A sodium iron pyrophosphate cathode material and its preparation method, and its application in sodium-ion batteries.

Abstract

This invention provides a sodium iron pyrophosphate cathode material, its preparation method, and a sodium-ion battery. The sodium iron pyrophosphate cathode material comprises a sodium iron pyrophosphate / carbon composite core and a carbon coating layer; the chemical formula of the sodium iron pyrophosphate / carbon composite core is Na. 4‑x A x Fe y B 2.91‑y (PO4) 2‑z / 3 P2O7F z / C, where 0.01≤x≤1, 2.7≤y<2.91, 1≤z≤3, A includes any one or at least two combinations of Li, K, or Ca, and B includes any one or at least two combinations of Mg, Al, Zn, V, Co, Mn, Zr, Ti, Cu, or Y. This invention comprehensively improves the conductivity, rate performance, cycle life, and stability of sodium iron pyrophosphate cathode materials through a dual carbon structure consisting of bulk composite carbon and surface carbon coating, combined with the regulation of the sodium iron pyrophosphate composition.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to sodium-ion battery cathode materials, and more particularly to a sodium iron pyrophosphate cathode material, its preparation method, and a sodium-ion battery. Background Technology

[0002] The demand for lithium-ion batteries is constantly growing, but lithium resources are limited, leading to a growing shortage. Sodium, abundant in the Earth's crust and inexpensive, has become an effective solution to reduce dependence on lithium. However, sodium iron pyrophosphate (NFPP), as the cathode material for sodium-ion batteries, faces challenges such as poor intrinsic electronic conductivity, low sodium-ion diffusion coefficient, and poor kinetic performance. Furthermore, it suffers from insufficient structural stability during cycling, resulting in rapid capacity decay.

[0003] CN116344772A discloses a spherical sodium iron pyrophosphate cathode material and its preparation method. The cathode material consists of a sodium iron pyrophosphate matrix and a carbon coating layer, and its chemical formula is Na₄Fe₂O₃. 3-x-y Ce x Nb y (PO4)2(P2O7), 0≤x≤1, 0≤y≤1; This invention uses a co-precipitation method to prepare spherical sodium iron pyrophosphate cathode materials. The prepared sodium iron pyrophosphate cathode materials have uniform particle size distribution and uniform particle size; the diffusion path of sodium ions is shortened, giving the material better cycle performance and sodium storage performance.

[0004] CN117819507A discloses a method for preparing spherical sodium iron pyrophosphate cathode material, comprising mixing a sodium source, an iron source, a phosphorus source, and a solvent, performing a hydrothermal reaction to obtain a primary particulate precursor, mixing it with a carbon source, and then spray-drying it to obtain a spherical precursor, and finally sintering it in an inert atmosphere. Through a simple hydrothermal reaction, spray drying, and sintering process, a spherical sodium iron pyrophosphate cathode material with good electrochemical performance is obtained.

[0005] CN119008916A discloses a method for preparing sodium iron pyrophosphate cathode material. This method involves sintering a carbon source composed of organic and inorganic carbon sources to form a carbon coating layer on the surface of the sodium iron pyrophosphate cathode material. Furthermore, by selecting the types of organic and inorganic carbon sources and controlling their weight ratio, a sodium iron pyrophosphate cathode material with high purity is prepared, reducing the content of impurities and improving the specific capacity, initial efficiency, and cycle stability of the sodium iron pyrophosphate cathode material.

[0006] Therefore, it is of great significance to provide a sodium iron pyrophosphate cathode material with good conductivity, fast sodium ion migration rate, and stable structure, as well as its preparation method. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention aims to provide a sodium iron pyrophosphate cathode material, its preparation method, and a sodium-ion battery. This invention regulates the composition of the sodium iron pyrophosphate cathode material to incorporate Fe vacancies and introduces doping elements to construct a high-entropy solid solution composition. This regulates the configurational entropy S of the sodium iron pyrophosphate cathode material to 1.5R~3.1R, effectively improving its electron and ion diffusion kinetics. Simultaneously, the invention employs a dual carbon structure of bulk composite carbon and surface carbon coating, addressing the poor intrinsic electronic conductivity of the sodium iron pyrophosphate cathode material from both intraparticle and interparticle dimensions. The porous structure of the surface carbon coating also increases the contact area between the electrolyte and the cathode material, shortening the sodium ion diffusion path (reducing the distance from the electrolyte to the particle surface) and accelerating the sodium ion migration rate. Through the synergistic effect of multiple mechanisms, this invention comprehensively improves the conductivity, rate performance, cycle life, and stability of the sodium iron pyrophosphate cathode material.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a sodium iron pyrophosphate cathode material, the sodium iron pyrophosphate cathode material comprising a sodium iron pyrophosphate / carbon composite core and a carbon coating layer covering the surface of the core; the chemical formula of the sodium iron pyrophosphate / carbon composite core is Na. 4-x A x Fe y B 2.91-y (PO4) 2-z / 3 P2O7F z / C, where 0.01≤x≤1, 2.7≤y<2.91, 1≤z≤3, A includes any one or at least two combinations of Li, K or Ca, and B includes any one or at least two combinations of Mg, Al, Zn, V, Co, Mn, Zr, Ti, Cu or Y; the entropy value S of the sodium iron pyrophosphate cathode material is 1.5R~3.1R, where R is the molar gas constant, R=8.314J / (mol·K).

[0010] First, this invention modulates the composition of sodium iron pyrophosphate cathode material to give it a certain number of Fe vacancies. By introducing element A to replace Na sites and element B to replace Fe sites, a high-entropy solid solution composition is constructed. The entropy value S of its configuration entropy is controlled to be 1.5R~3.1R. With its diversified local structure, the phase transition reaction in the sodium ion insertion / extraction reaction is delayed, which can effectively suppress the phase transition during the charge and discharge process and effectively suppress the volume expansion during the charge and discharge process, thereby improving the structural stability and thus improving the cycle performance.

[0011] Meanwhile, high-entropy solid solution components can improve electron and ion diffusion kinetics, and work in conjunction with F, which has stronger electronegativity in anionic groups. - via F - Influences on the outer electronic structure of the material enhances its kinetic properties, improves the electronic conductivity and ion diffusion rate of sodium iron pyrophosphate cathode material, and thus improves rate performance; the introduction of high-entropy components can also reduce surface defects of sodium iron pyrophosphate and suppress side reactions with the electrolyte.

[0012] Secondly, this invention improves the electronic conductivity and ion diffusion rate of sodium iron pyrophosphate cathode material from both intraparticle and interparticle dimensions through a dual carbon structure of bulk composite carbon and surface carbon coating. The bulk composite carbon improves the problem of low intrinsic conductivity of sodium iron pyrophosphate cathode material, while the outer carbon coating layer connects adjacent active particles, forming an electronic conduction pathway across particles, avoiding electron transport obstruction caused by poor interparticle contact. The outer carbon coating layer has a porous structure, which can increase the contact area between the electrolyte and the active material, shorten the diffusion path of sodium ions, and accelerate the migration rate of sodium ions.

[0013] In addition, the dual carbon structure maintains the structural integrity of the cathode material. The bulk composite carbon effectively alleviates stress concentration inside the particles through mechanical support, while the outer carbon coating provides a flexible protective shell that wraps and connects the active particles, inhibiting particle aggregation and breakage during cycling. It can also act as a physical barrier to reduce side reactions between the electrolyte and the material surface.

[0014] This invention comprehensively improves the conductivity, rate performance, cycle life, and stability of sodium iron pyrophosphate cathode material through the synergistic effect of multiple mechanisms.

[0015] Preferably, the peak intensities I1 of the (211) crystal plane, I2 of the (031) crystal plane, I3 of the (022) crystal plane, and I4 of the (222) crystal plane of the sodium iron pyrophosphate cathode material satisfy: 0.2≤(I1+I2) / (I3+I4)≤0.6.

