Lithium ferrous silicate positive electrode material and preparation method thereof

Abstract

The application discloses a lithium ferrous silicate positive material and a preparation method thereof, and belongs to the technical field of lithium ion batteries. The application comprises the following steps: firstly, preparing a precursor: lithium source, iron source, silicon source and fluorine source are weighed according to the stoichiometric ratio of Li, Fe, Si and F as 2:1:1:(0.01-0.04), mixed with a carbon source in a solvent, dried to obtain the precursor; then, two-stage calcination is carried out, and the lithium ferrous silicate positive material is obtained after cooling to room temperature. The application adopts fluorine doping, compared with metal ion doping in the prior art, the cost is reduced, and the effect of improving the ion and electron conductivity of the lithium ferrous silicate positive material is obvious.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and more specifically, relates to a lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries possess advantages such as high energy density, long cycle life, low self-discharge rate, and no pollution, making them widely used in power batteries and grid energy storage. The rapid development of lithium-ion batteries has increased the demand for high-performance, low-cost cathode materials. Lithium iron silicate cathode materials have advantages such as low cost, abundant raw materials, high theoretical specific capacity (330 mAh / g), good thermal stability, and overcharge resistance, making them ideal cathode materials for lithium-ion batteries. However, the low ionic and electronic conductivity of lithium iron silicate cathode materials limits their application in power batteries and energy storage. Currently, researchers often use ion doping, nano-sizing, and carbon coating techniques to improve the ionic and electronic conductivity of lithium iron silicate cathode materials. Carbon coating is the most commonly used modification method; it can not only improve the electronic conductivity of lithium iron silicate cathode materials but also inhibit grain growth, thereby achieving particle nano-sizing.

[0003] Fluorine has strong electronegativity, and fluorine doping can reduce the transfer impedance of lithium iron silicate cathode materials, thereby improving their ionic and electronic conductivity and enhancing the electrochemical performance of the electrode material. Carbon coating can enable the nano-sizing of lithium iron silicate cathode materials, shortening the lithium ion transport distance within the electrode material and further improving the rate performance of the electrode material.

[0004] Existing technologies utilize metal ions such as Ni 2+ Co 2+ Mn 2+ Cr 3+ Zn 2+ Mg 2+ These techniques aim to improve the ionic and electronic conductivity of lithium iron silicate cathode materials. However, they are not only costly but also do not provide ideal results, hindering the development of low-cost, high-performance lithium iron silicate cathode materials.

[0005] Chinese patent document 201010507828.8 discloses a method for preparing lithium iron phosphate cathode material that is simultaneously doped with fluorine and zirconium. However, this method requires the doping of zirconium into the cathode material, which increases the production cost. Furthermore, zirconium may affect the stability and other electrochemical properties of the cathode material. Therefore, there is still considerable room for improvement in the overall performance of lithium iron phosphate cathode material. Summary of the Invention

[0006] 1. The problem to be solved

[0007] Regarding existing technologies that utilize metal ions such as Ni 2+ Co2+ Mn 2+ Cr 3+ Zn 2+ Mg 2+ To address the issues of high cost and poor effectiveness in improving the ionic and electronic conductivity of lithium iron silicate cathode materials, this invention provides a lithium iron silicate cathode material and its preparation method.

[0008] 2. Technical Solution

[0009] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0010] In a first aspect, the present invention provides a method for preparing lithium iron phosphate cathode material, comprising the following steps:

[0011] S1. Providing a precursor: Providing a precursor containing a lithium source, an iron source, a silicon source, a fluorine source, and a carbon source, wherein the lithium source, iron source, silicon source, and fluorine source are weighed in a stoichiometric ratio of Li, Fe, Si, and F of 2:1:1:(0.01 to 0.04);

[0012] S2. Calcination: Under an inert gas atmosphere, the precursor is heated at a constant temperature of 300-400°C for 3-4 hours, and then heated at a constant temperature of 700-750°C for 5-6 hours. After cooling, lithium iron phosphate cathode material is obtained.

