Lithium iron phosphate positive electrode material with excellent low-temperature performance and synthesis method thereof

Abstract

The application relates to a lithium iron phosphate positive electrode material with excellent low-temperature performance and a synthesis method thereof. The lithium iron phosphate positive electrode material has a core with a chemical formula of LiFe1-x-y-zMxNyBzPO4, wherein M is a combination of Ti and Nb, the doping amount of x is 0.5<=x<=1.0, the doping amount of y is 0.1<=y<=0.8, the doping amount of z is 0.03<=z<=0.08, the core surface of the lithium iron phosphate positive electrode material further contains a coating layer, the coating layer is a mixed layer of nano boron carbide and amorphous carbon, and the average particle size of the lithium iron phosphate positive electrode material is 80-110 nm. Through specific component and structure design, the low-temperature performance and the electrochemical performance of the positive electrode material are improved, the 0.2C initial discharge specific capacity is 158-164 mAh / g, the capacity retention rate after 1000 cycles is 90-95%, and the discharge capacity retention rate at -20 DEG C is 85-91%.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, specifically relating to a lithium iron phosphate cathode material with excellent low-temperature performance and its synthesis method. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles, energy storage devices, and other fields due to their advantages such as high energy density and long cycle life. Lithium iron phosphate (LFP) has become a core cathode material for new energy vehicles and energy storage systems due to its high safety, long cycle life, and low cost. However, its inherent low electronic conductivity and small lithium-ion diffusion coefficient lead to significant capacity decay and reduced rate performance in low-temperature environments (such as below -20°C), severely limiting its application in cold regions.

[0003] Elemental doping, such as single metal ion doping like Mg or Al, can optimize the crystal structure, but it offers insufficient control over the electronic structure. Non-metallic element doping can improve conductivity, but the synergistic effect of single doping is limited. Traditional carbon coating can improve electronic conductivity, but the interfacial impedance remains high. Single ceramic coatings can improve structural stability, but sacrifice some conductivity. Nanoparticle size control can shorten the lithium-ion diffusion path, but it easily leads to agglomeration during preparation, resulting in decreased cycle performance. Therefore, there is an urgent need to develop a novel lithium iron phosphate cathode material designed through multi-element synergistic doping and composite coating structures to improve the low-temperature performance of lithium iron phosphate cathode materials. Summary of the Invention

[0004] This invention provides a lithium iron phosphate cathode material with excellent low-temperature performance and its synthesis method, which solves the problems of slow ion diffusion, poor electronic conductivity and high interface impedance of current lithium iron phosphate cathode materials at low temperatures, and significantly improves the low-temperature electrochemical performance of the material.

[0005] In a first aspect, the present invention relates to a lithium iron phosphate cathode material with excellent low-temperature performance, comprising: The core of the lithium iron phosphate cathode material has the following chemical formula: The general chemical formula is LiFe1-xy-zMxNyBzPO4, where: M represents a combination of Ti and Nb, with a doping concentration of 0.5 ≤ x ≤ 1.0. The doping concentration of N is 0.1 ≤ y ≤ 0.8; the doping concentration of B is 0.03 ≤ z ≤ 0.08. The core surface of the lithium iron phosphate cathode material also contains a coating layer: The coating layer is a mixture of nano-boron carbide and amorphous carbon, in which the mass percentage of nano-boron carbide is 50% to 65%; the thickness is 20 to 25 nm; and the average particle size of the lithium iron phosphate cathode material is 80 to 110 nm.

[0006] Preferably, M is a combination of Ti and Nb, and the molar ratio of Ti to Nb is 0.7 to 1.3:1.

[0007] Preferably, M is a combination of Ti and Nb, x=0.8; the doping amount of N is 0.4≤y≤0.6; and the doping amount of B is 0.05≤z≤0.06.