[0016] Preferably, in the unit cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 17.52 Å to 18.10 Å, the length of the b-axis is 6.48 Å to 6.53 Å, and the length of the c-axis is 10.55 Å to 10.65 Å.

[0017] Preferably, the carbon content in the core of the sodium iron pyrophosphate / carbon composite material is 1.0 wt.% to 2.0 wt.%.

[0018] Preferably, in the sodium iron pyrophosphate cathode material, the carbon coating layer has a mass percentage content of 0.2 wt.% to 1 wt.%.

[0019] Preferably, the D50 particle size of the core of the sodium iron pyrophosphate / carbon composite material is 15μm~20μm.

[0020] Preferably, the true density of the sodium iron pyrophosphate / carbon composite core is 2.5 g / cm³. 3 ~3.1g / cm 3 .

[0021] Preferably, the true density of the carbon coating layer is 1.5 g / cm³. 3 ~2.5g / cm 3 .

[0022] Preferably, the core of the sodium iron pyrophosphate / carbon composite material is a secondary particle, and the D50 particle size of the primary particle is 100nm~500nm.

[0023] In a second aspect, the present invention provides a method for preparing the sodium iron pyrophosphate cathode material as described in the first aspect, the method comprising:

[0024] Sodium source, iron source, phosphorus source, fluorine source, first carbon source, source A, and source B are dispersed in a solvent according to stoichiometric ratios and milled to obtain a dispersion. The dispersion is then spray-dried to obtain a first precursor. The precursor is mixed with a second carbon source to obtain a second precursor. The second precursor is sintered under an inert atmosphere to prepare the sodium iron pyrophosphate cathode material. Source A includes any one or a combination of at least two of lithium-containing compounds, potassium-containing compounds, or calcium-containing compounds. Source B includes any one or a combination of at least two of magnesium-containing compounds, aluminum-containing compounds, zinc-containing compounds, vanadium-containing compounds, cobalt-containing compounds, manganese-containing compounds, zirconium-containing compounds, titanium-containing compounds, copper-containing compounds, or yttrium-containing compounds.

[0025] Preferably, during the sand milling process, the pressure inside the tank is 0.3MPa~0.5MPa.

[0026] Preferably, after sand milling, the D50 particle size of the particles in the dispersion is 300nm~500nm.

[0027] Preferably, the total mass percentage of the raw materials in the dispersion is 30 wt.% to 50 wt.%.

[0028] Preferably, the sanding includes sanding at speeds of 300 rpm to 500 rpm and 800 rpm to 1200 rpm in sequence.

[0029] Preferably, the grinding balls used in the sand mill include zirconium beads of three different sizes mixed in a mass ratio of (2~3):(1~2):1, wherein the D50 particle sizes of the zirconium beads are 0.2mm~0.5mm, 0.8mm~1.2mm and 1.8mm~2.5mm, respectively.

[0030] Preferably, the air inlet temperature of the spray dryer is 150℃~170℃.

[0031] Preferably, the outlet temperature of the spray dryer is 90℃~100℃.

[0032] Preferably, the D50 particle size of the first precursor is 15μm~20μm.

[0033] Preferably, the sintering temperature is 450℃~600℃.

[0034] Preferably, the sintering time is 10h to 16h.

[0035] Thirdly, the present invention provides a positive electrode sheet comprising the sodium iron pyrophosphate positive electrode material as described in the first aspect.

[0036] Fourthly, the present invention provides a sodium-ion battery comprising a sodium iron pyrophosphate cathode material as described in the first aspect, or a cathode sheet as described in the third aspect.

[0037] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] This invention modulates the composition of sodium iron pyrophosphate cathode material to create a certain number of Fe vacancies and constructs a high-entropy solid solution component, controlling its configurational entropy S to be between 1.5R and 3.1R, effectively improving its electron and ion diffusion kinetics. Simultaneously, by employing a dual carbon structure of bulk composite carbon and surface carbon coating, the poor intrinsic electronic conductivity of sodium iron pyrophosphate cathode material is addressed from both intraparticle and interparticle dimensions. The porous structure of the surface carbon coating layer also increases the contact area between the electrolyte and the cathode material, shortening the sodium ion diffusion path (reducing the distance from the electrolyte to the particle surface) and accelerating the sodium ion migration rate. Through the synergistic effect of multiple mechanisms, this invention comprehensively improves the conductivity, rate performance, cycle life, and stability of sodium iron pyrophosphate cathode material. Attached Figure Description

[0040] Figure 1 The image shows the XRD pattern of the sodium iron pyrophosphate cathode material provided in Example 1.

[0041] Figure 2 This is a SEM image of the sodium iron pyrophosphate cathode material provided in Example 1.

[0042] Figure 3 This is a SEM image of the carbon coating layer of the sodium iron pyrophosphate cathode material provided in Example 1.

[0043] Figure 4 The first charge-discharge curve of the sodium-ion battery prepared from the sodium iron pyrophosphate cathode material provided in Example 1 is shown.

[0044] Figure 5 This describes the rate performance of a sodium-ion battery prepared using the sodium iron pyrophosphate cathode material provided in Example 1. Detailed Implementation

[0045] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. In this invention, "a combination of at least two" means, unless otherwise specified, a quantity greater than or equal to two. For example, "any combination of one or at least two" means one or more of two. It is understood that when referring to "a combination of at least two," it means any suitable combination of multiple items, i.e., a combination of "at least two" items carried out in a manner that does not conflict with and allows for the implementation of the invention.

[0047] The "scope" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given scope is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific scope. The scope defined in this way can include end values ​​or not include end values. Any end value can be included or excluded independently, and they can be combined arbitrarily. That is, any lower limit can be combined with any upper limit to form a scope.

[0048] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0049] In one specific embodiment, the present invention provides a sodium iron pyrophosphate cathode material, the sodium iron pyrophosphate cathode material comprising a sodium iron pyrophosphate / carbon composite core and a carbon coating layer covering the surface of the core; the chemical formula of the sodium iron pyrophosphate / carbon composite core is Na. 4-x A x Fe y B 2.91-y (PO4) 2-z / 3P2O7F z / C, where 0.01≤x≤1, for example, can be 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1; 2.7≤y<2.91, for example, can be 2.7, 2.75, 2.8, 2.85 or 2.9; 1≤z≤3, for example, can be 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75 or 3; A includes any one of Li, K or Ca, or includes combinations of Li and K, combinations of Li and Ca, combinations of K and Ca or combinations of Li, K and Ca. B includes any one or at least two combinations of Mg, Al, Zn, V, Co, Mn, Zr, Ti, Cu, or Y. Typical but non-limiting combinations include combinations of Mg and Al, Zn and V, Co and Mn, Zr and Ti, or Y and Mg. The entropy value S of the sodium iron pyrophosphate cathode material is 1.5R to 3.1R, for example, it can be 1.5R, 1.7R, 1.9R, 2.1R, 2.3R, 2.5R, 2.7R, 2.9R, or 3.1R, where R is the molar gas constant, R = 8.314 J / (mol·K).

[0050] This invention modulates the composition of sodium iron pyrophosphate cathode material to create a certain number of Fe vacancies. By introducing element A to replace Na sites and element B to replace Fe sites, a high-entropy solid solution composition is constructed. The entropy value S of its configuration entropy is controlled to be 1.5R~3.1R. With its diversified local structure, the phase transition reaction in the sodium ion insertion / extraction reaction is delayed, which can effectively suppress the phase transition during the charge and discharge process and effectively suppress the volume expansion during the charge and discharge process, thereby improving structural stability and thus improving cycle performance.

