NFPP-based positive electrode material, preparation method thereof and sodium ion battery

Abstract

The invention provides an NFPP-based positive electrode material, a preparation method thereof and a sodium ion battery, the NFPP-based positive electrode material comprises an NFPP positive electrode material core and a coating layer coating the outer part of the NFPP positive electrode material core, and the coating layer comprises metal-doped coal-based porous carbon. According to the NFPP-based positive electrode material provided by the invention, the metal-doped coal-based porous carbon is adopted as a component of the coating layer, the coating layer not only constructs an efficient conductive network, but also the doped metal and the coal-based porous carbon generate a synergistic effect, so that the conductivity of the NFPP-based positive electrode material is synergistically improved; the porous structure of the coating layer provides abundant rapid diffusion channels for sodium ions, so that the ion diffusion kinetics is effectively improved; the metal sites on the surface of the coating layer can also optimize electrolyte infiltration and promote interface charge transfer. Therefore, the sodium ion battery prepared from the NFPP-based positive electrode material shows excellent rate capability.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and relates to an NFPP-based cathode material, and more particularly to an NFPP-based cathode material, its preparation method, and sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries, due to their similar working principle to lithium-ion batteries, are considered a highly promising next-generation large-scale energy storage technology, serving as a supplement and replacement for lithium-ion batteries. Firstly, sodium is extremely abundant and widely distributed in the Earth's crust, far exceeding the relative scarcity and uneven distribution of lithium resources, significantly reducing raw material costs. Secondly, sodium-ion battery production lines are highly compatible with existing lithium-ion battery production lines, resulting in relatively low industrialization conversion costs. Thirdly, in terms of performance, sodium-ion batteries offer better safety and environmental friendliness, and some systems (such as polyanionic types) exhibit excellent low-temperature performance. These advantages make sodium-ion batteries a promising application prospect.

[0003] Among numerous sodium-ion battery cathode materials, the polyanionic compound sodium iron pyrophosphate (NFPP cathode material) exhibits outstanding comprehensive advantages. The advantages of NFPP cathode material are mainly as follows: First, it possesses a high theoretical specific capacity and a moderate operating voltage, thus exhibiting good energy density. Second, it has a unique crystal structure, containing both tetrahedrons and pyrophosphate ions, forming a robust three-dimensional framework and open sodium-ion diffusion channels. This results in minimal volume change during charge and discharge, leading to excellent cycle stability, significantly superior to layered oxides and Prussian blue materials. Finally, its core raw materials include iron, sodium, and phosphorus sources, which are abundant, inexpensive, and have relatively simple synthesis processes, resulting in lower production costs for NFPP cathode materials.

[0004] However, despite the numerous advantages of NFPP cathode materials, their inherent conductivity is poor, resulting in low electronic conductivity and relatively slow sodium-ion diffusion kinetics. This leads to high internal impedance in sodium-ion batteries fabricated with NFPP cathode materials, resulting in lower rate performance. To improve the performance of NFPP cathode materials, existing technologies employ methods such as carbon coating, elemental doping, and grain nanostructuring to modify them. However, these methods all have some drawbacks. For example, carbon coating is a complex process that requires expensive solvents and an inert atmosphere, thus increasing costs. While grain nanostructuring can shorten the ion diffusion path, it can easily lead to a decrease in the tap density of the material, adversely affecting the energy density of the battery, and it is also prone to generating impurity phases during the fabrication process.

[0005] For example, CN118851130A discloses a sodium iron pyrophosphate / carbon composite cathode material and its preparation, electrode sheet and battery. The preparation method of the sodium iron pyrophosphate / carbon composite cathode material includes the following steps: (1) adding starch to a mixed solution of sodium salt, iron salt and phosphate, stirring, gelatinizing and aging to obtain starch hydrogel; (2) drying the starch hydrogel to obtain NFPP@C aerogel precursor, and then performing heat treatment to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0006] For example, CN119400840A discloses a borate-substituted modified sodium iron pyrophosphate cathode material, its preparation method, and its application, belonging to the field of sodium-ion batteries. The method includes the following steps: S1. Adding a boron source to an aqueous solution of sodium iron pyrophosphate, mixing evenly to obtain a first solution, and drying to obtain a dry gel; S2. Calcining the dry gel under inert gas protection to replace part of the phosphate with borate, thereby obtaining the borate-substituted modified sodium iron pyrophosphate cathode material.

[0007] For example, CN116565165A discloses a dual continuous phase coated sodium iron pyrophosphate cathode material, which includes sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7)) and a coating layer (a continuous composite phase formed by uniformly mixing metal oxide TiNb2O7 and a carbon layer) on its surface. Its preparation method is as follows: 1) a slurry is prepared by mixing sodium source, iron source, phosphorus source, and solvent; 2) the slurry is spray-dried to obtain a precursor; 3) the precursor is mixed with carbon source, titanium source, and niobium source and then ground; 4) the ground precursor is sintered at high temperature and then cooled.

[0008] In summary, existing NFPP-based cathode materials all have certain defects, including insufficient conductivity, which leads to inadequate rate performance in sodium-ion batteries made from NFPP-based cathode materials. Therefore, developing a novel NFPP-based cathode material and its preparation method is crucial for sodium-ion batteries. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide an NFPP-based cathode material, its preparation method, and a sodium-ion battery. The NFPP-based cathode material provided by this invention uses metal-doped coal-based porous carbon as the component of the coating layer. This coating layer not only constructs a highly efficient conductive network, but the doped metal and the coal-based porous carbon also produce a synergistic effect, jointly improving the conductivity of the NFPP-based cathode material. The porous structure of the coating layer provides abundant and rapid diffusion channels for sodium ions, effectively improving ion diffusion kinetics. Furthermore, the metal sites on the surface of the coating layer optimize electrolyte wetting and promote interfacial charge transfer. Therefore, the sodium-ion battery prepared with the NFPP-based cathode material exhibits excellent rate performance.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] In a first aspect, the present invention provides an NFPP-based cathode material, the NFPP-based cathode material comprising an NFPP cathode material core and a coating layer covering the outside of the NFPP cathode material core, the coating layer comprising metal-doped coal-based porous carbon.

