Sodium ferric phosphate pyrophosphate-carbon composite material, battery containing same and preparation method of sodium ferric phosphate pyrophosphate-carbon composite material

Abstract

The invention discloses a sodium ferric phosphate pyrophosphate-carbon composite material, a battery containing the composite material and a preparation method of the composite material. The preparation method comprises the following steps: sintering a mixture containing an iron source, a sodium source, a carbon source, a phosphorus source and a solvent, wherein the iron source comprises FePO4. 2H2O, the FePO4. 2H2O has a crystal phase, and in the crystal phase, the mass ratio of a monoclinic phase is not less than 95%. The ferric sodium pyrophosphate-carbon composite material comprises an inner core and a carbon coating layer coating the surface of the inner core, wherein the phase composition of the inner core comprises ferric sodium pyrophosphate, the chemical formula of the inner core is as follows: x is equal to 3-5; the phase purity of the ferric sodium pyrophosphate in the phase composition is not lower than 95%; and the sodium-iron antisite defect of the ferric sodium phosphate pyrophosphate is not higher than 1%. After the ferric sodium pyrophosphate-carbon composite material is actually applied to a sodium ion battery, the specific capacity, the cycle performance and the first effect of the sodium ion battery can be effectively improved at the same time.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode materials, specifically relating to a sodium iron pyrophosphate-carbon composite material, a battery containing the same, and a method for preparing the same. Background Technology

[0002] Sodium-ion batteries are considered an ideal choice for energy storage materials due to their similar working principle to lithium-ion batteries and the abundance and low cost of sodium resources. Existing research mainly includes transition metal oxides, Prussian blue analogues, and polyanionic compounds. Polyanionic compounds possess ultra-long cycle durability and excellent rate performance, showing significant advantages in energy storage applications. Among these, sodium iron phosphate pyrophosphate is a prime example. , P2O7 (denoted as NFPP) has a high theoretical specific capacity and strong electronegativity. 4- This results in an average voltage of 3.1V for the material, demonstrating a high energy density, and thus it is considered a promising cathode material for sodium-ion batteries.

[0003] However, the actual specific capacity of NFPP (approximately 100 mAh g) -1 The specific capacity of NFPP is often lower than its theoretical capacity. This capacity limitation is mainly due to the formation of impurity phases NaFePO4 and Na2FeP2O7 during the NFPP production process. Phase purity issues severely affect the reversible specific capacity and cycle stability of this material. To address this problem, Professor Cao Yuliang's research group, by designing an iron-phosphorus ratio and introducing an iron defect regulation strategy (Nano Energy 2022, 91, 106680), prepared a cathode material with excellent electrochemical performance. However, an imbalance in the Na-Fe ratio during synthesis can easily lead to Fe-Na exchange and the resulting irreversible structural changes. Furthermore, the occupation of Na sites by transition metals can further affect voltage hysteresis and structural degradation during cycling, ultimately resulting in capacity decay. Patent application CN119601618A discloses a dual-site doped sodium iron pyrophosphate / carbon composite cathode material and its preparation method, introducing tungsten to occupy Fe and PO4 sites. While this reduces sodium-iron antisite defects, the tungsten occupation of Fe sites inevitably reduces the Fe redox number, ultimately leading to a decrease in specific capacity and cycle performance. Moreover, under this method, the initial discharge capacity of the resulting battery is very likely to exceed the charging capacity, meaning the initial efficiency is very likely to exceed 100%, which will further affect its specific capacity and cycle performance.

[0004] Therefore, there is an urgent need for a process to prepare sodium iron pyrophosphate to effectively improve phase purity and sodium-iron antisite defects, thereby improving the specific capacity, cycle performance and first-time efficiency of the resulting sodium-ion battery. Summary of the Invention

[0005] To address the limitations of existing sodium iron pyrophosphate (NFPP) preparation processes in simultaneously improving phase purity and reducing sodium-iron antisite defects, this invention provides a sodium iron pyrophosphate-carbon composite material, a battery containing the composite material, and a method for preparing the same. This sodium iron pyrophosphate-carbon composite material exhibits high NFPP phase purity and low sodium-iron antisite defects. When applied to sodium-ion batteries, it can effectively improve their specific capacity, cycle performance, and initial efficiency.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows.

[0007] This invention provides a method for preparing sodium iron pyrophosphate (NFPP)-carbon composite material, which includes the following steps:

[0008] The sodium iron pyrophosphate-carbon composite material is obtained by sintering a mixture containing an iron source, a sodium source, a carbon source, a phosphorus source and a solvent.

[0009] The iron source includes FePO4·2H2O, which has a crystalline phase, and the monoclinic phase accounts for no less than 95% of the mass of the crystalline phase.

