A modified sodium iron phosphate pyrophosphate cathode material, its preparation method and application

Abstract

The present invention relates to the technical field of sodium ion batteries, and particularly to a modified sodium iron pyrophosphate cathode material, a preparation method thereof, and an application thereof. The modified sodium iron pyrophosphate cathode material comprises a Na x Fe y Al z (PO4)2P2O7 core and a NaAlO2 coating layer coated on its surface; in the Na x Fe y Al z (PO4)2P2O7 core, the content of Al 3+ decreases in a gradient from the outer surface layer to the core of the core, wherein 4≤x≤4.18, 2.9≤y+z≤3, 0<z≤0.1, and x+2y+3z=10. The modified sodium iron pyrophosphate cathode material provided by the present invention has a high tap density, good structural stability, thermal stability and chemical stability, can effectively avoid the dissolution of Fe 2+ , and at the same time has excellent conductivity, a relatively high discharge capacity, and excellent rate performance and cycle performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a modified sodium iron phosphate pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] Compared with traditional lithium-ion batteries, sodium has inherent advantages such as low resource cost, wide distribution and abundant reserves. Therefore, sodium-ion batteries are expected to replace lithium-ion batteries and achieve large-scale application. Among them, the development of electrode materials is one of the most important goals in improving sodium-ion batteries.

[0003] Among sodium-ion battery cathode materials, sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP) has advantages such as low raw material cost, stable structure, high theoretical capacity (~129mAh / g), and high operating voltage (~3.1V vs. Na2PO4). + With advantages such as low sodium-ion battery density (Na₂O) and low volume expansion (~4%), NFPP materials are considered the most promising cathode material for sodium-ion batteries. However, existing NFPP materials still suffer from problems such as low compaction density, poor high-temperature cycling stability, and easy dissolution of Fe elements in the electrolyte.

[0004] Therefore, it is desirable to provide a modified sodium iron phosphate pyrophosphate cathode material to solve the above problems. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a modified sodium iron phosphate pyrophosphate cathode material, its preparation method, and its applications. The modified sodium iron phosphate pyrophosphate cathode material provided by this invention exhibits high compaction density, good structural stability, thermal stability, and chemical stability, and can effectively prevent Fe... 2+ It dissolves while exhibiting excellent conductivity, high discharge capacity, and excellent rate performance and cycle performance.

[0006] In a first aspect, the present invention provides a modified sodium iron phosphate pyrophosphate cathode material, wherein the modified sodium iron phosphate pyrophosphate cathode material comprises Na x Fe y Al z (PO4)2P2O7 core and NaAlO2 coating layer covering its surface;

[0007] The Na x Fe y Al z Al in the (PO4)2P2O7 kernel 3+ The content of decreases from the outer layer of the core to the core, where 4≤x≤4.18, 2.9≤y+z≤3, and 0 <z≤0.1,x+2y+3z=10。

[0008] Where x can be 4, 4.05, 4.1, 4.15, 4.18, etc.; z can be 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, etc.; and y+z can be 2.9, 2.92, 2.94, 2.96, 2.98, 3, etc.

[0009] The modified sodium iron phosphate pyrophosphate cathode material provided by this invention has high compaction density, good structural stability, thermal stability, and chemical stability, and can effectively avoid Fe 2+ The dissolution process exhibits excellent conductivity, high discharge capacity, and superior rate and cycle performance. Specifically:

[0010] The modified sodium iron phosphate pyrophosphate cathode material provided by this invention includes Al 3+ Gradient-doped sodium iron phosphate pyrophosphate core and sodium aluminate coating layer on its surface:

[0011] 1. Structural Stability: The crystal structure of sodium aluminate differs from that of NFPP. The NaAlO2 coating layer interacts with the NFPP lattice to a certain extent, providing support and helping the NFPP maintain the integrity of its crystal structure during charging and discharging. This reduces lattice distortion caused by sodium ion insertion and extraction, thereby improving structural stability. Simultaneously, the sodium aluminate coating layer also acts as a buffer between the NFPP and the electrolyte, reducing direct contact between them and thus preventing Fe... 2+ The dissolution of sodium aluminate reduces side reactions, contributing to the formation of a more stable SEI film and improving battery cycle performance and coulombic efficiency. Furthermore, the sodium aluminate coating alters the charge distribution on the NFPP surface, making the electric field on the electrode surface more uniform, which is beneficial for the uniform adsorption and desorption of sodium ions on the electrode surface, improving the kinetics of the electrode reaction. Meanwhile, the gradient-doped Al in the core... 3+ The ability to form more stable chemical bonds with surrounding ions helps to enhance the crystal structure of NFPP. During charging and discharging, it can better resist the structural stress caused by the insertion and extraction of sodium ions, reduce the possibility of lattice distortion and structural collapse, and improve the cycling stability of the material.

[0012] 2. Thermal Stability: The sodium aluminate coating acts as a thermal insulation layer, slowing down heat transfer between the NFPP and the surrounding environment during battery charging and discharging, thus providing a thermal buffer and helping to maintain a relatively stable internal battery temperature, improving thermal stability. Sodium aluminate can also interact with active sites on the NFPP surface, reducing the rate of thermal decomposition reactions and improving the battery's safety and stability under high-temperature environments. Meanwhile, the gradient-doped Al in the core... 3+This can enhance the crystal structure of NFPP, making it less susceptible to thermal decomposition and phase transitions at high temperatures. Simultaneously, the improved electronic structure and chemical bond energies after doping also contribute to enhanced thermal stability, reducing the risk of thermal runaway and other safety issues in batteries at high temperatures, thus improving battery safety and reliability.

