Magnesium / zirconium co-doped and manganese / iron concentration gradient sodium ferromanganese pyrophosphate material and preparation method thereof

Abstract

The invention discloses a magnesium / zirconium co-doped manganese / iron concentration gradient sodium ferromanganese pyrophosphate material and a preparation method thereof, the chemical formula of the sodium ferromanganese pyrophosphate material is Na < 4 + z > (MnxFe < 1-x >) < 3-y > MgyP < 4-z > ZrzO15, x is greater than or equal to 0.5 and less than or equal to 0.95, 0 lt; y is less than or equal to 0.05, 0 lt; z < = 0.03; the pyrophosphate sodium ferromanganese phosphate material is a microspherical secondary particle, the interior of the pyrophosphate sodium ferromanganese phosphate material has a continuous Mn / Fe concentration gradient from a core to a shell, and Mg and Zr elements are uniformly distributed in the whole secondary particle. The magnesium / zirconium co-doped and manganese / iron concentration gradient sodium ferromanganese pyrophosphate material and the preparation method thereof have the characteristics of high structural stability, good interface stability and excellent rate capability.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a magnesium / zirconium co-doped sodium manganese phosphate pyrophosphate material with manganese / iron concentration gradient and its preparation method. Background Technology

[0002] Sodium manganese iron pyrophosphate (SOP) has become a highly promising cathode material for sodium-ion batteries due to its high theoretical capacity, abundant raw materials, stable structure, and high operating voltage platform. However, its practical application still faces significant challenges:

[0003] Low intrinsic conductivity leads to poor rate performance.

[0004] For example, the dissolution of manganese and the Ginger-Taylor effect: During the charging and discharging process, the Ginger-Taylor distortion of Mn³⁺ will cause local distortion and destruction of the crystal structure, resulting in severe capacity decay during cycling. (3) Stress concentration inside the particles: During the charging and discharging process, the insertion and extraction of sodium will cause changes in the volume of the material, generating internal stress inside the uniformly composed particles, which will lead to particle cracking and performance degradation under long-term cycling.

[0005] To address the above problems, existing technologies typically employ the following methods:

[0006] Bulk doping: By introducing trace amounts of heterovalent elements (such as Mg²⁺, Zn²⁺, Ti) 4 Doping with Mn / Fe or P sites (such as ⁺) can stabilize the crystal structure and improve electronic / ionic conductivity. However, single bulk doping is insufficient to effectively address side reactions and internal stress at particle interfaces.

[0007] For example, surface coating: a layer of carbon or metal oxide is coated on the surface of material particles to suppress interfacial side reactions and manganese dissolution. However, the coating layer may hinder ion diffusion, and the sharp interface between the coating layer and the bulk phase may peel off after long-term cycling. Summary of the Invention

[0008] The purpose of this invention is to provide a magnesium / zirconium co-doped sodium manganese phosphate pyrophosphate material with a manganese / iron concentration gradient and its preparation method, which has the characteristics of high structural stability, good interface stability and excellent rate performance.

[0009] This invention can be achieved through the following technical solutions:

[0010] This invention relates to a magnesium / zirconium co-doped sodium manganese phosphate pyrophosphate material with a manganese / iron concentration gradient, the chemical formula of which is Na. 4+z (Mn x Fe 1-x ) 3-y Mg y P 4-z Zrz O 15 The 0.5 ≤ x ≤ 0.95, 0 < y ≤ 0.05, and 0 < z ≤ 0.03 are specified. The sodium manganese iron pyrophosphate material is a microspherical secondary particle with a continuous Mn / Fe concentration gradient from the core to the shell. Mg and Zr elements are uniformly distributed throughout the secondary particle.

[0011] Preferably, in the microspherical secondary particles, the x-value at the core is 0.8-0.95, and the x-value at the shell is 0.5-0.7, with the x-value decreasing continuously from the core to the shell.

[0012] Another aspect of the present invention relates to a method for preparing the above-mentioned sodium manganese phosphate pyrophosphate material, comprising the following steps:

[0013] S1. Preparation of Mn-rich ion solution: Dissolve manganese source, iron source, zirconium dopant and magnesium dopant in water to form Mn-rich ion solution;

[0014] S2. Preparation of Fe-rich solution: Dissolve manganese source, iron source, zirconium dopant and magnesium dopant in water to form Fe-rich solution;

[0015] S3. Preparation of gradient precursors: In a buffer solution, a controlled co-precipitation method was used to prepare hydroxide precursor microspheres with a core-shell concentration gradient structure by controlling the relative flow rates of Mn-rich and Fe-rich solutions through a program.

[0016] S4. Preparation of precursor slurry: The above gradient precursor is mixed with sodium source, phosphorus source and carbon source and water to obtain a uniform precursor slurry.

[0017] S5. Preparation of precursor dry powder: The above precursor slurry is dried to remove moisture and obtain dry precursor powder.

[0018] S6. High-temperature sintering: Under a protective atmosphere, the above precursor powder is calcined at high temperature, and after natural cooling, Na is obtained. 4+z (Mn x Fe 1-x ) 3-y Mg y P 4-z Zr z O 15 / C material.

[0019] Preferably, in the Mn-rich solution, the molar ratio of Mn to Fe is (8-9.5):(0.5-2); and in the Fe-rich solution, the molar ratio of Mn to Fe is (0.5-2):(8-9.5).

