Preparation method of olivine-type sodium iron phosphate, sodium iron phosphate of olivine type and sodium-ion battery

Abstract

This invention provides a method for preparing olivine-type sodium iron phosphate, olivine-type sodium iron phosphate, and a sodium-ion battery, specifically relating to the field of sodium-ion secondary battery technology. The preparation method involves dissolving sodium propionate and sodium salt, adding olivine-type iron phosphate, and grinding to obtain a precursor slurry. The precursor slurry is then spray-dried and sintered at 200-400°C to obtain the olivine-type sodium iron phosphate. In this preparation method, sodium propionate not only serves as a sodium source, but propionate ions also promote the kinetics of the solid-phase reaction, resulting in purer olivine-type sodium iron phosphate and further improved capacity. Furthermore, to reduce the high ineffective carbon content caused by the difficulty in propionate decomposition at low temperatures, an appropriate metal oxide precursor is introduced to facilitate the effective decomposition of carbonaceous species during pyrolysis, providing a feasible path for the low-cost, large-scale preparation of high-performance NaFePO4 materials.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion secondary battery technology, and in particular to a method for preparing olivine-type sodium iron phosphate, olivine-type sodium iron phosphate, and sodium-ion batteries. Background Technology

[0002] Among various sodium-ion battery cathode materials, olivine-type sodium iron phosphate has become one of the most watched cathode materials due to its stable structural characteristics similar to LiFePO4, as well as its smooth charge-discharge curve and high capacity.

[0003] Currently, olivine-structured NaFePO4 cannot be directly synthesized by solid-state sintering. This is due to the thermodynamic instability of olivine-structured NaFePO4, which typically undergoes a phase transformation at 480℃, changing from an olivine-type structure to an electrochemically inactive phosphatidylcholine-sodium phosphate-type structure, leading to a sharp decline in electrochemical performance. Therefore, the mainstream synthesis method for olivine-structured NaFePO4 remains the ion exchange method, which mainly involves two steps: delithiation and sodium intercalation of olivine-structured LiFePO4 to transform it into olivine-structured NaFePO4.

[0004] In ion exchange methods, although chemical delithiation can be easily achieved through oxidation with oxidants, the sodium intercalation process presents significant technical challenges: First, existing sodium intercalation processes struggle to achieve complete sodium ion insertion at low temperatures, and increasing the temperature risks triggering the aforementioned phase transformation. Second, the range of sodium salts suitable for intercalation is limited; single sodium salts or conventional compound sodium salts cannot simultaneously achieve both low-temperature reactivity and sodium intercalation efficiency. Furthermore, there is a lack of effective auxiliaries to promote low-temperature solid-state sintering reactions and facilitate complete sodium ion insertion. In other words, low-temperature sintering makes it difficult to achieve complete sodium ion insertion, hindering the synthesis of stoichiometric olivine-type NaFePO4.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing olivine-type sodium iron phosphate, olivine-type sodium iron phosphate, and a sodium-ion battery, aiming to solve at least one of the above-mentioned technical problems in the prior art.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0008] The first aspect of the present invention provides a method for preparing olivine-type iron sodium phosphate, wherein sodium propionate and sodium salt are dissolved, olivine-type iron phosphate is added and ground to obtain a precursor slurry; the precursor slurry is spray-dried and then sintered at 200~400°C to obtain the olivine-type iron sodium phosphate.

[0009] Furthermore, the sodium salt includes at least one of sodium formate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium citrate, and sodium oxalate.

[0010] Furthermore, the molar ratio of the sodium propionate to the sodium salt is (0.1~1):1.

[0011] And / or, the molar ratio of the olivine-type iron phosphate to sodium is 1:(1~1.2).

[0012] Furthermore, the grinding speed is 100~400 rpm and the time is 20~60 min.

[0013] Furthermore, the inlet temperature of the spray dryer is 150~250℃, and the outlet temperature is 80~120℃.

[0014] Furthermore, the sintering time is 10~25h.

[0015] And / or, the sintering heating rate is 1~3℃ / min.