[0010] Based on the aforementioned technical deficiencies in existing sodium iron pyrophosphate (NFPP)-carbon composite materials, the applicant unexpectedly discovered through research that by improving the type of iron source and crystal phase structure, especially by selecting monoclinic FePO4·2H2O, the formation of impurity phases such as NaFePO4 and Na2FeP2O7 during NFPP production can be effectively suppressed, thereby increasing the formation barrier and ultimately improving the performance of the resulting composite material and battery. However, in existing technologies, the configurational stability of orthorhombic FePO4·2H2O is often higher than that of monoclinic FePO4·2H2O. That is, orthorhombic FePO4·2H2O is generally considered to be in a stable state, while monoclinic FePO4·2H2O is considered to be in a metastable state. Therefore, there are almost no known technologies using monoclinic FePO4·2H2O for the synthesis of NFPP.

[0011] In some embodiments, the monoclinic phase accounts for 95%-100% of the mass of the crystal phase, for example, 95.54% or 98.98%.

[0012] In some implementations, the PDF card number for the monoclinic phase is 33-0667. The PDF card is sourced from the ICDD PDF-4 database.

[0013] In some embodiments, the crystalline phase also includes an orthorhombic phase.

[0014] The orthorhombic phase preferably accounts for 0-5% of the mass of the crystal phase, for example, 4.46% or 1.02%.

[0015] Preferably, the PDF card number for the orthogonal phase is 33-0666. The PDF card is sourced from the ICDD PDF-4 database.

[0016] In some implementations, the FePO4·2H2O may be a commercially available product or a homemade product.

[0017] In some preferred embodiments, the preparation of FePO4·2H2O includes the following steps:

[0018] S1, react with the first mixed solution containing iron powder and phosphoric acid, filter and collect the filtrate;

[0019] S2 is prepared by reacting a second mixed solution containing the filtrate, oxidant, and water.

[0020] In step S1, the amount of phosphoric acid used is such that the iron powder reacts fully to obtain Fe. 2+ Those skilled in the art can choose according to their needs; for example, when the iron powder mass is 50.0-60.0g, the amount of phosphoric acid used is 1L and the mass concentration is 25%.

[0021] In step S1, the reaction is preferably carried out under stirring conditions.

[0022] In step S2, the oxidant is preferably hydrogen peroxide.

[0023] In step S2, the amount of oxidant used is preferably such that the Fe in the filtrate is reduced. 2+ Fe reacts completely 3+ Those skilled in the art can choose according to their needs; for example, when the iron powder mass is 50.0-60.0g, the amount of oxidant used is 250mL and the volume concentration is 9%.

[0024] In step S2, the preparation of the second mixed solution preferably includes: first mixing the filtrate and water, and then adding the oxidant; preferably, the oxidant is added after the mixture is heated to 50°C; preferably, the oxidant is added dropwise.

[0025] In step S2, the reaction temperature is preferably 90-100℃, for example 95℃; and the reaction time is preferably 2-4h, for example 3h.

[0026] In some implementations, the iron source comprises primary particles and / or secondary particles.

[0027] Preferably, the primary particles are in the form of nanosheets.

[0028] Preferably, the secondary particles are composed of agglomerates of multiple primary particle nanosheets.

[0029] The average particle size of the secondary particles is preferably 5-50 μm, and more preferably 5-30 μm.

[0030] In some embodiments, the molar ratio of iron to phosphorus in the iron source is 0.90-1.05, for example, 0.978:1, 0.99:1, 1:1, or 1.03:1. Here, "the molar ratio of iron to phosphorus is 0.90-1.05" may refer to the actual operation where the iron source used is a mixture containing FePO4·2H2O and other unavoidable impurities. These impurities may be due to factors such as raw material purity, reaction conditions, and washing efficiency; ultimately resulting in the molar ratio of iron to phosphorus in the iron source not being the theoretical 1:1. Those skilled in the art will understand its specific meaning.

[0031] In some embodiments, the sodium source is one or more of sodium carbonate, sodium hydroxide, sodium pyrophosphate, disodium dihydrogen pyrophosphate, sodium oxalate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium chloride, and sodium acetate.

[0032] In some embodiments, the phosphorus source is one or more of sodium dihydrogen phosphate, sodium pyrophosphate, disodium hydrogen phosphate, disodium dihydrogen pyrophosphate, and sodium phosphate.

[0033] In some embodiments, the carbon source is one or more of glucose, sucrose, starch, citric acid, ascorbic acid, oxalic acid, and polyvinyl alcohol.

