Low-band-gap ferric sodium pyrophosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium-ion battery positive electrode materials, and relates to a low-band-gap ferric sodium pyrophosphate positive electrode material and a preparation method and application thereof. The chemical formula of the low-band-gap phosphoric acid and sodium ferric pyrophosphate positive electrode material is NaxFex-1-ny / 2My (PO4) x-2P2O7 / C, x = 3, 4 or 5, M is a doped low-electronegativity element, n is the valence state of the element M, and y is larger than or equal to 0.2 and smaller than or equal to 0.01. By adjusting the type and content of the low-electronegativity doping element, the electron state density of the Fe 3d orbit near the Fermi level is increased, and the band gap width is smaller than or equal to 1 eV. The low-electronegativity element partially replaces a bivalent Fe element, so that the band gap can be reduced to improve the electronic conductivity of the polyanion material, the electrochemical activity of the polyanion material is improved, and the specific capacity and the rate capability of the polyanion material are improved; meanwhile, lattice distortion during deep sodium removal of the polyanion material can be inhibited, and the cycling stability of the polyanion material is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium-ion battery cathode materials, and relates to a low-bandgap sodium iron pyrophosphate phosphate cathode material and a preparation method and application thereof. BACKGROUND

[0002] Sodium iron pyrophosphate phosphate (NFPP) is an iron-based polyanion material, which has significant advantages in the field of sodium-ion battery cathode materials: it has a higher theoretical capacity and a moderate working voltage, and due to the unique three-dimensional sodium ion diffusion channel and the mixed ion stable structure formed by phosphate and pyrophosphate, it has excellent potential for cycle performance. However, NFPP has two major defects: low electronic conductivity and lattice distortion during charging and discharging. These two problems directly lead to the difficulty of meeting the application requirements of the rate performance and actual cycle performance of sodium-ion batteries based on NFPP, which becomes a key bottleneck restricting its industrialization.

[0003] To improve the above problems, at present, organic carbon sources are mainly introduced into the material to form a carbon coating layer after carbonization to enhance the conductivity. However, due to the low temperature of NFPP itself, the graphitization degree of the organic carbon source is low when carbonized at this temperature, the carbon layer formed is arranged in disorder and has limited conductivity, which cannot fundamentally solve the problem of insufficient electronic conductivity. Patent CN118919716A proposes to replace Fe elements with monovalent positive ions M, but the selection range of monovalent positive ions is narrow, and it is difficult to flexibly control the material properties according to actual needs, which limits the further expansion and performance improvement space of this scheme. Patent CN118790969A improves the electronic conductivity by adjusting the content of Na vacancies and Fe defects in NFPP, but the ratio of Na and Fe is easy to be unbalanced during the synthesis process, which further causes the Fe-Na ion exchange phenomenon, leading to irreversible changes in the crystal structure of the material, thereby destroying the cycle stability.

[0004] Patent CN119409153A discloses an iron-based phosphate sodium-ion cathode material and a preparation method thereof. The method first reacts iron sources with different valence states and sodium-containing phosphorus sources to obtain a precursor, and then obtains the final product by mechanical mixing, drying and sintering treatment of the precursor with a sodium source, a carbon source and a dopant. Although the material shows good electrochemical performance in sodium-ion batteries and has industrialization potential, the doping elements are not explicitly shown in the stoichiometric relationship, and the doping method is closer to "mechanical mixing" or "random substitution", which lacks precise regulation of the charge balance of the crystal structure. This random doping easily introduces additional cation vacancies and other defects as carrier scattering centers, which not only makes it difficult to effectively improve the electronic conductivity, but also may even exacerbate the lattice distortion. In addition, the process uses a multi-step synthesis, and the process is relatively complex.

[0005] Patent CN116692810A discloses a preparation method of a sodium iron manganese pyrophosphate positive electrode material, which introduces sodium source, iron source, manganese source, phosphorus source and carbon source in the ball milling process, and obtains a porous positive electrode material by means of molten salt assisted sintering. The core of the method is to use Mn 2+ / Mn 3+ electronic pairs to improve the working voltage, but its doping strategy mainly serves the high voltage characteristics, and the influence of the doping elements on the electronic conductivity and structural stability of the material is not systematically considered. Especially, Mn has Jahn-Teller effect, which can induce local lattice distortion, and poses a potential threat to the long cycle stability, and it is difficult to balance the high conductivity and structural stability.

