Polyanionic positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a polyanionic positive electrode material as well as a preparation method and application thereof. The polyanionic positive electrode material comprises a core particle and a carbon coating layer coating the core particle, the core particle comprises Mn and Mg co-doped iron-based sodium pyrophosphate; the carbon coating layer comprises amorphous carbon and graphitized carbon. According to the invention, the double-doped Mn and Mg can achieve a synergistic effect, Mn doping is beneficial for increasing the average working voltage, Mg can be used as a lattice stabilizer to be beneficial for inhibiting distortion caused by Mn doping, and the structural stability is improved while the capacity is increased; the carbon coating layer contains amorphous carbon and graphitized carbon, so that filling of pores and interface gaps in the manganese and magnesium synergistically doped iron-based sodium pyrophosphate is facilitated. In a word, through the combination of the synergistic doping of manganese and magnesium and specific carbon coating, the gram capacity, the compaction density and the energy density of the material can be effectively improved, and the material is suitable for a high-power sodium ion battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and relates to a polyanionic positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Sodium-ion batteries have become an important cornerstone for the steady development of the new energy industry due to their abundant resources and low cost. Among the core material systems of sodium-ion batteries, the positive electrode material plays a decisive role in battery performance and cost. Current sodium-ion positive electrode materials have formed three major technical routes: layered oxides, polyanionic compounds, and Prussian blue compounds. Among them, polyanionic compounds have shown excellent adaptability in the energy storage field due to their unique advantages: they have a stable three-dimensional framework structure, excellent thermal stability and safety, long cycle life, and a wide working temperature range.

[0003] Among the many polyanionic compounds, sodium iron pyrophosphate phosphate (Na4Fe3(PO4)2P2O7, NFPP) stands out due to its unique structural advantages. Specifically, it has a high working voltage platform (3.2V~3.4V) and a small volume change during charging and discharging. The raw material cost is only about 60% of that of layered oxides, and it is non-toxic and environmentally friendly. These characteristics make NFPP a new generation of energy storage positive electrode material with great development potential. With the progress of material modification technology and the realization of large-scale production, NFPP-based sodium-ion batteries are expected to play an important role in the energy storage market and provide key technical support for promoting energy structure transformation.

[0004] However, the pure-phase NFPP material provided by the prior art has problems such as insufficient electronic conductivity, Fe 2+ / Fe 3+ low redox potential, low tap density, and insufficient specific capacity. SUMMARY

[0005] In view of the problems in the prior art, the purpose of the present application is to provide a polyanionic positive electrode material and a preparation method and application thereof. The polyanionic positive electrode material comprises core particles and a carbon coating layer covering the core particles; the core particles comprise Mn and Mg co-doped sodium iron pyrophosphate phosphate; and the carbon coating layer comprises amorphous carbon and graphitized carbon. Through the synergistic doping of manganese and magnesium and the combination of specific carbon coating, the specific capacity, tap density, and energy density of the material can be effectively improved, and it is suitable for high-power sodium-ion batteries.

[0006] To achieve this purpose, the present application adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a polyanionic cathode material comprising core particles and a carbon coating layer covering the core particles; the core particles comprising iron-based sodium pyrophosphate co-doped with Mn and Mg; and the carbon coating layer comprising amorphous carbon and graphitized carbon.

[0008] The polyanionic cathode material of this invention comprises iron-based sodium pyrophosphate co-doped with manganese and magnesium. The dual-doped Mn and Mg exhibit a synergistic effect. 2+ Partial Fe replacement 2+ It can expand Na + Diffusion channels are beneficial for improving rate performance; at the same time, Mn 2+ / Mn 3+ with Fe 2+ / Fe 3+ It can form a redox complementary effect, which is beneficial to improving the average operating voltage; Mg 2+ Mn doping can act as a lattice stabilizer, which helps suppress Jahn-Teller distortion caused by Mn doping, thus improving capacity without sacrificing structural stability. Compared to single-element doping, Mn doping can effectively improve the problem of generating electrochemically inert impurity phases (such as Mn3(PO4)2), and also helps to improve the problem of reduced cycle stability caused by lattice distortion due to excessive doping. Furthermore, the iron-based sodium pyrophosphate co-doped with manganese and magnesium in this invention is coated with a specific carbon coating layer, containing both amorphous carbon and graphitized carbon. Without amorphous carbon, the interfacial impedance increases and cycle performance decreases; without graphitized carbon, conductivity decreases and mechanical strength weakens. The combination of amorphous and graphitized carbon also helps to fill the pores and interfacial gaps in the manganese-magnesium co-doped iron-based sodium pyrophosphate, effectively improving conductivity and compaction density. In summary, the combination of manganese and magnesium co-doping and carbon coating effectively improves the specific capacity, compaction density, and energy density of the material, making it suitable for high-power sodium-ion batteries.