[0016] In another method for preparing olivine-type sodium iron phosphate according to the present invention, a metal oxide precursor is used to replace the sodium salt in the above preparation method; wherein, the amount of the metal oxide precursor is 1 to 5 wt% of the mass of sodium propionate.

[0017] Furthermore, the metal oxide includes at least one of titanium oxide, cerium oxide, and manganese oxide.

[0018] And / or, the titanium oxide precursor includes at least one of tetrabutyl titanate, isopropyl titanate, ethyl titanate, titanium citrate, and titanium acetate.

[0019] And / or, the cerium oxide precursor includes at least one of cerium citrate, cerium acetate, and cerium oxalate.

[0020] And / or, the manganese oxide precursor includes at least one of manganese acetate, manganese oxalate, manganese citrate, and manganese stearate.

[0021] The second aspect of the present invention provides an olivine-type sodium iron phosphate, which is prepared by the preparation method described in the first aspect.

[0022] A third aspect of the present invention provides a sodium-ion battery that uses the olivine-type sodium iron phosphate described in the second aspect as the positive electrode active material.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects:

[0024] The preparation method provided by this invention uses olivine-type iron phosphate (FePO4) as raw material and sodium salt and sodium propionate as sodium sources, achieving complete sodium ion insertion at low temperatures of 200-400℃. Sodium propionate not only serves as a sodium source, but the propionate ion also promotes the kinetics of the solid-state reaction, lowers the diffusion barrier of sodium ions, and increases the insertion rate of sodium ions in olivine-type iron phosphate, resulting in purer sodium olivine-type iron phosphate with further improved capacity. Furthermore, to reduce the high ineffective carbon content caused by the difficulty in propionate decomposition at low temperatures, an appropriate metal oxide precursor needs to be introduced to facilitate the effective decomposition of carbonaceous species during pyrolysis, providing a feasible path for the low-cost, large-scale preparation of high-performance NaFePO4 materials.

[0025] The olivine-type sodium iron phosphate provided by this invention achieves a specific capacity of 147.11 mAh / g at 0.1C charging, which is very close to the theoretical specific capacity of the material (approximately 154 mAh / g), indicating that the sodium ion insertion process in the crystal lattice is highly reversible and sufficient. Simultaneously, the material exhibits an extremely high initial coulombic efficiency of 97.7%, indicating minimal irreversible capacity loss during the first charge-discharge process and good electrode / electrolyte interface stability, which is beneficial for improving the overall energy density and cycle life of the battery.

[0026] The sodium-ion battery provided by this invention, given the advantages of the aforementioned olivine-type sodium iron phosphate, enables the prepared sodium-ion battery to combine high safety, long cycle life, good rate capability, and low cost, making it a promising new generation of electrochemical energy storage device. Attached Figure Description

[0027] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0028] Figure 1 Here is a SEM image of the olivine-type sodium iron phosphate obtained in Example 1;

[0029] Figure 2 The image shows a SEM image of sodium iron phosphate of the olivine type obtained in Comparative Example 1. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0031] In the following, the terms “comprising,” “having,” and their cognates, which may be used in various embodiments of the invention, are intended only to indicate a particular feature, number, step, operation, element, component, or combination thereof, and should not be construed as excluding, firstly, the presence of one or more other features, numbers, steps, operations, elements, components, or combinations thereof, or adding the possibility of one or more features, numbers, steps, operations, elements, components, or combinations thereof.

[0032] According to a first aspect of the present invention, a method for preparing olivine-type iron sodium phosphate is provided, wherein sodium propionate and sodium salt are dissolved, and then olivine-type iron phosphate is added and ground to obtain a precursor slurry; the precursor slurry is spray-dried and then sintered at 200~400°C to obtain the olivine-type iron sodium phosphate.