[0006] Therefore, there is an urgent need in the art to provide a simple and efficient NFPP positive electrode material with a simple process flow, which can effectively improve the electronic conductivity of the material and inhibit the lattice distortion in the charging and discharging process, thereby simultaneously optimizing the rate performance and cycle stability of the sodium ion battery. SUMMARY

[0007] The purpose of the present application is to overcome the defects of the prior art and provide a low-bandgap sodium iron pyrophosphate positive electrode material and a preparation method and application thereof.

[0008] The purpose of the present application can be achieved by the following technical solutions: A low-bandgap sodium iron pyrophosphate positive electrode material, the chemical formula of the positive electrode material is Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, the band gap width is ≤1 eV; wherein, x=3, 4, 5, M is a low electronegativity element for promoting the migration of electrons to the O atom in the Fe-O-M bridging structure, n is the valence state of the M element, 0.2≤y≤0.01.

[0009] Further, the M element is an element with lower electronegativity than Fe and its doping position is Fe site, the electronegativity of M is <1.83.

[0010] Further, the M element is selected from one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Zn, Ga, Y, Zr, Nb, Cd and In.

[0011] Preferably, the M element is selected from one or more of Mg, Zn and Nb.

[0012] Among them, the electronegativity of Mg is 1.31, which is much lower than that of Fe, and the electron donating ability is more excellent; and Mg 2+ is far away from Fe 2+The ion radius is highly matched, the lattice distortion degree caused by doping is minimum, it is an ideal doping element with dual functions of 'high-efficiency electron donor' and 'excellent structure stabilizer', and can simultaneously optimize electron conductivity and structure stability; Zn (electronegativity 1.65) has 3d 10 The stable electron configuration is full, no valence change occurs in the working state of the material, the side reaction and energy band structure disturbance caused by the valence change can be avoided, the purest and most reliable energy band regulation effect is provided for the material, and the long-term stability of the electron conductivity is ensured; Nb (electronegativity 1.60) is used as a dopant in +5 valence state, the corresponding ny / 2 charge compensation term value is larger, and only a lower doping amount can realize the strong charge compensation effect and structure 'pinning' effect, and the most outstanding technical effect of inhibiting the lattice distortion in the process of material deintercalation sodium and improving the cycle stability.

[0013] The application also provides a preparation method of the low-bandgap sodium iron phosphate positive electrode material according to any one of the above, and the preparation method is characterized by comprising the following steps: (1) mixing a sodium source, an iron source, a phosphorus source, a carbon source and a dopant with a solvent, mechanically dispersing through ball milling or sand milling, and then centrifugally separating to obtain solid particles; drying the solid particles to obtain a precursor powder; (2) calcining the precursor powder obtained in the step (1) in an inert atmosphere to obtain a sodium iron phosphate / carbon composite material.

[0014] Further, in the step (1), the total mass ratio of the carbon source to the sodium source, the iron source, the phosphorus source and the dopant is (0.05-0.15):1.

[0015] Further, the sodium source is selected from one or more of sodium carbonate, sodium hydroxide, sodium pyrophosphate, disodium pyrophosphate, sodium oxalate, sodium dihydrogen phosphate, sodium hydrogen phosphate, sodium phosphate, sodium chloride and sodium acetate; The iron source is selected from one or more of iron acetate, iron nitrate, ferrous oxalate, iron oxide, iron powder and iron phosphate; The phosphorus source is selected from one or more of sodium dihydrogen phosphate, sodium pyrophosphate, sodium hydrogen phosphate, disodium pyrophosphate and sodium phosphate; The carbon source is selected from one or more of glucose, sucrose, starch, citric acid, ascorbic acid, oxalic acid and polyvinyl alcohol; The dopant is an oxide, carbonate or oxalate of an M element.

[0016] The solvent is deionized water; Further, the dopant is selected from one or more of MgO, MnCO3, ZnO, Nb2O5, Al2O3, TiO2, V2O5, Cr2O3, CdO, ZrO2, MgC2O4, MnC2O4, ZnC2O4, CdC2O4, Al2(C2O4)3, Sc2(C2O4)3, Ga2(C2O4)3, In2(C2O4)3 and Y2(C2O4)3.

