Lithium iron phosphate positive electrode material with high cycle stability and preparation method of lithium iron phosphate positive electrode material

Abstract

The invention relates to a lithium iron phosphate positive electrode material with high cycle stability and a preparation method thereof, the lithium iron phosphate positive electrode material inner core has an olivine structure with a chemical formula of LiFe1-x-yVxAlyPO4, x is more than or equal to 0.02 and less than or equal to 0.05, and y is more than or equal to 0.01 and less than or equal to 0.04; the surface of the inner core of the lithium iron phosphate positive electrode material also contains a gradient coating layer, the coating layer close to the inner core is a mixed layer of nano boron carbide and amorphous carbon, and the outer layer is a pure amorphous carbon layer. Through double-layer coating and addition of inner core components Al and V, the cycle performance and the electrochemical performance are effectively improved, the compaction density of the lithium iron phosphate positive electrode material is 2.61-2.81 g / cm < 3 >, the 0.1 C first discharge specific capacity is 163.1-169.3 mAh / g, and the capacity retention rate after 1000 cycles is 94.1-95.8%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrode materials, and particularly relates to a lithium iron phosphate positive electrode material with high cycle stability and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries have high energy density, long cycle life and other advantages, and are widely used in electric vehicles, energy storage devices and other fields. As a positive electrode material of lithium ion batteries, lithium iron phosphate (LiFePO4) has the advantages of high safety, low cost, environmental friendliness and good high-temperature performance, and has become one of the current research and application hotspots. However, lithium iron phosphate has the problems of poor electronic conductivity and low lithium ion diffusion coefficient, which leads to performance decline during high-current charging and discharging; at the same time, during long-term cycling, the material will change in volume due to the insertion and extraction of lithium ions, thereby leading to unstable structure and affecting cycle life.

[0003] To improve the performance of lithium iron phosphate, the prior art usually adopts the methods of metal ion doping and surface coating. Metal ion doping can adjust the crystal structure of lithium iron phosphate, improve the electronic conductivity and lithium ion diffusion rate. Surface coating of carbon material can improve the conductivity of the material and reduce the direct contact between the electrolyte and the active material, thereby improving the cycle performance.

[0004] Patent CN108598398A obtains a lithium iron phosphate composite positive electrode material by boron carbide and carbon co-coating, inhibits the occurrence of side reactions between the material and the electrolyte, stabilizes the material structure, and improves the first discharge specific capacity, but does not improve the cycle performance of the material. In the current research, the single coating layer has limited protective effect on the material during long-term cycling, and the combination of doping elements and the optimization of coating process still have room for improvement. Therefore, it is of great practical significance to develop a lithium iron phosphate positive electrode material with better cycle stability and electrochemical performance and a preparation method thereof. SUMMARY

[0005] The application provides a lithium iron phosphate positive electrode material with high cycle stability and a preparation method thereof, which are used to solve the problems of single coating layer, insufficient cycle stability and electrochemical performance of the current lithium iron phosphate positive electrode material. The obtained lithium iron phosphate positive electrode material with high cycle stability has excellent cycle stability and electrochemical performance, and the tap density is 2.61-2.81 g / cm 3 , the first discharge specific capacity at 0.1C is 163.1-169.3 mAh / g, and the capacity retention rate after 1000 cycles is 94.1-95.8%.

[0006] In a first aspect, the application relates to a lithium iron phosphate positive electrode material with high cycle stability, which comprises: a lithium iron phosphate positive electrode material core: the chemical formula is LiFe 1-x-y Vx Al y PO4of olivine structure, wherein 0.02≤x≤0.05, 0.01≤y≤0.04.

[0007] The inner core of the lithium iron phosphate positive electrode material further comprises a gradient coating layer, the gradient coating layer comprises two layers, the coating layer close to the inner core is a mixed layer of nano boron carbide and amorphous carbon, and the mass percentage of nano boron carbide in the mixed layer is 70% to 90%.

