B / mg synergistically modified lithium iron manganese phosphate positive electrode material and preparation method and application thereof

Abstract

The application discloses B / Mg synergistically modified manganese iron lithium phosphate positive electrode material and a preparation method and application thereof. The method comprises the following steps: adding a carbon source and a dispersing agent into water to form a carbon source dispersion liquid; adding a lithium source, a manganese source, an iron source, a phosphorus source, a boron source and a magnesium source into the carbon source dispersion liquid and stirring and mixing; performing sand milling treatment, spray drying and sectional sintering, so as to obtain the modified positive electrode material. The positive electrode material takes manganese iron lithium phosphate as a main body, B elements are mainly distributed on the surface or the near-surface area of the material and form B-O bonds, boron oxides or borate structures, and are used for reducing the corrosion of an electrolyte on an active material; Mg elements participate in the regulation of a local lattice environment of the material and are used for stabilizing a Mn-O local coordination structure and relieving the Jahn-Teller distortion of Mn 3+ induced by Mn and Fe transition metals. Through the synergy of the B surface protection and the Mg lattice regulation, the application can reduce the charge transfer impedance, improve the lithium ion diffusion capacity, reduce the dissolution of Mn and Fe transition metals, and thus improve the rate performance, the high-temperature cycle stability and the comprehensive electrochemical performance of the manganese iron lithium phosphate positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a method for preparing a B / Mg synergistic modified lithium manganese iron phosphate cathode material, as well as the lithium manganese iron phosphate cathode material prepared by this method and a lithium-ion battery containing the material. Background Technology

[0002] With the rapid development of new energy vehicles and the energy storage industry, lithium-ion batteries are placing higher demands on the energy density, rate performance, cycle life, and safety of cathode materials. Lithium iron phosphate (LFP) materials have advantages such as low cost, high safety, and long cycle life, but their relatively low operating voltage limits further improvements in battery energy density. Lithium manganese iron phosphate (MnFeP) materials, by introducing the Mn element into the lithium iron phosphate structure, form MnFeP... 2+ / Mn 3+ High-voltage redox platforms can improve the working voltage and energy density of olivine phosphate materials while maintaining their structural stability and safety, thus showing promising application prospects.

[0003] However, lithium manganese iron phosphate materials still face some challenges in practical applications. Firstly, olivine-type phosphate materials exhibit low intrinsic electronic conductivity, and Li... + Mn primarily migrates along one-dimensional channels, limiting the material's rate performance. Secondly, during charge and discharge, Mn... 2+ Oxidized to Mn 3+ This can easily induce Jahn-Teller distortion, causing localized structural distortion and lattice stress accumulation in the MnO6 octahedron, thereby reducing the material's structural stability. Furthermore, under high voltage and high temperature environments, electrolyte side reactions intensify, and acidic substances such as HF easily corrode the Mn-O bonds on the material surface, promoting Mn dissolution and resulting in loss of active material, increased impedance, and capacity decay. Therefore, improving the structural stability, interfacial stability, and electrochemical reaction kinetics of lithium manganese iron phosphate materials is a crucial issue that urgently needs to be addressed in their application.

[0004] Existing modification methods mainly include carbon coating, ion doping, surface coating, and morphology control. Single modification methods typically only improve material properties in one aspect. For example, carbon coating can improve electronic conductivity, but its control over the local structural stability of Mn-O is limited; metal ion doping can improve lattice stability, but its barrier effect against surface electrolyte erosion is insufficient; surface coating can reduce interfacial side reactions, but if the coating layer is too thick, it may hinder Li... + Therefore, a synergistic modification method capable of simultaneously controlling the surface interface and bulk structure is needed to improve the overall performance of lithium manganese iron phosphate cathode materials.

