A method for preparing a manganese-doped sodium iron phosphate pyrophosphate precursor and a manganese-doped sodium iron phosphate pyrophosphate

Abstract

The application discloses a preparation method of manganese-doped sodium iron phosphate pyrophosphate precursor and the manganese-doped sodium iron phosphate pyrophosphate, and the preparation method comprises the following steps: (1) dissolving a phosphoric acid source, a pyrophosphoric acid source, a sodium source and an oxidizing agent in water to prepare a clear phosphorus salt solution; (2) dissolving an iron source and a manganese source in water to prepare a clear iron-manganese solution; (3) feeding the phosphorus salt solution and the iron-manganese solution in parallel dropwise, and reacting; after the reaction is completed, a filter cake is collected by filtration, and the filter cake is washed and dried to obtain the manganese-doped sodium iron phosphate pyrophosphate precursor. The application realizes the uniform mixing of Fe and Mn elements at an atomic level, ensures the consistency of the electrochemical activity of each particle, thereby stably playing the role of the Mn element in improving the voltage platform, inhibiting the manganese elution, stably arranging the crystal lattice, avoiding the Jahn-Teller effect, and finally optimizing the electrochemical performance of the manganese-doped sodium iron phosphate pyrophosphate positive electrode material.

Description

Technical Field

[0001] This invention relates to a method for preparing a sodium ferric pyrophosphate precursor and sodium ferric pyrophosphate, and more particularly to a method for preparing a manganese-doped sodium ferric pyrophosphate precursor and manganese-doped sodium ferric pyrophosphate. Background Technology

[0002] Sodium iron pyrophosphate (NFPP) operates at a voltage plateau of approximately 3.0-3.1V, while sodium manganese iron pyrophosphate (NFMPP) can achieve an operating voltage of approximately 3.3-3.4V through Mn doping. Under the same capacity conditions, NFMPP batteries can achieve an energy density nearly 30% higher than NFPP batteries, making them commercially valuable for energy storage and small-power applications. However, existing NFMPP precursor preparation processes still have significant shortcomings. Traditional processes generally employ a solid-state mixture of a manganese-free NFPP precursor and a manganese source, resulting in a severely uneven distribution of Mn / Fe elements. This unevenness exacerbates the Jahn-Teller effect, causing lattice distortion and structural strain, leading to accelerated manganese dissolution, structural instability, and ultimately deterioration of cycle performance.

[0003] To overcome the problem of uniformity in solid-phase mixing, CN119503750A discloses a precursor for a polyanionic sodium-ion battery cathode material and its preparation method. Ferric oxide and manganese oxide are dissolved in acid, oxidized with an oxidant, and then co-precipitated with a phosphorus source and a sodium source at pH ≥ 3.0 to obtain a precursor with the general formula Na. x M y H z O a (PO b ) c Amorphous precursor of mH2O. Although this method improves elemental distribution compared to solid-phase mixing, the process is complex, requiring sequential acid dissolution, pre-oxidation with an external oxidant (such as hydrogen peroxide), pH adjustment precipitation with alkali, solid-liquid separation, and washing and drying. CN121516842A discloses a high-voltage composite sodium iron phosphate precursor and its preparation method, using a hydrothermal method to synthesize the high-voltage composite sodium iron phosphate precursor Na. x Fe 3- y Mn y(PO4)2P2O7 (both iron and manganese are trivalent) aims to achieve atomic-level uniform Mn / Fe doping. Although this method increases the operating voltage at 0.1C rate to over 3.27V, it requires the use of concentrated nitric acid (1.2-2 times the amount of manganese source) to oxidize divalent manganese, followed by hydrothermal reaction in a reactor at 80-150℃ for 7-15 hours. These conditions are harsh and pose a risk of residual nitrogen oxides. Furthermore, while increasing the manganese doping ratio to improve electrochemical performance, this exacerbates the Jahn-Teller effect, leading to significant capacity decay. CN121484030A discloses a polyanionic cathode material and its preparation method and applications, which involves mixing iron, manganese, magnesium, phosphate, pyrophosphate, and a complexing agent in the liquid phase for co-precipitation to obtain a coprecipitate. However, this method mainly relies on introducing Mg doping to suppress the Jahn-Teller effect, failing to address the inherent distortion problem of manganese-doped NFPP.

