Method for preparing low porosity pyrophosphate sodium iron phosphate material from sacrificial template method and the material

Abstract

The application discloses a method for preparing a low-porosity pyrophosphate sodium ferric phosphate material by a self-sacrificial template method and the material, and comprises the following steps: S1, precursor mixing: mixing and grinding an iron source, a sodium source, a phosphoric acid source, a carbon source and a self-sacrificial template agent to obtain a uniform precursor slurry; S2, precursor drying: drying the precursor slurry to obtain a dried precursor powder; S3, first-stage heat treatment: under a protective atmosphere, the precursor powder is heated to a first temperature T1 and kept warm to form an intermediate body with a uniform porous structure; S4, second-stage heat treatment: under a negative pressure protective atmosphere, the intermediate body is heated to a second temperature T2 higher than the first temperature T1 and kept warm, and is naturally cooled to room temperature to obtain the low-porosity pyrophosphate sodium ferric phosphate material. The method for preparing the low-porosity pyrophosphate sodium ferric phosphate material by the self-sacrificial template method has the characteristics of low porosity, comprehensive performance and simple and efficient preparation process.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a method for preparing low-porosity sodium iron phosphate pyrophosphate material using a self-sacrificial template method, and the material itself. Background Technology

[0002] Sodium iron pyrophosphate, as a positive electrode material for polyanionic sodium-ion batteries, has received widespread research and attention in recent years due to its abundant resources, stable structure, and outstanding safety performance. However, in traditional synthesis processes such as solid-state methods, sol-gel methods, and hydrothermal methods, the material often forms a rich porous structure. While these pores facilitate electrolyte wetting and ion transport to some extent, they also significantly reduce the material's tap density and compaction density, making it difficult to improve the electrode's volumetric energy density. Furthermore, excessive porosity increases the contact area between the material and the electrolyte, accelerating interfacial side reactions (such as electrolyte decomposition and transition metal dissolution), leading to accelerated capacity decay during cycling. This severely limits the material's application prospects in high-energy-density, long-life sodium-ion batteries.

[0003] To reduce material porosity, existing methods typically rely on high-temperature, long-duration sintering and hot isostatic pressing (HIP). However, excessively high sintering temperatures can induce grain coarsening, compositional segregation, and even material decomposition. While HIP offers significant densification, its high equipment costs and complex processes make it unsuitable for large-scale production. Furthermore, these external pressure methods often fail to effectively eliminate closed-pore structures within the material, resulting in limited densification effects that are difficult to precisely control. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing low-porosity sodium iron phosphate pyrophosphate material using a self-sacrificial template method, and the material itself, which features low porosity, superior overall performance, and a simple and efficient preparation process.

[0005] This invention can be achieved through the following technical solutions:

[0006] The present invention discloses a method for preparing low-porosity sodium iron phosphate pyrophosphate material using a self-sacrificial template method, comprising the following steps:

[0007] S1. Precursor mixing: Iron source, sodium source, phosphate source, carbon source and self-sacrificing template agent are mixed and ground to obtain a uniform precursor slurry;

[0008] S2. Precursor drying: The above precursor slurry is dried to remove excess moisture and obtain dried precursor powder.

[0009] S3. First stage heat treatment: Under a protective atmosphere, the precursor powder is heated to the first temperature T1 and held at that temperature to decompose the template agent, create pores, and form an intermediate with a uniform porous structure.

[0010] S4. Second stage heat treatment: Under a negative pressure protective atmosphere, the intermediate is heated to a second temperature T2 that is higher than the first temperature T1 and held at that temperature. The volume shrinkage internal stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. The material is then naturally cooled to room temperature to obtain the low porosity sodium iron phosphate pyrophosphate material.

[0011] Furthermore, in step S1, the self-sacrificing template agent is one or more of the following: ferrous ammonium citrate, ferrous ammonium oxalate, ferrous ammonium oxalate, ferrous ammonium sulfate, ferrous ammonium sulfate, ferrous ammonium carbonate, and ferrous ammonium phosphate. These are all iron-containing compounds that are easily decomposed and produce gas. During heat treatment, they can decompose to produce gas and leave behind active iron-oxygen catalytic components, which can enhance the subsequent low-temperature nucleation, crystallization, and densification growth process of uniform porous materials.

[0012] Furthermore, in step S1, the amount of self-sacrificing template agent added is 0.10-0.25 times the molar amount of iron in the iron source. Within this range, the gas generated by the decomposition of the self-sacrificing template agent can leave a relatively uniform and regularly distributed nanoscale pore network inside the material, which is beneficial to the uniform release of stress during the subsequent material crystallization shrinkage process.

[0013] Furthermore, in step S3, the first temperature T1 is 350-450℃, and the holding time is ≥0.1H; the purpose is to ensure that the self-sacrificing template agent decomposes uniformly at this temperature, leaving behind iron-oxygen catalytic components and uniform pores. The resulting "active" porous framework is beneficial to the subsequent crystallization and densification of materials.

[0014] Furthermore, in step S4, the second temperature T2 is 500-750℃, and the holding time is ≥0.1H; the purpose is to transform the "active" porous framework into a thermodynamically stable phase with higher crystallinity and density at high temperature, thereby realizing a self-shrinking closed-pore structure and the preparation of low-porosity materials; the negative pressure is -50 to 0Pa, the purpose of which is to provide an external driving force to the pores in the "active" porous framework during the sintering process at the second temperature T2, making it easier for them to migrate towards the pore center, collapse, and contact and fuse with the opposite pore wall during crystal growth, forming a closed-pore structure.

