Modified manganese iron lithium phosphate cathode material, preparation method and application thereof

Abstract

The application provides a modified lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. The chemical general formula of the modified lithium manganese iron phosphate positive electrode material is Li 1‑x Mg x (Mn 1‑a Fe a ) 1‑y Nb y (PO4) 1‑ z B z , wherein 0

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a modified lithium manganese iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries (LIBs), as the core technology of modern electrochemical energy storage systems, have deeply penetrated consumer electronics, electric vehicles, and large-scale energy storage fields due to their advantages such as high energy density, long cycle life, and low self-discharge rate. Today, the demand for energy density in power batteries is growing exponentially, which places higher demands on the iterative upgrading of key LIB material systems, especially cathode materials. As the "energy heart" of LIBs, cathode materials directly determine the battery's operating voltage, energy density, and cycle stability; breakthroughs in their performance have become a key bottleneck driving battery technology innovation.

[0003] Currently commercially available cathode materials mainly include layered lithium cobalt oxide (LiCoO2), spinel lithium manganese oxide (LiMn2O4), and ternary materials (LiNi). 0.8 Co 0.1 Mn 0.1 Lithium iron phosphate (LiFePO4, LFP) is a type of lithium iron phosphate battery. LFP holds a significant market share in the power battery and energy storage fields due to its excellent thermal stability, low cost, and ultra-long cycle life (>2000 cycles). However, limited by its inherent characteristics, LFP has a lower theoretical specific capacity (170mAh / g) and average operating voltage (3.45V vs. Li / Li⁺), resulting in a volumetric energy density of only about 580Wh / L, which is insufficient to meet the stringent range requirements of high-end electric vehicles. Furthermore, LFP's intrinsic electronic conductivity (10⁻⁶ mAh / g) is relatively low. -9 ~10 -10 S / cm) and lithium-ion diffusion coefficient (~10) -14 cm 2 ·s -1 Its extremely low efficiency significantly limits its rate performance and restricts the realization of fast charging technology.

[0004] To break through the performance ceiling of lithium manganese phosphate (LMP), which also belongs to the olivine structure, researchers have turned their attention to lithium manganese phosphate (LiMnPO4, LMP). LMP possesses a high delithiation potential of 4.1V (vs. Li / Li). + LMP (Liquid Polymer) has a theoretical energy density that is about 20% higher than that of LFP (Liquid Polymer), and is considered a candidate material for next-generation high-voltage cathodes. However, the electronic / ionic conductivity of LMP is further reduced compared to LFP (electronic conductivity <10). -11 S / cm), and Mn 3+The Jahn-Teller distortion effect of will cause lattice distortion and phase transition stress, leading to rapid capacity attenuation during the material cycling process. In lithium manganese iron phosphate (LiMn 1-x Fe x PO4, LMFP) in the original lithiated state, the transition metal sites are jointly occupied by Fe 2+ and Mn 2+ (ionic radius: Fe 2+ : 0.78Å, Mn 2+ : 0.83Å). The two form a homogeneous olivine-type structure (orthorhombic crystal system, Pnma space group) in the form of a solid solution. In this structure, the MnO6 / FeO6 octahedra and PO4 tetrahedra are connected by shared vertices to form a rigid three-dimensional framework. This structure not only suppresses oxygen release through strong P-O covalent bonds to ensure safety, but also weakens the Jahn-Teller effect by the introduction of Fe, and simultaneously widens the one-dimensional diffusion channel for Li⁺. During the delithiation process, Fe 2+ is preferentially oxidized to Fe 3+ at a potential of 3.4-3.5V (vs. Li / Li⁺), while Mn 2+ is partially oxidized to Mn 3+ at a higher potential (~4.1V). However, the latter is prone to induce lattice stress due to Jahn-Teller distortion, resulting in cycle capacity attenuation. Therefore, by adjusting the Fe / Mn ratio (0<x≤0.5), LMFP can balance the high-voltage characteristics (theoretical energy density ~650Wh / kg) and structural stability, but its actual energy density is limited by the reversibility of the Mn 3+ oxidation reaction, which is usually 550-620Wh / kg. In conclusion, the actual performance of LMFP is still limited by insufficient intrinsic conductivity and the difference in electrochemical activity of Mn / Fe sites.

[0005] In recent years, ion doping has been proven to be an effective means to improve the electrochemical performance of materials. For example, introducing large-radius cations at Li sites can expand the Li⁺ diffusion channel and reduce the migration energy barrier, or aliovalent metal doping at Mn / Fe sites can induce a charge compensation effect to generate carriers and improve electronic conductivity. However, most existing studies focus on single-element doping, which is difficult to synergistically optimize electron / ion transport and structural stability, resulting in the working voltage platform of LMFP fluctuating (3.4-4.1V) and the reversible capacity still being significantly lower than the theoretical value. In addition, the traditional doping process is prone to cause element segregation or lattice distortion, which affects the cycle life of the material.

[0006] Therefore, how to simultaneously solve the problems of low actual specific capacity and low utilization rate of the manganese oxidation platform of LMFP is an important research direction that needs to be urgently tackled in the field of cathode materials for lithium-ion batteries at present. Contents of the Invention

[0007] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a modified lithium iron manganese phosphate cathode material, its preparation method and application. The present invention designs a synergistic doping strategy of three elements: Mg (Li site substitution), Nb (Mn / Fe site substitution) and B (PO4 site substitution). On the basis of ensuring the structural integrity of the olivine, the intrinsic electronic / ionic conductivity and structural stability of LMFP are synchronously improved, and the diffusion channels for lithium ions are broadened, achieving a comprehensive performance breakthrough with high specific capacity, high voltage platform proportion, long cycle life and excellent rate performance, meeting the demand for high energy density batteries, providing a material basis for the development of high specific energy and high power lithium-ion batteries, enabling the battery to exhibit better performance in fast charging and fast discharging scenarios, and expanding its applicability in the field of high-power applications.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a modified lithium iron manganese phosphate cathode material, and the chemical general formula of the modified lithium iron manganese phosphate cathode material is Li 1-x Mg x (Mn 1-a Fe a ) 1-y Nb y (PO4) 1-z B z , where 0<x≤0.03, 0<y≤0.03, 0<z≤0.03, and 0<a≤0.5.

[0010] The present invention designs a synergistic doping strategy of three elements: Mg (Li site substitution), Nb (Mn / Fe site substitution) and B (PO4 site substitution). On the basis of ensuring the structural integrity of the olivine, the intrinsic electronic / ionic conductivity and structural stability of LMFP are synchronously improved, and the diffusion channels for lithium ions are broadened, achieving a comprehensive performance breakthrough with high specific capacity, high voltage platform proportion, long cycle life and excellent rate performance, meeting the demand for high energy density batteries, providing a material basis for the development of high specific energy and high power lithium-ion batteries, enabling the battery to exhibit better performance in fast charging and fast discharging scenarios, and expanding its applicability in the field of high-power applications.

[0011] In the present invention, 0<x≤0.03, for example, can be 0.01, 0.015, 0.02, 0.025 or 0.03, etc.; 0<y≤0.03, for example, can be 0.01, 0.015, 0.02, 0.025 or 0.03, etc.; 0<z≤0.03, for example, can be 0.01, 0.015, 0.02, 0.025 or 0.03, etc.; 0<a≤0.5, for example, can be 0.1, 0.2, 0.3, 0.4 or 0.5, etc.

