A sodium iron phosphate pyrophosphate composite material, its preparation method and application

Abstract

This invention belongs to the field of sodium-ion battery technology, specifically relating to a sodium iron phosphate pyrophosphate composite material, its preparation method, and its application. The invention provides a method for preparing the sodium iron phosphate pyrophosphate composite material, comprising the following steps: adding a precipitant to a mixed solution of an iron source, a phosphorus source, and a sodium source; heat-treating the obtained precipitate to obtain a sodium iron phosphate pyrophosphate precursor; mixing the sodium iron phosphate pyrophosphate precursor with a carbon source and then heat-treating the mixture to obtain a sodium iron phosphate pyrophosphate carbon precursor; and sintering the sodium iron phosphate pyrophosphate carbon precursor to obtain the sodium iron phosphate pyrophosphate composite material. In the preparation method of the sodium iron phosphate pyrophosphate composite material provided by this invention, the sodium iron phosphate pyrophosphate precursor is synthesized using a liquid-phase method. By adjusting the process and adding a precipitant to in-situ coat the sodium iron phosphate pyrophosphate precursor with carbon, the sodium iron phosphate pyrophosphate composite material can achieve a higher capacity.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a sodium iron phosphate pyrophosphate composite material, its preparation method, and its application. Background Technology

[0002] In recent years, with the widespread application of lithium-ion batteries in electric vehicles, the problem of lithium resource shortage has become increasingly prominent. Sodium resources are well-known to be abundant and more suitable for large-scale energy storage. Sodium-ion batteries, due to their similar operating principles to lithium-ion batteries and their cost advantage, are widely considered a promising next-generation battery. Sodium-ion batteries have become a research hotspot in recent years, with numerous material systems being widely reported.

[0003] Among numerous sodium battery materials, polyanionic sodium battery cathode materials stand out as high-value and promising due to their excellent structural stability, suitable voltage platform, and cost advantages. Iron-based phosphate materials, in particular, contain unique tetrahedral PO4 units with strong covalent bonds. The relative separation of valence electrons from the polyanion and their distinctive three-dimensional framework structure facilitates rapid sodium ion insertion / extraction. Because of its abundant phosphorus and iron resources, three-dimensional ion diffusion channels, and long-cycle performance, sodium iron pyrophosphate has become the preferred cathode material for energy storage in the sodium battery industry.

[0004] Currently, according to published patent reports on sodium iron pyrophosphate, Chinese patent application number 202210256280.7 discloses a carbon-coated sodium-ion battery cathode material, Na4Fe. 3-x M x The (PO4)2P2O7 / C and its preparation method involve multiple sintering processes, making the process relatively complex, and the discharge specific capacity is lower than 70 mAh / g@0.5C (1.5-4.5V). Given the above, there is still room for further improvement in the material capacity of sodium iron phosphate pyrophosphate sodium-ion battery cathode materials. Summary of the Invention

[0005] This invention provides a sodium iron phosphate pyrophosphate composite material, its preparation method, and its application. The purpose is to address the issue that the material capacity of sodium iron phosphate pyrophosphate sodium-ion battery cathode materials still has room for further improvement in the prior art.

[0006] To address the aforementioned technical deficiencies, one objective of this invention is to provide a method for preparing a sodium iron phosphate pyrophosphate composite material; another objective is to provide a sodium iron phosphate pyrophosphate composite material obtained by the aforementioned method; and a third objective is to provide a sodium-ion battery using the aforementioned sodium iron phosphate pyrophosphate composite material.

[0007] In a first aspect, the present invention provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0008] Preparation of sodium iron phosphate pyrophosphate precursor: A precipitant is added to a mixed solution of iron source, phosphorus source and sodium source, and the resulting precipitate is heat-treated to obtain sodium iron phosphate pyrophosphate precursor;

[0009] Preparation of sodium iron phosphate carbon precursor for pyrophosphate: The obtained sodium iron phosphate precursor for pyrophosphate is mixed with a carbon source and then subjected to heat treatment to obtain sodium iron phosphate carbon precursor for pyrophosphate.