[0015] Furthermore, in step S1, the mixing and grinding method is one or more of sand milling, ball milling, and high-speed dispersion, which achieves particle pulverization, crushing, or interatomic separation in the form of shear force, gravity, and friction. The purpose is to uniformly disperse the self-sacrificing template agent into the precursor slurry to ensure that uniformly distributed pores are generated in the subsequent decomposition process.

[0016] Furthermore, in step S2, the drying method is one or more of the following: spray drying, vacuum drying, flash drying, heating evaporation, freeze drying, and forced air drying.

[0017] Further, in steps S3 and S4, the protective atmosphere is nitrogen, argon, or a mixture of the other with hydrogen at a volume fraction of 0.1-5%.

[0018] Furthermore, the iron source is one or more of the following: ferric oxide, ferrous oxide, ferric tetroxide, ferrous oxalate, ferric phosphate, ferric sulfate, ferrous sulfate, ferric nitrate, and ferric hydroxide.

[0019] Furthermore, the sodium source is one or more of sodium formate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium sulfate, and sodium nitrate.

[0020] Furthermore, the phosphoric acid source is one or more of the following phosphoric acid compounds: phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, pyrophosphate and its salt derivatives, metaphosphoric acid and its salt derivatives, polyphosphoric acid and its salt derivatives.

[0021] Furthermore, the carbon source is one or more of sucrose, glucose, citric acid, cyclodextrin, lactose, maltose, polyvinyl alcohol, and polyethylene glycol. It should be noted that there are no special requirements for the amount of carbon added in this invention; the amount can be flexibly adjusted according to actual needs, taking into account the differences in the final residual carbon content of different carbon sources.

[0022] Another aspect of the present invention is to protect a low-porosity sodium iron phosphate pyrophosphate material, which is prepared by the above-described preparation method.

[0023] This invention discloses a method for preparing low-porosity sodium iron phosphate pyrophosphate material using a self-sacrificial template method, and the material itself, which has the following beneficial effects:

[0024] First, the porosity is low. This invention innovatively utilizes a "self-sacrificing template agent" and a "two-stage heat treatment system" to achieve a perfect temporal connection and unity between the two opposing processes of "first creating pores" and "later eliminating pores." Simultaneously, the iron oxide produced by the decomposition of the "self-sacrificing template agent" catalyzes the material's own transformation from a metastable to a stable phase. This process, coupled with negative pressure and the material's own volume shrinkage, serves as the driving force for densification. This internal-to-external shrinkage stress can uniformly and effectively eliminate internal pores, especially closing pores, with better results than simple external heat treatment.

[0025] Secondly, it exhibits excellent overall performance. The sodium iron phosphate pyrophosphate material prepared by this invention has low porosity, small specific surface area, uniform grain size, and significantly improved tap density and compaction density. When used as a cathode in sodium-ion batteries, it can reduce side reactions between the material and the electrolyte, and is also conducive to the preparation of electrodes with high areal density, thereby significantly improving the battery's high-temperature performance, cycle performance, and energy density.

[0026] Third, the preparation process is simple and efficient. This method does not require complex and expensive equipment and can be completed in a conventional tube furnace or box furnace. The process is simple and easy to scale up, and has high industrial application value. Attached Figure Description

[0027] Figure 1 To apply the low porosity Na4Fe of Example 1 2.91 SEM of (PO4)2P2O7 / C material;

[0028] Figure 2 For Comparative Example 1, Na4Fe was prepared using the solid-state method. 2.91 SEM of (PO4)2P2O7 / C material. Detailed Implementation

[0029] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0030] The present invention discloses a method for preparing low-porosity sodium iron phosphate pyrophosphate material using a self-sacrificial template method, comprising the following steps:

[0031] S1. Precursor mixing: Iron source, sodium source, phosphate source, carbon source and self-sacrificing template agent are mixed and ground to obtain a uniform precursor slurry;

[0032] S2. Precursor drying: The above precursor slurry is dried to remove excess moisture and obtain dried precursor powder.

[0033] S3. First stage heat treatment: Under a protective atmosphere, the precursor powder is heated to the first temperature T1 and held at that temperature to decompose the template agent, create pores, and form an intermediate with a uniform porous structure.

[0034] S4. Second stage heat treatment: Under a negative pressure protective atmosphere, the intermediate is heated to a second temperature T2 that is higher than the first temperature T1 and held at that temperature. The volume shrinkage internal stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. The material is then naturally cooled to room temperature to obtain the low porosity sodium iron phosphate pyrophosphate material.

[0035] Further, in step S1, the self-sacrificing template agent is one or more of ferrous ammonium citrate, ferrous ammonium oxalate, ferrous ammonium oxalate, ferrous ammonium sulfate, ferrous ammonium sulfate, ferrous ammonium carbonate, and ferrous ammonium phosphate.

[0036] Furthermore, in step S1, the amount of self-sacrificing template agent added is 0.10-0.25 times the molar amount of iron in the iron source.

[0037] Furthermore, in step S3, the first temperature T1 is 350-450℃, and the holding time is ≥0.1H.

