A sodium ferric pyrophosphate positive electrode material, a preparation method thereof and a sodium ion battery

Abstract

The application provides a sodium pyrophosphate ferric phosphate positive electrode material, a preparation method thereof and a sodium ion battery. a Fe b (PO4) c (P2O7) d Wherein, 2 <= a <= 4, 1.5 <= b <= 4, 1 <= c <= 2, 0.5 <= d <= 2; the chemical general formula of the shell is Na a‑x Li x Fe b (PO4) c (P2O7) d Wherein, 0

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage device technology, specifically relating to a sodium iron phosphate pyrophosphate cathode material, its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries are considered an ideal choice for large-scale electrochemical energy storage systems due to the abundance, wide distribution, and low cost of sodium resources. Among the many cathode material systems for sodium-ion batteries, pyrophosphate phosphate-based materials (such as sodium iron phosphate pyrophosphate, hereinafter collectively referred to as NFPPs) exhibit excellent thermal stability and extremely low volume expansion rate due to their robust three-dimensional polyanionic framework structure, demonstrating outstanding performance in terms of long cycle life and safety.

[0003] Although these NFPP materials have relatively stable bulk crystal structures, their surface and near-surface regions still face specific air sensitivity challenges during actual preparation, storage, and application. This sensitivity does not stem from the overall instability of the material, but is mainly attributed to the weak binding energy of sodium ions at specific sites in the crystal lattice (usually Na3 sites or similar low-binding-energy sites), resulting in a metastable state. In addition, unmodified NFPP materials are extremely sensitive to water, undergoing a violent hydrolysis reaction upon contact with water, leading to the precipitation of large amounts of elements such as Na, P, and Fe from the crystal lattice, causing irreversible structural damage and performance degradation. Therefore, the current preparation of NFPP electrodes must strictly rely on expensive and toxic organic solvent systems (such as NMP / PVDF), resulting in high raw material costs, the need for complex solvent recovery systems, and extremely high requirements for workshop environmental humidity (dew point).

[0004] To address the poor conductivity and surface sensitivity of NFPPs, existing research reports mainly focus on the following improvement directions, but all have certain limitations: (1) Precision coating techniques such as atomic layer deposition (ALD): Existing technologies attempt to coat the surface of NFPPs with a dense oxide layer (Al2O3, ZnO) through ALD or chemical vapor deposition.

[0005] While precision coating can physically isolate moisture, it primarily relies on van der Waals forces or weak chemical bonds for adhesion, failing to specifically repair the Na3 sites with the lowest binding energy from a thermodynamic perspective. Furthermore, such methods typically require expensive vacuum equipment, and the resulting passivation film often significantly increases charge transfer resistance during cycling, leading to a decrease in rate performance.

[0006] (2) Bulk metal element doping strategy: Transition metals (such as Mn, Mg, V) are used to replace Fe sites in an attempt to stabilize sodium ions by adjusting lattice parameters.

[0007] However, doping is typically a holistic bulk behavior, lacking the ability to precisely lock onto metastable sodium ions directly exposed to air on the outermost surface. Furthermore, due to the uniform distribution of dopant elements, achieving satisfactory surface stability often requires a high doping level, which dilutes the active components and results in an intrinsic loss of energy density.

[0008] (3) Sacrificial passivation protection: Some technologies use additives with strong oxidizing or strong binding properties to pre-treat the surface during the synthesis stage to form an irreversible inert shell.

[0009] This type of strategy is called "sacrificial" protection. Its essence is to permanently "delock" the active Na3 sites. Although it improves stability, these sites cannot be activated and released during the first charge of the battery, resulting in extremely low coulombic efficiency in the first charge. Furthermore, the voltage plateau and specific capacity of the material cannot reach the theoretical expectations, which seriously restricts the commercialization potential of NFPPs.

[0010] In summary, while existing technologies improve the stability of NFPPs, they often come at the cost of loss of electrochemical activity or decrease in specific energy. Therefore, there is an urgent need to develop a surface engineering technology that can both "anchor" unstable Na3 sites on the surface to isolate them from air and moisture erosion and overcome the activation barrier during the first charge of the battery to achieve reversible activation of these sites. Summary of the Invention

[0011] To address the problem that existing technologies improve the stability of NFPPs while simultaneously causing a loss of electrochemical activity or a decrease in specific energy, this invention provides a sodium iron phosphate pyrophosphate cathode material, its preparation method, and a sodium-ion battery.