[0033] The preparation method provided by this invention uses olivine-type iron phosphate (FePO4) as raw material and sodium salt and sodium propionate as sodium sources, achieving complete sodium ion intercalation under low-temperature conditions. Sodium propionate not only serves as a sodium source, but the propionate ion also promotes the kinetics of the solid-state reaction, lowers the diffusion barrier of sodium ions, and increases the intercalation rate of sodium ions in olivine-type iron phosphate, resulting in purer sodium olivine-type iron phosphate with further improved capacity. Furthermore, to reduce the high ineffective carbon content caused by the difficulty in propionate decomposition at low temperatures, an appropriate metal oxide precursor needs to be introduced to facilitate the effective decomposition of carbonaceous species during pyrolysis, providing a feasible path for the low-cost, large-scale preparation of high-performance NaFePO4 materials.

[0034] Sodium propionate is chosen as the essential sodium source in this invention because of its unique thermodynamic properties. Sodium propionate undergoes multi-stage phase transitions, and during low-temperature sintering (<290℃), it undergoes solid-to-solid (the crystal transforms from a "rigid solid" to a "plastic solid") and solid-to-liquid phase transitions. These phase transitions help to form dynamic ion transport channels and micro-melting environments locally, significantly enhancing the diffusion kinetics of sodium ions and promoting their insertion into the olivine-type iron phosphate (FePO4) lattice. At the same time, the energy released by the phase transition can reduce the activation energy of the solid-phase reaction, alleviating the problem of restricted ion migration caused by low temperature, thereby promoting the sodium insertion reaction to be more complete, obtaining NaFePO4 close to the stoichiometric ratio, improving the structural integrity of the material and the reversibility of sodium insertion / extraction, and ultimately resulting in higher charge / discharge specific capacity and electrochemical performance.

[0035] Meanwhile, the use of sodium propionate as the essential sodium source also showed unexpected improvements in physical properties. Due to its solid-liquid melting process, the sodium iron phosphate particles can be more tightly bonded, thus giving olivine-type NaFePO4 a higher compaction density and a lower specific surface area.

[0036] Typically, but not limitingly, the sintering temperature can be, for example, 200°C, 230°C, 260°C, 290°C, 320°C, 350°C, or 400°C, or any value within the range of 200°C to 400°C.

[0037] Furthermore, the sodium salt includes at least one of sodium formate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium citrate, and sodium oxalate. These sodium salts can be completely decomposed during high-temperature sintering, and their anionic components escape in gaseous form, ultimately leaving only sodium elements to participate in the reaction or be doped into the crystal lattice of the target material, without introducing any impurity elements as residues.

[0038] Furthermore, the molar ratio of the sodium propionate to the sodium salt is (0.1~1):1.

[0039] If the amount of sodium propionate is too low, it will result in insufficient propionate ions, and the role of sodium propionate as an additive will not be fully realized. If the amount of sodium propionate is too high, propionate ions will remain on the surface of sodium iron phosphate and be difficult to decompose, and the proportion of active matter in the material will be reduced. At the same time, these residual sodium propionate will also increase the powder resistance of the material, which is not conducive to the performance of electrical properties.

[0040] Typically, but not limitingly, the molar ratio of sodium propionate to sodium salt can be, for example, 0.1:1, 0.2:1, 0.4:1, 0.6:1, 0.8:1 or 1:1, or any value in the range of (0.1 to 1):1.

[0041] And / or, the molar ratio of the olivine-type iron phosphate to sodium is 1:(1~1.2).

[0042] Typically, but not limitingly, the molar ratio of the olivine-type iron phosphate to sodium can be, for example, 1:1, 1:1.05, 1:1.1, 1:1.15 or 1:1.2, or any value in the range of 1:(1~1.2).

[0043] Furthermore, the grinding speed is 100~400 rpm and the time is 20~60 min.

[0044] Typically, but not limitingly, the grinding speed can be, for example, 100 rpm, 150 rpm, 200 rpm, 250 rpm, 300 rpm, 350 rpm or 400 rpm, or any value in the range of 100 to 400 rpm, and the grinding time can be, for example, 20 min, 30 min, 40 min, 50 min or 60 min, or any value in the range of 20 to 60 min.

[0045] Furthermore, the inlet temperature of the spray dryer is 150~250℃, and the outlet temperature is 80~120℃.