[0005] To address the aforementioned problems, this invention proposes a B / Mg synergistic modification strategy: Utilizing B to form BO bonds, boron oxides, or borates on or near the material surface, an interfacial protective layer is constructed to reduce the direct erosion of the active material by the electrolyte; simultaneously, Mg is used to regulate the local coordination environment of Mn-O, mitigating the effects of Mn... 3+ Induced Jahn-Teller distortion, thereby achieving synergistic regulation of surface interface protection and bulk lattice stability. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing lithium manganese iron phosphate cathode materials in practical applications, such as insufficient electron / ion transport kinetics and Mn. 3+ To address issues such as induced Jahn-Teller distortion, Mn dissolution, and insufficient high-temperature cycling stability, this paper proposes a method for preparing B / Mg synergistically modified lithium manganese iron phosphate cathode materials. This method simultaneously introduces both B and Mg sources during the lithium manganese iron phosphate preparation process. B elements form BO bonds, boron oxides, or borates as protective structures on or near the material surface to reduce electrolyte erosion of the active material. Simultaneously, Mg elements regulate the local coordination environment of Mn-O, stabilizing the transition metal-oxygen octahedral structure and mitigating Mn dissolution. 3+ Induced Jahn-Teller distortion enables synergistic regulation of interface protection and lattice stability, thereby improving the rate performance, cycle stability and high-temperature electrochemical stability of the material.

[0007] In a first aspect, the present invention provides a method for preparing a B / Mg synergistic modified lithium manganese iron phosphate cathode material, specifically comprising the following steps: (1) Add the carbon source and dispersant to water, stir to dissolve, and prepare a carbon source dispersion; (2) Add lithium source, manganese source, iron source, phosphorus source, boron source and magnesium source to the carbon source dispersion, and mix them according to the stoichiometric ratio of lithium manganese iron phosphate (Li∶Mn∶Fe∶P=1∶(0.5~0.8)∶(0.2~0.5)∶1) to obtain a mixed slurry; (3) The mixed slurry obtained in step (2) is subjected to sand milling to refine the raw material particles and disperse them evenly. (4) Spray dry the sand-milled slurry obtained in step (3) to obtain precursor powder; (5) The precursor powder obtained in step (4) is sintered in an inert atmosphere and cooled to obtain B / Mg synergistic modified lithium manganese iron phosphate cathode material.

[0008] In step (1), the carbon source is selected from at least one of glucose, sucrose, and citric acid, and the dispersant is selected from at least one of polyethylene glycol and polyvinylpyrrolidone. Preferably, the carbon source is glucose and the dispersant is polyethylene glycol.

[0009] In step (2), the lithium source is selected from at least one of lithium dihydrogen phosphate, lithium hydroxide, and lithium carbonate; the manganese source is selected from at least one of manganese carbonate, manganese dioxide, manganese tetroxide, and manganese oxide; the iron source is selected from at least one of ferric phosphate, ferrous oxalate, and ferric oxide; and the phosphorus source is selected from at least one of ferric phosphate and lithium dihydrogen phosphate. Preferably, the manganese source is manganese carbonate, the iron source is ferric phosphate, and the lithium source includes lithium dihydrogen phosphate and lithium hydroxide monohydrate.

[0010] In step (2), the boron source is selected from at least one of boric acid, boron oxide, and borate, and the magnesium source is selected from at least one of magnesium oxide, magnesium carbonate, magnesium acetate, and magnesium nitrate. Preferably, the boron source is boric acid, and the magnesium source is magnesium oxide.

[0011] In step (2), the amount of boron source added is 0.6-3.0 wt.% of the theoretical mass of lithium manganese iron phosphate product, and the amount of magnesium source added is 1.2-3.0 wt.% of the theoretical mass of lithium manganese iron phosphate product. Preferably, the amount of boric acid added is 1.2 wt.% of the theoretical mass of lithium manganese iron phosphate product, and the amount of magnesium oxide added is the theoretical mass of LiMn. 0.6 Fe 0.4 2.4 wt.% of the PO4 product.