[0020] Preferably, in step S3, the buffer solution is an ammonia-ammonium salt buffer system, the reaction pH is controlled at 9.0-13.0, and the reaction temperature is 30-80℃. Within this pH and temperature range, the nucleation rate of co-precipitated hydroxides and the uniformity of crystal epitaxial growth are better, which plays a decisive role in the subsequent formation of the concentration gradient sodium manganese phosphate pyrophosphate phase.

[0021] Preferably, in step S3, the total flow rate of the Mn-rich solution and the Fe-rich solution is constant. The flow rate of the Mn-rich solution is linearly reduced from its initial maximum value to zero by program control, while the flow rate of the Fe-rich solution is linearly increased from zero to its maximum value.

[0022] Preferably, in step S4, the molar ratio of Na in the sodium source to the total metal ions (Mn+Fe+Mg) in the precursor during mixing is (4.0-4.2):1, and the molar ratio of P in the phosphorus source to the total metal ions is (4-2):1.

[0023] Preferably, in step S6, the solid-state sintering conditions are: a high-temperature calcination temperature of 500-700℃ and a holding time of >0.1H; within this temperature range, elements such as sodium and phosphorus can dissolve and migrate uniformly into the interior of the secondary microspheres, and combine with transition metal ions to form crystal nuclei, growing into a relatively complete material with a gradient distribution of transition metal concentration; the protective atmosphere is one or two of nitrogen, argon, nitrogen-hydrogen, or argon-hydrogen mixture, all of which are non-oxidizing gases.

[0024] Preferably, in step S5, the drying method is one or more of spray drying, flash drying, and vacuum drying.

[0025] Preferably, the manganese source is one or more of manganese sulfate, manganese acetate, manganese chloride, manganese citrate, and manganese gluconate.

[0026] Preferably, the iron source is one or more of ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, ferric acetate, ferrous ammonium sulfate, and ferric citrate.

[0027] Preferably, the magnesium dopant is one or more of magnesium sulfate, magnesium acetate, magnesium citrate, and magnesium chloride.

[0028] Preferably, the zirconium dopant is one or more of zirconium oxychloride, zirconium sulfate, zirconium acetate, ammonium zirconium carbonate, and zirconium citrate.

[0029] Preferably, the sodium source is one or more of sodium formate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium oxalate, sodium citrate, and sodium nitrate.

[0030] Preferably, the phosphorus source is one or more of phosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0031] Preferably, the carbon source is one or more of citric acid, glucose, sucrose, maltose, and soluble starch.

[0032] This invention discloses a magnesium / zirconium co-doped sodium manganese phosphate pyrophosphate material with a manganese / iron concentration gradient and its preparation method, which has the following beneficial effects:

[0033] First, it exhibits high structural stability. Mg²⁺ (ionic radius ~0.072 nm) doping into the Mn²⁺ (~0.083 nm) / Fe²⁺ (~0.078 nm) sites effectively suppresses Mn dissolution, effectively supports the crystal framework, suppresses lattice distortion and the Jamie-Taylor effect during charge and discharge, and improves structural stability and the material's cycle stability. Simultaneously, in this invention, the continuous concentration gradient from the material core to the shell avoids the sharp compositional interfaces present in traditional core-shell structures, enabling a smooth transition of volume changes during charge and discharge, significantly reducing internal stress, and preventing particle cracking during long-term cycling, thereby achieving an ultra-long cycle life.

[0034] Secondly, the interface stability is improved. In this invention, the high Mn content in the core material provides the material with high capacity and high operating voltage; the low Mn and high Fe content in the shell, due to the more stable Fe²⁺ / Fe³⁺ redox couple, effectively reduces the side reactions between the particle surface and the electrolyte, thereby improving the interface stability and cycle life.

[0035] Third, Zr has excellent rate performance. 4 ⁺Due to its high electricity price and large radius, it may partially replace P. 5 The ⁺ sites (forming [ZrO6] octahedra) or tend to segregate at grain boundaries, which effectively pins the grain boundaries and inhibits excessive grain growth during high-temperature sintering, thereby obtaining finer and more uniform primary nanocrystals, shortening the Na⁺ diffusion path, and improving rate performance. At the same time, this invention increases structural defects and changes carrier concentration through heteroelement doping and gradient structure design, effectively reducing the electron transition energy barrier and improving the electronic conductivity of the material. In addition, the introduction of elements with different ionic radii broadens the ion diffusion channels, which is also conducive to the rapid insertion and extraction of sodium ions in the structure, thus improving the rate performance of the material. Detailed Implementation

[0036] 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.

[0037] This invention relates to a magnesium / zirconium co-doped sodium manganese phosphate pyrophosphate material with a manganese / iron concentration gradient, the chemical formula of which is Na. 4+z (Mn x Fe 1-x ) 3-y Mg y P 4-z Zr z O 15 The 0.5 ≤ x ≤ 0.95, 0 < y ≤ 0.05, and 0 < z ≤ 0.03 are specified. The sodium manganese iron pyrophosphate material is a microspherical secondary particle with a continuous Mn / Fe concentration gradient from the core to the shell. Mg and Zr elements are uniformly distributed throughout the secondary particle.

[0038] Preferably, in the microspherical secondary particles, the x-value at the core is 0.8-0.95, and the x-value at the shell is 0.5-0.7, with the x-value decreasing continuously from the core to the shell.

