Preferably, the coating material forms a coating layer on the surface of the boron-doped lithium manganese iron phosphate material.

[0015] It should be noted that the coating material forms a coating layer on the surface of the boron-doped lithium manganese iron phosphate material, which is more conducive to separating the boron-doped lithium manganese iron phosphate material from the electrolyte, reducing side reactions with the electrolyte, effectively inhibiting manganese dissolution, and improving the stability of the lithium manganese iron phosphate composite material.

[0016] Furthermore, the boron-doped lithium manganese iron phosphate material has the chemical formula LiFe. 1-x-y Mn x B y PO4, where 0.01 < x < 0.9, 0.01 < y ≤ 0.1;

[0017] The boron-containing coating material has the chemical formula LiMn. 2-z B z O4, where 0.01 < z ≤ 0.1.

[0018] It should be noted that the specific chemical formula of the boron-doped lithium manganese iron phosphate material is as described above. Boron atoms occupy the octahedral interstitial positions in the transition metal oxide crystal to form doping and bond with lattice oxygen, thereby improving the structural stability of the lithium manganese iron phosphate composite material; the boron-coated material LiMn 2-z B z O4 is electrochemically active and can enhance lithium-ion or electron migration, thereby improving the rate performance of lithium manganese iron phosphate composite materials.

[0019] Preferably, the boron-doped lithium manganese iron phosphate material has the chemical formula LiFe. 1-x-y Mn x B y PO4, where 0.01 < x < 0.9, 0.01 < y < 0.1;

[0020] The boron-containing coating material has the chemical formula LiMn. 2-z Bz O4, where 0.01 < z < 0.1.

[0021] Furthermore, the molar amount of the boron-containing coating material accounts for the molar amount of the lithium manganese iron phosphate composite material, and the percentage is m, where 0.5% ≤ m ≤ 5%.

[0022] It should be noted that controlling the molar amount of boron-containing coating material relative to the molar amount of lithium manganese iron phosphate composite material within the above-mentioned range can enable the lithium manganese iron phosphate composite material to have better cycle performance.

[0023] Furthermore, the particle size of the lithium manganese iron phosphate composite material is D50≤15μm.

[0024] For example, it can be 15μm, 12μm, 9μm, 6μm, 3μm or 1μm.

[0025] It should be noted that the lithium manganese iron phosphate composite material has a relatively suitable particle size, which is beneficial to further improving the overall performance of the material.

[0026] Furthermore, the thickness of the boron-containing coating material is 0.1–3 mm.

[0027] This invention also provides a method for preparing a lithium manganese iron phosphate composite material, comprising the following steps:

[0028] S1. Prepare a mixed solution A containing iron, manganese, phosphorus and boron elements;

[0029] S2. Adjust the pH value of mixed solution A;

[0030] S3. Dry the mixed solution A to obtain the dried product B;

[0031] S4. The dried product B is mixed with lithium and then calcined to obtain a lithium manganese iron phosphate composite material.

[0032] It should be noted that by adopting a simple sol-heat treatment process, the preparation method using a one-step sintering process is simple and suitable for large-scale industrial production.

[0033] Further, in step S4, the calcination includes a first calcination and a second calcination performed sequentially.

[0034] The temperature of the first calcination is 400-600℃, the time is 3-8h, and the heating rate is 3℃ / min-10℃ / min;

[0035] The secondary calcination temperature is 600–900℃, the time is 8–20 h, and the heating rate is 3℃ / min–10℃ / min.

[0036] It should be noted that during high-temperature calcination, due to the small ionic radius of boron atoms, they easily diffuse into the crystal phase of boron-doped lithium manganese iron phosphate materials and boron-coated materials, making boron atoms uniformly distributed in both materials. In these materials, boron atoms occupy octahedral interstitial positions in the transition metal oxide crystals, forming dopants and bonding with lattice oxygen, thereby improving the structural stability of the lithium manganese iron phosphate composite material.

[0037] By performing a first calcination and a second calcination, and controlling the temperature, time, and heating rate of the first and second calcinations within the aforementioned ranges, boron atoms can diffuse into the crystal phase of boron-doped lithium manganese iron phosphate materials and boron-coated materials, thereby improving the structural stability of lithium manganese iron phosphate composite materials and exhibiting excellent rate capability and cycle performance.