[0012] The sand milling process described in step (3) is carried out using a horizontal pin mill with a milling speed of 2000-3000 r·min. -1 The grinding time is 60-180 minutes. Preferably, the grinding speed is 2600 r·min. -1 The sand milling time is 120 minutes. Sand milling can improve the mixing uniformity of each component, refine the raw material particles, and provide a uniform precursor for subsequent spray granulation and high-temperature sintering.

[0013] The inlet temperature of the spray dryer in step (4) is 260-300℃, and the outlet temperature is 100-150℃. Spray drying can achieve rapid granulation and drying of slurry, so that the precursor forms a relatively complete spherical secondary particle structure.

[0014] The segmented sintering described in step (5) adopts a two-stage sintering process: the first stage is sintered at 2-5℃·min. -1 The temperature is raised to 500-560℃ and held for 1-3 hours for precursor pre-decomposition and preliminary carbonization of organic carbon sources; the second stage involves further heating to 700-780℃ and holding for 6-10 hours for olivine-type lithium manganese iron phosphate (e.g., LiMn).0.6 Fe 0.4 The formation of the PO4 crystal phase, the stabilization of the carbon layer, and the formation of the B / Mg synergistic modified structure are achieved. Preferably, the first stage is carried out at 3℃·min. -1 The temperature was raised to 530℃ and held for 2 hours. The second stage involved raising the temperature to 750℃ and holding for 8 hours.

[0015] The inert atmosphere is a nitrogen atmosphere or an argon atmosphere. Preferably, the inert atmosphere is a nitrogen atmosphere.

[0016] Secondly, this invention provides a B / Mg synergistic modified lithium manganese iron phosphate cathode material prepared according to the above method. The material is based on lithium manganese iron phosphate and includes a carbon coating layer, a B-containing surface protection structure, and a Mg-regulated Mn-O local structure. Specifically, B is mainly distributed on the surface or near-surface region of the material, existing as BO bonds, boron oxides, or borates, which reduces the direct erosion of the active material by the electrolyte; Mg participates in the regulation of the local lattice environment of the material, stabilizing the Mn-O local coordination structure and reducing Mn content. 3+ Induced Jahn-Teller distortion.

[0017] In this invention, the amounts of boron and magnesium sources can be adjusted within a certain range. Based on the theoretical mass of lithium manganese iron phosphate product, the amount of boron source added is 0.6-3.0 wt.%, and the amount of magnesium source added is 1.2-3.0 wt.%. Within this range, boron can form BO bonds, boron oxides, or borates as protective structures on or near the material surface, while magnesium can participate in regulating the local coordination environment of Mn-O, thereby achieving a synergistic effect of interface protection and lattice stability. More preferably, the boron source is boric acid, with an addition amount of 1.2 wt.%; the magnesium source is magnesium oxide, with an addition amount of 2.4 wt.%, and the resulting sample is denoted as LMFP / B-Mg3.

[0018] Preferably, the B / Mg synergistic modified lithium manganese iron phosphate cathode material has a first-cycle discharge specific capacity of 157.3 mAh·g at a 0.1C rate. -1 The initial coulombic efficiency was 98.7%, and the discharge specific capacity at 5C rate was 131.8 mAh·g. -1 The capacity retention rate after 200 cycles at 1C rate is 93.1%.

[0019] Thirdly, the present invention provides a lithium-ion battery cathode material, wherein the cathode material comprises the above-mentioned B / Mg synergistic modified lithium manganese iron phosphate cathode material as the cathode active material.

[0020] Fourthly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the above-mentioned lithium-ion battery positive electrode sheet.