[0038] Furthermore, in step S4, the second temperature T2 is 500-750℃, the holding time is ≥0.1H, and the negative pressure is -50 to 0Pa.

[0039] Furthermore, in step S1, the mixing and grinding method is one or more of sand milling, ball milling, and high-speed dispersion.

[0040] Furthermore, in step S2, the drying method is one or more of the following: spray drying, vacuum drying, flash drying, heating evaporation, freeze drying, and forced air drying.

[0041] Further, in steps S3 and S4, the protective atmosphere is nitrogen, argon, or a mixture of the other with hydrogen at a volume fraction of 0.1-5%.

[0042] Furthermore, the iron source is one or more of the following: ferric oxide, ferrous oxide, ferric tetroxide, ferrous oxalate, ferric phosphate, ferric sulfate, ferrous sulfate, ferric nitrate, and ferric hydroxide.

[0043] Furthermore, the sodium source is one or more of sodium formate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium sulfate, and sodium nitrate.

[0044] Furthermore, the phosphoric acid source is one or more of the following phosphoric acid compounds: phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, pyrophosphate and its salt derivatives, metaphosphoric acid and its salt derivatives, polyphosphoric acid and its salt derivatives.

[0045] Furthermore, the carbon source is one or more of the following: sucrose, glucose, citric acid, cyclodextrin, lactose, maltose, polyvinyl alcohol, and polyethylene glycol.

[0046] Another aspect of the present invention is to protect a low-porosity sodium iron phosphate pyrophosphate material, which is prepared by the above-described preparation method.

[0047] Example 1

[0048] This embodiment relates to a method for preparing low-porosity sodium iron phosphate pyrophosphate materials using a self-sacrificial template method, including the following steps:

[0049] S1. Precursor Mixing: Iron source, sodium source, phosphoric acid source, carbon source, and self-sacrificing template agent are mixed in stoichiometric ratios and milled to obtain a uniform precursor slurry. Specifically, the self-sacrificing template agent is ferric ammonium citrate, ferrous ammonium oxalate, ferrous ammonium sulfate, or ferrous ammonium sulfate, and the amount of self-sacrificing template agent added is 0.25 times the molar amount of iron in the iron source; the iron source is ferric oxide, ferrous oxide, magnetite, or ferrous oxalate; the sodium source is sodium formate, sodium acetate, or sodium carbonate; the phosphoric acid source is phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, or sodium phosphate; and the carbon source is sucrose, glucose, or citric acid.

[0050] S2. Precursor drying: The above precursor slurry is spray-dried to remove excess moisture and obtain dried precursor powder.

[0051] S3, First stage heat treatment: Under nitrogen as a protective atmosphere, the precursor powder is heated to the first temperature T1 of 450℃ and held for 1 hour to decompose the template agent to create pores and form an intermediate with a uniform porous structure.

[0052] S4. Second stage heat treatment: Under nitrogen as a protective atmosphere and with the pressure controlled at -50 Pa, the intermediate is heated to a second temperature T2 of 750℃, which is higher than the first temperature T1, and held at that temperature for 2 hours. The volume shrinkage internal stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. After natural cooling to room temperature, the low porosity sodium iron phosphate pyrophosphate material is obtained.

[0053] Example 2

[0054] This embodiment relates to a method for preparing low-porosity sodium iron phosphate pyrophosphate materials using a self-sacrificial template method, including the following steps:

[0055] S1. Precursor Mixing: Iron source, sodium source, phosphoric acid source, carbon source, and self-sacrificing template agent are ball-milled according to stoichiometric ratios to obtain a uniform precursor slurry. Specifically, the self-sacrificing template agent is ferric ammonium citrate, ferrous ammonium oxalate, ferrous ammonium carbonate, or ferrous ammonium phosphate, and the amount of self-sacrificing template agent added is 0.15 times the molar amount of iron in the iron source; the iron source is ferrous sulfate, ferric nitrate, or ferric hydroxide; the sodium source is sodium bicarbonate, sodium hydroxide, sodium sulfate, or sodium nitrate; the phosphoric acid source is metaphosphoric acid and its salt derivatives, polyphosphoric acid and its salt derivatives; and the carbon source is lactose, maltose, polyvinyl alcohol, or polyethylene glycol.

[0056] S2. Precursor drying: The above precursor slurry is flash-dried and forced-air-dried to remove excess moisture and obtain dry precursor powder.

[0057] S3, First stage heat treatment: Under the protective atmosphere of argon, the precursor powder is heated to the first temperature T1 of 400℃ and held for 3 hours to decompose the template agent to create pores and form an intermediate with a uniform porous structure.

[0058] S4. Second stage heat treatment: Under the protective atmosphere of argon and the pressure controlled at -30 Pa, the intermediate is heated to a second temperature T2 of 750℃, which is higher than the first temperature T1, and held at that temperature for 5 hours. The volume shrinkage internal stress generated during the phase transformation or composition homogenization of the material causes the porous structure to shrink and densify. After natural cooling to room temperature, the low porosity sodium iron phosphate pyrophosphate material is obtained.

