Negative enthalpy element multi-element doped composite sodium ferric phosphate material, sodium ion battery and preparation method of sodium ion battery

Abstract

The invention relates to the technical field of sodium ion battery positive electrode materials, and discloses a negative enthalpy element multi-element doped composite sodium ferric phosphate material, a sodium ion battery and a preparation method of the sodium ion battery. The chemical structural formula of the negative enthalpy element multi-element doped composite sodium iron phosphate material is Na < 2 + z-x > Fe < 1 + z-y > M < y > (PO4) < z > P < 2 > O < 7 > (at) C, wherein M is at least three of Ti, Zr, V, Ni, Nb, Al or Cr; 0 < x < = 0.3, and 0.03 < y < = 0.3. According to the negative enthalpy element multi-element doped composite sodium iron phosphate material, system energy is reduced through the negative enthalpy effect of the doped elements, conductivity is improved through energy band engineering, structural integrity and redox activity are improved through the multi-element synergistic effect, and a transmission channel is widened, so that the material has excellent stability, rate capability and specific capacity; the specific capacity can reach 123 mAh / g under the 0.1 C multiplying power, and the specific capacity still reaches 106 mAh / g under the 10 C multiplying power; and the capacity retention rate is close to 100% after 300 times of circulation under 10C multiplying power.

Description

Technical Field

[0001] This application relates to the field of sodium-ion battery cathode material technology, specifically to a negative enthalpy element-doped composite sodium iron phosphate material, a sodium-ion battery and its preparation method. Background Technology

[0002] Sodium-ion batteries are considered a highly promising large-scale energy storage technology after lithium-ion batteries due to their abundant resources, low cost, and excellent low-temperature performance. Among the many cathode material systems for sodium-ion batteries, polyanionic materials have attracted much attention due to their robust three-dimensional framework structure and excellent thermal stability. Among them, the composite sodium iron phosphate Na4Fe3(PO4)2P2O7 (NFPP) exhibits a high theoretical capacity (approximately 129 mAh / g) and a suitable operating voltage (~3.1V vs Na4Fe3(PO4)2P2O7). + With its low Na content and extremely small volume change rate (< 4%), it is a cathode material with great commercial potential.

[0003] However, NFPP materials themselves have significant drawbacks. First, NFPP materials have low intrinsic electronic conductivity. The [PO4] and [P2O7] polyanionic groups isolate the [FeO6] octahedron, blocking electron transport channels and resulting in poor rate performance. Second, NFPP materials exhibit lattice distortion and phase transitions. During deep charge-discharge processes, repeated insertion and extraction of sodium ions can cause local lattice distortion, especially under high-rate or long-cycle conditions, leading to irreversible structural degradation and affecting cycle life. Third, the synthesis process of NFPP materials is complex. Existing improvement methods often employ traditional solid-state or sol-gel methods, which frequently require multi-step sintering or complex secondary doping processes (such as organic solvent dispersion and secondary impregnation), increasing production costs and process control difficulties.

[0004] To address the aforementioned issues, elemental doping is currently one of the most effective modification strategies. Traditional doping often focuses on single or dual elements, primarily utilizing differences in ionic radii to increase interlayer spacing. However, this simple geometric doping approach fails to fundamentally solve the structural stability problem from a thermodynamic perspective. Summary of the Invention

[0005] This application provides a negative enthalpy element-doped composite sodium iron phosphate material, a sodium-ion battery and its preparation method, aiming to solve the technical problems of poor conductivity and structural instability at high rates of existing Na4Fe3(PO4)2P2O7 cathode materials.

[0006] To achieve the above objectives, the present application adopts the following technical solution.

[0007] The first aspect of this application provides a negative enthalpy element-doped composite sodium iron phosphate material with the chemical formula Na. 2+z-x Fe 1+z-y M y (PO4) z P2O7@C;

[0008] Where M is at least three of Ti, Zr, V, Ni, Nb, Al or Cr; 1≤z≤2, 0<x≤0.3, 0.03<y≤0.3.