[0021] Compared with the prior art, the present invention has at least one of the following beneficial effects: (1) This invention employs a sand milling-spray drying-segmented sintering process to prepare B / Mg synergistic modified lithium manganese iron phosphate cathode material, which is beneficial for achieving uniform mixing of raw materials, control of particle morphology, and uniform carbon layer coating. This process is relatively simple and can maintain the properties of olivine-type lithium manganese iron phosphate (e.g., LiMn). 0.6 Fe 0.4 Based on the stable main structure of PO4, the synergistic introduction of B and Mg elements has good potential for large-scale preparation.

[0022] (2) This invention introduces element B to form BO bonds, boron oxides, or borate structures on or near the surface of the material, thus constructing a surface protective layer. This protective structure can reduce the direct erosion of the active material by the electrolyte, reduce interfacial side reactions, and inhibit the dissolution of transition metals such as Mn and Fe to a certain extent, thereby improving the interfacial stability of the material.

[0023] (3) This invention introduces Mg element to regulate the local coordination environment of Mn-O, which helps to stabilize the transition metal-oxygen octahedral structure and reduce Mn. 3+ The induced Jahn-Teller distortion improves the structural stability of the material during charge and discharge, thereby enhancing its cycle performance.

[0024] (4) This invention achieves simultaneous improvement in interface stability and bulk structural stability through the synergistic effect of B surface protection and Mg lattice regulation. Within the range of B and Mg source addition amounts, B / Mg synergistic modification can improve the interfacial charge transfer and Li+ of lithium manganese iron phosphate materials to varying degrees. + Diffusion behavior, thereby improving the rate performance and cycle stability of the material.

[0025] (5) The B / Mg synergistic modified samples obtained within the range of B and Mg source addition amounts of the present invention can all regulate the electrochemical performance of lithium manganese iron phosphate materials to varying degrees; among them, the preferred embodiment LMFP / B-Mg3 has both high capacity, better rate performance and good cycle stability, with a first-cycle discharge specific capacity of 157.3 mAh·g at 0.1C. -1 The discharge specific capacity at a 5C rate is 131.8 mAh·g. -1 The capacity retention rate after 200 cycles at 1C is 93.1%. Meanwhile, this preferred embodiment exhibits less polarization growth and lower transition metal dissolution under high-temperature cycling conditions, indicating that the B / Mg synergistic modification method can effectively improve the overall electrochemical performance of lithium manganese iron phosphate cathode materials. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments are briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 The XRD patterns are of the lithium manganese iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2.

[0028] Figure 2 The diagram shows the 1C cycle performance of electrodes obtained using lithium manganese iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2.

[0029] Figure 3 The graph shows the rate performance of electrodes obtained using lithium manganese iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2. Detailed Implementation

[0030] The following examples illustrate the preparation method of B / Mg synergistically modified lithium manganese iron phosphate cathode material, using boric acid and magnesium oxide as preferred boron and magnesium sources, respectively. It should be noted that the amount of boron and magnesium source added is not limited to the single values ​​shown in the specific examples. Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention are commercially available or can be prepared using existing methods. The invention is further illustrated below with reference to specific examples. These examples are for illustrative purposes only and are not intended to limit the scope of protection of this invention.