[0059] Example 3

[0060] This embodiment relates to a method for preparing low-porosity sodium iron phosphate pyrophosphate materials using a self-sacrificial template method, including the following steps:

[0061] S1. Precursor Mixing: Iron source, sodium source, phosphate source, carbon source, and self-sacrificing template agent are dispersed at high speed according to stoichiometric ratio to obtain a uniform precursor slurry. Specifically, the self-sacrificing template agent is ferric ammonium citrate, ferrous ammonium carbonate, or ferrous ammonium phosphate, and the amount of self-sacrificing template agent added is 0.10 times the molar amount of iron in the iron source; the iron source is ferric oxide, ferrous oxide, ferric oxide, ferrous oxalate, ferric nitrate, or ferric hydroxide; the sodium source is sodium formate, sodium acetate, sodium carbonate, sodium phosphate, ammonium dihydrogen phosphate, pyrophosphate, and their salt derivatives; the carbon source is sucrose, glucose, citric acid, polyvinyl alcohol, or polyethylene glycol.

[0062] S2. Precursor drying: The above precursor slurry is dried by blowing air to remove excess moisture and obtain dry precursor powder.

[0063] S3. First stage heat treatment: Under the protective atmosphere of a mixture of nitrogen and hydrogen with a volume fraction of 1%, the precursor powder is heated to the first temperature T1 of 390°C and held for 2 hours to decompose the template agent and form an intermediate with a uniform porous structure.

[0064] S4. Second stage heat treatment: Under the protective atmosphere of a mixture of nitrogen and hydrogen with a volume fraction of 3% and a pressure controlled at -10Pa, the intermediate is heated to a second temperature T2 of 500℃, which is higher than the first temperature T1, and held at that temperature for 11 hours. The volume shrinkage internal stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. After natural cooling to room temperature, the low porosity sodium iron phosphate pyrophosphate material is obtained.

[0065] Example 4

[0066] This embodiment relates to a method for preparing low-porosity sodium iron phosphate pyrophosphate materials using a self-sacrificial template method, including the following steps:

[0067] S1. Precursor Mixing: Iron source, sodium source, phosphoric acid source, carbon source, and self-sacrificing template agent are mixed in stoichiometric ratios and milled to obtain a uniform precursor slurry. Specifically, the self-sacrificing template agent is ferric ammonium citrate, ferrous ammonium oxalate, ferric ammonium oxalate, or ferrous ammonium phosphate, and the amount of self-sacrificing template agent added is 0.15 times the molar amount of iron in the iron source; the iron source is ferric oxide, ferrous oxide, magnetite, ferric nitrate, or ferric hydroxide; the sodium source is sodium formate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium hydroxide, polyphosphoric acid, and their salt derivatives; the carbon source is sucrose, glucose, or citric acid.

[0068] S2. Precursor drying: The above precursor slurry is spray-dried to remove excess moisture and obtain dried precursor powder.

[0069] S3. First stage heat treatment: Under the protective atmosphere of a mixture of nitrogen and hydrogen with a volume fraction of 0.1-5%, the precursor powder is heated to the first temperature T1 of 390℃ and held for 6 hours to decompose the template agent, create pores, and form an intermediate with a uniform porous structure.

[0070] S4. Second stage heat treatment: Under the protective atmosphere of a mixture of nitrogen and hydrogen with a volume fraction of 0.1-5% and a pressure controlled at -20Pa, the intermediate is heated to a second temperature T2 of 700℃, which is higher than the first temperature T1, and held at that temperature for 12 hours. The volume shrinkage internal stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. After natural cooling to room temperature, the low porosity sodium iron phosphate pyrophosphate material is obtained.

[0071] Example 5

[0072] This embodiment relates to a method for preparing low-porosity sodium iron phosphate pyrophosphate materials using a self-sacrificial template method, including the following steps:

[0073] S1. Precursor Mixing: Iron source, sodium source, phosphoric acid source, carbon source, and self-sacrificing template agent are mixed in stoichiometric ratios and milled to obtain a uniform precursor slurry. Specifically, the self-sacrificing template agent is ferric ammonium citrate, ferrous ammonium oxalate, ferric ammonium oxalate, or ferrous ammonium sulfate, and the amount of self-sacrificing template agent added is 0.20 times the molar amount of iron in the iron source; the iron source is ferric oxide, ferrous oxide, or magnetite; the sodium source is sodium hydroxide, sodium sulfate, or sodium nitrate; the phosphoric acid source is phosphoric acid, sodium monohydrogen phosphate, or sodium dihydrogen phosphate; and the carbon source is sucrose, glucose, citric acid, or cyclodextrin.

[0074] S2. Precursor drying: The above precursor slurry is spray-dried to remove excess moisture and obtain dried precursor powder.

[0075] S3. First stage heat treatment: Under the protective atmosphere of a mixture of nitrogen and hydrogen with a volume fraction of 0.1-5%, the precursor powder is heated to the first temperature T1 of 420℃ and held for 3 hours to decompose the template agent, create pores, and form an intermediate with a uniform porous structure.

[0076] S4. Second stage heat treatment: Under the protective atmosphere of a mixture of nitrogen and hydrogen with a volume fraction of 0.5% and a pressure controlled at -24Pa, the intermediate is heated to a second temperature T2 of 650℃, which is higher than the first temperature T1, and held at that temperature for 10 hours. The volume shrinkage internal stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. After natural cooling to room temperature, the low porosity sodium iron phosphate pyrophosphate material is obtained.