[0017] Further, in step (1), the drying condition is vacuum drying at 60-80 ℃ for 10-16 h.

[0018] Further, in step (2), the calcination condition is calcination at 250-350 ℃ for 3-6 h under an inert atmosphere, and sintering at 450-550 ℃ for 10-12 h.

[0019] The application also provides a use of the low-bandgap sodium iron phosphate pyrophosphate positive electrode material as described in any one of the above in the preparation of a sodium ion battery positive electrode material.

[0020] The application also includes the use of the low-bandgap sodium iron phosphate pyrophosphate positive electrode material in the preparation of a sodium ion battery positive electrode, and the electrochemical performance thereof is tested.

[0021] Specifically, the low-bandgap sodium iron phosphate pyrophosphate positive electrode material is uniformly mixed with a conductive agent and a binder, and then coated on an aluminum foil by coating the slurry, and the aluminum foil is dried in a vacuum oven, and the dried foil is cut into a circular electrode sheet of a suitable diameter for the preparation of a sodium ion button cell.

[0022] Preferably, the Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C material, conductive carbon black and PVDF binder are weighed according to a mass ratio of 8:1:1, the positive electrode material and the conductive agent Super-P are weighed according to a mass ratio of 8:1, mixed uniformly in a mortar, and then mixed with the binder PVDF in an appropriate amount of NMP solvent to form a homogeneous slurry. The slurry is uniformly coated on an Al foil current collector by using a doctor blade. After drying, the Al foil is cut as a positive electrode, a sodium sheet is used as a negative electrode, and an electrolyte is 1M NaFP6 EC-DEC@5% FEC (1:1, V / V) to prepare a sodium button cell for testing the electrochemical performance thereof.

[0023] Compared with the prior art, the application has the following beneficial effects: (1) The application realizes the synergistic optimization of the performance of the material by introducing a low electronegativity doping element into the sodium iron pyrophosphate phosphate positive electrode material, and the specific effects are as follows: on the one hand, the low electronegativity element can promote more electrons to migrate to the bridging O atom (O atom in the Fe-O-M structure), thereby causing Fe 3d-O 2p electron redistribution, which can not only effectively increase the electron density around the Fe center, that is, increase the electron state density of the Fe 3d orbital near the Fermi level, but also optimize the electronic environment of the material and precisely control the band gap width, thereby significantly improving the electronic conductivity; on the other hand, the low electronegativity doping element plays a “pillar role” in the crystal structure of the material, which can effectively inhibit the lattice distortion in the deep sodium removal process of the sodium iron pyrophosphate phosphate polyanion material, thereby improving the cycle stability of the material, and finally realizing the synergistic optimization of the comprehensive performance of the sodium iron pyrophosphate phosphate (NFPP) positive electrode material.

[0024] (2) The low-bandgap sodium iron pyrophosphate phosphate positive electrode material of the application produces a synergistic optimization effect from the two aspects of electronic structure and crystal structure by introducing a low electronegativity doping element (M) at the Fe site, thereby significantly reducing the Na / Fe anti-site defects, the material band gap and the sodium ion diffusion barrier, as follows: First, at the level of crystal structure, the introduction of the low electronegativity doping element plays a “lattice pillar” role: by virtue of its specific ionic radius and bonding ability, it stabilizes the local crystal field, firmly locks Fe 2+ Na / Fe ion exchange energy barrier, thereby inhibiting the formation of anti-site defects; at the same time, the ny / 2 charge compensation term introduced in the chemical formula ensures that the lattice can still maintain precise electrical neutrality after doping, eliminating the driving force for defects induced by charge imbalance from the thermodynamic level, thereby further enhancing the structural stability.

[0025] Secondly, at the level of electronic structure, the introduction of the low electronegativity element (M) causes the electrons in the Fe-O-M bridging structure to be more biased towards the O atom, causing Fe 3d and O 2p orbital electron rearrangement and enhanced hybridization. This process directly increases the electron state density of the Fe 3d orbital near the Fermi level, effectively reduces the energy gap between the valence band and the conduction band, and thereby significantly improves the intrinsic electronic conductivity of the material.