[0039] Another aspect of the present invention relates to a method for preparing the above-mentioned sodium manganese phosphate pyrophosphate material, comprising the following steps:

[0040] S1. Preparation of Mn-rich ion solution: Dissolve manganese source, iron source, zirconium dopant and magnesium dopant in water to form Mn-rich ion solution;

[0041] S2. Preparation of Fe-rich solution: Dissolve manganese source, iron source, zirconium dopant and magnesium dopant in water to form Fe-rich solution;

[0042] S3. Preparation of gradient precursors: In a buffer solution, a controlled co-precipitation method was used to prepare hydroxide precursor microspheres with a core-shell concentration gradient structure by controlling the relative flow rates of Mn-rich and Fe-rich solutions through a program.

[0043] S4. Preparation of precursor slurry: The above gradient precursor is mixed with sodium source, phosphorus source and carbon source and water to obtain a uniform precursor slurry.

[0044] S5. Preparation of precursor dry powder: The above precursor slurry is dried to remove moisture and obtain dry precursor powder.

[0045] S6. High-temperature sintering: Under a protective atmosphere, the above precursor powder is calcined at high temperature, and after natural cooling, Na is obtained. 4+z (Mn x Fe 1-x ) 3-y Mg y P 4-z Zr z O 15 / C material.

[0046] Preferably, in the Mn-rich solution, the molar ratio of Mn to Fe is (8-9.5):(0.5-2); and in the Fe-rich solution, the molar ratio of Mn to Fe is (0.5-2):(8-9.5).

[0047] Preferably, in step S3, the buffer solution is an ammonia-ammonium salt buffer system, the reaction pH is controlled at 9.0-13.0, and the reaction temperature is 30-80℃. Within this pH and temperature range, the nucleation rate of co-precipitated hydroxides and the uniformity of crystal epitaxial growth are better, which plays a decisive role in the subsequent formation of the concentration gradient sodium manganese phosphate pyrophosphate phase.

[0048] Preferably, in step S3, the total flow rate of the Mn-rich solution and the Fe-rich solution is constant. The flow rate of the Mn-rich solution is linearly reduced from its initial maximum value to zero by program control, while the flow rate of the Fe-rich solution is linearly increased from zero to its maximum value.

[0049] Preferably, in step S4, the molar ratio of Na in the sodium source to the total metal ions (Mn+Fe+Mg) in the precursor during mixing is (4.0-4.2):1, and the molar ratio of P in the phosphorus source to the total metal ions is (4-2):1.

[0050] Preferably, in step S6, the solid-state sintering conditions are: a high-temperature calcination temperature of 500-700℃ and a holding time of >0.1H; within this temperature range, elements such as sodium and phosphorus can dissolve and migrate uniformly into the interior of the secondary microspheres, and combine with transition metal ions to form crystal nuclei, growing into a relatively complete material with a gradient distribution of transition metal concentration; the protective atmosphere is one or two of nitrogen, argon, nitrogen-hydrogen, or argon-hydrogen mixture, all of which are non-oxidizing gases.

[0051] Preferably, in step S5, the drying method is one or more of spray drying, flash drying, and vacuum drying.

[0052] Preferably, the manganese source is one or more of manganese sulfate, manganese acetate, manganese chloride, manganese citrate, and manganese gluconate.

[0053] Preferably, the iron source is one or more of ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, ferric acetate, ferrous ammonium sulfate, and ferric citrate.

[0054] Preferably, the magnesium dopant is one or more of magnesium sulfate, magnesium acetate, magnesium citrate, and magnesium chloride.

[0055] Preferably, the zirconium dopant is one or more of zirconium oxychloride, zirconium sulfate, zirconium acetate, ammonium zirconium carbonate, and zirconium citrate.

[0056] Preferably, the sodium source is one or more of sodium formate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium oxalate, sodium citrate, and sodium nitrate.

[0057] Preferably, the phosphorus source is one or more of phosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0058] Preferably, the carbon source is one or more of citric acid, glucose, sucrose, maltose, and soluble starch.

[0059] Application Example 1: Concentration Gradient Na 4.01 (Mn 0.6 Fe 0.4 ) 2.98 Mg 0.02 P 3.99 Zr 0.01 O 15 Synthesis and Electrochemical Properties of @C Materials

[0060] This embodiment involves a concentration gradient Na 4.01 (Mn 0.6 Fe 0.4 ) 2.98 Mg 0.02 P 3.99 Zr 0.01 O 15 @C material, the preparation method of which includes the following steps:

[0061] Step 1: Dissolve manganese acetate, ferrous sulfate, magnesium acetate, and zirconium acetate in water at a molar ratio of 9.0:1.0:0.0067:0.0034 to form a transparent Mn-rich solution;

[0062] Step 2: Dissolve manganese acetate, ferrous sulfate, magnesium acetate, and zirconium acetate in water at a molar ratio of 1.0:9.0:0.0067:0.0034 to form a transparent Fe ion-rich solution;

[0063] Step 3: In an ammonia-ammonium salt buffer system with a pH of 10, the temperature is controlled at 50℃. The relative flow rates of the Mn-rich solution and the Fe-rich solution are controlled by a program so that the flow rate of the Mn-rich solution linearly decreases from the initial maximum value to zero, while the flow rate of the Fe-rich solution linearly increases from zero to the maximum value to prepare hydroxide precursor microspheres with a core-shell concentration gradient structure.

[0064] Step 4: Add water to sodium formate, phosphoric acid, and precursor at a molar ratio of Na:P:total metal ions (Mn+Fe+Mg):4.1:3.99:1 and stir. At the same time, add glucose as a carbon source (the amount added is 5wt% of the total weight of the above precursors) to obtain a uniform precursor slurry.