[0009] Preferably, in the chemical structural formula of the negative enthalpy element-doped composite sodium iron phosphate material, M represents Ni, Ti, and V;

[0010] Alternatively, M can be Ni, Ti, V, or Zr;

[0011] Alternatively, M can be Ni, Ti, V, Zr, or Al;

[0012] Alternatively, M can be Ni, Ti, V, Al, Nb, or Cr.

[0013] More preferably, the chemical structural formula of the negative enthalpy element-doped composite sodium iron phosphate material is any one of the following chemical structural formulas:

[0014] Na 3.91 Fe 2.91 Ni 0.03 V 0.03 Ti 0.03 (PO4)2P2O7@C;

[0015] Na 3.89 Fe 2.90 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03 (PO4)2P2O7@C;

[0016] Na 3.86 Fe 2.87 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03 Al 0.03 (PO4)2P2O7@C;

[0017] Na 3.82 Fe 2.84 Ni 0.03 V 0.03 Nb 0.01 Ti 0.03 Al 0.03 Cr 0.03(PO4)2P2O7@C;

[0018] Na 2.82 Fe 1.84 Ni 0.03 V 0.03 Nb 0.01 Ti 0.03 Al 0.03 Cr 0.03 (PO4)P2O7@C.

[0019] A second aspect of this application provides a method for preparing the aforementioned negative enthalpy element-doped composite sodium iron phosphate material, comprising:

[0020] S1, Fe source, Na source, P source, organic carbon source, doped element precursor and dispersant are dispersed in water and ground to obtain a uniform precursor slurry;

[0021] S2, the precursor slurry is spray-dried to obtain precursor powder;

[0022] S3, the precursor powder is calcined at high temperature under an inert atmosphere to obtain a negative enthalpy element-doped composite sodium iron phosphate material.

[0023] Preferably, the Fe source includes at least one of ferrous sulfate, ferric phosphate, ferric oxide, ferrous oxalate, or ferrous phosphate;

[0024] The Na source includes at least one of sodium carbonate, sodium hydroxide, sodium sulfate, sodium dihydrogen phosphate, or sodium citrate.

[0025] The P source includes at least one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, or triammonium phosphate.

[0026] The organic carbon source includes at least one of ascorbic acid, citric acid monohydrate, glucose, sucrose, fructose, or oxalic acid dihydrate.

[0027] The dispersing agent includes at least one of polyvinylpyrrolidone or polyethylene glycol.

[0028] Preferably, the precursor of the dopant element is at least one selected from oxides, hydroxides, chlorides, nitrates, acetates, sulfates, oxalates, or ammonium salts of the dopant element.

[0029] The doping element is at least three of the following: Ti, Zr, V, Ni, Nb, Al, or Cr.

[0030] More preferably, the dopant precursor is at least three of the following: titanium tetrachloride, zirconium acetate, ammonium metavanadate, nickel acetate, aluminum acetate, chromium nitrate, or niobium pentoxide.

[0031] Preferably, the mass of the organic carbon source is 5% to 15% of the total mass of the Fe, Na, and P sources; and the mass of the auxiliaries is 1% to 5% of the total mass of the Fe, Na, and P sources.

[0032] The total molar proportion of the doping elements at the Fe sites is 1% to 10% of the Fe sites, and the number of doping elements is 3 to 7.

[0033] A third aspect of this application provides the application of the aforementioned negative enthalpy element-doped composite sodium iron phosphate material in sodium-ion batteries.

[0034] In a fourth aspect, this application provides a sodium-ion battery in which the positive electrode active material is the aforementioned multi-element doped composite sodium iron phosphate material with negative enthalpy elements.

[0035] Compared with the prior art, the beneficial effects of this application are as follows:

[0036] The negative enthalpy element-doped composite sodium iron phosphate material of this application reduces the system energy through the negative enthalpy effect of the doping elements, improves conductivity through band engineering, and enhances structural integrity, redox activity, and broadens transport channels through the synergistic effect of multiple elements, thereby exhibiting excellent stability, rate performance, and specific capacity.