[0026] Finally, the above-mentioned optimization effects of crystal structure and electronic structure synergistically affect the sodium ion transport process: the improvement of structural stability ensures the smoothness of the sodium ion diffusion channel, and the redistribution of electronic density weakens the local electrostatic field around the sodium ion migration site, thereby effectively reducing the sodium ion diffusion energy barrier. In summary, by using a single doping strategy, the application simultaneously realizes the triple core effects of “suppressing defects, reducing band gap and promoting ion transport”, and finally synergistically improves the rate performance, specific capacity and cycle stability of the material.

[0027] (3) The electronic conductivity of the low-bandgap sodium iron phosphate pyrophosphate positive electrode material of the application is 5*10 -4 5*10 -3 S / cm, and the powder compaction density (300 MPa) of the positive electrode material is 2.2-2.5 g / cm 3 .

[0028] (4) The preparation method of the low-bandgap sodium iron phosphate pyrophosphate positive electrode material of the application has a short process flow, simple process, is green and environmentally friendly, and has good repeatability. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The first circle charge-discharge curve of the sample prepared in Example 1 and Comparative Example 1 of the application.

[0030] Figure 2 The rate performance graph of the sample prepared in Example 1 and Comparative Example 1 of the application. DETAILED DESCRIPTION

[0031] The application will be described in detail below in combination with the drawings and specific examples. The present embodiment is implemented on the premise of the technical solution of the application, and detailed implementation modes and specific operation processes are given, but the protection scope of the application is not limited to the following examples.

[0032] The raw materials used in the application are all commercially available products unless otherwise specified.

[0033] Example 1 A low-bandgap sodium iron phosphate pyrophosphate positive electrode material Na4Fe 2.95 Mg 0.05 (PO4)2P2O7 / C, namely Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C (x=4, y=0.05, n=2) is prepared, including the following steps: (1) 1 mol of FeC2O4 2H2O, 0.68 mol of Na2CO3, 1.36 mol of NH4H2PO4 and 0.017 mol of MgO are weighed so that x=4 and y=0.05 in Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, and 10% of the total mass of the above four raw materials is added as a carbon coating agent. Deionized water is selected as a solvent and all the solid components are sand-milled at a speed of 1000 rpm for 5 h, and the obtained mixture is dried at 60 ℃ for 12 h.

[0034] (2) The sanding product is sintered at 300°C for 5 h in an argon atmosphere, and then sintered at 500°C for 10 h, the heating and cooling rate is 5°C / min, and then naturally cooled to room temperature to obtain Na4Fe 2.95 Mn 0.05 (PO4)2P2O7 / C positive electrode material.

[0035] Example 2 A low-bandgap sodium iron phosphate pyrophosphate positive electrode material Na4Fe 2.98 Mn 0.02 (PO4)2P2O7 / C, namely Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C (x=4, y=0.02, n=2) is prepared by the following steps: (1) 1 mol of FeC2O4 2H2O, 0.67 mol of Na2CO3, 1.34 mol of NH4H2PO4 and 0.0067 mol of MnCO3 are weighed so that Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, x=4, y=0.02, and 10% of the total mass of the above four raw materials is added as a carbon coating agent. Deionized water is selected as a solvent and all the solid components are sand-milled at a speed of 1000 rpm for 5 h, and the obtained mixture is dried at 60°C for 12 h.

[0036] (2) The sanding product is sintered at 300°C for 5 h in an argon atmosphere, and then sintered at 500°C for 10 h, the heating and cooling rate is 5°C / min, and then naturally cooled to room temperature to obtain Na4Fe 2.98 Mn 0.02 (PO4)2P2O7 / C positive electrode material.

[0037] Example 3 A low-bandgap sodium iron phosphate pyrophosphate positive electrode material Na4Fe 2.95 Zn 0.05 (PO4)2P2O7 / C, namely Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C (x=4, y=0.05, n=2) is prepared by the following steps: (1) 0.02 mol of FePO4, 0.0072 mol of Na2HPO4, 0.0064 mol of Na2CO3 and 0.0003 mol of ZnO are weighed so that Na x Fex-1-ny / 2 M y (PO4) x-2 P2O7 / C, wherein x = 4, y = 0.05, and 10% of the total mass of the four raw materials above is added as glucose as a carbon coating agent. Ethanol is selected as a solvent and all the solid components above are ball milled at a speed of 300 rpm for 12 h. The obtained mixture after ball milling is dried at 60 °C for 12 h.