[0009] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following technical solutions.

[0010] As a preferred embodiment of the present invention, the chemical formula of the polyanionic cathode material is denoted as Na₄Fe₂O₃. 3-x- y Mn x Mg y(PO4)2P2O7@C, where 0.05≤x≤0.35, 0.01≤y≤0.05. For example, x can be 0.05, 0.08, 0.10, 0.13, 0.15, 0.18, 0.20, 0.22, 0.25, 0.28, 0.3, 0.33, or 0.35, etc.; for example, y can be 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, or 0.05, etc.

[0011] Preferably, x / y = 1~7, for example 1, 2, 3, 4, 5, 6 or 7, more preferably 1~3, and even more preferably 2.

[0012] Preferably, the D of the nuclear particle 50 The particle size is <10μm, for example 9.8μm, 9μm, 8μm, 7μm, 6μm, 5μm, 4μm, 3μm, 2μm or 1μm, preferably 5μm to 7μm, and more preferably 6μm.

[0013] Preferably, the thickness of the carbon coating layer is 5nm to 30nm, for example, it can be 5nm, 8nm, 10nm, 12nm, 14nm, 16nm, 18nm, 20nm, 22nm, 24nm, 26nm or 30nm, etc.

[0014] Preferably, the compaction density of the polyanionic cathode material is ≥2.8 g / cm³. 3 For example, it could be 2.8 g / cm³ 3 2.82 g / cm 3 2.85g / cm 3 2.88g / cm 3 2.9g / cm 3 2.92g / cm 3 Or 2.94 g / cm 3 wait.

[0015] In a second aspect, the present invention provides a method for preparing a polyanionic cathode material according to the first aspect, comprising the following steps:

[0016] Iron source, manganese source, magnesium source, phosphate source, pyrophosphate source and complexing agent are mixed in the liquid phase and coprecipitated to obtain coprecipitate;

[0017] The coprecipitate was dried, mixed with a sodium source, and prepared into a slurry. It was then spray-dried to obtain the precursor.

[0018] The precursor is crushed, mixed with a first carbon source, and subjected to first carbon coating calcination. Then it is mixed with a second carbon source and subjected to second carbon coating calcination to obtain a polyanionic cathode material.

[0019] The preparation method of this invention uses wet mixing of raw materials and co-precipitation, which facilitates the uniform distribution and doping of Mn and Mg elements. After the obtained precursor is crushed, it is mixed with carbon source, which is beneficial for filling the interparticle gaps while carbon coating. The carbon coating is carried out in stages. During the first carbon coating, amorphous carbon is formed. At this time, complex chemical reactions occur, releasing small molecule gases. The escape of the gases leaves a large number of pores and defects. The second carbon coating involves graphitization, which causes some carbon to undergo a melting-carbonization-graphitization process at high temperature. This allows it to penetrate, wet, and fill the pores and interparticle gaps in the doped iron-based sodium pyrophosphate particles, as well as the pores and defects caused by the gas generated by the pyrolysis of the carbon source during the first carbon coating. Ultimately, this helps to fully improve the conductivity and the compaction density of the polyanionic cathode material.

[0020] The precursor described in this invention can be denoted as NFMAPP, where MA represents Mn and Mg doped in NFPP.

[0021] As a preferred technical solution of the present invention, the preparation method includes: providing a solution A containing an iron source, a manganese source and a magnesium source, a solution B containing a phosphate source, a solution C containing a pyrophosphate source and a solution D containing a complexing agent; mixing solutions B and C in parallel into solution A, adding solution D to a first pH value to carry out a first coprecipitation reaction, and then adding solution D again to raise the pH value to a second coprecipitation reaction to obtain a coprecipitate.

[0022] The co-precipitation reaction in the preparation method of this invention is carried out in two steps. First, Mg is preferentially precipitated at a first pH value. 2+ -PO4 3- The complex was then co-precipitated with Fe at the second pH value. 2+ / Mn 2+ This is beneficial for improving the atomic-level distribution of elements and the uniformity of doping, and also helps to avoid the formation of impurity phases.

[0023] Preferably, the first pH value is 3 to 5, for example, it can be 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8 or 5, etc.

[0024] Preferably, the first coprecipitation reaction is carried out under stirring for 1 to 3 hours, for example, 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, or 3 hours.