[0010] Preparation of sodium iron phosphate pyrophosphate composite material: The obtained sodium iron phosphate pyrophosphate carbon precursor was sintered to obtain sodium iron phosphate pyrophosphate composite material.

[0011] Furthermore, in the step of preparing sodium iron phosphate carbon precursor of pyrophosphate, the mixing method is to first grind the sodium iron phosphate precursor of pyrophosphate, and then add the carbon source for secondary grinding.

[0012] The grinding method includes at least one of ball milling and sand mill grinding, and the secondary grinding method includes at least one of ball milling and sand mill grinding;

[0013] And / or, the carbon source is added intermittently;

[0014] And / or, the mass ratio of the carbon source to the sodium iron phosphate pyrophosphate precursor is (1-40):15;

[0015] And / or, the temperature of the heat treatment is 120-180°C.

[0016] Furthermore, in the step of preparing sodium iron phosphate carbon pyrophosphate precursor, the grinding time is 5-120 min;

[0017] And / or, the grinding rate is 100-800 rpm;

[0018] And / or, the secondary grinding time is 5-120 min;

[0019] And / or, the secondary grinding rate is 100-800 rpm.

[0020] Furthermore, in the step of preparing sodium iron phosphate carbon precursor for pyrophosphate, the carbon source includes one or more of citric acid, glucose, sucrose, oxalic acid, polyvinylpyrrolidone, polyethylene glycol, and lactic acid.

[0021] And / or, the carbon source is added in solution form;

[0022] The mass-volume fraction of the carbon source in the carbon source solution is 0.1-1 g / mL.

[0023] Furthermore, in the step of preparing sodium iron phosphate pyrophosphate precursor, the iron source includes one or more of ferric nitrate, ferric nitrate nonahydrate, ferrous nitrate, ferrous carbonate, and ferrous phosphate.

[0024] And / or, the phosphoric acid includes one or more of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and ammonium dihydrogen phosphate;

[0025] And / or, the sodium source includes one or more of sodium phosphorus salts, sodium carbonate, and sodium chloride, wherein the sodium phosphorus salt is preferably sodium phosphate, disodium hydrogen phosphate, or sodium dihydrogen phosphate;

[0026] And / or, the molar ratio of iron, phosphorus, and sodium in the iron source, phosphoric acid, and sodium source is (2.6-3.2):4:(4-4.1).

[0027] Furthermore, in the step of preparing sodium iron phosphate pyrophosphate precursor, the precipitant is an alkaline substance, preferably one or more of sodium hydroxide, ammonia, and sodium carbonate.

[0028] The precipitant is added in solution form;

[0029] And / or, based on the mass of the iron source, the amount of the precipitant solution added is 0.5-1.0 mL / g;

[0030] And / or, the mass fraction of the precipitant in the solution is 28.0-30.0%;

[0031] And / or, the precipitant solution is added dropwise;

[0032] And / or, the temperature of the heat treatment is 80-150°C.

[0033] Furthermore, in the step of preparing sodium iron phosphate pyrophosphate composite material, the sintering includes a primary heat preservation and a secondary heat preservation.

[0034] The temperature for the first heat preservation is 100-500℃;

[0035] And / or, the duration of the single heat preservation is 1-8 hours;

[0036] And / or, the temperature of the secondary insulation is 500-800℃;

[0037] And / or, the secondary insulation time is 1-24 hours.

[0038] Furthermore, in the step of preparing sodium iron phosphate composite material, both the primary and secondary heat preservation are carried out in an inert atmosphere;

[0039] And / or, the inert atmosphere includes one or more of argon, nitrogen, and a hydrogen-argon mixture.

[0040] A sodium iron phosphate pyrophosphate composite material is obtained by the above preparation method.

[0041] A sodium-ion battery comprising a sodium iron pyrophosphate composite material obtained by the above preparation method or the above sodium iron pyrophosphate composite material.

