Doped ferric sodium pyrophosphate precursor, preparation method thereof and battery

Abstract

The invention provides a doped ferric sodium pyrophosphate precursor, a preparation method thereof and a battery, and the preparation method comprises the following steps: mixing doped ferrite, a chromium source and a first solvent to obtain a base solution, and mixing phosphate and pyrophosphate with a second solvent to obtain a mixed phosphorus-containing solution; the mixed phosphorus-containing solution, an oxidizing agent and alkali liquor are injected into the base solution in a parallel flow mode for a co-precipitation reaction, and the doped ferric sodium pyrophosphate precursor is obtained; wherein doping elements in the doped ferrite comprise magnesium and manganese. According to the preparation method, the mixed solution of the doped metal and the ferrous iron is used as the base solution, so that uniform doping of various elements is realized, various elements in the prepared doped ferric sodium phosphate pyrophosphate precursor have a synergistic effect, and the rate capability and the cycling stability of the prepared ferric sodium phosphate pyrophosphate positive electrode material are greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery material technology, and relates to a doped sodium iron pyrophosphate precursor, its preparation method, and the battery thereof. Background Technology

[0002] Energy storage systems are a core component in promoting the large-scale development and efficient utilization of renewable energy. Electrochemical energy storage technology based on rechargeable batteries, with its advantages of fast response, high energy density, and wide applicability, has become one of the mainstream development directions in the current energy storage field. Lithium-ion batteries, with their mature technology, have been widely used in new energy vehicles, consumer electronics, and other fields, but their development faces severe resource constraints. Against this backdrop, sodium, belonging to Group IA like lithium, with its similar chemical properties, virtually unlimited Earth reserves, and low mining costs, makes sodium-ion batteries an ideal technological route to replace lithium-ion batteries and achieve large-scale energy storage applications, demonstrating extremely high commercial potential.

[0003] As a core component determining the energy density, cycle life, and safety of sodium-ion batteries, the performance and cost of cathode materials directly impact the overall competitiveness of the battery. Currently, sodium-ion battery cathode materials are mainly classified into four categories: layered oxides, polyanionic cathodes, Prussian blue cathodes, and organic materials. Among these, layered oxides and polyanionic cathodes have become the focus of current research and industrialization due to their outstanding comprehensive performance. Polyanionic cathode materials, in particular, have become the preferred material for large-scale commercial energy storage applications due to their excellent cycle stability, superior thermal stability, and inexpensive raw materials. Depending on the type of anion, polyanionic cathode materials can be further subdivided into sulfates, phosphates, pyrophosphates, silicates, and mixed salts of phosphate and pyrophosphate. Among these subcategories, sodium iron pyrophosphate, as the most extensively researched iron-based polyanionic cathode material, not only possesses high theoretical capacity and stable operating voltage but also excellent cycle performance. Furthermore, relying on low-cost elements such as iron and phosphorus, it has a natural advantage in cost control, making it a cathode material with both promising application prospects and cost-effectiveness.

[0004] CN118684208A discloses a high-entropy sodium iron pyrophosphate precursor and a high-entropy sodium iron pyrophosphate cathode material. The preparation method of the precursor includes: mixing an iron source, an alloy source, a phosphorus source and a carbon source, drying and sintering to obtain the high-entropy sodium iron pyrophosphate precursor.

[0005] CN120573667A discloses a porous sodium iron pyrophosphate cathode precursor, its preparation method, and its application. The preparation method includes the following steps: co-precipitating an iron source solution, a phosphorus source solution, a pyrophosphate source solution, an oxidant solution, a precipitant solution, and an organic polymer solution to obtain an intermediate material; and pre-calcining the intermediate material to obtain the porous sodium iron pyrophosphate cathode precursor.

[0006] The elemental distribution uniformity or structural stability of the sodium iron pyrophosphate precursor synthesized by the above scheme is poor, resulting in low electronic conductivity, poor rate performance, or poor cycle performance of the cathode material, making it difficult to meet the fast charging and discharging requirements of energy storage systems. Moreover, the material particles have irregular morphology, severe agglomeration, and low compaction density, which limits the improvement of battery energy density. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a doped sodium iron pyrophosphate precursor, its preparation method, and a battery thereof. By using a mixed solution of doped metal and ferrous iron as the base liquid, the present invention achieves uniform doping of multiple elements, resulting in a doped sodium iron pyrophosphate precursor in which various elements work synergistically to significantly improve the rate performance and cycle stability of the prepared sodium iron pyrophosphate cathode material.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing a doped sodium iron pyrophosphate precursor, the method comprising the following steps:

[0010] A base solution is obtained by mixing a doped ferrous salt, a chromium source and a first solvent, and a mixed phosphorus-containing solution is obtained by mixing a phosphate and a pyrophosphate with a second solvent.