[0025] Preferably, the second pH value is 5 to 7, for example, it can be 5, 5.2, 5.4, 5.6, 5.8, 6, 6.2, 6.4, 6.6, 6.8 or 7, etc.

[0026] Preferably, the second coprecipitation reaction is carried out under stirring for 1 to 3 hours, for example, 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, or 3 hours.

[0027] As a preferred embodiment of the present invention, the iron source, manganese source and magnesium source include at least one of the sulfate, chloride or nitrate of the corresponding metal element.

[0028] Preferably, the total molar concentration of the iron source, manganese source and magnesium source in solution A is 0.1 mol / L to 1 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L, etc.

[0029] Preferably, the phosphate source includes phosphoric acid.

[0030] Preferably, the molar concentration of phosphate source in solution B is 1 mol / L to 3 mol / L, for example, it can be 1 mol / L, 1.2 mol / L, 1.4 mol / L, 1.6 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, 2.4 mol / L, 2.6 mol / L, 2.8 mol / L or 3 mol / L, etc.

[0031] Preferably, the pyrophosphate source includes sodium pyrophosphate.

[0032] Preferably, the molar concentration of pyrophosphate in solution C is 0.1 mol / L to 1 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L, etc.

[0033] Preferably, the complexing agent comprises ammonia monohydrate.

[0034] Preferably, the mass percentage concentration of the complexing agent in the solution D is 10% to 30%, for example, it can be 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, or 30%, etc.

[0035] As a preferred technical solution of the present invention, after the coprecipitation reaction is completed, centrifugation and washing are performed in sequence, followed by drying to obtain a dry powder, and the dry powder is mixed with the sodium source.

[0036] Preferably, the centrifugation speed is 5000rpm~10000rpm, for example, it can be 5000rpm, 6000rpm, 7000rpm, 8000rpm, 9000rpm or 10000rpm, etc.; the time is 5min~15min, for example, it can be 5min, 6min, 7min, 8min, 9min, 10min, 11min, 12min, 13min, 14min or 15min, etc.

[0037] Preferably, the detergent used for washing includes water, and the washing is performed at least three times.

[0038] Preferably, the drying includes vacuum drying.

[0039] Preferably, the drying temperature is 80℃~120℃, for example, it can be 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 11℃ or 120℃, etc.; the time is 6h~18h, for example, it can be 6h, 8h, 10h, 12h, 14h, 16h or 18h, etc.

[0040] As a preferred embodiment of the present invention, the sodium source includes sodium carbonate.

[0041] Preferably, the amounts of the precursor and the sodium source are controlled according to the stoichiometric ratio of iron to sodium in iron-based sodium pyrophosphate (Na:Fe=4:3).

[0042] Preferably, the solvent of the slurry includes water.

[0043] Preferably, the solid content of the slurry is 30% to 60%, for example, it can be 30%, 33%, 35%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58% or 60%, etc.

[0044] Preferably, the slurry is ball-milled before being spray-dried.

[0045] Preferably, the rotational speed of the ball mill is 200 rpm to 400 rpm, for example, 200 rpm, 220 rpm, 240 rpm, 260 rpm, 280 rpm, 300 rpm, 320 rpm, 340 rpm, 360 rpm, 380 rpm, or 400 rpm; the time is 3 hours to 5 hours, for example, 3 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours, 4 hours, 4.2 hours, 4.4 hours, 4.6 hours, 4.8 hours, or 5 hours.

[0046] Preferably, the inlet temperature of the spray dryer is 180℃~220℃, for example, 180℃, 190℃, 200℃, 210℃ or 220℃, and the outlet temperature is 80℃~120℃, for example, 80℃, 90℃, 100℃, 110℃ or 120℃.

[0047] Preferably, the crushing method includes airflow crushing.

[0048] Preferably, the particle size D of the precursor is... 50 ≤10μm, for example, can be 10μm, 9μm, 8μm, 7μm, 6μm, 5μm, 4μm, 3μm, 2μm or 1μm, etc., preferably ≤6μm.

[0049] In this invention, controlling the particle size of the precursor is beneficial for obtaining a particle size D. 50 The ultrafine precursor with a diameter of ≤10μm can be combined with a double-layer carbon coating to further improve electronic / ionic conductivity and achieve high-rate charge and discharge.

[0050] As a preferred embodiment of the present invention, the first carbon source includes at least one of citric acid, oxalic acid, acetic acid, or tartaric acid.

[0051] Preferably, the process of mixing the precursor with the first carbon source includes providing a solution E containing the first carbon source, mixing the precursor with the solution E, ball milling, and then performing first carbon coating calcination.

[0052] Preferably, in the solution E, the mass percentage concentration of the first carbon source is 20% to 40%, for example, it can be 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, or 40%, etc.