[0042] Compared with the prior art, the present invention has at least one of the following beneficial effects:

[0043] 1. In the preparation method of sodium iron phosphate pyrophosphate composite material provided by the present invention, the sodium iron phosphate pyrophosphate precursor is synthesized by liquid phase method. By adjusting the process and adding a precipitant, the sodium iron phosphate pyrophosphate precursor is coated with carbon in situ, which can make the sodium iron phosphate pyrophosphate composite material have a higher capacity.

[0044] 2. In the carbon coating process, this invention enables more uniform carbon coating. Compared with conventional methods, the sodium iron phosphate pyrophosphate composite material has a higher main phase content, more uniform particles, and better conductivity. Under the same conditions, the sodium iron phosphate pyrophosphate composite material of this application can achieve a higher capacity.

[0045] 3. The preparation method of the present invention is simple, low in cost, and has obvious advantages for mass production (industrial production).

[0046] 4. The carbon source of the sodium iron phosphate pyrophosphate composite material of the present invention is uniformly distributed, has good conductivity, and has obvious advantages in electrochemical performance (high discharge capacity and good cycle stability). Attached Figure Description

[0047] Figure 1 This is a phase composition diagram of the sodium iron phosphate pyrophosphate composite material prepared in Example 1;

[0048] Figure 2 The graph shows the charge-discharge curves of the sodium iron phosphate composite material prepared in Example 1 after being assembled into a sodium-ion battery for the first three weeks.

[0049] Among them, 1st is the charge-discharge curve of the battery in the first week, 2nd is the charge-discharge curve of the battery in the second week, and 3rd is the charge-discharge curve of the battery in the third week. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0051] The embodiments of the present invention are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. The process parameters in the following embodiments that do not specify specific conditions are generally in accordance with conventional conditions.

[0052] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0053] A specific embodiment of the present invention provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0054] Preparation of sodium iron phosphate pyrophosphate precursor: A precipitant is added to a mixed solution of iron source, phosphorus source and sodium source, and the resulting precipitate is heat-treated to obtain sodium iron phosphate pyrophosphate precursor;

[0055] Preparation of sodium iron phosphate carbon precursor for pyrophosphate: The obtained sodium iron phosphate precursor for pyrophosphate is mixed with a carbon source and then subjected to heat treatment to obtain sodium iron phosphate carbon precursor for pyrophosphate.

[0056] Preparation of sodium iron phosphate pyrophosphate composite material: After sintering the obtained sodium iron phosphate pyrophosphate carbon precursor, a sodium iron phosphate pyrophosphate composite material is obtained.

[0057] The present invention relates to the preparation of sodium iron phosphate pyrophosphate composite material by first preparing sodium iron phosphate pyrophosphate (NFPP) precursor by reacting iron source, phosphorus source, sodium source and precipitant, then coating carbon source in situ on the NFPP precursor to obtain NFPP precursor / carbon, and finally sintering to obtain sodium iron phosphate pyrophosphate composite material.

[0058] NFPP precursors refer to precursors containing phosphorus, iron, and sodium.

[0059] NFPP material itself has a problem with weak electrical conductivity, as detailed below:

[0060] 1) Conventional materials exhibit uneven carbon coating, resulting in an island-like distribution. This invention employs a self-made NFPP precursor for in-situ carbon coating, achieving uniform coating and effectively improving the electronic conductivity of the material.

[0061] 2) Conventional materials require secondary refining using equipment, which is a complex process. The precursor produced in this invention has adjustable particle size, resulting in uniform and controllable particle size in a single synthesis. Furthermore, the small particle size at the first stage effectively improves the ionic conductivity of the material.

[0062] Issues with NFPP material phase purity: Materials obtained by conventional methods have low NFPP phase content. The composite material prepared by the method of this invention has higher purity of the target product and higher discharge capacity.

[0063] In this invention, as an optional embodiment, in the step of preparing sodium iron phosphate carbon precursor of pyrophosphate, the mixing method is to first grind the sodium iron phosphate precursor of pyrophosphate, and then add a carbon source for secondary grinding.

[0064] The grinding method includes at least one of ball milling and sand milling; the secondary grinding method includes at least one of ball milling and sand milling; the carbon source is added intermittently; in this invention, when the precursor is coated with carbon in situ, the auxiliary means of mechanical mixing by ball milling or sand milling can make the carbon coating more uniform.