[0077] Application Example 1: Low Porosity Na₄Fe 2.91 Synthesis and electrochemical properties of (PO4)2P2O7 / C

[0078] This embodiment involves low-porosity Na4Fe 2.91 The preparation method of (PO4)2P2O7 / C material by self-sacrificial template method includes the following steps:

[0079] Step 1: Add water and grind ferric phosphate, sodium acetate, phosphoric acid, and ferric ammonium citrate (self-sacrificing template agent, added at a molar ratio of 0.15 times the molar amount of iron in ferric phosphate) in a molar ratio of 2.4735:4:1.09:0.4365. Add sucrose (added at a molar ratio of 6.0 wt% of the mass of ferric phosphate) as a carbon source to obtain a uniform yellow precursor slurry.

[0080] Step 2: Spray dry the above yellow precursor slurry, set the inlet air temperature to 300℃ and the outlet air temperature to 100℃, remove the solvent, achieve solid-liquid separation, and obtain dry precursor powder.

[0081] Step 3: In a nitrogen atmosphere, the above precursor powder is kept at 400°C for 5 hours to decompose the ferric ammonium citrate template agent to produce gas and form an intermediate with a uniform porous structure.

[0082] Step 4: Under a negative pressure of -10 Pa, the above porous intermediate is kept at 550°C for 10 hours. The volume shrinkage stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. Step 5: Allow it to cool naturally to room temperature to obtain low-porosity Na4Fe. 2.91 (PO4)2P2O7 / C material.

[0083] Low porosity Na4Fe 2.91After mixing (PO4)2P2O7 / C material, SurP, and PVDF5130 in a mass ratio of 9.5:0.2:0.3 and homogenizing, the viscosity of the discharged slurry with a solid content of 60% could be stably controlled below 2000 Pa·s, exhibiting good flow properties. This phenomenon is closely related to the obvious two-phase separation between the solid material and the liquid during the pulping process, indicating that the liquid does not easily penetrate or adsorb into the interior of the solid particles, reflecting that the material itself has a small pore structure and low porosity. Thanks to this low porosity characteristic, the slurry exhibits excellent stability and operability in subsequent processing, making it suitable for various molding process requirements. The black slurry was then coated onto aluminum foil using a 150µm four-sided coating tool, and the film was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6 mm using a punching machine. Sodium metal was used as the counter electrode, and 1 mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte. The separator was a PP / PE / PP three-layer separator. The CR2016 button cell was assembled in a glove box.

[0084] Figure 1 The image shows low porosity Na4Fe 2.91 Scanning electron microscope (SEM) images of the (PO4)2P2O7 / C material. The images reveal a clear surface interface and dense structure, exhibiting distinct molten closed-cell morphology. Compared to the sample in Comparative Example 1, the pore structure is significantly reduced, resulting in a substantial decrease in porosity. The pore structure data in Table 1 further demonstrate that the pore volume of this material is only 0.1 cm³. 3 The density of this material is significantly lower than that of the control sample, indicating a high degree of suppression of internal porosity. Furthermore, the physicochemical property test results listed in Table 1 show that the compacted density of this material reaches 2.39 g / cm³. 3 Its specific surface area is only 2.5m². 2 The density and specific surface area of ​​the material were significantly improved compared to the comparative material. These data collectively demonstrate that the process optimization, through self-sacrificing template agent induction combined with gradient heating treatment, effectively filled and closed the pores during sintering, significantly improved the structural density, and correspondingly reduced the specific surface area, thereby achieving a significant increase in compaction density. This lays a solid structural foundation for its electrochemical performance and mechanical stability in subsequent applications.

[0085] Table 1 data shows that low porosity Na4Fe 2.91The (PO4)2P2O7 / C electrode exhibits a reversible discharge capacity of 118 mAh / g at a rate of 0.1C (1C = 129 mAh / g), comparable to the comparative material, indicating that the reduction in porosity did not significantly affect its capacity performance. More importantly, after 1000 cycles at room temperature (25℃) and high temperature (55℃), the electrode retains a capacity of 99.3% and 97.2%, respectively, showing almost no capacity decay. This excellent cycling stability is closely related to the optimization of the material's pore structure. Low porosity effectively reduces the contact interface between the electrode material and the electrolyte, thereby significantly suppressing side reactions between them and reducing the loss of active materials and structural degradation during cycling. Therefore, the structural integrity of the material is maintained over a long period, demonstrating excellent electrochemical durability, making it particularly suitable for high-temperature or long-cycle applications with stringent cycle life requirements.

[0086] Application Example 2: Low Porosity Na4Fe 2.91 Synthesis and electrochemical properties of (PO4)2P2O7 / C

[0087] This embodiment involves low-porosity Na4Fe 2.91 The preparation method of (PO4)2P2O7 / C material by self-sacrificial template method includes the following steps:

[0088] Step 1: Nano-ferric hydroxide, sodium formate, phosphoric acid, and ferrous ammonium sulfate (self-sacrificing template agent, added at a molar ratio of 0.2 times the molar amount of iron in nano-ferric hydroxide) are ground with water in a molar ratio of 2.328:4:4:0.582. Glucose (added at a molar ratio of 5.5 wt% of the mass of nano-ferric hydroxide) is added as a carbon source to obtain a uniform reddish-brown precursor slurry.

[0089] Step 2: Spray dry the above reddish-brown precursor slurry, setting the inlet air temperature to 280℃ and the outlet air temperature to 110℃ to remove the solvent, achieve solid-liquid separation, and obtain dry precursor powder.