[0038] For example, the temperature of the first calcination can be 400℃, 500℃ or 600℃, the time can be 3h, 5h, 7h or 8h, and the heating rate can be 3℃ / min, 5℃ / min, 7℃ / min or 10℃ / min.

[0039] The secondary calcination temperature can be 600℃, 700℃, 800℃ or 900℃, the time can be 8h, 10h, 12h, 14h, 16h, 18h or 20h, and the heating rate can be 3℃ / min, 5℃ / min, 7℃ / min or 10℃ / min.

[0040] Furthermore, the molar ratio of lithium, iron, manganese, phosphorus, and boron is (1–1.5):[(1-m)×(1-xy)]:[(1-m)×x+m×(2-z)]:1:[m×z+(1-m)×y], where 0.01 < x < 0.9, 0.01 < y ≤ 0.1, 0.01 < z ≤ 0.1, and 0.5 ≤ m ≤ 5%.

[0041] It should be noted that by keeping the molar ratio of lithium, iron, manganese, phosphorus and boron within the above range, the prepared boron-doped lithium manganese iron phosphate material and boron-coated material can have the above chemical formula, so as to obtain lithium manganese iron phosphate composite material with good stability and good rate performance.

[0042] In addition, by adjusting the molar ratio of lithium, iron, manganese, phosphorus and boron, the molar proportion of boron-containing coating material in lithium manganese iron phosphate composite materials can be controlled, thereby adjusting the performance of lithium manganese iron phosphate composite materials.

[0043] Further, in step S3, the chemical formula of the dried product B is Mn. 2-z B z (OH)2@Fe 1-x-y Mn x By PO4, where 0.01 < x < 0.9, 0.01 < y ≤ 0.1, and 0.01 < z ≤ 0.1.

[0044] Furthermore, in step S4, the lithium element is derived from one, two, or more of lithium carbonate, lithium hydroxide, and lithium oxalate.

[0045] Furthermore, in step S1, the iron element comes from one, two, or more of iron oxide, iron hydroxide, iron sulfate, and iron oxalate.

[0046] Furthermore, in step S1, the manganese element comes from one, two, or more of manganese carbonate, manganese sulfate, and manganese oxide.

[0047] Furthermore, in step S1, the phosphorus element is derived from one, two, or more of phosphoric acid, lithium dihydrogen phosphate, and lithium hydrogen phosphate.

[0048] Furthermore, in step S1, the boron element is derived from one or both of boric acid and boron oxide.

[0049] Further, step S1 can specifically be: dispersing the iron source, manganese source, phosphorus source and boron source into the solvent, stirring evenly to obtain a mixed solution A.

[0050] Furthermore, the stirring speed is 500–700 rpm;

[0051] The stirring can be carried out under heating conditions of 40-50°C.

[0052] Furthermore, the solvent of the mixed solution A is one or more of water and alcohol-based solvents.

[0053] Furthermore, the alcohol-based solvent is ethanol.

[0054] Furthermore, in step S2, the pH value of the mixed solution A is adjusted to 0 to 6.0 using acid.

[0055] It should be noted that adjusting the pH of mixed solution A to 0-6.0 can prevent the formation of metal hydroxide precipitates in mixed solution A.

[0056] Furthermore, the acid solution is one, two, or more of citric acid, sulfuric acid, hydrochloric acid, and nitric acid.

[0057] Furthermore, in step S3, the drying temperature is 110–200°C, and the drying time is 24–48 hours.

[0058] It should be noted that the mixed solution A can remove excess solvent by drying, and the dried raw material has better stability, which is beneficial to improving the effect of high-temperature calcination, so as to obtain a high-performance lithium manganese iron phosphate composite material.

[0059] In step S4, the mixing method is dry grinding, and the grinding time is 30 to 60 minutes.

[0060] For example, an agate mortar and pestle can be used for grinding.

[0061] Furthermore, both the primary and secondary calcinations are carried out under a protective atmosphere.

[0062] It should be noted that the protective atmosphere can protect the deposited products from oxidation or other chemical reactions during primary and secondary calcination.

[0063] Furthermore, the protective atmosphere can be an inert gas atmosphere.

[0064] For example, the inert gas atmosphere can be a nitrogen atmosphere.

[0065] The present invention also provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode and a negative electrode; the positive electrode comprising the above-mentioned lithium iron manganese phosphate composite material or the lithium iron manganese phosphate composite material prepared by the above method.