[0012] This invention achieves the following synergistic effect by precisely controlling the doping ratio of the three elements: utilizing Mg 2+ Partially substituted Li site (ionic radius: Mg) 2+ =0.72Å, Li + =0.76Å), broadening the Li + A one-dimensional diffusion channel along the

[010] direction allows Li to... + Improved diffusion coefficient; utilizing Nb 5+ Partial substitution of Mn / Fe sites (ionic radius: Nb) 5+ =0.69Å, Mn 2+ =0.83Å), high-valence doping induces charge compensation, which is beneficial for generating electron-hole pairs, improving electronic conductivity, and simultaneously suppressing Mn. 3+ Jahn-Teller distortion; using B 3+ Partially replaces the PO4 bit (B) 3+ =0.23Å, P 5+ =0.31Å), forming a BO4-PO4 composite tetrahedral network, which enhanced structural rigidity and suppressed phase transition stress during cycling. Mg doping broadened the Li + Channels, Nb doping enhances electron transport, which can enable 4.1V (Mn 2+ / Mn 3+ The platform capacity contribution has increased to over 55%, significantly increasing the proportion of high-voltage platforms, and the operating voltage window has been expanded to 3.0-4.5V.

[0013] Preferably, the surface of the modified lithium manganese iron phosphate cathode material is further coated with a carbon layer.

[0014] This invention, through carbon layer coating, not only improves the conductivity of the material but also forms a synergistic regulation effect with tri-element doping. On the one hand, it synergistically suppresses Mn doping with Nb doping. 3+ Jahn-Teller distortion further enhances the cycling stability of the material. On the other hand, the three-dimensional conductive network constructed by the carbon layer can interact with the Li broadened by Mg in the bulk phase. + The diffusion channels and Nb-induced carrier enhancement effect work synergistically to accelerate interfacial charge transfer and lithium-ion transport. At the same time, the B-doped optimized PO4 tetrahedral structure enhances the manganese oxidation activity, significantly improving the utilization rate of the manganese oxidation platform.

[0015] Preferably, the thickness of the carbon layer is 4-10 nm, for example, it can be 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm or 10 nm.

[0016] Preferably, a = 0.3-0.4, for example, it can be 0.3, 0.32, 0.34, 0.36, 0.38 or 0.4, etc.

[0017] In a second aspect, the present invention provides a method for preparing the modified lithium manganese iron phosphate cathode material as described in the first aspect, the method comprising the following steps:

[0018] Solution A containing Nb, Mn, Fe, B, and P sources is provided; solution B containing Li and Mg sources is provided.

[0019] The A solution, B solution, dispersant, reducing agent and pH adjuster are mixed and subjected to a hydrothermal reaction to obtain the precursor precipitate.

[0020] The precursor precipitate was sintered to obtain the modified lithium manganese iron phosphate cathode material.

[0021] This invention introduces a dispersant into the preparation process because the dispersant can suppress the mixing of cations (Mn) after solution A and solution B are combined. 2+ Fe 2+ 、Nb 5+ Mg 2+ B 3+ The aggregation of P source and P source can ensure that each dopant element is uniformly dispersed in the reaction system, avoiding element segregation due to excessive local concentration, laying the foundation for uniform lattice occupancy of dopant elements in the subsequent precursor, and ensuring the synergistic optimization effect of bulk doping on electron / ion transport and structural stability.

[0022] This invention introduces a reducing agent into the preparation process because the reducing agent can maintain Fe 2+ Mn 2+ The stable price state, while ensuring Nb 5+ The valence state is stable in a hydrothermal environment. By maintaining a reducing environment during the co-precipitation stage, the valence states of Fe, Mn, and Nb in the precursor can be matched with the target lattice, promoting the formation of a precursor with accurate stoichiometry and uniform element distribution, thus laying the foundation for the electrochemical activity of the three-element synergistic doping.

[0023] This invention introduces a pH adjuster into the preparation process because it balances the various cations (Mn) by regulating the acidity and alkalinity of the reaction system. 2+ Fe 2+ 、Nb 5+ Mg 2+ B 3+ The hydrolysis-precipitation rate of the P source ensures that each dopant element participates uniformly in the construction of the precursor lattice, maintains the integrity of the olivine structure, enhances structural stability, and ultimately achieves a synergistic improvement in the overall performance of the material.

[0024] Preferably, the Nb source includes NbO(CH3COO)3 and C 10 H5NbO 20Any one or a combination of at least two of Nb2O5, NbOPO4, or (NH4)2(Nb(C2O4)4)·2H2O.

[0025] Preferably, the Mn source is any one or a combination of at least two of the following: MnSO4·H2O, MnCO3, Mn(NO3)2, Mn(PO4)3, MnO2, Mn3O4, MnC2O4, MnCl2, MnCl3, or (CH3COO)2Mn.

[0026] Preferably, the Fe source includes a divalent iron source, which includes any one or a combination of at least two of FeSO4·7H2O, Fe(NO3)2, Fe(PO4)2, Fe(CO3)2, FeCl2, FeC2O4, (CH3COO)2Fe, or FeO.

[0027] Preferably, the B source includes H3BO3, NaBO2, KBO2, NH4BO2, B2O3, Na2B4O7·10H2O, K2B4O7, Li2B4O7, LiBO2·2H2O, C3H9BO3, or C6H 15 Any one or at least two of BO3.

[0028] Preferably, the P source includes any one or a combination of at least two of H3PO4, NH4H2PO4, Li4H2PO4, (NH4)2HPO4, (NH4)3PO4, Li3PO4, or P2O5.

[0029] Preferably, the Li source includes any one or a combination of at least two of LiOH·H2O, Li2CO3, CH3COOLi, Li3PO4, LiH2PO4, Li2HPO4, LiNO3, Li2C2O4 or Li2SO4.

[0030] Preferably, the Mg source includes any one or a combination of at least two of Mg(NO3)2·6H2O, MgSO4·H2O, MgSO4·7H2O, or MgSO4.

[0031] Preferably, the dispersant includes any one of surfactant dispersants, organic solvent dispersants, or polymeric dispersants.

[0032] Preferably, the surfactant dispersant includes any one or a combination of at least two of CTAB (hexadecyltrimethylammonium bromide), PAA (polyacrylic acid), or PAANH4 (ammonium polyacrylate).

[0033] Preferably, the organic solvent dispersant includes any one or a combination of at least two of methanol, ethylene glycol, or glycerol.

[0034] Preferably, the polymeric dispersant includes PVP (polyvinylpyrrolidone) and / or PEG (polyethylene glycol).

[0035] Preferably, the reducing agent includes ascorbic acid and / or sodium ascorbate.

[0036] Preferably, the pH adjuster comprises any one or a combination of at least two of ammonia, sulfamic acid, citric acid, fumaric acid, succinic acid, adipic acid, maleic acid, ammonium hydroxide, sodium hydroxide, or potassium hydroxide.

[0037] Preferably, the amount of dispersant added is 5-20 wt%, based on the total mass of the element sources in solution A and solution B, for example, 5 wt%, 10 wt%, 15 wt%, or 20 wt%. It should be noted that the element sources in solution A refer to the sum of Nb, Mn, Fe, B, and P sources, and the element sources in solution B refer to the sum of Li and Mg sources.

[0038] Preferably, the amount of reducing agent added is 1-10 wt%, based on the total mass of the element sources in solution A and solution B, for example, it can be 1 wt%, 3 wt%, 5 wt%, 7 wt%, or 10 wt%.