[0046] Typically, but not limitingly, the inlet temperature of the spray dryer can be, for example, 150°C, 170°C, 190°C, 210°C, 230°C, or 250°C, or any value within the range of 150°C to 250°C, and the outlet temperature can be, for example, 80°C, 90°C, 100°C, 110°C, or 120°C, or any value within the range of 80°C to 120°C.

[0047] Furthermore, the sintering time is 10~25h.

[0048] Typically, but not limitingly, the sintering time can be, for example, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 18 h, 20 h, 22 h, 24 h or 25 h, or any value in the range of 10 to 25 h.

[0049] And / or, the sintering heating rate is 1~3℃ / min, mainly to ensure that the precursor achieves a uniform and controllable transformation during pyrolysis and solid-state reaction. This heating rate avoids excessively rapid heating that could lead to localized and violent decomposition of organic sodium salts such as sodium propionate, generating a large amount of gas and causing particle agglomeration or structural cracking. It also prevents prolonged process cycles and reduced efficiency due to excessively slow heating. More importantly, under this mild heating condition, sodium propionate can undergo multi-stage phase transformation in an orderly manner, gradually releasing active sodium ions and forming a brief micro-molten state. This effectively promotes the uniform diffusion and embedding of sodium ions into the FePO4 lattice, while maintaining moderate decomposition of the carbon source to build a conductive network. This ensures the phase purity, microstructure compactness, and electrochemical performance stability of the final product, olivine-type NaFePO4.

[0050] Typically, but not limitingly, the heating rate of the sintering can be, for example, 1 °C / min, 1.5 °C / min, 2 °C / min, 2.5 °C / min or 3 °C / min, or any value in the range of 1 to 3 °C / min.

[0051] The inventors discovered that using sodium propionate as the sole sodium source results in a high carbon content in the final olivine-type sodium iron phosphate, which increases the carbon content and thus affects the specific capacity. Therefore, introducing other sodium sources in combination with it can maximize the specific capacity.

[0052] In another method for preparing olivine-type sodium iron phosphate according to the present invention, a metal oxide precursor is used to replace the sodium salt in the above preparation method; wherein, the amount of the metal oxide precursor is 1 to 5 wt% of the mass of sodium propionate.

[0053] When the amount of metal oxide precursor is too low, the propionate and anions in the added sodium salt will decompose without replenishment, resulting in the accumulation of anions on the material surface. At the same time, the amount of residual carbon on the surface is very large, which is not conducive to reducing the powder resistance and improving the capacity. When the amount of metal oxide precursor is too high, the proportion of active material will decrease, which will directly affect the capacity of the material.

[0054] Typically, but not limitingly, the amount of the metal oxide precursor may be, for example, 1 wt%, 2 wt%, 3 wt%, 4 wt%, or 5 wt% of the mass of sodium propionate, or any value in the range of 1 to 5 wt%.

[0055] Furthermore, the metal oxide includes at least one of titanium oxide, cerium oxide, and manganese oxide.

[0056] And / or, the titanium oxide precursor includes at least one of tetrabutyl titanate, isopropyl titanate, ethyl titanate, titanium citrate, and titanium acetate.

[0057] And / or, the cerium oxide precursor includes at least one of cerium citrate, cerium acetate, and cerium oxalate.

[0058] And / or, the manganese oxide precursor includes at least one of manganese acetate, manganese oxalate, manganese citrate, and manganese stearate.

[0059] The second aspect of the present invention provides an olivine-type sodium iron phosphate, which is prepared by the preparation method described in the first aspect.

[0060] The olivine-type sodium iron phosphate provided by this invention achieves a specific capacity of 147.11 mAh / g at 0.1C charging, which is very close to the theoretical specific capacity of the material (approximately 154 mAh / g), indicating that the sodium ion insertion process in the crystal lattice is highly reversible and sufficient. Simultaneously, the material exhibits an extremely high initial coulombic efficiency of 97.7%, indicating minimal irreversible capacity loss during the first charge-discharge process and good electrode / electrolyte interface stability, which is beneficial for improving the overall energy density and cycle life of the battery.