[0066] It should be noted that the positive electrode of the lithium battery includes the aforementioned lithium manganese iron phosphate composite material, which gives the lithium battery good electrochemical performance such as initial charge-discharge efficiency and cycle performance.

[0067] 3. Beneficial effects

[0068] Compared with the prior art, the technical solution provided by this invention has the following advantages:

[0069] (1) The lithium manganese iron phosphate composite material provided by the present invention includes boron-doped lithium manganese iron phosphate material and boron-containing coating material coated on the surface of boron-doped lithium manganese iron phosphate material. Boron atoms occupy the octahedral interstitial positions in the transition metal oxide crystal to form doping and bond with lattice oxygen, thereby improving the structural stability of the lithium manganese iron phosphate composite material and enhancing its cycle performance and rate performance.

[0070] On the other hand, boron-containing coating materials can separate boron-doped lithium manganese iron phosphate materials from the electrolyte, reducing side reactions with the electrolyte, effectively inhibiting manganese dissolution, and improving the stability of lithium manganese iron phosphate composite materials.

[0071] (2) The lithium manganese iron phosphate composite material provided by the present invention, LiMn 2-z B zO4 is electrochemically active and can enhance lithium-ion or electron migration, thereby improving the rate performance of lithium manganese iron phosphate composite materials.

[0072] (3) The preparation method of lithium manganese iron phosphate composite material provided by the present invention, during high-temperature calcination, due to the small ionic radius of boron atoms, easily diffuses into the crystal phase of boron-doped lithium manganese iron phosphate material and boron-coated material, occupies the octahedral interstitial position in the transition metal oxide crystal in the boron-doped lithium manganese iron phosphate material and boron-coated material to form doping and bond with lattice oxygen, thereby improving the structural stability of lithium manganese iron phosphate composite material.

[0073] (4) By adopting a simple sol-heat treatment process, the preparation method of one-step sintering is simple and suitable for industrial mass production.

[0074] (5) The lithium battery provided by the present invention has a positive electrode comprising the aforementioned lithium manganese iron phosphate composite material, which enables the lithium battery to have good electrochemical performance such as first charge and discharge efficiency and cycle performance. Detailed Implementation

[0075] This disclosure can be more readily understood by referring to the following description in conjunction with examples, all of which form part of this disclosure. It should be understood that this disclosure is not limited to the specific products, methods, conditions, or parameters described and / or shown herein. Furthermore, the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting, unless otherwise stated.

[0076] It should also be understood that, for clarity, certain features of this disclosure may be described herein in the context of individual embodiments, but may also be provided in combination with each other in individual embodiments. That is, unless obviously incompatible or specifically excluded, each individual embodiment is considered to be combinable with any other embodiment, and such combination is considered to represent another different embodiment. Conversely, for brevity, various features of this disclosure described in the context of individual embodiments may also be provided individually or in any sub-combination. Finally, while a particular embodiment may be described as part of a series of steps or part of a more general structure, each step or substructure may also be considered an independent embodiment in itself.

[0077] Unless otherwise stated, it should be understood that each individual element in the list and each combination of individual elements in the list will be interpreted as a different embodiment. For example, a list of embodiments denoted as "A, B, or C" should be interpreted as including embodiments "A", "B", "C", "A or B", "A or C", "B or C", or "A, B, or C".

[0078] When an item is described using the combined terms “...and / or ...", the description should be understood to include any one of the listed items and all combinations thereof.

[0079] Generally, the use of the term "about" indicates an approximation that can vary depending on the desired characteristics obtained from the disclosed subject matter and will be interpreted in a context-dependent manner based on function. Therefore, those skilled in the art will be able to interpret a degree of difference on a case-by-case basis. In some cases, the number of significant figures used when expressing a particular value can be a representative technique for determining the difference allowed by the term "about." In other cases, a gradient within a range of values ​​can be used to determine the range of differences allowed by the term "about." Furthermore, all ranges in this disclosure are inclusive and composable, and references to values ​​within a range include every value within that range.

[0080] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terms used herein and / or include any and all combinations of one or more of the associated listed items.

[0081] Unless otherwise specified in the following examples, the conditions were performed under standard conditions or as recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0082] The present invention will be further illustrated below with reference to specific embodiments, but these embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field. The essential features and significant effects of the present invention can be seen from the following embodiments. The described embodiments are some, but not all, embodiments of the present invention, and therefore do not limit the present invention in any way. Any non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are within the protection scope of the present invention.