[0051] Introducing Li, K, or Ca elements into sodium iron pyrophosphate cathode materials to substitute Na sites is beneficial for inducing lattice distortion, reducing the volume effect, and improving structural stability. It can also expand the Na... + and the spacing between transition metal layers, enhancing Na + Diffusion rate is improved, enhancing the conductivity of the material. Introducing Fe vacancies effectively suppresses the formation of the NaFePO4 impurity phase and increases the entropy value of the sodium iron pyrophosphate cathode material. An appropriate concentration of Fe vacancies helps adapt to local changes and alleviates volume strain during charging and discharging. Fe vacancies cause chemical bond distortion, affecting the electron cloud distribution of nearby atoms, resulting in a lower band gap and migration barrier, thereby increasing the diffusion rate of electrons and Na+. + Electrical conductivity.

[0052] Meanwhile, high-entropy solid solution components can improve electron and ion diffusion kinetics, and work in conjunction with F, which has stronger electronegativity in anionic groups. - via F- Influences on the outer electronic structure of the material enhances its kinetic properties, improves the electronic conductivity and ion diffusion rate of sodium iron pyrophosphate cathode material, and thus improves rate performance; the introduction of high-entropy components can also reduce surface defects of sodium iron pyrophosphate and suppress side reactions with the electrolyte.

[0053] Secondly, this invention improves the electronic conductivity and ion diffusion rate of sodium iron pyrophosphate cathode material from both intraparticle and interparticle dimensions through a dual carbon structure of bulk composite carbon and surface carbon coating. The bulk composite carbon improves the problem of low intrinsic conductivity of sodium iron pyrophosphate cathode material, while the outer carbon coating layer connects adjacent active particles, forming an electronic conduction pathway across particles, avoiding electron transport obstruction caused by poor interparticle contact. The outer carbon coating layer has a porous structure, which can increase the contact area between the electrolyte and the active material, shorten the diffusion path of sodium ions, and accelerate the migration rate of sodium ions.

[0054] In addition, the dual carbon structure maintains the structural integrity of the cathode material. The bulk composite carbon effectively alleviates stress concentration inside the particles through mechanical support, while the outer carbon coating provides a flexible protective shell that wraps and connects the active particles, inhibiting particle aggregation and breakage during cycling. It can also act as a physical barrier to reduce side reactions between the electrolyte and the material surface.

[0055] This invention comprehensively improves the conductivity, rate performance, cycle life, and stability of sodium iron pyrophosphate cathode material through the synergistic effect of multiple mechanisms.

[0056] In this invention, the formula for calculating the entropy S of the configuration entropy of sodium iron pyrophosphate cathode material is based on the general formula for the configuration entropy of high-entropy materials. conf As derived from the formula, in the sodium iron pyrophosphate cathode material provided by this invention, the anionic group is the structural core of the sodium iron pyrophosphate cathode material, with a disorder degree of almost 0. The change in entropy value caused by the disorder is negligible. Therefore, in this invention, the entropy value S of the configuration entropy of the sodium iron pyrophosphate cathode material is the sum of the entropy values ​​of Na-substituted element A, Fe, and Fe-substituted element B, calculated according to the following formula 1:

[0057] Formula 1.

[0058] Formula 1 is the core formula for calculating the configuration entropy of a mixed system, used to describe the entropy increase when multiple particles are randomly mixed at equivalent lattice sites. Where R is the gas constant, N is the number of particle types involved in the mixture (e.g., when Na-substituted element A is one type and Fe-substituted element B is one type, N = 1 (types of Na-substituted elements) + 1 (types of Fe-substituted elements) + 1 (Fe) = 3), x iLet be the mole fraction of the i-th type of particle.

[0059] In sodium iron pyrophosphate cathode materials, entropy affects the material's structural stability and electrical conductivity. Lower entropy values ​​are detrimental to suppressing phase transitions during charge and discharge, hindering the achievement of stable matrix structures, improved electronic and ionic conductivity, and reduced surface defects. However, excessively high entropy values ​​also lead to structural instability. Regulating the entropy within a certain range provides a flexible local environment for the atomic arrangement, ion distribution, and bonding states within the material's microscopic regions. This helps the cathode material adapt to local changes caused by microscopic dynamic fluctuations due to ion migration and charge transfer during charge and discharge, mitigating volumetric strain and thus improving the material's cycle stability.

[0060] This invention increases the entropy of sodium iron pyrophosphate cathode material by introducing Fe vacancies. Simultaneously, a certain concentration of high-entropy doping can stabilize the structural instability caused by Fe vacancies, allowing the internal defects of the material to reach a self-consistent level. However, if the entropy is too high, it may damage the crystal structure of the material, leading to structural collapse and reducing the material's stability.

[0061] This invention regulates the composition of sodium iron pyrophosphate cathode material, thereby controlling its cell structure. By limiting the cell parameters a, b, and c axes within a certain range, it achieves the following beneficial effects:

[0062] (1) Optimization of ion transport performance: The unit cell parameters determine the size and connectivity of ion transport channels inside the crystal. By adjusting the lengths of the a, b, and c axes, the ion transport performance of Na can be optimized. + The space of the transport channels for charge carriers in the crystal is more adapted, which effectively reduces the barrier to ion migration, increases the ion transport rate, and thus improves the rate performance of the material, enabling the battery to maintain good performance under high current charging and discharging conditions.

[0063] (2) Enhanced structural stability: A suitable range of unit cell parameters enables the crystal structure of sodium iron pyrophosphate cathode material to withstand stress changes during ion insertion / extraction during charge-discharge cycles. During charge-discharge, it will not expand or contract excessively, thereby reducing lattice distortion, suppressing the formation of secondary phases, enhancing the structural stability of the material, and extending the cycle life of the battery.

[0064] (3) Enhanced electrochemical activity: Cell parameters affect the bonding environment and electronic structure of atoms in a crystal. By adjusting the lengths of the a, b, and c axes within a specific range, the electronic structure of the material becomes more conducive to charge transfer and storage, which can improve the electrochemical activity of the electrode material, enabling the battery to achieve higher capacity utilization during charging and discharging, and improving key performance indicators such as the battery's energy density.

[0065] In some embodiments, the unit cell of the sodium iron pyrophosphate cathode material has an a-axis length of 17.52 Å to 18.10 Å, for example, 17.52 Å, 17.55 Å, 17.60 Å, 17.70 Å, 17.80 Å, 17.90 Å, 18.00 Å, or 18.10 Å; a b-axis length of 6.48 Å to 6.53 Å, for example, 6.48 Å, 6.49 Å, 6.50 Å, 6.51 Å, 6.52 Å, or 6.53 Å; and a c-axis length of 10.55 Å to 10.65 Å, for example, 10.55 Å, 10.57 Å, 10.59 Å, 10.61 Å, 10.63 Å, or 10.65 Å.

[0066] In sodium iron pyrophosphate cathode materials, the ratio (I1+I2) / (I3+I4) between the peak intensities I1 of the (211) crystal plane, I2 of the (031) crystal plane, I3 of the (022) crystal plane, and I4 of the (222) crystal plane reflects the phase purity of the sodium iron pyrophosphate cathode material. The presence of impurities or lattice distortion will cause this ratio to change. This invention regulates the above-mentioned peak intensity ratio by controlling the composition of the sodium iron pyrophosphate cathode material, thereby improving the performance of the sodium iron pyrophosphate cathode material.

[0067] In some embodiments, the peak intensities I1 of the (211) crystal plane, I2 of the (031) crystal plane, I3 of the (022) crystal plane, and I4 of the (222) crystal plane of the sodium iron pyrophosphate cathode material satisfy: 0.2≤(I1+I2) / (I3+I4)≤0.6, for example, 0.2, 0.3, 0.4, 0.5 or 0.6.