[0013] The reason for adopting the above technical solution is that, compared with the existing technology that incorporates metals such as Mg, Zn, Ni, Co, Nr, Cu, Ge, Ti, and Cr into lithium iron phosphate cathode materials, or simultaneously incorporates fluorine and metals, the present invention only incorporates fluorine, which can maintain or even improve the electrochemical performance of lithium iron phosphate cathode materials while saving costs.

[0014] Furthermore, the determination of calcination temperature and time in step S2 has a significant impact on the electrochemical performance of lithium iron phosphate cathode material. In the two-stage calcination, there are several possibilities for how to design the time and temperature conditions for each calcination to better improve the material's performance. In this invention, the temperature and time of both calcinations are limited to a small range. The lithium iron phosphate cathode material prepared under these conditions has better performance than that under other conditions. This may be because calcination involves heating the precursor material at high temperature to induce phase transition and crystal growth. By controlling the temperature and time of the two calcinations within a small range, the desired crystal phase can be obtained preferentially, the formation of unwanted phases can be reduced, the purity of the crystal phase can be increased, and the grain growth can be better controlled to form smaller and more uniform nanoparticles.

[0015] Furthermore, in step S1, the lithium source includes one, two, or more of lithium carbonate, lithium oxide, lithium hydroxide, lithium chloride, lithium oxalate, lithium acetate, lithium nitrate, lithium sulfate, and lithium dihydrogen phosphate.

[0016] Furthermore, in step S1, the iron source includes one, two, or more of ferrous oxalate, ferrous acetate, ferrous chloride, ferrous sulfate, ferrous hydroxide, ferrous nitrate, ferrous citrate, and ferrous oxide.

[0017] Furthermore, in step S1, the silicon source includes silicon dioxide.

[0018] Furthermore, in step S1, the silicon source is fumed silicon dioxide with a size range of 20–30 nm.

[0019] The reason for adopting the above technical solution is that using fumed silica with a size range of 20-30 nm, compared to larger-sized non-fumed silica, facilitates control over the particle size of the final lithium iron phosphate cathode material, resulting in a smaller particle size, specifically 25-55 nm. This shortens the path for lithium ions to enter the active material from the electrolyte or return to the electrolyte, helping to reduce the diffusion resistance and migration time of lithium ions and improve the charge and discharge efficiency of lithium-ion batteries. In contrast, the particle size of existing lithium iron phosphate cathode materials is typically around 100 nm, and their overall conductivity is relatively poor.

[0020] Furthermore, in step S1, the fluorine source is selected from one, two, or more of lithium fluoride, silicon fluoride, sodium fluoride, potassium fluoride, and ammonium fluoride.

[0021] Furthermore, in step S1, the fluorine source is selected from one or both of lithium fluoride and silicon fluoride.

[0022] The reason for adopting the above technical solution is that, since lithium and silicon are the components in the lithium iron silicate cathode material, the target product of this invention, lithium fluoride or silicon fluoride will not introduce impurity elements compared to other fluorine sources.

[0023] Furthermore, in step S1, the carbon source is an organic carbon source.

[0024] The reason for adopting the above technical solution is that inorganic carbon sources are usually in an amorphous state after carbonization, and the coating uniformity is poor. In contrast, organic carbon sources can coat the target product surface more uniformly after carbonization, which can prevent the electrode material from being corroded in the electrolyte, better protect the electrode material, and form an effective conductive transport channel on the material surface, thereby improving the electrochemical performance of the material.

[0025] Furthermore, the organic carbon source is selected from high molecular weight organic compounds.

[0026] The reason for adopting the above technical solution is that, due to its macromolecular chain structure, the polymeric organic carbon source has better flexibility and fluidity. During the carbonization process, this characteristic allows the polymeric organic carbon source to more uniformly cover the surface of the target product, forming a continuous and dense coating layer. For example, the carbon source is polyvinylpyrrolidone.

[0027] Furthermore, in step S1, the carbon layer formed after the carbon source is carbonized accounts for 3wt% to 10wt% of the lithium iron silicate cathode material.