[0039] Preferably, the amount of pH adjuster added meets the following requirements: the pH of the mixed solution system is 6.5-7.5, for example, it can be 6.5, 6.8, 7, 7.2 or 7.5, etc.

[0040] Preferably, the temperature of the hydrothermal reaction is 180-220℃, for example, 180℃, 190℃, 200℃, 210℃ or 220℃, and the heat preservation time is 18-22h, for example, 18h, 19h, 20h, 21h or 22h.

[0041] Preferably, before the precursor precipitate is sintered, it undergoes a carbon coating pretreatment, the steps of which include:

[0042] (a) Provide a C solution containing the precursor precipitate and a D solution containing a carbon source.

[0043] (b) The C solution and the D solution are mixed so that the carbon source is dispersed around and on the surface of the precursor precipitate particles, and the product to be sintered is obtained after drying.

[0044] Preferably, the carbon source includes organic carbon.

[0045] Preferably, the organic carbon includes any one or a combination of at least two of glucose, sucrose, fructose, cellulose, maltose, citric acid, soluble starch, salicylic acid, oxalic acid, polyacrylic acid, polyethylene glycol, or dopamine.

[0046] Preferably, the mass ratio of the carbon source in solution D to the precursor precipitate in solution C is (0-0.5):1, for example, it can be 0.1:1, 0.2:1, 0.3:1, 0.4:1 or 0.5:1, etc.

[0047] Preferably, the atmosphere for the sintering process is an inert atmosphere. For example, it may be a nitrogen atmosphere or an argon atmosphere.

[0048] Preferably, the gas flow rate in the inert atmosphere is 50-150 cm⁻¹. 3 / min, for example, could be 50cm 3 / min, 75cm 3 / min, 100cm 3 / min, 125cm 3 / min or 150cm 3 / min etc.

[0049] Preferably, the sintering process includes first-stage sintering and second-stage sintering performed by sequentially increasing the temperature.

[0050] Preferably, the temperature of the first-stage sintering is 250-400℃, for example, 250℃, 300℃, 350℃ or 400℃, and the holding time is 0.5-4h, for example, 0.5h, 1h, 2.5h, 2h, 2.5h, 3h, 3.5h or 4h.

[0051] This invention performs first-stage sintering within the aforementioned parameter range, which facilitates the full decomposition and pre-carbonization of the carbon source. This prevents the precursor powder from splashing out of the crucible due to rapid vaporization and expansion caused by excessively high temperatures, which could potentially damage the already formed material structure. This process enables the carbon source to initially form a uniform conductive network, laying the foundation for the formation of a continuous conductive network at subsequent high temperatures, and contributing to improved electronic conductivity of the final three-element synergistic doped material. Furthermore, this process removes residual moisture and water of crystallization, preventing rapid evaporation of moisture during subsequent high-temperature stages from damaging the material structure.

[0052] Preferably, the temperature of the second-stage sintering is 500-800℃, for example, 500℃, 600℃, 700℃ or 800℃, and the holding time is 3-8h, for example, 3h, 4h, 5h, 6h, 7h or 8h.

[0053] This invention employs a two-stage sintering process within the aforementioned parameter range, which facilitates crystal growth and enhances crystallinity. This allows for the full diffusion and migration of elemental ions such as lithium, magnesium, iron, manganese, niobium, phosphorus, and boron, resulting in a highly ordered olivine-type crystal structure. This process further carbonizes the "precursor carbon" formed during the first stage of pre-carbonization, creating an amorphous carbon layer that ultimately coats the surface of the LMFP particles. Furthermore, this process suppresses manganese disproportionation, reduces oxygen vacancies that may arise during sintering, and improves the structural stability of the final three-element co-doped material.

[0054] Preferably, the preparation method includes the following steps:

[0055] (1) At a temperature of 70-90℃ (e.g., 70℃, 80℃ or 90℃, etc.), Nb source, Mn source and Fe source are added to the first solvent and stirred and mixed. Then B source and P source are added and stirred and mixed to obtain solution A. The first solvent includes alcohol (exemplary, such as ethylene glycol, etc.) and water.

[0056] At a temperature of 70-90℃ (e.g., 70℃, 80℃, or 90℃), the Li source and Mg source are added to a second solvent and stirred to obtain solution B; wherein, the second solvent is an alcohol (exemplary, for example, ethylene glycol).

[0057] (2) Stir and mix the A solution and the B solution, then add the dispersant and the reducing agent, and add the pH adjuster to adjust the pH of the solution system to 6.5-7.5 to obtain a mixed solution; wherein, based on the total mass of the element sources in the A solution and the B solution, the amount of the dispersant added is 5-20 wt%; based on the total mass of the element sources in the A solution and the B solution, the amount of the reducing agent added is 1-10 wt%.

[0058] The mixed solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 180-220°C for 18-22 hours. After the reaction was completed, the solution was cooled, washed, and dried to obtain the precursor precipitate.

[0059] (3) The precursor precipitate and the third solvent are ultrasonically mixed to obtain solution C; organic carbon and the fourth solvent are ultrasonically mixed to obtain solution D; wherein the third solvent and the fourth solvent each independently include an alcohol (exemplary, for example, anhydrous ethanol, etc.).

[0060] The C solution and D solution are ultrasonically mixed to disperse organic carbon around and on the surface of the precursor precipitate particles, and then rapidly dried to obtain the product to be sintered.

[0061] (4) Place the product to be sintered in an inert atmosphere and heat it to 250-400°C at a heating rate of 1-4°C / min (e.g., 1°C / min, 2°C / min, 3°C / min or 4°C / min, etc.) and hold it at the temperature for 0.5-4 hours for first-stage sintering. Then heat it to 500-800°C at a heating rate of 4-10°C / min (e.g., 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min or 10°C / min, etc.) and hold it at the temperature for 3-8 hours for second-stage sintering. After the second-stage sintering is completed, cool it to obtain a modified lithium manganese iron phosphate cathode material with a carbon coating layer.

[0062] Thirdly, the present invention provides a positive electrode sheet, wherein the positive electrode sheet includes the modified lithium manganese iron phosphate positive electrode material as described in the first aspect, or includes the modified lithium manganese iron phosphate positive electrode material prepared by the preparation method described in the second aspect.

[0063] Fourthly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a positive electrode as described in the third aspect.

[0064] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0065] Compared with the prior art, the present invention has the following beneficial effects:

[0066] (1) This invention designs a three-element synergistic doping strategy of Mg (Li-site substitution), Nb (Mn / Fe-site substitution), and B (PO4-site substitution), utilizing Mg 2+ Partially replacing Li sites, broadening the Li range. + A one-dimensional diffusion channel along the

[010] direction allows Li to... + Improved diffusion coefficient; utilizing Nb 5+ Partial substitution of Mn / Fe sites and high-valence doping induce charge compensation, which is beneficial for generating electron-hole pairs, improving electronic conductivity, and simultaneously suppressing Mn. 3+ Jahn-Teller distortion; using B 3+ Partial substitution of PO4 sites forms a BO4-PO4 composite tetrahedral network, enhancing structural rigidity and suppressing phase transformation stress during cycling. Mg doping broadens the Li... + Channels, Nb doping enhances electron transport, which can enable 4.1V (Mn 2+ / Mn 3+ The platform capacity contribution has increased to over 55%, and the operating voltage window has been expanded to 3.0-4.5V.