[0061] A third aspect of the present invention provides a sodium-ion battery that uses the olivine-type sodium iron phosphate described in the second aspect as the positive electrode active material.

[0062] The sodium-ion battery provided by this invention, given the advantages of the aforementioned olivine-type sodium iron phosphate, enables the prepared sodium-ion battery to combine high safety, long cycle life, good rate capability, and low cost, making it a promising new generation of electrochemical energy storage device.

[0063] The present invention is further illustrated below with specific embodiments and comparative examples. However, it should be understood that these embodiments are merely for illustrative purposes and should not be construed as limiting the invention in any way. Unless otherwise specified, the raw materials used in the embodiments and comparative examples of the present invention were carried out under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0064] Example 1

[0065] This embodiment provides an olivine-type sodium iron phosphate, and the specific preparation process is as follows:

[0066] (1) Weigh out 7.28g of sodium propionate and 30.93g of sodium acetate trihydrate (m 丙酸钠 :m 三水乙酸钠 Place the 1:2 ratio of water in a beaker, add 160 mL of deionized water, and stir until completely dissolved.

[0067] (2) Add 45.25g of olivine-type ferric phosphate to the above solution, stir evenly, pour into a sand mill, and sand mill at 200rpm for 0.5h to disperse the raw materials evenly and obtain the precursor slurry.

[0068] (3) The above precursor slurry is spray-dried by a spray dryer with an inlet temperature of 200°C and an outlet temperature of 100°C to obtain precursor powder.

[0069] (4) Place the precursor powder obtained above in a porcelain boat and sinter it in a protective atmosphere. Heat it to 300°C at a heating rate of 2°C / min and hold it for 12 hours to obtain olivine-type sodium iron phosphate.

[0070] Comparative Example 1

[0071] This comparative example provides an olivine-type sodium iron phosphate. Unlike Example 1, only sodium propionate is used as the sodium source, and the amount of sodium propionate used is 29.11g. The rest of the preparation method is the same as that in Example 1, and will not be described again here.

[0072] Comparative Example 2

[0073] This comparative example provides an olivine-type sodium iron phosphate. Unlike Example 1, only sodium acetate trihydrate is used as the sodium source. The amount of sodium acetate trihydrate is 41.24g. The rest of the preparation method is the same as in Example 1, and will not be repeated here.

[0074] Characterization Example 1

[0075] Morphology analysis was performed on the olivine-type sodium iron phosphate obtained in Example 1 and Comparative Example 1, and the corresponding SEM images are shown below. Figure 1 and Figure 2As shown.

[0076] from Figure 1 It can be seen that the surface of the primary particles is smooth and no excess impurity particles are produced, indicating that the sodium ions are well embedded and there are no obvious undecomposed compounds.

[0077] from Figure 2 It can be seen that excessive sodium propionate makes it difficult for propionate ions to decompose at low temperatures, resulting in an insulating coating on the particle surface, which makes the powder resistance of the material abnormally high and is not conducive to the insertion and extraction of sodium ions.

[0078] Test Example 1

[0079] 1. Physical performance testing

[0080] Compacted density: Tested using a powder tap density meter. The olivine-type sodium iron phosphate sample is placed in a standard graduated cylinder and compacted under a fixed vibration frequency and pressure until the volume no longer changes. The compacted density (g / cm³) is calculated based on the sample mass and final volume. 3 ).

[0081] Specific surface area: determined by nitrogen adsorption-desorption (BET method). The specific surface area (m²) of the olivine-type sodium iron phosphate sample was calculated using the BET equation by measuring the amount of nitrogen adsorbed by the sample at liquid nitrogen temperature. 2 / g).

[0082] C content test: Using an elemental analyzer, the olivine-type sodium iron phosphate sample is completely burned in a high-temperature oxygen stream to convert the carbon in it into carbon dioxide. The amount of CO2 generated is then quantitatively determined by infrared absorption or thermal conductivity detectors to accurately obtain the mass percentage (%) of carbon in the material.

