A carbon-coated and metal-doped mixed sodium phosphate cathode material and a preparation method thereof

Abstract

The present invention provides a sodium-ion battery cathode material of hybrid phosphate with carbon coating and metal-site doping and a preparation method thereof. The structural general formula of the sodium-ion battery cathode material is Na 4‑x M 3‑x‑y A y (PO4) 2‑x P2O7@C, wherein M is one or more of Fe, Ni, Mn, and Co; A is one or more of Mg, Al, Cr, Zr, and Ti; 0 ≤ x < 2, 0.01 ≤ y ≤ 0.2, @ represents a coating layer, and C represents the carbon material for coating to form a carbon coating layer; the raw materials for forming the carbon coating layer are organic carbon source cellulose and inorganic carbon source, and the inorganic carbon source is carbon nanotube or graphene; the carbon coating layer accounts for 1% - 15% of the weight of the hybrid phosphate sodium-ion battery cathode material; the carbon material with sp 2 hybrid carbon structure in the carbon coating layer accounts for 65 at.% - 85 at.% of the carbon coating layer material. The sodium-ion battery cathode material of hybrid phosphate with carbon coating and metal-site doping of the present invention has more excellent capacitance, rate performance, and cycling performance.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and in particular to a carbon-coated and metal-doped mixed sodium phosphate positive electrode material and a preparation method thereof. Background Art

[0002] In order to further get rid of people's excessive dependence on traditional fossil energy, countries have begun to vigorously develop renewable energy many years ago. At present, common wind, photovoltaic and hydropower are inevitably facing the embarrassing situation of intermittent power generation. To solve this problem, people have explored various energy storage technologies that can convert intermittent renewable energy into stable electricity. Among them, electrochemical energy storage technology has become the mainstream way of adding new energy storage capacity due to its advantages of not being restricted by terrain and environment and being able to directly store and release electrical energy.

[0003] In 2021, CATL released the first generation of sodium-ion batteries, and the industry generally believes that the sodium-ion industry has ushered in an excellent period of development. In recent years, with the vigorous promotion of the industrialization process, the problems of large-scale production hidden deep inside have been exposed. Simply put, it is nothing more than that different types of positive electrode materials have their limitations. On the basis of controlling costs, the application scenarios of different materials are fully explored according to their characteristics. At present, the three mainstream systems of sodium-ion positive electrode materials have their own strengths. Among them, iron-based phosphoric acid pyrophosphate (NFPP) in the polyanion system has good rate performance and cycle stability, and its structure is composed of Fe, P and O elements. Therefore, it can rely on low cost. Become a highly potential technical solution in the field of energy storage. However, this type of material has the problem of low theoretical specific capacity and intrinsic conductivity, and it is easy to produce impurities during the synthesis process, resulting in reduced material performance.

[0004] At present, in order to improve the problem of low intrinsic conductivity of NFPP, researchers have done a lot of research work, including the use of carbon coating, metal doping, defect engineering and the introduction of high entropy concepts. Generally speaking, by thermally decomposing organic compounds to form exothermic in-situ carbon-coated NFPP materials, since its internal structure remains the same as the original, the low conductivity of the internal structure of the material cannot be changed; in addition, the research group of Mai Liqiang has proved through Mg metal doping experiments that the electrochemical properties of NFPP such as rate performance and cycle life can be improved (Adv. Funct. Mater. 2023, 33, 2211257), but the associated problem of impure phases cannot be solved. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a carbon-coated and metal-doped mixed sodium phosphate cathode material and a preparation method thereof.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a carbon-coated and metal-doped mixed sodium phosphate positive electrode material, the structure of the mixed sodium phosphate positive electrode material is Na 4-x M 3-x-y A y (PO4) 2-x P2O7@C, wherein M is one or more of Fe, Ni, Mn, and Co, A is one or more of Mg, Al, Cr, Zr, and Ti, 0≤x<2, 0.01≤y≤0.2, @C represents the coating layer, and C represents the coated carbon material forming a carbon coating layer;

[0007] The raw materials for forming the carbon coating layer are organic carbon source cellulose and inorganic carbon source, the inorganic carbon source is carbon nanotubes or graphene, and the carbon coating layer accounts for 1.0% to 15% of the weight of the mixed sodium phosphate positive electrode material;

[0008] sp in carbon coating 2 The carbon material with hybrid carbon structure accounts for 65at.% to 85at.% of the carbon coating layer material.