[0053] Preferably, the mass of the first carbon source accounts for 3% to 8% of the mass of the precursor, for example, it can be 3%, 4%, 5%, 6%, 7% or 8%, etc.

[0054] Preferably, the first carbon coating calcination is carried out in an oxygen-free atmosphere.

[0055] Preferably, the first carbon coating calcination is carried out under an inert atmosphere.

[0056] Preferably, the heating rate of the first carbon coating calcination is 1℃ / min to 10℃ / min, for example, it can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min, etc., and the holding temperature is 250℃ to 400℃, for example, it can be 250℃, 260℃, 270℃, 280℃, 300℃, etc. Temperatures range from 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, or 400℃, etc.; the heat preservation time is from 1 hour to 8 hours, for example, 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 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, etc.

[0057] As a preferred embodiment of the present invention, the second carbon source includes at least one of sucrose, glucose, or starch.

[0058] The preparation method described in this invention uses a wet process for co-precipitation and can use inexpensive carbon sources such as sucrose to achieve carbon coating. The overall cost is only 60% of that of layered oxides, making it suitable for large-scale production.

[0059] Preferably, the process of mixing with the second carbon source includes: calcining the first carbon coating to obtain a coating body; providing a solution F containing the second carbon source; mixing the coating body with the solution F; spray drying; and then performing the second carbon coating calcination.

[0060] Preferably, the mass percentage concentration of the second carbon source in the solution F is 20% to 40%, for example, it can be 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, or 40%, etc.

[0061] Preferably, the mass of the second carbon source accounts for 3% to 8% of the mass of the coating, for example, it can be 3%, 4%, 5%, 6%, 7% or 8%.

[0062] Preferably, the second carbon coating calcination is carried out in an oxygen-free atmosphere.

[0063] Preferably, the second carbon coating calcination is carried out in a mixed atmosphere of inert atmosphere and hydrogen, wherein the volume percentage of hydrogen is 1% to 10%, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, etc.

[0064] Preferably, the heating rate of the second carbon coating calcination is 5℃ / min to 15℃ / min, for example, it can be 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min, 12℃ / min, 13℃ / min, 14℃ / min, or 15℃ / min, etc.; the holding temperature is 1500℃ to 2000℃, for example, it can be... The temperatures can be 1500℃, 1550℃, 1600℃, 1650℃, 1700℃, 1750℃, 1800℃, 1850℃, 1900℃, 1950℃, or 2000℃, etc.; the heat preservation time is 5h~20h, for example, it can be 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, or 20h, etc.

[0065] Preferably, the second carbon coating is calcined and then ground to obtain a polyanionic cathode material.

[0066] Thirdly, the present invention provides a sodium-ion battery containing the polyanionic cathode material described in the first aspect, or containing the polyanionic cathode material obtained by the preparation method described in the second aspect.

[0067] It should be noted that, due to space limitations and to avoid redundancy, this invention does not exhaustively list all point values ​​within the above numerical range, but it is not limited to the listed values ​​either; other unlisted values ​​within the above numerical range are also applicable.

[0068] Compared with existing technical solutions, the present invention has at least the following beneficial effects:

[0069] The polyanionic cathode material of this invention is co-doped with manganese and magnesium, utilizing Mn 2+ Expand Na + Diffusion channels are beneficial for improving rate performance; utilizing Mn 2+ / Mn 3+ with Fe 2+ / Fe 3+ The redox complementary effect of Mg helps to increase the average operating voltage to above 3.4V; 2+ It can act as a lattice stabilizer to suppress Jahn-Teller distortion caused by Mn doping, thus improving capacity without sacrificing structural stability. Simultaneously, it forms a specific carbon coating layer containing amorphous carbon and graphitized carbon, which helps fill the pores and interfacial gaps in the manganese and magnesium co-doped iron-based sodium pyrophosphate, effectively improving conductivity and compaction density. In summary, the combination of manganese and magnesium co-doping and specific carbon coating effectively improves the material's specific capacity, compaction density, and energy density, making it suitable for high-power sodium-ion batteries. Detailed Implementation

[0070] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0071] Those skilled in the art will understand that the embodiments described are merely illustrative of the invention and should not be construed as limiting the invention.

[0072] Example 1

[0073] This embodiment provides a polyanionic cathode material, comprising core particles and a carbon coating layer covering the core particles; the core particles comprise iron-based sodium pyrophosphate co-doped with Mn and Mg; the carbon coating layer comprises an amorphous carbon inner layer and a graphitized carbon outer layer. The chemical formula of the polyanionic cathode material is denoted as Na₄Fe₄. 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C; the D of the nuclear particle 50 The particle size is 6 μm; the thickness of the amorphous carbon inner layer is 2 nm to 3 nm; the thickness of the graphitized carbon outer layer is 16 nm; the compaction density of the polyanionic cathode material is ≥2.8 g / cm³. 3 .