[0083] 2. Electrochemical performance testing

[0084] Using the olivine-type sodium iron phosphate obtained in Example 1 and Comparative Examples 1-2 as the positive electrode active material, 80 parts of the positive electrode active material, 10 parts of acetylene black, and 10 parts of binder were dissolved in N-methylpyrrolidone and coated onto treated aluminum foil. The mixture was then vacuum-dried at 90°C to obtain the positive electrode sheet. Subsequently, in an anhydrous and oxygen-free glove box, the prepared positive electrode sheet, sodium sheet, separator, and NC-008 electrolyte (1 mol / L sodium salt of NaClO4 dissolved in a 1:1 volume ratio of ethylene carbonate (EC) and propylene carbonate (PC), with an additional 5% fluoroethylene carbonate (FEC)) were assembled into a coin cell for electrochemical performance testing.

[0085] First-time performance test: The battery was charged and discharged at a current of 129 mA / g at room temperature. The charge and discharge voltage range was 1.8~4.2V, and a complete charge and discharge cycle was performed at 0.1C. The data obtained are shown in Table 1 below.

[0086] The obtained data is shown in Table 1 below.

[0087] Table 1

[0088]

[0089] As can be seen from Table 1, sodium iron phosphate prepared by sodium propionate and sodium acetate intercalation has the best electrochemical performance, and it is the best in terms of both compaction and specific surface area. However, although sodium propionate is theoretically more perfect in intercalation, it faces the problem of propionate decomposition, which leads to the proportion of active material and hinders the sodium ion shuttle during charge and discharge, thus the capacity decreases. Using only sodium acetate results in insufficient sodium intercalation, which makes it difficult to increase the capacity.

[0090] Example 2

[0091] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that an equimolar amount of sodium formate is used to replace sodium acetate trihydrate. The rest of the preparation method is the same as in Example 1, and will not be repeated here.

[0092] Example 3

[0093] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that an equimolar amount of sodium carbonate is used to replace sodium acetate trihydrate. The rest of the preparation method is the same as in Example 1, and will not be repeated here.

[0094] Example 4

[0095] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that an equimolar amount of sodium bicarbonate is used to replace sodium acetate trihydrate. The rest of the preparation method is the same as in Example 1, and will not be repeated here.

[0096] Example 5

[0097] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that an equimolar amount of sodium citrate is used to replace sodium acetate trihydrate. The rest of the preparation method is the same as in Example 1, and will not be repeated here.

[0098] Example 6

[0099] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that an equimolar amount of sodium oxalate is used to replace sodium acetate trihydrate. The rest of the preparation method is the same as in Example 1, and will not be repeated here.

[0100] Test Example 2

[0101] The olivine-type sodium iron phosphate obtained in Examples 2-6 were subjected to physical and electrochemical performance tests according to the test method of Test Example 1, and the data obtained are recorded in Table 2.

[0102] Table 2

[0103]

[0104] As can be seen from Table 2, sodium formate and sodium acetate have relatively optimal electrochemical performance and lower residual carbon content. This is mainly because acetate and formate volatilize during the sand milling and spraying process, consuming a large number of anions. Furthermore, during the sintering process, acetate and formate are more easily decomposed and volatilized at low temperatures, while other anions are more difficult to volatilize, especially carbonate, which is relatively stable at this temperature. We need a certain amount of anion carbonization, and the excess carbon is used to reduce the ferric iron in ferric phosphate, providing better electrochemical potential energy for sodium intercalation and increasing the amount of sodium intercalation. Therefore, the capacity of carbonate is lower.

[0105] Example 7

[0106] This embodiment provides an olivine-type sodium iron phosphate, which differs from Example 1 in that m 丙酸钠 :m 三水乙酸钠 The molar ratio is 0.2:1, and the remaining steps are the same as in Example 1, so they will not be repeated here.

[0107] Example 8

[0108] This embodiment provides an olivine-type sodium iron phosphate, which differs from Example 1 in that m 丙酸钠 :m 三水乙酸钠 The molar ratio is 1:1, and the remaining steps are the same as in Example 1, so they will not be repeated here.