[0008] Secondly, the present invention relates to a method for synthesizing a lithium iron phosphate cathode material with excellent low-temperature performance, comprising the following steps: Step 1: Preparation of nitrogen-boron co-doped iron phosphate precursor Iron, phosphorus, nitrogen, and boron sources were dissolved in deionized water according to the elemental ratio of the core chemical formula, and the mixture was stirred at 70–80 °C for 2–3 h to form a homogeneous solution. The pH was adjusted to 4.5–5 with ammonia water, and then the mixture was reacted at 150–180 °C for 12–18 h. The resulting N and B co-doped FePO4 precursor powder with a D50 particle size of 60–80 nm was obtained by spray drying. The phosphorus source was a combination of H3PO4 and ethylenediaminetetramethylenephosphonic acid. Step 2: Lithium source composite and sintering (1) The precursor powder is ball-milled and mixed with Li2CO3 powder, nano boron carbide, amorphous carbon source and doped element M source, and sintered at 700-750°C for 8-10 h under argon atmosphere to form a coating layer, and then cooled to room temperature to obtain the crystalline product. (2) Anneal the crystallized product at 350-400℃ for 2-3 hours, cool it to room temperature, crush it and sieve it to obtain a positive electrode material with an average particle size of 80-110nm.

[0009] Preferably, the molar ratio of phosphorus from H3PO4 to ethylenediaminetetramethylenephosphonic acid in the FePO4 precursor powder is 0.4 to 0.5:1.

[0010] Preferably, the iron source is any one or a combination of at least two of ferrous sulfate, ferrous oxalate, and ferrous chloride.

[0011] Preferably, the carbon source for amorphous carbon is selected from any one or a combination of at least two of glucose, sucrose, fructose, lactose, or maltose.

[0012] Preferably, the dopant element M is selected from an oxide of M.

[0013] Preferably, the nitrogen source is melamine.

[0014] Preferably, the inlet air temperature for spray drying in step one is 80-90°C, and the atomization pressure is 0.5-0.8 MPa.

[0015] The beneficial effects of this invention are as follows: By doping with specific amounts of nitrogen and boron, and using a specific ratio of Ti to Nb, and through the design of a mixed coating layer structure of nano-boron carbide and amorphous carbon, as well as the selection of a phosphorus source, the interfacial impedance and electron transport efficiency at low temperatures are simultaneously optimized. This refines the particle size of the cathode material, improves conductivity, and enhances its low-temperature and electrochemical performance. The initial discharge specific capacity at 0.2C is 158–164 mAh / g, the capacity retention rate after 1000 cycles is 90–95%, and the discharge capacity retention rate at -20℃ is 85–91%. The overall performance at -20℃ reaches industry-leading levels, providing key technical support for the large-scale application of lithium iron phosphate cathode materials in cold regions. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the process flow for synthesizing a lithium iron phosphate cathode material with excellent low-temperature performance, as disclosed in an embodiment of the present invention. Detailed Implementation

[0018] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] In existing research on lithium iron phosphate (LFP) cathode materials, battery capacity decays significantly and rate performance declines at low temperatures (such as below -20°C), severely limiting their application in cold regions. There is an urgent need to develop a novel LFP cathode material designed through multi-element synergistic doping and composite coating structures to improve the low-temperature performance of LFP cathode materials, meet market demands, and drive industry development.

[0020] To address the aforementioned technical problems, embodiments of the present invention provide a lithium iron phosphate cathode material with excellent low-temperature performance, comprising: The core of the lithium iron phosphate cathode material has the following chemical formula: The general chemical formula is LiFe 1-x-y-z M x N y B z PO4, where: M represents a combination of Ti and Nb, with a doping concentration of 0.5 ≤ x ≤ 1.0. The doping concentration of N is 0.1 ≤ y ≤ 0.8; the doping concentration of B is 0.03 ≤ z ≤ 0.08. The core surface of the lithium iron phosphate cathode material also contains a coating layer: The coating layer comprises a mixed layer of nano-boron carbide and amorphous carbon, wherein the mass percentage of nano-boron carbide in the mixed layer is 50% to 65%; the thickness is 20 to 25 nm; and the average particle size of the lithium iron phosphate cathode material is 80 to 110 nm.

[0021] The dopant element M is a combination of Ti and Nb. Ti and Nb enter the Fe sites, forming octahedral distortions and constructing anisotropic Li ion migration channels, increasing the lithium-ion diffusion coefficient by 30-50%. The doping amount is controlled within 0.5 ≤ x ≤ 1.0. Excess Ti blocks the migration channels. Controlling the Ti to Nb molar ratio at 0.7–1.3:1 fully utilizes the low valence state of Ti and the high valence state of Nb to synergistically regulate the band structure, further reducing the material's band gap. This lays the foundation for rapid electron transport at low temperatures, balancing conductivity and structural stability, and improving the low-temperature performance of the lithium iron phosphate cathode material.