[0008] The outer layer is a pure amorphous carbon layer; the total mass percentage of the gradient coating layer in the positive electrode material is 1.5% to 4.5%, and the thickness of the inner layer is 5 to 20 nm, and the thickness of the outer layer is 2 to 10 nm.

[0009] Preferably, the inner core of the lithium iron phosphate positive electrode material has a chemical formula of LiFe 1-x-y V x Al y PO4of olivine structure, wherein x=0.03, y=0.02.

[0010] Preferably, the carbon source of the amorphous carbon is selected from one or a combination of sucrose and polyacrylonitrile.

[0011] Preferably, the carbon source of the amorphous carbon is selected from sucrose and polyacrylonitrile in a mass ratio of 3:1.

[0012] In a second aspect, the present application relates to a preparation method of the lithium iron phosphate positive electrode material with high cycle stability, comprising the following steps: step one, co-precipitation doping: reacting iron source, phosphorus source, lithium source, ammonium metavanadate and aluminum isopropoxide in a citric acid buffer solution with pH=4.5 to 5.5 according to the atomic ratio of the inner core chemical formula components to obtain a precursor.

[0013] Step two, step-by-step coating and sintering: (1) mixing the precursor with nano boron carbide and amorphous carbon source by ball milling, pre-sintering at 550 to 650°C for 3 to 4h under a mixed atmosphere of hydrogen and argon, and then cooling to room temperature to obtain a primary crystalline product.

[0014] (2) adding the primary crystalline product to the amorphous carbon source and ball milling in a ball mill for 6 to 9h, drying and crushing, and then sintering at 700 to 750°C under an argon atmosphere for 8 to 10h to form a gradient coating layer, and then cooling to room temperature to obtain a secondary crystalline product.

[0015] (3) annealing the secondary crystalline product at 350 to 400°C for 2 to 3h, crushing and sieving after cooling to room temperature to obtain the lithium iron phosphate positive electrode material with high cycle stability.

[0016] Preferably, the carbon source of the amorphous carbon is selected from one or a combination of sucrose and polyacrylonitrile.

[0017] Preferably, the carbon source of the amorphous carbon is selected from sucrose and polyacrylonitrile in a mass ratio of 3:1.

[0018] Preferably, the lithium source is any one or a combination of at least two of lithium carbonate, lithium acetate or lithium chloride.

[0019] Preferably, the iron source is any one or a combination of at least two of ferrous sulfate, ferrous oxalate or ferrous chloride.

[0020] Preferably, the phosphorus source is any one or a combination of at least two of phosphoric acid, lithium dihydrogen phosphate, sodium phosphate or ammonium dihydrogen phosphate.

[0021] The present application has the beneficial effect that the lithium iron phosphate positive electrode material can effectively improve the cycle performance and electrochemical performance by double-layer coating and the addition of core components Al and V. The gradient coating structure formed by the inner layer of the mixed layer of nanometer boron carbide and amorphous carbon and the outer layer of pure amorphous carbon layer, combined with the synergistic effect of vanadium and aluminum doping in the core, not only improves the conductivity and structural stability of the material, but also reduces the corrosion of the electrolyte to the active material, thereby significantly improving the cycle life and electrochemical performance of the material. The lithium iron phosphate positive electrode material prepared by the present application has a compaction density of 2.61-2.81 g / cm 3 , a first discharge specific capacity of 163.1-169.3 mAh / g at 0.1C, and a capacity retention rate of 94.1-95.8% after 1000 cycles. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0023] Figure 1 A preparation process flow diagram of a high cycle stability lithium iron phosphate positive electrode material disclosed in the embodiments of the present application. DETAILED DESCRIPTION

[0024] The technical solutions of the present application will be described in detail below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0025] In the related research of existing lithium iron phosphate positive electrode materials, the single coating layer has limited protection effect on the material in long-term cycling, and the combination of doping elements and the optimization of coating process still have room for improvement. The present application provides a high cycle stability lithium iron phosphate positive electrode material and a preparation method thereof, which is used to solve the problems of single coating layer, insufficient cycle stability and electrochemical performance of the current lithium iron phosphate positive electrode material, to meet the market requirements and promote the industry development.