[0009] The carbon-coated and metal-doped mixed sodium phosphate positive electrode material is mainly composed of sodium phosphate positive electrode material, and uses Fe, Ni, Mn, and Co as metal elements, and Mg, Al, Cr, Zr, and Ti are selected to be trace doped in the metal position in the material crystal structure. The structures of Mg, Al, Cr, Zr, and Ti replace part of Fe, Ni, Mn, and Co. On the basis of not destroying the original structure, the material structure defects caused by the main element defects are used to increase the generation entropy of the doped and modified material, and effectively solve the problem of maricite-NaFePO4 impurity phase being easily generated in the original structure of the material. At the same time, the metal-doped mixed sodium phosphate positive electrode material is coated by combining cellulose organic carbon source with graphene and carbon nanotube inorganic carbon source, and the ratio of organic carbon source to graphene and carbon nanotube inorganic carbon source is controlled, so that the organic carbon source and the inorganic carbon source produce a synergistic effect, so that the carbon coating layer material forms sp accounting for 65at.% to 85at.% of the carbon coating layer material. 2 Carbon materials with hybrid carbon structures, sp 2 The carbon material with hybrid carbon structure has high electronic conductivity. 2The carbon material with hybrid carbon structure wraps the carbon layer, which improves the specific capacity and rate performance of the material, and also overcomes the problem of expensive graphene-coated materials. The contradiction between the cost of sodium phosphate cathode materials and battery performance is also an inherent difficulty of this material. The above-mentioned carbon-coated and metal-doped mixed sodium phosphate cathode materials improve the structural stability and conductivity of the material from both internal and external aspects at the same time, and achieve higher actual specific capacity and cycle stability performance of the material at a lower raw material cost, further providing good electrochemical performance support for large-scale application of products.

[0010] Preferably, the carbon coating layer has sp 2 The carbon material with the hybrid carbon structure accounts for 70 at. % to 80 at. % of the carbon coating layer material.

[0011] Preferably, the mass ratio of the organic carbon source to the inorganic carbon source is (5-25):1.

[0012] In the above-mentioned carbon-coated and metal-doped mixed sodium phosphate cathode material, the mass ratio of organic carbon source to inorganic carbon source can better balance the electrical conductivity of the battery material and better coating effect, and ensure the stability of the coating layer structure, which not only reduces the material cost, but also further improves the actual specific capacity and cycle stability of the material, and improves the application range and service life of the material.

[0013] Preferably, the mass ratio of the organic carbon source to the inorganic carbon source is (10-20):1.

[0014] Preferably, 0≤x≤1.

[0015] More preferably, 0.5≤x≤0.8.

[0016] The present invention also adjusts the ratio of phosphate and pyrophosphate configurations in the carbon-coated and metal-doped mixed sodium phosphate cathode material, and finds that when 0.5≤x≤0.8, the controlled ratio of phosphate and pyrophosphate configurations makes the carbon-coated and metal-doped mixed sodium phosphate cathode material have higher conductivity and better cycle stability.

[0017] Preferably, 0.01≤y≤0.1.

[0018] Preferably, M is one or two of Fe and Mn, and A is one or more of Mg, Zr and Ti.

[0019] Preferably, the mixed sodium phosphate positive electrode material has the general structural formula of Na 4-x Fe 3-x-y A y (PO4) 2-xP2O7@C, wherein A is one or more of Mg, Al, Cr, Zr, and Ti.

[0020] Preferably, the carbon-coated and metal-doped mixed sodium phosphate cathode material is a polycrystalline material with a D50 of 2.0 to 8.5 μm.

[0021] Preferably, the carbon-coated and metal-doped mixed sodium phosphate cathode material is a polycrystalline material with a D50 of 4.0 to 6.0 μm.

[0022] Preferably, the organic carbon source cellulose is one or more of hydroxypropyl methylcellulose, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, and ethyl cellulose.

[0023] The present invention also provides a method for preparing any of the above-mentioned carbon-coated and metal-doped mixed sodium phosphate cathode materials, the method comprising the following steps:

[0024] (1) Mix the sodium source material powder, M source material powder, A source material powder, and phosphorus source material powder, and record the mass as M1. Take the carbon source premixed with the organic carbon source cellulose and the inorganic carbon source, and record the mass as M2. Mix all the materials, and perform the first sintering in an inert gas atmosphere, heating to 200-400° C. at a heating rate of 2-10° C. / min, and sinter at a constant temperature for 1-6 h;

[0025] (2) performing a second sintering in an inert gas atmosphere, heating the material to 500-650° C. at a heating rate of 2-10° C. / min, and sintering the material at a constant temperature for 2-15 hours; and then cooling the material to below 30° C. at a cooling rate of 2-10° C. / min to obtain the carbon-coated and metal-doped mixed sodium phosphate cathode material.

[0026] Preferably, the sodium source is one of sodium dihydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium acetate and sodium citrate.

[0027] Preferably, the M source material powder is one or more of nitrates, oxalates, oxides and phosphates of the corresponding elements.

[0028] Preferably, the A source material powder is a metal salt or oxide of the corresponding element.

[0029] Preferably, the phosphorus source is ammonium phosphate, diammonium phosphate or diammonium hydrogen phosphate.

[0030] Preferably, the particle size range of the sodium source material powder, M source material powder, A source material powder, phosphorus source material powder, organic carbon source cellulose, and inorganic carbon source after grinding and before sintering is D50≤1.2μm and D100≤5μm.