[0013] 3. Chemical Stability: The sodium aluminate coating protects NFPP from corrosive or oxidizing components in the electrolyte, improving its chemical stability and preventing performance degradation due to chemical changes during long-term use. Simultaneously, sodium aluminate can regulate the pH value of the electrode surface to a certain extent, maintaining chemical balance and reducing electrode material dissolution or other adverse chemical reactions caused by pH changes, thereby extending battery life.

[0014] 4. Ion Transport Performance: Sodium aluminate itself possesses a certain ion conductivity. As a coating layer, it can form additional ion diffusion channels on the surface of NFPP particles, facilitating the transport of sodium ions within and on the surface of the electrode material, thereby improving the battery's charge / discharge efficiency and rate performance. Simultaneously, the uniform and dense sodium aluminate coating layer can fill some defects and pores on the surface of NFPP particles, preventing impurities or by-reaction products in the electrolyte from blocking the ion transport channels, thus ensuring smoother sodium ion transport. Furthermore, the gradient-doped Al in the core... 3+ The crystal structure of NFPP will be fine-tuned to make the sodium ion transport channels more regular and unobstructed. This can increase the size of the ion channels to a certain extent, reduce the obstacles in the sodium ion transport process, and improve the sodium ion diffusion coefficient, thereby improving the rate performance of the material.

[0015] 5. High compaction density: The modified sodium iron phosphate pyrophosphate cathode material provided by this invention can effectively control particle size and improve compaction density.

[0016] In summary, this invention achieves its goal by coating the surface of the sodium iron phosphate pyrophosphate core with a NaAlO2 coating layer and by gradient doping Al within the sodium iron phosphate pyrophosphate core. 3+ It can effectively isolate the Fe in the sodium iron phosphate core of pyrophosphate 2+ Direct contact with the electrolyte to avoid Fe 2+ The dissolution of sodium iron pyrophosphate effectively improves its conductivity, structural stability, thermal stability, and chemical stability, resulting in the modified sodium iron pyrophosphate cathode material provided by this invention having high discharge capacity and excellent rate performance and cycle performance.

[0017] As a preferred embodiment of the present invention, the NaAlO2 coating layer and Na x Fe y Al zThe mass ratio of (PO4)2P2O7 kernels is 0.5-3:100, such as 0.5:100, 1:100, 2:100, 3:100, etc.

[0018] When the NaAlO2 coating layer is in contact with Na x Fe y Al z When the mass ratio of the (PO4)2P2O7 core is within the above range, it can ① effectively prevent the material from reacting directly with the electrolyte, reduce by-products, and improve battery stability; ② form a conductive network, accelerate electron transport, and improve battery charge and discharge efficiency; ③ reduce volume expansion during charge and discharge, and extend cycle life. If the coating layer is too thin, the protection and conductivity are insufficient, the material is easily corroded by the electrolyte, and the performance improvement is not significant; if the coating layer is too thick, it will hinder sodium ion migration, increase the battery's internal resistance, leading to a decrease in capacity, slower charging, and even affecting the overall energy density of the battery.

[0019] In a second aspect, the present invention provides a method for preparing the modified sodium iron phosphate pyrophosphate cathode material described in the first aspect, the method comprising:

[0020] (1) The iron source, phosphoric acid source and pyrophosphate source are reacted in an aqueous solution to obtain a reaction solution;

[0021] (2) Add aluminum source and oxalic acid source dropwise to the reaction solution to carry out the reaction. After the reaction is complete, add alkaline solution dropwise to carry out the reaction to obtain the precursor.

[0022] (3) The precursor is mixed with sodium source, reducing agent and carbon source to obtain gel-like mixture, dried and sintered to obtain the modified sodium iron phosphate pyrophosphate cathode material.

[0023] This invention utilizes a co-precipitation method to prepare modified sodium iron phosphate pyrophosphate cathode materials. Specifically, this invention first uses an iron source, a phosphoric acid source, and a pyrophosphate source to react in an aqueous solution to obtain H+ with small and uniform particle size. 1-a Na a Fe3(PO4) 4-2b (P2O7) b (0≤a≤1, 0.94≤b<1.00) Crystalline material, then aluminum source and oxalic acid source are slowly added dropwise. Oxalate ions and aluminum ions can combine to form aluminum oxalate chelate [Al(C2O4)3]. 3- And along the above H 1-a Na a Fe3(PO4) 4-2b (P2O7) b Interstitial growth of the crystals occurs, with elements adhering to the surface via hydrogen bonding. Subsequently, an alkaline solution is added, and the Al in the aluminum oxalate chelate... 3+Nucleation and growth were induced in a controlled manner to form an Al(OH)3 coating layer on the surface, resulting in an Al(OH)3-coated NFPP precursor. The Al(OH)3-coated NFPP precursor material was then sintered together with a sodium source and other materials.