[0012] In this invention, the general chemical formula of the NFPP cathode material in the NFPP cathode material core is: Na 4+x Fe 3-y (PO4)2P2O7, where -0.3≤x≤0.3, 0≤y≤0.3.

[0013] In this invention, -0.3≤x≤0.3, and the value of x can be, for example, -0.3, -0.2, -0.1, 0, 0.1, 0.2 or 0.3, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0014] In this invention, 0 ≤ y ≤ 0.3, and the value of y can be, for example, 0, 0.05, 0.10, 0.15, 0.20, 0.25 or 0.30, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0015] The NFPP-based cathode material provided by this invention has a coating layer comprising metal-doped coal-based porous carbon. On one hand, the presence of the metal-doped coal-based porous carbon forms a conductive network, thereby enhancing the conductivity of the NFPP-based cathode material. On the other hand, the doped metal in the coal-based porous carbon can also synergistically enhance the conductivity of the NFPP-based cathode material. Furthermore, the porous structure of the coating layer caused by the metal-doped coal-based porous carbon not only increases the rapid diffusion channels for sodium ions, thus effectively improving the ion diffusion kinetics of sodium-ion batteries, but also optimizes the wetting effect of the electrolyte and promotes interfacial charge transfer at the interface. Therefore, the sodium-ion battery prepared with the NFPP-based cathode material exhibits high rate performance. In addition, the NFPP-based cathode material has a simple structure and low preparation cost, and can be prepared using existing processes and equipment.

[0016] Preferably, based on the mass of the NFPP-based cathode material, the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material is 1wt% to 3wt%, for example, it can be 1.0wt%, 1.2wt%, 1.4wt%, 1.6wt%, 1.8wt%, 2.0wt%, 2.2wt%, 2.4wt%, 2.6wt%, 2.8wt%, or 3.0wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0017] In this invention, by limiting the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material, the coating effect of the coating layer on the core of the NFPP cathode material is further optimized, thereby further improving the performance of the NFPP-based cathode material.

[0018] Preferably, the metal-doped coal-based porous carbon comprises a doped metal and coal-based porous carbon.

[0019] Preferably, in the metal-doped coal-based porous carbon, the mass ratio of the doped metal to the coal-based porous carbon is (0.5~1.5):10, for example, it can be 0.5:10, 0.7:10, 0.9:10, 1.1:10, 1.3:10 or 1.5:10, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0020] Preferably, the doped metal includes copper and / or aluminum, and more particularly copper and aluminum.

[0021] In the NFPP-based cathode material provided by this invention, copper powder and aluminum powder are used as raw materials to dope coal-based porous carbon to form metal-doped coal-based porous carbon. Therefore, the doped metals are copper and aluminum. Compared with other metals, copper oxides and aluminum oxides, when the doped metals are copper and aluminum, the coating layer has better conductivity, which is more conducive to improving the conductivity of the NFPP-based cathode material.

[0022] Preferably, the mass ratio of copper to aluminum in the doped metal is 1:(0.5~2), for example, it can be 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9 or 1:2, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0023] In this invention, by limiting the mass ratio of copper to aluminum in the doped metal, the performance of the coating layer is further optimized, thereby optimizing the performance of the NFPP-based cathode material.

[0024] In a second aspect, the present invention provides a method for preparing the NFPP-based cathode material described in the first aspect, the method comprising:

[0025] After mixing NFPP precursor, sodium carbonate, and metal-doped coal-based porous carbon, the mixture is sintered in a protective atmosphere to obtain NFPP-based cathode material.

[0026] In the preparation method of NFPP-based cathode material provided by the present invention, sodium carbonate is also mixed in while mixing NFPP precursor. The role of adding sodium carbonate is to supplement sodium, thereby forming sodium-sufficient NFPP-based cathode material.

[0027] Preferably, the method for preparing the NFPP precursor includes: a first mixed iron source, phosphorus source, pyrophosphate source, sodium source, oxidant and solvent, which are subjected to a co-precipitation reaction to obtain the NFPP precursor.

[0028] Preferably, the iron source includes any one or a combination of at least two of ferrous sulfate, ferrous chloride, ferrous nitrate, or ferrous oxalate. Typical but non-limiting combinations include a combination of ferrous sulfate and ferrous chloride, a combination of ferrous nitrate and ferrous oxalate, or a combination of ferrous sulfate, ferrous chloride, and ferrous nitrate.

[0029] Preferably, the phosphorus source includes any one or a combination of at least two of phosphoric acid, ammonium dihydrogen phosphate, disodium hydrogen phosphate, or sodium dihydrogen phosphate. Typical but non-limiting combinations include a combination of phosphoric acid and ammonium dihydrogen phosphate, a combination of disodium hydrogen phosphate and sodium dihydrogen phosphate, or a combination of phosphoric acid, ammonium dihydrogen phosphate, and disodium hydrogen phosphate.

[0030] Preferably, the pyrophosphate source includes any one or a combination of at least two of sodium pyrophosphate, potassium pyrophosphate, ammonium pyrophosphate, or pyrophosphate. Typical but non-limiting combinations include a combination of sodium pyrophosphate and potassium pyrophosphate, a combination of ammonium pyrophosphate and pyrophosphate, or a combination of sodium pyrophosphate, potassium pyrophosphate, and ammonium pyrophosphate.