[0038] (2) The ball-milled product is sintered at 300 °C for 5 h in an argon atmosphere, and then sintered at 500 °C for 10 h, with a temperature rising and falling rate of 5 °C / min. The carbon-coated Na4Fe 2.95 Zn 0.05 (PO4)2P2O7 / C positive electrode material.

[0039] Example 4 A low-bandgap sodium iron phosphate pyrophosphate positive electrode material Na4Fe 2.925 Nb 0.03 (PO4)2P2O7 / C, namely Na x Fe x-1-ny / 2M y (PO4) x-2 P2O7 / C (x = 4, y = 0.03, n = 5) is prepared, including the following steps: (1) 0.02 mol of FePO4, 0.0037 mol of Na4P2O7, 0.0063 mol of Na2CO3, and 0.0001 mol of Nb2O5 are weighed, so that Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, wherein x = 4, y = 0.03, 10% of the total mass of the four raw materials above is added as glucose as a carbon coating agent. Ethanol is selected as a solvent and all the solid components above are ball milled at a speed of 300 rpm for 12 h. The obtained mixture after ball milling is dried at 60 °C for 12 h.

[0040] (2) The ball-milled product is sintered at 300 °C for 5 h in an argon atmosphere, and then sintered at 500 °C for 10 h, with a temperature rising and falling rate of 5 °C / min. The carbon-coated Na4Fe 2.925 Nb 0.03 (PO4)2P2O7 / C positive electrode material.

[0041] Example 5 A low-bandgap sodium iron phosphate pyrophosphate positive electrode material Na5Fe 3.9 Mn 0.1 (PO4)3P2O7 / C, namely Nax Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C (x = 5, y = 0.1, n = 2) was prepared by the following steps: (1) 1 mol FeC2O42H2O, 0.64 mol Na2CO3, 1.28 mol NH4H2PO4 and 0.026 mol MnCO3 were weighed so that Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, x = 5, y = 0.1, and 10% of the total mass of the above four raw materials of glucose were added as a carbon coating agent. Deionized water was selected as the solvent and all the solid components were sand-milled at a speed of 1000 rpm for 5 h. The obtained mixture was dried at 60 °C for 12 h after sand-milling.

[0042] (2) The sand-milling product was sintered at 300 °C for 6 h in an argon atmosphere, and then sintered at 500 °C for 10 h, with a temperature rising and falling rate of 5 °C / min. The carbon-coated Na5Fe 3.9 Mn 0.1 (PO4)3P2O7 / C positive electrode material was obtained by natural cooling to room temperature.

[0043] Example 6 A low-bandgap sodium iron phosphate pyrophosphate positive electrode material Na3Fe 1.775 Al 0.15 (PO4)P2O7 / C, namely Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C (x = 3, y = 0.15, n = 3) was prepared by the following steps: (1) 0.02 mol FePO4, 0.014 mol Na2HPO4, 0.003 mol Na2CO3 and 0.0008 mol Al2O3 were weighed so that Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, x = 3, y = 0.15, and 10% of the total mass of the above four raw materials of glucose were added as a carbon coating agent. Ethanol was selected as the solvent and all the solid components were ball-milled at a speed of 500 rpm for 16 h. The obtained mixture was dried at 60 °C for 12 h after ball-milling.

[0044] (2) The ball-milling product is sintered at 300 DEG C for 5 h and at 500 DEG C for 10 h in an argon atmosphere, the temperature rising and falling rate is 5 DEG C / min, and the carbon-coated Na3Fe 1.775 Al 0.15 (PO4)P2O7 / C positive electrode material.

[0045] Comparative Example 1 A preparation method of a sodium iron pyrophosphate positive electrode material Na4Fe3(PO4)2P2O7 / C, comprising the following steps: (1) 1 mol of FeC2O4 2H2O, 0.67 mol of Na2CO3 and 1.33 mol of NH4H2PO4 are weighed, so that the molar ratio of Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, x=4, y=0, n=0, and 10% of glucose in the theoretical product mass fraction is added as a carbon coating agent. Deionized water is selected as a solvent and all the solid components are sand-milled at a speed of 1000 rpm for 5 h, and the obtained mixture is dried at 60 DEG C for 12 h.