[0024] Taking sodium carbonate as an example, the reaction process is as follows:

[0025]

[0026] Taking sodium hydroxide as an example, the reaction process is as follows:

[0027] NaOH + Al(OH)3 → NaAlO2 + 2H2O↑

[0028] Taking sodium bicarbonate as an example, the reaction process is as follows:

[0029] NaHCO3+Al(OH)3→NaAlO2+CO2↑+2H2O↑

[0030] That is, a NaAlO2 coating layer is formed on the surface of NFPP, and at the same time, high-temperature sintering can drive Al 3+ Diffusion to achieve Al in the NFPP kernel 3+ Gradient doping with concentration gradually decreasing from the outside to the inside (from the outer layer of the core to the core).

[0031] In addition, during the sintering process, some phosphate groups will undergo the following reaction:

[0032]

[0033] That is, some phosphate ions are converted into pyrophosphate ions. By pre-controlling the ratio of phosphate ions and pyrophosphate ions in the reaction raw materials, the present invention can obtain the final product n(PO4):n(P2O7)=2:1.

[0034] As a preferred technical solution of the present invention, step (1) involves adding an aqueous solution of phosphate source and pyrophosphate source dropwise to an aqueous solution of iron source for reaction. Otherwise, if the aqueous solution of iron source is added dropwise to the aqueous solution of phosphate source and pyrophosphate source, excess phosphate and pyrophosphate may react with iron to form other impurities such as iron phosphate and iron pyrophosphate.

[0035] This invention controls the droplet acceleration rate at H 1-a Na a Fe3(PO4) 4-2b (P2O7) b (0≤a≤1,0.94≤b<1.00) The size of spherical particles is controlled in the early stage of crystal material formation, thereby reducing the particle size of the final product and increasing the compaction density.

[0036] As a preferred embodiment of the present invention, the pH of the aqueous solution of the iron source is 1-1.5, such as 1, 1.1, 1.2, 1.3, 1.4, 1.5, etc.

[0037] As a preferred embodiment of the present invention, the pH adjuster for adjusting the pH in the aqueous solution of the iron source is selected from any one or more of sulfuric acid (H2SO4), nitric acid (HNO3) and hydrochloric acid (HCl).

[0038] As a preferred embodiment of the present invention, the iron source is selected from any one or more of ferric sulfate (Fe2(SO4)3), ferric nitrate (Fe(NO3)3), and ferric chloride (FeCl3).

[0039] As a preferred embodiment of the present invention, the pH adjuster is selected from the acid corresponding to the anion in the iron source. For example, when the iron source is ferric sulfate, the pH adjuster is sulfuric acid; when the iron source is ferric nitrate, the pH adjuster is nitric acid, so as to minimize the types of impurities in the solution and facilitate subsequent water washing to remove impurities.

[0040] As a preferred embodiment of the present invention, the phosphoric acid source is selected from any one or more of phosphoric acid (H3PO4), trisodium phosphate (Na3PO4), disodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), triammonium phosphate ((NH4)3PO4), diammonium dihydrogen phosphate (NH4H2PO4), and diammonium hydrogen phosphate ((NH4)2HPO4).

[0041] As a preferred embodiment of the present invention, the pyrophosphate source is selected from any one or more of pyrophosphate (H4P2O7), sodium pyrophosphate (Na4P2O7), disodium dihydrogen pyrophosphate (Na2H2P2O7), and ammonium pyrophosphate ((NH4)4P2O7).

[0042] As a preferred embodiment of the present invention, the Fe in the iron source 3+ PO4 in phosphate source 3- and P2O7 in pyrophosphate source 4- The molar ratio is (2.64-2.72):(2.2-2.3):1, for example 2.64:2.2:1, 2.66:2.21:1, 2.68:2.23:1, 2.7:2.25:1, 2.72:2.27:1, 2.64:2.3:1, etc.

[0043] As a preferred technical solution of the present invention, the reaction temperature in step (1) is 40-50℃, such as 40℃, 42℃, 44℃, 46℃, 48℃, 50℃, etc.

[0044] As a preferred technical solution of the present invention, step (2) of adding the aluminum source and the oxalic acid source to the reaction solution means adding the aqueous solution of the aluminum source and the aqueous solution of the oxalic acid source to the reaction solution respectively.

[0045] As a preferred embodiment of the present invention, the aluminum source is selected from any one or more of aluminum sulfate (Al2(SO4)3), aluminum nitrate (Al(NO3)3), and aluminum chloride (AlCl3). Preferably, the anion in the aluminum source is the same as the anion in the iron source. For example, when the iron source is ferric sulfate, the aluminum source is aluminum sulfate; when the iron source is ferric nitrate, the aluminum source is aluminum nitrate, so as to minimize the types of impurities in the solution and facilitate subsequent water washing to remove impurities.

[0046] As a preferred embodiment of the present invention, the oxalic acid source is selected from any one or more of oxalic acid (H2C2O4), ammonium oxalate ((NH4)2C2O4), and sodium oxalate (Na2C2O4).

[0047] As a preferred embodiment of the present invention, the alkaline solution is selected from aqueous solutions of sodium hydroxide (NaOH) and / or ammonium hydroxide (NH4OH).

[0048] As a preferred technical solution of the present invention, the aluminum source contains Al 3+ C2O4 in oxalic acid source 2- OH in alkaline solutions - The molar ratio is 1:3:3.

[0049] As a preferred embodiment of the present invention, the precursor solution is obtained after the alkaline solution is added dropwise and reacted, and the precursor solution is obtained after centrifugation and drying.