[0004] In summary, existing NFMPP precursor preparation technologies still cannot simultaneously meet the requirements of uniform elemental distribution, low manganese cyclic leaching, high voltage and high capacity matching, and simple and low-cost processes, and urgently need further improvement. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a method for preparing a manganese-doped sodium ferric phosphate precursor that can uniformly distribute Mn / Fe elements, suppress the Jahn-Teller effect and manganese leaching problems. This invention also provides the manganese-doped sodium ferric phosphate finally prepared from the precursor obtained by the method.

[0006] Technical solution: The preparation method of the manganese-doped sodium iron pyrophosphate precursor of the present invention includes the following steps:

[0007] (1) Dissolve the phosphoric acid source, pyrophosphate source, sodium source and oxidant in water to prepare a clear phosphate salt solution;

[0008] (2) Dissolve the iron and manganese sources in water to prepare a clear iron-manganese solution;

[0009] (3) The phosphate salt solution and the iron-manganese solution are added dropwise in parallel to the feed to react. After the reaction is completed, the filter cake is collected by filtration. The filter cake is washed and dried to obtain the manganese-doped sodium iron pyrophosphate precursor.

[0010] Among them, the iron source reacts with the phosphate source to form a precipitate, and the manganese source reacts with the oxidant to form a precipitate. However, the phosphate source, pyrophosphate source, sodium source, and oxidant do not react with each other, and the iron source and manganese source do not react with each other. By preparing the raw materials that do not react with each other into clear solutions separately and then mixing them, it is possible to ensure that all raw materials are completely dissolved while avoiding premature precipitation.

[0011] In step (1), the phosphoric acid source is phosphoric acid, ammonium phosphate, sodium phosphate, and / or their respective hydrates, which avoids the introduction of metal impurity ions and eliminates the need for subsequent impurity removal. The ammonium phosphate includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and triammonium phosphate, and the sodium phosphate includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, and trisodium phosphate.

[0012] In step (1), the pyrophosphate source is sodium pyrophosphate and / or its hydrate, used to provide pyrophosphate ions.

[0013] In step (1), the sodium source is a water-soluble sodium salt, including sodium hydroxide, sodium carbonate, sodium bicarbonate, etc.

[0014] In step (1), the oxidant is one of sodium persulfate, ammonium persulfate, or hydrogen peroxide. The amount of oxidant used is excessive, which is used to completely oxidize the divalent iron source and manganese source to trivalent while avoiding the introduction of metal impurity ions.

[0015] In step (2), the iron source is a water-soluble divalent ferrous salt and / or its hydrate, wherein the divalent ferrous salt is at least one of ferrous sulfate, ferrous sulfate heptahydrate, and ferrous chloride. Divalent ferrous salts have good water solubility, making it easy to prepare a clear solution; if trivalent ferrous salts are used, they will hydrolyze to form a precipitate, which is not conducive to achieving atomic-level uniform mixing of elements; the manganese source is a water-soluble divalent manganese salt and / or its hydrate, wherein the divalent manganese salt is at least one of manganese sulfate, manganese chloride, and manganese sulfate monohydrate.

[0016] In steps (1) and (2), the amounts of the phosphate source, pyrophosphate source, sodium source, iron source, and manganese source are such that the molar ratio of phosphate, pyrophosphate, sodium ions, iron ions, and manganese ions in the product system is (1.6-1.7):(0.5-1.2):(1.6-1.7):(0.5-0.7):(0.3-0.7), and the concentration of phosphate is 0.2-0.3 mol / L. This feed ratio determines the pH value of the reaction system, and no additional pH control is required after feeding the materials according to this ratio.