[0012] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a sodium iron phosphate pyrophosphate cathode material, comprising a core and a shell; The general chemical formula of the kernel is Na. a Fe b (PO4) c (P2O7) d Where 2≤a≤4, 1.5≤b≤4, 1≤c≤2, and 0.5≤d≤2; The chemical formula of the outer shell is Na. a-x Li x Fe b (PO4) c (P2O7) d , of which 0 <x<a。

[0013] Preferably, the thickness of the outer shell is 5~50 nm.

[0014] Preferably, the core is Na4Fe3(PO4)2P2O7, and the outer shell is Na. 4-x Li x Fe3(PO4)2P2O7, of which 0 <x<1。

[0015] Secondly, the present invention provides a method for preparing the sodium iron phosphate pyrophosphate cathode material, comprising: Sodium iron phosphate pyrophosphate matrix powder was added to a lithium salt solution to form a mixed slurry, which was then stirred and impregnated to initiate the Li... + / Na + Ion exchange reaction causes lithium ions to replace sodium ions on the surface of the sodium iron phosphate matrix lattice; The mixed slurry after the reaction is subjected to solid-liquid separation, and the obtained solid is washed and dried. The dried powder was placed in an inert protective atmosphere for high-temperature heat treatment to obtain the sodium iron phosphate pyrophosphate cathode material.

[0016] Preferably, the lithium salt in the lithium salt solution is selected from one or more of lithium nitrate, lithium hydroxide, lithium acetate, and lithium chloride.

[0017] Preferably, the solvent in the lithium salt solution is anhydrous ethanol, n-propanol, or isopropanol.

[0018] Preferably, the concentration of the lithium salt solution is controlled at 0.05~2 mol / L.

[0019] Preferably, the reaction temperature of the ion exchange reaction is controlled at 15~80℃, and the reaction time is controlled at 12~48 hours.

[0020] Preferably, the temperature of the high-temperature heat treatment is 200~500℃.

[0021] Thirdly, the present invention provides a sodium-ion battery comprising the sodium iron phosphate pyrophosphate cathode material as described above.

[0022] Compared with the prior art, the present invention has the following beneficial effects: The sodium iron phosphate pyrophosphate cathode material of this invention has a core of sodium iron phosphate pyrophosphate-based polyanionic material, while the outer shell is a lithium-rich surface layer formed by lithium ions partially replacing sodium ions on the crystal lattice surface of the sodium iron phosphate pyrophosphate-based polyanionic material. The introduction of this lithium-rich surface layer improves the material's air stability and overcomes the problems of electrochemical activity loss or specific energy decrease. Specifically, on the one hand, the sodium ion binding energy at the Na3 sites in existing NFPP materials is weak (metastable), making them easily induced to desorb by water molecules and oxygen when exposed to air, leading to lattice surface poisoning and the generation of a large amount of alkaline impurities (such as NaOH and Na2CO3). This invention, however, utilizes Li... +Occupying the highly active Na3 sites on the surface, a robust lithium-rich surface layer is constructed on the crystal lattice surface by replacing the weaker Na-O bonds (approximately 256 kJ / mol) with stronger Li-O bonds (approximately 341 kJ / mol). This effectively resists the erosion of water molecules in the air, significantly reduces the residual alkali content on the surface, and allows the material to maintain structural and performance stability even after prolonged (>7 days) air exposure. On the other hand, traditional surface coatings (such as Al2O3 or carbon coatings) or strong ion doping often employ a "sacrificial" strategy, which permanently locks or physically isolates the surface active sites, resulting in a decrease in capacity during the first cycle. Furthermore, the voltage plateau contributed by the Na3 sites cannot be recovered in subsequent cycles, and the energy density monotonically decays with each cycle. The lithium-rich surface layer containing Li-O bonds constructed in this invention possesses the intelligent characteristic of "locked during storage and activated during cycling." Although the Li-O bonds require a high activation energy during the first charge cycle (manifested as a temporary suppression of the Na3 oxidation peak intensity during the first cycle), during subsequent long cycles, with the breathing changes of the lattice volume and the repeated opening and closing of ion channels, the locked Na3 sites are gradually "awakened" and re-participate in the electrochemical reaction. Macroscopically, this manifests as an "energy climb" trend in the battery's discharge specific energy, which first stabilizes or gradually increases with the number of cycles, significantly improving the energy density throughout the entire life cycle. On the other hand, unmodified NFPP materials are extremely sensitive to water. When in contact with water, they undergo a violent hydrolysis reaction, causing a large amount of Na, P, Fe, and other elements to precipitate from the lattice, resulting in irreversible structural damage and performance degradation. Therefore, the existing NFPP electrode preparation must strictly rely on expensive and toxic organic solvent systems (such as NMP / PVDF), which not only results in high raw material costs but also requires a complex solvent recovery system and extremely high requirements for workshop environmental humidity (dew point). The cathode material of this invention benefits from the high-strength Li-O bond "anchoring" effect on its surface, exhibiting excellent hydrolytic stability. Even after prolonged immersion in aqueous solution, its crystal structure remains intact, with no significant elemental dissolution. This characteristic allows the material to be directly compatible with aqueous binders (such as PAA, CMC / SBR, sodium alginate, etc.) and deionized water solvents, achieving a leap from traditional "oil-based processes" to green and inexpensive "water-based processes." This not only completely eliminates the toxicity risks of NMP but also significantly reduces raw material costs and equipment maintenance costs, substantially improving the economic efficiency and environmental friendliness of battery manufacturing. In summary, this core-shell structure design of the present invention retains the rapid ion transport characteristics of the three-dimensional large pores inside NFPP while repairing surface defects, achieving a perfect balance between high-rate performance and high stability.