[0068] In some embodiments, the carbon content in the sodium iron pyrophosphate / carbon composite core is 1.0 wt.% to 2.0 wt.%, for example, it can be 1.0 wt.%, 1.2 wt.%, 1.4 wt.%, 1.6 wt.%, 1.8 wt.%, or 2.0 wt.%.

[0069] In some embodiments, the carbon coating layer in the sodium iron pyrophosphate cathode material has a mass percentage content of 0.2 wt.% to 1 wt.%, for example, it can be 0.2 wt.%, 0.4 wt.%, 0.6 wt.%, 0.8 wt.% or 1 wt.%.

[0070] In some embodiments, the D50 particle size of the core of the sodium iron pyrophosphate / carbon composite material is 15 μm to 20 μm, for example, it can be 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm.

[0071] In this invention, the true density of the core of the sodium iron pyrophosphate / carbon composite material affects the compaction density, charge transport efficiency, and cycle stability of the cathode material. When the true density is too high, it may cause blockage of the ion migration channels inside the cathode material, which in turn leads to a decrease in conductivity. At the same time, when the true density is too high, the structural rigidity of the cathode material is enhanced, resulting in weak resistance to volumetric strain and poor cycle stability. If the true density is too low, it is not conducive to improving the compaction density.

[0072] In some embodiments, the true density of the sodium iron pyrophosphate / carbon composite core is 2.5 g / cm³. 3 ~3.1g / cm 3 For example, it could be 2.5g / cm³ 3 2.6g / cm 3 2.7g / cm 3 2.8g / cm 3 2.9g / cm 3 3.0g / cm 3 Or 3.1g / cm 3 .

[0073] In some embodiments, the true density of the carbon coating is 1.5 g / cm³. 3 ~2.5g / cm 3 For example, it could be 1.5g / cm³ 3 1.7g / cm 3 1.9g / cm 3 2.1g / cm 3 2.3g / cm 3 Or 2.5g / cm 3 .

[0074] In some embodiments, the core of the sodium iron pyrophosphate / carbon composite material is a secondary particle, and the D50 particle size of the primary particle is 100nm~500nm, for example, it can be 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm or 500nm.

[0075] In another specific embodiment, the present invention provides a method for preparing the sodium iron pyrophosphate cathode material as described in one of the foregoing specific embodiments, the preparation method comprising:

[0076] Sodium source, iron source, phosphorus source, fluorine source, first carbon source, source A, and source B are dispersed in a solvent according to stoichiometric ratios and milled to obtain a dispersion. The dispersion is then spray-dried to obtain a first precursor. The precursor is mixed with a second carbon source to obtain a second precursor. The second precursor is sintered under an inert atmosphere to prepare the sodium iron pyrophosphate cathode material. Source A includes any one or a combination of at least two of lithium-containing compounds, potassium-containing compounds, or calcium-containing compounds. Source B includes any one or a combination of at least two of magnesium-containing compounds, aluminum-containing compounds, zinc-containing compounds, vanadium-containing compounds, cobalt-containing compounds, manganese-containing compounds, zirconium-containing compounds, titanium-containing compounds, copper-containing compounds, or yttrium-containing compounds.

[0077] The preparation method provided by this invention, which involves sand milling under specific pressure, is beneficial for refining and homogenizing the raw materials and improving synthesis conditions. This is mainly reflected in several aspects: improved dispersibility – under specific pressure, the grinding media (such as zirconia beads) in the sand mill exert stronger shear and impact forces on the precursor raw materials, more effectively breaking down large particles and significantly reducing the particle size of the raw materials. This helps improve the dispersion uniformity of each component, ensuring a more uniform chemical composition of the subsequently synthesized sodium iron pyrophosphate cathode material and avoiding performance inconsistencies caused by local component differences; increased specific surface area – the refined particles have a larger specific surface area, increasing the contact area between reactants in subsequent synthesis reactions, accelerating the reaction rate, and allowing the reaction to proceed more fully, thereby improving the purity and crystallinity of the product; and reduced reaction temperature – because the precursor particles are finer and more active after sand milling, the reaction can occur at a relatively lower temperature during the synthesis of the sodium iron pyrophosphate cathode material. This not only saves energy but also reduces the generation of impurities and crystal transformation that may occur at high temperatures, which is beneficial for controlling product quality and performance. It also shortens reaction time. After the precursor with higher activity is milled under specific pressure, the kinetics of the synthesis reaction can be accelerated, thereby significantly shortening the reaction time, improving production efficiency, and reducing production costs.

[0078] In some embodiments, the sodium source includes any one or a combination of at least two of sodium carbonate, trisodium phosphate, sodium hydroxide, sodium citrate, or sodium oxalate.

[0079] In some embodiments, the iron source includes any one or a combination of at least two of ferric phosphate, ferric nitrate, ferric sulfate, or ferric oxide.

[0080] In some embodiments, the phosphorus source includes any one or a combination of at least two of ferric phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, ferric pyrophosphate, or phosphoric acid.

[0081] In some embodiments, the fluorine source includes any one or a combination of at least two of sodium fluoride, lithium fluoride, or calcium fluoride.

[0082] In some embodiments, the lithium-containing compound includes any one or a combination of at least two of lithium hydroxide, lithium carbonate, lithium nitrate, or lithium acetate.

[0083] In some embodiments, the potassium-containing compound includes any one or a combination of at least two of potassium hydroxide, potassium carbonate, or potassium bicarbonate.

[0084] In some embodiments, the calcium-containing compound includes any one or a combination of at least two of calcium hydroxide, calcium carbonate, or calcium oxide.

[0085] In some embodiments, the magnesium-containing compound includes any one or a combination of at least two of magnesium oxide, magnesium carbonate, or magnesium sulfate.

[0086] In some embodiments, the aluminum-containing compound includes any one or a combination of at least two of aluminum oxide, aluminum hydroxide, or aluminum sulfate.

[0087] In some embodiments, the zinc-containing compound includes any one or a combination of at least two of zinc oxide, zinc carbonate, or zinc sulfate.

[0088] In some embodiments, the cobalt-containing compound includes any one or a combination of at least two of cobalt oxide, cobalt carbonate, or cobalt sulfate.

[0089] In some embodiments, the manganese-containing compound includes manganese dioxide and / or manganese trioxide.

[0090] In some embodiments, the zirconium-containing compound includes zirconium oxide.

[0091] In some embodiments, the titanium-containing compound includes titanium dioxide.

[0092] In some embodiments, the copper-containing compound includes copper oxide and / or copper sulfate.

[0093] In some embodiments, the vanadium-containing compound includes vanadium pentoxide and / or sodium vanadate.

[0094] In some embodiments, the yttrium-containing compound includes yttrium oxide and / or yttrium fluoride.

[0095] In some embodiments, the first carbon source and the second carbon source each independently comprise any one or a combination of at least two of glucose, citric acid, sucrose, or polyethylene glycol.

[0096] In some embodiments, the solvent includes water.

[0097] In some embodiments, during the sand milling process, the pressure inside the tank is 0.3MPa to 0.5MPa, for example, it can be 0.3MPa, 0.35MPa, 0.4MPa, 0.45MPa or 0.5MPa.

[0098] The preparation method provided by this invention controls the D50 particle size of the particles in the dispersion through sand milling, keeping it within a certain range. This promotes reaction uniformity, optimizes the microstructure of the material, and improves production efficiency. Maintaining the particles in the dispersion at a suitable D50 particle size ensures sufficient contact between the components of the precursor raw materials, resulting in a more uniform and complete reaction in subsequent synthesis reactions. During subsequent processing such as sintering, it promotes more regular crystal growth, forming a more ideal crystal form and microstructure. This helps to construct continuous and unobstructed ion transport channels, improves the sodium ion transport efficiency in the material, thereby improving the electrochemical performance of the material and enhancing the charge / discharge capacity and rate performance of the battery.