[0090] Step 3: In a nitrogen atmosphere, the above precursor powder is kept at 420°C for 3 hours to decompose the ferrous ammonium sulfate template agent to produce gas and form an intermediate with a uniform porous structure.

[0091] Step 4: Under a negative pressure of -30 Pa, the above porous intermediate is kept at 600℃ for 5 hours. The volume shrinkage stress generated during the material phase transformation or composition homogenization process causes the porous structure to shrink and densify. Step 5: Allow it to cool naturally to room temperature to obtain low-porosity Na4Fe. 2.91 (PO4)2P2O7 / C material.

[0092] Low porosity Na4Fe 2.91After mixing (PO4)2P2O7 / C material, SurP, and PVDF5130 in a mass ratio of 9.5:0.2:0.3, the slurry prepared from this material, even with a solid content as high as 60%, still maintained a stable viscosity below 2000 Pa·s, exhibiting excellent flow characteristics. This result is mainly attributed to the clear two-phase separation between the solid particles and the liquid phase, indicating that the liquid is not easily adsorbed or retained inside the particles, thus indirectly confirming the material's structural characteristics of low porosity and few pores. The low porosity of the material directly promotes the improvement of the slurry's processing performance: low viscosity and good fluidity not only make the slurry easier to handle during stirring, conveying, and coating, but also help improve coating uniformity and process stability, meeting the stringent requirements of various electrode forming processes for the slurry's rheological properties. Subsequently, a 150µm four-sided coating agent was used to coat the black slurry onto aluminum foil, and the film was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6 mm using a punching machine. Sodium metal was used as the counter electrode, and 1 mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte. The separator was a PP / PE / PP three-layer separator. The CR2016 button cell was assembled in a glove box.

[0093] The pore structure data in Table 1 show that the pore volume of the material induced by the self-sacrificing template agent combined with gradient heating is only 0.08 cm³ / g, far lower than that of the control sample, indicating that this process can significantly inhibit the formation and expansion of pores within the material. Furthermore, physicochemical property tests show that the compaction density of this material is increased to 2.40 g / cm³, while the specific surface area is only 3.2 m² / g. Both key indicators are significantly improved compared to the controllable material, indicating a significant increase in the material's densification. These data collectively demonstrate that the self-sacrificing template agent, through controlled decomposition during heat treatment, forms internal support and guidance, and, combined with the gradient heating strategy, gradually regulates grain growth and pore structure closure, achieving effective filling of pores and interfacial fusion. This synergistic process not only promotes close packing between particles, reducing porosity and specific surface area, but also significantly improves the overall compaction density of the material, thus laying a superior structural foundation for its subsequent processing performance, electrochemical stability, and mechanical strength in electrode fabrication.

[0094] Table 1 shows that low porosity Na4Fe 2.91The (PO4)2P2O7 / C electrode still provides a reversible discharge capacity of 117 mAh / g at a rate of 0.1C (1C = 129 mAh / g), comparable to the performance of comparative materials with higher porosity, indicating that pore structure optimization did not compromise its electrochemical capacity. More notably, after 1000 cycles at 25°C and 55°C, the electrode retains 99.1% and 98.5% of its capacity, respectively, exhibiting negligible capacity decay. This superior cycling stability is primarily due to the significant reduction in material porosity. The low-porosity structure drastically reduces the ineffective contact area between the electrode and the electrolyte, effectively suppressing interfacial side reactions, including continuous electrolyte decomposition, dissolution of transition metal ions, and excessive growth of the solid electrolyte interfacial film. The reduction in side reactions directly mitigates the loss of active materials and the degradation of the electrode structure, enabling the material to maintain high structural integrity and electrochemical activity during long-term cycling. Therefore, low porosity not only improves the cycle durability of the electrode under extreme temperature conditions, but also provides a key material basis for its application in high energy density and long life sodium-ion battery systems.

[0095] Comparative Example 1: Solid-phase method for Na₄Fe 2.91 Synthesis and electrochemical properties of (PO4)2P2O7 / C

[0096] This embodiment involves Na4Fe 2.91 The solid-state preparation method for (PO4)2P2O7 / C material includes the following steps:

[0097] Step 1: Add water to ferric phosphate, sodium acetate and phosphoric acid in a molar ratio of 2.91:4.0:1.09 and grind them together. Add sucrose (6.0 wt% of the mass of ferric phosphate) as a carbon source to obtain a uniform yellow precursor slurry.

[0098] Step 2: Spray dry the above yellow precursor slurry, set the inlet air temperature to 300℃ and the outlet air temperature to 100℃, remove the solvent, achieve solid-liquid separation, and obtain dry precursor powder.

[0099] Step 3: In a nitrogen atmosphere, the above precursor powder is kept at 550°C for 10 hours, and then naturally cooled to room temperature to obtain low-porosity Na4Fe. 2.91 (PO4)2P2O7 / C material.