[0037] The negative enthalpy effect reduces the system energy: Unlike traditional high-entropy materials that passively rely on entropy to offset positive enthalpy of mixing, this application actively selects and dops elements with strong chemical affinity that can form exothermic mixing reactions (with negative enthalpy of mixing), such as combinations of Ni, Ti, V, Al, Zr, Nb, and Cr. Negative enthalpy of mixing implies a strong attractive interaction between components, resulting in a more uniform solid solution. Furthermore, according to the Gibbs free energy formula: ΔG = ΔH - TΔS, when ΔH is negative, ΔG tends to be negative regardless of temperature. Therefore, the formed solid solution is the lowest-energy stable state, rather than a metastable state stable at high temperatures, thus constructing a robust lattice that is both high in configurational entropy and intrinsically stable in energy.

[0038] Band engineering improves conductivity: Doping elements are high-valence transition metals. After doping at the Fe site, they not only introduce sodium vacancies through charge compensation mechanisms, which is conducive to sodium ion diffusion, but also change the band structure of the material by introducing d orbitals, reducing the band gap and thus significantly improving the intrinsic electronic conductivity of the material.

[0039] Multi-element synergistic effect: Dopants with more negative mixing enthalpy (Al, Ti, Zr) can provide structural support. Their extremely high oxygen bond energy (MO) is significantly higher than that of the main element Fe-O, which is the primary contributor to the negative mixing enthalpy. They act as strong support points in the crystal lattice, preventing the collapse or distortion of FeO6 octahedra in the desodium state, thus achieving "zero strain" characteristics and allowing the material to maintain structural integrity at 10 C or even higher rates. V / Ni provides redox activity, contributing higher capacity, while Cr / Nb adjusts lattice parameters to broaden transport channels.

[0040] The negative enthalpy element-doped composite sodium iron phosphate material has excellent electrochemical performance. Its specific capacity can reach 123 mAh / g at 0.1 C rate and still reach 106 mAh / g at a high rate of 10 C. After 300 cycles at 10 C rate, the capacity retention rate is close to 100%. Attached Figure Description

[0041] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 This is a SEM image of the negative enthalpy element-doped composite sodium iron phosphate material from Example 2.

[0043] Figure 2 The XRD patterns are of the negative enthalpy element-doped composite sodium iron phosphate material of Example 2 and the undoped composite sodium iron phosphate material of Comparative Example 1.

[0044] Figure 3 The image shows the charge-discharge curves of the battery assembled from the negative enthalpy element multi-doped composite sodium iron phosphate material in Example 2 at a rate of 0.1 C.

[0045] Figure 4 The image shows the cycle performance of the battery assembled from the negative enthalpy element multi-doped composite sodium iron phosphate material of Example 2 at a high rate of 10 C. Detailed Implementation

[0046] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0047] In the following description of this embodiment, the terms "including", "comprising", "having", and "containing" are all open-ended terms, meaning that they include but are not limited to.

[0048] In the following description of this embodiment, the term "and / or" is used to describe the association relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, B existing alone, and A and B existing simultaneously. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0049] In the following description of this embodiment, the term "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0050] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0051] Those skilled in the art should understand that, in the following description of the embodiments of this application, the sequence of numbers does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0052] Those skilled in the art will understand that the numerical ranges in the embodiments of this application should be understood as each intermediate value between the upper and lower limits of the specifically disclosed range. Each smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this application. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0053] Unless otherwise stated, the technical / scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. While this application describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this application. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0054] Firstly, this application provides a negative enthalpy element-doped composite sodium iron phosphate material, the chemical structural formula of which is Na. 2+z-x Fe 1+z-y M y (PO4) z P2O7@C;

[0055] Where M is at least three of Ti, Zr, V, Ni, Nb, Al or Cr; 1≤z≤2, 0<x≤0.3, 0.03<y≤0.3.

[0056] As a preferred embodiment of this application, in the negative enthalpy element multi-doped composite sodium iron phosphate material, the doping element M can be a combination of Ni, Ti and V;

[0057] Alternatively, M can be a combination of Ni, Ti, V and Zr;

[0058] Alternatively, M can be a combination of Ni, Ti, V, Zr and Al;

[0059] Alternatively, M can be a combination of Ni, Ti, V, Al, Nb, and Cr.