[0065] Step 5: Spray dry the above precursor slurry with an inlet air temperature of 300°C and an outlet air temperature of 100°C to remove moisture and obtain dry precursor powder.

[0066] Step 6: Under a nitrogen atmosphere, the above precursor powder is calcined at 600°C for 8 hours, and then allowed to cool naturally to obtain Na. 4.01 (Mn 0.6 Fe 0.4 ) 2.98 Mg 0.02 P 3.99 Zr 0.01 O 15 @C material.

[0067] The test results in Table 1 show that Na 4.01 (Mn 0.6 Fe 0.4 ) 2.98 Mg 0.02 P 3.99 Zr 0.01 O 15 The electronic conductivity of the @C material in powder form reaches 8.5*10⁻⁶. -3 The S / cm ratio is five orders of magnitude higher than that of the materials prepared by the liquid-phase method in Comparative Example 1 and the solid-phase method in Comparative Example 2. This indicates that the doping of impurity elements such as Mg and Zr improves the energy level structure of elements such as Mn and Fe in the structure, narrows the band gap, lowers the electron transport barrier, and effectively improves the electron conductivity inside the material. Furthermore, under a pressure of 220 MPa, Na… 4.01 (Mn 0.6 Fe 0.4 ) 2.98 Mg 0.02 P 3.99 Zr 0.01 O 15 The powder compaction density of material @C is 2.35 g / cm³. 3 The material prepared by the liquid phase method in Comparative Example 1 is significantly superior to that prepared by the liquid phase method. This significant difference comes from the construction of hydroxide precursor microspheres with a concentration gradient structure. These microspheres act as templates during sintering, guiding the nucleation and growth of the material, promoting the growth of primary grains, forming large single crystals, reducing the porosity inside the grains, increasing the overall density of the material, and thus improving its compaction density.

[0068] Will Na 4.01 (Mn 0.6 Fe 0.4 ) 2.98 Mg 0.02 P 3.99 Zr 0.01 O15 @C material, SurP PVDF5130 according to the mass ratio is Mix NMP in a ratio of 9.3:0.4:0.3, and use a high-speed homogenizer to thoroughly mix the above materials to form a homogeneous and fluid mixture. Black slurry with high mobility, The black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used. The membrane was assembled into a CR2016 button cell in a glove box.

[0069] Table 1 shows the electrochemical performance test results for Na. 4.01 (Mn 0.6 Fe 0.4 ) 2.98 Mg 0.02 P 3.99 Zr 0.01 O 15 The @C electrode exhibits a discharge specific capacity of 125.7 mAh / g and a median discharge voltage of 3.32 V at a rate of 0.1C (1C = 129 mAh / g), significantly higher than the material in the comparative example. This may be related to the uniformity of elemental dispersion in the hydroxide precursor microspheres with a concentration gradient structure, which enhances the performance of the prepared material. Compared to the liquid-phase method, the material prepared by this method has larger single-crystal particles, more complete crystal structure, and higher activity of sodium storage sites. In contrast, the solid-phase method cannot guarantee elemental uniformity during the grinding process, resulting in a high impurity content and poor performance. Furthermore, Table 1 shows that the capacity retention rate of this electrode at 10C is as high as 95.7% compared to 0.1C, far exceeding that of the material in the comparative example. This is partly due to the modulation of the electron transition energy barrier by impurity elements in the material structure, effectively reducing the charge transfer internal resistance and thus improving the electron transport capability of the material. On the other hand, Zr... 4 The higher charge and larger radius of Na⁺ tend to segregate at grain boundaries, which effectively pins the grain boundaries and inhibits excessive grain growth during high-temperature sintering, resulting in finer, more uniform primary grains, shortening the Na⁺ diffusion path, and thus improving its rate performance. Ultimately, at 1C rate, after 1000 cycles, this electrode maintained a capacity retention of 98.5%, with less capacity decay than Comparative Examples 1 and 2. This may be related to the material's low polarization and high structural stability. The doping of impurity elements suppressed lattice distortion and the Jameer-Taylor effect of Mn during charge and discharge. Simultaneously, the low Mn and high Fe content in the outer shell, due to the more stable Fe²⁺ / Fe³⁺ redox couple, effectively reduced side reactions between the particle surface and the electrolyte, improving interface stability and cycle life.

[0070] Application Example 2: Concentration Gradient Na 4.025 (Mn0.8 Fe 0.2 ) 2.96 Mg 0.04 P 3.975 Zr 0.025 O 15 Synthesis and Electrochemical Properties of @C Materials

[0071] This embodiment involves a concentration gradient Na 4.025 (Mn 0.8 Fe 0.2 ) 2.96 Mg 0.04 P 3.975 Zr 0.025 O 15 @C material, the preparation method of which includes the following steps:

[0072] Step 1: Dissolve manganese sulfate, ferrous sulfate, magnesium sulfate, and zirconium sulfate in water at a molar ratio of 9.5:0.5:0.0135:0.00845 to form a transparent Mn-rich solution.

[0073] Step 2: Dissolve manganese acetate, ferrous sulfate, magnesium acetate, and zirconium acetate in water at a molar ratio of 0.5:9.5:0.0135:0.00845 to form a transparent Fe ion-rich solution.

[0074] Step 3: In an ammonia-ammonium salt buffer system with a pH of 12, the temperature is controlled at 70℃. The relative flow rates of the Mn-rich solution and the Fe-rich solution are controlled by a program so that the flow rate of the Mn-rich solution linearly decreases from the initial maximum value to zero, while the flow rate of the Fe-rich solution linearly increases from zero to the maximum value to prepare hydroxide precursor microspheres with a core-shell concentration gradient structure.