[0050] As a preferred technical solution of the present invention, the aluminum source contains Al 3+ Fe in iron source 3+ The molar ratio is (0.5-0.6):(2.8-3.0), for example, 0.5:2.8, 0.5:3, 0.55:3, 0.6:2.8, etc.

[0051] As a preferred embodiment of the present invention, the sodium source is selected from any one or more of sodium carbonate, sodium hydroxide, or sodium bicarbonate, preferably sodium carbonate.

[0052] As a preferred embodiment of the present invention, the reducing agent is selected from citric acid (C6H8O7), oxalic acid (C2H2O4), and sucrose (C6H8O7). 12 H 22 O 11 Any one or more of ) and ascorbic acid (C6H8O6).

[0053] As a preferred embodiment of the present invention, the carbon source is selected from any one or more of glucose, polyethylene glycol or carbon nanotubes (CNTs), preferably glucose.

[0054] As a preferred embodiment of the present invention, the mass ratio of the precursor, sodium source, reducing agent and carbon source is 37.31:(13.78-14.35):(12-16):1, for example 37.31:13.78:12:1, 37.31:14.01:14.5:1, 37.31:14.35:15.2:1, 37.31:14.35:16:1, etc.

[0055] As a preferred technical solution of the present invention, the mixing temperature in step (3) is 50-80℃, for example 50℃, 60℃, 70℃, 80℃, etc.

[0056] As a preferred embodiment of the present invention, the drying temperature is 110-125℃, such as 110℃, 115℃, 120℃, 125℃, etc., preferably 120℃; the drying time is 18-36h, such as 18h, 24h, 30h, 36h, etc., preferably 24h.

[0057] As a preferred embodiment of the present invention, the sintering is carried out in an inert gas atmosphere, wherein the inert gas can be any one or more of nitrogen, argon or helium.

[0058] As a preferred embodiment of the present invention, the sintering method is to sinter at 350°C for 3-8 hours, followed by sintering at 550-650°C for 8-18 hours. The 3-8 hours can be 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, etc. The 550-650°C can be 550°C, 580°C, 600°C, 620°C, 650°C, etc., and the 8-18 hours can be 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, etc.

[0059] As a preferred embodiment of the present invention, the sintering heating rate is 2-8℃ / min, such as 2℃ / min, 3℃ / min, 5℃ / min, 8℃ / min, etc., preferably 2℃ / min.

[0060] As a specific embodiment of the present invention, the preparation method includes the following steps:

[0061] A. Place the iron source in a solvent water to obtain an aqueous solution of the iron source and adjust the pH to 1-1.5; place the phosphoric acid source and pyrophosphate source in a solvent water to obtain a mixed aqueous solution of the phosphoric acid source and pyrophosphate source; place the aluminum source in a solvent water to obtain an aqueous solution of the aluminum source; place the oxalic acid source in a solvent water to obtain an aqueous solution of the oxalic acid source.

[0062] B. Add the mixed aqueous solution of phosphoric acid source and pyrophosphate source dropwise to the aqueous solution of iron source to carry out the reaction, and obtain the reaction solution;

[0063] C. Add aqueous solutions of aluminum source and aqueous solutions of oxalic acid source dropwise to the reaction solution to carry out the reaction. After the reaction is complete, add alkaline solution dropwise to carry out the reaction to obtain the precursor.

[0064] D. The precursor is mixed with a sodium source, a reducing agent and a carbon source to obtain a gel-like mixture, which is then dried and sintered to obtain the modified sodium iron phosphate pyrophosphate cathode material.

[0065] In some embodiments of the present invention, in the mixed aqueous solution of the phosphoric acid source and the pyrophosphate source, PO4 3- The concentration is 0.50-0.55 mol / L, P2O7 4- The concentration is 0.18-0.22 mol / L.

[0066] In some embodiments of the present invention, the aqueous solution of the iron source contains Fe 3+ The concentration is 0.25 mol / L.

[0067] In some embodiments of the present invention, the aqueous solution of the aluminum source contains Al 3+ The concentration is 0.2 mol / L

[0068] In some embodiments of the present invention, the aqueous solution of oxalic acid source contains C2O4. 2- The concentration of ions is 0.6 mol / L.

[0069] In some embodiments of the present invention, OH in the alkaline solution - The concentration is 0.6 mol / L.

[0070] In some embodiments of the present invention, the dropping rate of the mixed aqueous solution of the phosphoric acid source and the pyrophosphate source is 2-4 L / h.

[0071] In some embodiments of the present invention, the aqueous solution of the aluminum source and the aqueous solution of the oxalic acid source have the same dropping rate, which is 1-2 L / h.

[0072] In some embodiments of the present invention, the dropping rate of the alkaline solution is 1 L / h.

[0073] Thirdly, the present invention provides a positive electrode sheet, the positive electrode sheet comprising the modified sodium iron phosphate pyrophosphate positive electrode material described in the first aspect or the modified sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method described in the second aspect.

[0074] Fourthly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the positive electrode sheet described in the third aspect.

[0075] The technical solution provided by the embodiments of the present invention has the following advantages compared with the prior art:

[0076] The modified sodium iron phosphate pyrophosphate cathode material provided by this invention has high compaction density, good structural stability, thermal stability, and chemical stability, and can effectively avoid Fe 2+ It dissolves while exhibiting excellent conductivity, high discharge capacity, and excellent rate performance and cycle performance. Attached Figure Description

[0077] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0078] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0079] Figure 1 The image shows the XRD pattern of the modified sodium iron phosphate pyrophosphate cathode material prepared in Example 1.