[0046] (2) The sand-milling product is sintered at 300 DEG C for 6 h and at 500 DEG C for 10 h in an argon atmosphere, the temperature rising and falling rate is 5 DEG C / min, and the carbon-coated Na4Fe3(PO4)2P2O7 / C positive electrode material is obtained after natural cooling to room temperature.

[0047] Comparative Example 2 A preparation method of a sodium iron pyrophosphate positive electrode material Na5Fe4(PO4)3P2O7 / C, namely Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C (x=4, y=0.03, n=0), comprising the following steps: (1) 0.02 mol of FePO4, 0.0025 mol of Na4P2O7 and 0.0075 mol of Na2CO3 are weighed, so that the molar ratio of Na x Fe x-1-ny / 2M y (PO4) x-2 P2O7 / C, x=5, y=0, n=0 (undoped), and 10% of glucose in the theoretical product mass fraction is added as a carbon coating agent. Ethanol is selected as a solvent and all the solid components are ball-milled at a speed of 400 rpm for 12 h, and the obtained mixture is dried at 60 DEG C for 12 h.

[0048] (2) The ball-milling product was sintered at 350 °C for 5 h and at 550 °C for 10 h in an argon atmosphere, the temperature rising and falling rate was 5 °C / min, and the carbon-coated Na5Fe4(PO4)3P2O7 / C positive electrode material was obtained by natural cooling to room temperature.

[0049] To evaluate the comprehensive performance of the positive electrode materials prepared in Examples 1-6 and Comparative Examples 1-2 of the present application, the band gap, electrochemical performance and cycle stability of the positive electrode materials were tested, as follows: (1) Band gap test: The band gap was calculated by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) combined with the Tauc Plot method.

[0050] (2) Electronic conductivity test: The four-probe method was used to test the conductivity of the positive electrode material powder. First, the positive electrode material powder with a mass of m was loaded into a mold, and a pressure of 20 MPa was applied and maintained, and then the positive electrode material powder was pressed into a uniform sample sheet with a thickness of h and a diameter of d. In the pressure maintaining state, a constant current I was applied to the outer two probes, and the voltage V between the inner two probes was measured. The sample resistance was calculated according to the formula R=V / I. According to the cross-sectional area of the sample sheet A=π(d / 2) 2 , the conductivity σ was calculated according to the formula conductivity σ=h / (R×A).

[0051] (3) Compaction density test: The small disc and the empty aluminum foil (area A mm 2 ) after coating the positive electrode active material layer were punched out using a punching machine. The thickness of the small disc and the empty aluminum foil was measured using a micrometer, and was recorded as H, h (mm), respectively. The mass of the small disc and the empty aluminum foil was weighed, and was recorded as M, m (g), respectively. The compaction density was calculated as ((M-m) / (A×(H-h)))×1000 (g / cm 3 ) (4) Electrochemical performance The prepared positive electrode material was used as the positive electrode active material, and the positive electrode active material, the conductive agent and the binder were weighed according to the mass ratio of 8:1:1. First, 240 mg of the active material and 30 mg of Super-P conductive agent were mixed in a mortar for 10 min until they were uniformly mixed. In a 5 mL beaker, 600 mg of a PVDF solution with a concentration of 5% was weighed, and the mixed powder was added to the beaker. Three drops of NMP were added, and the beaker was sealed with plastic wrap and sealing film. The beaker was placed on a magnetic stirring table and stirred with a stirrer for 4 h to ensure that the components were fully mixed. The slurry was applied to an aluminum foil, which was vacuum dried at 120 °C for 12 h. The dried and cut circular electrode sheet was used as the sodium-ion battery positive electrode sheet for the button cell.

[0052] The above electrode sheet was used as the positive electrode, a sodium sheet was used as the negative electrode, and 1M NaPF6 EC-DEC@5% FEC (1:1, V / V) was used as the electrolyte. A CR2032 type button cell was assembled in the order of the positive electrode sheet, a separator, the sodium sheet, a gasket, and a spring, and the packaged battery was allowed to stand for 24 h to allow the electrodes to be fully soaked with the electrolyte. Subsequently, the battery was placed in a 25°C constant temperature environment and allowed to stand for 4 h to ensure stable testing temperature.