[0022] Nitrogen (N) doping replaces oxygen (O) in the PO4 tetrahedron, forming PON bonds, weakening the covalent nature of the PO bonds, and lowering the Li ion migration barrier. Boron (B) doping allows boron atoms to enter the interstitial spaces, creating numerous defects and forming BO-Li fast-conducting sites, further optimizing interfacial ion conduction. The electronic structure rearrangement caused by boron doping improves electron mobility, optimizes the charge transfer process on the electrode surface, and significantly enhances the rate of electrochemical reactions. Since B has lower electronegativity than C, while N has higher electronegativity, this allows electron-deficient holes near carbon atoms in the N-B co-doped carbon layer to combine with electron-donating atoms, resulting in a higher doping concentration. The unique synergistic coupling between B and N atoms further optimizes the band structure and surface state of lithium iron phosphate, improving ion migration rates at low temperatures and significantly enhancing its electrochemical and low-temperature performance. Therefore, the N doping content is controlled at 0.1-0.8 at%, and the B doping content at 0.03-0.08 at%.

[0023] In one embodiment, M is a combination of Ti and Nb, x=0.8; the doping amount of N is 0.4≤y≤0.6; and the doping amount of B is 0.05≤z≤0.06.

[0024] A hybrid coating layer of boron carbide nanoparticles and amorphous carbon is used, with boron carbide comprising 50%-65% of the total mass. The boron carbide nanoparticles are uniformly dispersed within the carbon layer, forming a bifunctional layer that integrates ion conduction channels and an electronic conductivity network. The BC bonds of the boron carbide nanoparticles also inhibit electrolyte penetration, while the amorphous carbon fills the gaps between the boron carbide nanoparticles, forming a continuous conductive network, further improving the material's conductivity and cycle stability. A ratio of too high or too low in the hybrid layer of boron carbide nanoparticles and amorphous carbon will compromise the dual functions of ion conduction channels and electronic conductivity networks; the ideal mass percentage of boron carbide nanoparticles is 50%–65%. A coating layer that is too thin will fail to form a continuous network, while one that is too thick will hinder the diffusion of Li ions; the thickness is controlled at 20-25 nm.

[0025] The average particle size of lithium iron phosphate cathode material is 80-110nm. Through spray drying and sintering processes, the particle size is precisely controlled, which shortens the Li ion diffusion path, enhances interfacial electron transport, and the fine particle size is more conducive to improving the low-temperature performance of lithium iron phosphate.

[0026] like Figure 1 As shown, an embodiment of the present invention provides a method for synthesizing a lithium iron phosphate cathode material with excellent low-temperature performance, comprising the following steps: Step 1: Preparation of nitrogen-boron co-doped iron phosphate precursor Iron, phosphorus, nitrogen, and boron sources were dissolved in deionized water according to the elemental ratio of the core chemical formula, and the mixture was stirred at 70–80 °C for 2–3 h to form a homogeneous solution. The pH was adjusted to 4.5–5 with ammonia water, and then the mixture was reacted at 150–180 °C for 12–18 h. The resulting N and B co-doped FePO4 precursor powder with a D50 particle size of 60–80 nm was obtained by spray drying. The phosphorus source was a combination of H3PO4 and ethylenediaminetetramethylenephosphonic acid. Step 2: Lithium source composite and sintering (1) The precursor powder is ball-milled and mixed with Li2CO3 powder, nano boron carbide, amorphous carbon source and doped element M source, and sintered at 700-750°C for 8-10 h under argon atmosphere to form a coating layer, and then cooled to room temperature to obtain the crystalline product. (2) Anneal the crystallized product at 350-400℃ for 2-3 hours, cool it to room temperature, crush it and sieve it to obtain a positive electrode material with an average particle size of 80-110nm.

[0027] In one embodiment, the molar ratio of phosphorus in the FePO4 precursor powder from H3PO4 to ethylenediaminetetramethylenephosphonic acid is 0.4 to 0.5:1.

[0028] The phosphorus source for FePO4 precursor powder preparation is a combination of H3PO4 and ethylenediaminetetramethylenephosphonic acid (EDTA): H3PO4 and EDTA are combined in a molar ratio of 0.4–0.5:1. EDTA is part of the phosphorus source. Furthermore, its molecular structure contains four phosphonic acid groups and an amino group, exhibiting a high electrical charge on its surface. The four phosphonic acid groups form chelates with Fe ions, resulting in a uniform distribution of iron, phosphorus, nitrogen, and boron at the molecular scale. This refines the particle size of the FePO4 precursor powder. Uniform atomic-level doping ensures the consistency of core lattice distortion after sintering, avoiding ion conduction bottlenecks caused by local defects, improving electrical conductivity and ion transport capacity, and enhancing low-temperature performance. This forms a spherical precursor with a D50 of 60–80 nm, providing an ideal carrier for subsequent uniform coating.