[0031] Example 1 B / Mg synergistic modification of LiMn 0.6 Fe 0.4 The preparation of PO4 / C cathode material includes the following steps: (1) Preparation of carbon source dispersion: Add glucose and polyethylene glycol to 1L of deionized water and stir for more than 20 minutes to fully dissolve them and obtain a uniform carbon source dispersion; (2) Ingredient mixing: Iron phosphate is used as the iron source and part of the phosphorus source, manganese carbonate is used as the manganese source, and lithium dihydrogen phosphate and lithium hydroxide are used as the lithium source and supplementary phosphorus source. The ingredients are mixed according to the molar ratio of Li:Mn:Fe:P = 1:0.6:0.4:1, and boric acid and magnesium oxide are added as modifiers; wherein, the amount of boric acid added is the theoretical amount of LiMn 0.6 Fe 0.4 The PO4 product mass is 1.2 wt.%, and the magnesium oxide addition is the theoretical LiMn content. 0.6 Fe 0.42.4 wt.% of the PO4 product mass. Add each raw material to the carbon source dispersion obtained in step (1) and stir at high speed for 1 hour to obtain a mixed slurry; (3) Sand milling: The mixed slurry obtained in step (2) is fed into a horizontal rod mill and milled at 2600 r·min -1 Circulate and grind for 120 minutes to fully refine and evenly disperse the raw material particles; (4) Spray drying: The slurry obtained in step (3) is spray dried at an inlet temperature of 260-300℃ and an outlet temperature of 100-150℃ to obtain precursor powder; (5) Segmented sintering: The precursor powder obtained in step (4) is placed under a nitrogen atmosphere and sintered at 3°C·min. -1 The temperature was raised to 530℃ and held for 2 hours, then raised to 750℃ and held for 8 hours. After cooling, B / Mg synergistic modified LiMn was obtained. 0.6 Fe 0.4 The PO4 / C cathode material is denoted as LMFP / B-Mg3.

[0032] The LMFP / B-Mg3 sample obtained in Example 1 was tested and found to have a first-cycle discharge specific capacity of 157.3 mAh·g at 0.1C. -1 The initial coulombic efficiency was 98.7%; the discharge specific capacity at 5C rate was 131.8 mAh·g. -1 The capacity retention after 200 cycles at 1C was 93.1%. Electrochemical impedance spectroscopy results showed that the charge transfer impedance Rct of this sample was 57.5 Ω, and the Li... + The diffusion coefficient is 2.75 × 10⁻⁶. -14 Cm²·s -1 .

[0033] Example 2 B / Mg synergistic modification of LiMn with different Mg addition amounts 0.6 Fe 0.4 The preparation of PO4 / C cathode material includes the following steps: Except for adjusting the amount of magnesium oxide added to the theoretical LiMn 0.6 Fe 0.4 Except for 1.2 wt.% of the PO4 product, the remaining steps were the same as in Example 1 to obtain B / Mg synergistic modified LiMn. 0.6 Fe 0.4 The PO4 / C cathode material is denoted as LMFP / B-Mg1.

[0034] The LMFP / B-Mg1 sample was tested and found to have a first-cycle discharge specific capacity of 156.3 mAh·g at 0.1C. -1The initial coulombic efficiency was 98.0%; the discharge specific capacity at 5C rate was 116.5 mAh·g. -1 The capacity retention rate after 200 cycles at 1C rate is 93.1%.

[0035] Example 3 B / Mg synergistic modification of LiMn with different Mg addition amounts 0.6 Fe 0.4 The preparation of PO4 / C cathode material includes the following steps: Except for adjusting the amount of magnesium oxide added to the theoretical LiMn 0.6 Fe 0.4 Except for 1.8 wt.% of the PO4 product, the remaining steps were the same as in Example 1 to obtain B / Mg synergistic modified LiMn. 0.6 Fe 0.4 The PO4 / C cathode material is denoted as LMFP / B-Mg2.

[0036] The LMFP / B-Mg2 sample was tested and found to have a first-cycle discharge specific capacity of 158.6 mAh·g at 0.1C. -1 The initial coulombic efficiency was 98.8%; the discharge specific capacity at 5C rate was 118.2 mAh·g. -1 The capacity retention rate after 200 cycles at 1C rate is 82.4%.

[0037] Example 4 B / Mg synergistic modification of LiMn with different Mg addition amounts 0.6 Fe 0.4 The preparation of PO4 / C cathode material includes the following steps: Except for adjusting the amount of magnesium oxide added to the theoretical LiMn 0.6 Fe 0.4 Except for 3.0 wt.% of the PO4 product, the remaining steps were the same as in Example 1 to obtain B / Mg synergistic modified LiMn. 0.6 Fe 0.4 The PO4 / C cathode material is denoted as LMFP / B-Mg4.