[0053] In a voltage range of 1.7-4.3 V (vs. Na + / Na), charge-discharge tests were performed at 25°C constant temperature: first, a charge-discharge cycle of “constant current charging to 4.3 V, then constant current discharging to 1.7 V” was performed at a rate of 0.1C, and the discharge specific capacity at this rate was recorded; the charge-discharge rate was switched to 0.2C, 0.5C, 1C, 2C, 3C, 5C and 10C in turn, and the cycle operation of “constant current charging to 4.3 V, then constant current discharging to 1.7 V” was repeated at each rate; the cycle was repeated 5 times at each rate, and the discharge specific capacity at different cycles and different rates was recorded to obtain Figure 2 the cycle discharge specific capacity curve at different rates.

[0054] In a voltage range of 1.7-4.3 V (vs. Na + / Na), the charge-discharge specific capacity at a current density of 0.1C was measured, respectively; the discharge specific capacity at the 1st cycle and the 1000th cycle was recorded after 1000 cycles at a current density of 5C, and the capacity retention rate was calculated according to the formula: capacity retention rate = (discharge specific capacity at the 1000th cycle / discharge specific capacity at the 1st cycle) x 100%.

[0055] The test results are shown in Table 1.

[0056] Table 1: Electrochemical performance of the materials Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C prepared in Examples 1-6 As shown in Table 1: the band gap of the NFPP prepared in Examples 1-6 was 0.28-0.98 eV, the discharge specific capacity at a current density of 0.1C was 108.5-123.5 mAh g + in a voltage range of 1.7-4.3 V (vs. Na -1 / Na), and the reversible specific capacity at 5C was 99.5-110.3 mAh g -1Meanwhile, the NFPPs prepared in Examples 1-6 exhibit excellent cycle stability: 96.9-98.9% retention rate at 5C high rate for 1000 cycles. The band gap of the undoped Comparative Example 1 NFPP is 3.29 eV, and the discharge specific capacity is 104.8 mAh g + at a current density of 0.1C in the voltage range of 1.7-4.3V (vs. Na -1 / Na), and the reversible specific capacity is 87.7 mAh g -1 at 5C, while the cycle stability is poor, with a retention rate of 85.6% for 1000 cycles at 5C high rate. The band gap of the NFPP of Comparative Example 2 is 3.33 eV, and the discharge specific capacity is 106.4 mAh g -1 at a current density of 0.1C, and the reversible specific capacity is 84.4 mAh g -1 at 5C, while the cycle stability is poor, with a retention rate of 83.9% for 1000 cycles at 5C high rate.

[0057] As can be seen, doping with low electronegativity elements can significantly reduce the band gap of the NFPP material, improve its electronic conductivity, discharge specific capacity and cycle stability. The core mechanism is that the doping elements promote the migration of electrons to O atoms through the Fe-O-M bridging structure, increase the Fe 3d orbital electron state density, reduce the band gap, and at the same time inhibit the lattice distortion during deep sodium extraction, and improve the structural stability.

[0058] Figure 1 The first cycle charge-discharge curves of the samples prepared in Example 1 (Mg doping) and Comparative Example 1 (undoped) of the present application show that the first cycle charge-discharge specific capacity of Example 1 is significantly higher than that of Comparative Example 1, indicating that the material after Mg doping has higher electronic conductivity and more stable sodium ion intercalation / extraction behavior. The curve characteristics of Comparative Example 1 reflect the problems of poor electronic conductivity and large sodium ion diffusion resistance of the undoped material. This is because Mg doping reduces the band gap of the material through the above mechanism, improves the electronic conductivity, and improves the electrode reaction kinetics.

[0059] Figure 2 The rate graph of Example 1 and Comparative Example 1 of the present application is shown. As can be seen from the graph, the capacity decay of Example 1 is smaller at different rates, and the capacity decay of Comparative Example 1 is more obvious, indicating that Example 1 has more excellent high rate performance. The reason is that the low band gap structure improves the electronic and ionic conduction ability, and enhances the rate performance of the material.