[0083] Example 1

[0084] Ferric sulfate, manganese oxide, phosphoric acid, and boric acid were placed in a 500 mL beaker containing 300 mL of water at a molar ratio of 0.15:0.619:1:0.101. The beaker was then placed in a 45°C water bath and stirred at 600 rpm until homogeneous. The pH was adjusted to 5 with sulfuric acid. The beaker containing the mixture was then dried in a forced-air drying oven at 110°C for 30 h. Lithium carbonate was then ground with the above-mentioned ferric sulfate, manganese oxide, phosphoric acid, and boric acid at a molar ratio of 0.52:0.15:0.619:1:0.101 and the dried product using an agate mortar for 60 min. The resulting product was then placed in a porcelain boat and calcined in a tube furnace at a heating rate of 5°C / min at 450°C for 5 h and then at 600°C for 12 h under a nitrogen atmosphere to obtain the coated LiMn. 1.9 B 0.1 LiMn with a molar percentage of 1% O4 1.9 B 0.1 O4@LiFe 0.3 Mn 0.6 B 0.1 PO4 lithium manganese iron phosphate composite material.

[0085] The prepared lithium manganese iron phosphate composite material, conductive carbon black, and PVDF (polyvinylidene fluoride) were mixed with NMP (N-methylpyrrolidone) at a mass ratio of 96%:2%:2%, and then coated and rolled to prepare a positive electrode sheet. Separately, graphite, conductive carbon black, SBR (styrene-butadiene rubber), and CMC (carboxymethyl cellulose) were mixed with deionized water at a mass ratio of 95%:2%:2%:1%, and then coated and rolled to prepare a negative electrode sheet. The positive and negative electrode sheets were assembled into a battery, and the battery's electrochemical performance was tested.

[0086] Table 1. Initial charge-discharge cycle conditions in Example 1

[0087]

[0088]

[0089] After testing, the battery's initial charge-discharge efficiency was 90.5%. After 600 cycles at 1C, its capacity retention was 90.5%.

[0090] Example 2

[0091] Ferric sulfate, manganese oxide, phosphoric acid, and boric acid were placed in a 500 mL beaker containing 300 mL of water at a molar ratio of 0.15:0.638:1:0.102. The beaker was then placed in a 45°C water bath and stirred at 600 rpm until homogeneous. The pH was adjusted to 5 with sulfuric acid. The beaker containing the mixture was then dried in a forced-air drying oven at 110°C for 30 h. Lithium carbonate was then ground with the above-mentioned ferric sulfate, manganese oxide, phosphoric acid, and boric acid at a molar ratio of 0.55:0.15:0.638:1:0.102 and the dried product using an agate mortar for 60 min. The resulting product was then placed in a porcelain boat and calcined in a tube furnace at a heating rate of 5°C / min at 450°C for 5 h and then at 600°C for 12 h under a nitrogen atmosphere to obtain the coated LiMn. 1.9 B 0.1 LiMn with a molar percentage of 2% O4 1.9 B 0.1 O4@LiFe 0.3 Mn 0.6 B 0.1 PO4 lithium manganese iron phosphate composite material.

[0092] The prepared lithium manganese iron phosphate, conductive carbon black, and PVDF were mixed with NMP at a mass ratio of 96%:2%:2%, and then coated and rolled to form a positive electrode sheet. Separately, graphite, conductive carbon black, SBR, and CMC were mixed with deionized water at a mass ratio of 95%:2%:2%:1%, and then coated and rolled to form a negative electrode sheet. The positive and negative electrode sheets were assembled into a battery, and the battery's electrochemical performance was tested.

[0093] Table 2. Initial charge-discharge cycle conditions in Example 2

[0094]

[0095] After testing, the battery's initial charge-discharge efficiency was 91.0%. After 600 cycles at 1C, its capacity retention was 91.2%.