[0031] Preferably, the sodium source includes any one or a combination of at least two of sodium hydroxide, sodium carbonate, sodium acetate, sodium nitrate, or sodium chloride. Typical but non-limiting combinations include a combination of sodium hydroxide and sodium carbonate, a combination of sodium acetate and sodium nitrate, or a combination of sodium hydroxide, sodium carbonate, and sodium chloride.

[0032] Preferably, the oxidant includes any one or a combination of at least two of hydrogen peroxide, sodium hypochlorite, or potassium permanganate. Typical but non-limiting combinations include a combination of hydrogen peroxide and sodium hypochlorite, a combination of sodium hypochlorite and potassium permanganate, or a combination of hydrogen peroxide, sodium hypochlorite, and potassium permanganate.

[0033] Preferably, the solvent includes any one or a combination of at least two of water, ethanol, isopropanol, or N-methylpyrrolidone. Typical but non-limiting combinations include a combination of water and ethanol, a combination of isopropanol and N-methylpyrrolidone, or a combination of water, ethanol, and isopropanol.

[0034] Preferably, in the first mixture, the molar ratio of iron source, phosphorus source, pyrophosphate source, sodium source and oxidant is 1:(0.1~0.4):(0.6~0.9):(2~2.5):(0.5~1.5).

[0035] In the first mixture of the present invention, the molar ratio of iron source to phosphorus source is 1:(0.1~0.4), for example, it can be 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, 1:0.35 or 1:0.4, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] In the first mixture of the present invention, the molar ratio of iron source to pyrophosphate source is 1:(0.6~0.9), for example, it can be 1:0.6, 1:0.65, 1:0.7, 1:0.75, 1:0.8, 1:0.85 or 1:0.9, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0037] In the first mixture of the present invention, the molar ratio of iron source to sodium source is 1:(2~2.5), for example, it can be 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4 or 1:2.5, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0038] In the first mixture of the present invention, the molar ratio of iron source to oxidant is 1:(0.5~1.5), for example, it can be 1:0.5, 1:0.7, 1:0.9, 1:1.1, 1:1.3 or 1:1.5, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] Preferably, during the first mixing, the iron source, phosphorus source, pyrophosphate source, sodium source, oxidant, and solvent form a reaction solution, and the concentration of iron ions in the reaction solution is controlled to be 0.5 mol / L to 2.0 mol / L, for example, 0.5 mol / L, 0.7 mol / L, 0.9 mol / L, 1.1 mol / L, 1.3 mol / L, 1.5 mol / L, 1.7 mol / L, 1.9 mol / L, or 2.0 mol / L, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0040] Preferably, in the coprecipitation reaction, the pH is controlled to be 2.0~4.0, for example, it can be 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8 or 4.0, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] Preferably, the method for preparing the metal-doped coal-based porous carbon includes: mixing metal powder with coal-based porous carbon in a second mixing process to obtain metal-doped coal-based porous carbon.

[0042] Preferably, the metal powder includes copper powder and / or aluminum powder, and more preferably copper powder and aluminum powder.

[0043] Preferably, the D50 particle size of the metal powder is 0.5μm to 1.0μm, for example, it can be 0.5μm, 0.55μm, 0.6μm, 0.65μm, 0.7μm, 0.75μm, 0.8μm, 0.85μm, 0.9μm, 0.95μm or 1.0μm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] Preferably, the second mixing method includes ball milling for not less than 2 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0045] Preferably, the coal-based porous carbon has a D50 particle size of 1μm to 3μm, a porosity of 50% to 70%, and a pore diameter of 50nm to 200nm.

[0046] The D50 particle size of the coal-based porous carbon described in this invention is 1μm to 3μm, for example, it can be 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2.0μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm or 3.0μm, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0047] The porosity of the coal-based porous carbon described in this invention is 50% to 70%, for example, it can be 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68% or 70%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0048] The pore diameter of the coal-based porous carbon described in this invention is 50nm~200nm, for example, it can be 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm or 200nm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0049] Preferably, the method for preparing the coal-based porous carbon includes:

[0050] After a third mixing of lignite powder and activator, calcination is carried out in a protective atmosphere to obtain coal-based porous carbon.

[0051] Preferably, the activator includes a potassium activator and / or a sodium activator.

[0052] Preferably, the potassium activator includes any one or a combination of at least two of potassium hydroxide, potassium carbonate, potassium chloride, or potassium nitrate. Typical but non-limiting combinations include a combination of potassium hydroxide and potassium carbonate, a combination of potassium chloride and potassium nitrate, a combination of potassium hydroxide, potassium carbonate, and potassium chloride, or a combination of potassium carbonate, potassium chloride, and potassium nitrate.

[0053] Preferably, the sodium activator includes any one or a combination of at least two of sodium hydroxide, sodium carbonate, sodium chloride, or sodium nitrate. Typical but non-limiting combinations include a combination of sodium hydroxide and sodium carbonate, a combination of sodium chloride and sodium nitrate, a combination of sodium hydroxide, sodium carbonate, and sodium chloride, or a combination of sodium carbonate, sodium chloride, and sodium nitrate.

[0054] Preferably, in the third mixture, the mass ratio of lignite powder to activator is (1~3):1, for example, it can be 1:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1, 2:1, 2.2:1, 2.4:1, 2.6:1, 2.8:1 or 3:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0055] Preferably, the protective atmosphere includes nitrogen and / or a protective gas.

[0056] Preferably, the calcination temperature is 600℃~800℃ and the time is 2h~4h.