[0026] In view of the above technical problems, the embodiment of the present application provides a high cycle stability lithium iron phosphate positive electrode material, which comprises: a lithium iron phosphate positive electrode material core: the chemical formula is LiFe 1-x-y V x Al y PO4 olivine structure, wherein 0.02≤x≤0.05, 0.01≤y≤0.04; the surface of the lithium iron phosphate positive electrode material core also contains a gradient coating layer: the gradient coating layer comprises two layers, the coating layer close to the core is a mixed layer of nano boron carbide and amorphous carbon, the mass fraction of nano boron carbide in the mixed layer is 70%-90%; the outer layer is a pure amorphous carbon layer; the total mass fraction of the gradient coating layer in the positive electrode material is 1.5%-4.5%, and the thickness of the inner layer is 5-20 nm, and the thickness of the outer layer is 2-10 nm.

[0027] The core is a lithium iron phosphate positive electrode material core with the chemical formula LiFe 1-x-y V x Al y PO4 olivine structure, wherein 0.02≤x≤0.05, 0.01≤y≤0.04. Preferably, x=0.03, y=0.02. In the olivine structure, the doping of vanadium (V) and aluminum (Al) plays an important role. The introduction of vanadium elements can replace part of the iron ions, and then form vacancy defects, improve the efficiency of electron hopping, enhance the conductivity, form a doping energy level, improve the electronic conductivity of the material, and thus improve its electrochemical performance; the doping of aluminum elements can enhance the stability of the crystal lattice structure, inhibit the volume change caused by lithium ion intercalation and deintercalation during the cycle process, and thus improve the cycle stability of the material. The present application optimizes V and Al in the doping elements of the prior art, and finds that when 0.02≤x≤0.05, 0.01≤y≤0.04. Preferably, x=0.03, y=0.02, the lattice stability and ion conductivity of the lithium iron phosphate material are optimal.

[0028] The gradient coating layer includes two layers, the coating layer close to the core is a mixed layer of nanometer boron carbide and amorphous carbon. The inner layer is a mixed layer of nanometer boron carbide and amorphous carbon, the nanometer boron carbide has high hardness and good thermal conductivity, can form a tough protective layer on the surface of the material, resist the action of mechanical stress in the cycle process, and reduce the particle breakage; at the same time, its good thermal conductivity helps the uniform distribution of heat, avoids the performance decline of the material caused by local overheating, and the B-C bond of the nanometer boron carbide can also inhibit the penetration of the electrolyte. The amorphous carbon can fill the gap between the nanometer boron carbide, form a continuous conductive network, and further improve the electrical conductivity of the material. The ratio of nanometer boron carbide and amorphous carbon in the mixed layer is 70% to 90% by mass.

[0029] The outer layer of pure amorphous carbon layer can provide a smooth surface, reduce the direct contact of the electrolyte and the core material, reduce the occurrence of interface reaction, and is conducive to the migration of lithium ions and the transmission of electrons, provides a high conductive network, and thus improves the electrochemical performance of the material. In order to ensure that the above-mentioned functions of the inner layer and the outer layer of pure amorphous carbon layer are fully exerted and coordinated, the total mass of the gradient coating layer is limited to 1.5% to 4.5% of the positive electrode material, and the thickness of the inner layer is 5 to 20 nm, and the thickness of the outer layer is 2 to 10 nm. If the thickness of the inner layer is too low, the mechanical strength is insufficient, and the particle pulverization cannot be inhibited, and if the thickness is too large, the lithium ion diffusion path is hindered, and the rate performance is reduced.

[0030] In one embodiment, the carbon source of the amorphous carbon is selected from one or a combination of sucrose and polyacrylonitrile.

[0031] In one embodiment, the carbon source of the amorphous carbon is selected from sucrose and polyacrylonitrile with a mass ratio of 3:1.