[0028] The reason for adopting the above technical solution is that if the content of carbon source is too low, it is impossible to effectively coat lithium iron silicate; if the content of polyvinylpyrrolidone is too high, the gaps that are originally for lithium ion transport will be blocked in large quantities, which will greatly affect the electrode reaction kinetics. Therefore, the carbon layer formed after carbonization of carbon source accounts for 3wt% to 10wt% of lithium iron silicate cathode material, which is a reasonable range.

[0029] Furthermore, in step S1, the solvent is at least one of deionized water, distilled water, ethanol, and acetone.

[0030] Further, step S1 specifically involves: weighing Li2CO3, Fe2CO4·4H2O, SiO2, and LiF according to the stoichiometric ratio of Li, Fe, Si, and F as (2-x):1:1:x, mixing them with polyvinylpyrrolidone in an ethanol solution, and drying them to obtain the precursor;

[0031] The value of x ranges from 0.01 to 0.04.

[0032] The reason for adopting the above technical solution is that: by selecting the above formula as the lithium iron silicate cathode material, lithium fluoride can not only provide lithium source but also fluorine source, without introducing impurities. At the same time, by controlling the stoichiometric ratio of Li2CO3 and LiF within a specific range, the conductivity of the lithium iron silicate cathode material can be further improved.

[0033] Furthermore, in step S2, the inert gas is selected from at least one of argon, nitrogen, carbon monoxide, carbon dioxide, and hydrogen.

[0034] Furthermore, in step S2, the inert gas is argon and nitrogen.

[0035] Compared to carbon monoxide, carbon dioxide, and hydrogen, argon and nitrogen are more advantageous in terms of safety and cost as inert gases.

[0036] Furthermore, the volume ratio of argon to nitrogen is (3-5):1.

[0037] Preferably, the volume ratio of argon to nitrogen is 4:1.

[0038] The reason for adopting the above technical solution is that: firstly, argon is completely inert, has a higher density, and can be used at extremely high temperatures without causing risks; secondly, nitrogen is less expensive than argon. Considering the combination of safety and cost advantages, the overall proportion of argon is controlled at 80%, and nitrogen at 20%.

[0039] Furthermore, in step S1, the mixing conditions include: mixing method: ball milling; ball mill speed: 400-500 r / min; ball-to-material ratio: (6-8):1.

[0040] For example, in step S1, the ball mill speed is 400 r / min and the ball-to-material ratio is 7:1.

[0041] Secondly, the present invention provides a lithium iron phosphate cathode material, which is prepared by the aforementioned method.

[0042] Furthermore, the particle size of the lithium iron phosphate cathode material is 25–55 nm.

[0043] Furthermore, the surface of the lithium iron silicate cathode material is coated with a carbon layer.

[0044] 3. Beneficial effects

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] (1) The cathode material of the present invention uses only fluorine doping. Compared with the metal ion doping in the prior art, it can significantly improve the ionic and electronic conductivity of lithium iron silicate cathode material while reducing the raw material cost. The electrochemical performance of the cathode material of the present invention is significantly improved compared with that without fluorine doping. At a charge-discharge rate of 0.1C, the discharge specific capacity can reach 215.5mAh / g.

[0047] (2) The method for preparing the fluorine-doped lithium iron phosphate cathode material of the present invention is simple. The prepared lithium iron phosphate cathode material has a particle size of approximately 25–55 nm, and its surface is uniformly coated with a layer of carbon material. The uniform carbon coating layer has the following effect: the carbon layer can inhibit the growth of grains during the preparation process, promote the formation of smaller nanoparticles, and shorten the Li-P of the material. + The diffusion path; secondly, it can enhance the conductivity of the material and improve the Li- content of the material. + It can reduce the diffusion rate; finally, it can reduce electrode / solution side reactions and enhance the electrochemical stability of the material. Attached Figure Description

[0048] Figure 1This is a SEM image of the lithium iron silicate cathode material in Example 1.