[0057] In this invention, the calcination temperature is 600℃~800℃, for example, it can be 600℃, 625℃, 650℃, 675℃, 700℃, 725℃, 750℃, 775℃ or 800℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0058] In this invention, the calcination time is 2h to 4h, for example, it can be 2.0h, 2.2h, 2.4h, 2.6h, 2.8h, 3.0h, 3.2h, 3.4h, 3.6h, 3.8h or 4.0h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0059] Preferably, the method for preparing the coal-based porous carbon further includes, prior to the third mixing, the following steps: sequentially pulverizing, removing ash from, washing, and drying the lignite powder.

[0060] Preferably, the pulverized lignite powder obtained after pulverization has a D50 particle size of no more than 50 μm. For example, it can be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm or 50 μm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0061] Preferably, the ash removal process includes soaking in a hydrochloric acid solution.

[0062] Preferably, the mass concentration of the hydrochloric acid solution is 2wt% to 15wt%, for example, it can be 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt% or 15wt%, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0063] Preferably, the soaking time is 6h to 30h, for example, it can be 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h, 26h, 28h or 30h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0064] Preferably, the washing liquid used in the washing process includes water.

[0065] Preferably, the drying temperature is 70℃~140℃ and the drying time is 5h~18h.

[0066] In this invention, the drying temperature is 70℃~140℃, for example, it can be 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃ or 140℃, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0067] In this invention, the drying time is 5h to 18h, for example, it can be 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h or 18h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0068] Preferably, in the preparation method of the NFPP-based cathode material, the mixing method includes ball milling for 4 to 6 hours.

[0069] Preferably, in the mixture, the mass ratio of NFPP precursor to sodium carbonate is (4~3):1; for example, it can be 4.0:1, 3.8:1, 3.6:1, 3.4:1, 3.2:1 or 3.0:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0070] Preferably, the protective atmosphere includes nitrogen and / or a protective gas.

[0071] Preferably, the sintering temperature is 650℃~750℃ and the time is 8h~12h.

[0072] In this invention, the sintering temperature is 650℃~750℃, for example, it can be 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 730℃, 740℃ or 750℃, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0073] In this invention, the sintering time is 8h to 12h, for example, it can be 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h or 12h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0074] As a preferred embodiment of the preparation method of the present invention, the preparation method includes:

[0075] (1) Preparation of NFPP precursor: Iron source, phosphorus source, pyrophosphate source, sodium source, oxidant and solvent with a first mixing molar ratio of 1:(0.1~0.4):(0.6~0.9):(2~2.5):(0.5~1.5) are mixed to form a reaction solution and the concentration of iron ions in the reaction solution is controlled to be 0.5mol / L~2.0mol / L and the pH is 2.0~4.0. Co-precipitation reaction is carried out to obtain NFPP precursor;

[0076] (2) Preparation of metal-doped coal-based porous carbon: The lignite powder is pulverized to obtain pulverized lignite powder with a D50 particle size of no more than 50 μm; then the pulverized lignite powder is soaked in a hydrochloric acid solution with a mass concentration of 2wt%~15wt% for 6h~30h, then washed with water, and then dried at 70℃~140℃ for 5h~18h to obtain dried lignite powder;

[0077] The dried lignite powder was mixed with potassium activator and / or sodium activator in a third mixing process, and the mass ratio of lignite powder to potassium activator and / or sodium activator was controlled to be (1~3):1 to obtain a mixture. The mixture was then calcined at 600℃~800℃ for 2h~4h in a protective atmosphere, followed by water washing and drying to obtain coal-based porous carbon with D50 particle size of 1μm~3μm, porosity of 50%~70% and pore diameter of 50nm~200nm.

[0078] Copper powder with a D50 particle size of 0.5μm~1.0μm and aluminum powder with a D50 particle size of 0.5μm~1.0μm are mixed with the obtained coal-based porous carbon by ball milling for no less than 2 hours to obtain metal-doped coal-based porous carbon.

[0079] (3) After mixing NFPP precursor, sodium carbonate and metal-doped coal-based porous carbon by ball milling for 4h~6h, sintering is carried out at a temperature of 650℃~750℃ for 8h~12h in a protective atmosphere to obtain NFPP-based cathode material.

[0080] Thirdly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the NFPP-based cathode material described in the first aspect.

[0081] 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.

[0082] Compared with the prior art, the present invention has the following beneficial effects:

[0083] The NFPP-based cathode material provided by this invention has a coating layer comprising metal-doped coal-based porous carbon. On one hand, the presence of the metal-doped coal-based porous carbon forms a conductive network, thereby enhancing the conductivity of the NFPP-based cathode material. On the other hand, the doped metal in the coal-based porous carbon can also synergistically enhance the conductivity of the NFPP-based cathode material. Furthermore, the porous structure of the coating layer caused by the metal-doped coal-based porous carbon not only increases the rapid diffusion channels for sodium ions, thus effectively improving the ion diffusion kinetics of the sodium-ion battery. Moreover, the doped metal sites on the surface of the coating layer can also optimize the wetting effect of the electrolyte and promote interfacial charge transfer. Therefore, the sodium-ion battery prepared with the NFPP-based cathode material exhibits high rate performance. Detailed Implementation

[0084] 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.

[0085] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0086] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0087] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0088] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0089] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0090] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0091] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0092] 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.

[0093] Example 1

[0094] This embodiment provides an NFPP-based cathode material, which includes an NFPP cathode material core and a coating layer covering the outside of the NFPP cathode material core. The coating layer is composed of metal-doped coal-based porous carbon.

[0095] Based on the mass of the NFPP-based cathode material, the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material is 1.5 wt%.

[0096] The metal-doped coal-based porous carbon comprises a doping metal (including copper and aluminum in a mass ratio of 1:1) and coal-based porous carbon in a mass ratio of 1.0:10.