[0065] The mass ratio of the carbon source to the sodium iron phosphate pyrophosphate precursor is (1-40):15 (e.g., 2:15, 5:15, 10:15, 15:15, 20:15, 25:15, 30:15, 35:15); the heat treatment temperature is 120-180℃ (e.g., 130℃, 140℃, 160℃, 170℃). Heat treatment can be performed using a muffle furnace or by forced-air drying. After heat treatment, a solid precipitate or gel-like substance remains. The purpose of heating is to remove residual moisture from the sodium iron phosphate pyrophosphate carbon precursor.

[0066] The purpose of intermittent addition is to ensure that the sodium iron phosphate pyrophosphate precursor and the carbon source are mixed evenly, eliminating the presence of large particles. The purpose of adding the carbon source is to provide carbon-reducing gas for subsequent processing, and the residual carbon source can improve the conductivity of the material.

[0067] In this invention, the grinding machine is a conventional grinding machine equipment and does not require special customization.

[0068] In this invention, the specific method of intermittently adding carbon source is as follows: first, the sodium iron phosphate pyrophosphate precursor is ground, and after grinding for a certain period of time, 70-90% by mass of carbon source and sodium iron phosphate pyrophosphate precursor are added for grinding and mixing treatment, and finally 10-30% by mass of carbon source is added for grinding.

[0069] In this invention, as an optional embodiment, in the step of preparing the sodium iron phosphate carbon precursor for pyrophosphate, the grinding time is 5-120 min (e.g., 20 min, 40 min, 60 min, 80 min, 100 min); the grinding rate is 100-800 rpm (e.g., 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm); the secondary grinding time is 5-120 min (e.g., 20 min, 40 min, 60 min, 80 min, 100 min); the secondary grinding rate is 100-800 rpm (e.g., 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm). After processing, the precursor is uniformly in contact with the carbon source, resulting in better carbon coating.

[0070] In this invention, as an optional embodiment, the carbon source includes one or more of citric acid, glucose, sucrose, oxalic acid, polyvinylpyrrolidone, polyethylene glycol, and lactic acid; the carbon source is added in solution form. The mass-volume fraction of the carbon source in the solution is 0.1-1 g / mL (e.g., 0.2 g / mL, 0.4 g / mL, 0.5 g / mL, 0.6 g / mL, 0.7 g / mL, 0.8 g / mL, 0.9 g / mL).

[0071] If the carbon source is not added in solution form, it will result in uneven distribution. If it is added directly, the carbon source will not mix evenly with the sodium iron phosphate pyrophosphate precursor, leading to insufficient coating and easy adhesion to the walls during processing, causing material loss.

[0072] In this invention, as an optional implementation, the concentration of the carbon source solution affects the carbon content and other properties of the material. If the carbon source concentration is too low, there will be less carbon and weak conductivity, while if the carbon source concentration is too high, the cost will be high and the performance will be poor.

[0073] In this invention, as an optional embodiment, in the step of preparing sodium iron phosphate pyrophosphate precursor, the iron source includes one or more of ferric nitrate, ferric nitrate nonahydrate, ferrous nitrate, ferrous carbonate, and ferrous phosphate.

[0074] And / or, the phosphoric acid includes one or more of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and ammonium dihydrogen phosphate;

[0075] And / or, the sodium source includes one or more of sodium phosphorus salts, sodium carbonate, and sodium chloride, wherein the sodium phosphorus salt is preferably sodium phosphate, disodium hydrogen phosphate, or sodium dihydrogen phosphate;

[0076] And / or, the molar ratio of iron, phosphorus, and sodium in the iron source, phosphoric acid, and sodium source is (2.6-3.2):4:(4-4.1).

