A lithium iron phosphate positive electrode material with high cycle stability and a preparation method thereof

Abstract

The application relates to a high-cycle-stability lithium iron phosphate positive electrode material and a preparation method thereof. 1‑x‑y V x Al y PO4 of an olivine structure, wherein 0.02<=x<=0.05 and 0.01<=y<=0.04; the lithium iron phosphate positive electrode material core surface further contains a gradient coating layer, the coating layer close to the 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 the components Al and V in the core, 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 initial discharge specific capacity is 163.1-169.3 mAh / g at 0.1 C, and the capacity retention rate is 94.1-95.8% after 1000 cycles.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, specifically relating to a lithium iron phosphate cathode material with high cycle stability and its preparation 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 (LiFePO4), as the cathode material for lithium-ion batteries, has become a hot topic in current research and application due to its advantages such as high safety, low cost, environmental friendliness, and good high-temperature performance. However, lithium iron phosphate suffers from poor electronic conductivity and a low lithium-ion diffusion coefficient, leading to performance degradation during high-current charge and discharge. Furthermore, during long-term cycling, the material undergoes volume changes due to lithium-ion insertion and extraction, resulting in structural instability and affecting cycle life.

[0003] To improve the performance of lithium iron phosphate (LFP), existing technologies typically employ metal ion doping and surface coating methods. Metal ion doping can modulate the crystal structure of LFP, improving electronic conductivity and lithium-ion diffusion rate. Surface coating with carbon materials can enhance the conductivity of the material and reduce direct contact between the electrolyte and the active material, thereby improving cycle performance.

[0004] Patent CN108598398A describes a lithium iron phosphate composite cathode material obtained through boron carbide and carbon co-coating. This method suppresses side reactions between the material and the electrolyte, stabilizing the material structure and improving the initial discharge specific capacity, but it does not improve the material's cycle performance. Current research shows that a single coating layer offers limited protection during long-term cycling, and there is still room for improvement in the combination of doping elements and the optimization of the coating process. Therefore, developing a lithium iron phosphate cathode material with superior cycle stability and electrochemical performance, along with its preparation method, is of significant practical importance. Summary of the Invention

[0005] This invention provides a high-cycle-stability lithium iron phosphate cathode material and its preparation method, addressing the problems of insufficient cycle stability and electrochemical performance in current lithium iron phosphate cathode materials due to their single coating layer. The obtained high-cycle-stability lithium iron phosphate cathode material exhibits excellent cycle stability and electrochemical performance, with a compaction density of 2.61–2.81 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%.

[0006] In a first aspect, the present invention relates to a lithium iron phosphate cathode material with high cycle stability, comprising: a lithium iron phosphate cathode material core: having the chemical formula LiFe 1-x-y Vx Al y The olivine structure of PO4, where 0.02≤x≤0.05, 0.01≤y≤0.04.

[0007] The core surface of the lithium iron phosphate cathode material also contains a gradient coating layer: the gradient coating layer includes two layers, the coating layer near the core is a mixed layer of nano-boron carbide and amorphous carbon, in which the mass ratio of nano-boron carbide is 70% to 90%.

[0008] 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 inner layer thickness is 5 to 20 nm, while the outer layer thickness is 2 to 10 nm.

[0009] Preferably, 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.

[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] Secondly, the present invention relates to 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.

[0013] 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.

[0014] (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.

[0015] (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.

[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, and 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 beneficial effects of this invention are as follows: By employing a double-layer coating and the addition of core components Al and V, the lithium iron phosphate cathode material can effectively improve its cycle life and electrochemical performance. The gradient coating structure formed by the inner layer of mixed boron carbide nanoparticles and amorphous carbon, and the outer layer of pure amorphous carbon, 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 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. 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%. Attached Figure Description

[0022] 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.

[0023] Figure 1 This is a schematic diagram of the preparation process of a lithium iron phosphate cathode material with high cycle stability disclosed in an embodiment of the present invention. Detailed Implementation

[0024] 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.

[0025] In existing research on lithium iron phosphate cathode materials, the protective effect of a single coating layer on the material during long-term cycling is limited, and there is still room for improvement in the combination of doping elements and the optimization of the coating process. This invention provides a lithium iron phosphate cathode material with high cycle stability and its preparation method, addressing the problems of single coating layers and insufficient cycle stability and electrochemical performance in current lithium iron phosphate cathode materials, thereby meeting market demands and promoting industry development.