[0097] The preparation method of the NFPP-based cathode material is as follows:

[0098] (1) Preparation of NFPP precursor: The first mixture of iron source (ferrous sulfate), phosphorus source (phosphoric acid), pyrophosphate source (sodium pyrophosphate), sodium source (sodium carbonate), oxidant (hydrogen peroxide) and solvent (water) in a molar ratio of 1:0.2:0.7:2.2:1.0 forms a reaction solution and controls the concentration of iron ions in the reaction solution to be 1.2 mol / L and the pH to be 3.0. The coprecipitation reaction is carried out to obtain the NFPP precursor;

[0099] (2) Preparation of metal-doped coal-based porous carbon: The lignite powder was pulverized to obtain pulverized lignite powder with a D50 particle size of 30 μm; the pulverized lignite powder was then soaked in hydrochloric acid solution with a mass concentration of 8 wt% for 18 h, washed with water, and then dried at 110℃ for 12 h to obtain dried lignite powder.

[0100] The dried lignite powder was mixed with potassium activator (potassium hydroxide) in a third mixing process, and the mass ratio of lignite powder to potassium activator (potassium hydroxide) was controlled at 2:1 to obtain a mixture. The mixture was then calcined at 700℃ for 3 hours in an argon atmosphere, followed by water washing and drying to obtain coal-based porous carbon with a D50 particle size of 2μm, a porosity of 60%, and a pore diameter range of 100nm~180nm.

[0101] Copper powder with a D50 particle size of 0.8 μm and aluminum powder with a D50 particle size of 0.8 μm were mixed with the obtained coal-based porous carbon by ball milling for 3 hours to obtain metal-doped coal-based porous carbon.

[0102] (3) The NFPP precursor, sodium carbonate and metal-doped coal-based porous carbon were mixed by ball milling for 5 hours, and the mass ratio of NFPP precursor to sodium carbonate was 3.5:1. Then, the mixture was sintered at 700°C for 10 hours in an argon atmosphere to obtain the NFPP-based cathode material.

[0103] Example 2

[0104] This embodiment provides an NFPP-based cathode material, which includes an NFPP cathode material core and a coating layer covering the outside of the NFPP cathode material core. The coating layer is composed of metal-doped coal-based porous carbon.

[0105] Based on the mass of the NFPP-based cathode material, the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material is 1 wt%.

[0106] The metal-doped coal-based porous carbon comprises a doping metal (including copper and aluminum in a mass ratio of 1:0.5) and coal-based porous carbon in a mass ratio of 1.5:10.

[0107] The preparation method of the NFPP-based cathode material is as follows:

[0108] (1) Preparation of NFPP precursor: The first mixture of iron source (ferrous chloride), phosphorus source (ammonium dihydrogen phosphate), pyrophosphate source (sodium pyrophosphate), sodium source (sodium hydroxide), oxidant (sodium hypochlorite) and solvent (water) in a molar ratio of 1:0.2:0.7:2.2:1.0 forms a reaction solution and the concentration of iron ions in the reaction solution is controlled at 2.0 mol / L and the pH is 2.0. Co-precipitation reaction is carried out to obtain NFPP precursor;

[0109] (2) Preparation of metal-doped coal-based porous carbon: The lignite powder was pulverized to obtain pulverized lignite powder with a D50 particle size of 40 μm; then the pulverized lignite powder was soaked in a 15 wt% hydrochloric acid solution for 6 h and washed with water, and then dried at 140℃ for 5 h to obtain dried lignite powder.

[0110] The dried lignite powder was mixed with sodium activator (sodium hydroxide) in a third mixing process, and the mass ratio of lignite powder to sodium activator (sodium hydroxide) was controlled at 3:1 to obtain a mixture. The mixture was then calcined at 800℃ for 2 hours in a nitrogen atmosphere, followed by water washing and drying to obtain coal-based porous carbon with a D50 particle size of 3μm, a porosity of 50%, and a pore diameter range of 50nm~120nm.

[0111] Copper powder with a D50 particle size of 1.0 μm and aluminum powder with a D50 particle size of 1.0 μm were mixed with the obtained coal-based porous carbon by ball milling for 5 hours to obtain metal-doped coal-based porous carbon.

[0112] (3) The NFPP precursor, sodium carbonate and metal-doped coal-based porous carbon were mixed by ball milling for 6 hours, and the mass ratio of NFPP precursor to sodium carbonate was kept to be 4:1. Then, the mixture was sintered at 750°C for 8 hours in an argon atmosphere to obtain the NFPP-based cathode material.

[0113] Example 3

[0114] This embodiment provides an NFPP-based cathode material, which includes an NFPP cathode material core and a coating layer covering the outside of the NFPP cathode material core. The coating layer is composed of metal-doped coal-based porous carbon.

[0115] Based on the mass of the NFPP-based cathode material, the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material is 3 wt%.

[0116] The metal-doped coal-based porous carbon comprises a doping metal (including copper and aluminum in a mass ratio of 1:2) and coal-based porous carbon in a mass ratio of 0.5:10.

[0117] The preparation method of the NFPP-based cathode material is as follows:

[0118] (1) Preparation of NFPP precursor: The first mixture of iron source (ferrous chloride), phosphorus source (ammonium dihydrogen phosphate), pyrophosphate source (sodium pyrophosphate), sodium source (sodium hydroxide), oxidant (hydrogen peroxide) and solvent (water) in a molar ratio of 1:0.2:0.7:2.2:1.0 forms a reaction solution and the concentration of iron ions in the reaction solution is controlled at 0.5 mol / L and the pH is 4.0. Co-precipitation reaction is carried out to obtain NFPP precursor;

[0119] (2) Preparation of metal-doped coal-based porous carbon: The lignite powder was pulverized to obtain pulverized lignite powder with a D50 particle size of 20 μm; then the pulverized lignite powder was soaked in a 2 wt% hydrochloric acid solution for 30 h and washed with water, and then dried at 70 °C for 18 h to obtain dried lignite powder.