[0017] In steps (1) and (2), the preparation of the phosphate salt solution and the iron-manganese solution is carried out at 40-60℃, which promotes dissolution and reduces the additional heating time in the subsequent reaction. If the temperature is too low, the generated particles will be small, which is not conducive to filtration and washing. If the temperature is too high, it will cause severe self-decomposition of the oxidant, increasing the consumption of the oxidant. At the same time, high temperature promotes the hydrolysis of pyrophosphate, resulting in an imbalance in the molar ratio of pyrophosphate, and a side reaction will occur under high temperature conditions, generating ferric ammonium hydroxyphosphate.

[0018] In step (3), the reaction temperature is 40-60℃ and the reaction time is 0.5-3h. During the reaction, iron and manganese are oxidized and precursor precipitates are generated. The precursor particles generated in the early stage of the reaction are small and dispersed, which act as precipitation inducers to induce the subsequent precipitation reaction to occur on the surface of the inducer, so that the final reaction product is uniform and consistent.

[0019] In step (3), the feed rates of the phosphate salt solution and the iron-manganese solution in the co-current drip addition are both 500-1500 g / h and the flow rates are equal. The co-current drip addition allows the phosphate salt solution and the iron-manganese solution to come into full contact, achieving sufficient precipitation of each element. This ensures that the manganese element is uniformly incorporated into the sodium iron pyrophosphate precursor, avoiding uneven element distribution caused by excessively high local concentrations. The same feed rate ensures that the pH, ferrous ion, manganese ion, phosphate, and pyrophosphate concentrations are stable throughout the reaction process, resulting in mild reaction conditions. If the co-current drip addition is not performed, since both iron and manganese ions catalyze the decomposition of the oxidant, the oxidant in the phosphate salt solution will prematurely oxidize the iron and manganese elements, leading to a higher concentration of trivalent manganese and iron ions in the initial stage of the reaction. This does not meet the requirement of stable control of the manganese and iron ion concentrations during the reaction process, and the oxidant itself will decompose.

[0020] Preferably, in step (3), before the co-current dripping, water is added to the reactor as a base, and the amount of water is 10-60% of the volume of the phosphate salt solution. If the amount of water used as a base is too large, the particle size of the precursor particles generated in the early stage of the reaction will increase, and the particle size of the aggregates formed later will decrease; if the amount of water used as a base is too small, the particle size of the precursor particles generated in the early stage of the reaction will be too small, and the particle size of the aggregates formed later will increase.

[0021] In step (3), the drying temperature is 100-160℃ and the drying time is 10-24h.

[0022] The manganese-doped sodium ferric phosphate of the present invention is prepared from the precursor obtained by the aforementioned method.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) By preparing the phosphoric acid source, pyrophosphate source, sodium source, oxidant, iron source and manganese source into clear solutions respectively, and feeding them in parallel, the atomic level uniform mixing of Fe and Mn elements is achieved, ensuring that the electrochemical activity of each particle is consistent, thereby stabilizing the role of Mn element in improving the voltage platform, inhibiting manganese dissolution, stabilizing the lattice, avoiding the Jahn-Teller effect, and finally optimizing the electrochemical performance of manganese-doped iron sodium pyrophosphate cathode material; (2) No additional doping elements need to be introduced, the preparation process is simple, and it is suitable for industrial-scale production. Attached Figure Description

[0024] Figure 1This is a scanning electron microscope image of the manganese-doped sodium iron pyrophosphate precursor prepared in Example 1 of the present invention. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to the embodiments and comparative examples. Unless otherwise stated, all reagents used are commercially available and are used directly without purification.

[0026] Example 1

[0027] 1. Preparation of phosphate salt solution: Weigh out 85% phosphoric acid, sodium pyrophosphate, sodium ions, iron ions, and manganese ions according to the molar ratio of phosphate, pyrophosphate, sodium ions, iron ions, and manganese ions in the system of 1.6:0.5:1.6:0.5:0.3. Dissolve 85% phosphoric acid, sodium pyrophosphate, sodium hydroxide, and excess 30% hydrogen peroxide in 2250g of pure water at 40℃ to obtain a clear solution.