[0023] The present invention provides a method for preparing sodium iron phosphate pyrophosphate cathode material, utilizing Li + Small radius (0.76 Å vs Na) +The characteristics of 1.02 Å and high charge density, driven by liquid-phase ion exchange, allow Li to achieve high efficiency. + By occupying the highly active Na3 sites on the surface, a robust lithium-rich surface layer was successfully formed in situ on the crystal lattice surface. The process is simple and controllable.

[0024] Furthermore, existing water-washing de-alkali removal processes easily lead to excessive loss of sodium ions and collapse of the bulk structure, while high-temperature solid-phase doping makes it difficult to precisely control element distribution only on the surface. This invention uses organic solvents (such as ethanol and propanol) as a medium for a liquid-phase impregnation process. Organic solvents limit the amount of Li... + Excessive diffusion into the depths of the particles enables precise exchange modification limited to the surface layer (e.g., 5–50 nm), avoiding damage to the bulk structure.

[0025] Furthermore, the material preparation method of the present invention has mild reaction conditions (15~80℃) and is easy to implement for industrial application.

[0026] The sodium-ion battery based on the sodium iron phosphate pyrophosphate cathode material of this invention exhibits excellent overall electrochemical performance. Under preferred implementation conditions, the modified cathode material achieves an initial efficiency of 90.26% after 7 days of exposure to air (compared to only 89.04% for the unmodified sample); after 1000 cycles at 10C, the capacity retention is 98.39% (compared to 80.95% for the unmodified sample), and it demonstrates a significant characteristic of energy density retention or increase, showing great potential for commercial application. Attached Figure Description

[0027] 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, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 The image shows the XRD pattern of the sodium iron phosphate pyrophosphate cathode material of this invention.

[0029] Figure 2 The graph shows a comparison of the coulombic efficiency and long-cycle performance of the sodium iron phosphate pyrophosphate cathode material of this invention and the unmodified original sample, which were assembled into batteries using oil-based slurry.

[0030] Figure 3 The graph shows a comparison of the coulombic efficiency and long-cycle performance of the sodium iron phosphate pyrophosphate cathode material of this invention and the unmodified original sample, which were assembled into batteries using an aqueous slurry and tested.

[0031] Figure 4The dQ / dV curves of the sodium iron phosphate pyrophosphate cathode material of the present invention and the unmodified original sample were obtained by assembling a battery with an aqueous slurry and testing; (a) the original sample; (b) the sodium iron phosphate pyrophosphate cathode material of the present invention.

[0032] Figure 5 The constant current charge-discharge curves and rate performance of the sodium iron phosphate pyrophosphate cathode material of the present invention and the unmodified original sample were obtained by assembling a battery with an aqueous slurry and testing; (a) constant current charge-discharge curves; (b) rate performance. Detailed Implementation

[0033] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0034] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0035] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or apparatuses. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not intended to limit the order of the method steps or define the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0036] In a first aspect, the present invention provides a sodium iron phosphate pyrophosphate cathode material, the cathode material having a core-shell structure, comprising a sodium iron phosphate pyrophosphate (NFPP) bulk core and an in-situ formed lithium-rich surface shell. The core is a sodium iron phosphate pyrophosphate-based polyanionic material, whose general chemical formula is Na. a Fe b (PO4) c (P2O7) d Where 2≤a≤4, 1.5≤b≤4, 1≤c≤2, and 0.5≤d≤2; The outer shell is chemically represented by Na. a-x Li x Fe b (PO4)c (P2O7) d , of which 0 <x<a。

[0037] In some embodiments of the present invention, the core has the chemical formula Na4Fe3(PO4)2P2O7, and the outer shell has the chemical formula Na 4-x Li x Fe3(PO4)2P2O7, where 0 <x<1。

[0038] The outer shell is a lithium-rich surface layer formed in situ by partially replacing sodium ions on the crystal lattice surface of sodium iron phosphate pyrophosphate-based polyanionic material with lithium ions through ion exchange. This ion exchange is limited to the material surface layer, and the thickness of the outer shell, i.e., the lithium-rich surface layer, is 5-50 nm. Lithium ions in the lithium-rich surface layer mainly occupy Na3 sites in the crystal lattice, causing the Na-O bonds on the material surface to transform into higher-energy Li-O bonds. Within the outer shell, the lithium concentration decreases gradually from the surface to the center.