[0075] Step 4: Add water to sodium hydroxide, phosphoric acid, and precursor at a molar ratio of Na:P:total metal ions (Mn+Fe+Mg):4.15:3.975:1 and stir. At the same time, add sucrose as a carbon source (the amount added is 4wt% of the total weight of the above precursors) to obtain a uniform precursor slurry.

[0076] Step 5: Spray dry the above precursor slurry with an inlet air temperature of 300°C and an outlet air temperature of 100°C to remove moisture and obtain dry precursor powder.

[0077] Step 6: Under a nitrogen atmosphere, the above precursor powder is calcined at 650°C for 10 hours, and then allowed to cool naturally to obtain Na. 4.025 (Mn 0.8 Fe 0.2 ) 2.96 Mg 0.04 P 3.975 Zr 0.025 O15 @C material.

[0078] The test results in Table 1 show that Na 4.025 (Mn 0.8 Fe 0.2 ) 2.96 Mg 0.04 P 3.975 Zr 0.025 O 15 The electronic conductivity of the @C material in powder form reaches 4.1*10⁻⁶. -4 Compared to the liquid-phase method used in Comparative Example 1 and the solid-phase method used in Comparative Example 2, the material prepared in this study achieved a significant improvement of up to five orders of magnitude in electronic conductivity. This result indicates that by effectively doping with impurity elements such as Mg and Zr, the electronic energy level structure of key elements such as Mn and Fe in the material was significantly optimized, resulting in a smaller band gap and a lower energy barrier for electron transitions and transport. This significantly enhanced the electron migration efficiency within the material, ultimately leading to a qualitative leap in its conductivity. Furthermore, under a pressure of 220 MPa, Na… 4.025 (Mn 0.8 Fe 0.2 ) 2.96 Mg 0.04 P 3.975 Zr 0.025 O 15 The powder compaction density of material @C is 2.30 g / cm³. 3 Compared to the liquid-phase material in Comparative Example 1, this material exhibits a significant performance leap, primarily due to the introduction of hydroxide precursor microspheres with a concentration gradient structure. This special precursor acts as a self-sacrificing template during heat treatment, inducing the directional alignment and growth of crystal nuclei, preferentially promoting the merging and growth of primary grains, directly resulting in large single crystals with fewer internal pores and higher crystallinity. This fundamental structural optimization significantly enhances the overall density of the material, ultimately manifesting as a substantial breakthrough in compaction density. While the compaction density is comparable to that of the material prepared by the solid-phase method in Comparative Example 2, indicating that the material prepared by the pure solid-phase method also has high density, its poor homogeneity leads to suboptimal material performance.

[0079] Will Na 4.025 (Mn 0.8 Fe 0.2 ) 2.96 Mg 0.04 P 3.975 Zr 0.025 O 15 @C material SurP and PVDF5130 are in the following mass ratios: Mix NMP in a ratio of 9.4:0.3:0.3, and use a high-speed homogenizer to thoroughly mix the above materials to form a homogeneous and fluid mixture. Black slurry with high mobility,The black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used. The membrane was assembled into a CR2016 button cell in a glove box.

[0080] Table 1 shows the electrochemical performance test results for Na. 4.025 (Mn 0.8 Fe 0.2 ) 2.96 Mg 0.04 P 3.975 Zr 0.025 O 15 The @C electrode exhibits a discharge capacity of 123.1 mAh / g and a median discharge voltage of 3.42 V at a rate of 0.1C (1C = 129 mAh / g), significantly outperforming the materials in the comparative examples. This result can be attributed to the uniform elemental dispersion provided by the concentration gradient precursor microspheres, which effectively enhances the material's performance. Compared to the liquid-phase method, this method produces larger, more crystalline single-crystal particles with higher activity at sodium storage sites. In contrast, the solid-phase method suffers from poor performance due to the difficulty in ensuring uniform elemental mixing during the grinding process, resulting in a high impurity content in the product. Furthermore, the data in Table 1 show that the electrode maintains a capacity retention of 96.2% at a high rate of 10C, far exceeding the performance of the two comparative materials. Its superior rate performance is mainly attributed to two mechanisms: firstly, the doping of impurity elements modulates the electronic band structure, lowers the electronic transition energy barrier, thereby effectively reducing charge transfer resistance and enhancing electronic conductivity; secondly, Zr... 4 Due to its high valence and large ionic radius, Na⁺ tends to segregate at grain boundaries, resulting in a grain boundary pinning effect. This suppresses excessive grain growth during high-temperature sintering, helps form fine and uniform primary grains, shortens the Na⁺ diffusion path, and further improves the material's rate performance. Regarding long-term cycle stability, this electrode maintained a capacity retention of 97.9% after 1000 cycles at 1C, with a significantly lower rate decay than the comparative example. This is mainly attributed to the material's low overall polarization and high structural stability. Specifically, the doping of impurity elements effectively suppressed lattice distortion and the Jameer-Taylor effect of Mn³⁺ during charge and discharge. Simultaneously, the low Mn and high Fe composition of the outer shell, utilizing the high electrochemical stability of the Fe²⁺ / Fe³⁺ couple, reduced side reactions between the particle surface and the electrolyte, thus significantly improving interfacial stability and cycle life.