[0049] Figure 2 The graph shows the charge-discharge curves of the lithium iron silicate cathode material in Example 1 at 0.1C. Detailed Implementation

[0050] It should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0051] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0052] As used herein, the term “about” is used to provide for the flexibility and imprecision associated with a given term, measure, or value. Those skilled in the art can readily determine the degree of flexibility for a particular variable.

[0053] As used herein, “adjacent” means that two structures or elements are close to each other. Specifically, elements identified as “adjacent” may be adjacent or connected. Such elements may also be close to or near each other without necessarily touching. In some cases, the precision of proximity may depend on the specific context.

[0054] As used herein, the term “at least one of…” is intended to be synonymous with “one or more of…”. For example, “at least one of A, B, and C” explicitly includes only A, only B, only C, and combinations thereof.

[0055] Concentration, amount, and other numerical data may be presented in range format herein. It should be understood that such range format is used solely for convenience and brevity and should be flexibly interpreted to include not only the values ​​explicitly stated as the limits of the range, but also all individual values ​​or subranges encompassed within the range, as if each value and subrange were explicitly stated. For example, a range of values ​​from about 1 to about 4.5 should be interpreted to include not only the explicitly stated limits of 1 to 4.5, but also individual numbers (such as 2, 3, 4) and subranges (such as 1 to 3, 2 to 4, etc.). The same principle applies to ranges that describe only a single value, such as “less than about 4.5,” which should be interpreted to include all the aforementioned values ​​and ranges. Furthermore, this interpretation should apply regardless of the breadth of the range or characteristic described.

[0056] Any step described in any method or process claim (e.g., steps S1, S2, S3... or steps (1), (2), (3)... or steps 1), 2), 3)...) may be performed in any order, and is not limited to the order set forth in the claims.

[0057] The limitation of method + function or step + function is used only if all of the following conditions are met in a particular claim: a) it expressly states "a method for..." or "a step for..."; b) it expressly states the corresponding function. The structures, materials, or actions supporting the method + function are expressly described in the description herein. Therefore, the scope of the invention should be determined solely by the appended claims and their legal equivalents, and not by the description and examples given herein.

[0058] The present invention will be further described below with reference to specific embodiments.

[0059] Example 1

[0060] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0061] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and F of 1.99:1:1:0.01. 1.475 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.0052 g of LiF were weighed and mixed with 2.117 g of polyvinylpyrrolidone in an ethanol solution by ball milling. The ball mill was a YXQM-1L model ball mill produced by Changsha Miqi Instrument Equipment Co., Ltd. The ball milling speed was 400 r / min; the ball milling time was 5 hours; the ball-to-material ratio was 7:1; and then dried at 85℃ for 12 h to obtain the precursor.

[0062] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.99 FeSiO4F 0.01 / C represents the target product, fluorine-doped nano-Li₂FeSiO₄ / C. Fluorine-doped nano-Li₂FeSiO₄ / C was assembled into a coin cell, and its specific capacity was tested at different rates, ranging from 1.5 to 4.8 V. The test results showed that its discharge specific capacity at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C was 215.5 mAh / g, 170.7 mAh / g, 165.6 mAh / g, and 100.2 mAh / g, respectively. Figure 1 This is a SEM image of fluorine-doped nano-Li₂FeSiO₄ / C in this embodiment. Figure 2This is a charge-discharge curve of fluorine-doped nano-Li2FeSiO4 / C in this embodiment at 0.1C.

[0063] Example 2

[0064] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0065] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and F of 1.98:1:1:0.02. 1.474 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.01 g of LiF were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in ethanol solution and dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0066] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.98 FeSiO4F 0.02 / C represents the target product, fluorine-doped nano-Li₂FeSiO₄ / C. Fluorine-doped nano-Li₂FeSiO₄ / C was assembled into a coin cell, and its specific capacity was tested at different rates, ranging from 1.5 to 4.8 V. The test results showed that its discharge specific capacity at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C was 223.5 mAh / g, 180.4 mAh / g, 175.8 mAh / g, and 105.3 mAh / g, respectively.