[0080] Figure 2 The first discharge curves at 0.1C are for coin cells assembled with the cathode materials prepared in Examples 1-3 and Comparative Example 1.

[0081] Figure 3 The rate performance diagrams show the coin cell assembled from the cathode materials prepared in Examples 1-3 and Comparative Example 1.

[0082] Figure 4 The graph shows the long-cycle performance of coin cells assembled with the cathode materials prepared in Examples 1-3 and Comparative Example 1 at 1C. Detailed Implementation

[0083] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0084] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0085] Example 1

[0086] This embodiment provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material includes Al. 3+ Gradient-doped Na 4.12 Fe 2.91 Al 0.02 The (PO4)2P2O7 core and the NaAlO2 coating layer covering its surface, and the NaAlO2 coating layer is related to Na 4.12 Fe 2.91 Al 0.02 The mass ratio of the (PO4)2P2O7 core is 2:100;

[0087] The preparation method includes the following steps:

[0088] (1) According to the molar ratio n(Fe) 3+ ):n(PO4 3- ):n(P2O7 4- Calculate and weigh ferric sulfate, sodium dihydrogen phosphate, and disodium dihydrogen pyrophosphate using the ratio 2.64:2.25:1.00. Add sodium dihydrogen phosphate and disodium dihydrogen pyrophosphate to deionized water to prepare PO4. 3- The concentration is 0.50 mol / L, P2O7 4- Solution A has a concentration of 0.22 mol / L. Ferric sulfate is added to deionized water to prepare Fe... 3+ Solution B has a concentration of 0.25 mol / L, and an appropriate amount of sulfuric acid is added to adjust the pH to 1.20.

[0089] (2) According to the molar ratio n(Al) 3+ ):n(C2O4 2- ):n(OH - ):n(Fe 3+ ) = 1:3:3:5.4 Calculate and weigh aluminum sulfate, oxalic acid, and sodium hydroxide. Add aluminum sulfate, oxalic acid, and sodium hydroxide separately to deionized water to prepare Al 3+ A 0.2 mol / L solution of C and C₂O₄ 2- A 0.6 mol / L solution of D and OH - Solution E has a concentration of 0.6 mol / L.

[0090] (3) Add solution B to the reactor and continuously introduce solution A into the reactor at a constant temperature of 40°C and a flow rate of 2 L / h. After solution A has been completely introduced, stir the reaction for another 1 h and simultaneously introduce solutions C and D at a flow rate of 1 L / h. After solutions C and D have been completely introduced, stir the reaction for another 1 h and introduce solution E at a flow rate of 1 L / h. After the reaction is complete, centrifuge and dry to obtain the precipitate precursor.

[0091] (4) Weigh the above raw materials according to the mass ratio of precursor to sodium carbonate, citric acid, and glucose of 37.31:14.01:14.45:1, add deionized water, stir at 50°C to form a gel, and dry in an oven at 120°C for 24 hours to obtain an amorphous powder. Place the powder into a crucible and sinter in a box furnace under argon protection. The heating rate is 2°C / min, the first stage is held at 350°C for 5 hours, and the second stage is held at 550°C for 12 hours. After that, grind and pass through a 300-mesh sieve to obtain the modified sodium iron phosphate pyrophosphate cathode material.

[0092] Example 2

[0093] This embodiment provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material includes Al. 3+ Gradient-doped Na 4.03 Fe 2.91 Al 0.05 The (PO4)2P2O7 core and the NaAlO2 coating layer covering its surface, and the NaAlO2 coating layer is related to Na 4.03 Fe 2.91 Al 0.05 The mass ratio of the (PO4)2P2O7 core is 0.5:100;

[0094] The preparation method includes the following steps:

[0095] (1) According to the molar ratio n(Fe) 3+ ):n(PO4 3- ):n(P2O7 4- Calculate and weigh ferric nitrate, trisodium phosphate, and sodium pyrophosphate using the ratio 2.69:2.30:1.00. Add trisodium phosphate and sodium pyrophosphate to deionized water to prepare PO4. 3- The concentration is 0.53 mol / L, P2O7 4- Solution A has a concentration of 0.20 mol / L. Ferric sulfate is added to deionized water to prepare Fe... 3+ Solution B has a concentration of 0.25 mol / L, and an appropriate amount of nitric acid is added to adjust the pH to 1.00.

[0096] (2) According to the molar ratio n(Al) 3+ ):n(C2O4 2- ):n(OH - ):n(Fe 3+ Calculate and weigh aluminum nitrate, sodium oxalate, and sodium hydroxide using a ratio of 1:3:3:6. Add aluminum nitrate, sodium oxalate, and sodium hydroxide separately to deionized water to prepare Al... 3+ A 0.2 mol / L solution of C and C₂O₄ 2- A 0.6 mol / L solution of D and OH- Solution E has a concentration of 0.6 mol / L.

[0097] (3) Add solution B to the reactor and continuously introduce solution A into the reactor at a flow rate of 1.5 L / h at a constant temperature of 40°C. After solution A has been completely introduced, stir the reaction for another 1 h. Then, simultaneously introduce solutions C and D at a flow rate of 1 L / h. After solutions C and D have been completely introduced, stir the reaction for another 1 h. Then, introduce solution E at a flow rate of 1 L / h. After the reaction is complete, centrifuge and dry to obtain the precipitate precursor.