[0099] Furthermore, when the particle size in the dispersion is appropriate, the time and energy consumption required for sand milling are within a reasonable range. This achieves the expected dispersion and refinement effect without reducing production efficiency due to excessive grinding. It also facilitates the smooth progress of subsequent processes and reduces production costs.

[0100] If the D50 particle size of the particles in the dispersion is less than 300 nm, it will increase the risk of agglomeration, increase production costs, and affect material performance. When the particle size is too small, the specific surface area increases significantly, the surface energy increases significantly, and the particles are more likely to agglomerate due to strong surface forces (such as van der Waals forces). Agglomerated particles will lose some of the advantages of fine grinding, affecting the uniformity of the reaction and the performance of the material. Furthermore, reducing the particle size to an excessively small level requires more grinding time, grinding media, and energy consumption, and may also require the addition of more dispersants to prevent agglomeration, which will significantly increase production costs and reduce production efficiency. In the synthesis of sodium iron pyrophosphate cathode material, excessively small particle size may lead to excessively rapid crystal growth, generating more lattice defects, which is not conducive to the formation of a stable and ordered crystal structure, thereby affecting the conductivity and ion transport performance of the material, and ultimately leading to a decrease in the cycle stability and charge / discharge capacity of the battery.

[0101] If the D50 particle size in the dispersion exceeds 500 nm, it will cause incomplete material reaction, hindered ion transport, and affect material stability. Larger particle sizes have a smaller specific surface area, resulting in limited contact area between reactants. This makes it difficult for the reactants to react fully during synthesis, leading to reduced product purity, the presence of more impurity phases, and negatively impacting the electrochemical performance of the material. Furthermore, large particle sizes make it difficult to construct efficient ion transport channels when forming the microstructure of the material, increasing the resistance to sodium ion transport within the material. This slows down the sodium ion diffusion rate, degrades the battery's rate performance, and prevents the battery from fully utilizing its capacity under high-current charge and discharge conditions. In addition, the uneven volume change of large particle sizes during charge and discharge makes it easier for the material structure to be damaged, reducing structural stability and shortening the battery's cycle life.

[0102] In some embodiments, after milling, the D50 particle size of the particles in the dispersion is 300nm~500nm, for example, it can be 300nm, 350nm, 400nm, 450nm or 500nm.

[0103] In some embodiments, the total mass percentage of the raw materials in the dispersion is 30 wt.% to 50 wt.%, for example, it can be 30 wt.%, 35 wt.%, 40 wt.%, 45 wt.% or 50 wt.%.

[0104] In some embodiments, the sand milling includes sequentially performing sand milling at speeds of 300 rpm to 500 rpm and 800 rpm to 1200 rpm. First, the speed is set to 300 rpm to 500 rpm, for example, 300 rpm, 325 rpm, 350 rpm, 375 rpm, 400 rpm, 425 rpm, 450 rpm, 475 rpm, or 500 rpm. The purpose is to coarsely grind large particles of raw material, achieving initial dispersion and uniform mixing, avoiding material blockage caused by directly using high-speed sand milling. Then, the speed is set to 800 rpm to 1200 rpm, for example, 800 rpm, 850 rpm, 900 rpm, 950 rpm, 1000 rpm, 1050 rpm, 1100 rpm, 1150 rpm, or 1200 rpm. The purpose is to provide strong shearing / impact forces to achieve deep particle refinement, reaching the required particle size. Furthermore, higher speeds can improve the overall efficiency of the sand milling and reduce overall energy consumption.

[0105] In some embodiments, the grinding balls used in the sand mill comprise zirconium beads of three different sizes mixed in a mass ratio of (2~3):(1~2):1, wherein the D50 particle sizes of the zirconium beads are 0.2mm~0.5mm, 0.8mm~1.2mm, and 1.8mm~2.5mm, respectively, and the mass ratio of the three zirconium beads can be, for example, 2:1:1, 2.2:1.2:1, 2.4:1.4:1, 2.6:1.6:1, 2.8:1.8:1, or 3:2:1.

[0106] In some embodiments, the inlet temperature of the spray dryer is 150°C to 170°C, for example, it can be 150°C, 155°C, 160°C, 165°C or 170°C.

[0107] In some embodiments, the outlet temperature of the spray dryer is 90°C to 100°C, for example, 90°C, 92°C, 94°C, 96°C, 98°C or 100°C.

[0108] In some embodiments, the D50 particle size of the first precursor is 15 μm to 20 μm, for example, it can be 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm.

[0109] In some embodiments, the inert atmosphere includes nitrogen and / or an inert gas, wherein the inert gas includes argon and / or helium.

[0110] In the preparation method provided by this invention, the sintering temperature affects the full diffusion of elements such as Na, Fe, P, and O in the precursor, thereby affecting the regularity of the lattice arrangement of the cathode material. A suitable sintering temperature is conducive to the formation of the target crystal phase of the cathode material, suppresses the generation of impurity phases, and improves the purity of the cathode material. At the same time, the sintering temperature affects the formation and growth process of the cathode material lattice, affects the size, dispersion, and pore structure of the cathode material particles, and thus relates to the specific surface area and ion diffusion path of the material. A suitable sintering temperature is conducive to improving the integrity of the cathode material crystal structure, optimizing the microstructure, and reducing agglomeration.

[0111] If the sintering temperature exceeds 600℃, the volatile components in the precursor will decompose or volatilize excessively, leading to an imbalance in the proportion of elements such as Na and P in the system. This results in an impure crystal phase and an excessive number of impurities in the cathode material. At the same time, excessive grain growth and lattice parameter imbalance cause a significant increase in the diffusion path of sodium ions, a sharp drop in specific surface area, and abnormal expansion and contraction, which causes severe lattice distortion in the cathode material and a significant decrease in structural stability. Consequently, the rate performance and cycle performance of the cathode material deteriorate.

[0112] If the sintering temperature is below 450℃, the energy driving ion diffusion and lattice arrangement is insufficient, the cathode material is not fully crystallized, and the proportion of amorphous phase with low sodium ion diffusion coefficient is high. At the same time, at the lower sintering temperature, the crystal growth rate is too slow, the grains are small and the defects are dense, and the irreversible capacity increases during charge and discharge, resulting in a decrease in the material's capacity. In addition, at too low a sintering temperature, the precursor decomposes incompletely and remains in the cathode material, resulting in insufficient material density, poor mechanical strength, and even triggering interfacial side reactions, leading to a rapid decay of the material's capacity, which is not conducive to improving cycle life.

[0113] In some embodiments, the sintering temperature is 450°C to 600°C, for example, 450°C, 475°C, 500°C, 525°C, 550°C, 575°C or 600°C.

[0114] In some embodiments, the sintering time is 10h to 16h, for example, 10h, 11h, 12h, 13h, 14h, 15h or 16h.

[0115] In yet another embodiment, the present invention provides a positive electrode sheet comprising the sodium iron pyrophosphate positive electrode material as described in the preceding embodiment.

[0116] In another specific embodiment, the present invention provides a sodium-ion battery comprising a sodium iron pyrophosphate cathode material as described in one of the preceding specific embodiments, or comprising a cathode sheet as described in yet another specific embodiment.

[0117] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0118] Example 1

[0119] This embodiment provides a sodium iron pyrophosphate cathode material, including Na 3.5 K 0.5 Fe 2.8 Co 0.11 (PO4) 1.3 P2O7F 2.1 / C kernel, and a carbon coating layer covering the surface of the kernel.