[0067] Example 3

[0068] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0069] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and F of 1.97:1:1:0.03. 1.473 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.016 g of LiF were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in ethanol solution and dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0070] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.97 FeSiO4F 0.03 / C represents the target product, fluorine-doped nano-Li₂FeSiO₄ / C. Fluorine-doped nano-Li₂FeSiO₄ / C was assembled into a coin cell, and its specific capacity was tested at different rates, ranging from 1.5 to 4.8 V. The test results showed that its discharge specific capacity at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C was 213.8 mAh / g, 168.2 mAh / g, 160.4 mAh / g, and 96.5 mAh / g, respectively.

[0071] Example 4

[0072] This embodiment provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0073] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and F of 1.96:1:1:0.04. 1.472 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.021 g of LiF were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in ethanol solution and dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0074] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.96 FeSiO4F 0.04 / C represents the target product, fluorine-doped nano-Li₂FeSiO₄ / C. Fluorine-doped nano-Li₂FeSiO₄ / C was assembled into a coin cell, and its specific capacity was tested at different rates, ranging from 1.5 to 4.8 V. The test results showed that its discharge specific capacity at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C was 205.7 mAh / g, 157.6 mAh / g, 142.8 mAh / g, and 87.3 mAh / g, respectively.

[0075] Comparative Example 1

[0076] This comparative example provides a method for preparing lithium iron phosphate cathode material, including the following steps:

[0077] (1) The raw materials were weighed according to the molar ratio of Li, Fe and Si of 2:1:1. 1.478 g of Li2CO3, 4.32 g of FeC2O4·4H2O and 1.2 g of nano SiO2 were weighed and mixed with 2.117 g of polyvinylpyrrolidone in ethanol solution by ball milling and drying to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0078] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300℃ for 4 hours and then at 700℃ for 6 hours. After cooling to room temperature, Li2FeSiO4 / C was obtained, which is the target product, nano-Li2FeSiO4 / C. The nano-Li2FeSiO4 / C was assembled into a coin cell, and its specific capacity was tested at different rates, with a voltage range of 1.5–4.8V. The test results showed that its discharge specific capacity at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C was 170.7 mAh / g, 120.6 mAh / g, 102.8 mAh / g, and 47.3 mAh / g, respectively.

[0079] As can be seen from this comparative example, when the molar fractions of lithium, iron and silicon remain unchanged, if all lithium sources are lithium carbonate and no fluorine is added, the electrochemical performance of the cathode material will decrease significantly, and the discharge specific capacity will decrease by about one-quarter.

[0080] Comparative Example 2

[0081] (1) The raw materials were weighed according to the molar ratio of Li, Fe and Si of 2:1:1. 1.04 g of LiF, 4.32 g of FeC2O4·4H2O and 1.2 g of nano SiO2 were weighed and mixed with 2.117 g of polyvinylpyrrolidone in ethanol solution by ball milling and drying to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0082] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300℃ for 4 hours and then at 700℃ for 6 hours. After cooling to room temperature, Li2FeSiO4 / C was obtained, which is the target product, nano-Li2FeSiO4 / C. The nano-Li2FeSiO4 / C was assembled into a coin cell, and its specific capacity was tested at different rates, with a voltage range of 1.5–4.8V. The test results showed that its discharge specific capacity at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C was 120.7 mAh / g, 70.6 mAh / g, 52.8 mAh / g, and 27.3 mAh / g, respectively.

[0083] As can be seen from this comparative example, when the molar fractions of lithium, iron, and silicon remain unchanged, if lithium fluoride is used as the lithium source, the molar fraction of fluorine will be too high and exceed the limit value by a large margin. The electrochemical performance of the cathode material will decrease significantly, and the discharge specific capacity will decrease by about half.

[0084] Comparative Example 3

[0085] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and F of 1.995:1:1:0.005. 1.4739 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.0026 g of LiF were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in ethanol solution and dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0086] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.995 FeSiO4F 0.005 / C, which is the target product nano-Li 1.995 FeSiO4F 0.005 / C. Nano Li 1.995 FeSiO4F 0.005 The cells were assembled into button cells using the / C assembly method, and their specific capacity was tested at different rates, with a voltage range of 1.5–4.8V. The test results showed that the discharge specific capacity was 180.5mAh / g, 131.4mAh / g, 106.8mAh / g, and 55.3mAh / g at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C, respectively.