[0120] The dried lignite powder was mixed with potassium activator (potassium hydroxide) and sodium activator (sodium hydroxide) in a third mixing process, and the mass ratio of lignite powder to potassium activator (potassium hydroxide) and sodium activator (sodium hydroxide) was controlled to be 1:1 to obtain a mixture. The mixture was then calcined at 600℃ for 4 hours in an argon atmosphere, followed by water washing and drying to obtain coal-based porous carbon with a D50 particle size of 1μm, a porosity of 70%, and a pore diameter of 80nm~200nm.

[0121] Copper powder with a D50 particle size of 0.5 μm and aluminum powder with a D50 particle size of 0.5 μm were mixed with the obtained coal-based porous carbon by ball milling for 2 hours to obtain metal-doped coal-based porous carbon.

[0122] (3) The NFPP precursor, sodium carbonate and metal-doped coal-based porous carbon were mixed by ball milling for 4 hours, and the mass ratio of NFPP precursor to sodium carbonate was kept to be 3:1. Then, the mixture was sintered at 650°C for 12 hours in an argon atmosphere to obtain the NFPP-based cathode material.

[0123] Example 4

[0124] This embodiment provides an NFPP-based cathode material, which is the same as in Example 1 except that the mass of the NFPP-based cathode material is 100% and the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material is 0.5 wt%.

[0125] Example 5

[0126] This embodiment provides an NFPP-based cathode material, which is the same as in Example 1 except that the mass of the NFPP-based cathode material is 100% and the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material is 5 wt%.

[0127] Example 6

[0128] This embodiment provides an NFPP-based cathode material, which is the same as in Example 1 except that the metal-doped coal-based porous carbon contains doped metals (including copper and aluminum in a mass ratio of 1:1) in a mass ratio of 0.2:10 and coal-based porous carbon.

[0129] Example 7

[0130] This embodiment provides an NFPP-based cathode material, which is the same as in Example 1 except that the metal-doped coal-based porous carbon contains doped metals (including copper and aluminum in a mass ratio of 1:1) in a mass ratio of 2.5:10 and coal-based porous carbon.

[0131] Example 8

[0132] This embodiment provides an NFPP-based cathode material, wherein, apart from the metal-doped coal-based porous carbon, the doped metal contains only copper.

[0133] Except for replacing the aluminum powder in step (2) of the preparation method of the NFPP-based cathode material with copper powder of equal mass and D50 particle size, the rest is the same as in Example 1.

[0134] Example 9

[0135] This embodiment provides an NFPP-based cathode material, wherein, apart from the metal-doped coal-based porous carbon, the doped metal contains only aluminum;

[0136] Except for replacing the copper powder in step (2) of the preparation method of the NFPP-based cathode material with aluminum powder of equal mass and D50 particle size, the rest is the same as in Example 1.

[0137] Example 10

[0138] This embodiment provides an NFPP-based cathode material. Except for step (2) of the preparation method of the NFPP-based cathode material, in which the D50 particle size of both copper powder and aluminum powder is 0.1 μm, the rest is the same as in Example 1.

[0139] Example 11

[0140] This embodiment provides an NFPP-based cathode material. Except for step (2) of the preparation method of the NFPP-based cathode material, in which the D50 particle size of both copper powder and aluminum powder is 3.0 μm, the rest is the same as in Example 1.

[0141] Example 12

[0142] This embodiment provides an NFPP-based cathode material. Except for step (2) of the preparation method of the NFPP-based cathode material, in which the obtained mixture is calcined at 450°C to obtain coal-based porous carbon with a particle size of 0.6 μm, a porosity of 75%, and a pore diameter range of 230 nm to 400 nm, the rest is the same as in Example 1.

[0143] Example 13

[0144] This embodiment provides an NFPP-based cathode material. Except for step (2) of the preparation method of the NFPP-based cathode material, in which the obtained mixture is calcined at 950°C to obtain coal-based porous carbon with a particle size of 4.5 μm, a porosity of 42%, and a pore diameter range of 30 nm to 150 nm, the rest is the same as in Example 1.

[0145] Comparative Example 1

[0146] This comparative example provides an NFPP-based cathode material, except that the metal-doped coal-based porous carbon is replaced with coal-based porous carbon;

[0147] That is, the step (2) of the preparation method of the NFPP-based cathode material is omitted, which is "mixing copper powder with a D50 particle size of 0.8 μm and aluminum powder with a D50 particle size of 0.8 μm with the obtained coal-based porous carbon by ball milling for 3 hours to obtain metal-doped coal-based porous carbon", and the metal-doped coal-based porous carbon in step (3) is replaced with coal-based porous carbon, and the rest is the same as in Example 1.

[0148] Comparative Example 2

[0149] This comparative example provides an NFPP-based cathode material, except that the coating layer covering the core of the NFPP cathode material is omitted;

[0150] That is, step (2) of the preparation method of the NFPP-based cathode material is omitted, and step (3) is replaced with "sintering the NFPP precursor obtained in step (1) and sodium carbonate at a temperature of 700°C for 10 hours in an argon atmosphere to obtain the NFPP-based cathode material". The rest is the same as in Example 1.