[0039] The core of this invention lies in resolving the contradiction between the air stability and electrochemical activity of sodium iron phosphate (NFPP) materials through "surface ion exchange engineering." Specifically, this is reflected in the following two points: (1) The “Li-O anchoring” surface stabilization mechanism based on bond energy difference: The principle of this invention is: utilizing lithium ions (Li + Small radius (0.76 Å vs Na) + The high charge density (1.02 Å) drives the preferential thermodynamic replacement of the least stable Na3 sites on the NFPP lattice surface. This results in the formation of stronger Li-O bonds (341 kJ / mol) to replace the weaker Na-O bonds (256 kJ / mol), creating a dense "lithium-rich surface layer." This lithium-rich surface layer effectively resists the erosion of the lattice by water molecules and carbon dioxide in the air, fundamentally inhibiting the formation of surface residual alkali (NaOH / Na2CO3), improving the air stability of the NFPP material, and mitigating its surface sensitivity. Meanwhile, the cathode material of this invention benefits from the "anchoring" effect of the high-strength Li-O bonds on its surface, exhibiting excellent hydrolytic stability. Even after prolonged immersion in aqueous solution, its crystal structure remains intact with no significant elemental dissolution. This characteristic enables the material to be directly compatible with aqueous binders (such as CMC / SBR, sodium alginate, etc.) and deionized water solvents, achieving a leap from the traditional "oil-based process" to the green and inexpensive "water-based process." This not only completely eliminates the toxicity risks of NMP but also significantly reduces raw material costs and equipment maintenance costs, thereby significantly improving the economic efficiency and environmental friendliness of battery manufacturing.

[0040] (2) Unique "kinetically constrained-cyclically activated" energy compensation effect: Unlike traditional inert coatings (such as Al2O3) or deadlock doping (which leads to permanent capacity loss in the first and subsequent cycles), the Li-O bonds introduced in this invention possess "reversible activation" characteristics. During the first charge cycle, due to the strong binding effect of the Li-O bonds, the extraction of Na3 sites requires overcoming a relatively high activation energy (manifested as a slight shift or weakening of the Na3 oxidation peak potential in the first cycle). However, in subsequent cycles, with the lattice breathing effect and repeated opening of ion channels, the "temporarily locked" Na3 sites are gradually activated and release their activity. This results in the material exhibiting an anomalous and superior characteristic of gradually recovering its discharge specific energy with increasing cycle count.

[0041] Therefore, this invention improves the stability of the material by using a lithium-rich surface layer on the material surface, while also overcoming the problems of loss of electrochemical activity or decrease in specific energy.

[0042] On the other hand, the present invention provides a method for preparing the above-mentioned sodium iron phosphate pyrophosphate cathode material, which employs a liquid-phase ion exchange method combined with inert atmosphere annealing, specifically including: Sodium iron phosphate pyrophosphate matrix powder was added to a lithium salt solution to form a mixed slurry, which was then stirred and impregnated to initiate the Li... + / Na + Ion exchange reaction causes lithium ions to replace sodium ions on the surface of the sodium iron phosphate matrix lattice; The mixed slurry after the reaction was subjected to solid-liquid separation, and was washed with solvent to remove residual lithium salts on the surface, followed by drying; The dried powder was placed in an inert protective atmosphere for high-temperature heat treatment to induce lattice fixation, thereby obtaining the sodium iron phosphate pyrophosphate cathode material.

[0043] To ensure that the ion exchange reaction occurs only in the surface layer of 5-50 nm without damaging the bulk structure, the present invention optimizes and limits the specific parameters in the preparation method.

[0044] In some preferred embodiments of the present invention, the lithium salt is selected from one or more of lithium nitrate, lithium hydroxide, lithium acetate, and lithium chloride.

[0045] In some preferred embodiments of the present invention, the concentration of the lithium salt solution is controlled at 0.05~2.0 mol / L. The solvent in the lithium salt solution is selected from organic solvents such as anhydrous ethanol, n-propanol, and isopropanol.

[0046] In some preferred embodiments of the present invention, the reaction temperature of the ion exchange reaction is controlled at 15~80°C and the reaction time is controlled at 12~48 hours.