[0026] To address the aforementioned technical problems, this invention provides a lithium iron phosphate cathode material with high cycle stability, comprising: a lithium iron phosphate cathode material core with 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.

[0027] The core is chemically formulated as LiFe 1-x-y V x Al y The olivine structure of PO4 has the following properties: 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 a crucial role. The introduction of vanadium can replace some iron ions, subsequently forming vacancy defects, improving electron hopping efficiency, enhancing conductivity, forming doping energy levels, and improving the electronic conductivity of the material, thereby improving its electrochemical performance. The doping of aluminum can enhance the stability of the crystal structure, suppress the volume change caused by lithium ion insertion and extraction during cycling, and thus improve the cycling stability of the material. This invention preferentially combines V and Al among the doping elements in the prior art, and found that when 0.02 ≤ x ≤ 0.05, 0.01 ≤ y ≤ 0.04, preferably x = 0.03, y = 0.02, the lithium iron phosphate material exhibits optimal lattice stability and ionic conductivity.

[0028] The gradient coating layer consists of two layers. The inner layer, closer to the core, is a mixture of boron carbide nanoparticles and amorphous carbon. The inner layer is also a mixture of boron carbide nanoparticles and amorphous carbon. Boron carbide nanoparticles possess high hardness and good thermal conductivity, forming a tough protective layer on the material surface to resist mechanical stress during cycling and reduce particle breakage. Simultaneously, its good thermal conductivity helps to distribute heat evenly, preventing material performance degradation due to localized overheating. The BC bonds in boron carbide nanoparticles also inhibit electrolyte penetration. Amorphous carbon fills the gaps between the boron carbide nanoparticles, forming a continuous conductive network and further improving the material's conductivity. A ratio of too high or too low in the mixture of boron carbide nanoparticles and amorphous carbon will compromise both the strength of the protective layer and its thermal and electrical conductivity. The mass percentage of boron carbide nanoparticles in this mixture is 70%–90%.

[0029] The outer amorphous carbon layer provides a smooth surface, reducing direct contact between the electrolyte and the core material, minimizing interfacial reactions, and facilitating lithium-ion migration and electron transport, thus providing a highly conductive network and enhancing the material's electrochemical performance. To ensure the full utilization and synergy of the aforementioned effects of the inner and outer amorphous carbon layers, the total mass of the gradient coating layer is limited to 1.5%–4.5% of the cathode material, with the inner layer thickness ranging from 5 to 20 nm and the outer layer thickness from 2 to 10 nm. If the inner layer thickness is too low, the mechanical strength will be insufficient to suppress particle pulverization; if the thickness is too high, it will hinder lithium-ion diffusion paths and reduce rate performance.

[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 in a mass ratio of 3:1.

[0032] The carbon source selected in this invention is one or a combination of sucrose and polyacrylonitrile. When sucrose is pyrolyzed in an inert atmosphere, it is completely converted into amorphous carbon with no residual ash. The pyrolysis process generates abundant pores, forming amorphous carbon with a high specific surface area, enhancing the lithium-ion diffusion interface. Compared to other carbon sources such as glucose, sucrose has higher osmotic pressure stability, which can inhibit precursor particle agglomeration and ensure uniform coating. The pyrolysis of polyacrylonitrile generates a partially graphitized carbon layer with an electrical conductivity 10 times that of pure amorphous carbon, forming a fibrous carbon network that significantly improves the mechanical strength of the coating layer and buffers the volumetric strain during cyclic charging and discharging. The combined use of sucrose and polyacrylonitrile as carbon sources allows full utilization of the ion diffusion channels and interfacial stability provided by sucrose. The pyrolysis of polyacrylonitrile constructs an electron conduction framework and provides volume buffering, improving overall conductivity and resistance to pulverization. Preferably, the amorphous carbon source is selected from sucrose and polyacrylonitrile in a mass ratio of 3:1.

[0033] like 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] Table 3 shows that 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 0.03≤x≤0.05 and 0.02≤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. The compacted density is between 2.61 and 2.81 g / cm³. 3 The capacity retention rate after 1000 cycles is 94.1%–95.8%.

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.

Images