[0011] A mixed phosphorus-containing solution, oxidant, and alkaline solution were injected concurrently into the base liquid to carry out a co-precipitation reaction, thereby obtaining the doped sodium iron pyrophosphate precursor.

[0012] The doping elements in the doped ferrous salt include magnesium and manganese.

[0013] This invention uses magnesium and manganese-doped ferrous salts mixed with a chromium source to prepare a base solution. During the precursor preparation stage, phosphate and pyrophosphate ions combine with the doping elements magnesium, manganese, and chromium to form a uniform precipitate, which is then incorporated into the sodium iron pyrophosphate precursor to obtain an amorphous sodium iron pyrophosphate precursor. The uniform doping of manganese in the sodium iron pyrophosphate precursor can increase the charge and discharge voltage of the cathode material, thereby increasing the energy density of the material. The uniform doping of chromium can introduce defect states at the top of the valence band, thereby increasing the electronic conductivity of the material and thus improving the rate performance. The uniform doping of magnesium can improve the structural stability of the cathode material, thereby improving the stability of the material under high voltage and high rate cycling. The sodium iron pyrophosphate cathode material prepared by sintering the doped sodium iron pyrophosphate precursor with a sodium source has high phase purity and good rate performance.

[0014] Preferably, the doped ferrous salt is obtained by removing titanium from industrial-grade ferrous salt.

[0015] The industrial-grade ferrous salt described in this invention is a byproduct of ferrous sulfate in the titanium dioxide industry. After simple titanium removal treatment, magnesium- and manganese-doped ferrous sulfate can be obtained. The magnesium and manganese content in the industrial-grade ferrous salt is stable, uniformly distributed, and can be controlled within a suitable ratio range with iron. It can be used as a raw material for direct precursor preparation, which not only realizes resource recovery and reduces production costs, but also improves the uniformity of dopant element distribution in the obtained precursor.

[0016] Preferably, the doped ferrous salt includes doped ferrous sulfate.

[0017] Preferably, the chromium source includes chromium nitrate and / or chromium sulfate.

[0018] Preferably, the first solvent includes water.

[0019] Preferably, the molar concentration of iron in the base liquid is 1 mol / L to 1.2 mol / L, for example: 1 mol / L, 1.05 mol / L, 1.1 mol / L, 1.15 mol / L or 1.2 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0020] Preferably, the molar ratio of iron to magnesium in the base liquid is (16~30):1, for example: 16:1, 18:1, 20:1, 25:1 or 30:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0021] Preferably, the molar ratio of iron to manganese in the base liquid is (100~300):1, for example: 100:1, 150:1, 200:1, 250:1 or 300:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0022] Preferably, the molar ratio of iron to chromium in the base liquid is (100~300):1, for example: 100:1, 150:1, 200:1, 250:1 or 300:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] Preferably, the phosphate comprises sodium phosphate.

[0024] Preferably, the pyrophosphate comprises sodium pyrophosphate.

[0025] Preferably, the second solvent comprises water.

[0026] Preferably, the molar concentration of phosphate in the mixed phosphorus-containing solution is 0.1 mol / L to 2 mol / L, for example: 0.1 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0027] Preferably, the molar concentration of pyrophosphate in the mixed phosphorus-containing solution is 0.1 mol / L to 2 mol / L, for example: 0.1 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0028] Preferably, the oxidant comprises an aqueous solution of hydrogen peroxide.

[0029] Preferably, the molar concentration of the hydrogen peroxide solution is 5 mol / L to 15 mol / L, for example: 5 mol / L, 8 mol / L, 10 mol / L, 12 mol / L or 15 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] Preferably, the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to ferrous ions in the base solution is (0.2~0.6):1, for example: 0.2:1, 0.3:1, 0.4:1, 0.5:1 or 0.6:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0031] Preferably, the alkaline solution comprises a sodium hydroxide solution.

[0032] Preferably, the molar concentration of the sodium hydroxide solution is 5 mol / L to 15 mol / L, for example: 5 mol / L, 8 mol / L, 10 mol / L, 12 mol / L or 15 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] Preferably, the pH of the coprecipitation reaction is 4 to 7, for example: 4, 4.5, 5, 6 or 7, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] Preferably, the temperature of the coprecipitation reaction is 30℃~90℃, for example: 30℃, 40℃, 60℃, 80℃ or 90℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0035] Preferably, the atmosphere for the coprecipitation reaction includes nitrogen and / or argon.