[0081] Comparative Example 1: Preparation of Na by Liquid Phase Method 4.0 (Mn 0.6 Fe0.4 ) 3.0 P 4.0 O 15 Synthesis and Electrochemical Properties of @C Materials

[0082] This embodiment involves Na 4.0 (Mn 0.6 Fe 0.4 ) 3.0 P 4.0 O 15 @C material, the preparation method of which includes the following steps:

[0083] Step 1: Dissolve manganese acetate, ferrous sulfate, phosphoric acid, and sodium acetate in water at a molar ratio of 1.8:1.2:4:4. Add glucose (5 wt% of the total solid content) as a carbon source to form a transparent brownish-red precursor solution.

[0084] Step 2: Spray dry the above precursor solution with an inlet air temperature of 300°C and an outlet air temperature of 100°C to remove moisture from the slurry and obtain dried precursor powder.

[0085] Step 3: Calcine the above precursor powder at 600℃ for 8 hours, then allow it to cool naturally to obtain Na. 4.0 (Mn 0.6 Fe 0.4 ) 3.0 P 4.0 O 15 @C material.

[0086] Table 1 shows the results for Na. 4.0 (Mn 0.6 Fe 0.4 ) 3.0 P 4.0 O 15 The electronic conductivity of the C material powder is only 4.7*10. -8 The S / cm ratio is significantly lower than that of the materials in the examples. This phenomenon is closely related to the structure of the material itself. In the sodium manganese iron pyrophosphate structure, the manganese and iron active center ions are separated and surrounded by large, covalently bonded pyrophosphate and phosphate anions. These anions are typical insulators; they act like “electron walls,” hindering direct jumps between different metal ions, thus confining electrons around their respective metal ions and making it difficult to form a delocalized electron cloud. Due to the aforementioned electron localization effect, from a solid-state physics perspective, the band gap between the valence band top and conduction band bottom of this material is relatively large. At room temperature, electrons in the valence band have difficulty gaining enough energy to jump to the conduction band, thus failing to participate in conduction. Furthermore, at a pressure of 220 MPa, Na… 4.0 (Mn 0.6 Fe 0.4 )3.0 P 4.0 O 15 The powder compaction density of material @C is 2.03 g / cm³. 3 The density is lower than that of the material in the examples. The main reason is that the material formed by the liquid phase method has finer particles and more micropores between the particles, which leads to a decrease in the density of the material and ultimately affects its compaction density.

[0087] Will Na 4.0 (Mn 0.6 Fe 0.4 ) 3.0 P 4.0 O 15 @C material SurP and PVDF5130 are mixed in a mass ratio of 9.4:0.3:0.3. The above materials are mixed with NMP in a specific ratio and then homogenized using a high-speed homogenizer to form a black mixture with a uniform color and high fluidity. Slurry, of The black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, and 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte. The separator was a PP / PE / PP three-layer separator. The membrane was assembled into a CR2016 button cell in a glove box.

[0088] Table 1 shows the Na 4.0 (Mn 0.6 Fe 0.4 ) 3.0 P 4.0 O 15The key electrochemical performance of the @C electrode is as follows: Under low-rate conditions of 0.1C (1C = 129 mAh / g), the electrode exhibits a discharge specific capacity of only 95.6 mAh / g, indicating low capacity utilization; simultaneously, its average discharge voltage is only 3.15V. This poor capacity and voltage performance is mainly attributed to the numerous impurity phases introduced during the liquid-phase preparation process. During the liquid-phase preparation of this material, the binding energy between sodium, iron, and phosphate ions is strong in the high-temperature sintering stage, making it easier to form thermodynamically stable sodium manganese pyrophosphate or sodium iron pyrophosphate phases. The presence of these impurity phases disrupts the stoichiometric balance of iron, manganese, sodium, phosphate, and pyrophosphate ions in local regions, thereby inducing the formation of various impurity phases. This results in a complex and inhomogeneous crystal phase composition in the final product. The presence of these impurity phases not only reduces the effective charge per unit mass of material that can participate in redox reactions, causing a decrease in capacity, but also exacerbates electrode polarization due to the hindering effect of grain boundaries between different crystal phases on ion migration, thus leading to a decrease in discharge voltage. Furthermore, Table 1 shows that at a high rate of 10C, the capacity retention of this electrode relative to 0.1C is only 64.9%. This indicates that the low intrinsic electronic conductivity and high impurity phase content of the material lead to a significant increase in the energy barriers for electron transport and sodium ion migration across different crystal phase interfaces. This not only increases the internal resistance of the system but also prolongs the ion migration time, ultimately resulting in a significant deterioration in rate performance. Finally, after 1000 cycles at a rate of 1C, the capacity retention of this electrode is only 68.4%, exhibiting severe capacity decay. This phenomenon is closely related to the uneven volume change rate of each phase region during sodium insertion / extraction processes due to the coexistence of multiple phases in the material. Due to the differences in expansion / contraction behavior between different phases, microcracks are easily generated at grain boundaries. These cracks not only destroy the structural integrity but also provide channels for electrolyte penetration and interfacial side reactions, thereby accelerating capacity decay and significantly reducing the cycling stability of the material. In addition, the Jameer-Taylor effect of Mn ions in the material structure and their dissolution in the electrolyte also exacerbate the collapse of the material structure and worsen the cycling stability of the material.