[0060] In summary, by introducing low electronegativity elements such as Mg, Mn, Zn, Nb, Al and the like into the positive electrode material for doping, the band gap structure of the material can be significantly optimized, and the discharge specific capacity, high rate performance and cycle stability thereof are improved; the undoped material (Comparative Examples 1 and 2) has a wide band gap, resulting in poor electronic conductivity, insufficient structural stability and poor electrochemical comprehensive performance. This is because the low electronegativity doping elements promote the migration of electrons to the O atom through the Fe-O-M bridging structure, adjust the Fe 3d-O 2p electron distribution, increase the electron state density near the Fermi level, thereby effectively reducing the band gap and improving the electronic conductivity; at the same time, the doping elements act as a "pillar" in the crystal lattice, inhibit the lattice distortion during deep sodium extraction, and enhance the structural stability and cycle life.

[0061] The preferred embodiments of the present application have been described above with the aid of drawings, but the present application is not limited to the above examples, and various modifications and changes can be made by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A low band gap sodium iron phosphates positive electrode material, characterized in that, The chemical formula of the positive electrode material is Na x Fe x-1-ny / 2 M y (PO4) x-2 P2O7 / C, the band gap width is less than or equal to 1 eV; wherein, x=3, 4, 5, M is a low electronegativity element for promoting the migration of electrons to the O atom in the Fe-O-M bridging structure, n is the valence of the M element, and 0.2<=y<=0.

01.

2. The low-bandgap sodium iron phosphatesepolyphosphate cathode material of claim 1, wherein, The M element is an element with lower electronegativity than Fe, and the doping position of M is Fe site, and the electronegativity of M is <1.

83.

3. The low-bandgap sodium iron phosphates pyrophosphates cathode material of claim 2, wherein, The M element is selected from one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Zn, Ga, Y, Zr, Nb, Cd, In.

4. A method for preparing the low band gap sodium iron phosphate pyrophosphate cathode material according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: (1) mixing a sodium source, an iron source, a phosphorus source, a carbon source, and a dopant with a solvent, mechanically dispersing by ball milling or sand milling, and centrifuging to obtain solid particles; drying the solid particles to obtain a precursor powder; (2) calcining the precursor powder obtained in step (1) in an inert atmosphere to obtain a sodium iron phosphate pyrophosphate / carbon composite material.

5. The method for preparing the low bandgap sodium iron pyrophosphate cathode material according to claim 4, characterized in that, In step (1), the total mass ratio of the carbon source to the sodium source, the iron source, the phosphorus source, and the dopant is (0.05-0.15):

1.

6. The method for preparing the low bandgap sodium iron pyrophosphate cathode material according to claim 4, characterized in that, The sodium source is selected from one or more of sodium carbonate, sodium hydroxide, sodium pyrophosphate, disodium pyrophosphate, sodium oxalate, sodium dihydrogen phosphate, sodium hydrogen phosphate, sodium phosphate, sodium chloride, and sodium acetate; The iron source is selected from one or more of iron acetate, iron nitrate, ferrous oxalate, iron oxide, iron powder, and iron phosphate; The phosphorus source is selected from one or more of sodium dihydrogen phosphate, sodium pyrophosphate, sodium hydrogen phosphate, disodium pyrophosphate, and sodium phosphate; The carbon source is selected from one or more of glucose, sucrose, starch, citric acid, ascorbic acid, oxalic acid, and polyvinyl alcohol; The dopant is an oxide, carbonate, or oxalate of an M element.

7. The method for preparing the low bandgap sodium iron pyrophosphate cathode material according to claim 6, characterized in that, The dopant is selected from one or more of MgO, MnCO3, ZnO, Nb2O5, Al2O3, TiO2, V2O5, Cr2O3, CdO, ZrO2, MgC2O4, MnC2O4, ZnC2O4, CdC2O4, Al2(C2O4)3, Sc2(C2O4)3, Ga2(C2O4)3, In2(C2O4)3, and Y2(C2O4)3.

8. The method for preparing the low bandgap sodium iron pyrophosphate cathode material according to claim 4, characterized in that, In step (1), the drying conditions are: vacuum drying at 60-80 ℃ for 10-16 h.

9. The method for preparing the low bandgap sodium iron pyrophosphate cathode material according to claim 4, characterized in that, In step (2), the calcination conditions are: calcining at 250-350 ℃ for 3-6 h under an inert atmosphere, and then sintering at 450-550 ℃ for 10-12 h.

10. Use of the low-bandgap sodium iron phosphate pyrophosphate positive electrode material according to any one of claims 1-3 in the preparation of a sodium ion battery positive electrode material.

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