[0109] Example 9

[0110] This embodiment provides an olivine-type sodium iron phosphate, which differs from Example 1 in that m 丙酸钠 :m 三水乙酸钠 The molar ratio is 1.5:1, and the remaining steps are the same as in Example 1, so they will not be repeated here.

[0111] Test Example 3

[0112] The olivine-type iron phosphate sodium obtained in Examples 7-9 was subjected to physical and electrochemical performance tests according to the test method of Test Example 1, and the data obtained are recorded in Table 3.

[0113] Table 3

[0114]

[0115] As can be seen from Table 3, when the amount of sodium propionate added is insufficient, although the residual carbon content is not large and the specific surface area is also low, the role of sodium propionate as a sodium coagulant cannot be fully exerted. It is difficult for sodium ions to be completely inserted into the crystal lattice at low temperature, so the capacity is low. However, as more sodium propionate is added, it can be found that the residual carbon content increases with the increase of sodium propionate, and the capacity gradually decreases. This further illustrates that too much sodium propionate is not conducive to the exertion of capacity.

[0116] Example 10

[0117] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that the sintering temperature in step (4) is 200°C. The rest of the preparation methods are the same as in Example 1, and will not be repeated here.

[0118] Comparative Example 3

[0119] This comparative example provides an olivine-type sodium iron phosphate. The difference from Example 1 is that the sintering temperature in step (4) is 320°C. The rest of the preparation methods are the same as those in Example 1, and will not be repeated here.

[0120] Comparative Example 4

[0121] This comparative example provides an olivine-type sodium iron phosphate. Unlike Example 1, only sodium acetate trihydrate is used as the sodium source. The amount of sodium acetate trihydrate is 41.24 g. The sintering temperature in step (4) is 320 °C. The rest of the preparation methods are the same as in Example 1, and will not be repeated here.

[0122] Test Example 4

[0123] The physical and electrochemical properties of the olivine-type sodium iron phosphate obtained in Example 10 and Comparative Examples 3-4 were tested according to the test method of Test Example 1, and the data obtained are recorded in Table 4.

[0124] Table 4

[0125]

[0126] As can be seen from Table 4, too low a sintering temperature is conducive to the thermodynamic diffusion of sodium ions, resulting in insufficient sodium intercalation. However, too high a temperature will make the structure of sodium iron phosphate olivine unstable, producing some sodium iron phosphate of the phosphate iron type, thus slightly reducing the capacity. Similarly, without the addition of sodium propionate, the capacity will still be lost at high temperatures.

[0127] Example 11

[0128] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that the heating rate in step (4) is 2℃ / min. The rest of the preparation methods are the same as in Example 1, and will not be repeated here.

[0129] Example 12

[0130] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that the heating rate in step (4) is 3℃ / min. The rest of the preparation methods are the same as in Example 1, and will not be repeated here.

[0131] Example 13

[0132] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that the heating rate in step (4) is 6℃ / min. The rest of the preparation methods are the same as in Example 1, and will not be repeated here.

[0133] Test Example 5

[0134] The olivine-type sodium iron phosphate obtained in Examples 11-13 were subjected to physical and electrochemical performance tests according to the test method of Test Example 1. The data obtained are recorded in Table 5.

[0135] Table 5

[0136]

[0137] As can be seen from Table 5, the heating rate has little effect on compaction, specific surface area, and carbon content. However, the capacity decreased slightly as the heating rate increased. This may be because the rapid heating rate caused the organic anions to shrink rapidly, which hindered the insertion channels of sodium ions, resulting in a slight decrease in capacity.

[0138] Example 14

[0139] This embodiment provides an olivine-type sodium iron phosphate. Unlike Example 1, it uses tetrabutyl titanate instead of sodium acetate trihydrate. The amount of tetrabutyl titanate added is 1.3g, and the amount of sodium propionate is 29.11g. The rest of the methods are the same as in Example 1, and will not be repeated here.