[0034] In some embodiments, the molar ratio of sodium, iron, and phosphorus in the mixture is (2-5):(1-4):(2-5), for example, 4:3:4, 5:4:5, or 3:2:3. The amount of sodium, iron, and phosphorus source can be controlled by adjusting the specific molar ratio of these elements; the specific meaning of this control is understood by those skilled in the art.

[0035] In some embodiments, the mass of the carbon source is 1%-20% of the theoretical mass of sodium ferric pyrophosphate, preferably 10%-15%, for example 12%, based on the theoretical mass of sodium ferric pyrophosphate generated from the iron, sodium, and phosphorus sources during sintering. The theoretical mass of sodium ferric pyrophosphate is calculated based on the absence of impurities (such as NaFePO4 and Na2FeP2O7), i.e., calculated assuming the product is entirely sodium ferric pyrophosphate.

[0036] In some embodiments, the solvent is one or more of ethanol, acetone, propanol, and deionized water.

[0037] In some embodiments, the preparation of the mixture includes mixing the iron source, sodium source, carbon source, phosphorus source and solvent.

[0038] The mixing is preferably carried out by ball milling and / or sand milling.

[0039] The milling speed is, for example, 1000 rpm, and the milling time is, for example, 5 hours.

[0040] The ball milling is carried out, for example, in a ball mill, at a rotational speed of, for example, 300 rpm, and for a duration of, for example, 12 hours.

[0041] In some implementations, the mixing process further includes a drying step.

[0042] The drying process is preferably vacuum drying.

[0043] The drying temperature is preferably 60-80℃.

[0044] The drying time is preferably 10-16 hours, for example 12 hours.

[0045] In some implementations, the sintering includes primary sintering and secondary sintering.

[0046] The first sintering is preferably carried out in an inert atmosphere, such as argon or a hydrogen-argon mixture.

[0047] The temperature of the first sintering is preferably 250-350℃, for example 300℃.

[0048] The preferred sintering time is 3-6 hours.

[0049] The heating rate to the primary sintering temperature is preferably 3-10 °C / min, for example 5 °C / min.

[0050] The secondary sintering is preferably carried out in an inert atmosphere, such as argon or a hydrogen-argon mixture.

[0051] The secondary sintering temperature is preferably 450-550℃, for example 500℃ or 520℃.

[0052] The secondary sintering time is preferably 10-12 hours.

[0053] The heating rate from the primary sintering temperature to the secondary sintering temperature is preferably 3-10℃ / min, for example 5℃ / min.

[0054] In some embodiments, the secondary sintering process further includes a cooling step to 25°C-100°C.

[0055] The present invention also provides a sodium iron pyrophosphate-carbon composite material, comprising a core and a carbon coating layer covering the surface of the core; wherein,

[0056] The core phase composition includes sodium iron pyrophosphate, the chemical formula of which is: x=3-5; the phase purity of the sodium ferric pyrophosphate in the phase composition is not less than 95%;

[0057] The sodium iron pyrophosphate sodium has a sodium iron antisite defect of no more than 1%.

[0058] In this invention, the phase composition of the core may include, in addition to the sodium ferric pyrophosphate, an impurity phase, such as NaFePO4 and / or Na2FeP2O7. The phase purity may refer to the mass percentage of the sodium ferric pyrophosphate in the phase composition.

[0059] In some embodiments, the chemical formula of the sodium ferric pyrophosphate is: , or .

[0060] In some embodiments, the mass ratio of the core to the carbon coating layer in the sodium iron pyrophosphate-carbon composite material is 95:5-99:1.

[0061] In some embodiments, the sodium ferric pyrophosphate in the phase composition has a phase purity of 95%-100%, for example 96.01% or 99.02%.

[0062] In some embodiments, the sodium iron pyrophosphate sodium-carbon composite material has a sodium iron antisite defect of 0.85%-1%, for example 0.88%, 0.91% or 0.98%.

[0063] The present invention also provides a sodium iron pyrophosphate-carbon composite material, which is prepared by the method described above for preparing sodium iron pyrophosphate-carbon composite materials.

[0064] In some embodiments, the sodium iron pyrophosphate-carbon composite material is as previously defined.

[0065] The present invention also provides a sodium-ion battery comprising the sodium iron pyrophosphate-carbon composite material as described above.

[0066] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0067] The reagents and raw materials used in this invention are all commercially available.