[0047] In some preferred embodiments of the present invention, the high-temperature heat treatment is performed at a temperature of 200~500℃ to enhance the Li-O bonding strength.

[0048] The NFPP matrix powder described in this invention can be obtained directly through commercial channels, or it can be prepared using conventional solid-phase synthesis, sol-gel method or hydrothermal method and other known techniques in the art. This invention does not limit its specific source or preceding synthesis process.

[0049] This invention uses specific organic solvents (such as anhydrous ethanol, n-propanol, and isopropanol) as the exchange medium, utilizing Li + The chemical potential difference between the organic solvent and the crystal lattice interface enabled ion exchange at the surface layer (5-50 nm), avoiding the degradation of Li. + Excessive penetration into the bulk phase undermines the three-dimensional macroporous transport advantage of NFPP. Combined with annealing, surface hydroxyl defects that might be introduced by wet chemical treatment are eliminated, thus improving surface crystallinity.

[0050] The present invention also provides a sodium-ion battery, characterized in that it includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the above-mentioned sodium iron phosphate pyrophosphate positive electrode material.

[0051] Thanks to the high air stability of the sodium iron phosphate pyrophosphate cathode material, the sodium-ion battery of the present invention has excellent air storage stability and long cycle life.

[0052] Example 1 This embodiment provides a surface-modified sodium iron phosphate pyrophosphate cathode material and its preparation method.

[0053] (1) Preparing the precursor: Prepare sodium iron phosphate (NFPP) matrix powder (purchased from Aladdin); (2) Preparation of ion exchange solution: Weigh out lithium nitrate (LiNO3) and dissolve it in anhydrous ethanol solvent. Stir until completely dissolved to prepare a lithium salt solution with a concentration of 1.0 mol / L.

[0054] (3) Liquid-phase ion exchange: The NFPP matrix powder from step (1) was added to the lithium salt solution from step (2) (solid-liquid ratio of 1g:50mL). Under an argon protective atmosphere, the reaction temperature was controlled at 25℃, and the mixture was magnetically stirred at 500rpm for 24 hours to initiate the Li... + / Na + Ion exchange reaction, causing Li + It occupies Na3 sites on the surface of the NFPP matrix.

[0055] (4) Post-processing: After the reaction, the ion-exchanged slurry was filtered to separate the solid and liquid phases, washed three times with anhydrous ethanol to remove residual lithium salts from the surface, and dried in a vacuum oven at 80°C for 12 hours. The dried powder was then placed in a tube furnace and heat-treated at 300°C for 4 hours under an argon atmosphere to obtain the surface-modified sodium iron phosphate pyrophosphate cathode material Na. 3.3 Li 0.7 Fe3(PO4)2P2O7 (denoted as NFPP-Li-1).

[0056] Example 2 This embodiment provides a surface-modified sodium iron phosphate pyrophosphate cathode material and its preparation method.

[0057] (1) Preparing the precursor: Same as Example 1.

[0058] (2) Preparation of ion exchange solution: Weigh out lithium hydroxide (LiOH), dissolve it in n-propanol solvent, stir until completely dissolved, and prepare a lithium salt solution with a concentration of 0.5 mol / L.

[0059] (3) Liquid-phase ion exchange: The NFPP matrix powder from step (1) was added to the lithium salt solution from step (2). Under an argon protective atmosphere, the reaction temperature was controlled at 50°C, and the reaction time was stirred for 12 hours to initiate the Li... + / Na + Ion exchange reaction.

[0060] (4) Post-processing: In the post-processing step, the heat treatment temperature was adjusted to 250℃, and the remaining operations were the same as in Example 1, to obtain the surface-modified sodium iron phosphate pyrophosphate cathode material Na. 3.2 Li 0.8 Fe3(PO4)2P2O7 (denoted as NFPP-Li-2).

[0061] Example 3 This embodiment provides a surface-modified sodium iron phosphate pyrophosphate cathode material and its preparation method.

[0062] (1) Preparing the precursor: Same as Example 1.

[0063] (2) Preparation of ion exchange solution: Weigh lithium acetate (CH3COOLi), dissolve it in anhydrous ethanol solvent, stir until completely dissolved, and prepare a lithium salt solution with a concentration of 2.0 mol / L.

[0064] (3) Liquid-phase ion exchange: The NFPP matrix powder from step (1) was added to the lithium salt solution from step (2). Under an argon protective atmosphere, the reaction temperature was controlled at 80°C, and the reaction time was stirred for 12 hours to initiate the Li... + / Na + Ion exchange reaction.