[0060] As a more preferred embodiment of this application, the chemical structural formula of the negative enthalpy element-doped composite sodium iron phosphate material is any one of the following chemical structural formulas:

[0061] Na 3.91 Fe 2.91 Ni 0.03 V 0.03 Ti 0.03 (PO4)2P2O7@C;

[0062] Na 3.89 Fe 2.90 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03 (PO4)2P2O7@C;

[0063] Na 3.86 Fe 2.87 Ni 0.03 V0.03 Zr 0.01 Ti 0.03 Al 0.03 (PO4)2P2O7@C;

[0064] Na 3.82 Fe 2.84 Ni 0.03 V 0.03 Nb 0.01 Ti 0.03 Al 0.03 Cr 0.03 (PO4)2P2O7@C;

[0065] Na 2.82 Fe 1.84 Ni 0.03 V 0.03 Nb 0.01 Ti 0.03 Al 0.03 Cr 0.03 (PO4)P2O7@C.

[0066] The negative enthalpy element-doped composite sodium iron phosphate material of this application reduces the system energy through the negative enthalpy effect of the doping elements, improves conductivity through band engineering, and enhances structural integrity, redox activity, and broadens transport channels through the synergistic effect of multiple elements, thereby exhibiting excellent stability, rate performance, and specific capacity.

[0067] Unlike traditional high-entropy materials that passively rely on entropy to offset positive entropy, this application actively selects and dops elements with strong chemical affinity that can form exothermic mixing reactions (with negative entropy), such as combinations of Ni, Ti, V, Al, Zr, Nb, and Cr. Negative entropy implies strong attraction between components, resulting in more uniform solid solution. Furthermore, according to the Gibbs free energy formula: ΔG = ΔH - TΔS, when ΔH is negative, ΔG tends to be negative regardless of temperature. Therefore, the formed solid solution is a stable state with the lowest energy, rather than a metastable state stable at high temperatures, thus constructing a robust lattice that is both high in configurational entropy and intrinsically stable in energy.

[0068] In the negative enthalpy element-doped composite sodium iron phosphate material of this application, the doping elements are high-valence transition metals. After doping at the Fe site, they not only introduce sodium vacancies through the charge compensation mechanism, which is conducive to sodium ion diffusion, but also change the band structure of the material by introducing the d orbitals of these elements, reducing the band gap, thereby significantly improving the intrinsic electronic conductivity of the material.

[0069] In the negative enthalpy element-doped composite sodium iron phosphate material of this application, multiple dopants form a synergistic effect. Dopants with more negative mixed enthalpy values ​​(Al, Ti, Zr) can play a structural support role, possessing extremely high oxygen bond energies (MO) significantly higher than the Fe-O bond energies of the main element, such as Ti-O bond energy ~662 kJ / mol and Zr-O bond energy ~776 kJ / mol, which are much higher than Fe-O's ~407 kJ / mol. They act as strong support points in the crystal lattice, playing a "pinning" role and preventing the collapse or distortion of FeO6 octahedra in the desodium state within the crystal lattice, achieving "zero-strain" characteristics, allowing the material to maintain structural integrity even at 10 C or even higher rates. V / Ni provides redox activity, contributing higher capacity, while Cr / Nb adjusts lattice parameters to broaden transport channels.

[0070] Secondly, this application provides a method for preparing the above-mentioned negative enthalpy element-doped composite sodium iron phosphate material, including:

[0071] S1, Fe source, Na source, P source, organic carbon source, doped element precursor and dispersant are dispersed in water and ground to obtain a uniform precursor slurry;

[0072] Specifically, a dispersion is obtained by dispersing Fe source, Na source, P source, organic carbon source, dopant precursor, and dispersant in water and stirring thoroughly at room temperature until homogeneous. The dispersion is then transferred to a nano-mill and milled for 6–10 hours at a temperature below 35°C and a rotation speed of 1800–2200 r / min, controlling the slurry particle size D50 to < 0.5 μm, resulting in a uniformly mixed precursor slurry. The organic carbon source comprises 5%–15% of the total mass of Fe, Na, and P sources; the dispersant comprises 1%–5% of the total mass of Fe, Na, and P sources; and the total molar proportion of the dopant elements at Fe sites is 1%–10% of the Fe sites, with 3–7 different dopant elements present.