[0087] As can be seen from this comparative example, when the molar fractions of lithium, iron, and silicon remain unchanged, if the molar fraction of fluorine is lower than the limit, the electrochemical performance of the cathode material will decrease significantly, and the discharge specific capacity will decrease by about 10%.

[0088] Comparative Example 4

[0089] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and F of 1.95:1:1:0.05. 1.44 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.026 g of LiF were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in ethanol solution and dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0090] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.95 FeSiO4F 0.05 / C, which is the target product nano-Li 1.95 FeSiO4F 0.05 / C. Nano Li 1.95 FeSiO4F 0.05The cells were assembled into button cells using the / C assembly method, and their specific capacity was tested at different rates, with a voltage range of 1.5–4.8V. The test results showed that the discharge specific capacity was 197.5mAh / g, 140.4mAh / g, 115.8mAh / g, and 65.2mAh / g at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C, respectively.

[0091] As shown in this comparative example, when the mole fractions of lithium, iron, and silicon remain constant, if the mole fraction of fluorine exceeds the limit,

[0092] The electrochemical performance of the cathode material will decrease, and its discharge specific capacity at a charge-discharge rate of 0.1C will drop to below 200mAh / g.

[0093] Comparative Example 5

[0094] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and F of 1.94:1:1:0.06. 1.433 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.0312 g of LiF were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in ethanol solution and dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0095] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.94 FeSiO4F 0.06 / C, which is the target product nano-Li 1.94 FeSiO4F 0.06 / C. Nano Li 1.94 FeSiO4F 0.06 The cells were assembled into button cells using the / C assembly method, and their specific capacity was tested at different rates, with a voltage range of 1.5–4.8V. The test results showed that the discharge specific capacity was 183.5mAh / g, 134.4mAh / g, 107.8mAh / g, and 60.6mAh / g at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C, respectively.

[0096] A comprehensive analysis of this comparative example and comparative example 4 shows that when the molar fractions of lithium, iron, and silicon remain constant, the more the molar fraction of fluorine exceeds the limit, the more significant the decrease in the electrochemical performance of the cathode material. Its discharge specific capacity at a charge-discharge rate of 0.1C even drops below 190mAh / g.

[0097] Comparative Example 6

[0098] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si and Mg as 2:0.98:1:0.02. 1.478 g of Li2CO3, 4.23 g of FeC2O4·4H2O, 1.2 g of nano SiO2 and 0.033 g of MgCO3 were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in ethanol solution and dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0099] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li₂Fe₂ was obtained. 0.98 Mg 0.02 SiO4 / C, which is the target product, nano-Li2Fe. 0.98 Mg 0.02 SiO4 / C. Nano-Li2Fe 0.98 Mg 0.02 SiO4 / C was assembled into button half-cells, and their specific capacity was tested at different rates, with a voltage range of 1.5–4.8V. The test results showed that the discharge specific capacity was 170.5mAh / g, 124.4mAh / g, 87.8mAh / g, and 53.6mAh / g at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C, respectively.

[0100] As can be seen from this comparative example, if magnesium is used to replace fluorine in the cathode material, and the molar fraction of silicon (one-quarter) and the molar fraction of metals (including lithium, iron, and magnesium) (three-quarters) remain unchanged, it can be seen that the electrochemical performance will decrease significantly due to the lack of fluorine doping, and the discharge specific capacity will decrease by about one-quarter.

[0101] Comparative Example 7

[0102] (1) The raw materials were weighed according to the molar ratio of Li, Fe, Si, Mg, F of 1.98:1:1:0.01:0.01. 1.462 g of Li2CO3, 4.32 g of FeC2O4·4H2O, 1.2 g of nano SiO2, 0.016 g of MgCO3, and 0.0052 g of LiF were weighed and ball-milled with 2.117 g of polyvinylpyrrolidone in an ethanol solution and then dried to obtain the precursor. The ball milling and drying conditions were the same as in Example 1.