[0151] Sodium-ion batteries were prepared using the NFPP-based cathode materials provided in the above embodiments and comparative examples. The preparation method of the sodium-ion battery is as follows: NFPP-based cathode material, acetylene black conductive agent and polyvinylidene fluoride binder are mixed in N-methylpyrrolidone solvent at a mass ratio of 8:1:1. After grinding and stirring until a uniform slurry is formed, the slurry is uniformly coated on aluminum foil current collector using a coating machine to obtain an electrode sheet. Subsequently, the obtained electrode sheet is dried in a vacuum oven at 120°C for 12 hours. After cutting and pressing, it is assembled into a CR2032 type button battery in an argon-filled glove box, using a sodium metal sheet as the counter electrode, a glass fiber membrane as the separator, and a 1 mol / L NaClO4 solution (the solvent includes EC and PC in a volume ratio of 1:1) as the electrolyte.

[0152] The obtained sodium-ion batteries were subjected to rate performance testing. The testing method was as follows: at 25°C, using a Blue Battery testing system, the sodium-ion batteries were tested at 2.0V~3.6V (vs. Na). + Within the voltage window of / Na), the battery was charged and discharged at different rates (0.2C, 1C, 2C, 5C) to obtain the discharge specific capacity at each rate, as shown in Table 1.

[0153] Table 1

[0154]

[0155] From Table 1, we can obtain:

[0156] (1) The sodium-ion batteries prepared with the NFPP-based cathode materials provided in Examples 1-3 of the present invention all have high discharge specific capacity at 0.2C, 1C, 2C and 5C rates, showing excellent rate performance.

[0157] (2) By comparing Example 1 with Examples 4 and 5, it can be seen that in this invention, when the mass fraction of the NFPP-based cathode material is 1 wt% to 3 wt%, the NFPP-based cathode material and sodium-ion battery have better performance. This is because when the mass fraction of the metal-doped coal-based porous carbon is in this range, the coating layer can form a continuous conductive network and optimize its porous structure. If the mass fraction of the metal-doped coal-based porous carbon is too low, the coating layer will not be fully coated, and the conductive network and ion channel construction will be insufficient. If the mass fraction of the metal-doped coal-based porous carbon is too high, the thick coating layer will increase the ion transport resistance, which will lead to a decrease in rate performance.

[0158] (3) By comparing Example 1 with Examples 6 and 7, it can be seen that in the present invention, when the mass ratio of the doped metal (including copper and aluminum with a mass ratio of 1:1) to the coal-based porous carbon is (0.5~1.5):10, the NFPP-based cathode material and sodium-ion battery have better performance. This is because this mass ratio can maximize the synergistic doping effect of copper and aluminum. If the mass ratio is too low, the doped metal content is insufficient, and the improvement of conductivity and interface dynamics is limited. If the mass ratio is too high, the excessive doped metal is prone to agglomeration, which will cause some pores of the coal-based porous carbon to be blocked, increasing the ion transport resistance and hindering the improvement of rate performance.

[0159] (4) By comparing Example 1 with Examples 8 and 9, it can be seen that in the present invention, when the metal-doped coal-based porous carbon includes both copper and aluminum as the doping metal, the NFPP-based cathode material and sodium-ion battery have better performance than when the doping metal includes only copper or aluminum. This is because copper and aluminum produce a synergistic enhancement effect. The main role of copper is to improve electronic conductivity and build an efficient conductive network; the main role of aluminum is to promote the formation of a more stable SEI film at the interface and optimize sodium ion diffusion dynamics. The synergistic effect of copper and aluminum, compared with a single metal, can more comprehensively optimize the electronic conduction and ion migration process of the NFPP-based cathode material, thereby improving the rate performance of the material while ensuring structural stability.

[0160] (5) By comparing Example 1 with Examples 10 and 11, it can be seen that in step (2) of the preparation method of the NFPP-based cathode material in this invention, when the D50 particle size of the metal powder is 0.5μm~1.0μm, the NFPP-based cathode material and sodium-ion battery have better performance. This is because when the D50 particle size of the metal powder is within this particle size range, it is beneficial to achieve the optimal dispersion and bonding of the metal powder in coal-based porous carbon. If the D50 particle size of the metal powder is too small, the metal powder is prone to agglomeration, making it difficult to form uniform doping and affecting the continuity of the conductive network. If the D50 particle size of the metal powder is too large, it will lead to insufficient uniformity of the coating layer, and the excessively large metal particles will easily hinder the ion diffusion channel.

[0161] (6) By comparing Example 1 with Examples 12 and 13, it can be seen that in the preparation method of the NFPP-based cathode material in this invention, when the calcination temperature is 600℃~800℃, coal-based porous carbon with D50 particle size of 1μm~3μm, porosity of 50%~70% and pore diameter of 50nm~200nm is obtained, so that the NFPP-based cathode material and sodium-ion battery have better performance. This is because when calcined in this temperature range, lignite powder can be fully graphitized to form a good conductive network, and volatiles can be moderately released to generate pores. The mesopores formed by 50nm~200nm form a fast diffusion channel for sodium ions, while the particle size of 1μm~3μm ensures a high tap density when the ion transport path on the surface is too long. Thus, the conductivity and ion diffusion dynamics of the NFPP-based cathode material are synergistically optimized, so that the sodium-ion battery exhibits better rate performance.

[0162] (7) As can be seen from the comparison between Example 1 and Comparative Examples 1 and 2, the NFPP-based cathode material provided by the present invention has a coating layer containing metal-doped coal-based porous carbon. On the one hand, the presence of metal-doped coal-based porous carbon forms a conductive network, thereby enhancing the conductivity of the NFPP-based cathode material. On the other hand, the doped metal in the coal-based porous carbon can also have a synergistic effect with the coal-based porous carbon, further enhancing the conductivity of the NFPP-based cathode material. Furthermore, the coating layer has a porous structure caused by metal-doped coal-based porous carbon, which not only increases the rapid diffusion channels of sodium ions, but also effectively improves the ion diffusion kinetics of sodium-ion batteries. Moreover, the doped metal sites on the surface of the coating layer can also optimize the wetting effect of the electrolyte and promote interfacial charge transfer. Therefore, the sodium-ion battery prepared with the NFPP-based cathode material exhibits high rate performance.