[0036] Preferably, the coprecipitation reaction time is 20h to 50h, for example: 20h, 25h, 30h, 40h or 50h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0037] In a second aspect, the present invention provides a doped sodium iron pyrophosphate precursor, which is prepared by the preparation method described in the first aspect.

[0038] Secondly, the present invention provides a doped sodium iron pyrophosphate cathode material, wherein the doped sodium iron pyrophosphate cathode material is prepared by mixing the doped sodium iron pyrophosphate precursor as described in the second aspect with a sodium source, followed by spray granulation and sintering.

[0039] In a second aspect, the present invention provides a sodium-ion battery comprising a doped sodium iron pyrophosphate cathode material as described in the third aspect.

[0040] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] (1) By using a mixed solution of doped metal and ferrous iron as the base liquid, the present invention achieves uniform doping of multiple elements, and the synergistic effect of various elements in the doped sodium iron pyrophosphate precursor significantly improves the rate performance and cycle stability of the prepared sodium iron pyrophosphate cathode material.

[0043] (2) When the doped sodium iron pyrophosphate precursor prepared by the method of the present invention is used to prepare a cathode material, the discharge specific capacity at 10 mA / g can reach more than 110.06 mAh / g, the discharge specific capacity at 100 mA / g can reach more than 102.11 mAh / g, the 1C / 0.1C discharge capacity ratio can reach more than 92.73%, and the capacity retention rate after 10,000 cycles can reach more than 86.45%. It can be seen that the rate performance and cycle performance of the cathode material prepared by the present invention are significantly better than those of conventional sodium iron pyrophosphate cathode materials. Attached Figure Description

[0044] Figure 1 This is the XRD pattern of the doped sodium iron pyrophosphate precursor prepared in Example 1 of the present invention. Detailed Implementation

[0045] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0046] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0047] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0048] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0049] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0050] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0051] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0052] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0053] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0054] In this invention, "optional" means that something is optional, that is, it refers to any one of the two parallel solutions of "having" or "not having". If there are multiple "optional" options in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, then each "optional" option is independent.

[0055] In this invention, "room temperature" generally refers to 4℃~35℃, and can refer to 20℃±5℃. In some embodiments of this invention, room temperature refers to 20℃~30℃.

[0056] The magnesium-manganese-doped ferrous sulfate used in the embodiments and comparative examples of this invention was obtained by the following method:

[0057] First, dissolve ferrous sulfate, a byproduct of the titanium dioxide industry, in water to obtain a solution with a pH of 2.5. Add ammonia water to adjust the pH to 5.0. After filtration, evaporation, and crystallization, titanium-free ferrous sulfate crystals can be obtained.

[0058] The proportions of iron, magnesium, and manganese can be controlled within a certain range. In this invention, ferrous sulfate of a suitable proportion is selected as a raw material, and the specific proportions are as described in each embodiment.

[0059] Example 1

[0060] This embodiment provides a doped sodium iron pyrophosphate precursor, which is prepared by the following method:

[0061] Magnesium-manganese-doped ferrous sulfate, chromium nitrate, and water are mixed to obtain a base solution. Sodium phosphate, sodium pyrophosphate, and water are mixed to obtain a mixed phosphorus-containing solution. In the base solution, the molar concentration of ferrous ions is 1.1 mol / L, the molar ratio of iron to magnesium is 22:1, the molar ratio of iron to manganese is 200:1, the molar ratio of iron to chromium is 200:1, the molar concentration of phosphate in the mixed phosphorus-containing solution is 0.4 mol / L, and the molar concentration of pyrophosphate in the mixed phosphorus-containing solution is 0.2 mol / L.

[0062] A mixed phosphorus-containing solution, a 12 mol / L hydrogen peroxide solution, and a 10 mol / L sodium hydroxide solution were injected concurrently into the base solution. The molar ratio of hydrogen peroxide in the hydrogen peroxide solution to ferrous ions in the base solution was 0.4:1. The pH was controlled at 6.5, the temperature at 50°C, and the co-precipitation reaction was carried out under a nitrogen atmosphere for 25 h. After filtration, washing, and drying, the doped sodium pyrophosphate phosphate precursor was obtained.