[0029] The crystalline product is annealed at 350–400℃ for 2–3 hours, cooled to room temperature, crushed, and sieved to obtain cathode material with an average particle size of 80–110 nm. Annealing can eliminate internal stress generated during sintering, improve the crystal structure and interfacial bonding of the material, and further enhance the cycle stability and low-temperature performance of the material. The annealing temperature of 350–400℃ will not cause decomposition or structural damage to the coating layer, and can optimize the internal structure of the material without affecting the coating effect.

[0030] In one embodiment, the iron source is any one or a combination of at least two of ferrous sulfate, ferrous oxalate, and ferrous chloride.

[0031] In one embodiment, the carbon source for amorphous carbon is selected from any one or a combination of at least two of glucose, sucrose, fructose, lactose, or maltose.

[0032] In one embodiment, the dopant element M is selected from an oxide of M.

[0033] In one embodiment, the nitrogen source is melamine.

[0034] In one embodiment, the inlet air temperature for spray drying in step one is 80–90°C, and the atomization pressure is 0.5–0.8 MPa.

[0035] By employing specific amounts of nitrogen and boron, as well as doping with Ti and Nb, and through the design of a mixed coating layer structure of nano-boron carbide and amorphous carbon, along with the selection of a phosphorus source, the interfacial impedance and electron transport efficiency at low temperatures were simultaneously optimized. This refined the particle size of the cathode material, improved conductivity, and enhanced its low-temperature and electrochemical performance. The initial discharge specific capacity at 0.2C was 158–164 mAh / g, with a capacity retention of 90–95% after 1000 cycles, and a discharge capacity retention of 85–91% at -20℃. The overall performance at -20℃ reached industry-leading levels, providing crucial technical support for the large-scale application of lithium iron phosphate cathode materials in cold regions.

[0036] The embodiments of the present invention are described in detail below. The composition of the lithium iron phosphate cathode material of Embodiments 1 to 5 and Comparative Examples 1 to 2 is shown in Table 1.

[0037] Table 1: Composition of lithium iron phosphate cathode materials in Examples 1-5 and Comparative Examples 1-2

[0038] The process parameters used in the synthesis methods of Examples 1-5 and Comparative Examples 3-4 of this invention are shown in Table 2.

[0039] Table 2: Process parameters used in the synthesis methods of Examples 1-5 and Comparative Examples 3-4

[0040] The M source in Examples 1-5 and Comparative Examples 1-4 all used TiO2 and Nb2O5.

[0041] Comparative Examples 1 and 2 are identical to Example 4 in terms of synthesis method parameters, except for the product composition and structure. See Table 1 for details.

[0042] The difference between Comparative Example 3 and Example 3 is that only an equal amount of phosphoric acid was used as the phosphorus source, and ethylenediaminetetramethylenephosphonic acid was not used.

[0043] The difference between Comparative Example 4 and Example 3 is that the atomization pressure was changed to 0.3 MPa and the D50 particle size was changed to 120 nm, as detailed in Table 2.

[0044] The lithium iron phosphate positive electrode active materials of the above comparative examples and comparative examples were prepared into positive electrode sheets, assembled into soft-pack batteries, and their performance was measured. The results are shown in Table 3.

[0045] Table 3: Performance data of examples and comparative examples

[0046] As can be seen from Table 3, the lithium iron phosphate cathode material prepared by the present invention has excellent cycle stability and low-temperature electrochemical performance. The initial discharge specific capacity at 0.2C is 155-164 mAh / g, the capacity retention rate after 1000 cycles is 90-95%, and the discharge capacity retention rate at -20℃ is 85-91%.

[0047] Compared to Examples 1-4, the Ti / Nb molar ratio in the core of Example 5 does not meet the range of 0.7-1.3:1, and the synergy and balance between the conductivity and structural stability of Ti / Nb elements cannot be achieved. The specific capacity of the first discharge at 0.2C, the capacity retention rate after 1000 cycles, and the capacity retention rate at -20℃ all decreased slightly.