[0098] (4) Weigh the above raw materials according to the mass ratio of precursor to sodium carbonate, citric acid, and glucose of 37.31:14.35:15.19:1, add deionized water, stir at 50°C to form a gel, and dry in an oven at 125°C for 18 hours to obtain an amorphous powder. Place the powder into a crucible and sinter in a box furnace under argon protection. The heating rate is 2°C / min, the first stage is held at 350°C for 6 hours, and the second stage is held at 580°C for 10 hours. After that, grind and pass through a 300-mesh sieve to obtain the modified sodium iron phosphate pyrophosphate cathode material.

[0099] Example 3

[0100] This embodiment provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material includes Al. 3+ Gradient-doped Na4Fe 2.91 Al 0.06 The (PO4)2P2O7 core and the NaAlO2 coating layer covering its surface, and the NaAlO2 coating layer is related to Na x Fe y Al z The mass ratio of the (PO4)2P2O7 core is 3:100;

[0101] The preparation method includes the following steps:

[0102] (1) According to the molar ratio n(Fe) 3+ ):n(PO4 3- ):n(P2O7 4- Calculate and weigh ferric chloride, ammonium dihydrogen phosphate, and ammonium pyrophosphate using the ratio 2.70:2.20:1.00. Add ammonium dihydrogen phosphate and ammonium pyrophosphate to deionized water to prepare PO4. 3- The concentration is 0.50 mol / L, P2O7 4- Solution A has a concentration of 0.19 mol / L. Ferric sulfate is added to deionized water to prepare Fe... 3+ Prepare a 0.25 mol / L solution B and adjust the pH to 1.00 with an appropriate amount of hydrochloric acid.

[0103] (2) According to the molar ratio n(Al) 3+ ):n(C2O4 2- ):n(OH - ):n(Fe 3+ Calculate and weigh aluminum chloride, ammonium oxalate, and sodium hydroxide using a ratio of 1:3:3:6. Add aluminum chloride, ammonium oxalate, and sodium hydroxide separately to deionized water to prepare Al... 3+ A 0.2 mol / L solution of C and C₂O₄ 2- A 0.6 mol / L solution of D and OH - Solution E has a concentration of 0.6 mol / L.

[0104] (3) Add solution B to the reactor, and continuously introduce solution A into the reactor at a flow rate of 1.0 L / h at a constant temperature of 45°C. After solution A has been completely introduced, stir the reaction for another 1 h, and simultaneously introduce solutions C and D at a flow rate of 1 L / h. After solutions C and D have been completely introduced, stir the reaction for another 1 h, and then introduce solution E at a flow rate of 1 L / h. After the reaction is complete, centrifuge and dry to obtain the precipitate precursor.

[0105] (4) Weigh the above raw materials according to the mass ratio of precursor to sodium carbonate, citric acid, and glucose of 37.31:13.78:13.18:1, add deionized water, stir at 50°C to form a gel, and dry in an oven at 120°C for 36 hours to obtain an amorphous powder. Place the powder into a crucible and sinter in a box furnace under nitrogen protection. The heating rate is 2°C / min, the first stage is held at 350°C for 8 hours, and the second stage is held at 640°C for 10 hours. After that, grind and pass through a 300-mesh sieve to obtain the modified sodium iron phosphate pyrophosphate cathode material.

[0106] Example 4

[0107] This embodiment provides a modified sodium iron phosphate pyrophosphate cathode material and its preparation method. The modified sodium iron phosphate pyrophosphate cathode material includes Al. 3+ Gradient-doped Na 4.12 Fe 2.91 Al 0.02 The (PO4)2P2O7 core and the NaAlO2 coating layer covering its surface, and the NaAlO2 coating layer is related to Na 4.12 Fe 2.91 Al 0.02 The mass ratio of the (PO4)2P2O7 core is 2:100;

[0108] The preparation method is the same as in Example 1. The difference from Example 1 is that sodium carbonate is replaced with sodium hydroxide in step (4) of this example. The mass ratio of the precursor to sodium hydroxide, citric acid and glucose is 37.31:14.01:14.45:1. The sintering heating rate in this example is 5℃ / min.

[0109] Comparative Example 1

[0110] This comparative example provides a sodium iron phosphate pyrophosphate cathode material Na4Fe3(PO4)2P2O7 and its preparation method. The preparation method employs a co-precipitation method, comprising the following steps:

[0111] (1) According to the molar ratio n(Fe) 3+ ):n(PO4 3- ):n(P2O7 4- Calculate and weigh ferric sulfate, sodium dihydrogen phosphate, and disodium dihydrogen pyrophosphate using the ratio 2.91:2:1. Add sodium dihydrogen phosphate and disodium dihydrogen pyrophosphate to deionized water to prepare PO4. 3- The concentration is 0.50 mol / L, P2O7 4- Solution A has a concentration of 0.22 mol / L. Ferric sulfate is added to deionized water to prepare Fe... 3+ Solution B has a concentration of 0.25 mol / L, and an appropriate amount of sulfuric acid is added to adjust the pH to 1.50.

[0112] (2) Add solution B to the reaction vessel and continuously introduce solution A into the reaction vessel at a flow rate of 1.0 L / h at a constant temperature of 45°C. After the reaction is complete, centrifuge and dry to obtain the precipitate precursor.