[0074] This embodiment provides a method for preparing a polyanionic cathode material, including the following steps:

[0075] Step S1, prepare the solution:

[0076] Mixed metal source solution (solution A): Weigh 2.85 mol of ferrous sulfate heptahydrate (FeSO4·7H2O), 0.1 mol of manganese sulfate (MnSO4·H2O), and 0.05 mol of magnesium chloride (MgCl2) to make the molar ratio of the elements Mn / (Fe+Mn+Mg)=3.33% and Mg / (Fe+Mn+Mg)=1.67%, dissolve in 10 L of deionized water, and stir until completely dissolved;

[0077] Phosphate source solution (solution B): Dissolve 6 mol of phosphoric acid (H3PO4) in 5 L of deionized water;

[0078] Pyrophosphate source solution (solution C): Weigh 1 mol of sodium pyrophosphate (Na4P2O7) and dissolve it in 5 L of deionized water;

[0079] Complexing agent solution (solution D): Prepare 25% concentration of ammonia monohydrate (NH3·H2O);

[0080] Step S2, coprecipitation reaction:

[0081] After mixing solutions B and C, slowly add the mixture dropwise to solution A while stirring. Adjust the pH of the mixture to 4.0 ± 0.1 with solution D and continue stirring for 2 hours to carry out the first coprecipitation reaction. Then add solution D to adjust the pH to 7.0 ± 0.1 and continue stirring for 2 hours to carry out the second coprecipitation reaction, forming a coprecipitate and obtaining a reddish-brown suspension.

[0082] The suspension was centrifuged at 8000 rpm for 10 min, washed three times with deionized water, and then vacuum dried at 100 °C for 12 h to obtain NFMAPP precursor powder.

[0083] Step S3: Ball milling, spray drying, and crushing:

[0084] The NFMAPP precursor was mixed with sodium carbonate (Na2CO3) at a stoichiometric ratio of Na:Fe = 4:3, and deionized water was added to prepare a slurry with a solid content of 50%. The slurry was ball-milled at 3000 rpm for 4 hours. Then, it was spray-dried (inlet temperature 200℃, outlet temperature 100℃) to obtain a dried powder, which was then air-jet crushed to a particle size D. 50 The precursor was obtained with a diameter of 6.0 μm.

[0085] Step S4: Perform graded carbon coating calcination.

[0086] The precursor was mixed with 3 wt% citric acid, which was added in solution form, and ball milled for 2 h to obtain the first material to be calcined. The first material to be calcined was heated to 350°C at 5°C / min under Ar atmosphere and held for 2 h to carry out the first carbon coating calcination to form an amorphous carbon layer with a thickness of 2 nm to 3 nm to obtain the coated material.

[0087] The coating material was mixed with 5 wt% sucrose, which was added as a 30% solids solution. The mixture was then spray-dried to obtain a spray-dried material. This spray-dried material was then heated to 250°C at a rate of 10°C / min and held for 4 hours in an Ar / H2 (95:5 volume ratio) mixed atmosphere to remove residual organic matter. The temperature was then further increased to 1750°C and held for 10 hours for a second carbon coating calcination to promote partial graphitization of the formed carbon layer. After cooling, the mixture was ground to obtain a black powder product, namely the polyanionic cathode material Na4Fe. 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C.

[0088] Example 2

[0089] This embodiment provides a polyanionic cathode material, which differs from Embodiment 1 in that: the total doping amount of Mn and Mg remains unchanged, but the ratio of Mn to Mg is adjusted so that the chemical formula of the polyanionic cathode material changes from Na4Fe2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.85 Mn 0.02 Mg 0.13 (PO4)2P4O7@C, except for the above, the other conditions are exactly the same as in Example 1.

[0090] Example 3

[0091] This embodiment provides a polyanionic cathode material, which differs from Embodiment 1 in that: the total doping amount of Mn and Mg remains unchanged, but the ratio of Mn to Mg is adjusted so that the chemical formula of the polyanionic cathode material changes from Na4Fe 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.85 Mn 0.05 Mg 0.1 (PO4)2P4O7@C, except for the above, the other conditions are exactly the same as in Example 1.