[0068] The positive and progressive effects of this invention are as follows:

[0069] The preparation method of this invention effectively improves the NFPP phase purity (not less than 95%) and sodium-iron antisite defects (not more than 1%) of the obtained sodium iron pyrophosphate-carbon composite material by using high-purity monoclinic FePO4·2H2O, thereby improving the electronic conductivity and sodium ion diffusion rate of the obtained sodium-ion battery. Ultimately, it improves the capacity, rate performance, and cycle performance while maintaining high initial efficiency. The entire preparation process is short, simple, feasible, environmentally friendly, and highly reproducible. Attached Figure Description

[0070] Figure 1 This is a SEM image of FePO4·2H2O obtained in Example 1.

[0071] Figure 2 The X-ray diffraction (XRD) patterns of FePO4·2H2O obtained in Examples 1, 2, and 1 are shown.

[0072] Figure 3 The images show the X-ray diffraction (XRD) patterns of the sodium iron pyrophosphate-carbon composite materials obtained in Examples 1, 2, and 1.

[0073] Figure 4 The first charge-discharge curves at a current density of 0.1C are shown for the sodium iron pyrophosphate-carbon composite materials obtained in Examples 1, 2 and Comparative Example 1 after being used in sodium-ion batteries.

[0074] Figure 5 The data is shown in the rate curves of sodium iron pyrophosphate-carbon composite materials obtained in Example 1 and Comparative Example 1 after being used in sodium-ion batteries.

[0075] Figure 6 The graph shows the cycling performance of the sodium iron pyrophosphate-carbon composite material obtained in Example 1 at a current density of 5C after being used in a sodium-ion battery.

[0076] Figure 7 The graph shows the cycling performance of the sodium iron pyrophosphate-carbon composite material obtained in Example 1 at a current density of 1C after being used in a sodium-ion battery. Detailed Implementation

[0077] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0078] In the following examples and comparative examples, FePO4·2H2O was prepared in-house according to the present invention. Due to factors such as the purity of raw materials and reaction conditions, the molar ratio of iron to phosphorus in the final product is not the theoretical value of 1:1.

[0079] Among them, the PDF card number of the monoclinic phase in FePO4·2H2O is 33-0667, and the PDF card number of the orthorhombic phase is 33-0666. The DF card is from the ICDD·PDF-4 database.

[0080] In the following examples and comparative examples, the obtained Na4Fe3(PO4)2P2O7, Na5Fe4(PO4)3P2O7, and Na3Fe2(PO4)P2O7 can all be referred to as NFPP.

[0081] Example 1

[0082] (1) Weigh out pure monoclinic FePO4·2H2O (monoclinic phase ratio is 100%, Fe / P=0.978), NaH2PO4 and Na2CO3, so that the molar ratio of Na:Fe:P is 4:3:4, and then add 10% glucose as a carbon coating agent based on the theoretical product mass fraction. Use deionized water as solvent and mill all the above solid components together at a speed of 1000 rpm for 5 h. After milling, the mixture is dried at 60 ℃ for 12 h.

[0083] (2) The sand-milled product was sintered at 300℃ for 6 h and 500℃ for 10 h in an argon atmosphere. The heating and cooling rate was 5℃ / min. The product was then naturally cooled to room temperature (25℃) to obtain carbon-coated Na4Fe3(PO4)2P2O7 composite material.

[0084] The specific preparation method of FePO4·2H2O used in this embodiment is as follows:

[0085] Place 55.85g of iron powder into a beaker. Add 1L of 25% (w / v) dilute phosphoric acid to the beaker. Stir with a glass rod; this process is exothermic and produces hydrogen gas. After stirring for 3 hours, observe that the iron powder at the bottom of the beaker no longer reacts, indicating the reaction is complete. Filter the solution using a vacuum filtration flask and collect the filtrate for the next reaction. Pour the filtrate into a three-necked flask and add 537mL of deionized water. Heat to 50℃ and add 250mL of 9% (v / v) hydrogen peroxide dropwise over 60 minutes. Heat to 95℃ and maintain the temperature for 3 hours. After the reaction is complete, unload the mixture.

[0086] Example 2

[0087] (1) Weigh FePO4·2H2O (monoclinic phase content 95.54%, Fe / P=0.990), NaH2PO4 and Na2CO3, so that the molar ratio of Na:Fe:P = 4:3:4, and then add 10% of the theoretical product mass fraction of glucose as a carbon coating agent. Use deionized water as solvent and grind all the above solid components together at a speed of 1000 rpm for 5 h. After grinding, the mixture is dried at 80 ℃ for 12 h.

[0088] (2) The sand-milled product was sintered at 250℃ for 6 h and 550℃ for 12 h in an argon atmosphere. The heating and cooling rate was 5℃ / min. The product was then naturally cooled to room temperature (25℃) to obtain carbon-coated Na4Fe3(PO4)2P2O7 composite material.