[0063] The XRD pattern of the doped sodium iron pyrophosphate precursor is shown below. Figure 1 As shown, by Figure 1 It can be seen that the doped sodium iron pyrophosphate precursor has an amorphous structure.

[0064] Example 2

[0065] This embodiment provides a doped sodium iron pyrophosphate precursor, which is prepared by the following method:

[0066] Magnesium-manganese doped ferrous sulfate, chromium nitrate, and water are mixed to obtain a base solution. Sodium phosphate, sodium pyrophosphate, and water are mixed to obtain a mixed phosphorus-containing solution. In the base solution, the molar concentration of ferrous ions is 1 mol / L, the molar ratio of iron to magnesium is 16:1, the molar ratio of iron to manganese is 100:1, the molar ratio of iron to chromium is 100:1, the molar concentration of phosphate ions in the mixed phosphorus-containing solution is 0.2 mol / L, and the molar concentration of pyrophosphate ions in the mixed phosphorus-containing solution is 0.1 mol / L.

[0067] A mixed phosphorus-containing solution, a 5 mol / L hydrogen peroxide solution, and a 5 mol / L sodium hydroxide solution were injected concurrently into the base solution. The molar ratio of hydrogen peroxide in the hydrogen peroxide solution to ferrous ions in the base solution was 0.2:1. The pH was controlled at 7.0, the temperature at 90°C, and the co-precipitation reaction was carried out under a nitrogen atmosphere for 20 h. After filtration, washing, and drying, the doped sodium pyrophosphate precursor was obtained.

[0068] Example 3

[0069] This embodiment provides a doped sodium iron pyrophosphate precursor, which is prepared by the following method:

[0070] Magnesium-manganese doped ferrous sulfate, chromium nitrate, and water are mixed to obtain a base solution. Sodium phosphate, sodium pyrophosphate, and water are mixed to obtain a mixed phosphorus-containing solution. In the base solution, the molar concentration of ferrous ions is 1.2 mol / L, the molar ratio of iron to magnesium is 30:1, the molar ratio of iron to manganese is 300:1, the molar ratio of iron to chromium is 300:1, the molar concentration of phosphate ions in the mixed phosphorus-containing solution is 2 mol / L, and the molar concentration of pyrophosphate ions in the mixed phosphorus-containing solution is 1 mol / L.

[0071] A mixed phosphorus-containing solution, a 15 mol / L hydrogen peroxide solution, and a 15 mol / L sodium hydroxide solution were injected concurrently into the base solution. The molar ratio of hydrogen peroxide in the hydrogen peroxide solution to ferrous ions in the base solution was 0.6:1. The pH was controlled at 4, the temperature at 30°C, and the co-precipitation reaction was carried out under a nitrogen atmosphere for 50 h. After filtration, washing, and drying, the doped sodium ferric pyrophosphate precursor was obtained.

[0072] Example 4

[0073] The only difference between this embodiment and Embodiment 1 is that the molar ratio of iron to magnesium in the base liquid is adjusted to 10:1; all other conditions and parameters are exactly the same as in Embodiment 1.

[0074] Example 5

[0075] The only difference between this embodiment and Embodiment 1 is that the molar ratio of iron to magnesium in the base liquid is adjusted to 50:1. All other conditions and parameters are exactly the same as in Embodiment 1.

[0076] Example 6

[0077] The only difference between this embodiment and Embodiment 1 is that the molar ratio of iron to manganese in the base liquid is adjusted to 50:1. All other conditions and parameters are exactly the same as in Embodiment 1.

[0078] Example 7

[0079] The only difference between this embodiment and Embodiment 1 is that the molar ratio of iron to manganese in the base liquid is adjusted to 400:1. All other conditions and parameters are exactly the same as in Embodiment 1.

[0080] Example 8

[0081] The only difference between this embodiment and Embodiment 1 is that the molar ratio of iron to chromium in the base liquid is adjusted to 50:1. All other conditions and parameters are exactly the same as in Embodiment 1.

[0082] Example 9

[0083] The only difference between this embodiment and Embodiment 1 is that the molar ratio of iron to chromium in the base solution is adjusted to 400:1. All other conditions and parameters are exactly the same as in Embodiment 1.

[0084] Comparative Example 1

[0085] The only difference between this comparative example and Example 1 is that ferrous sulfate, magnesium nitrate and manganese sulfate are used in equal proportions to replace the doped ferrous sulfate. All other conditions and parameters are exactly the same as in Example 1.