[0073] In this application, the Fe source is selected from at least one of ferrous sulfate, ferric phosphate, ferric oxide, ferrous oxalate, or ferrous phosphate, preferably ferrous oxalate or ferric phosphate.

[0074] The Na source is selected from at least one of sodium carbonate, sodium hydroxide, sodium sulfate, sodium dihydrogen phosphate, or sodium citrate, preferably sodium carbonate.

[0075] The P source is selected from at least one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, or triammonium phosphate, preferably ammonium dihydrogen phosphate.

[0076] The organic carbon source is selected from at least one of ascorbic acid, citric acid monohydrate, glucose, sucrose, fructose or oxalic acid dihydrate, preferably glucose.

[0077] The dispersing agent is selected from at least one of polyvinylpyrrolidone or polyethylene glycol.

[0078] In this application, the precursor of the doping element is at least one of the following: oxide, hydroxide, chloride, nitrate, acetate, sulfate, oxalate, or ammonium salt of the doping element. The doping element is at least three of the following: Ti, Zr, V, Ni, Nb, Al, or Cr, such as any three, four, five, six, or seven of the following. Specifically, the precursor of the doping element is preferably at least three of the following: titanium tetrachloride, zirconium acetate, ammonium metavanadate, nickel acetate, aluminum acetate, chromium nitrate, or niobium pentoxide.

[0079] S2, the precursor slurry is spray-dried to obtain precursor powder;

[0080] Specifically, the precursor slurry is granulated a second time using a spray dryer to obtain precursor powder. During spray drying, the inlet temperature is 200~240℃ and the outlet temperature is 90~110℃.

[0081] S3, the precursor powder is calcined at high temperature under an inert atmosphere to obtain a negative enthalpy element-doped composite sodium iron phosphate material.

[0082] Specifically, the precursor powder is calcined at high temperature under an inert protective atmosphere and cooled to room temperature to obtain a negative enthalpy element-doped iron-based mixed phosphate cathode material with a uniform carbon layer on the surface. The protective gas is one of nitrogen, argon, or argon / hydrogen mixture (hydrogen content 5%).

[0083] The specific process of the calcination treatment is as follows: at 2~10℃ min -1 Heat to 300-400℃ at a heating rate, hold for 5-10 hours, and then continue heating at a rate of 2-5℃ / min. -1 The heating rate is increased to 500~600℃, and the temperature is maintained for 8~12 h; after cooling to room temperature, the sample is taken out for grinding and sieving.

[0084] The negative enthalpy element-doped composite sodium iron phosphate material of this application has excellent electrochemical performance. Its specific capacity can reach 123 mAh / g at 0.1C rate and still reach 106 mAh / g at a high rate of 10C. After 300 cycles at 10C rate, the capacity retention rate is close to 100%, and it can be used as a positive electrode active material in sodium-ion batteries.

[0085] This application also provides a sodium-ion battery, wherein the active material in the positive electrode is the aforementioned negative enthalpy element-doped composite sodium iron phosphate material. This application does not impose any special limitations on the conductive agent, binder, and current collector of the positive electrode; commonly used conductive agents, binders, and current collectors in the art can all be used.

[0086] In the sodium-ion battery of this application, no special limitations are made on the negative electrode, electrolyte and separator, and commonly used negative electrodes, electrolytes and separators in the art can be used.

[0087] The present application will be further illustrated by the following examples.

[0088] Example 1

[0089] Example 1 provides a negative enthalpy element-doped composite sodium iron phosphate material, whose chemical structural formula is Na. 3.91 Fe 2.91 Ni 0.03 V 0.03 Ti 0.03 (PO4)2P2O7@C. Its preparation methods include:

[0090] S1, 1.455 mol of ferrous oxalate, 0.9775 mol of sodium carbonate, and 2 mol of ammonium dihydrogen phosphate were weighed as the main raw materials; 42 g of glucose was weighed as the carbon source, and 18 g of PEG-4000 was weighed as the auxiliary agent; 0.015 mol of nickel acetate, 0.015 mol of titanium tetrachloride, and 0.015 mol of ammonium metavanadate were weighed as the dopant.