[0113] (3) The above raw materials were weighed according to the mass ratio of precursor to sodium carbonate, citric acid and glucose of 37.31:14.01:14.45:1 and added to deionized water. The mixture was stirred at 50°C to form a gel, and then dried in an oven at 120°C for 48 hours to obtain an amorphous powder. The powder was placed in a crucible and sintered in a box furnace under nitrogen protection. The heating rate was 2°C / min, the first stage was held at 350°C for 8 hours, and the second stage was held at 560°C for 10 hours. After that, the mixture was ground and passed through a 300-mesh sieve to obtain uncoated and undoped sodium iron pyrophosphate cathode material.

[0114] Comparative Example 2

[0115] This comparative example provides a sodium iron phosphate pyrophosphate cathode material Na4Fe3(PO4)2P2O7 and its preparation method. The preparation method adopts the sol-gel method and includes the following steps:

[0116] Based on the molar ratio of elements n(Na):n(Fe):n(P) = 4:3:4, calculate and weigh sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate; add the above substances together to deionized water to prepare Fe 3+ A 3 mol / L solution was stirred until homogeneous, and citric acid in an equimolar amount of Fe was added. The solution was heated and stirred at 50°C until it became gel-like, and then dried in an oven at 120°C for 48 hours to obtain an amorphous powder. The powder was placed in a crucible and sintered in a box furnace under nitrogen protection at a heating rate of 2°C / min. The first stage was held at 350°C for 8 hours, and the second stage was held at 560°C for 10 hours. Afterward, the powder was ground and passed through a 300-mesh sieve to obtain uncoated and undoped sodium iron pyrophosphate cathode material.

[0117] Performance Test 1

[0118] (1) The modified sodium iron phosphate pyrophosphate cathode material prepared in the examples was subjected to XRD testing. The XRD pattern of the modified sodium iron phosphate pyrophosphate cathode material prepared in Example 1 is shown below. Figure 1 As shown.

[0119] (2) The compaction density of the cathode materials prepared in the examples and comparative examples was tested using a powder compaction density meter.

[0120] Take 1-2g of positive electrode material powder, put it into a powder compaction density meter, set the minimum applied pressure to 80MPa, the minimum unloaded pressure to 3MPa, and the holding time to 10s, and the compaction density can be measured. The results are shown in Table 1.

[0121] Table 1

[0122] Example 1 2.25 Example 2 2.29 Example 3 2.24 Example 4 2.23 Comparative Example 1 2.18 Comparative Example 2 1.83

[0123] As can be seen from the results in Table 1, the modified sodium iron phosphate pyrophosphate cathode material prepared by this invention has a higher compaction density.

[0124] Performance Test 2

[0125] (1) Assembly of coin cells: The positive electrode materials prepared in the examples and comparative examples were used as positive electrode active materials and mixed with conductive acetylene black (AB) and binder (4 wt% polyvinylidene fluoride) in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 to prepare positive electrode slurry. The slurry was homogenized using a vibratory homogenizer for at least 15 min, and then uniformly coated onto aluminum foil with a 150 μm doctor blade. It was dried in a vacuum oven at 100 °C for 10 h until NMP and residual moisture were completely evaporated. The loading of the positive electrode active material was approximately 2.5 mg / cm³. 2In a glove box under an argon atmosphere (H2O / O2 < 0.01 ppm), a coin cell was fabricated using the above-mentioned electrode, sodium sheet, glass fiber membrane, and electrolyte (5 vol% fluoroethylene carbonate added to a 1 mol / L NaClO4 propylene carbonate solution) as the reference electrode, counter electrode, separator, and electrolyte, respectively.

[0126] (2) Conduct constant current charge and discharge experiments at different rates on the battery testing system.

[0127] The nominal specific capacity is 129mAh g. -1 The voltage window is 1.80-4.20V, and the charge / discharge rates are 0.1C, 1C, 5C, 10C, 20C, 30C, 40C, and 50C in sequence, with each rate cycling 5 times.

[0128] The test results are shown in Table 2 and Figure 2-4 As shown, where, Figure 2 The first discharge curves at 0.1C are shown for coin cells assembled using the cathode materials prepared in Examples 1-3 and Comparative Example 1. Figure 3 The graph shows the rate performance of coin cells assembled from the cathode materials prepared in Examples 1-3 and Comparative Example 1. Figure 4 The long-cycle performance of coin cells assembled with the cathode materials prepared in Examples 1-3 and Comparative Example 1 at 1C.

[0129] The results in Table 2 are shown below:

[0130] Table 2

[0131]

[0132]

[0133] From Table 2 and Figure 2-4 The results show that the modified sodium iron phosphate pyrophosphate cathode material prepared by this invention has higher discharge capacity, better rate performance and cycle performance.

[0134] Performance Test 3

[0135] (1) The positive electrode materials obtained in the examples and comparative examples were used as positive electrode active materials, and coin cells were assembled according to the method described in step (1) of performance test 2. Then, the cells were tested on a battery testing system according to a 129mAh g / g standard. -1 The nominal specific capacity is determined by first activating the battery by charging and discharging it three times at a rate of 0.1C, and then charging it at a rate of 1C for 0 min, 12 min, 24 min, 36 min, and 48 min respectively. After the program is completed, the battery is disassembled and the electrode plates are removed to obtain electrode plates with a charge SOC of 0%, 20%, 40%, 60%, and 80%.