[0092] Example 4

[0093] This embodiment provides a polyanionic cathode material, which differs from Embodiment 1 in that: the ratio of Mn to Mg remains unchanged, but the total doping amount of Mn and Mg is adjusted, so that the chemical formula of the polyanionic cathode material changes from Na4Fe 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.97 Mn 0.02 Mg 0.01 (PO4)2P4O7@C, i.e., without Mg doping, and other conditions are exactly the same as in Example 1.

[0094] Example 5

[0095] This embodiment provides a polyanionic cathode material, which differs from Embodiment 1 in that: the ratio of Mn to Mg remains unchanged, but the total doping amount of Mn and Mg is adjusted, so that the chemical formula of the polyanionic cathode material changes from Na4Fe 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.925 Mn 0.05 Mg 0.025 (PO4)2P4O7@C, i.e., without Mg doping, and other conditions are exactly the same as in Example 1.

[0096] Example 6

[0097] This embodiment provides a polyanionic cathode material, which differs from Embodiment 1 in that: the ratio of Mn to Mg remains unchanged, but the total doping amount of Mn and Mg is adjusted, so that the chemical formula of the polyanionic cathode material changes from Na4Fe 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.775 Mn 0.15 Mg 0.075 (PO4)2P4O7@C, i.e., without Mg doping, and other conditions are exactly the same as in Example 1.

[0098] Example 7

[0099] This embodiment provides a polyanionic cathode material, which differs from Embodiment 1 in that: the ratio of Mn to Mg remains unchanged, but the total doping amount of Mn and Mg is adjusted, so that the chemical formula of the polyanionic cathode material changes from Na4Fe 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.7 Mn 0.2 Mg 0.1 (PO4)2P4O7@C, i.e., without Mg doping, and other conditions are exactly the same as in Example 1.

[0100] Example 8

[0101] This embodiment provides a polyanionic cathode material. The difference from Embodiment 1 is that in step S2 of the preparation method, the first pH value is directly adjusted from 4.0±0.1 to 3.0±0.1. Apart from the above, the other conditions are exactly the same as those in Embodiment 1.

[0102] Example 9

[0103] This embodiment provides a polyanionic cathode material. The difference from Embodiment 1 is that in step S2 of the preparation method, the first pH value is directly adjusted from 4.0±0.1 to 5.0±0.1. Apart from the above, the other conditions are exactly the same as those in Embodiment 1.

[0104] Example 10

[0105] This embodiment provides a polyanionic cathode material. The difference from Embodiment 1 is that in step S2 of the preparation method, the first pH value is directly adjusted from 4.0±0.1 to 6.0±0.1. Apart from the above, the other conditions are exactly the same as those in Embodiment 1.

[0106] Example 11

[0107] This embodiment provides a polyanionic cathode material. The difference from Embodiment 1 is that in step S2 of the preparation method, the pH is directly adjusted to the second pH value of 7.0±0.1 without stepwise precipitation. Apart from the above, the other conditions are exactly the same as those in Embodiment 1.

[0108] Comparative Example 1

[0109] This comparative example provides a polyanionic cathode material, which differs from Example 1 in that: the total doping amount of Mn and Mg remains unchanged, but the ratio of Mn to Mg is adjusted so that the chemical formula of the polyanionic cathode material changes from Na4Fe 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.85 Mg 0.15 (PO4)2P4O7@C, i.e., without Mn doping, and other conditions are exactly the same as in Example 1.

[0110] Comparative Example 2

[0111] This comparative example provides a polyanionic cathode material, which differs from Example 1 in that: the total doping amount of Mn and Mg remains unchanged, but the ratio of Mn to Mg is adjusted so that the chemical formula of the polyanionic cathode material changes from Na4Fe 2.85 Mn 0.1 Mg 0.05 (PO4)2P4O7@C is adjusted to Na4Fe 2.85 Mn 0.15 (PO4)2P4O7@C, i.e., without Mg doping, and other conditions are exactly the same as in Example 1.

[0112] Comparative Example 3

[0113] This comparative example provides a polyanionic cathode material. The difference from Example 1 is that in step S4 of the preparation method, carbon coating is not graded; only the first carbon source is used for carbon coating to form an amorphous carbon layer. Apart from the above, the other conditions are exactly the same as in Example 1.

[0114] Comparative Example 4

[0115] This comparative example provides a polyanionic cathode material. The difference from Example 1 is that in step S4 of the preparation method, carbon coating is not graded, but only a second carbon source is used for carbon coating to form a graphitized carbon layer. Apart from the above, the other conditions are exactly the same as those in Example 1.