[0091] The above raw materials were added to deionized water and stirred to mix. The mixture was then poured into the chamber of a sand mill and ground at 2000 rpm for 6 hours to obtain a uniform precursor slurry.

[0092] S2, the precursor slurry is spray-dried (inlet air 230℃ / outlet air 100℃) and granulated to obtain precursor powder.

[0093] S3 was heated to 350℃ and held for 6 hours at a heating rate of 10℃ / min under a nitrogen atmosphere, and then heated to 550℃ and calcined for 10 hours at a heating rate of 2℃ / min to obtain the negative enthalpy element-doped composite sodium iron phosphate cathode material.

[0094] Example 2

[0095] Example 2 provides a negative enthalpy element-doped composite sodium iron phosphate material, whose chemical structural formula is Na. 3.89 Fe 2.90 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03(PO4)2P2O7@C. Its preparation methods include:

[0096] S1, weigh out 1.45 mol of iron phosphate, 0.9725 mol of sodium carbonate, and 0.55 mol of ammonium dihydrogen phosphate as main raw materials; weigh out 28 g of glucose as carbon source and 20 g of PVP as auxiliary agent; weigh out 0.015 mol of nickel acetate, 0.015 mol of titanium tetrachloride, 0.015 mol of ammonium metavanadate, and 0.005 mol of zirconium acetate as dopants;

[0097] The above raw materials were added to deionized water and stirred to mix. The mixture was then poured into the chamber of a sand mill and ground at 2000 rpm for 6 hours to obtain a uniform precursor slurry.

[0098] S2 is the same as in Example 1;

[0099] S3 is the same as in Example 1.

[0100] Example 3

[0101] Example 3 provides a negative enthalpy element-doped composite sodium iron phosphate material, whose chemical structural formula is Na. 3.86 Fe 2.87 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03 Al 0.03 (PO4)2P2O7@C. Its preparation methods include:

[0102] S1, using 1.435 mol of iron phosphate, 0.965 mol of sodium carbonate, and 0.565 mol of ammonium dihydrogen phosphate as the main raw materials; using 28 g of glucose as the carbon source and 20 g of PVP as an auxiliary agent; and using 0.015 mol of nickel acetate, 0.015 mol of titanium tetrachloride, 0.015 mol of ammonium metavanadate, 0.005 mol of zirconium acetate, and 0.015 mol of aluminum acetate as dopants;

[0103] The above raw materials were added to deionized water and stirred to mix. The mixture was then poured into the chamber of a sand mill and ground at 2000 rpm for 6 hours to obtain a uniform precursor slurry.

[0104] S2 is the same as in Example 1;

[0105] S3 is the same as in Example 1.

[0106] Example 4

[0107] Example 4 provides a negative enthalpy element-doped composite sodium iron phosphate material, whose chemical structural formula is Na. 3.82 Fe 2.84 Ni0.03 V 0.03 Nb 0.01 Ti 0.03 Al 0.03 Cr 0.03 (PO4)2P2O7@C. Its preparation methods include:

[0108] S1, using 1.42 mol of iron phosphate, 0.955 mol of sodium carbonate, and 0.58 mol of ammonium dihydrogen phosphate as the main raw materials; using 28 g of glucose as the carbon source and 20 g of PVP as the auxiliary agent; and using 0.015 mol of nickel acetate, 0.015 mol of titanium tetrachloride, 0.015 mol of ammonium metavanadate, 0.0025 mol of niobium pentoxide, 0.015 mol of aluminum acetate, and 0.015 mol of chromium nitrate as dopants;

[0109] The above raw materials were added to deionized water and stirred to mix. The mixture was then poured into the chamber of a sand mill and ground at 2000 rpm for 6 hours to obtain a uniform precursor slurry.

[0110] S2 is the same as in Example 1;

[0111] S3 is the same as in Example 1.