[0067] (2) The three-element synergistic doping strategy provided by the present invention, while ensuring the integrity of the olivine structure, simultaneously improves the intrinsic electronic / ionic conductivity and structural stability of LMFP, and broadens the diffusion channels of lithium ions, achieving a comprehensive performance breakthrough with high specific capacity, high voltage plateau ratio, long cycle life and excellent rate performance, meeting the needs of high energy density batteries, providing a material basis for the development of high specific energy and high power lithium-ion batteries, making the battery perform better in fast charging and fast discharging scenarios, and expanding its applicability in high power application fields. Attached Figure Description

[0068] Figure 1 This is a SEM image of the modified lithium manganese iron phosphate cathode material with a carbon coating provided in Example 1 of this invention.

[0069] Figure 2 The image shows the XRD pattern of the modified lithium manganese iron phosphate cathode material with a carbon coating provided in Example 1 of this invention.

[0070] Figure 3 The charge-discharge curves of the full battery prepared according to Example 1 of this invention are shown.

[0071] Figure 4 The present invention provides the cycle performance curve of the full cell prepared based on Example 1. Detailed Implementation

[0072] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0073] Example 1

[0074] This embodiment provides a modified lithium iron phosphate cathode material, the chemical formula of which is Li. 0.99 Mg 0.01 (Mn 0.6 Fe 0.4 ) 0.99 Nb 0.01 (PO4) 0.99 B 0.01 .

[0075] The surface of the modified lithium manganese iron phosphate cathode material is also coated with a carbon layer with a thickness of 5 nm.

[0076] This embodiment also provides a method for preparing the above-mentioned modified lithium manganese iron phosphate cathode material, the preparation method comprising the following steps:

[0077] (1) At a temperature of 80℃, C 10 H5NbO 20MnSO4·H2O and FeSO4·7H2O were added to 40 mL of the first solvent and stirred to mix. Then H3BO3 and H3PO4 were added and the mixture was stirred for 20 min to obtain solution A. The first solvent consisted of ethylene glycol and water in a volume ratio of 3:1.

[0078] At a temperature of 80℃, LiOH·H2O and MgSO4·H2O were added to 40 mL of a second solvent and stirred to obtain solution B; wherein, the second solvent was ethylene glycol.

[0079] (2) Stir and mix the A solution and the B solution for 0.5 h, then add 2 g of CTAB and 0.5 g of L-ascorbic acid, and add ammonia water to adjust the pH of the solution system to 7 to obtain a mixed solution; wherein, based on the total mass of the element source in the A solution and the element source in the B solution, the amount of CTAB added is 7 wt%; based on the total mass of the element source in the A solution and the element source in the B solution, the amount of L-ascorbic acid added is 2 wt%.

[0080] The mixed solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 200°C for 20 hours. After the reaction was completed, the solution was cooled, washed, and dried to obtain the precursor precipitate. The washing was performed by alternating between ethanol and water.

[0081] (3) The precursor precipitate and the third solvent are ultrasonically mixed for 30 min to obtain solution C; glucose and the fourth solvent are ultrasonically mixed for 30 min to obtain solution D; wherein the third solvent and the fourth solvent are both anhydrous ethanol.

[0082] The C solution and D solution were ultrasonically mixed for 30 minutes to disperse glucose around and on the surface of the precursor precipitate particles, and then rapidly dried to obtain the product to be sintered; wherein the mass ratio of glucose to precursor precipitate was 0.3:1.

[0083] (4) The product to be sintered is placed in a tube furnace under an inert atmosphere and heated to 350°C at a heating rate of 4°C / min and then kept at a constant temperature for 4 hours for first-stage sintering. Then, the product is heated to 700°C at a heating rate of 8°C / min and kept at a constant temperature for 8 hours for second-stage sintering. After the second-stage sintering is completed, the product is cooled to obtain a modified lithium manganese iron phosphate cathode material with a carbon coating layer.

[0084] Figure 1The SEM image of the modified lithium manganese iron phosphate cathode material with carbon coating provided in this embodiment is shown. As can be seen from the figure, the material morphology shows micron-sized secondary particles composed of nano-sized primary particle clusters. The primary particles are irregularly ellipsoidal, dispersed and uniformly distributed, which is no different from conventional lithium manganese iron phosphate particles. This indicates that the synergistic doping of magnesium, niobium and boron does not change the growth mode of lithium manganese iron phosphate microcrystals. The particle size distribution of the primary particles is in the range of 50-500 nm, and the particle size distribution of the secondary particles is in the range of 1-20 μm.

[0085] Figure 2 The XRD pattern of the modified lithium manganese iron phosphate cathode material with carbon coating provided in this embodiment is shown. As can be seen from the figure, the material exhibits a typical olivine crystal structure with pnma space group. Its diffraction peaks have high intensity and sharp shape, indicating that each sample has high crystallinity. The main peaks can be aligned with the diffraction peaks of the LMFP standard card (PDF#13-0336). No obvious impurity peaks were observed, which indicates that the generated phases are all pure phases. Moreover, no carbon peaks appeared in the curves of each sample, indicating that carbon exists in a non-crystalline structure.

[0086] Figure 3 The first charge-discharge curves of the full cell prepared according to this embodiment at 0.2C are shown. This full cell mainly consists of a positive electrode sheet made by homogenizing and coating magnesium, niobium, and boron-doped lithium manganese iron phosphate positive electrode powder onto an aluminum current collector, a polypropylene separator, and a graphite-coated carbon copper negative electrode. The test voltage window is 2.5-4.5V, and the ambient temperature is 25℃. Figure 3 It can be seen that both the charge and discharge curves show two different flat voltage plateaus, corresponding to Fe... 2+ / Fe 3+ (3.5V vs. Li) + ) and Mn 2+ / Mn 3+ (4.1V vs. Li) + The redox pair of Li. 0.99 Mg 0.01 (Mn 0.6 Fe 0.4 ) 0.99 Nb 0.01 (PO4) 0.99 B 0.01 The cathode material has a discharge specific capacity of 159.21 mAh / g at 0.2C and a coulombic efficiency of up to 98%.

[0087] Figure 4 The charge-discharge cycle performance curves of the full battery prepared according to this embodiment at 0.2C are shown. The test voltage window is 2.5-4.5V, and the ambient temperature is 25°C. Figure 4 It can be seen that Li0.99 Mg 0.01 (Mn 0.6 Fe 0.4 ) 0.99 Nb 0.01 (PO4) 0.99 B 0.01 The cathode material exhibited excellent cycle stability at 0.2C, with a capacity retention of 98.17% after 200 cycles and a coulombic efficiency close to 100%.

[0088] Example 2

[0089] This embodiment provides a modified lithium iron phosphate cathode material, the chemical formula of which is Li. 0.98 Mg 0.02 (Mn 0.6 Fe 0.4 ) 0.98 Nb 0.02 (PO4) 0.98 B 0.02 .

[0090] The surface of the modified lithium manganese iron phosphate cathode material is also coated with a carbon layer with a thickness of 5 nm.

[0091] This embodiment also provides a method for preparing the above-mentioned modified lithium manganese iron phosphate cathode material, the preparation method comprising the following steps:

[0092] (1) At a temperature of 70℃, C 10 H5NbO 20 MnSO4·H2O and FeSO4·7H2O were added to 40 mL of the first solvent and stirred to mix. Then H3BO3 and H3PO4 were added and the mixture was stirred for 20 min to obtain solution A. The first solvent consisted of ethylene glycol and water in a volume ratio of 3:1.