[0089] The specific preparation method of FePO4·2H2O used in this embodiment is as follows:

[0090] Place 55.85g of iron powder into a beaker. Add 1L of 25% (w / v) dilute phosphoric acid to the beaker. Stir with a glass rod; this process is exothermic and produces hydrogen gas. After stirring for 3 hours, observe that the iron powder at the bottom of the beaker no longer reacts, indicating the reaction is complete. Filter the solution using a vacuum filtration flask and collect the filtrate for the next reaction. Pour the filtrate into a three-necked flask and add 537mL of deionized water. Heat to 60℃ and add 250mL of 9% (v / v) hydrogen peroxide dropwise over 60 minutes. Heat to 95℃ and maintain the temperature for 3 hours. Once the reaction is complete, unload the mixture.

[0091] Example 3

[0092] (1) Weigh FePO4·2H2O (monoclinic phase percentage 100%, Fe / P=1.00), Na2HPO4 and NaOH, so that the molar ratio of Na:Fe:P = 5:4:5, and then add 12% glucose as a carbon coating agent based on the theoretical product mass fraction. Use anhydrous ethanol as solvent and add it together with all the above solid components into a ball mill jar, and add an appropriate amount of zirconium beads. Place the ball mill jar symmetrically in a planetary ball mill and ball mill at 300 rpm for 12 h. After separation, vacuum dry the obtained solid at 60 ℃ for 12 h.

[0093] (2) The ball-milled product was sintered at 300℃ for 6 h and 520℃ for 12 h in an argon atmosphere. The heating and cooling rate was 5℃ / min. The product was then naturally cooled to room temperature (25℃) to obtain carbon-coated sodium iron pyrophosphate Na5Fe4(PO4)3P2O7 composite material.

[0094] The specific preparation method of FePO4·2H2O used in this embodiment is as follows:

[0095] Place 58.80g of iron powder into a beaker. Add 1L of 25% (w / v) dilute phosphoric acid to the beaker. Stir with a glass rod; this process is exothermic and produces hydrogen gas. After stirring for 3 hours, observe that the iron powder at the bottom of the beaker no longer reacts, indicating the reaction is complete. Filter the solution using a vacuum filtration flask and collect the filtrate for the next reaction. Pour the filtrate into a three-necked flask and add 537mL of deionized water. Heat to 50℃ and add 250mL of 9% hydrogen peroxide (volume concentration) dropwise over 60 minutes. Heat to 95℃ and maintain the temperature for 3 hours. Once the reaction is complete, unload the mixture.

[0096] Example 4

[0097] (1) Weigh FePO4·2H2O (monoclinic phase content 98.98%, Fe / P=1.03), Na2HPO4 and Na2CO3, so that the molar ratio of Na:Fe:P is 3:2:3, and then add 10% of the theoretical product mass fraction of glucose as a carbon coating agent. Use deionized water as solvent and mill all the above solid components together at a speed of 1000 rpm for 5 h. After milling, the mixture is dried at 60 ℃ for 12 h.

[0098] (2) The sand-milled product was sintered at 300℃ for 6 h and 500℃ for 10 h in an argon atmosphere. The heating and cooling rate was 5℃ / min. The product was then naturally cooled to room temperature (25℃) to obtain carbon-coated Na3Fe2PO4P2O7 composite material.

[0099] The specific preparation method of FePO4·2H2O used in this embodiment is as follows:

[0100] Place 55.85g of iron powder into a beaker. Add 1L of 25% (w / v) dilute phosphoric acid to the beaker. Stir with a glass rod; this process is exothermic and produces hydrogen gas. After stirring for 3 hours, observe that the iron powder at the bottom of the beaker no longer reacts, indicating the reaction is complete. Filter the solution using a vacuum filtration flask and collect the filtrate for the next reaction. Pour the filtrate into a three-necked flask and add 537mL of deionized water. Heat to 50℃ and add 250mL of 9% hydrogen peroxide (volume concentration) dropwise over 30 minutes. Heat to 95℃ and maintain the temperature for 3 hours. After the reaction is complete, unload the mixture.

[0101] Comparative Example 1

[0102] (1) Weigh FePO4·2H2O (monoclinic phase content 68.96%, Fe / P=0.978), Na2HPO4 and Na2CO3, so that the molar ratio of Na:Fe:P is 4:3:4, and then add 10% of the theoretical product mass fraction of glucose as a carbon coating agent. Use deionized water as solvent and grind all the above solid components together at a speed of 1000 rpm for 5 h. After grinding, the mixture is dried at 80 ℃ for 12 h.

[0103] (2) The sand-milled product was sintered at 350℃ for 6 h and 500℃ for 12 h in an argon atmosphere. The heating and cooling rate was 5℃ / min. The product was then naturally cooled to room temperature (25℃) to obtain carbon-coated Na4Fe3(PO4)2P2O7 composite material.