[0112] Example 5

[0113] Example 5 provides a negative enthalpy element-doped composite sodium iron phosphate material, whose chemical structural formula is Na. 2.82 Fe 1.84 Ni 0.03 V 0.03 Nb 0.01 Ti 0.03 Al 0.03 Cr 0.03 (PO4)P2O7@C.

[0114] The difference between Example 5 and Example 4 is the amount of the main raw materials used. Specifically, the amount of iron phosphate is 0.92 mol, sodium carbonate is 0.705 mol, and ammonium dihydrogen phosphate is 0.58 mol; the rest are the same as in Example 4.

[0115] Comparative Example 1

[0116] The undoped composite sodium iron phosphate material has the chemical formula Na4Fe3(PO4)2P2O7@C.

[0117] Comparative Example 2

[0118] The undoped composite sodium iron phosphate material has the chemical formula Na3Fe2(PO4)P2O7@C.

[0119] The negative enthalpy element-doped composite sodium iron phosphate material Na prepared in Example 2 3.89 Fe 2.90 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03 The morphology test and performance evaluation of (PO4)2P2O7@C are as follows:

[0120] 1. Morphological testing

[0121] Its SEM image is as follows Figure 1 As shown. From Figure 1 It can be seen that it is a micrometer-sized sphere.

[0122] 2. XRD Testing

[0123] Na in Example 2 3.89 Fe 2.90 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03 The XRD patterns of (PO4)2P2O7@C and Na4Fe3(PO4)2P2O7@C in Comparative Example 1 are shown below. Figure 2 As shown. From Figure 2 It can be seen that the negative enthalpy multi-component doped cathode material in Example 2 has fewer impurities and increased characteristic peak intensity compared to the undoped cathode material in Comparative Example 1, indicating that the negative enthalpy effect and multi-component synergistic effect can effectively improve the phase purity and crystallinity of the cathode material.

[0124] 3. Electrochemical performance testing

[0125] The positive electrode material Na from Example 2 was weighed according to a mass ratio of 7:2:1. 3.89 Fe 2.90 Ni 0.03 V 0.03 Zr 0.01 Ti 0.03 One g of (PO4)2P2O7@C, Super P carbon black, and PVDF binder were mixed evenly with an appropriate amount of NMP and coated onto the surface of a bright aluminum foil to a thickness of 150-200 micrometers. After vacuum drying, the mixture was stamped to obtain a positive electrode sheet with a diameter of 12 mm. A coin cell was assembled using sodium metal as the negative electrode and a 1 mol / L NaClO4 / EC:PC (1:1 Vol%) / 5% FEC electrolyte.

[0126] The electrochemical performance of this coin cell was tested using the Blue Lightning system. Its charge-discharge curves at 0.1 C rate (1C = 129 mA / g, 1.5V~4.3V) are shown below. Figure 3As shown, its specific capacity reached 123mAh / g, which is close to the theoretical value.

[0127] The coin cell was subjected to cycle performance testing at a high rate of 10 C, and the results are as follows: Figure 4 As shown, the cycling efficiency is close to 100%, and there is basically no decay after 300 cycles at 10 C, demonstrating excellent cycling stability.

[0128] Following the above method, positive electrode sheets were prepared from the negative enthalpy element-doped composite sodium iron phosphate positive electrode materials of Examples 1-5 and the undoped composite sodium iron phosphate positive electrode materials of Comparative Examples 1-2, and then assembled into coin cells for electrochemical performance testing. The test results are shown in Table 1.

[0129] Table 1. Electrochemical performance test results of the cathode materials in the examples and comparative examples.

[0130]

[0131] As shown in Table 1, compared with the comparative examples, the cycling stability and specific capacity of the samples doped with negative enthalpy elements were significantly improved. Specifically, Examples 2 and 3 achieved 100% capacity retention after 300 cycles, demonstrating that Al / Zr / Ti, as high oxygen bond energy elements, can enhance lattice structure rigidity and thus significantly improve cycling stability. The sample prepared in Example 4 exhibited the highest 10 C specific capacity, proving that Cr / Nb doping can effectively broaden sodium ion transport channels, resulting in a more significant improvement in high-rate performance.

[0132] Although this application has been described in detail in this specification with general descriptions and specific embodiments, some modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, such modifications or improvements made without departing from the spirit of this application are all within the scope of protection claimed in this application.