[0096] Example 3

[0097] Ferric sulfate, manganese oxide, phosphoric acid, and boric acid were placed in a 500 mL beaker containing 300 mL of water at a molar ratio of 0.15:0.657:1:0.103. The beaker was then placed in a 45°C water bath and stirred at 600 rpm until homogeneous. The pH was adjusted to 5 with sulfuric acid. The beaker containing the mixture was then dried in a forced-air drying oven at 110°C for 30 h. Lithium carbonate was then ground with the above-mentioned ferric sulfate, manganese oxide, phosphoric acid, and boric acid at a molar ratio of 0.58:0.15:0.657:1:0.103 and the dried product using an agate mortar for 60 min. The resulting product was then placed in a porcelain boat and calcined in a tube furnace at a heating rate of 5°C / min at 450°C for 5 h and then at 600°C for 12 h under a nitrogen atmosphere to obtain the coated LiMn. 1.9 B 0.1 LiMn with a molar percentage of 3% O4 1.9 B 0.1 O4@LiFe 0.3 Mn 0.6 B 0.1 PO4 lithium manganese iron phosphate composite material.

[0098] The prepared lithium manganese iron phosphate composite material, conductive carbon black, and PVDF were mixed with NMP at a mass ratio of 96%:2%:2%, and then coated and rolled to form a positive electrode sheet. Separately, graphite, conductive carbon black, SBR, and CMC were mixed with deionized water at a mass ratio of 95%:2%:2%:1%, and then coated and rolled to form a negative electrode sheet. The positive and negative electrode sheets were assembled into a battery, and the battery's electrochemical performance was tested.

[0099] Table 3. Initial charge-discharge cycle conditions in Example 3

[0100]

[0101] After testing, the battery's initial charge-discharge efficiency was 92.3%. After 600 cycles at 1C, its capacity retention was 92.5%.

[0102] Example 4

[0103] Ferric sulfate, manganese oxide, phosphoric acid, and boric acid were placed in a 500 mL beaker containing 300 mL of water at a molar ratio of 0.15:0.676:1:0.104. The beaker was then placed in a 45°C water bath and stirred at 600 rpm until homogeneous. The pH was adjusted to 5 with sulfuric acid. The beaker containing the mixture was then dried in a forced-air drying oven at 110°C for 30 h. Lithium carbonate was then ground with the above-mentioned ferric sulfate, manganese oxide, phosphoric acid, and boric acid at a molar ratio of 0.6:0.15:0.676:1:0.104 and the dried product using an agate mortar for 60 min. The resulting product was then placed in a porcelain boat and calcined in a tube furnace at a heating rate of 5°C / min at 450°C for 5 h and then at 600°C for 12 h under a nitrogen atmosphere to obtain the coated LiMn. 1.9 B 0.1 LiMn with a molar percentage of 4% O4 1.9 B 0.1 O4@LiFe 0.3 Mn 0.6 B 0.1 PO4 lithium manganese iron phosphate composite material.

[0104] The prepared lithium manganese iron phosphate composite material, conductive carbon black, and PVDF were mixed with NMP at a mass ratio of 96%:2%:2%, and then coated and rolled to form a positive electrode sheet. Separately, graphite, conductive carbon black, SBR, and CMC were mixed with deionized water at a mass ratio of 95%:2%:2%:1%, and then coated and rolled to form a negative electrode sheet. The positive and negative electrode sheets were assembled into a battery, and the battery's electrochemical performance was tested.

[0105] Table 4. Initial charge-discharge cycle conditions in Example 4

[0106]

[0107] After testing, the battery's initial charge-discharge efficiency was 90.1%. After 600 cycles at 1C, its capacity retention was 90.3%.

[0108] Comparative Example 1

[0109] Ferric sulfate, manganese oxide, and phosphoric acid were placed in a 500 mL beaker containing 300 mL of water at a molar ratio of 0.4:0.6:1. The beaker was then placed in a 45°C water bath and stirred at 600 rpm until homogeneous. The pH was adjusted to 5. The beaker containing the mixture was then dried in a forced-air drying oven at 110°C for 30 h. Lithium carbonate was then ground with the above-mentioned ferric sulfate, manganese oxide, phosphoric acid, and boric acid at a molar ratio of 0.5:0.4:0.6:1 and the dried product using an agate mortar for 60 min. The mixture was then placed in a porcelain boat and calcined in a tube furnace at a heating rate of 5°C / min at 450°C for 5 h and then at 600°C for 12 h under a nitrogen atmosphere to obtain LiFe. 0.4Mn 0.6 PO4 lithium manganese iron phosphate composite material.