[0038] The LMFP / B-Mg4 sample was tested and found to have a first-cycle discharge specific capacity of 156.6 mAh·g at 0.1C. -1 The initial coulombic efficiency was 98.6%; the discharge specific capacity at 5C rate was 137.2 mAh·g. -1 The capacity retention rate after 200 cycles at 1C rate is 87.5%.

[0039] Comparative Example 1 LiMn without B / Mg synergistic modification 0.6 Fe 0.4The preparation of PO4 / C cathode material includes the following steps: Without adding boric acid and magnesium oxide, the remaining steps are the same as in Example 1, yielding unmodified LiMn. 0.6 Fe 0.4 PO4 / C cathode material, denoted as LMFP.

[0040] The material exhibits a first-cycle discharge specific capacity of 152.8 mAh·g at 0.1C. -1 The initial coulombic efficiency was 98.1%; the discharge specific capacity at 5C rate was 117.4 mAh·g. -1 The capacity retention rate after 200 cycles at 1C rate is 90.7%.

[0041] Comparative Example 2 Single B-modified LiMn 0.6 Fe 0.4 The preparation of PO4 / C cathode material includes the following steps: Except for the addition of boric acid as a modifier and the omission of magnesium oxide, the remaining steps are the same as in Example 1; wherein, the amount of boric acid added is the theoretical LiMn 0.6 Fe 0.4 1.2 wt.% of the PO4 product yielded B-modified LiMn. 0.6 Fe 0.4 The PO4 / C cathode material is designated as LMFP-B2.

[0042] According to the test, the LMFP-B2 sample obtained in Comparative Example 2 had a first-cycle discharge specific capacity of 155.7 mAh·g at 0.1C. -1 The initial coulombic efficiency was 98.8%; the discharge specific capacity at 5C rate was 121.0 mAh·g. -1 The capacity retention rate after 200 cycles at 1C rate is 96.8%.

[0043] Compared with the LMFP / B-Mg3 obtained in Example 1, the LMFP-B2 obtained in Comparative Example 2 has better cycle stability, but its 5C rate discharge capacity is lower than that of LMFP / B-Mg3, indicating that the improvement of the rate performance and reaction kinetics of the material by B modification alone is still limited.

[0044] The lithium manganese iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2 and their electrochemical performance were characterized as follows: Figure 1 The XRD patterns of the lithium manganese iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2 are shown in comparison. Figure 1 It can be seen that the main diffraction peaks of each sample are all related to olivine-type LiMn. 0.6 Fe 0.4The PO4 standard phases match and belong to the orthorhombic Pnma space group. No obvious impurity phase diffraction peaks were observed, indicating that neither B modification nor B / Mg synergistic modification damaged the main crystal structure of lithium manganese iron phosphate material. Specifically, Comparative Example 1 is unmodified LiMn 0.6 Fe 0.4 The PO4 / C materials, Comparative Example 2 is a single B-modified material, and Examples 1-4 are B / Mg synergistic modified materials under different Mg addition conditions. All samples maintain a typical olivine-type phosphate structure, indicating that the introduction of boron and magnesium sources does not cause significant structural phase transitions.

[0045] The positive electrode materials obtained in Examples 1-4 and Comparative Examples 1-2 were mixed with conductive agent Super P and binder PVDF at a mass ratio of 8:1:1, respectively. N-methylpyrrolidone was added to prepare a positive electrode slurry, which was then uniformly coated onto aluminum foil. After drying and cutting, a positive electrode sheet with a diameter of 12 mm was obtained. Using lithium metal sheet as the counter electrode and LiPF6-based carbonate electrolyte, coin cells were assembled in a glove box, and constant current charge-discharge tests were performed within a voltage range of 2.0-4.5 V.