[0065] (4) Post-processing: In the post-processing steps, the heat treatment temperature was adjusted to 400℃, and the remaining operations were the same as in Example 1, to obtain the surface-modified sodium iron phosphate pyrophosphate cathode material Na. 3.8 Li 0.2 Fe3(PO4)2P2O7 (denoted as NFPP-Li-3).

[0066] Example 4 This embodiment provides a surface-modified sodium iron phosphate pyrophosphate cathode material and its preparation method.

[0067] (1) Preparing the precursor: Same as Example 1.

[0068] (2) Preparation of ion exchange solution: Weigh out lithium chloride (LiCl), dissolve it in isopropanol solvent, and prepare a lithium salt solution with a concentration of 0.1 mol / L.

[0069] (3) Liquid-phase ion exchange: The NFPP matrix powder from step (1) was added to the lithium salt solution from step (2). Under an argon protective atmosphere, the reaction temperature was controlled at 15°C, and the reaction time was stirred for 48 hours to initiate the Li... + / Na + Ion exchange reaction.

[0070] (4) Post-processing: Similar to Example 1, the surface-modified sodium iron phosphate pyrophosphate cathode material was obtained: Na 3.6 Li 0.4 Fe3(PO4)2P2O7 (denoted as NFPP-Li-4).

[0071] like Figure 1 The image shown is the X-ray diffraction (XRD) pattern of the surface-modified sodium iron phosphate pyrophosphate cathode material (NFPP-Li-1) prepared in Example 1 of this invention. Figure 1 It can be seen that the peak intensity ratio of the (011) crystal plane diffraction peak to the (210) crystal plane diffraction peak in the low-angle region of NFPP-Li-1 satisfies 0.8 ≤ I （011） / I （210）≤1.5, higher than the peak intensity ratio of unmodified raw material pyrophosphate sodium iron phosphate 0≤ I （011） / I （210） ≤0.8. This significant change in the relative intensity of the diffraction peaks on this specific crystal plane is due to Li + with Na + This is due to the change in lattice electron density distribution caused by ion exchange. In X-ray diffraction crystallography, the intensity of diffraction peaks is closely related to the scattering ability of electrons outside the atomic nucleus. Because Li... + The number of electrons outside the nucleus is significantly less than that of Na. + Its ability to scatter X-rays is relatively weak. During the modification process, Li... + After successfully replacing specific Na sites in the surface or near-surface lattice structure of the original NFPP matrix, the electron density arrangement on specific crystal planes such as (011) and (210) changes, directly leading to relative changes in its structure factor and diffraction peak intensity. Therefore, this I (011) / I (210) The significant increase in the ratio strongly confirms, from the crystal structure level, that Li + It has been successfully introduced into the NFPP lattice through ion exchange, achieving effective modification of its surface structure.

[0072] To verify the electrochemical performance of the surface-modified sodium iron phosphate pyrophosphate (NFPP-Li) cathode material, it was assembled into a coin cell for testing. The specific battery assembly and testing conditions were as follows: The cathode material prepared in Example 1 of this invention (after exposure to air for 7 days), the conductive agent (Super P), and the binder (polyvinylidene fluoride PVDF) were mixed in a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added and ground into a uniform slurry. This slurry was coated onto aluminum foil, vacuum dried, rolled, and punched to form the cathode sheet. A CR2032 coin cell was assembled in an argon-filled glove box, using a sodium metal sheet as the counter electrode, glass fiber as the separator, and 1.0 M NaClO4 dissolved in EC / DEC at a volume ratio of 1:1 as the electrolyte. After the assembled battery was allowed to stand at room temperature for 12 hours, a constant current charge-discharge test was performed on the battery testing system, with a voltage window of 2.0 V ~ 4.0 V (vs. NaClO4). + / Na), the test current density was 10 C, and the results were as follows: Figure 2 As shown.

[0073] from Figure 2As can be seen, the modified sample achieved an initial efficiency of 90.26%, while the unmodified sample only achieved 89.04%. After 900 cycles at 10C, the modified sample retained 98.39% of its capacity, while the unmodified sample retained only 80.95%. These results demonstrate that the modified NFPP-Li-1 obtained in this invention significantly improves the stability of the battery.

[0074] To verify the water resistance of the modified cathode material of this invention, a slurry was prepared using a water-based polyacrylic acid (PAA) binder and pure water solvent, and batteries were assembled and subjected to cycle performance testing. The specific battery assembly and testing conditions are as follows: The cathode material (NFPP-Li-1), conductive agent (Super P), and PAA prepared in Example 1 of this invention were mixed at a mass ratio of 8:1:1, and an appropriate amount of water was added to grind it into a uniform slurry. This slurry was coated onto aluminum foil, vacuum dried, rolled, and punched to form the cathode sheet. A CR2032 coin cell was assembled in an argon-filled glove box, using a sodium metal sheet as the counter electrode, glass fiber as the separator, and 1.0 M NaClO4 dissolved in EC / DEC at a volume ratio of 1:1 as the electrolyte. After the assembled battery was allowed to stand at room temperature for 12 hours, cycle testing was performed on the battery testing system, with a voltage window of 2.0 V ~ 4.0 V (vs. NaClO4). + / Na), the test current density was 10 C. The test results are as follows: Figures 3-5 As shown.