[0140] Example 15

[0141] This embodiment provides an olivine-type sodium iron phosphate. Unlike Example 1, it uses tetrabutyl titanate instead of sodium acetate trihydrate. The amount of tetrabutyl titanate added is 0.29g, and the amount of sodium propionate is 29.11g. The rest of the methods are the same as in Example 1, and will not be repeated here.

[0142] Example 16

[0143] This embodiment provides an olivine-type sodium iron phosphate. Unlike Example 1, it uses tetrabutyl titanate instead of sodium acetate trihydrate. The amount of tetrabutyl titanate added is 1.45g, and the amount of sodium propionate is 29.11g. The rest of the methods are the same as in Example 1, and will not be repeated here.

[0144] Example 17

[0145] This embodiment provides an olivine-type sodium iron phosphate. The difference from Example 1 is that tetrabutyl titanate is used instead of sodium acetate trihydrate. The amount of tetrabutyl titanate added is 2g, and the amount of sodium propionate is 29.11g. The rest of the methods are the same as in Example 1, and will not be repeated here.

[0146] Example 18

[0147] This embodiment provides an olivine-type sodium iron phosphate, which differs from Example 14 in that cerium oxide is used instead of tetrabutyl titanate. The rest of the methods are the same as in Example 14 and will not be described again here.

[0148] Example 19

[0149] This embodiment provides an olivine-type sodium iron phosphate, which differs from Example 14 in that manganese acetate is used instead of tetrabutyl titanate. The rest of the methods are the same as in Example 14 and will not be described again here.

[0150] Test Example 6

[0151] The olivine-type iron sodium phosphate obtained in Examples 14-19 was subjected to physical and electrochemical performance tests according to the test method of Test Example 1. The data obtained are recorded in Table 6.

[0152] Table 6

[0153]

[0154] As shown in Table 6, the addition of titanium can effectively reduce the overall carbon content of the material, indicating that titanium helps decompose residual sodium propionate. Different amounts of titanium have different effects. When the amount added is too low, the overall improvement effect is not obvious. When the amount added is too high, the excess titanium will form titanium oxide, which reduces the purity of the active material and thus slightly reduces the capacity. When manganese and cerium are used, due to the different effects of different metal elements, the decomposition effect on carbon is poor, which makes the overall capacity lower than before modification.

[0155] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the technical scope disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing olivine-type sodium iron phosphate, characterized in that, Sodium propionate and sodium salt in a molar ratio of (0.5~1):1 were dissolved, and then olivine-type ferric phosphate was added and ground to obtain a precursor slurry. After spray drying the precursor slurry, it is sintered at 300°C with a heating rate of 1~3°C / min to obtain the olivine-type sodium iron phosphate. The sodium salt is sodium formate or sodium acetate.

2. The preparation method according to claim 1, characterized in that, The molar ratio of olivine-type iron phosphate to sodium is 1:(1~1.2).

3. The preparation method according to claim 1 or 2, characterized in that, The grinding speed is 100~400 rpm, and the time is 20~60 min.

4. The preparation method according to claim 1 or 2, characterized in that, The inlet temperature of the spray dryer is 150~250℃, and the outlet temperature is 80~120℃.

5. The preparation method according to claim 1 or 2, characterized in that, The sintering time is 10~25h.

6. A method for preparing olivine-type sodium iron phosphate, characterized in that, The sodium salt in the preparation method according to any one of claims 1 to 5 is replaced with a metal oxide precursor; The amount of the metal oxide precursor used is 1-5 wt% of the mass of sodium propionate; The metal oxide includes at least one of titanium oxide, cerium oxide, and manganese oxide; Titanium oxide precursors include at least one of tetrabutyl titanate, isopropyl titanate, ethyl titanate, titanium citrate, and titanium acetate. Cerium oxide precursors include at least one of cerium citrate, cerium acetate, and cerium oxalate; Manganese oxide precursors include at least one of manganese acetate, manganese oxalate, manganese citrate, and manganese stearate.

7. A sodium iron phosphate of the olivine type, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 6.

8. A sodium-ion battery, characterized in that, The olivine-type sodium iron phosphate described in claim 7 is used as the positive electrode active material.

Images