[0046] Figure 2 The graph shows the cycling performance of the electrodes obtained from the lithium manganese iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2 at a 1C rate. Figure 2 It can be seen that the unmodified LMFP sample in Comparative Example 1 exhibited significant capacity decay during cycling, with a capacity retention of 90.7% after 200 cycles at 1C. In Comparative Example 2, the capacity retention of the single-B modified sample LMFP-B2 increased to 96.8%, indicating that B modification can improve the material's cycling stability. This may be related to the formation of BO bonds, boron oxides, or borates as protective structures on or near the material surface by the B element. The LMFP / B-Mg3 sample obtained in Example 1 maintained a capacity retention of 93.1% after 200 cycles at 1C, demonstrating superior rate performance and reaction kinetics while maintaining good cycling stability. Although Examples 3 and 4 showed certain advantages in some capacity indicators, the cycle retention decreased significantly, indicating that excessive or inappropriate Mg addition may affect the long-term cycling stability of the material.

[0047] Figure 3 The graph shows the rate performance of the electrodes obtained from the lithium manganese iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2. Figure 3 It can be seen that the discharge specific capacity of each sample decreased to varying degrees with increasing rate, but the LMFP / B-Mg3 sample obtained in Example 1 showed better capacity retention at high rates. Comparative Example 1 had a discharge specific capacity of 117.4 mAh·g at 5C rate. -1 Comparative Example 2 has a capacity of 121.0 mAh·g -1In Example 1, the discharge specific capacity was increased to 131.8 mAh·g at a 5C rate. -1 This demonstrates that B / Mg synergistic modification can effectively improve the high-rate reaction kinetics of the material. Example 4 showed a discharge specific capacity of 137.2 mAh·g at 5C. -1 However, its cycle retention rate was lower than that of Example 1, indicating that while excessive Mg is beneficial for increasing rate capacity, it is detrimental to structural stability during cycling. Considering both rate performance and cycling performance, Examples 1-4 all fall within the specific implementation range of the B / Mg synergistic modification described in this invention. Different Mg addition amounts have different effects on material capacity utilization, rate performance, and cycle stability. Among them, Example 1 exhibits a good balance between capacity utilization, rate performance, and cycle retention rate, and can be considered one of the preferred embodiments of this invention.

[0048] The electrochemical properties of the materials obtained in the examples and comparative examples are shown in Table 1.

[0049] Table 1 As shown in Table 1, compared to Comparative Example 1, the first-cycle discharge specific capacity and cycle retention rate of the single B-modified sample in Comparative Example 2 are both improved, indicating that B modification can improve the capacity utilization and cycle stability of lithium manganese iron phosphate materials. This improvement may be related to the formation of BO bonds, boron oxide, or borate structures by B on or near the material surface. The resulting surface protective structure can weaken the direct erosion of the material surface by the electrolyte and reduce interfacial side reactions to a certain extent.

[0050] As shown in Table 1, Examples 1-4 all fall within the scope of the B / Mg synergistic modification described in this invention. Different amounts of Mg added have different effects on the material's capacity utilization, rate performance, and cycle stability. Specifically, the LMFP / B-Mg3 sample obtained in Example 1 exhibits a first-cycle discharge specific capacity of 157.3 mAh·g at 0.1C. -1 The specific capacity at 5C discharge is 131.8 mAh·g. -1 After 200 cycles at 1C, the capacity retention rate was 93.1%, demonstrating a good balance between capacity utilization, rate performance, and cycle retention, making it a preferred embodiment of the present invention. The performance differences in Examples 2, 3, and 4 indicate that the amount of Mg added within the limits defined by the present invention can adjust the electrochemical performance of the material; however, excessively low or high Mg additions may lead to a decrease in rate performance or cycle stability.

[0051] Analysis revealed that, within the B and Mg source addition ranges defined in this invention, B primarily reduces electrolyte erosion and interfacial side reactions by forming BO bonds, boron oxides, or borates as surface protective structures; while Mg primarily alleviates Mn degradation by regulating the local Mn-O coordination environment. 3+ Induced Jahn-Teller distortion. The synergistic effect of B and Mg achieves a combined optimization of surface interface protection and bulk structure stability, thereby improving the rate performance and cycle stability of lithium manganese iron phosphate cathode materials.