[0089] Comparative Example 2: Preparation of Na by Solid-State Method 4.0 (Mn 0.8 Fe 0.2 ) 3.0 P 4.0 O 15 @C materials and their electrochemical properties

[0090] This embodiment involves Na 4.0 (Mn 0.8 Fe 0.2 ) 3.0 P 4.0 O 15 @C material, the preparation method of which includes the following steps:

[0091] Step 1: Add manganese sulfate, ferrous sulfate, pyrophosphate, and sodium acetate to a grinding jar in a molar ratio of 2.4:0.6:2:4. Use sucrose (4 wt% of the total mass of the solids) as a carbon source. Perform high-energy grinding with a ball-to-particle ratio of 20:1 and a grinding time of 2 hours to form a uniform light yellow precursor powder.

[0092] Step 2: Calcine the above precursor powder at 650℃ for 10 hours, then allow it to cool naturally to obtain Na. 4.0 (Mn 0.8 Fe 0.2 ) 3.0 P 4.0 O 15 @C material.

[0093] The test results in Table 1 show that Na 4.0 (Mn 0.8 Fe 0.2 ) 3.0 P 4.0 O 15 The electronic conductivity of the @C material in powder form is only 3.8*10. -10 The conductivity (S / cm) is significantly lower than that of the material in the application examples. This insufficient conductivity stems from microstructural defects and intrinsic electronic properties formed during the synthesis process. At the microscale, the solid-state synthesis route has inherent limitations: manganese acetate and ferrous acetate, as metal sources, are difficult to mix uniformly at the molecular level during mechanical grinding, resulting in spatial heterogeneity of manganese and iron elements in the precursor. This initial inhomogeneity is further amplified during subsequent heat treatment, ultimately forming compositional fluctuations and localized enrichment regions in the crystal lattice, disrupting the continuity of the conductive network. At the electronic scale, the d-electron clouds of transition metal ions exhibit highly localized characteristics with low orbital overlap, failing to form widely delocalized electronic channels. Electrons must repeatedly traverse these localized regions via thermally activated transitions, facing high effective energy barriers, leading to impaired overall conduction paths and significantly increased charge migration resistance, ultimately resulting in a significant decrease in the material's macroscopic electronic conductivity.

[0094] Will Na 4.0 (Mn 0.8 Fe 0.2 ) 3.0 P 4.0 O 15 @C material SurP and PVDF5130 are mixed in a mass ratio of 9.3:0.4:0.3. The above materials are mixed with NMP in a specific ratio and then homogenized using a high-speed homogenizer to form a black mixture with a uniform color and high fluidity. Slurry, ofThe black paste was then coated onto aluminum foil using a 150µm four-sided coating apparatus, and the membrane was dried in a vacuum drying oven at 100℃ for 2 hours. The electrode membrane was punched into a disc with a radius of 0.6mm using a punching machine. Sodium metal was used as the counter electrode, and 1mol / L NaClO4EC+DEC (1:1 vol%)+5% FEC was used as the electrolyte. The separator was a PP / PE / PP three-layer separator. The membrane was assembled into a CR2016 button cell in a glove box.

[0095] Table 1 shows the Na prepared by the solid-state method. 4.0 (Mn 0.8 Fe 0.2 ) 3.0 P 4.0 O 15The electrochemical performance of the @C electrode at 0.1C rate (1C = 129 mAh / g) was tested. The results showed that the electrode's discharge specific capacity was only 74.5 mAh / g, and the average discharge voltage was 2.83 V, significantly lower than the corresponding material in the application example in both capacity and voltage. This performance gap mainly stems from the uneven distribution of manganese and iron ions at the microscale within the material. During solid-state preparation, the limitations of ion-level mixing led to phase separation and regional enrichment of transition metal elements during sintering, resulting in localized charge distribution. This impurity phase and structural inhomogeneity, on the one hand, inhibited the effective redox reaction of manganese ions in electrochemical cycling, reducing the utilization rate of active sites and leading to insufficient capacity utilization; on the other hand, the localized charge distribution also hindered the continuous transport of electrons and sodium ions within the material, increasing the internal resistance of the electrode reaction, exacerbating polarization, and causing a decrease in the discharge voltage plateau. The electrochemical data in Table 1 further indicate that the capacity retention rate of this electrode at a high rate of 10C is only 53.7%, far lower than the material in application example 1. The poor rate performance can also be attributed to the presence of impurity phases and insufficient uniformity of manganese and iron ion mixing in the material. Impurity phases and inhomogeneous structures disrupt the continuity of electron conduction paths, forming numerous electrically insulating regions and significantly increasing the internal resistance of the electrode. Under high-rate discharge conditions, the rapidly increasing current load induces strong polarization, limiting the rapid insertion / extraction kinetics of sodium ions, ultimately leading to a significant decrease in capacity retention. Furthermore, after 1000 cycles at 1C, the capacity retention of this electrode is only 42.1%, a significantly higher rate of decay than in Application Example 1. The poor cycle stability is due to the amplification of impurity phases and uneven elemental distribution introduced during solid-state synthesis during long-term cycling. This microstructural inhomogeneity causes inconsistent lattice volume changes in different regions during repeated sodium ion insertion / extraction, generating significant internal stress. The continuous accumulation of stress easily induces microcracks and structural pulverization in active particles, which not only disrupts the conductive paths but also continuously exposes new active surfaces, exacerbating side reactions. Meanwhile, structural instability also significantly increases the dissolution rate of manganese ions, causing loss of active materials and irreversible damage to the crystal structure. Multiple mechanisms work together to ultimately lead to accelerated capacity decay and significant deterioration of cycling performance.