[0120] In the aforementioned sodium iron pyrophosphate cathode material, the carbon coating layer has a mass percentage content of 0.6 wt.%, the carbon content in the core is 1.5 wt.%, and the true density of the core is 2.7 g / cm³. 3 The true density of the carbon coating is 1.9 g / cm³. 3The core has a D50 particle size of 18μm and is a secondary particle constructed from primary particles with a D50 particle size of 400nm.

[0121] In the unit cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 17.82 Å, the length of the b-axis is 6.50 Å, and the length of the c-axis is 10.61 Å. The peak intensities I1 of the (211) crystal plane, I2 of the (031) crystal plane, I3 of the (022) crystal plane, and I4 of the (222) crystal plane satisfy (I1+I2) / (I3+I4)=0.4. The entropy value S of the sodium iron pyrophosphate cathode material is 2.3R.

[0122] This embodiment also provides a method for preparing the above-mentioned sodium iron pyrophosphate cathode material, including:

[0123] (1) Sodium carbonate, ferric nitrate, phosphoric acid, sodium fluoride, glucose, potassium hydroxide and cobalt oxide were dispersed in a solvent according to the stoichiometric ratio. Zirconium beads with D50 particle sizes of 0.3 mm, 1 mm and 2 mm were selected and mixed in a mass ratio of 2.5:1.5:1 as grinding balls. The pressure inside the sand mill was set to 0.4 MPa. The sand mill was first ground at 400 rpm for 2 h and then at 1000 rpm for 5 h to obtain a dispersion with a D50 particle size of 400 nm.

[0124] (2) The inlet temperature of the spray drying is set to 160℃ and the outlet temperature is set to 95℃. The first precursor with a D50 particle size of 18μm is obtained by spraying.

[0125] (3) The first precursor and citric acid are dry-mixed using a high-speed mixer to obtain the second precursor, which is then sintered at 500°C for 12 hours under an argon atmosphere to obtain the sodium iron pyrophosphate cathode material.

[0126] The XRD pattern of the sodium iron pyrophosphate cathode material provided in this embodiment is as follows: Figure 1 As shown, the SEM image is as follows: Figure 2 As shown, the SEM image of the carbon coating layer is as follows: Figure 3 As shown, its surface has a porous structure.

[0127] Example 2

[0128] This embodiment provides a sodium iron pyrophosphate cathode material, including Na 3.99 Li 0.01 Fe 2.7 Zn 0.21 (PO4) 1.7 The P2O7F1 / C core, and the carbon coating layer covering the surface of the core.

[0129] In the aforementioned sodium iron pyrophosphate cathode material, the carbon coating layer has a mass percentage content of 0.2 wt.%, the carbon content in the core is 1.0 wt.%, and the true density of the core is 3.1 g / cm³. 3 The true density of the carbon coating is 2.5 g / cm³. 3 The core has a D50 particle size of 15μm and is a secondary particle constructed from primary particles with a D50 particle size of 300nm.

[0130] In the cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 17.52 Å, the length of the b-axis is 6.48 Å, and the length of the c-axis is 10.55 Å. The peak intensities I1 of the (211) crystal plane, I2 of the (031) crystal plane, I3 of the (022) crystal plane, and I4 of the (222) crystal plane satisfy: (I1+I2) / (I3+I4)=0.2. The entropy value S of the sodium iron pyrophosphate cathode material is 1.5R.

[0131] This embodiment also provides a method for preparing the above-mentioned sodium iron pyrophosphate cathode material, including:

[0132] (1) Disperse trisodium phosphate, ferric phosphate, ammonium dihydrogen phosphate, lithium fluoride, sucrose, lithium carbonate and zinc oxide in a solvent according to the stoichiometric ratio. Select zirconium beads with D50 particle sizes of 0.2 mm, 0.8 mm and 1.8 mm and mix them in a mass ratio of 2:1:1 as grinding balls. Set the pressure inside the sand mill to 0.3 MPa. First, sand mill at 300 rpm for 2.5 h, and then sand mill at 800 rpm for 4 h to obtain a dispersion with a D50 particle size of 300 nm.

[0133] (2) Set the inlet temperature of the spray dryer to 150°C and the outlet temperature to 90°C to obtain the first precursor with a D50 particle size of 15μm.

[0134] (3) The first precursor and the carbon source are dry-mixed using a high-speed mixer to obtain the second precursor, which is then sintered at 450°C for 10 hours under a nitrogen atmosphere to obtain the sodium iron pyrophosphate cathode material.

[0135] Example 3

[0136] This embodiment provides a sodium iron pyrophosphate cathode material, comprising Na3Ca1Fe 2.9 Y 0.01 The PO4P2O7F3 / C core, and the carbon coating layer covering the surface of the core.

[0137] In the aforementioned sodium iron pyrophosphate cathode material, the carbon coating layer has a mass percentage of 1 wt.%, the carbon content in the core is 2.0 wt.%, and the true density of the core is 2.5 g / cm³. 3The true density of the carbon coating is 1.5 g / cm³. 3 The core has a D50 particle size of 20μm, and the core is a secondary particle constructed from primary particles with a D50 particle size of 500nm.

[0138] In the unit cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 18.10 Å, the length of the b-axis is 6.53 Å, and the length of the c-axis is 10.65 Å. The peak intensities I1 of the (211) crystal plane, I2 of the (031) crystal plane, I3 of the (022) crystal plane, and I4 of the (222) crystal plane satisfy: (I1+I2) / (I3+I4)=0.6. The entropy value S of the sodium iron pyrophosphate cathode material is 3.1R.

[0139] This embodiment also provides a method for preparing the above-mentioned sodium iron pyrophosphate cathode material, including:

[0140] (1) Sodium carbonate, ferric oxide, sodium dihydrogen phosphate, calcium fluoride, polyethylene glycol, calcium carbonate and yttrium fluoride were dispersed in water according to the stoichiometric ratio. Zirconium beads with D50 particle sizes of 0.3 mm, 1 mm and 2 mm were selected and mixed in a mass ratio of 2.5:1.5:1 as grinding balls. The pressure inside the sand mill was set to 0.5 MPa. The sand mill was first ground at 500 rpm for 2 h and then at 1200 rpm for 6 h to obtain a dispersion with a D50 particle size of 500 nm.

[0141] (2) Set the inlet temperature of the spray drying to 170°C and the outlet temperature to 100°C to obtain the first precursor with a D50 particle size of 20μm.

[0142] (3) The first precursor and the carbon source are dry mixed using a high-speed mixer to obtain the second precursor, which is then sintered at 600°C for 16 hours under an argon atmosphere to obtain the sodium iron pyrophosphate cathode material.

[0143] Example 4

[0144] This embodiment provides a sodium iron pyrophosphate cathode material, wherein the carbon content in the sodium iron pyrophosphate / carbon composite core is 0.8 wt.%, and the true density of the sodium iron pyrophosphate / carbon composite core is 3.2 g / cm³. 3 Except for the above, everything else is the same as in Example 1.

[0145] Example 5

[0146] This embodiment provides a sodium iron pyrophosphate cathode material, wherein the carbon content in the sodium iron pyrophosphate / carbon composite core is 2.2 wt.%, and the true density of the sodium iron pyrophosphate / carbon composite core is 2.3 g / cm³. 3 Except for the above, everything else is the same as in Example 1.

[0147] Example 6

[0148] This embodiment provides a sodium iron pyrophosphate cathode material with a true density of 1.3 g / cm³ excluding the carbon coating layer. 3 Except for the above, everything else is the same as in Example 1.

[0149] Example 7

[0150] This embodiment provides a sodium iron pyrophosphate cathode material with a true density of 2.8 g / cm³ excluding the carbon coating layer. 3 Except for the above, everything else is the same as in Example 1.