[0052] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a B / Mg synergistic modified lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: A carbon source and a dispersant are added to water and stirred to dissolve, thus preparing a carbon source dispersion. Lithium, manganese, iron, phosphorus, boron, and magnesium sources are added to the carbon source dispersion in a stoichiometric ratio according to the stoichiometric ratio of lithium manganese iron phosphate to obtain a mixed slurry; wherein the amount of boron source added is 0.6-3.0 wt.% of the theoretical mass of lithium manganese iron phosphate product, and the amount of magnesium source added is 1.2-3.0 wt.% of the theoretical mass of lithium manganese iron phosphate product. The mixed slurry is subjected to sand milling to refine the raw material particles and disperse them evenly. Then, the sand-milled slurry is spray-dried to obtain precursor powder. The precursor powder is sintered in segments under an inert atmosphere. The segmented sintering includes a first-stage pre-sintering and a second-stage high-temperature sintering. The first-stage pre-sintering temperature is 500-560℃ and the holding time is 1-3h. The second-stage high-temperature sintering temperature is 700-780℃ and the holding time is 6-10h. After cooling, B / Mg synergistic modified lithium manganese iron phosphate cathode material is obtained.

2. The method according to claim 1, characterized in that, The carbon source is selected from at least one of glucose, sucrose, and citric acid, and the dispersant is selected from at least one of polyethylene glycol and polyvinylpyrrolidone.

3. The method according to claim 1, characterized in that, The lithium source is selected from at least one of lithium dihydrogen phosphate, lithium hydroxide, and lithium carbonate; the manganese source is selected from at least one of manganese carbonate, manganese dioxide, manganese tetroxide, and manganese oxide; the iron source is selected from at least one of iron phosphate, ferrous oxalate, and iron oxide; and the phosphorus source is selected from at least one of iron phosphate and lithium dihydrogen phosphate.

4. The method according to claim 1, characterized in that, The boron source is selected from at least one of boric acid, boron oxide, and borate, and the magnesium source is selected from at least one of magnesium oxide, magnesium carbonate, magnesium acetate, and magnesium nitrate.

5. The method according to claim 1 or 4, characterized in that, The amount of boron source added is 0.9-1.8 wt.% of the theoretical mass of lithium manganese iron phosphate product, and the amount of magnesium source added is 1.8-3.0 wt.% of the theoretical mass of lithium manganese iron phosphate product.

6. The method according to claim 1, characterized in that, The sand milling process is carried out using a horizontal pin mill with a milling speed of 2000-3000 r·min. - ¹, The grinding time is 60-180 min.

7. The method according to claim 1, characterized in that, The spray dryer has an inlet temperature of 260-300℃ and an outlet temperature of 100-150℃.

8. The method according to claim 1, characterized in that, The first stage of pre-sintering is used for precursor pre-decomposition and preliminary carbonization of organic carbon sources, while the second stage of high-temperature sintering is used for the formation of olivine-type lithium manganese iron phosphate crystal phase, carbon layer stabilization, and the formation of B / Mg synergistic modified structure.

9. A B / Mg synergistic modified lithium manganese iron phosphate cathode material, characterized in that, The cathode material is prepared by the method described in any one of claims 1–8, wherein the cathode material is mainly composed of lithium manganese iron phosphate and includes a carbon coating layer, a boron-containing surface protection structure, and a Mg-regulated Mn-O local structure; wherein the boron-containing surface protection structure includes BO bonds, boron oxides, or borate structures.

10. A lithium-ion battery, characterized in that, The B / Mg synergistic modified lithium manganese iron phosphate cathode material as described in claim 9 is used as the cathode active material.

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