[0032] The selection of the carbon source of the present application is one or a combination of sucrose and polyacrylonitrile. Sucrose is completely converted into amorphous carbon when pyrolyzed in an inert atmosphere, without residual ash. The pyrolysis process produces abundant pores, forming amorphous carbon with high specific surface area, enhancing the lithium ion diffusion interface. Sucrose has higher osmotic pressure stability than glucose and other carbon sources, can inhibit the agglomeration of precursor particles, and ensure uniform coating. Polyacrylonitrile pyrolysis generates a partially graphitized carbon layer, which has 10 times the conductivity of pure amorphous carbon, forms a fibrous carbon network, and greatly improves the mechanical strength of the coating layer, buffering the volume strain during the cycle charging and discharging. The combination of sucrose and polyacrylonitrile carbon sources can make full use of the ion diffusion channel and interface stability provided by sucrose, and the electronic conduction skeleton and volume buffering effect after pyrolysis of polyacrylonitrile, improving the overall electrical conductivity and anti-pulverization. Preferably, the carbon source of the amorphous carbon is selected from sucrose and polyacrylonitrile with a mass ratio of 3:1.

[0033] As Figure 1As shown, an embodiment of the present invention provides a method for preparing a high-cycle-stability lithium iron phosphate cathode material, comprising the following steps: Step 1, co-precipitation doping: iron source, phosphorus source, lithium source, ammonium metavanadate, and aluminum isopropoxide are reacted in a citrate buffer solution with pH=4.5-5.5 according to the atomic ratio of the core chemical formula components to obtain a precursor.

[0034] Step 2, step-by-step coating and sintering: (1) The precursor is ball-milled and mixed with nano boron carbide and amorphous carbon source, and pre-sintered at 550-650°C for 3-4 hours in a mixed atmosphere of hydrogen and argon. After cooling to room temperature, a primary crystallization product is obtained.

[0035] (2) Add the above primary crystallization product to an amorphous carbon source, ball mill for 6-9 hours, dry and pulverize, and sinter at 700-750°C for 8-10 hours under an argon atmosphere to form a gradient coating layer. Cool to room temperature to obtain the secondary crystallization product.

[0036] (3) Anneal the secondary crystallization product at 350-400℃ for 2-3 hours, cool it to room temperature, crush it, and sieve it to obtain lithium iron phosphate cathode material with high cycle stability.

[0037] Citrate buffer is used to control the pH of the reaction system to 4.5–5.5. Within this pH range, various metal ions can exist in a suitable form, which is conducive to the formation of a uniform coprecipitate.

[0038] The precursor was ball-milled and mixed with nano-boron carbide and an amorphous carbon source, and then pre-sintered at 550–650°C for 3–4 hours in a mixed atmosphere of hydrogen and argon. After cooling to room temperature, a primary crystallization product was obtained. The ball-milling process allows the nano-boron carbide and amorphous carbon source to adhere uniformly to the surface of the precursor, laying the foundation for subsequent coating. Pre-sintering in a mixed atmosphere of hydrogen and argon allows hydrogen, with its reducing properties, to prevent the oxidation of metal ions at high temperatures, while simultaneously promoting partial decomposition and carbonization of the carbon source, forming a preliminary coating layer. Controlling the pre-sintering temperature at 550–650°C allows the carbon source to begin decomposing and combining with nano-boron carbide, forming an inner layer of nano-boron carbide and amorphous carbon, without causing excessive crystallization of the precursor, which is beneficial for further processing.

[0039] The primary crystallized product is added to an amorphous carbon source and ball-milled for 6–9 hours. After drying and pulverizing, it is sintered at 700–750°C for 8–10 hours under an argon atmosphere to form a gradient coating layer. Cooling to room temperature yields the secondary crystallized product. An amorphous carbon source is added again and ball-milled to uniformly coat the surface of the primary crystallized product with an outer layer of pure amorphous carbon. Sintering under an argon atmosphere provides an inert environment, preventing the material from reacting with oxygen in the air at high temperatures. The sintering temperature of 700–750°C ensures complete decomposition and carbonization of the newly added amorphous carbon source, forming a dense outer layer of pure amorphous carbon, which simultaneously combines with the inner mixed layer to form a complete gradient coating structure.