[0110] The prepared lithium manganese iron phosphate composite material, conductive carbon black, and PVDF were mixed with NMP at a mass ratio of 96%:2%:2%, and then coated and rolled to form a positive electrode sheet. Separately, graphite, conductive carbon black, SBR, and CMC were mixed with deionized water at a mass ratio of 95%:2%:2%:1%, and then coated and rolled to form a negative electrode sheet. The positive and negative electrode sheets were assembled into a battery, and the battery's electrochemical performance was tested.

[0111] Table 5 shows the initial charge-discharge cycle conditions in Comparative Example 1.

[0112]

[0113] After testing, the battery's initial charge-discharge efficiency was 89.0%. After 600 cycles at 1C, the capacity retention was 88.6%, which may be because Mn dissolution during cycling damaged the SEI film on the negative electrode, leading to the loss of active lithium.

[0114] As can be seen from the data of the above embodiments and comparative examples, the lithium manganese iron phosphate composite material can effectively improve the electrochemical performance of the battery, such as the first charge-discharge efficiency and cycle performance.

Claims

1. A lithium manganese iron phosphate composite material, characterized in that: The lithium manganese iron phosphate composite material includes boron-doped lithium manganese iron phosphate material and a boron-containing coating material coating the surface of the boron-doped lithium manganese iron phosphate material. The boron-doped lithium manganese iron phosphate material has the chemical formula LiFe 1-x-y Mn x B y PO4, where 0.01 < x < 0.9, 0.01 < y ≤ 0.1; The boron-containing coating material has the chemical formula LiMn. 2-z B z O4, where 0.01 < z ≤ 0.1; The preparation method of the lithium manganese iron phosphate composite material includes the following steps: S1. Prepare a mixed solution A containing iron, manganese, phosphorus and boron elements; S2. Adjust the pH value of mixed solution A; S3. Dry the mixed solution A to obtain the dried product B; S4. The dried product B is mixed with lithium and then calcined to obtain a lithium manganese iron phosphate composite material.

2. The lithium manganese iron phosphate composite material according to claim 1, characterized in that: The molar amount of the boron-containing coating material accounts for the molar amount of the lithium manganese iron phosphate composite material, and 0.5%≤m≤5%.

3. The lithium manganese iron phosphate composite material according to claim 1, characterized in that: In step S4, the calcination includes a first calcination and a second calcination performed sequentially. The temperature of the first calcination is 400~600℃, the time is 3~8h, and the heating rate is 3℃ / min~10℃ / min; The secondary calcination temperature is 600~900℃, the time is 8~20h, and the heating rate is 3℃ / min~10℃ / min.

4. The lithium manganese iron phosphate composite material according to claim 1, characterized in that: The molar ratio of lithium, iron, manganese, phosphorus and boron in the lithium manganese iron phosphate composite material is (1~1.5):[(1-m)×(1-xy)]:[(1-m)×x+m×(2-z)]:1:[m×z+(1-m)×y], where 0.01<x<0.9, 0.01<y≤0.1, 0.01<z≤0.1, and 0.5≤m≤5%.

5. The lithium manganese iron phosphate composite material according to claim 3, characterized in that: In step S2, the pH of mixed solution A is adjusted to 0-6.0 using acid; and / or, In step S4, the mixing method is dry grinding, and the grinding time is 30~60 minutes.

6. The lithium manganese iron phosphate composite material according to claim 3 or 5, characterized in that: In step S4, the lithium element is derived from one or more of lithium carbonate, lithium hydroxide, and lithium oxalate; and / or, In step S1, the iron element comes from one or more of iron oxide, iron hydroxide, iron sulfate, and iron oxalate; and / or, In step S1, the manganese element is derived from one or more of manganese carbonate, manganese sulfate, and manganese oxide; and / or, In step S1, the phosphorus element is derived from one or more of phosphoric acid, lithium dihydrogen phosphate, and lithium hydrogen phosphate; and / or, In step S1, the boron element comes from one or both of boric acid and boron oxide.

7. The lithium manganese iron phosphate composite material according to claim 6, characterized in that: In step S4, both the primary and secondary calcinations are carried out under a protective atmosphere.

8. A lithium-ion battery, the lithium-ion battery comprising a positive electrode and a negative electrode, characterized in that: The positive electrode comprises the lithium iron manganese phosphate composite material as described in any one of claims 1 to 7.

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