[0075] Figure 3 This is a long-cycle comparison of NFPP-Li-1 (modified sample) and NFPP (original sample) obtained by pulping with water-based polyacrylic acid (PAA) binder and pure water solvent. The test results show that the unmodified original sample experiences a precipitous capacity decay and a decrease in coulombic efficiency after approximately 450 cycles, revealing a structural defect where its lattice is highly susceptible to moisture erosion and collapse. In contrast, the modified sample exhibits almost zero capacity decay over 500 cycles, and its coulombic efficiency remains consistently close to 100%. This significant contrast strongly confirms that surface Li... + The ion exchange not only acts as a "pillar" within the crystal lattice but also constructs a robust structural protective layer on the material surface, completely blocking water molecule invasion and effectively inhibiting the dissolution of active elements. Therefore, this data conclusively proves that the modified material of this invention possesses excellent water stability, perfectly compatible with environmentally friendly water-based binders (such as PAA) and pure water processing technologies, greatly reducing the cost and environmental sensitivity of electrode production.

[0076] Figure 4 This shows the dQ / dV curves for NFPP-Li-1 (modified sample) and NFPP (original sample) of this invention. Figure 4As shown, the dQ / dV curves of NFPP-Li-1 (modified sample) and NFPP (original sample) exhibit two significant electrochemical differences, which fully confirm the beneficial effects of the modification method of this invention from a microscopic mechanism perspective: (1) Weakening of characteristic peaks in the first cycle: The characteristic oxidation peak intensity of the modified sample at approximately 2.95V (corresponding to sodium removal at the Na3 site) in the first cycle was significantly lower than that of the original sample in the first cycle, and also lower than that of its own characteristic oxidation peak intensity in subsequent cycles (cycles 2-5). This anomaly directly proves that during the liquid-phase exchange stage, Li + It has successfully and preferentially occupied the most thermodynamically unstable Na3 site in the crystal lattice, resulting in Na3 being able to be extracted from this site during the first charge. + The activity was significantly reduced (the characteristic oxidation peak at approximately 2.95V was extremely weak in the first cycle), but recovered after subsequent cycles with re-sodium insertion. Li + Precise anchoring of the Na3 site plays a crucial "pillar" role, effectively suppressing lattice collapse and phase transition caused by deep charge and discharge.

[0077] (2) Decreased peak potential difference (ΔV): Further comparison shows that the potential difference (ΔV) between the oxidation and reduction peaks of the modified sample is significantly smaller than that of the unmodified original sample. This indicates that the modification effectively reduces the electrochemical polarization of the battery. Due to the doping of Li... + The radius (0.76 Å) is smaller than that of Na. + (1.02 Å) is smaller, and its site-directed doping not only broadens the alkali metal ion transport channels inside the lattice, but also reduces the activation energy of ion diffusion and the interfacial charge transfer impedance.

[0078] In summary, the weakening of the characteristic oxidation peak intensity and the decrease in polarization voltage in the dQ / dV curve conclusively prove that Li + Ion exchange modification endows the material with both extremely high structural stability and excellent ion transport kinetics. This dual optimization at the microscopic level perfectly echoes and explains the macroscopic beneficial effect mentioned earlier, which is that "the specific energy of the modified material remains highly stable or shows an upward trend in the first 5 discharge cycles."