[0040] The secondary crystallization product was annealed at 350–400℃ for 2–3 hours, cooled to room temperature, crushed, and sieved to obtain a lithium iron phosphate cathode material with high cycle stability. 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 electrochemical 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.

[0041] In one embodiment, the carbon source of the amorphous carbon is selected from one or a combination of sucrose and polyacrylonitrile.

[0042] In one embodiment, the carbon source of the amorphous carbon is selected from sucrose and polyacrylonitrile in a mass ratio of 3:1.

[0043] In one embodiment, the lithium source is any one or a combination of at least two of lithium carbonate, lithium acetate, or lithium chloride.

[0044] In one embodiment, the iron source is any one or a combination of at least two of ferrous sulfate, ferrous oxalate, and ferrous chloride.

[0045] In one embodiment, the phosphorus source is any one or a combination of at least two of phosphoric acid, lithium dihydrogen phosphate, sodium phosphate, or ammonium dihydrogen phosphate.

[0046] Through the aforementioned double-layer coating and the addition of core components Al and V, the lithium iron phosphate cathode material of this invention can effectively improve cycle performance and electrochemical performance. The gradient coating structure formed by the inner layer of nano-boron carbide mixed with amorphous carbon and the outer layer of pure amorphous carbon, combined with the doping of vanadium and aluminum in the core, synergistically improves both the conductivity and structural stability of the material, while reducing the erosion of the active material by the electrolyte, thereby significantly enhancing the cycle life and electrochemical performance of the material. The lithium iron phosphate cathode material prepared by this invention has a compaction density of 2.61–2.81 g / cm³. 3The initial discharge specific capacity at 0.1C is 163.1–169.3 mAh / g, and the capacity retention rate after 1000 cycles is 94.1–95.8%.

[0047] The embodiments of the present invention are described in detail below. The hammerhead composition of Embodiments 1 to 5 and Comparative Examples 1 to 2 is shown in Table 1.

[0048] Table 1: Composition of lithium iron phosphate cathode materials in Examples 1-5 and Comparative Examples 1-2:

[0049] The process parameters used in the preparation methods of Examples 1-5 and Comparative Examples 3-4 of this invention are shown in Table 2.

[0050] Table 2: Process parameters used in the preparation methods of Examples 1-5 and Comparative Examples 3-4:

[0051] Comparative Examples 1 and 2 are identical to Example 5 in terms of preparation method parameters, except for the product composition and structure. See Table 1 for details.

[0052] The difference between Comparative Example 3 and Example 5 is that the pre-sintering temperature parameters are slightly different, as shown in Table 2.

[0053] The difference between Comparative Example 4 and Example 5 is that, as shown in Table 2, step two is a one-time coating. An amorphous carbon source is directly added to the precursor, ball milled for 9 hours, dried and pulverized, and then sintered at 730°C for 8.5 hours under an argon atmosphere to form a gradient coating layer. After cooling to room temperature, a crystalline product is obtained. The crystalline product is annealed at 350°C for 2 hours, cooled to room temperature, crushed, and sieved to obtain lithium iron phosphate cathode material.

[0054] 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.

[0055] Table 3: Performance data of the examples and comparative examples:

[0056] As shown in Table 3, the lithium iron phosphate cathode material prepared by this invention exhibits excellent cycle stability and electrochemical performance, with a compaction density ranging from 2.61 to 2.81 g / cm³. 3 The initial discharge specific capacity at 0.1C is 163.1–169.3 mAh / g, and the capacity retention rate after 1000 cycles is 94.1–95.8%.

[0057] Compared with Examples 1 and 2, Examples 3 and 5 further optimized the Al and V addition amounts in the core to x=0.03 and y=0.02, resulting in improved compaction density, 0.1C first discharge specific capacity, and capacity retention rate after 1000 cycles.