[0116] Characterization and testing:

[0117] The polyanionic cathode materials obtained in the examples and comparative examples were used to prepare cathode sheets, and then assembled into batteries with cathode sheets, separators, and electrolytes. The following tests were then conducted:

[0118] At 25℃ and within a voltage range of 1.5V to 4.0V, the discharge specific capacity of the battery at 1C and 5C rates was tested respectively, and the 5C capacity retention rate was calculated. The 5C capacity retention rate = 5C discharge specific capacity / 1C discharge specific capacity.

[0119] At 25℃, within a voltage range of 1.5V to 4.0V, after 1000 charge-discharge cycles at 1C, the cycle capacity retention rate was tested. The cycle capacity retention rate = 1000th discharge specific capacity / 1st discharge specific capacity.

[0120] The test results are recorded in Table 1.

[0121] Table 1

[0122]

[0123] As can be seen from Table 1:

[0124] A comparison of Examples 1, 2-3, and Comparative Examples 1-2 shows that adjusting the ratio of Mg to Mn, with a molar ratio of Mn to Mg of 2:1, can better balance energy density, capacity performance, and cycle performance.

[0125] A comparison between Example 1 and Examples 4-7 shows that when the total doping amount is too small, the optimization and improvement effect on the cathode material is not obvious, while when the total doping amount is too large, the performance will be deteriorated.

[0126] A comparison of Example 1 and Examples 8-11 shows that the present invention performs precipitation in two steps and controls the two-step precipitation at a suitable pH value, which is beneficial to improving the atomic-level distribution of elements and the uniformity of doping, and helps to avoid the formation of impurity phases, thereby improving the electrochemical performance of the cathode material.

[0127] A comparison of Example 1 and Comparative Examples 3-4 shows that the carbon coating in this invention is carried out in stages, which is beneficial for filling pores and defects and improving coating uniformity. Compared with coating only once, it can better improve the electrochemical performance of the cathode material.

[0128] In summary, the polyanionic cathode material of this invention undergoes synergistic co-doping with manganese and magnesium, utilizing Mn 2+ Expand Na + Diffusion channels are beneficial for improving rate performance; utilizing Mn 2+ / Mn 3+ with Fe 2+ / Fe 3+The redox complementary effect of Mg helps to increase the average operating voltage to above 3.4V; 2+ It can act as a lattice stabilizer to suppress Jahn-Teller distortion caused by Mn doping, thus improving capacity without sacrificing structural stability. Simultaneously, it forms a specific carbon coating layer containing amorphous carbon and graphitized carbon, which helps fill the pores and interfacial gaps in the manganese and magnesium co-doped iron-based sodium pyrophosphate, effectively improving conductivity and compaction density. In summary, the combination of manganese and magnesium co-doping and specific carbon coating effectively improves the material's specific capacity, compaction density, and energy density, making it suitable for high-power sodium-ion batteries.

[0129] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0130] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0131] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A polyanionic cathode material, characterized in that, It includes a core particle and a carbon coating layer covering the core particle; the core particle includes iron-based sodium pyrophosphate co-doped with Mn and Mg; the carbon coating layer includes amorphous carbon and graphitized carbon.

2. The polyanionic cathode material according to claim 1, characterized in that, The chemical formula of the polyanionic cathode material is Na₄Fe₂ 3-x-y Mn x Mg y (PO4)2P2O7@C, where 0.05≤x≤0.35, 0.01≤y≤0.05; Preferably, x / y = 1~7, more preferably 1~3; Preferably, the D of the nuclear particle 50 Particle size <10μm, preferably 5μm~7μm; Preferably, the thickness of the carbon coating layer is 5 nm to 30 nm; Preferably, the compaction density of the polyanionic cathode material is ≥2.8 g / cm³. 3 .

3. A method for preparing the polyanionic cathode material according to claim 1 or 2, characterized in that, Includes the following steps: Iron source, manganese source, magnesium source, phosphate source, pyrophosphate source and complexing agent are mixed in the liquid phase and coprecipitated to obtain coprecipitate; The coprecipitate was dried, mixed with a sodium source, and prepared into a slurry. It was then spray-dried and crushed to obtain the precursor. The precursor is crushed, mixed with a first carbon source, and subjected to first carbon coating calcination. Then it is mixed with a second carbon source and subjected to second carbon coating calcination to obtain a polyanionic cathode material.

4. The method for preparing the polyanionic cathode material according to claim 3, characterized in that, The preparation method includes: providing a solution A containing an iron source, a manganese source and a magnesium source, a solution B containing a phosphate source, a solution C containing a pyrophosphate source and a solution D containing a complexing agent; mixing solutions B and C in parallel flow to solution A, adding solution D to a first pH value to carry out a first coprecipitation reaction, and then adding solution D again to raise the pH value to a second coprecipitation reaction to obtain a coprecipitate; Preferably, the first pH value is 3-5; Preferably, the first coprecipitation reaction is carried out under stirring for 1 to 3 hours; Preferably, the second pH value is 5-7; Preferably, the second coprecipitation reaction is carried out under stirring for 1 to 3 hours.