Claims

1. A composite sodium iron phosphate material doped with negative enthalpy elements, characterized in that, Its chemical structural formula is Na 2+z- x Fe 1+z-y M y (PO4) z P2O7@C; Where M is at least three of Ti, Zr, V, Ni, Nb, Al or Cr; 1≤z≤2, 0<x≤0.3, 0.03<y≤0.

3.

2. The negative enthalpy element-doped composite sodium iron phosphate material according to claim 1, characterized in that, M represents Ni, Ti, and V; Alternatively, M can be Ni, Ti, V, or Zr; Alternatively, M can be Ni, Ti, V, Zr, or Al; Alternatively, M can be Ni, Ti, V, Al, Nb, or Cr.

3. The negative enthalpy element-doped composite sodium iron phosphate material according to claim 2, characterized in that, The chemical structural formula of the negative enthalpy element-doped composite sodium iron phosphate material is any one of the following chemical structural formulas: Na 3.91 Fe 2.91 Ni 0.03 V 0.03 You 0.03 (PO4)2P2O7@C; Na 3.89 Fe 2.90 Ni 0.03 V 0.03 Zr 0.01 You 0.03 (PO4)2P2O7@C; Na 3.86 Fe 2.87 Ni 0.03 V 0.03 Zr 0.01 You 0.03 Al 0.03 (PO4)2P2O7@C; Na 3.82 Fe 2.84 Ni 0.03 V 0.03 Nb 0.01 You 0.03 Al 0.03 Cr 0.03 (PO4)2P2O7@C; Na 2.82 Fe 1.84 Ni 0.03 V 0.03 Nb 0.01 You 0.03 Al 0.03 Cr 0.03 (PO4)P2O7@C.

4. The preparation method of the negative enthalpy element-doped composite sodium iron phosphate material according to claim 1, characterized in that, include: S1, Fe source, Na source, P source, organic carbon source, doped element precursor and dispersant are dispersed in water and ground to obtain a uniform precursor slurry; S2, the precursor slurry is spray-dried to obtain precursor powder; S3, the precursor powder is calcined at high temperature under an inert atmosphere to obtain a negative enthalpy element-doped composite sodium iron phosphate material.

5. The preparation method according to claim 4, characterized in that, The Fe source includes at least one of ferrous sulfate, ferric phosphate, ferric oxide, ferrous oxalate, or ferrous phosphate. The Na source includes at least one of sodium carbonate, sodium hydroxide, sodium sulfate, sodium dihydrogen phosphate, or sodium citrate. The P source includes at least one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphoric acid, or triammonium phosphate. The organic carbon source includes at least one of ascorbic acid, citric acid monohydrate, glucose, sucrose, fructose, or oxalic acid dihydrate. The dispersing agent includes at least one of polyvinylpyrrolidone or polyethylene glycol.

6. The preparation method according to claim 4, characterized in that, The precursor of the dopant element is at least one of the following: oxide, hydroxide, chloride, nitrate, acetate, sulfate, oxalate or ammonium salt of the dopant element. The doping element is at least three of the following: Ti, Zr, V, Ni, Nb, Al, or Cr.

7. The preparation method according to claim 6, characterized in that, The dopant precursor is at least three of the following: titanium tetrachloride, zirconium acetate, ammonium metavanadate, nickel acetate, aluminum acetate, chromium nitrate, or niobium pentoxide.

8. The preparation method according to claim 4, characterized in that, The organic carbon source accounts for 5% to 15% of the total mass of the Fe, Na, and P sources; the auxiliaries account for 1% to 5% of the total mass of the Fe, Na, and P sources. The total molar proportion of the doping elements at the Fe sites is 1% to 10% of the Fe sites, and the number of doping elements is 3 to 7.

9. The application of the negative enthalpy element multi-doped composite sodium iron phosphate material according to any one of claims 1-3 in sodium-ion batteries.

10. A sodium-ion battery, characterized in that, Its positive electrode active material is the negative enthalpy element multi-doped composite sodium iron phosphate material as described in any one of claims 1-3.

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