[0086] Comparative Example 2

[0087] The only difference between this comparative example and Example 1 is that water is used as the base liquid, and a mixed salt solution containing ferrous, magnesium, manganese and chromium (the proportions of each element are the same as those in the base liquid in the example) is injected into the base liquid in parallel with a mixed phosphorus-containing solution, oxidant and alkali solution. All other conditions and parameters are exactly the same as in Example 1.

[0088] Comparative Example 3

[0089] The only difference between this comparative example and Example 1 is that manganese is not doped, and ferrous sulfate and magnesium nitrate are used to replace the doped ferrous sulfate in equal proportions. All other conditions and parameters are exactly the same as in Example 1.

[0090] Comparative Example 4

[0091] The only difference between this comparative example and Example 1 is that magnesium is not doped, and ferrous sulfate and manganese sulfate are used in equal proportions to replace the doped ferrous sulfate. All other conditions and parameters are exactly the same as in Example 1.

[0092] Comparative Example 5

[0093] The only difference between this comparative example and Example 1 is that it does not contain chromium; all other conditions and parameters are exactly the same as in Example 1.

[0094] Performance testing:

[0095] The doped sodium iron pyrophosphate precursors prepared in the examples and comparative examples were mechanically mixed with sodium carbonate at a molar ratio of 1:2.5, wet-milled for 4 hours, spray-granulated, and sintered at 600°C for 8 hours under a nitrogen atmosphere to obtain the doped sodium iron pyrophosphate cathode material. Using the cathode material prepared above as the main cathode material and sodium metal sheets as the anode, CR2032 coin cells were assembled at 25°C. Electrochemical performance tests were then conducted at discharge current densities of 10 mA / g and 100 mA / g within a voltage range of 2.0–3.8 V. The test results are shown in Table 1.

[0096] Table 1

[0097]

[0098] As shown in Table 1, and based on Examples 1-9, the doped sodium iron pyrophosphate precursor prepared using the method of this invention, when used as a cathode material, exhibits a discharge capacity of over 105.78 mAh / g at 10 mA / g, a discharge capacity of over 91.23 mAh / g at 100 mA / g, a 1C / 0.1C discharge capacity ratio of over 85.58%, and a capacity retention rate of over 80.29% after 10,000 cycles. By adjusting the preparation conditions, the doped sodium iron pyrophosphate precursor, when used as a cathode material, exhibits a discharge capacity of over 110.06 mAh / g at 10 mA / g, a discharge capacity of over 102.11 mAh / g at 100 mA / g, a 1C / 0.1C discharge capacity ratio of over 92.73%, and a capacity retention rate of over 86.45% after 10,000 cycles. Therefore, it can be seen that the rate performance and cycle performance of the cathode material prepared by this invention are significantly better than those of conventional sodium iron pyrophosphate cathode materials.

[0099] A comparison of Examples 1 and 4-5 shows that the magnesium content in the base solution affects the performance of the doped sodium iron pyrophosphate precursor described in this invention. Controlling the molar ratio of iron to magnesium in the base solution to 16-30:1 results in a better performance of the doped sodium iron pyrophosphate precursor. If the magnesium content is too high, the conductivity of the cathode material decreases, leading to a significant reduction in both rate performance and cycle performance. If the magnesium content is too low, the structural stability of the cathode material decreases, which also leads to a decrease in electrical performance.

[0100] A comparison of Examples 1 and 6-7 shows that the manganese content in the substrate affects the performance of the doped sodium iron pyrophosphate precursor described in this invention. Controlling the molar ratio of iron to manganese in the substrate to 100-300:1 yields a better-performing doped sodium iron pyrophosphate precursor. If the manganese content is too high, the effective capacity of the cathode material within the voltage range decreases, leading to a decline in electrochemical performance. Conversely, if the manganese content is too low, the average discharge voltage of the cathode material decreases significantly, also resulting in a decline in electrical performance.

[0101] A comparison of Examples 1 and 8-9 shows that the chromium content in the substrate affects the performance of the doped sodium iron pyrophosphate precursor described in this invention. Controlling the molar ratio of iron to chromium in the substrate to 100-300:1 yields a better-performing doped sodium iron pyrophosphate precursor. If the chromium content is too high, the proportion of active components in the resulting cathode material will be low, and the charge / discharge capacity per unit mass of the cathode material will decrease. Conversely, if the chromium content is too low, the conductivity of the cathode material will significantly decrease, resulting in a significant reduction in the rate performance of the obtained cathode material.