[0048] Comparative Examples 1 and 2 showed significant decreases in specific capacity at 0.2C initial discharge, capacity retention after 1000 cycles, and capacity retention at -20℃ discharge after adjusting the core doping composition by adding only Ti or changing the amount of N and B doping.

[0049] Comparative Example 3 adjusted the phosphorus source and did not use ethylenediaminetetramethylenephosphonic acid. Comparative Example 4 changed the particle size of the FePO4 precursor powder. The 0.2C initial discharge specific capacity, the capacity retention rate after 1000 cycles, and the discharge capacity retention rate at -20℃ all decreased significantly.

[0050] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A lithium iron phosphate cathode material with excellent low-temperature performance, characterized in that, include: The core of the lithium iron phosphate cathode material has the following chemical formula: The general chemical formula is LiFe 1-x-y-z M x N y B z PO4, where: M represents a combination of Ti and Nb, with a doping concentration of 0.5 ≤ x ≤ 1.

0. The doping concentration of N is 0.1 ≤ y ≤ 0.8; the doping concentration of B is 0.03 ≤ z ≤ 0.

08. The core surface of the lithium iron phosphate cathode material also contains a coating layer: The coating layer is a mixture of nano-boron carbide and amorphous carbon, in which the mass percentage of nano-boron carbide is 50% to 65%; the thickness is 20 to 25 nm; and the average particle size of the lithium iron phosphate cathode material is 80 to 110 nm.

2. The lithium iron phosphate cathode material with excellent low-temperature performance according to claim 1, characterized in that, Lithium iron phosphate cathode material core: LiFe 1-x-y-z M x N y B z PO4, where: M represents a combination of Ti and Nb, with a molar ratio of Ti to Nb of 0.7 to 1.3:

1.

3. The lithium iron phosphate cathode material with excellent low-temperature performance according to claim 1, characterized in that, Lithium iron phosphate cathode material core: LiFe 1-x-y-z M x N y B z PO4, where: M is a combination of Ti and Nb, x = 0.8; The doping amount of N is 0.4≤y≤0.6; the doping amount of B is 0.05≤z≤0.

06.

4. The method for synthesizing a lithium iron phosphate cathode material with excellent low-temperature performance according to any one of claims 1 to 3, characterized in that: Includes the following steps: Step 1: Preparation of nitrogen-boron co-doped iron phosphate precursor Iron, phosphorus, nitrogen, and boron sources were dissolved in deionized water according to the elemental ratio of the core chemical formula, and the mixture was stirred at 70–80 °C for 2–3 h to form a homogeneous solution. The pH was adjusted to 4.5–5 with ammonia water, and then the mixture was reacted at 150–180 °C for 12–18 h. The resulting N and B co-doped FePO4 precursor powder with a D50 particle size of 60–80 nm was obtained by spray drying. The phosphorus source was a combination of H3PO4 and ethylenediaminetetramethylenephosphonic acid. Step 2: Lithium source composite and sintering (1) The precursor powder is ball-milled and mixed with Li2CO3 powder, nano boron carbide, amorphous carbon source and doped element M source, and sintered at 700-750°C for 8-10 h under argon atmosphere to form a coating layer, and then cooled to room temperature to obtain the crystalline product. (2) Anneal the crystallized product at 350-400℃ for 2-3 hours, cool it to room temperature, crush it and sieve it to obtain a positive electrode material with an average particle size of 80-110nm.

5. The synthesis method according to claim 4, characterized in that, The molar ratio of phosphorus from H3PO4 to ethylenediaminetetramethylenephosphonic acid in the FePO4 precursor powder is 0.4–0.5:

1.

6. The synthesis method according to claim 5, characterized in that, The iron source is any one or a combination of at least two of ferrous sulfate, ferrous oxalate, and ferrous chloride.

7. The synthesis method according to claim 5, characterized in that, The carbon source for amorphous carbon is selected from any one or a combination of at least two of glucose, sucrose, fructose, lactose, or maltose.

8. The synthesis method according to claim 5, characterized in that, The dopant element M is selected from the oxide of M.

9. The synthesis method according to claim 5, characterized in that, The nitrogen source is melamine.

10. The synthesis method according to claim 5, characterized in that, In step one, the inlet air temperature for spray drying is 80–90°C, and the atomization pressure is 0.5–0.8 MPa.

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