[0093] At a temperature of 70°C, LiOH·H2O and MgSO4·H2O were added to 40 mL of a second solvent and stirred to obtain solution B; wherein, the second solvent was ethylene glycol.

[0094] (2) Stir and mix the A solution and the B solution for 0.5 h, then add 2 g of CTAB and 0.25 g of L-ascorbic acid, and add ammonia water to adjust the pH of the solution system to 6.5 to obtain a mixed solution; wherein, based on the total mass of the element source in the A solution and the element source in the B solution, the amount of CTAB added is 7 wt%; based on the total mass of the element source in the A solution and the element source in the B solution, the amount of L-ascorbic acid added is 2 wt%.

[0095] The mixed solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 180°C for 22 hours. After the reaction was completed, the solution was cooled, washed, and dried to obtain the precursor precipitate. The washing was performed by alternating between ethanol and water.

[0096] (3) The precursor precipitate and the third solvent are ultrasonically mixed for 30 min to obtain solution C; glucose and the fourth solvent are ultrasonically mixed for 30 min to obtain solution D; wherein the third solvent and the fourth solvent are both anhydrous ethanol.

[0097] The C solution and D solution were ultrasonically mixed for 30 minutes to disperse glucose on the surface of the precursor precipitate particles, and then rapidly dried to obtain the product to be sintered; wherein the mass ratio of glucose to precursor precipitate was 0.1:1.

[0098] (4) The product to be sintered is placed in a tube furnace under an inert atmosphere and heated to 350°C at a heating rate of 4°C / min and then kept at a constant temperature for 4 hours for first-stage sintering. Then, the product is heated to 700°C at a heating rate of 4°C / min and kept at a constant temperature for 8 hours for second-stage sintering. After the second-stage sintering is completed, the product is cooled to obtain modified lithium manganese iron phosphate cathode material with carbon coating.

[0099] Example 3

[0100] This embodiment provides a modified lithium iron phosphate cathode material, the chemical formula of which is Li. 0.97 Mg 0.03 (Mn 0.6 Fe 0.4 ) 0.97 Nb 0.03 (PO4) 0.97 B 0.03 .

[0101] The surface of the modified lithium manganese iron phosphate cathode material is also coated with a carbon layer, the thickness of which is approximately 5 nm.

[0102] This embodiment also provides a method for preparing the above-mentioned modified lithium manganese iron phosphate cathode material, the preparation method comprising the following steps:

[0103] (1) At a temperature of 90℃, C 10 H5NbO 20 MnSO4·H2O and FeSO4·7H2O were added to 40 mL of the first solvent and stirred to mix. Then H3BO3 and H3PO4 were added and the mixture was stirred for 20 min to obtain solution A. The first solvent consisted of ethylene glycol and water in a volume ratio of 3:1.

[0104] At a temperature of 90℃, LiOH·H2O and MgSO4·H2O were added to 40 mL of a second solvent and stirred to obtain solution B; wherein, the second solvent was ethylene glycol.

[0105] (2) Stir and mix the A solution and the B solution for 0.5 h, then add 2 g of CTAB and 0.5 g of L-ascorbic acid, and add ammonia water to adjust the pH of the solution system to 7.5 to obtain a mixed solution; wherein, based on the total mass of the element source in the A solution and the element source in the B solution, the amount of CTAB added is 7 wt%; based on the total mass of the element source in the A solution and the element source in the B solution, the amount of L-ascorbic acid added is 2 wt%.

[0106] The mixed solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 220°C for 18 hours. After the reaction was completed, the solution was cooled, washed, and dried to obtain the precursor precipitate. The washing was performed by alternating between ethanol and water.

[0107] (3) The precursor precipitate and the third solvent are ultrasonically mixed for 30 min to obtain solution C; glucose and the fourth solvent are ultrasonically mixed for 30 min to obtain solution D; wherein the third solvent and the fourth solvent are both anhydrous ethanol.

[0108] The C solution and D solution were ultrasonically mixed for 30 minutes to disperse glucose on the surface of the precursor precipitate particles, and then rapidly dried to obtain the product to be sintered; wherein the mass ratio of glucose to precursor precipitate was 0.5:1.

[0109] (4) The product to be sintered is placed in a tube furnace under an inert atmosphere and heated to 350°C at a heating rate of 4°C / min and then kept at a constant temperature for 4 hours for first-stage sintering. Then, the product is heated to 700°C at a heating rate of 4°C / min and kept at a constant temperature for 8 hours for second-stage sintering. After the second-stage sintering is completed, the product is cooled to obtain modified lithium manganese iron phosphate cathode material with carbon coating.

[0110] Example 4

[0111] The difference between this embodiment and Embodiment 1 is that in the chemical formula of the modified lithium manganese iron phosphate cathode material, the subscript x of Mg is 0.02.

[0112] The remaining preparation methods and parameters are consistent with those in Example 1.

[0113] Example 5

[0114] The difference between this embodiment and Embodiment 1 is that in the chemical formula of the modified lithium manganese iron phosphate cathode material, the subscript x of Mg is 0.03.

[0115] The remaining preparation methods and parameters are consistent with those in Example 1.

[0116] Example 6

[0117] The difference between this embodiment and Embodiment 1 is that in the chemical formula of the modified lithium manganese iron phosphate cathode material, the subscript y of Nb is 0.02.

[0118] The remaining preparation methods and parameters are consistent with those in Example 1.

[0119] Example 7

[0120] The difference between this embodiment and Embodiment 1 is that in the chemical formula of the modified lithium manganese iron phosphate cathode material, the subscript y of Nb is 0.03.

[0121] The remaining preparation methods and parameters are consistent with those in Example 1.

[0122] Example 8

[0123] The difference between this embodiment and Embodiment 1 is that in the chemical formula of the modified lithium manganese iron phosphate cathode material, the subscript z of B is 0.02.

[0124] The remaining preparation methods and parameters are consistent with those in Example 1.

[0125] Example 9

[0126] The difference between this embodiment and Embodiment 1 is that in the chemical formula of the modified lithium manganese iron phosphate cathode material, the subscript z of B is 0.03.

[0127] The remaining preparation methods and parameters are consistent with those in Example 1.

[0128] Example 10

[0129] The difference between this embodiment and embodiment 1 is that the amount of glucose added in step (3) is adjusted so that the thickness of the carbon layer is 12 nm.

[0130] The remaining preparation methods and parameters are consistent with those in Example 1.

[0131] Example 11

[0132] The difference between this embodiment and embodiment 1 is that no dispersant is added in step (2).

[0133] The remaining preparation methods and parameters are consistent with those in Example 1.

[0134] Example 12

[0135] The difference between this embodiment and embodiment 1 is that L-ascorbic acid is not added in step (2).

[0136] The remaining preparation methods and parameters are consistent with those in Example 1.

[0137] Example 13

[0138] The difference between this embodiment and embodiment 1 is that in step (2), the pH of the mixed solution is 5.5.

[0139] The remaining preparation methods and parameters are consistent with those in Example 1.

[0140] Example 14

[0141] The difference between this embodiment and embodiment 1 is that in step (2), the pH of the mixed solution is 8.5.

[0142] The remaining preparation methods and parameters are consistent with those in Example 1.

[0143] Example 15

[0144] The difference between this embodiment and embodiment 1 is that the temperature of the hydrothermal reaction in step (2) is 150°C.

[0145] The remaining preparation methods and parameters are consistent with those in Example 1.

[0146] Example 16

[0147] The difference between this embodiment and embodiment 1 is that the temperature of the hydrothermal reaction in step (2) is 250°C.