[0028] 2. Preparation of iron and manganese solution: Dissolve ferrous sulfate heptahydrate and manganese sulfate monohydrate in 705g of pure water at 40℃ to obtain a clear solution.

[0029] 3. Reaction: Add 500 mL of pure water to the reactor as a base. Add the aforementioned phosphate salt solution and iron-manganese solution dropwise in parallel at a flow rate of 500 g / h. After the addition is complete, continue stirring and maintaining the temperature for 0.5 h. The amount of pure water used can be selected from 10-60% of the volume of the phosphate salt solution.

[0030] 4. Washing and drying: After the reaction is complete, filter and collect the filter cake. Wash the filter cake with pure water until the conductivity of the washing water is less than 1000 μs / cm. Place the filter cake in an oven at 120℃ and dry for 20 h to obtain the manganese-doped sodium iron pyrophosphate precursor.

[0031] 5. Preparation of manganese-doped sodium iron pyrophosphate cathode material: 100g of the precursor, 32g of sodium carbonate, and 8g of glucose were dispersed in 200g of deionized water and continuously stirred to obtain a mixture. The mixture was then milled using zirconium beads with a particle size of 0.3±0.1mm until the slurry particle size D50 was approximately 0.4±0.05μm. The milled slurry was spray-dried at an inlet air temperature of 225~230℃ and an outlet air temperature of 90~95℃ to obtain a dry powder. The dry powder was sintered under inert gas protection at a heating rate of 3℃ / min and a sintering temperature of 530℃ for 4 hours, followed by cooling to obtain a manganese-doped sodium iron pyrophosphate cathode material with a carbon content of approximately 1.6%.

[0032] Example 2

[0033] The difference from Example 1 is that in step (1), sodium monophosphate, sodium pyrophosphate decahydrate, sodium carbonate, ferrous sulfate and manganese sulfate are weighed according to the molar ratio of phosphate, pyrophosphate, sodium ion, iron ion and manganese ion in the system of 1.7:1.2:1.7:0.7:0.7, respectively. 30% hydrogen peroxide is replaced with excess sodium persulfate. In step (3), phosphate salt solution and iron-manganese solution are added dropwise in parallel at a flow rate of 750 g / h. The manganese-doped sodium iron pyrophosphate precursor is prepared into manganese-doped sodium iron pyrophosphate cathode material according to the same method as in Example 1.

[0034] Example 3

[0035] The difference from Example 1 is that in step (2), monoammonium phosphate, sodium pyrophosphate, sodium bicarbonate, ferrous chloride and manganese chloride are weighed according to the molar ratio of phosphate, pyrophosphate, sodium ion, iron ion and manganese ion in the system of 1.65:0.7:1.65:0.6:0.5, and 30% hydrogen peroxide is replaced with excess ammonium persulfate. In step (3), the phosphate salt solution and the iron and manganese solution are added dropwise in parallel at a flow rate of 1000 g / h. The manganese-doped sodium iron pyrophosphate precursor is prepared into manganese-doped sodium iron pyrophosphate cathode material according to the same method as in Example 1.

[0036] Example 4

[0037] The difference from Example 1 is that in steps (1) and (2), the preparation of phosphate salt solution and iron-manganese solution is carried out at 60°C. In step (3), phosphate salt solution and iron-manganese solution are added dropwise in parallel at a flow rate of 1500 g / h. After the addition is completed, stirring and keeping warm for 3 h are continued. The other steps are exactly the same. The manganese-doped sodium iron pyrophosphate precursor obtained is prepared into manganese-doped sodium iron pyrophosphate cathode material in the same way as in Example 1.

[0038] Comparative Example 1

[0039] The difference from Example 1 is that in step (2), the manganese source is changed to manganese trioxide (the molar amount of manganese in the system remains unchanged), and an iron-manganese suspension is obtained. The other steps are exactly the same. The manganese-doped sodium iron pyrophosphate precursor obtained is prepared into manganese-doped sodium iron pyrophosphate cathode material in the same way as in Example 1.