[0079] Figure 5 The constant current charge-discharge curves (a) and rate performance (b) of NFPP-Li-1 (modified sample) and NFPP (original sample) prepared in Example 1 of this invention are shown. Figure 2 As can be seen in (a), under the stringent aqueous slurry (deionized water solvent) coating process, the surface-modified sodium iron pyrophosphate phosphate (modified sample) and the unmodified original sample exhibit significant differences in the first charge-discharge test. This provides profound confirmation of the macroscopic electrochemical behavior of Li +Microscopic mechanism of doping modification: (1) Improvement of first-cycle capacity and water erosion resistance mechanism: It can be seen from the first-cycle charge-discharge curve that the first-cycle discharge specific capacity of the modified sample is significantly higher than that of the original sample. The core reason for this capacity advantage is that the lithium-rich surface layer obtained by modification endows the material with a "water erosion barrier" and a "pillar effect" inside the lattice. The original NFPP lattice is extremely sensitive to moisture. During the long-term stirring of water-based slurry preparation, the surface Na + It is highly susceptible to dissolution and irreversible surface phase transitions (forming an electrochemically inert layer), leading to capacity loss. This invention addresses this by introducing Li... + Replacing specific Na sites on the surface not only acts as a "rivet" support inside the structure, but also builds a robust protective interface on the material's periphery, completely isolating water molecules from invasion and maximizing the retention of reversible intercalation and deintercalation active sodium ions. (2) Reduction of voltage plateau spacing (ΔV) and kinetic optimization: Further observation of the plateau distribution of the constant current charge-discharge curves shows that the charging plateau of the modified sample decreases slightly, while the discharging plateau increases significantly, meaning that the spacing (ΔV) between the charge-discharge voltage plateaus is significantly reduced. This means that the electrochemical polarization of the modified sample is significantly suppressed. First, due to the effective protection of the surface Li layer, the thick, high-resistivity surface byproduct layer ("dead layer") irreversibly generated by water erosion in the original sample is avoided, thus ensuring extremely efficient charge transfer at the electrode / electrolyte interface. Second, the Li that replaces the Li entering the lattice + The ionic radius (0.76 Å) is significantly smaller than that of Na. + (1.02 Å), this size difference effectively broadens the three-dimensional diffusion channels of alkali metal ions within the lattice, reducing the Na... + The activation energy for migration. Therefore, the modified sample exhibits a lower overpotential (polarization) and a smoother, more sustained voltage plateau. From Figure 2 As can be seen in (b), unlike the original sample, the modified sample showed a stable or increasing discharge specific energy (Wh / kg) in the first 5 cycles of the half-cell test, indicating that the locked Na3 sites in the modified sample were gradually "awakened" and re-participated in the electrochemical reaction.

[0080] In summary, this invention utilizes lithium / sodium ion exchange surface engineering to control Na3 sites. This not only anchors unstable Na3 sites on the surface during storage using strong chemical bonds (such as Li-O bonds) to prevent air erosion and improve the air stability of the sodium iron phosphate pyrophosphate cathode material, but also overcomes the activation barrier during the first charge of the battery, achieving reversible activation of these sites. This non-sacrificial modification strategy not only solves the air stability problem but also exhibits an excellent trend of increasing or maintaining energy density as Na3 sites are gradually released in subsequent cycles, thus significantly outperforming traditional sacrificial potential modification methods.

[0081] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A sodium iron phosphate pyrophosphate cathode material, characterized in that, Includes the kernel and the shell; The general chemical formula of the kernel is Na. a Fe b (PO4) c (P2O7) d Where 2≤a≤4, 1.5≤b≤4, 1≤c≤2, and 0.5≤d≤2; The chemical formula of the outer shell is Na. a-x Li x Fe b (PO4) c (P2O7) d , of which 0 <x<a。 2. The sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The thickness of the outer shell is 5~50 nm.

3. The sodium iron phosphate pyrophosphate cathode material according to claim 1, characterized in that, The core is Na4Fe3(PO4)2P2O7, and the outer shell is Na. 4-x Li x Fe3(PO4)2P2O7, of which 0 <x<1。 4. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to any one of claims 1 to 3, characterized in that, include: Sodium iron phosphate pyrophosphate matrix powder was added to a lithium salt solution to form a mixed slurry, which was then stirred and impregnated to initiate the Li... + / Na + Ion exchange reaction causes lithium ions to replace sodium ions on the surface of the sodium iron phosphate matrix lattice; The mixed slurry after the reaction is subjected to solid-liquid separation, and the obtained solid is washed and dried. The dried powder was placed in an inert protective atmosphere for high-temperature heat treatment to obtain the sodium iron phosphate pyrophosphate cathode material.

5. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 4, characterized in that, The lithium salt is selected from one or more of lithium nitrate, lithium hydroxide, lithium acetate, and lithium chloride.

6. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 4, characterized in that, The solvent in the lithium salt solution is anhydrous ethanol, n-propanol, or isopropanol.

7. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 4, characterized in that, The concentration of the lithium salt solution is controlled at 0.05~2 mol / L.

8. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 4, characterized in that, The reaction temperature of the ion exchange reaction is controlled at 15~80℃, and the reaction time is controlled at 12~48 hours.

9. The method for preparing the sodium iron phosphate pyrophosphate cathode material according to claim 4, characterized in that, The high-temperature heat treatment temperature is 200~500℃.

10. A sodium-ion battery, characterized in that, The cathode material includes sodium iron phosphate pyrophosphate as described in any one of claims 1 to 3.

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