[0096] Table 1 Performance Test Results

[0097] Application Example 1 Comparative Example 1 Application Example 2 Comparative Example 2 <![CDATA[Compaction density of the material powder at 220 MPa (g / cm 3 )]]> 2.35 2.03 2.30 2.29 Electron conductivity (S / cm) <![CDATA[8.5*10 -3 ]]> <![CDATA[4.7*10 -8 ]]> <![CDATA[4.1*10 -4 ]]> <![CDATA[3.8*10 -10 ]]> Discharge median voltage (V) 3.32 3.15 3.42 2.83 0.1C discharge specific capacity (mAh / g) 125.7 95.6 123.1 74.5 Capacity retention at 10C rate (10C / 0.1C %) 95.7 64.9 96.2 53.7 Retention rate after 1000 cycles at 1C ratio (%) 98.5 68.4 97.9 42.1

[0098] 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 magnesium / zirconium co-doped sodium manganese phosphate pyrophosphate material with a manganese / iron concentration gradient, characterized in that: the chemical formula of the sodium manganese phosphate pyrophosphate material is Na 4+z (Mn x Fe 1-x ) 3-y Mg y P 4-z Zr z O 15 , where 0.5 ≤ x ≤ 0.95, 0 < y ≤ 0.05, 0 < z ≤ 0.03; the sodium manganese iron pyrophosphate material is a microspherical secondary particle with a continuous Mn / Fe concentration gradient from the core to the shell, and Mg and Zr elements are uniformly distributed throughout the secondary particle.

2. The sodium manganese phosphate pyrophosphate material with magnesium / zirconium co-doping and manganese / iron concentration gradient according to claim 1, characterized in that: In the microspherical secondary particles, the x-value at the core is 0.8-0.95, and the x-value at the shell is 0.5-0.7, with the x-value decreasing continuously from the core to the shell.

3. A method for preparing the sodium manganese phosphate pyrophosphate material according to claim 1 or 2, characterized in that... Includes the following steps: S1. Preparation of Mn-rich ion solution: Dissolve manganese source, iron source, zirconium dopant and magnesium dopant in water to form Mn-rich ion solution; S2. Preparation of Fe-rich solution: Dissolve manganese source, iron source, zirconium dopant and magnesium dopant in water to form Fe-rich solution; S3. Preparation of gradient precursors: In a buffer solution, a controlled co-precipitation method was used to prepare hydroxide precursor microspheres with a core-shell concentration gradient structure by controlling the relative flow rates of Mn-rich and Fe-rich solutions through a program. S4. Preparation of precursor slurry: The above gradient precursor is mixed with sodium source, phosphorus source and carbon source and water to obtain a uniform precursor slurry. S5. Preparation of precursor dry powder: The above precursor slurry is dried to obtain dry precursor powder. S6. High-temperature sintering: Under a protective atmosphere, the above precursor powder is calcined at high temperature, and after natural cooling, Na is obtained. 4+z (Mn x Fe 1-x ) 3-y Mg y P 4-z Zr z O 15 / C material.

4. The method for preparing sodium manganese ferric pyrophosphate material according to claim 3, characterized in that: In Mn-rich solutions, the molar ratio of Mn to Fe is (8-9.5):(0.5-2); in Fe-rich solutions, the molar ratio of Mn to Fe is (0.5-2):(8-9.5).

5. The method for preparing sodium manganese phosphate pyrophosphate material according to claim 3, characterized in that: In step S3, the buffer solution is an ammonia-ammonium salt buffer system, the reaction pH is controlled between 9.0 and 13.0, and the reaction temperature is between 30 and 80°C.

6. The method for preparing sodium manganese ferric pyrophosphate material according to claim 3, characterized in that: In step S3, the total flow rates of the Mn-rich solution and the Fe-rich solution are kept constant. The flow rate of the Mn-rich solution is linearly reduced from its initial maximum value to zero by the program, while the flow rate of the Fe-rich solution is linearly increased from zero to its maximum value.

7. The method for preparing sodium manganese ferric pyrophosphate material according to claim 3, characterized in that: In step S4, the molar ratio of Na in the sodium source to the total metal ions (Mn+Fe+Mg) in the precursor during mixing is (4.0-4.2):1, and the molar ratio of P in the phosphorus source to the total metal ions is (4-2):

1.

8. The method for preparing sodium manganese ferric pyrophosphate material according to claim 3, characterized in that: In step S6, the solid-state sintering conditions are: high-temperature calcination temperature of 500-700℃, holding time > 0.1H, and protective atmosphere of nitrogen, argon, nitrogen-hydrogen, or a mixture of argon and hydrogen.

9. The method for preparing sodium manganese ferric pyrophosphate material according to claim 3, characterized in that: In step S5, the drying method is one or more of spray drying, flash drying, and vacuum drying.

10. The method for preparing sodium manganese phosphate pyrophosphate material according to claim 3, characterized in that: The manganese source is one or more of manganese sulfate, manganese acetate, manganese chloride, manganese citrate, and manganese gluconate; The iron source is one or more of the following: ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, ferric acetate, ferrous ammonium sulfate, and ferric citrate. The magnesium dopant is one or more of magnesium sulfate, magnesium acetate, magnesium citrate, and magnesium chloride; The zirconium dopant is one or more of zirconium oxychloride, zirconium sulfate, zirconium acetate, ammonium zirconium carbonate, and zirconium citrate; The sodium source is one or more of sodium formate, sodium acetate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium oxalate, sodium citrate, and sodium nitrate. The phosphorus source is one or more of the following: phosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate. The carbon source is one or more of citric acid, glucose, sucrose, maltose, and soluble starch.