5. The method for preparing the polyanionic cathode material according to claim 4, characterized in that, The iron source, manganese source and magnesium source include at least one of the sulfate, chloride or nitrate of the corresponding metal element; Preferably, in solution A, the total molar concentration of the iron source, manganese source and magnesium source is 0.1 mol / L to 1 mol / L; Preferably, the phosphate source includes phosphoric acid; Preferably, in solution B, the molar concentration of phosphate source is 1 mol / L to 3 mol / L; Preferably, the pyrophosphate source includes sodium pyrophosphate; Preferably, in solution C, the molar concentration of pyrophosphate is 0.1 mol / L to 1 mol / L; Preferably, the complexing agent comprises ammonia monohydrate; Preferably, the complexing agent in solution D has a mass percentage concentration of 10% to 30%.

6. The method for preparing the polyanionic cathode material according to any one of claims 3-5, characterized in that, After the coprecipitation reaction is completed, the product is centrifuged and washed in sequence, and then dried to obtain a dry powder. The dry powder is then mixed with the sodium source. Preferably, the centrifugation speed is 5000 rpm to 10000 rpm, and the time is 5 min to 15 min; Preferably, the detergent used for washing includes water, and the washing is performed at least three times; Preferably, the drying includes vacuum drying; Preferably, the drying temperature is 80℃~120℃ and the time is 6h~18h.

7. The method for preparing the polyanionic cathode material according to any one of claims 3-6, characterized in that, The sodium source includes sodium carbonate; Preferably, the amounts of the precursor and the sodium source are controlled according to the stoichiometric ratio of iron to sodium in iron-based phosphate sodium pyrophosphate; Preferably, the solvent of the slurry includes water; Preferably, the solid content of the slurry is 30% to 60%; Preferably, the slurry is ball-milled before spray drying; Preferably, the ball mill rotates at a speed of 200 rpm to 400 rpm for 3 hours to 5 hours. Preferably, the inlet temperature of the spray dryer is 180℃~220℃, and the outlet temperature is 80℃~120℃; Preferably, the crushing method includes airflow crushing; Preferably, the particle size D of the precursor is... 50 ≤10μm; preferably ≤6μm.

8. The method for preparing the polyanionic cathode material according to any one of claims 3-7, characterized in that, The first carbon source includes at least one of citric acid, oxalic acid, acetic acid, or tartaric acid; Preferably, the process of mixing the precursor with the first carbon source includes providing a solution E containing the first carbon source, mixing the precursor with the solution E, ball milling, and then performing first carbon coating calcination. Preferably, in the solution E, the mass percentage concentration of the first carbon source is 20% to 40%; Preferably, the mass of the first carbon source accounts for 3% to 8% of the mass of the precursor; Preferably, the first carbon coating calcination is carried out in an oxygen-free atmosphere; Preferably, the first carbon coating calcination is carried out under an inert atmosphere; Preferably, the heating rate of the first carbon coating calcination is 1℃ / min to 10℃ / min, the holding temperature is 250℃ to 400℃, and the holding time is 1h to 8h.

9. The method for preparing the polyanionic cathode material according to any one of claims 3-8, characterized in that, The second carbon source includes at least one of sucrose, glucose, or starch; Preferably, the process of mixing with the second carbon source includes: calcining the first carbon coating to obtain a coating body; providing a solution F containing the second carbon source; mixing the coating body with the solution F; spray drying; and then performing the second carbon coating calcination. Preferably, in the solution F, the mass percentage concentration of the second carbon source is 20% to 40%; Preferably, the second carbon source accounts for 3% to 8% of the mass of the coating body; Preferably, the second carbon coating calcination is carried out in an oxygen-free atmosphere; Preferably, the second carbon coating calcination is carried out in a mixed atmosphere of inert atmosphere and hydrogen, wherein the volume percentage of hydrogen is 1% to 10%. Preferably, the heating rate of the second carbon coating calcination is 5℃ / min~15℃ / min, the holding temperature is 1500℃~2000℃, and the holding time is 5h~20h; Preferably, the second carbon coating is calcined and then ground to obtain a polyanionic cathode material.

10. A sodium-ion battery, characterized in that, The material contains the polyanionic cathode material according to claim 1 or 2, or the polyanionic cathode material obtained by the preparation method according to any one of claims 3-9.

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