[0104] The specific preparation method of FePO4·2H2O used in this comparative example is as follows:

[0105] Place 55.85g of iron powder into a beaker. Add 1L of 25% (w / v) dilute phosphoric acid to the beaker. Stir with a glass rod; this process is exothermic and produces hydrogen gas. After stirring for 3 hours, observe that the iron powder at the bottom of the beaker no longer reacts, indicating the reaction is complete. Filter the solution using a vacuum filtration flask and collect the filtrate for the next reaction. Pour the filtrate into a three-necked flask and add 537mL of deionized water. Heat to 80℃ and add 250mL of 9% hydrogen peroxide (volume concentration) dropwise over 60 minutes. Heat to 95℃ and maintain the temperature for 3 hours. Once the reaction is complete, unload the mixture.

[0106] Comparative Example 2

[0107] (1) Weigh FePO4·2H2O (monoclinic phase content 70.63%, Fe / P=0.969), Na2HPO4 and NaOH, so that the molar ratio of Na:Fe:P = 5:4:5, and then add 10% of the theoretical product mass fraction of glucose as a carbon coating agent. Use anhydrous ethanol as solvent and add it together with all the above solid components into a ball mill jar, and add an appropriate amount of zirconium beads. Place the ball mill jar symmetrically in a planetary ball mill and ball mill at 300 rpm for 12 h. After separation, vacuum dry the obtained solid at 60 ℃ for 12 h.

[0108] (2) The ball-milled product was sintered at 300℃ for 6 h and 520℃ for 12 h in an argon atmosphere. The heating and cooling rate was 5℃ / min. The product was then naturally cooled to room temperature (25℃) to obtain carbon-coated Na5Fe4(PO4)3P2O7 composite material.

[0109] The specific preparation method of FePO4·2H2O used in this embodiment is as follows:

[0110] Place 53.55g of iron powder into a beaker. Add 1L of 25% (w / v) dilute phosphoric acid to the beaker. Stir with a glass rod; this process is exothermic and produces hydrogen gas. After stirring for 3 hours, observe that the iron powder at the bottom of the beaker no longer reacts, indicating the reaction is complete. Filter the solution using a vacuum filtration flask and collect the filtrate for the next reaction. Pour the filtrate into a three-necked flask and add 537mL of deionized water. Heat to 70℃ and add 250mL of 9% hydrogen peroxide (volume concentration) dropwise over 60 minutes. Heat to 95℃ and maintain the temperature for 3 hours. Once the reaction is complete, unload the mixture.

[0111] Effect Example

[0112] The carbon-coated iron-sodium pyrophosphate composite materials obtained in Examples 1-4 and Comparative Examples 1-2, and the iron source FePO4·2H2O used, were characterized as follows:

[0113] 1. Microscopic morphology characterization

[0114] The characterization was performed using scanning electron microscopy.

[0115] 2. Characterization of phase purity and phase proportion

[0116] The tests were performed using X-ray diffraction (XRD). Specifically, the crystal structure of the sample was characterized using an X-ray powder diffractometer (Bruker D8); Cu Kα was used as the X-ray source; and the scan rate was 5° min. -1 The corresponding lattice parameters were refined and fitted to the XRD pattern using GSAS software.

[0117] 3. Characterization of Na / Fe antisite defects

[0118] The test was performed using an X-ray diffractometer (XRD).

[0119] 4. Electrochemical performance characterization

[0120] (1) Preparation of electrode sheets:

[0121] Sodium iron pyrophosphate-carbon composite material was used as the positive electrode active material, and it was weighed with a conductive agent and a binder at a mass ratio of 8:1:1. First, 240 mg of the positive electrode active material and 30 mg of the conductive agent Super-P were weighed and mixed in a mortar for 10 min until homogeneous. Then, 600 mg of a 5% PVDF binder solution was weighed into a 5 mL beaker. The mixed powder was added to the beaker, along with 3 drops of NMP. The beaker was sealed with plastic wrap and sealing film, and stirred on a magnetic stir bar for 4 h to ensure thorough mixing of the components. The slurry was coated onto aluminum foil and vacuum dried at 120 ℃ for 12 h. After drying, it was cut into circular electrode sheets to obtain the positive electrode sheet for sodium-ion batteries used in coin cells.

[0122] (2) Battery fabrication:

[0123] Using a sodium sheet as the negative electrode and a 1M NaPF6 EC-DEC@5% FEC (1:1, V / V) electrolyte, a CR2032 coin cell was assembled by placing the positive electrode, separator, sodium sheet, gasket, and spring in that order. The encapsulated cell was left to stand for 24 hours to allow the electrodes to be fully wetted by the electrolyte, in preparation for subsequent electrochemical performance testing.