[0148] The remaining preparation methods and parameters are consistent with those in Example 1.

[0149] Example 17

[0150] The difference between this embodiment and embodiment 1 is that in step (4), first-stage sintering is not performed, but sintering is carried out directly at a constant temperature of 650°C for 6.5 hours.

[0151] The remaining preparation methods and parameters are consistent with those in Example 1.

[0152] Comparative Example 1

[0153] The difference between this comparative example and Example 1 is that the modified lithium manganese iron phosphate cathode material has the chemical formula Li. 0.99 Na 0.01 (Mn 0.6 Fe 0.4 ) 0.99 Nb 0.01 (PO4) 0.99 B 0.01 The MgSO4·H2O in step (1) is replaced with sodium sulfate.

[0154] The remaining preparation methods and parameters are consistent with those in Example 1.

[0155] Comparative Example 2

[0156] The difference between this comparative example and Example 1 is that the modified lithium manganese iron phosphate cathode material has the chemical formula Li. 0.99 Mg 0.01 (Mn 0.6 Fe 0.4 ) 0.99 Nb 0.01 (PO4) 0.99 F 0.01 In step (1), H3PO4 is replaced with ammonium fluoride.

[0157] The remaining preparation methods and parameters are consistent with those in Example 1.

[0158] Comparative Example 3

[0159] The difference between this comparative example and Example 1 is that the modified lithium manganese iron phosphate cathode material has the chemical formula Li. 0.95 Mg 0.05 (Mn 0.6 Fe 0.4 ) 0.99 Nb 0.01 (PO4) 0.99 B 0.01 .

[0160] The remaining preparation methods and parameters are consistent with those in Example 1.

[0161] Comparative Example 4

[0162] The difference between this comparative example and Example 1 is that the modified lithium manganese iron phosphate cathode material has the chemical formula Li. 0.99 Mg 0.01 (Mn 0.6 Fe 0.4 ) 0.95 Nb 0.05 (PO4) 0.99 B 0.01 .

[0163] The remaining preparation methods and parameters are consistent with those in Example 1.

[0164] Comparative Example 5

[0165] The difference between this comparative example and Example 1 is that the modified lithium manganese iron phosphate cathode material has the chemical formula Li. 0.99 Mg 0.01 (Mn 0.6 Fe 0.4 ) 0.99 Nb0.01 (PO4) 0.95 B 0.05 .

[0166] The remaining preparation methods and parameters are consistent with those in Example 1.

[0167] Performance testing

[0168] The modified lithium manganese iron phosphate cathode materials with carbon coating prepared in the above embodiments and comparative examples are assembled into coin cells. The specific steps include:

[0169] Modified lithium manganese iron phosphate cathode material with carbon coating, acetylene black and polyvinylidene fluoride were mixed in a molar ratio of 92:4:4 and added to N-methylpyrrolidone to obtain a cathode slurry. The cathode slurry was then coated onto aluminum foil to obtain a cathode sheet. The lithium sheet was used as the anode sheet, and CR2430 button batteries were manufactured in an argon-filled glove box.

[0170] The charge-discharge performance of the CR2430 button cell was tested to obtain the initial discharge specific capacity. The test conditions included 0.1C.

[0171] Cycle performance tests were conducted on CR2430 button cells to obtain capacity retention. The test conditions included: 0.2C, 200 cycles.

[0172] Rate performance tests were conducted on CR2430 button cells to obtain the reversible capacity at various rates. The test conditions included: 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and then back to 0.1C. Each rate was cycled for 5 cycles, and after returning to 0.1C, the cells were kept in a long cycle.

[0173] The test results are shown in Table 1.

[0174] Table 1

[0175]

[0176] analyze:

[0177] As shown in Table 1, this invention achieves significant improvements in specific capacity, voltage plateau ratio, cycle stability, and rate performance of LMFPs through a three-element synergistic doping strategy involving Mg (Li-site substitution), Nb (Mn / Fe-site substitution), and B (PO4-site substitution). Compared to traditional LMFP materials (whose actual specific capacity is typically below 150 mAh / g), the synergistically doped material achieves a discharge specific capacity of over 160 mAh / g at 0.1C, approaching the theoretical specific capacity (170 mAh / g), thus meeting the requirements of high-energy-density batteries. This is achieved by broadening the Li... + Diffusion channels and enhanced electron transport, 4.1V (Mn 2+ / Mn3+ The high-voltage platform capacity contribution is significantly improved, increasing from less than 30% in traditional materials to over 55%, and the operating voltage window is extended to 3-4.5V, enabling the battery to store and release energy more effectively in the high-voltage range, thus improving the energy output efficiency in practical applications. After 200 cycles at 0.2C, the capacity retention exceeds 90%, and the cycle stability is greatly improved. This is mainly due to the structural stability after doping optimization and the suppression of side reactions, ensuring the reliability of the battery in long-term use. At 2C high rates, the discharge specific capacity can still reach over 125mAh / g, a significant improvement compared to the 120mAh / g of traditional materials at the same rate. This is attributed to the improvement in electronic conductivity (from the extremely low level of traditional LMFPs to a more suitable level) and Li + The increased diffusion coefficient makes the battery perform better in fast charging and discharging scenarios, expanding its applicability in high-power applications.

[0178] A comparison of Examples 1 and 10 shows that if the carbon layer is too thick, it will hinder the diffusion of lithium ions, leading to a significant decrease in the material's rate performance, especially at high charge and discharge rates. It will also increase internal resistance and electrode polarization, resulting in poorer cycle performance. An excessively thick carbon layer will reduce the material's tap density and compaction density, thus affecting the overall energy density of the battery. Furthermore, it will result in an excessively large specific surface area, making the material prone to agglomeration and affecting dispersibility and stability during the homogenization process.

[0179] A comparison of Examples 1 and 11-12 shows that if no dispersant is added in step (2), the strong van der Waals forces between the particles will cause these nanoscale particles to easily agglomerate, forming large agglomerates. This severely affects the dispersibility and uniformity of the material, making it difficult to achieve the ideal nanostructure. It will also increase charge transfer impedance, leading to a significant decrease in electrochemical performance. The low stacking density of the agglomerated particles makes it difficult to increase the electrode compaction density, limiting the energy density of the battery and increasing the difficulty of subsequent slurry preparation. If L-ascorbic acid is not added in step (2), it will cause Fe in the solution to... 2+ and Mn 2+ Oxidized to a higher oxidation state (e.g., Fe) 3+ Mn 3+ This not only generates non-target impurity phases (such as MnPO4), disrupting the olivine structure of the material, but also triggers a severe Jahn-Teller effect during charge and discharge, causing manganese dissolution and structural distortion, ultimately leading to a significant reduction in the material's capacity, a sharp deterioration in cycle performance, and a decrease in rate performance.