[0163] 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. An NFPP-based cathode material, characterized in that, The NFPP-based cathode material includes an NFPP cathode material core and a coating layer covering the outside of the NFPP cathode material core. The coating layer is composed of metal-doped coal-based porous carbon.

2. The NFPP-based cathode material according to claim 1, characterized in that, Based on the mass of the NFPP-based cathode material, the mass fraction of metal-doped coal-based porous carbon in the NFPP-based cathode material is 1wt%~3wt%.

3. The NFPP-based cathode material according to claim 1 or 2, characterized in that, The metal-doped coal-based porous carbon comprises both doped metal and coal-based porous carbon. Preferably, in the metal-doped coal-based porous carbon, the mass ratio of the doped metal to the coal-based porous carbon is (0.5~1.5):10; Preferably, the doped metal includes copper and / or aluminum, and more particularly copper and aluminum; Preferably, the mass ratio of copper to aluminum in the doped metal is 1:(0.5~2).

4. A method for preparing the NFPP-based cathode material according to any one of claims 1 to 3, characterized in that, The preparation method includes: After mixing NFPP precursor, sodium carbonate, and metal-doped coal-based porous carbon, the mixture is sintered in a protective atmosphere to obtain NFPP-based cathode material.

5. The preparation method according to claim 4, characterized in that, The method for preparing the NFPP precursor includes: a first mixed iron source, phosphorus source, pyrophosphate source, sodium source, oxidant and solvent, which are subjected to a co-precipitation reaction to obtain the NFPP precursor; Preferably, in the first mixture, the molar ratio of iron source, phosphorus source, pyrophosphate source, sodium source and oxidant is 1:(0.1~0.4):(0.6~0.9):(2~2.5):(0.5~1.5).

6. The preparation method according to claim 4, characterized in that, The method for preparing the metal-doped coal-based porous carbon includes: mixing metal powder with coal-based porous carbon in a second mixing process to obtain metal-doped coal-based porous carbon; Preferably, the metal powder includes copper powder and / or aluminum powder, and more preferably copper powder and aluminum powder; Preferably, the D50 particle size of the metal powder is 0.5μm~1.0μm.

7. The preparation method according to claim 6, characterized in that, The coal-based porous carbon has a D50 particle size of 1μm to 3μm, a porosity of 50% to 70%, and a pore diameter of 50nm to 200nm. Preferably, the method for preparing the coal-based porous carbon includes: After a third mixing of lignite powder and activator, calcination is carried out in a protective atmosphere to obtain coal-based porous carbon. Preferably, the activator includes a potassium activator and / or a sodium activator; Preferably, the potassium activator includes any one or a combination of at least two of potassium hydroxide, potassium carbonate, potassium chloride, or potassium nitrate; Preferably, the sodium activator includes any one or a combination of at least two of sodium hydroxide, sodium carbonate, sodium chloride, or sodium nitrate; Preferably, in the third mixture, the mass ratio of lignite powder to activator is (1~3):1; Preferably, the calcination temperature is 600℃~800℃ and the time is 2h~4h.

8. The preparation method according to any one of claims 4 to 7, characterized in that, In the mixture, the mass ratio of NFPP precursor to sodium carbonate is (4~3):1; Preferably, the sintering temperature is 650℃~750℃ and the time is 8h~12h.

9. The preparation method according to claim 4, characterized in that, The preparation method includes: (1) Preparation of NFPP precursor: Iron source, phosphorus source, pyrophosphate source, sodium source, oxidant and solvent with a first mixing molar ratio of 1:(0.1~0.4):(0.6~0.9):(2~2.5):(0.5~1.5) are mixed to form a reaction solution and the concentration of iron ions in the reaction solution is controlled to be 0.5mol / L~2.0mol / L and the pH is 2.0~4.

0. Co-precipitation reaction is carried out to obtain NFPP precursor; (2) Preparation of metal-doped coal-based porous carbon: The lignite powder is pulverized to obtain pulverized lignite powder with a D50 particle size of no more than 50 μm; then the pulverized lignite powder is soaked in a hydrochloric acid solution with a mass concentration of 2wt%~15wt% for 6h~30h, then washed with water, and then dried at 70℃~140℃ for 5h~18h to obtain dried lignite powder; The dried lignite powder was mixed with potassium activator and / or sodium activator in a third mixing process, and the mass ratio of lignite powder to potassium activator and / or sodium activator was controlled to be (1~3):1 to obtain a mixture. The mixture was then calcined at 600℃~800℃ for 2h~4h in a protective atmosphere, followed by water washing and drying to obtain coal-based porous carbon with D50 particle size of 1μm~3μm, porosity of 50%~70% and pore diameter of 50nm~200nm. Copper powder with a D50 particle size of 0.5μm~1.0μm and aluminum powder with a D50 particle size of 0.5μm~1.0μm are mixed with the obtained coal-based porous carbon by ball milling for no less than 2 hours to obtain metal-doped coal-based porous carbon. (3) The NFPP precursor, sodium carbonate and metal-doped coal-based porous carbon are mixed by ball milling for 4h~6h, and the mass ratio of NFPP precursor to sodium carbonate is (4~3):

1. Then, the mixture is sintered in a protective atmosphere at a temperature of 650℃~750℃ for 8h~12h to obtain the NFPP-based cathode material.

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

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