[0151] Example 8

[0152] This embodiment provides a sodium iron pyrophosphate cathode material, except that the core has the chemical formula Na. 4.0 Fe 2.8 Co 0.11 (PO4) 1.3 P2O7F 2.1 In the unit cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 17.98 Å, the length of the b-axis is 6.51 Å, and the length of the c-axis is 10.41 Å, while the rest are the same as in Example 1.

[0153] Example 9

[0154] This embodiment provides a sodium iron pyrophosphate cathode material, except that the core has the chemical formula Na. 4.0 Fe 2.8 Co 0.11 (PO4) 1.3 P2O7Cl 2.1 In the unit cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 17.72 Å, the length of the b-axis is 6.65 Å, and the length of the c-axis is 10.39 Å, while the rest are the same as in Example 1.

[0155] Example 10

[0156] This embodiment provides a sodium iron pyrophosphate cathode material, except that the core has the chemical formula Na. 4.0 Fe 2.3 Co 0.61 (PO4) 1.3 P2O7Cl 2.1 In the unit cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 17.23 Å, the length of the b-axis is 6.71 Å, and the length of the c-axis is 10.45 Å, while the rest are the same as in Example 1.

[0157] Example 11

[0158] This embodiment provides a sodium iron pyrophosphate cathode material. Except that, in the preparation process, after sand milling in step (1), the D50 particle size of the particles in the dispersion is 250 nm, the rest is the same as in Example 1.

[0159] Example 12

[0160] This embodiment provides a sodium iron pyrophosphate cathode material. Except that, in the preparation process, after sand milling in step (1), the D50 particle size of the particles in the dispersion is 550 nm, the rest is the same as in Example 1.

[0161] Example 13

[0162] This embodiment provides a sodium iron pyrophosphate cathode material. Except for the sintering temperature of step (3) being 400°C during the preparation process, the rest is the same as in Example 1.

[0163] Example 14

[0164] This embodiment provides a sodium iron pyrophosphate cathode material. Except for the sintering temperature of step (3) being 650°C during the preparation process, the rest is the same as in Example 1.

[0165] Comparative Example 1

[0166] This comparative example provides a sodium iron pyrophosphate cathode material, except that the chemical formula is Na. 3.5 K 0.5 Fe 2.8 Co 0.11 (PO4) 1.3 P2O7F 2.1 In the preparation process of the sodium iron pyrophosphate cathode material, except that no carbon source is added in steps (1) and (3), the rest is the same as in Example 1.

[0167] Comparative Example 2

[0168] This comparative example provides a sodium iron pyrophosphate cathode material with the chemical formula Na. 3.5 K 0.5 Fe 2.8 Co 0.11 (PO4) 1.3 P2O7F 2.1 / C, except that no carbon source is added in step (3) during the preparation of the sodium iron pyrophosphate cathode material, the rest is the same as in Example 1.

[0169] Comparative Example 3

[0170] This comparative example provides a sodium iron pyrophosphate cathode material, which, in addition to including Na... 3.5 K 0.5 Fe 2.8 Co0.11 (PO4) 1.3 P2O7F 2.1 The core and the carbon coating layer covering the surface of the core are the same as in Example 1 except that no carbon source is added in step (1) during the preparation of the sodium iron pyrophosphate cathode material.

[0171] Comparative Example 4

[0172] This comparative example provides a sodium iron pyrophosphate cathode material, except that the core chemical formula is Na₄Fe. 2.8 Co 0.11 (PO4) 1.3 P2O7F 2.1 / C, the entropy value S of sodium iron pyrophosphate cathode material is 2.64R. In the preparation process of sodium iron pyrophosphate cathode material, except for the absence of potassium hydroxide in step (1), the rest is the same as in Example 1.

[0173] Comparative Example 5

[0174] This comparative example provides a sodium iron pyrophosphate cathode material, except that the core chemical formula is Na. 3.5 K 0.5 Fe 2.91 (PO4) 1.3 P2O7F 2.1 / C, the entropy value S of sodium iron pyrophosphate cathode material is 2.76R. In the preparation process of sodium iron pyrophosphate cathode material, except for the absence of cobalt oxide in step (1), the rest is the same as in Example 1.

[0175] Comparative Example 6

[0176] This comparative example provides a sodium iron pyrophosphate cathode material, except that the core chemical formula is Na. 3.5 K 0.5 Fe 2.89 Co 0.11 (PO4) 1.3 P2O7F 2.1 / C, the entropy value S of the sodium iron pyrophosphate cathode material is 2.5R. In the preparation process of the sodium iron pyrophosphate cathode material, except for adjusting the amount of ferric nitrate added according to the stoichiometric ratio of this embodiment in step (1), the rest are the same as in Example 1.

[0177] Comparative Example 7

[0178] This comparative example provides a sodium iron pyrophosphate cathode material, except that the core chemical formula is Na. 3.5 K 0.5 Fe 2.8 Co 0.11(PO4)2P2O7 / C, the entropy value S of sodium iron pyrophosphate cathode material is 2.3R. In the preparation process of the sodium iron pyrophosphate cathode material, except for step (1) where sodium fluoride is not added, the rest is the same as in Example 1.

[0179] Comparative Example 8

[0180] This comparative example provides a sodium iron pyrophosphate cathode material, except that the core has the chemical formula Na. 3.5 K 0.5 Fe 2.35 Co 0.56 (PO4) 1.3 P2O7F 2.1 Except for the entropy value S of the sodium iron pyrophosphate cathode material being 1.3R, everything else is the same as in Example 1.

[0181] Comparative Example 9

[0182] This comparative example provides a sodium iron pyrophosphate cathode material, except that the core has the chemical formula Na. 2.75 K 1.25 Fe 2.88 Co 0.03 (PO4) 1.3 P2O7F 2.1 Except for the entropy value S of the sodium iron pyrophosphate cathode material being 3.2R, everything else is the same as in Example 1.

[0183] Performance testing:

[0184] The sodium iron pyrophosphate cathode material provided in all the above embodiments and comparative examples was dispersed in NMP with carbon nanotubes, conductive carbon black and PVDF in a mass ratio of 94:1:2:3 to prepare a slurry. This slurry was then coated onto the surface of aluminum foil, dried, and rolled to obtain an areal density of 9 mg / cm³. 2 The compacted density is 2.2 g / cm³. 3 The positive electrode plate.

[0185] In an argon-filled glove box, a sodium-ion battery was assembled using metallic sodium as the counter electrode and matched with the aforementioned positive electrode. Electrochemical performance was tested at 25°C and within a voltage range of 1.5V to 4.0V.

[0186] (1) The initial discharge specific capacity was tested under 0.1C charging and 0.1C discharging conditions. The test results are shown in Table 1. The initial charge-discharge curves of the sodium-ion battery prepared from the sodium iron pyrophosphate cathode material provided in Example 1 are shown in Table 1. Figure 4 As shown;

[0187] (2) Under the conditions of 0.5C charging and 1C discharging, the cycle performance was tested after 50 charge-discharge cycles. The test results are shown in Table 1.

[0188] (3) Charge at 0.5C and discharge at nC rate, perform rate discharge tests, and calculate the rate discharge capacity retention rate. Rate discharge capacity retention rate = nC discharge specific capacity / 0.1C discharge specific capacity, n = 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9. The test results are shown in Table 2. The rate discharge performance of the sodium-ion battery prepared from the sodium iron pyrophosphate cathode material provided in Example 1 is as follows: Figure 5 As shown.