[0180] A comparison of Examples 1 and 13-14 shows that if the pH of the mixed solution in step (2) is too low, manganese and iron ions will have difficulty forming stable phosphate precipitates in an acidic environment, resulting in impurity phases, reduced material purity, and severely affecting the electrochemical performance of the final product. Specifically, when the pH is too low, phosphate ions (PO4) will have difficulty forming stable phosphate precipitates in an acidic environment. 3- ) will be with H + Combine to form H2PO4 - Or H3PO4, resulting in free PO4 in the solution. 3- The concentration decreased, making it unable to react with Mn. 2+ and Fe 2+ The reaction proceeds fully to generate the lithium manganese iron phosphate precursor. This can lead to the presence of non-target impurity phases in the product, such as iron phosphate and manganese phosphate, affecting the phase purity and crystal structure integrity of the material. Simultaneously, the acidic environment accelerates the disproportionation reaction of manganese ions, generating Mn. 3+ and Mn 4+ High-valence manganese ions further degrade the electrochemical stability of the material, leading to capacity decay and deterioration of cycle performance. If the pH of the mixed solution in step (2) is too high, it will cause premature precipitation of manganese and iron ions, forming a non-uniform hydroxide precursor, thereby affecting the phase purity and electrochemical performance of the final product. Specifically, when the pH value is too high, the manganese ions (Mn) in the solution will precipitate prematurely, forming a non-uniform hydroxide precursor, thus affecting the phase purity and electrochemical performance of the final product. 2+ ) and iron ions (Fe 2+ It will rapidly react with hydroxide ions (OH-) - The carbon binds with Fe to form Mn(OH)₂ and Fe(OH)₂ precipitates. This precipitation process is often uneven, easily forming large agglomerates rather than ideal nanoscale precursors. These large particles are difficult to react completely during subsequent sintering, leading to impurity phases in the final product, such as unreacted oxides or non-target phosphate compounds. Furthermore, the uneven precursor also affects the uniformity of carbon coating, further reducing the material's electronic conductivity and lithium-ion diffusion rate, ultimately resulting in a decrease in battery capacity, rate performance, and cycle life.

[0181] A comparison of Examples 1 and 15-16 shows that if the temperature of the hydrothermal reaction in step (2) is too low, sufficient energy cannot be provided to drive the rapid formation of crystal nuclei and perfect crystal growth. Specifically, this leads to incomplete precursor reaction, poor crystallinity, excessively large and uneven grain size, and uneven distribution of manganese and iron elements, ultimately severely degrading the electrochemical performance of the material. In particular, when the temperature is insufficient, the activity of reactant ions decreases, making it difficult to overcome energy barriers and achieve effective intermolecular bonding, resulting in a small number of nuclei and slow crystal growth. This not only causes the reaction to remain in the initial stage, generating a large number of amorphous or defective intermediate products, but also prevents the formation of regular nanoscale particles that should have been formed, replacing them with large particles of uneven size and irregular morphology. These structural defects directly hinder the diffusion of lithium ions in the material, resulting in a significant decrease in battery capacity, rate performance and cycle stability. If the temperature of the hydrothermal reaction in step (2) is too high, it will cause the pressure of the reaction system to increase sharply, exacerbating equipment corrosion and safety risks. At the same time, it may cause excessive reaction of the precursor, resulting in excessively fast crystal growth, coarse grains, and irregular morphology. It may also induce abnormal dissolution or valence state changes of manganese ions, thereby damaging the structural stability and electrochemical performance of the final product.

[0182] A comparison of Examples 1 and 17 shows that if, in step (4), first-stage sintering is not performed, but instead sintering is carried out at a constant temperature of 700°C for 8 hours, the carbon source will react violently due to its inability to decompose gradually, generating a large amount of gas that causes the precursor to splash, and may form too many pores or damage the already formed structure. At the same time, uneven carbon coating will seriously degrade the conductivity of the material. Specifically, the core function of the 350°C pre-sintering stage is to allow the carbon source to decompose gently and be initially carbonized. If this step is skipped and the temperature is raised directly to 700°C, the carbon source will burn or gasify rapidly, which will not only waste resources, but may also cause the material to be lost due to splashing from the crucible due to the large amount of gas generated instantly, resulting in a loose structure and uneven carbon coating of the final product, which will seriously affect the electrochemical performance of the final lithium manganese iron phosphate cathode material.

[0183] A comparison of Example 1 and Comparative Example 1 shows that partial substitution with Na at the Li site can lead to potential challenges in rate performance and cycle stability: although Na +While sodium doping can broaden lithium channels, its relatively large ionic radius may also cause greater local lattice distortion. During charge and discharge, Na⁺ may occupy some lithium sites and be difficult to extract, forming "inert sites" that hinder the long-term rapid migration of lithium ions and pose a potential risk to cycle stability. It may also lead to a decrease in voltage plateau and energy density fluctuations: the introduction of sodium may slightly lower the average operating voltage plateau of the material. Although sodium doping may compensate for this by increasing reversible capacity, the overall energy density change is uncertain, and its final effect needs to be optimized by precisely controlling the doping amount. Magnesium doping, on the other hand, typically has a more stable effect in this regard.

[0184] A comparison of Example 1 and Comparative Example 2 shows that partial substitution with F at the PO4 site presents potential challenges to rate performance and process complexity: excessive F doping can lead to a significant increase in primary particle size, resulting in a longer lithium-ion migration path, which in turn increases polarization and causes a decrease in high-rate performance. Furthermore, F doping requires more precise control of the doping amount and process conditions to avoid the formation of non-conductive impurity phases such as LiF, which places more stringent requirements on process control than B doping. It may also lead to a trade-off between voltage plateau fluctuations and energy density: F entering the lattice may affect Fe... 2+ / Fe 3+ and Mn 2+ / Mn 3+ The energy levels of redox couples are fine-tuned, which in some cases may cause a slight shift in the discharge voltage plateau (potentially increasing or decreasing), thus affecting the battery's energy density output. Furthermore, unlike boron doping, which may form borate complexes, fuchsinium doping primarily affects the crystal structure and interfaces, exhibiting a different mechanism for inhibiting transition metal dissolution. Therefore, it requires synergistic design with electrolyte additives (such as fluoroborates) to maximize its advantages.

[0185] As can be seen from the comparison between Example 1 and Comparative Examples 3-5, if the molar percentage of Mg is too high, it will cause problems due to Mg. 2+ Occupying too many lithium sites and failing to undergo redox reactions reduces the number of active lithium atoms available for insertion and extraction, directly lowering the material's specific capacity. Simultaneously, excessive magnesium blocks lithium-ion migration channels, increasing internal resistance and polarization, severely degrading rate performance and cycle stability; if the molar percentage of Nb is too high, it will further impair Nb content. 5+ Ionic radius (0.064 nm) and Mn 2+ (0.083nm) or Fe 2+ Significant differences exist at (0.078nm), leading to severe lattice distortion and inducing significant internal stress. This can not only generate impurity phases that hinder lithium-ion diffusion but also weaken the material's inherent capacity and rate performance. If the molar proportion of B is too large, it will... 3+ The ionic radius (0.012 nm) is much smaller than that of PO4.3 The ionic radius (0.017 nm) leads to PO4 3 Excessive distortion of the tetrahedral structure disrupts lattice integrity, hinders lithium-ion diffusion, increases charge transfer impedance, and may generate inactive impurity phases, ultimately leading to a significant decrease in the material's specific capacity and rate performance.