[0100] Figure 2 The image shows Na₄Fe prepared by solid-state method. 2.91Scanning electron microscopy (SEM) images of the (PO4)2P2O7 / C material reveal numerous pores within the material, exhibiting irregular particle morphology and uneven size distribution. This structural characteristic is closely related to the random and non-uniform pores formed during sintering. These pores, lacking effective densification driving forces during phase transformation, are difficult to spontaneously eliminate, ultimately leading to the material's typical porous morphology. The physicochemical property test data in Table 1 further confirm its structural defects: the material's compacted density is only 1.90 g / cm³, its specific surface area is as high as 15.4 m² / g, and its porosity is as high as 12.9 cm³. 3 The specific surface area is significantly lower than that of the material in subsequent structurally optimized application examples, while the lower compaction density directly reflects the hollow characteristics and widespread pore distribution within the material. These structural factors collectively limit its processing performance and volumetric energy density expression in electrode fabrication.

[0101] Na4Fe 2.91 After mixing (PO4)2P2O7 / C material, SurP, and PVDF5130 in a mass ratio of 9.5:0.2:0.3, and homogenizing the mixture, the viscosity of the output slurry exceeded 10,000 Pa·s when the solid content was controlled at 60%. This resulted in a significant decrease in fluidity, with some areas even exhibiting a jelly-like gel state, making uniform coating difficult and severely impacting subsequent electrode processing. This phenomenon is closely related to the high porosity of the material: during slurry preparation, a large amount of liquid is adsorbed through the pores and retained within the solid particles, reducing the proportion of freely flowing liquid phase in the slurry and causing a sharp increase in system viscosity. Ultimately, this leads to viscous slurry with deteriorated rheological properties, limiting its application in large-scale electrode manufacturing. Subsequently, a 150µm four-sided coating apparatus was used to coat the black slurry onto aluminum foil. A smooth and uniform film was selected and dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6 mm using a punching machine. Sodium metal was used as the counter electrode, and 1 mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte. The separator was a PP / PE / PP three-layer separator. The CR2016 button cell was assembled in a glove box.

[0102] Table 1 shows the electrochemical performance test data for Na4Fe. 2.91The (PO4)2P2O7 / C electrode exhibits a reversible discharge capacity of 117 mAh / g at a rate of 0.1C (1C = 129 mAh / g), which is essentially the same as in the application example, indicating that porosity has little impact on the initial capacity of the material. However, in terms of long-term cycling performance, after 1000 cycles at 25°C and 55°C, the capacity retention rates are only 89.4% and 82.4%, respectively, significantly lower than the porosity-optimized material in the application example, showing a clear capacity decay trend. This decrease in cycling stability is closely related to the high porosity of the material. The high porosity structure significantly increases the actual contact area between the electrode and the electrolyte, providing more active interfaces for side reactions. During cycling, the electrolyte can more easily penetrate into the pores, continuously triggering interfacial side reactions, such as the oxidative decomposition of the electrolyte, the dissolution of transition metal ions, and the repeated formation and rupture of unstable solid electrolyte interfacial films. These side reactions not only consume the active sodium source but also lead to gradual damage to the electrode structure and increased internal resistance, thereby accelerating capacity decay. Especially under high-temperature conditions, the kinetics of side reactions accelerate, further exacerbating the dissolution of materials in the electrolyte and interfacial degradation, leading to a significant deterioration in cycle performance. Therefore, while high porosity does not affect initial capacity, it severely restricts the long-term cycle stability of the electrode, particularly under harsh high-temperature environments. This highlights the importance of controlling pore structure and reducing interfacial side reactions in material design for improving battery cycle life.

[0103] Comparative Example 2: Liquid Phase Method for Na₄Fe 2.91 Synthesis and electrochemical properties of (PO4)2P2O7 / C

[0104] This embodiment involves Na4Fe 2.91 The liquid-phase preparation method for (PO4)2P2O7 / C material includes the following steps:

[0105] Step 1: Add water and grind ferrous sulfate, sodium formate and phosphoric acid in a molar ratio of 2.91:4.0:4.0. Add glucose (5.5 wt% of the mass of ferrous sulfate) as a carbon source to obtain a uniform yellow precursor slurry.

[0106] Step 2: Spray dry the above yellow precursor slurry, set the inlet air temperature to 300℃ and the outlet air temperature to 100℃, remove the solvent, achieve solid-liquid separation, and obtain dry precursor powder.

[0107] Step 3: In a nitrogen atmosphere, the above precursor powder is kept at 550°C for 10 hours, and then naturally cooled to room temperature to obtain low-porosity Na4Fe. 2.91 (PO4)2P2O7 / C material.

[0108] The physicochemical performance test data in Table 1 show that the material has a compaction density of only 1.82 g / cm³, a specific surface area as high as 10.9 m² / g, and a porosity of 8.6 cm³ / g, which is significantly different from the material in the subsequent application examples after structural optimization. This structural feature is closely related to the random and non-uniform pores formed during sintering: in the absence of an effective densification driving force, these pores are difficult to spontaneously eliminate during phase transition, ultimately forming a typical disordered porous morphology. This combined characteristic of high specific surface area, low compaction density, and high porosity reflects the widespread distribution of hollow structures and pores within the material. During electrode preparation, this porous structure not only increases the internal resistance and viscosity of the slurry, reducing coating uniformity, but also hinders the close packing of particles during the electrode compaction stage, thus restricting the electrode's processing performance and finished product consistency. Simultaneously, excessive pores occupy a large amount of active volume, severely limiting the electrode's compaction density and volumetric energy density, making it difficult to meet the design requirements of high-energy-density batteries.