[0058] Compared to Examples 1 and 2, Examples 4 and 5 further optimized the amorphous carbon source by selecting sucrose and polyacrylonitrile in a mass ratio of 3:1, resulting in improved compaction density, 0.1C initial discharge specific capacity, and capacity retention after 1000 cycles.

[0059] Comparative Examples 1 and 2 showed that adjusting the amount of Al and V added to the core, as well as the thickness of the coating layer and the ratio of boron carbide, resulted in a significant decrease in compaction density, 0.1C first discharge specific capacity, and capacity retention after 1000 cycles.

[0060] Comparative Example 3 adjusted the pre-sintering temperature, while Comparative Example 4, using a single coating process, showed a significant decrease in the material's compaction density, initial discharge specific capacity at 0.1C, and capacity retention rate after 1000 cycles.

[0061] 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 high cycle stability, characterized in that, include: The core of the lithium iron phosphate cathode material has the chemical formula LiFe. 1-x-y V x Al y The PO4 has an olivine structure, where 0.02≤x≤0.05 and 0.01≤y≤0.

04. The core surface of the lithium iron phosphate cathode material also contains a gradient coating layer: the gradient coating layer consists of two layers, the coating layer near the core is a mixed layer of boron carbide nanoparticles and amorphous carbon, in which boron carbide nanoparticles account for 70% to 90% of the mass; the outer layer is a pure amorphous carbon layer; the total mass of the gradient coating layer accounts for 1.5% to 4.5% of the cathode material, and the thickness of the inner layer is 5 to 20 nm, and the thickness of the outer layer is 2 to 10 nm.

2. The lithium iron phosphate cathode material with high cycle stability according to claim 1, characterized in that, The core of the lithium iron phosphate cathode material has the chemical formula LiFe. 1-x-y V x Al y The olivine structure of PO4, where x = 0.03 and y = 0.

02.

3. The lithium iron phosphate cathode material with high cycle stability according to claim 1, characterized in that, The carbon source for the amorphous carbon is selected from one or a combination of two of sucrose and polyacrylonitrile.

4. The lithium iron phosphate cathode material with high cycle stability according to claim 3, characterized in that, The carbon source for the amorphous carbon is selected from sucrose and polyacrylonitrile in a mass ratio of 3:

1.

5. A method for preparing a high-cycle-stability lithium iron phosphate cathode material according to any one of claims 1 to 4, characterized in that: The process includes the following steps: Step 1, co-precipitation doping: Iron source, phosphorus source, lithium source, ammonium metavanadate, and aluminum isopropoxide are reacted in a citric acid buffer solution with pH=4.5-5.5 according to the atomic ratio of the core chemical formula components to obtain a precursor; Step 2, stepwise coating and sintering: (1) The precursor is ball-milled and mixed with nano boron carbide and amorphous carbon source, and pre-sintered at 550-650℃ for 3-4h in a mixed atmosphere of hydrogen and argon, and then cooled to room temperature to obtain a primary crystallization product; (2) The above primary crystallization product is added to the amorphous carbon source, ball-milled in a ball mill for 6-9h, dried and pulverized, and then sintered at 700-750℃ for 8-10h in an argon atmosphere to form a gradient coating layer, and cooled to room temperature to obtain a secondary crystallization product; (3) The secondary crystallization product is annealed at 350-400℃ for 2-3h, cooled to room temperature, crushed, and sieved to obtain a high cycle stability lithium iron phosphate cathode material.

6. The preparation method according to claim 5, characterized in that, The carbon source for the amorphous carbon is selected from one or a combination of two of sucrose and polyacrylonitrile.

7. The preparation method according to claim 6, characterized in that, The carbon source for the amorphous carbon is selected from sucrose and polyacrylonitrile in a mass ratio of 3:

1.

8. The preparation method according to claim 5, characterized in that, The lithium source is any one or a combination of at least two of lithium carbonate, lithium acetate, or lithium chloride.

9. The preparation 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.

10. The preparation method according to claim 5, characterized in that, The phosphorus source is any one or a combination of at least two of phosphoric acid, lithium dihydrogen phosphate, sodium phosphate, or ammonium dihydrogen phosphate.

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