[0077] In this invention, as an optional embodiment, in the step of preparing the sodium iron phosphate pyrophosphate precursor, the precipitant is an alkaline substance, preferably one or more of sodium hydroxide, ammonia, and sodium carbonate; the precipitant is added in solution form; based on the mass of the iron source, the amount of precipitant solution added is 0.5-1.0 mL / g (e.g., 0.6 mL / g, 0.7 mL / g, 0.8 mL / g, 0.9 mL / g); the mass fraction of the precipitant in the solution is 28.0-30.0%;

[0078] The purpose of adding a precipitant is to precipitate metal ions. The concentration is related to the amount added. The precipitant is added dropwise. The purpose of dropwise addition is to ensure more uniform particle precipitation, preventing a large number of particles of varying sizes from causing unevenness in the precursor and affecting the performance of subsequent products.

[0079] In this invention, as an optional embodiment, the heat treatment temperature in the step of preparing the sodium iron phosphate pyrophosphate precursor is 80-150℃ (e.g., 90℃, 100℃, 110℃, 120℃, 130℃, 140℃). The heat treatment can be performed using a muffle furnace or by forced-air drying. After heat treatment, a solid precipitate or gel-like substance is obtained. The purpose of heating is to remove moisture from the sodium iron phosphate pyrophosphate precursor.

[0080] In this invention, as an optional embodiment, the sintering includes a primary heat treatment and a secondary heat treatment. The temperature of the primary heat treatment is 100-500℃ (e.g., 150℃, 200℃, 250℃, 350℃, 400℃, 450℃); the duration of the primary heat treatment is 1-8h (e.g., 2h, 4h, 5h, 6h, 7h); the temperature of the secondary heat treatment is 500-800℃ (e.g., 600℃, 650℃, 700℃, 750℃); and the duration of the secondary heat treatment is 1-24h (e.g., 2h, 4h, 6h, 8h, 12h, 14h, 16h, 18h, 20h, 22h).

[0081] In this invention, as an optional embodiment, in the step of preparing sodium iron phosphate composite material, both the primary and secondary heat preservation are carried out in an inert atmosphere; the inert atmosphere includes one or more of argon, nitrogen, and a hydrogen-argon mixture.

[0082] Both the primary and secondary heat treatments are carried out in a tube furnace. The process of heat treatment, heating, and then heat treatment again in the tube furnace is accompanied by the purification of raw materials, the removal of residual moisture, and the formation of product nascent crystals. As the heat treatment time is extended, impurities are thoroughly removed and a large amount of product is generated.

[0083] Compared with conventional methods, the preparation method of the present invention produces a sodium iron phosphate pyrophosphate composite material with a higher main phase content, more uniform particles, and better conductivity. Under the same conditions, the sodium iron phosphate pyrophosphate composite material of the present application can achieve a higher capacity.

[0084] In this invention, the main phase is sodium iron phosphate pyrophosphate. Since sodium iron phosphate pyrophosphate composites inevitably contain impurity sodium iron phosphate, the technical solution of this invention can reduce impurities in the composite material and increase the content of the main phase. The main phase content in the composite material prepared by this invention can reach as high as 98.3%.

[0085] A specific embodiment of the present invention also provides a sodium iron phosphate pyrophosphate composite material, which is obtained by the above preparation method.

[0086] A specific embodiment of the present invention also provides a sodium-ion battery, comprising a sodium iron pyrophosphate composite material obtained by the above preparation method or the above sodium iron pyrophosphate composite material.

[0087] Example 1

[0088] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0089] 1) Dissolve 0.15 mol of ferric nitrate nonahydrate and 0.20 mol of sodium dihydrogen phosphate in 50 mL of deionized water to form solution A. Add 50 mL of 28% ammonia solution dropwise to solution A to induce precipitation. After the reaction is complete, heat-treat at 80 °C to obtain the sodium ferric phosphate pyrophosphate composite precursor.

[0090] 2) Take 150g of the above-mentioned sodium iron phosphate pyrophosphate composite precursor and put it into a ball mill jar for processing at 250rpm for 20min. Then, add 320mL of 0.3g / mL citric acid solution and process at 250rpm for 10min. Then add 80mL of 0.3g / mL citric acid solution and process at 250rpm for 10min. The solution is then set aside. After heat treatment at 150℃, the sodium iron phosphate carbon pyrophosphate composite precursor is obtained.