[0102] As can be seen from the comparison between Example 1 and Comparative Example 1, the present invention uses ferrous sulfate by-products from the titanium dioxide industry. After simple titanium removal treatment, magnesium and manganese-doped ferrous sulfate can be obtained. The magnesium and manganese content in the industrial-grade ferrous salt is stable, uniformly distributed, and within a suitable ratio with iron. It can be used as a raw material to directly prepare a precursor, which not only realizes resource recovery and reduces production costs, but also improves the uniformity of dopant element distribution in the obtained precursor. The performance of the obtained precursor is basically the same as that of the precursor prepared using a variety of high-purity doped metal salts.

[0103] A comparison of Example 1 and Comparative Example 2 shows that when conventional water is used as the base liquid, the system conditions fluctuate greatly during the reaction process, the consistency of the products obtained at different stages of the reaction is poor, and the electrochemical performance of the final cathode material will also decrease significantly.

[0104] As can be seen from the comparison between Example 1 and Comparative Examples 3-5, the uniform doping of manganese in the sodium iron pyrophosphate precursor of the present invention can improve the charge and discharge voltage of the cathode material, thereby improving the energy density of the material. The uniform doping of chromium can introduce defect states at the top of the valence band, thereby improving the electronic conductivity of the material and thus improving the rate performance of the material. The uniform doping of magnesium can improve the structural stability of the cathode material and thus improve the stability of the material under high voltage and high rate cycling.

[0105] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a doped sodium iron pyrophosphate precursor, characterized in that, The preparation method includes the following steps: A base solution is obtained by mixing a doped ferrous salt, a chromium source and a first solvent, and a mixed phosphorus-containing solution is obtained by mixing a phosphate and a pyrophosphate with a second solvent. A mixed phosphorus-containing solution, oxidant, and alkaline solution were injected concurrently into the base liquid to carry out a co-precipitation reaction, thereby obtaining the doped sodium iron pyrophosphate precursor. The doping elements in the doped ferrous salt include magnesium and manganese.

2. The preparation method according to claim 1, characterized in that, The doped ferrous salt is prepared by removing titanium from industrial-grade ferrous salt; Preferably, the doped ferrous salt includes doped ferrous sulfate; Preferably, the chromium source includes chromium nitrate and / or chromium sulfate; Preferably, the first solvent includes water.

3. The preparation method according to claim 1 or 2, characterized in that, The molar concentration of iron in the underlying solution is 1 mol / L to 1.2 mol / L; Preferably, the molar ratio of iron to magnesium in the bottom liquid is (16~30):1; Preferably, the molar ratio of iron to manganese in the bottom liquid is (100~300):1; Preferably, the molar ratio of iron to chromium in the bottom liquid is (100~300):

1.

4. The preparation method according to any one of claims 1-3, characterized in that, The phosphate includes sodium phosphate; Preferably, the pyrophosphate comprises sodium pyrophosphate; Preferably, the second solvent comprises water.

5. The preparation method according to any one of claims 1-4, characterized in that, The molar concentration of phosphate in the mixed phosphorus-containing solution is 0.1 mol / L to 2 mol / L; Preferably, the molar concentration of pyrophosphate in the mixed phosphorus-containing solution is 0.1 mol / L to 2 mol / L.

6. The preparation method according to any one of claims 1-5, characterized in that, The oxidant includes an aqueous solution of hydrogen peroxide; Preferably, the molar concentration of the hydrogen peroxide aqueous solution is 5 mol / L to 15 mol / L; Preferably, the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to ferrous ions in the base solution is (0.2~0.6):1; Preferably, the alkaline solution comprises a sodium hydroxide solution; Preferably, the molar concentration of the sodium hydroxide solution is 5 mol / L to 15 mol / L.

7. The preparation method according to any one of claims 1-6, characterized in that, The pH of the coprecipitation reaction is 4-7; Preferably, the temperature of the co-precipitation reaction is 30℃~90℃; Preferably, the atmosphere for the coprecipitation reaction includes nitrogen and / or argon; Preferably, the coprecipitation reaction takes 20 to 50 hours.

8. A doped sodium iron pyrophosphate precursor, characterized in that, The doped sodium iron pyrophosphate precursor is prepared by the preparation method according to any one of claims 1-7.

9. A doped sodium iron pyrophosphate cathode material, characterized in that, The doped sodium iron pyrophosphate cathode material is prepared by mixing the doped sodium iron pyrophosphate precursor as described in claim 8 with a sodium source, followed by spray granulation and sintering.

10. A sodium-ion battery, characterized in that, The sodium-ion battery comprises the doped sodium iron pyrophosphate cathode material as described in claim 9.

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