[0040] Comparative Example 2

[0041] The difference from Example 1 is that in step (1), hydrogen peroxide was not added when preparing the phosphate salt solution, and excess hydrogen peroxide was added dropwise at a rate of 3 g / min when preparing the iron-manganese solution to ensure that ferrous iron and divalent manganese were completely oxidized to trivalent manganese. The other steps are exactly the same. The manganese-doped sodium iron pyrophosphate precursor obtained is prepared into manganese-doped sodium iron pyrophosphate cathode material in the same way as in Example 1.

[0042] Comparative Example 3

[0043] The difference from Example 1 is that in step (1), when preparing the iron-manganese solution, an equimolar amount of ferric chloride is used instead of ferrous sulfate heptahydrate. The other steps are exactly the same. The manganese-doped sodium iron pyrophosphate precursor obtained is prepared into manganese-doped sodium iron pyrophosphate cathode material in the same way as in Example 1.

[0044] Comparative Example 4

[0045] The difference from Example 1 is that in step (3), the phosphate salt solution is added dropwise to the iron-manganese solution. The other steps are exactly the same. The manganese-doped sodium iron pyrophosphate precursor obtained is prepared into manganese-doped sodium iron pyrophosphate cathode material in the same way as in Example 1.

[0046] The precursors prepared in the examples and comparative examples were subjected to principal component and impurity analysis. The contents of Na, Mn and metal impurity ions were detected by ICP; the Fe content was detected by potassium dichromate titration; the P content was detected by the quinoline molybdenum gravimetric method; BET is the specific surface area, which was detected by the BET specific surface area detection method; and the particle size was detected by Malvern particle size analyzer. The results are shown in Table 1.

[0047] Table 1. Detection results of precursors prepared in the examples and comparative examples.

[0048] The manganese-doped sodium iron pyrophosphate cathode materials prepared in the examples and comparative examples were electrochemically characterized using a blue electric test system to measure charging voltage, 0.2C discharge capacity, 0.5C discharge capacity, and capacity retention after 100 cycles at 1C. ICP was used to detect manganese dissolution after 100 cycles at 1C. The results are shown in Table 2.

[0049] Table 2. Electrochemical performance results of manganese-doped sodium iron pyrophosphate cathode materials prepared in the examples and comparative examples. Example 1 3.3 120.1 117.3 97.3 35 Example 2 3.3 120.2 116.9 97.5 32 Example 3 3.5 125.6 121.5 96.8 42 Example 4 3.3 120.2 117.1 96.2 34 Comparative Example 1 3.1 115.45 108.9 88.5 152 Comparative Example 2 3.2 118.32 112.5 90.8 81 Comparative Example 3 3.2 119.04 112.4 91.0 79 Comparative Example 4 3.1 116.03 109.3 89.1 123

[0050] The morphology of the manganese-doped sodium iron pyrophosphate precursor prepared in Example 1 was observed by scanning electron microscopy (SEM), and the results are as follows: Figure 1As shown in the figure, electron microscopy reveals that the precursor particles have a diameter between 100 and 200 nm, exhibiting a thick, lamellar structure that aggregates into relatively loose secondary aggregates through stacking. When using manganese-doped sodium iron pyrophosphate precursor to prepare NFPP, the milling performance is excellent, easily achieving the target particle size control range quickly through milling. The resulting NFPP exhibits high sintering activity, and compared to traditional methods, the sintering temperature is reduced by 20-30 °C to achieve the same compaction density, demonstrating advantages in electrical properties.

[0051] Comparing Examples 1 and 2, it can be found that within the scope defined by this invention, changing different phosphoric acid sources does not significantly affect the synthesized precursor and NFPP material. Comparing Examples 1 and 3, it can be found that increasing the manganese content is beneficial to improving the charging voltage and discharging capacity of the NFPP material. Comparing Examples 1 and 4, it can be found that higher reaction temperature and increased feed flow rate lead to larger precursor particle size and BET, increased filter cake washing difficulty, and higher impurities. This is because the feed rate in Example 4 is fast, and the precursor particles generated in the early stage of the reaction are smaller, resulting in a larger BET. At the same time, subsequent agglomeration is severe, so the measured agglomerate particle size is large. In addition, the NFPP material prepared in this example has a slightly lower 0.5C discharge capacity and 1C 100-cycle capacity retention rate, but it still maintains a higher electrochemical performance than the comparative example and is still within an acceptable range.