[0091] 3) Take the above-mentioned sodium iron phosphate carbon composite precursor and transfer it to a tube furnace filled with hydrogen and argon atmosphere. Sinter it by first holding it at 300℃ for 3 hours and then raising the temperature to 550℃ and holding it for 10 hours to obtain a sodium iron phosphate carbon composite material with uniform thickness.

[0092] Phase analysis was performed on the sodium iron phosphate@carbon pyrophosphate composite material prepared in Example 1, such as... Figure 1 As shown, from Figure 1It can be seen that the main phase of the synthesized material is the target substance, namely sodium iron phosphate pyrophosphate. The sodium iron phosphate pyrophosphate@carbon composite material (positive electrode main material), Ketjen Black (negative electrode main material), and Whatman glass fiber (separator) prepared in Example 1 were assembled into a sodium-ion battery. Charge-discharge tests were conducted on the sodium-ion battery (test conditions: 1.7-4.3V, 0.5C, 1C = 120mAh / g, constant current charge-discharge). The test results are as follows. Figure 2 As shown, from Figure 2 It can be seen that the specific discharge capacity of the sodium-ion battery at 0.5C is 102.9 mAh / g, and the coulombic efficiency is 96%.

[0093] Example 2

[0094] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0095] The difference between this embodiment and Embodiment 1 is that the amount of ferric nitrate nonahydrate in step 1) is 0.13 mol, while the rest of the steps are the same as in Embodiment 1.

[0096] Example 3

[0097] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0098] The difference between this embodiment and Embodiment 1 is that the amount of ferric nitrate nonahydrate in step 1) is 0.14 mol, while the rest of the steps are the same as in Embodiment 1.

[0099] Example 4

[0100] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0101] The difference between this embodiment and Embodiment 1 is that the amount of ferric nitrate nonahydrate in step 1) is 0.16 mol, while the rest of the steps are the same as in Embodiment 1.

[0102] Example 5

[0103] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0104] The difference between this embodiment and Embodiment 1 is that the mass-volume fraction of citric acid in step 2) is 0.1 g / mL, while the rest of the steps are the same as in Embodiment 1.

[0105] Example 6

[0106] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0107] The difference between this embodiment and Embodiment 1 is that the mass-volume fraction of citric acid in step 2) is 1.0 g / mL, while the rest of the steps are the same as in Embodiment 1.

[0108] Example 7

[0109] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0110] The difference between this embodiment and embodiment 1 is that the heat treatment temperature in step 2) is 180°C, while the other steps are the same as in embodiment 1.

[0111] Example 8

[0112] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0113] The difference between this embodiment and Embodiment 1 is that the heat treatment temperature in step 2) is 120°C, while the other steps are the same as in Embodiment 1.

[0114] Example 9

[0115] This embodiment provides a method for preparing a sodium iron phosphate pyrophosphate composite material, including the following steps:

[0116] The difference between this embodiment and Embodiment 1 is that the inert gas in step 3) is nitrogen, while the rest of the steps are the same as in Embodiment 1.

[0117] Comparative Example 1

[0118] This comparative example provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0119] The difference between this comparative example and Example 1 is that step 1) is as follows: 0.15 mol of ferric nitrate nonahydrate and 0.20 mol of sodium dihydrogen phosphate are dissolved in 50 mL of deionized water, and then heat-treated at 80 °C to obtain the sodium iron phosphate pyrophosphate composite precursor. The remaining steps are the same as in Example 1.

[0120] Comparative Example 2

[0121] This comparative example provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0122] The difference between this comparative example and Example 1 is that step 1) is as follows: 0.15 mol of ferric nitrate nonahydrate and 0.20 mol of sodium dihydrogen phosphate are dissolved in 50 mL of deionized water to form solution A. 50 mL of 28% ammonia solution is added dropwise to solution A to begin precipitation. After the reaction is complete, the solution is heat-treated at 80°C and finally sintered at 300°C to obtain the sodium iron phosphate pyrophosphate composite precursor. The remaining steps are the same as in Example 1.