[0189] Table 1

[0190]

[0191] Table 2

[0192]

[0193] Based on the test results in Tables 1 and 2, this invention addresses the problem of poor intrinsic electronic conductivity of sodium iron pyrophosphate cathode materials from both intraparticle and interparticle dimensions through a dual carbon structure consisting of bulk composite carbon and surface carbon coating. Simultaneously, the porous structure of the surface carbon coating increases the contact area between the electrolyte and the cathode material, shortens the diffusion path of sodium ions (reducing the distance from the electrolyte to the particle surface), and accelerates the migration rate of sodium ions. Furthermore, the composition of the sodium iron pyrophosphate cathode material is regulated to include certain Fe vacancies, constructing a high-entropy solid solution composition, and controlling the entropy value S to be 1.5R~3.1R. These multiple mechanisms work synergistically to improve the structural stability of the sodium iron pyrophosphate cathode material and further enhance its electronic and ionic conductivity, comprehensively improving its conductivity, rate performance, cycle life, and stability.

[0194] According to the test results of Example 1 and Comparative Examples 1 to 3, if a dual carbon structure of bulk composite carbon and surface carbon coating is not introduced simultaneously, the electronic conductivity and ion diffusion rate of sodium iron pyrophosphate cathode material cannot be improved in a synchronous manner, resulting in a significant deterioration of the electrochemical performance of sodium iron pyrophosphate cathode material.

[0195] Based on the test results of Example 1, Comparative Examples 4 and 7, if the composition of the sodium iron pyrophosphate cathode material does not meet the requirements of this invention, and multiple mechanisms are not used simultaneously for coordinated regulation, the structural stability of the sodium iron pyrophosphate cathode material and the improvement of electronic and ionic conductivity are not ideal, resulting in a decrease in the electrochemical performance of the cathode material.

[0196] According to the test results of Example 1, Comparative Examples 8 and 9, if the entropy value S of sodium iron pyrophosphate cathode material is as low as 1.3R or as high as 3.2R, it cannot adequately alleviate the volume strain during the charging and discharging process, thereby improving the cycle stability of the material and causing the cycle performance of the cathode material to deteriorate.

[0197] Based on the test results of Examples 1, 4, and 5, both excessively high and low carbon content in the sodium iron pyrophosphate / carbon composite core are detrimental to capacity performance. A carbon content higher than 2.2 wt.% will hinder sodium ion diffusion to some extent, increasing polarization during charging and discharging, thus affecting its discharge specific capacity. A carbon content lower than 0.8 wt.% cannot effectively improve the material's electronic conductivity, leading to excessively high charging resistance. Furthermore, excessively low carbon content may cause Fe in the material to... 2+ It is more easily oxidized, resulting in reduced sample purity and capacity decay.

[0198] Based on the test results of Examples 1, 6, and 7, an excessively high or low true density of the carbon coating layer is detrimental to capacity utilization and cycle life improvement. A true density higher than 2.8 g / cm³ is undesirable. 3 This means the carbon layer structure is too dense, which hinders the diffusion of sodium ions and increases the migration resistance of sodium ions during charging and discharging, thereby reducing the battery's charging and discharging efficiency and specific capacity. The true density of the carbon coating layer is less than 1.3 g / cm³. 3 The carbon layer structure is porous and cannot provide effective protection for the core material.

[0199] According to the test results of Examples 1 and 8 to 10, if the lengths of the a-axis, b-axis and c-axis in the cell of the sodium iron pyrophosphate cathode material do not meet the range in this invention, the ion transport performance of the cathode material cannot be effectively optimized, the structural stability of the cathode material cannot be enhanced, and the electrochemical activity of the cathode material cannot be effectively improved, resulting in a decrease in the electrochemical performance of the cathode material.

[0200] According to the test results of Examples 1, 11 and 12, if the D50 particle size of the particles in the dispersion is too small (to 250 nm) or too large (to 550 nm), it is not conducive to promoting reaction uniformity, optimizing the microstructure of the material and improving production efficiency. As a result, it is impossible to effectively construct a continuous and unobstructed ion transport channel, leading to a decrease in the charge and discharge capacity and rate performance of the cathode material.

[0201] According to the test results of Examples 1, 13 and 14, if the sintering temperature is as low as 400℃ or as high as 650℃, it is not conducive to improving the integrity and purity of the phase structure of the cathode material, and a good microstructure and morphology cannot be obtained, resulting in a decrease in the cycle performance and rate performance of the cathode material.

[0202] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A sodium iron pyrophosphate cathode material, characterized in that, The sodium iron pyrophosphate cathode material includes a sodium iron pyrophosphate / carbon composite core and a carbon coating layer covering the surface of the core. The chemical formula of the core of the sodium iron pyrophosphate / carbon composite material is Na. 4-x A x Fe y B 2.91-y (PO4) 2-z / 3 P2O7F z / C, where 0.01≤x≤1, 2.7≤y<2.91, 1≤z≤3, A includes any one or at least two of Li, K or Ca, and B includes any one or at least two of Mg, Al, Zn, V, Co, Mn, Zr, Ti, Cu or Y; The entropy value S of the sodium iron pyrophosphate cathode material is 1.5R~3.1R, where R is the molar gas constant, R=8.314J / (mol·K).

2. The sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, In the unit cell of the sodium iron pyrophosphate cathode material, the length of the a-axis is 17.52 Å to 18.10 Å, the length of the b-axis is 6.48 Å to 6.53 Å, and the length of the c-axis is 10.55 Å to 10.65 Å.

3. The sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The peak intensities I1 of the (211) crystal plane, I2 of the (031) crystal plane, I3 of the (022) crystal plane, and I4 of the (222) crystal plane of the sodium iron pyrophosphate cathode material satisfy the following: 0.2≤(I1+I2) / (I3+I4)≤0.

6.

4. The sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The carbon content in the core of the sodium iron pyrophosphate / carbon composite material is 1.0 wt.%~2.0 wt.% by mass. And / or, in the sodium iron pyrophosphate cathode material, the carbon coating layer has a mass percentage content of 0.2 wt.% to 1 wt.%.

5. The sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The D50 particle size of the core of the sodium iron pyrophosphate / carbon composite material is 15μm~20μm; And / or, the true density of the sodium iron pyrophosphate / carbon composite core is 2.5 g / cm³. 3 ~3.1g / cm 3 ; And / or, the true density of the carbon coating is 1.5 g / cm³. 3 ~2.5g / cm 3 .

6. A method for preparing the sodium iron pyrophosphate cathode material as described in any one of claims 1 to 5, characterized in that, The preparation method includes: Sodium source, iron source, phosphorus source, fluorine source, first carbon source, source A and source B are dispersed in a solvent according to stoichiometric ratio, and milled to obtain a dispersion; the dispersion is spray-dried to obtain a first precursor; the precursor is mixed with a second carbon source to obtain a second precursor; the second precursor is sintered under an inert atmosphere to prepare the sodium iron pyrophosphate cathode material. The A source includes any one or a combination of at least two of lithium-containing compounds, potassium-containing compounds, or calcium-containing compounds; The B source includes any one or a combination of at least two of the following: magnesium-containing compounds, aluminum-containing compounds, zinc-containing compounds, vanadium-containing compounds, cobalt-containing compounds, manganese-containing compounds, zirconium-containing compounds, titanium-containing compounds, copper-containing compounds, or yttrium-containing compounds.

7. The preparation method according to claim 6, characterized in that, During the grinding process, the pressure inside the tank is 0.3MPa~0.5MPa; And / or, after the sand milling, the D50 particle size of the particles in the dispersion is 300nm~500nm.

8. The preparation method according to claim 6, characterized in that, The inlet temperature of the spray dryer is 150℃~170℃; And / or, the outlet temperature of the spray dryer is 90℃~100℃; And / or, the D50 particle size of the first precursor is 15μm~20μm; And / or, the sintering temperature is 450℃~600℃; And / or, the sintering time is 10h~16h.

9. A positive electrode plate, characterized in that, The positive electrode includes the sodium iron pyrophosphate positive electrode material as described in any one of claims 1 to 5.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes the sodium iron pyrophosphate cathode material as described in any one of claims 1 to 5, or includes the cathode sheet as described in claim 9.

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