[0103] (2) Under an atmosphere of 80% argon and 20% nitrogen, the precursor was heated at 300°C for 4 hours and then at 700°C for 6 hours. After cooling to room temperature, Li was obtained. 1.99 Fe 0.99 Mg 0.01 SiO4F 0.01 / C, which is the target product nano-Li 1.99 Fe 0.99 Mg0.01 SiO4F 0.01 / C. Nano Li 1.99 Fe 0.99 Mg 0.01 SiO4F 0.01 The cells were assembled into button cells using the / C assembly method, and their specific capacity was tested at different rates, with a voltage range of 1.5–4.8V. The test results showed that the discharge specific capacity was 183.5mAh / g, 136.4mAh / g, 97.8mAh / g, and 63.7mAh / g at charge / discharge rates of 0.1C, 0.5C, 1C, and 5C, respectively.

[0104] As can be seen from this comparative example, replacing half of the lithium fluoride in the cathode material with other metals means that fluorine and metals are doped at the same time. While ensuring that the molar fraction of silicon (one-quarter) and the molar fraction of metals (including lithium, iron, and magnesium) (three-quarters) remain unchanged, and the molar fraction of fluorine is also within the specified range, the electrochemical performance of the cathode material will still decrease. Its discharge specific capacity at a charge-discharge rate of 0.1C even drops below 190mAh / g. The effect of doping only with fluorine is better than doping with other metals or doping with both metals and fluorine at the same time.

[0105] The above description provides an illustrative overview of the present invention and its embodiments. This description is not restrictive, and the embodiments shown are merely one example of the invention's implementation. Actual implementations are not limited to these examples. Therefore, if those skilled in the art are inspired by this description and design similar implementations and examples without departing from the spirit of the invention, such designs should fall within the scope of protection of the present invention.

Claims

1. A method for preparing lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1. Providing a precursor: Providing a precursor containing a lithium source, an iron source, a silicon source, a fluorine source, and a carbon source, wherein the lithium source, iron source, silicon source, and fluorine source are weighed in a stoichiometric ratio of Li, Fe, Si, and F of 2:1:1:(0.01 to 0.04); S2. Calcination: Under an inert gas atmosphere, the precursor is heated at a constant temperature of 300-400°C for 3-4 hours, and then heated at a constant temperature of 700-750°C for 5-6 hours. After cooling, lithium iron phosphate cathode material is obtained. The lithium source includes one or more of lithium carbonate, lithium oxide, lithium hydroxide, lithium chloride, lithium oxalate, lithium acetate, lithium nitrate, lithium sulfate, and lithium dihydrogen phosphate. The iron source includes one or more of ferrous oxalate, ferrous acetate, ferrous chloride, ferrous sulfate, ferrous hydroxide, ferrous nitrate, ferrous citrate, and ferrous oxide. The silicon source includes silicon dioxide; the silicon source is fumed silicon dioxide with a size range of 20–30 nm. The fluorine source is selected from one or more of lithium fluoride, silicon fluoride, sodium fluoride, potassium fluoride and ammonium fluoride; The carbon source is polyvinylpyrrolidone; the carbon layer formed after carbonization of the carbon source accounts for 3wt% to 10wt% of the lithium iron silicate cathode material.

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the fluorine source is selected from one or both of lithium fluoride and silicon fluoride.

3. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step S2, the inert gas is selected from at least one of argon and nitrogen.

4. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, Step S1 specifically involves weighing Li2CO3, Fe2CO4·4H2O, SiO2, and LiF according to the stoichiometric ratio of Li, Fe, Si, and F as (2-x):1:1:x, mixing them with polyvinylpyrrolidone in an ethanol solution, and drying them to obtain the precursor; the value of x ranges from 0.01 to 0.

04.

5. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by the method described in any one of claims 1 to 4.

6. The lithium iron phosphate cathode material according to claim 5, characterized in that, The particle size of the lithium iron silicate cathode material is 25–55 nm.

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