[0123] Comparative Example 3

[0124] This comparative example provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0125] The difference between this comparative example and Example 1 is that in step 1), ammonia is directly added to solution A, while the remaining steps are the same as in Example 1.

[0126] Comparative Example 4

[0127] This comparative example provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0128] The difference between this comparative example and Example 1 is that in step 2), 100% of the carbon source is added at once, while the rest of the steps are the same as in Example 1.

[0129] Comparative Example 5

[0130] This comparative example provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0131] The difference between this comparative example and Example 1 is that no carbon source is added in step 2), while the rest of the steps are the same as in Example 1.

[0132] Comparative Example 6

[0133] This comparative example provides a method for preparing a sodium iron phosphate pyrophosphate composite material, comprising the following steps:

[0134] 1) Dissolve 0.15 mol of ferric nitrate nonahydrate, 0.20 mol of sodium dihydrogen phosphate, and 400 mL of 0.3 g / mL citric acid solution in 50 mL of deionized water to form solution A. Add 50 mL of 28% ammonia solution dropwise to solution A to induce precipitation. After the reaction is complete, heat-treat at 150 °C to obtain the sodium ferric phosphate carbon pyrophosphate composite precursor.

[0135] 2) Take the above-mentioned sodium iron phosphate carbon composite precursor and transfer it to a tube furnace filled with hydrogen and argon atmosphere. Hold it at 300℃ for 3 hours and then raise the temperature to 550℃ and hold it for 10 hours to obtain a sodium iron phosphate carbon composite material with uniform thickness.

[0136] Test case

[0137] The main phase of the sodium iron phosphate pyrophosphate composites prepared in Examples 1-9 and Comparative Examples 1-6 was detected by XRD (X-ray diffraction).

[0138] The sodium iron phosphate composite material (positive electrode material), Ketjen black (negative electrode material), and Whatman glass fiber (separator) prepared in Examples 1-9 and Comparative Examples 1-6 were assembled into sodium-ion batteries. The discharge specific capacity and capacity retention of the sodium-ion batteries were tested under constant current charge-discharge conditions of 1.7-4.3V, 0.5C, 1C=120mAh / g.

[0139] The test results are shown in the table below:

[0140]

[0141]

[0142]

[0143] The table above shows that: 1) Comparing the data from Examples 1 and 2-4, it can be seen that a suitable iron-phosphorus ratio can stabilize the material structure, improve the purity of the main phase, and further enhance the material's capacity and cycle stability. 2) Comparing the data from Examples 1 and 5 and 6, it can be seen that the introduction of a suitable carbon source helps improve the material's conductivity and enhance its electrochemical performance. 3) Comparing the data from Examples 1 and 7 and 8, it can be seen that a suitable precursor / carbon treatment temperature can fully carbonize and effectively solidify the material surface, further improving the material's microstructure and conductivity. 4) Comparing the data from Examples 1 and 9, it can be seen that a suitable sintering atmosphere also helps optimize the material's microstructure and physical properties, and its electrochemical performance also changes. 5) Comparing the data from Examples 1 and Comparative Examples 1-6, it can be seen that the results of the embodiments of the present invention all demonstrate excellent performance and process advantages.

[0144] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be within the scope of protection of the pending claims of the present invention.

Claims

1. A method for preparing a sodium pyrophosphate ferric phosphate complex material, characterized by, The method comprises the following steps: Preparation of sodium pyrophosphate iron phosphate precursor: adding a precipitant to a mixed solution of an iron source, a phosphorus source and a sodium source, and performing heat treatment on the obtained precipitate to obtain a sodium pyrophosphate iron phosphate precursor; Preparation of sodium pyrophosphate iron phosphate carbon precursor: mixing the obtained sodium pyrophosphate iron phosphate precursor with a carbon source, and performing heat treatment to obtain a sodium pyrophosphate iron phosphate carbon precursor; Preparation of sodium pyrophosphate iron phosphate composite material: sintering the obtained sodium pyrophosphate iron phosphate carbon precursor to obtain a sodium pyrophosphate iron phosphate composite material.