[0186] It should be noted that the present invention is illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A modified lithium iron phosphate cathode material, characterized in that, The modified lithium manganese iron phosphate cathode material has the general chemical formula Li. 1-x Mg x (Mn 1-a Fe a ) 1-y Nb y (PO4) 1-z B z , of which 0 <x≤0.03,0<y≤0.03,0<z≤0.025,0<a≤0.5; The surface of the modified lithium manganese iron phosphate cathode material is also coated with a carbon layer; the thickness of the carbon layer is 4-10 nm. The preparation method of the modified lithium manganese iron phosphate cathode material includes the following steps: Solution A containing Nb, Mn, Fe, B, and P sources is provided; Solution B containing Li and Mg sources is provided. The A solution, B solution, dispersant, reducing agent, and pH adjuster are mixed and subjected to a hydrothermal reaction to obtain a precursor precipitate; the temperature of the hydrothermal reaction is 180-220℃. The precursor precipitate was sintered to obtain the modified lithium manganese iron phosphate cathode material. Before the precursor precipitate is sintered, it undergoes a carbon coating pretreatment, the steps of which include: (a) Provide a C solution containing the precursor precipitate and a D solution containing a carbon source; (b) The C solution and the D solution are mixed so that the carbon source is dispersed around and on the surface of the precursor precipitate particles, and the product to be sintered is obtained after drying. The sintering process includes first-stage sintering and second-stage sintering performed by sequentially increasing the temperature.

2. The modified lithium manganese iron phosphate cathode material according to claim 1, characterized in that, a=0.3-0.4。 3. A method for preparing the modified lithium manganese iron phosphate cathode material as described in claim 1 or 2, characterized in that, The preparation method includes the following steps: Solution A containing Nb, Mn, Fe, B, and P sources is provided; Solution B containing Li and Mg sources is provided. The A solution, B solution, dispersant, reducing agent and pH adjuster are mixed and subjected to a hydrothermal reaction to obtain the precursor precipitate; The precursor precipitate was sintered to obtain the modified lithium manganese iron phosphate cathode material.

4. The preparation method according to claim 3, characterized in that, The Nb source includes NbO(CH3COO)3 and C 10 H5NbO 20 Any one or a combination of at least two of Nb2O5, NbOPO4, or (NH4)2(Nb(C2O4)4)·2H2O; And / or, the Mn source includes any one or a combination of at least two of MnSO4·H2O, MnCO3, Mn(NO3)2, Mn(PO4)3, MnO2, Mn3O4, MnC2O4, MnCl2, MnCl3 or (CH3COO)2Mn; And / or, the Fe source includes a divalent iron source, which includes any one or a combination of at least two of FeSO4·7H2O, Fe(NO3)2, Fe(PO4)2, Fe(CO3)2, FeCl2, FeC2O4, (CH3COO)2Fe, or FeO; And / or, the B source includes H3BO3, NaBO2, KBO2, NH4BO2, B2O3, Na2B4O7·10H2O, K2B4O7, Li2B4O7, LiBO2·2H2O, C3H9BO3, or C6H 15 Any one or at least two of BO3; And / or, the P source includes any one or a combination of at least two of H3PO4, NH4H2PO4, Li4H2PO4, (NH4)2HPO4, (NH4)3PO4, Li3PO4 or P2O5; And / or, the Li source includes any one or a combination of at least two of LiOH·H2O, Li2CO3, CH3COOLi, Li3PO4, LiH2PO4, Li2HPO4, LiNO3, Li2C2O4 or Li2SO4; And / or, the Mg source includes any one or a combination of at least two of Mg(NO3)2·6H2O, MgSO4·H2O, MgSO4·7H2O, or MgSO4.

5. The preparation method according to claim 3, characterized in that, The dispersant includes any one of surfactant dispersants, organic solvent dispersants, or polymeric dispersants; And / or, the surfactant dispersant includes any one or a combination of at least two of CTAB, PAA, or PAANH4; And / or, the organic solvent dispersant includes any one or a combination of at least two of methanol, ethylene glycol, or glycerol; And / or, the polymeric dispersant includes PVP and / or PEG; And / or, the reducing agent includes ascorbic acid and / or sodium ascorbate; And / or, the pH adjuster includes any one or a combination of at least two of ammonia, sulfamic acid, citric acid, fumaric acid, succinic acid, adipic acid, maleic acid, ammonium hydroxide, sodium hydroxide, or potassium hydroxide; And / or, based on the total mass of the element sources in solution A and solution B, the amount of the dispersant added is 5-20 wt%; And / or, based on the total mass of the element sources in solution A and solution B, the amount of the reducing agent added is 1-10 wt%; And / or, the amount of pH adjuster added meets the following requirement: the pH of the mixed solution system is 6.5-7.

5.

6. The preparation method according to claim 3, characterized in that, The hydrothermal reaction temperature is 180-220℃, and the holding time is 18-22h; And / or, before the precursor precipitate is sintered, it is pretreated with carbon coating, the steps of which include: (a) Provide a C solution containing the precursor precipitate and a D solution containing a carbon source; (b) The C solution and the D solution are mixed so that the carbon source is dispersed around and on the surface of the precursor precipitate particles, and the product to be sintered is obtained after drying. The carbon source includes organic carbon; The organic carbon includes any one or a combination of at least two of glucose, sucrose, fructose, cellulose, maltose, citric acid, soluble starch, salicylic acid, oxalic acid, polyacrylic acid, polyethylene glycol, or dopamine. The mass ratio of the carbon source in solution D to the precursor precipitate in solution C is (0-0.5):

1.

7. The preparation method according to claim 3, characterized in that, The atmosphere for the sintering process is an inert atmosphere; In the inert atmosphere, the gas flow rate is 50-150 cm⁻¹. 3 / min; And / or, the sintering process includes first-stage sintering and second-stage sintering performed by sequentially increasing the temperature; The first-stage sintering temperature is 250-400℃, and the holding time is 0.5-4h; The second-stage sintering temperature is 500-800℃, and the holding time is 3-8h.

8. The preparation method according to claim 3, characterized in that, The preparation method includes the following steps: (1) At a temperature of 70-90℃, Nb source, Mn source and Fe source are added to the first solvent and stirred and mixed. Then B source and P source are added and stirred and mixed to obtain solution A. The first solvent includes alcohol and water. At a temperature of 70-90℃, the Li source and Mg source are added to a second solvent and stirred to obtain solution B; wherein, the second solvent is an alcohol. (2) Stir and mix the A solution and the B solution, then add a dispersant and a reducing agent, and add a pH adjuster to adjust the pH of the solution system to 6.5-7.5 to obtain a mixed solution; wherein, based on the total mass of the element sources in the A solution and the B solution, the amount of the dispersant added is 5-20 wt%; based on the total mass of the element sources in the A solution and the B solution, the amount of the reducing agent added is 1-10 wt%. The mixed solution was transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 180-220°C for 18-22 hours. After the reaction was completed, the solution was cooled, washed, and dried to obtain the precursor precipitate. (3) The precursor precipitate and the third solvent are ultrasonically mixed to obtain solution C; organic carbon and the fourth solvent are ultrasonically mixed to obtain solution D; wherein the third solvent and the fourth solvent each independently include an alcohol; The C solution and D solution are ultrasonically mixed to disperse organic carbon around and on the surface of the precursor precipitate particles, and then rapidly dried to obtain the product to be sintered. (4) The product to be sintered is placed in an inert atmosphere and heated to 250-400°C at a heating rate of 1-4°C / min and then kept at a constant temperature for 0.5-4h for first-stage sintering. Then, the product is heated to 500-800°C at a heating rate of 4-10°C / min and kept at a constant temperature for 3-8h for second-stage sintering. After the second-stage sintering is completed, the product is cooled to obtain a modified lithium manganese iron phosphate cathode material with a carbon coating layer.

9. A positive electrode sheet, characterized in that, The positive electrode sheet includes the modified lithium manganese iron phosphate positive electrode material as described in claim 1 or 2, or includes the modified lithium manganese iron phosphate positive electrode material prepared by the preparation method described in any one of claims 3-8.

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

Images