[0052] Comparing Example 1 and Comparative Example 4, it was found that if the phosphate salt solution was added dropwise to the iron-manganese solution, the oxidant in the phosphate salt solution would oxidize the iron-manganese elements in advance, resulting in a higher concentration of trivalent manganese and iron ions in the initial stage of the reaction. This caused manganese and iron to segregate in the synthesized precursor, resulting in uneven mixing. Consequently, the charging voltage, discharge capacity, 0.5C discharge capacity, and 1C 100-cycle capacity retention of the final NFPP were significantly reduced, while manganese dissolution was significantly increased.

[0053] Comparing Example 1 with Comparative Example 1, it can be observed that the insolubility of manganese trioxide produces the same negative effects as in Comparative Example 4. Comparing Example 1 with Comparative Example 2, it can be observed that if iron and manganese are oxidized first, the levels of iron, manganese, phosphate, and pyrophosphate in the reaction system continuously decrease, while the pH continuously increases. The precipitation reaction process is constantly changing, resulting in poor uniformity and consistency of the synthesized precursor. Comparing Example 1 with Comparative Example 3, it can be observed that if ferric iron is used directly, the ferric solution is prone to hydrolysis and precipitation, leading to partial segregation of the iron salt precursor, which also correspondingly deteriorates the electrical properties of NFPP.

Claims

1. A method for preparing a manganese-doped sodium iron pyrophosphate precursor, characterized in that, Includes the following steps: (1) Dissolve the phosphoric acid source, pyrophosphate source, sodium source and oxidant in water to prepare a clear phosphate salt solution; (2) Dissolve the iron and manganese sources in water to prepare a clear iron-manganese solution; (3) The phosphate salt solution and the iron-manganese solution are added dropwise in parallel to the feed to react. After the reaction is completed, the filter cake is collected by filtration. The filter cake is washed and dried to obtain the manganese-doped sodium iron pyrophosphate precursor.

2. The preparation method according to claim 1, characterized in that, In step (1), the phosphoric acid source is phosphoric acid, ammonium phosphate, sodium phosphate and / or their respective hydrates.

3. The preparation method according to claim 1, characterized in that, In step (1), the pyrophosphate source is sodium pyrophosphate and / or its hydrate.

4. The preparation method according to claim 1, characterized in that, In step (1), the oxidant is one of sodium persulfate, ammonium persulfate, or hydrogen peroxide.

5. The preparation method according to claim 1, characterized in that, In step (2), the iron source is a water-soluble divalent iron salt and / or its hydrate; According to the preparation method of claim 1, the manganese source in step (2) is a water-soluble divalent manganese salt and / or its hydrate.

6. The preparation method according to claim 1, characterized in that, In steps (1) and (2), the amounts of the phosphate source, pyrophosphate source, sodium source, iron source, and manganese source are such that the molar ratio of phosphate, pyrophosphate, sodium ion, iron ion, and manganese ion in the product system is (1.6-1.7):(0.5-1.2):(1.6-1.7):(0.5-0.7):(0.3-0.7).

7. The preparation method according to claim 1, characterized in that, In steps (1) and (2), the preparation of the phosphate salt solution and the iron-manganese solution is carried out at 40-60℃.

8. The preparation method according to claim 1, characterized in that, In step (3), the reaction temperature is 40-60℃ and the reaction time is 0.5-3h.

9. The preparation method according to claim 1, characterized in that, In step (3), the feed rates of the phosphate salt solution and the iron-manganese solution in the parallel-flow addition are both 500-1500 g / h and the flow rates are equal.

10. A manganese-doped sodium iron pyrophosphate, characterized in that, It is prepared from a precursor obtained by any one of claims 1-9.

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