2. The method for preparing sodium iron pyrophosphate composite material according to claim 1, characterized in that, In the step of preparing the sodium pyrophosphate iron phosphate carbon precursor, the mixing is performed by first grinding the sodium pyrophosphate iron phosphate precursor, and then adding the carbon source and performing secondary grinding; The grinding method comprises at least one of ball milling and sand mill grinding, and the secondary grinding method comprises at least one of ball milling and sand mill grinding; The carbon source is added in the form of a solution; The mass ratio of the carbon source to the sodium pyrophosphate iron phosphate precursor is (1-40): 15; The heat treatment is performed at a temperature of 120-180℃.

3. The method for preparing sodium pyrophosphate ferric phosphate complex material according to claim 2, characterized in that, In the step of preparing the sodium pyrophosphate iron phosphate carbon precursor, the grinding time is 5-120 min; The grinding rate is 100-800 rpm; The secondary grinding time is 5-120 min; The secondary grinding rate is 100-800 rpm.

4. The method for preparing sodium iron pyrophosphate composite material according to claim 1, characterized in that, In the step of preparing the sodium pyrophosphate iron phosphate carbon precursor, the carbon source comprises one or more of citric acid, glucose, sucrose, oxalic acid, polyvinylpyrrolidone, polyethylene glycol and lactic acid; The carbon source is added in the form of a solution; The mass fraction of the carbon source in the carbon source solution is 0.1-1 g / mL.

5. The method for preparing the sodium iron phosphate pyrophosphate composite material according to claim 1, characterized in that, In the step of preparing the sodium pyrophosphate iron phosphate precursor, the iron source comprises one or more of ferric nitrate, ferric nitrate nonahydrate, ferrous nitrate, ferrous carbonate and ferrous phosphate; The phosphorus source comprises one or more of sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate and ammonium dihydrogen phosphate; The sodium source comprises one or more of a phosphorus-containing sodium salt, sodium carbonate and sodium chloride; The molar ratio of iron, phosphorus and sodium in the iron source, the phosphorus source and the sodium source is (2.6-3.2): 4: (4-4.1).

6. The method for preparing sodium iron pyrophosphate composite material according to claim 5, characterized in that, The phosphorus-containing sodium salt is sodium phosphate, disodium hydrogen phosphate or sodium dihydrogen phosphate.

7. The method for preparing the sodium iron phosphate pyrophosphate composite material according to claim 1, characterized in that, In the step of preparing the sodium pyrophosphate iron phosphate precursor, the precipitant is an alkaline substance; The precipitant is added in the form of a solution; The amount of the precipitant solution added is 0.5-1.0 mL / g based on the mass of the iron source; The mass fraction of the precipitant in the precipitant solution is 28.0-30.0%; The precipitant solution is added dropwise; The heat treatment is performed at a temperature of 80-150℃.

8. The method for preparing sodium pyrophosphate ferric phosphate complex material according to claim 7, characterized in that, The alkaline substance is one or more of sodium hydroxide, ammonia and sodium carbonate.

9. The method for preparing the sodium iron phosphate pyrophosphate composite material according to claim 1, characterized in that, In the step of preparing the sodium pyrophosphate iron phosphate composite material, the sintering comprises primary heat preservation and secondary heat preservation; The temperature of the primary heat preservation is 100-500℃; And / or, the first holding time is 1-8h; And / or, the second holding temperature is 500-800℃; And / or, the second holding time is 1-24h.

10. The method of claim 9, wherein the sodium pyrophosphate ferric phosphate complex is prepared by the process of: In the preparation of pyrophosphate sodium ferric phosphate composite material step, the first holding and the second holding are both carried out in an inert atmosphere; ​ The inert atmosphere comprises one or more of argon, nitrogen, hydrogen-argon mixed gas.

11. A sodium pyrophosphate ferric phosphate complex material, characterized by, The pyrophosphate sodium ferric phosphate composite material prepared by the preparation method of any one of claims 1-10.

12. A sodium-ion battery, characterized in that, The pyrophosphate sodium ferric phosphate composite material prepared by the preparation method of any one of claims 1-10 or the pyrophosphate sodium ferric phosphate composite material of claim 11.

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