[0109] Na4Fe 2.91 After mixing (PO4)2P2O7 / C material, SurP, and PVDF5130 in a mass ratio of 9.5:0.2:0.3, and homogenizing the mixture, the viscosity of the discharged slurry exceeded 8000 Pa·s when the solid content was controlled at 60%. This resulted in a significant decrease in fluidity, making uniform coating difficult and severely impacting subsequent electrode processing. This phenomenon is closely related to the high porosity of the material: during slurry preparation, a large amount of liquid is adsorbed through the pores and retained inside the solid particles, reducing the proportion of freely flowing liquid phase in the slurry and causing a sharp increase in system viscosity. Ultimately, this leads to viscous slurry and deteriorated rheological properties, limiting its application in large-scale electrode manufacturing. Subsequently, a 150µm four-sided coating apparatus was used to coat the black slurry onto aluminum foil. A smooth and uniform film was selected and dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6 mm using a punching machine. Sodium metal was used as the counter electrode, and 1 mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte. The separator was a PP / PE / PP three-layer separator. The CR2016 button cell was assembled in a glove box.

[0110] Table 1 shows the electrochemical performance test results for Na4Fe. 2.91The (PO4)2P2O7 / C electrode exhibits a reversible discharge capacity of 116 mAh / g at a rate of 0.1C (1C = 129 mAh / g), which is essentially the same as that in the application example, indicating that porosity has a relatively small impact on the initial capacity of the material. Furthermore, Table 1 shows that after 1000 cycles at 25°C (room temperature) and 55°C (high temperature), the capacity retention rates of this electrode are 92.3% and 88.9%, respectively, significantly lower than that of the material in the application example. This performance difference is closely related to the material's higher porosity. The porous structure increases the contact area between the electrode and the electrolyte, promoting electrolyte penetration and interfacial reactions, exacerbating the dissolution of active materials and side reactions, such as unstable SEI film growth and dissolution of transition metal ions, ultimately accelerating capacity decay and deterioration of cycling performance.

[0111] Table 1 Performance Test Results

[0112]

[0113] The above embodiments are merely specific examples of the present invention, and their descriptions are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A method for preparing low-porosity sodium iron phosphate pyrophosphate material using a self-sacrificial template method, characterized in that... Includes the following steps: S1. Precursor mixing: Iron source, sodium source, phosphate source, carbon source and self-sacrificing template agent are mixed and ground to obtain a uniform precursor slurry; S2. Precursor drying: The above precursor slurry is dried to obtain dried precursor powder. S3. First stage heat treatment: Under a protective atmosphere, the precursor powder is heated to the first temperature T1 and held at that temperature to decompose the template agent, create pores, and form an intermediate with a uniform porous structure. S4. Second stage heat treatment: Under a negative pressure protective atmosphere, the intermediate is heated to a second temperature T2 that is higher than the first temperature T1 and held at that temperature. It is then naturally cooled to room temperature to obtain the low-porosity sodium iron phosphate pyrophosphate material.

2. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by self-sacrificial template method according to claim 1, characterized in that: In step S1, the self-sacrificing template agent is one or more of the following: ferrous ammonium citrate, ferrous ammonium oxalate, ferrous ammonium oxalate, ferrous ammonium sulfate, ferrous ammonium sulfate, ferrous ammonium carbonate, and ferrous ammonium phosphate.

3. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by self-sacrificial template method according to claim 1, characterized in that: In step S1, the amount of self-sacrificing template agent added is 0.10-0.25 times the molar amount of iron in the iron source.

4. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by self-sacrificial template method according to claim 1, characterized in that: In step S3, the first temperature T1 is 350-450℃, and the holding time is ≥0.1H.

5. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by the self-sacrificial template method according to claim 1, characterized in that: In step S4, the second temperature T2 is 500-750℃, the holding time is ≥0.1H, and the negative pressure is -50 to 0Pa.

6. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by self-sacrificial template method according to claim 1, characterized in that: In step S1, the mixing and grinding method is one or more of the following: sand milling, ball milling, and high-speed dispersion.

7. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by self-sacrificial template method according to claim 1, characterized in that: In step S2, the drying method is one or more of the following: spray drying, vacuum drying, flash drying, heating evaporation, freeze drying, and forced air drying. In steps S3 and S4, the protective atmosphere is nitrogen, argon, or a mixture of the other with hydrogen at a volume fraction of 0.1-5%.

8. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by the self-sacrificial template method according to claim 7, characterized in that: The iron source is one or more of the following: ferric oxide, ferrous oxide, ferric tetroxide, ferrous oxalate, ferric phosphate, ferric sulfate, ferrous sulfate, ferric nitrate, and ferric hydroxide. The sodium source is one or more of sodium formate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium sulfate, and sodium nitrate. The phosphoric acid source is one or more of the following: phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, pyrophosphate and its salt derivatives, metaphosphoric acid and its salt derivatives, and polyphosphoric acid and its salt derivatives.

9. The method for preparing low-porosity sodium iron phosphate pyrophosphate material by self-sacrificial template method according to claim 1, characterized in that: The carbon source is one or more of the following: sucrose, glucose, citric acid, cyclodextrin, lactose, maltose, polyvinyl alcohol, and polyethylene glycol.

10. A low-porosity sodium iron phosphate pyrophosphate material, characterized in that: It is prepared by any one of the preparation methods described in claims 1-9.

Images