[0136] (2) Immerse the above electrodes in 10g of the electrolyte as described in Performance Test 1 (1 mol / L NaClO4 propylene carbonate solution with 5 vol% fluoroethylene carbonate added) and place at 40℃ for 168h. After removing the electrodes, take 10mL of electrolyte, filter it, and test ICP-Fe to obtain the amount of Fe dissolved in the electrolyte at 0%, 20%, 40%, 60%, and 80% charging SOC. Substitute into the following formula:

[0137]

[0138] The content of dissolved Fe in the active substance can be obtained, and the results are shown in Table 3:

[0139] Table 3

[0140]

[0141]

[0142] Table 3 shows that the modified sodium iron phosphate pyrophosphate cathode material provided by this invention can effectively avoid the presence of Fe. 2+ Contact with the electrolyte effectively prevents Fe 2+ Dissolution.

[0143] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0144] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A modified sodium iron phosphate pyrophosphate cathode material, characterized in that, The modified sodium iron phosphate pyrophosphate cathode material includes Na. x Fe y Al z (PO4)2P2O7 core and NaAlO2 coating layer covering its surface; The Na x Fe y Al z Al in the (PO4)2P2O7 kernel 3+ The content of decreases from the outer layer of the core to the core, where 4≤x≤4.18, 2.9≤y+z≤3, and 0 <z≤0.1,x+2y+3z=10。 2. The modified sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The NaAlO2 coating layer and Na x Fe y Al z The mass ratio of the (PO4)2P2O7 core is 0.5-3:

100.

3. The method for preparing the modified sodium iron phosphate pyrophosphate cathode material according to claim 1 or 2, characterized in that, The preparation method includes: (1) The iron source, phosphoric acid source and pyrophosphate source are reacted in an aqueous solution to obtain a reaction solution; (2) Add aluminum source and oxalic acid source dropwise to the reaction solution to carry out the reaction. After the reaction is complete, add alkaline solution dropwise to carry out the reaction to obtain the precursor. (3) The precursor is mixed with sodium source, reducing agent and carbon source to obtain gel-like mixture, dried and sintered to obtain the modified sodium iron phosphate pyrophosphate cathode material.

4. The preparation method according to claim 3, characterized in that, Step (1) involves adding aqueous solutions of phosphoric acid source and pyrophosphate source dropwise to an aqueous solution of iron source to carry out the reaction.

5. The preparation method according to claim 4, characterized in that, The pH of the aqueous solution of the iron source is 1-1.

5.

6. The preparation method according to claim 3 or 4, characterized in that, The iron source is selected from any one or more of ferric sulfate, ferric nitrate and ferric chloride; And / or, the phosphoric acid source is selected from any one or more of phosphoric acid, trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, triammonium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; And / or, the pyrophosphate source is selected from any one or more of pyrophosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, and ammonium pyrophosphate; And / or, Fe in the iron source 3+ PO4 in phosphate source 3- and P2O7 in pyrophosphate source 4- The molar ratio is (2.64-2.72):(2.2-2.3):

1.

7. The preparation method according to claim 3 or 4, characterized in that, Step (2) involves adding the aluminum source and oxalic acid source dropwise into the reaction solution, which means adding an aqueous solution of the aluminum source and an aqueous solution of the oxalic acid source dropwise into the reaction solution, respectively. And / or, the aluminum source is selected from any one or more of aluminum sulfate, aluminum nitrate, and aluminum chloride; And / or, the oxalic acid source is selected from any one or more of oxalic acid, ammonium oxalate, and sodium oxalate; And / or, the alkaline solution is selected from aqueous solutions of sodium hydroxide and / or ammonium hydroxide; And / or, the aluminum source contains Al 3+ C2O4 in oxalic acid source 2- OH in alkaline solutions - The molar ratio is 1:3:3; And / or, the aluminum source contains Al 3+ Fe in iron source 3+ The molar ratio is (0.5-0.6):(2.8-3.0).

8. The preparation method according to claim 3 or 4, characterized in that, The sodium source is selected from any one or more of sodium carbonate, sodium hydroxide, or sodium bicarbonate. And / or, the reducing agent is selected from any one or more of citric acid, oxalic acid, sucrose, and ascorbic acid; And / or, the carbon source is selected from any one or more of glucose, polyethylene glycol, or carbon nanotubes; And / or, the mass ratio of the precursor, sodium source, reducing agent and carbon source is 37.31:(13.78-14.35):(12-16):

1.

9. The preparation method according to claim 8, characterized in that, The carbon source is selected from glucose.

10. The preparation method according to claim 3 or 4, characterized in that, The mixing temperature in step (3) is 50-80°C; And / or, the drying temperature is 110-125°C, and the drying time is 18-36 h; And / or, the sintering is carried out in an inert gas atmosphere; And / or, the sintering method is to sinter at 350°C for 3-8 h, followed by sintering at 550-650°C for 8-18 h; And / or, the sintering heating rate is 2-8°C / min.

11. A positive electrode plate, characterized in that, The positive electrode sheet comprises the modified sodium iron phosphate pyrophosphate positive electrode material according to claim 1 or 2, or the modified sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method according to any one of claims 3-10.

12. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode sheet as described in claim 11.

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