[0124] (3) Performance testing:

[0125] Within a voltage range of 2-4V, tests were conducted at a test temperature of 25℃, and the following were measured: ① discharge specific capacity at a current density of 0.1C; ② first-time efficiency at a current density of 0.1C; ③ discharge specific capacity at a current density of 5C; ④ discharge specific capacity at a current density of 10C; ⑤ capacity retention rate after 500 cycles at a current density of 1C (in terms of discharge specific capacity); ⑥ capacity retention rate after 500 cycles at a current density of 5C (in terms of discharge specific capacity).

[0126] The cathode material has a nominal specific capacity of 129 mAh g at a 1C current density. -1 The electrode loading is approximately 2.0 mg / cm³. -2 .

[0127] The results are shown in Table 1 and , respectively. Figures 1-7 As shown.

[0128] Table 1

[0129]

[0130] In the table, the meaning of NFPP phase purity is: the obtained sodium iron pyrophosphate-carbon composite material contains a core and a carbon coating layer covering the surface of the core. In addition to NFPP, the phase composition of the core may also include impurity phases, such as NaFePO4 and / or Na2FeP2O7. The phase purity refers to the mass percentage of NFPP in the phase composition of the core.

[0131] like Figure 1 As shown, the FePO4·2H2O obtained in Example 1 exhibits distinct primary and secondary particle structures; the primary particles are in the form of nanosheets, while the secondary particles are composed of aggregates of multiple primary particle nanosheets. The average particle size of the secondary particles was measured to be approximately 30 μm.

[0132] like Figure 2 As shown, the monoclinic phase ratio of FePO4·2H2O obtained in Examples 1 and 2 is 100% and 95.54%, respectively, while the monoclinic phase ratio of FePO4·2H2O obtained in Comparative Example 1 is only 68.96%.

[0133] like Figures 3-7 As shown in Table 1, the sodium iron pyrophosphate-carbon composite materials obtained in the examples can simultaneously ensure an NFPP phase purity of over 95% and a Na / Fe antisite defect percentage of no more than 1% in NFPP. Furthermore, when used in sodium-ion batteries, they can simultaneously ensure a 0.1C discharge specific capacity of no less than 10³ mAh g⁻¹. -1 5C discharge specific capacity not less than 90 mAh g -1 10C discharge specific capacity not less than 89 mAh g -1 The capacity retention rate after 500 cycles at 1C is not less than 99.8%, and the capacity retention rate after 500 cycles at 5C is not less than 97%.

[0134] In contrast to Examples 1 and 2, where the monoclinic phase ratio of FePO4·2H2O was significantly lower, the purity of the NFPP phase, the Na / Fe antisite defects in the NFPP, and the specific capacity and cycle stability of the resulting sodium-ion battery all deteriorated significantly.

Claims

1. A method for preparing a sodium iron pyrophosphate-carbon composite material, characterized in that, It includes the following steps: The sodium iron pyrophosphate-carbon composite material is obtained by sintering a mixture containing an iron source, a sodium source, a carbon source, a phosphorus source and a solvent. The iron source includes FePO4·2H2O, which has a crystalline phase, and the monoclinic phase accounts for no less than 95% of the mass of the crystalline phase.

2. The preparation method of the sodium iron pyrophosphate-carbon composite material as described in claim 1, characterized in that, The monoclinic phase accounts for 95%-100% of the mass of the crystal phase, for example, 95.54% or 98.98%. And / or, the PDF card number of the monoclinic phase is 33-0667; And / or, the crystalline phase further includes an orthorhombic phase; The orthorhombic phase preferably accounts for 0-5% of the mass of the crystal phase, for example, 4.46% or 1.02%. The PDF card number of the orthogonal phase is preferably 33-0666.

3. The preparation method of the sodium iron pyrophosphate-carbon composite material as described in claim 1, characterized in that, The preparation of FePO4·2H2O includes the following steps: S1, react with the first mixed solution containing iron powder and phosphoric acid, filter and collect the filtrate; S2, reacting with a second mixed solution containing the filtrate, oxidant, and water to obtain the product; In step S1, the reaction is preferably carried out under stirring conditions; In step S2, the oxidant is preferably hydrogen peroxide; In step S2, the preparation of the second mixed solution preferably includes: first mixing the filtrate and water, and then adding the oxidant; preferably, the oxidant is added after the mixture is heated to 50°C; preferably, the oxidant is added dropwise. In step S2, the reaction temperature is preferably 90-100℃, for example 95℃; the reaction time is preferably 2-4h, for example 3h. And / or, the iron source includes primary particles and / or secondary particles; Preferably, the primary particles are in the form of nanosheets; Preferably, the secondary particles are composed of agglomerations of multiple primary particle nanosheets; The average particle size of the secondary particles is preferably 5-50 μm, and more preferably 5-30 μm; And / or, in the iron source, the molar ratio of iron to phosphorus is (0.90-1.05):1, for example 0.978:1, 0.99:1, 1:1 or 1.03:

1.

4. The preparation method of the sodium iron pyrophosphate-carbon composite material as described in claim 1, characterized in that, The sodium source is one or more of sodium carbonate, sodium hydroxide, sodium pyrophosphate, disodium dihydrogen pyrophosphate, sodium oxalate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium chloride, and sodium acetate. And / or, the phosphorus source is one or more of sodium dihydrogen phosphate, sodium pyrophosphate, disodium hydrogen phosphate, disodium dihydrogen pyrophosphate, and sodium phosphate; And / or, the carbon source is one or more of glucose, sucrose, starch, citric acid, ascorbic acid, oxalic acid, and polyvinyl alcohol; And / or, in the mixture, the molar ratio of sodium, iron and phosphorus is (2-5):(1-4):(2-5), for example 4:3:4, 5:4:5 or 3:2:3; And / or, based on the theoretical mass of sodium iron pyrophosphate generated by the iron source, sodium source and phosphorus source in the sintering, the mass of the carbon source is 1%-20% of the theoretical mass of the sodium iron pyrophosphate, preferably 10%-15%, for example 12%; And / or, the solvent is one or more of ethanol, acetone, propanol and deionized water.

5. The method for preparing the sodium iron pyrophosphate-carbon composite material as described in claim 1, characterized in that, The preparation of the mixture includes: mixing the iron source, sodium source, carbon source, phosphorus source and solvent; The mixing is preferably carried out by ball milling and / or sand milling; The milling speed is, for example, 1000 rpm, and the milling time is, for example, 5 hours; The ball milling is carried out, for example, in a ball mill, the ball milling speed is, for example, 300 rpm, and the ball milling time is, for example, 12 hours. And / or, the mixing process further includes a drying step; Preferably, the drying process is vacuum drying; The drying temperature is preferably 60-80℃; The drying time is preferably 10-16 hours, for example 12 hours.

6. The method for preparing the sodium iron pyrophosphate-carbon composite material as described in claim 1, characterized in that, The sintering includes primary sintering and secondary sintering; The primary sintering is preferably carried out in an inert atmosphere, such as argon or a hydrogen-argon mixture. The temperature of the first sintering is preferably 250-350°C, for example 300°C; The preferred sintering time is 3-6 hours. The heating rate to the primary sintering temperature is preferably 3-10°C / min, for example 5°C / min; The secondary sintering is preferably carried out in an inert atmosphere, such as argon or a hydrogen-argon mixture. The secondary sintering temperature is preferably 450-550℃, for example 500℃ or 520℃; The secondary sintering time is preferably 10-12 hours. The heating rate from the primary sintering temperature to the secondary sintering temperature is preferably 3-10℃ / min, for example 5℃ / min; And / or, the secondary sintering process further includes a cooling step to 25°C-100°C.

7. A sodium iron pyrophosphate-carbon composite material, characterized in that, It is prepared by the method of preparing sodium iron pyrophosphate-carbon composite material as described in any one of claims 1-6.

8. A sodium iron pyrophosphate-carbon composite material, characterized in that, It includes a core and a carbon coating layer covering the surface of the core; wherein, The core phase composition includes sodium iron pyrophosphate, the chemical formula of which is: x=3-5; the phase purity of the sodium ferric pyrophosphate in the phase composition is not less than 95%; The sodium iron pyrophosphate sodium has a sodium iron antisite defect of no more than 1%.

9. The sodium iron pyrophosphate-carbon composite material as described in claim 8, characterized in that, The chemical formula of the sodium ferric pyrophosphate is: , or ; And / or, in the sodium iron pyrophosphate-carbon composite material, the mass ratio of the core to the carbon coating layer is 95:5-99:1; And / or, the sodium ferric pyrophosphate in the phase composition has a phase purity of 95%-100%, for example 96.01% or 99.02%; And / or, the sodium iron pyrophosphate sodium-carbon composite material has a sodium iron antisite defect of 0.85%-1%, for example 0.88%, 0.91% or 0.98%.

10. A sodium-ion battery, characterized in that, The sodium-ion battery comprises a sodium iron pyrophosphate-carbon composite material as described in any one of claims 7-9.

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