[0031] Preferably, sodium source material powder, M source material powder, A source material powder, phosphorus source material powder, organic carbon source cellulose, and inorganic carbon source are dispersed in a solvent by ball milling, sand milling, and drying, and the dispersion solvent is one of water, methanol, ethanol, acetone, isopropanol, ethylenediamine, propanol, ethylene glycol, and n-butanol.

[0032] More preferably, the ball milling makes the solid content between 10% and 60%, more preferably 20% to 50%; the ball milling speed is 80 to 1000 rpm, more preferably 400 to 800 rpm; the ball-to-material ratio is 2 to 20:1, and the diameter of the zirconia ball is 0.1 to 0.5 mm.

[0033] More preferably, the rotation speed in the sand milling stage is 800-3000 rpm, preferably 1200-2200 rpm; the particle size of the solid in the slurry after sand milling is D50≤1.2um, D100≤5um, more preferably D50≤800nm, D100≤3um.

[0034] More preferably, the drying method adopted is spray drying, freeze drying, high temperature drying or vacuum drying.

[0035] More preferably, the drying method is spray drying; the inlet temperature of the spray drying is 180-280°C, the outlet temperature is 80-120°C, the spray rate is 2-15L / h, and the carrier gas is one or more of dry air, nitrogen, argon and helium.

[0036] Preferably, the mass ratio of the organic carbon source cellulose to the inorganic carbon source is (5-25):1.

[0037] Preferably, the ratio of M2 to M1 is (3.5-90):100

[0038] More preferably, the ratio of M2 to M1 is (6-55):100.

[0039] Preferably, the inert gas is a single atmosphere of nitrogen, argon, helium, or a mixed atmosphere of nitrogen / hydrogen and argon / hydrogen.

[0040] The beneficial effects of the present invention are as follows: the present invention provides a carbon-coated and metal-doped mixed sodium phosphate positive electrode material and a preparation method thereof. The carbon-coated and metal-doped mixed sodium phosphate positive electrode material of the present invention takes sodium phosphate positive electrode material as the main body, adopts Fe, Ni, Mn and Co as metal elements, and selects Mg, Al, Cr, Zr and Ti to be trace-doped at the metal position in the crystal structure of the material. The structures of Mg, Al, Cr, Zr and Ti replace part of Fe, Ni, Mn and Co. On the basis of not destroying the original structure, the material structure defects caused by the main element defects are used to increase the generation entropy of the doped and modified material, and effectively solve the problem that the maricite-NaFePO4 impurity phase is easily generated in the original structure of the material. At the same time, the metal-doped mixed sodium phosphate positive electrode material is coated by a cellulose organic carbon source and a graphene and carbon nanotube inorganic carbon source, and the ratio of the organic carbon source to the graphene and carbon nanotube inorganic carbon source is controlled so that the organic carbon source and the inorganic carbon source produce a synergistic effect, so that the carbon coating layer material forms a sp 2 Carbon materials with hybrid carbon structures, sp 2 The carbon material with hybrid carbon structure has high electronic conductivity. 2 The carbon material of the hybrid carbon structure wraps the carbon layer to achieve higher conductivity and better coating effect of the material, and ensures the stability of the coating layer structure, and improves the cycle stability and rate performance of the material. At the same time, it also overcomes the problem of expensive graphene coating materials. The above-mentioned carbon-coated and metal-doped mixed sodium phosphate positive electrode material improves the structural stability and conductivity of the material from both internal and external aspects at the same time, and achieves higher actual specific capacity and cycle stability performance of the material at a lower raw material cost, further providing good electrochemical performance support for the large-scale application of the product. The raw materials used in the preparation method of the present invention are all cheap and readily available bulk chemicals, and the material is prepared by a simple sand milling and solid phase sintering combined method, which does not require high sources of raw materials, has a simple process method, is easy to control process parameters, is easy to mass produce, and can greatly ensure the problem of product production consistency, and provides a feasible technical solution for the successful preparation of sodium polyanion positive electrode materials with high rate, low cost, high specific capacity and good air stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 X-ray diffraction patterns of samples prepared in Example 1 and Comparative Example 1 of the carbon-coated and metal-doped mixed sodium phosphate cathode material of the present invention; Intensity in the figure represents intensity.

[0042] Figure 2These are Raman test graphs of Example 2, Comparative Example 1 and Comparative Example 2 of the carbon-coated and metal-doped mixed sodium phosphate cathode material of the present invention; Intensity in the graph represents intensity.

[0043] Figure 3 This is a scanning electron microscope image of the sample prepared in Example 1 of the carbon-coated and metal-doped mixed sodium phosphate cathode material of the present invention (the magnification is indicated in the figure).

[0044] Figure 4 This is a high-resolution X-ray photoelectron spectrum of C1s of the samples prepared in Example 1 and Comparative Example 2 of the carbon-coated and metal-doped mixed sodium phosphate cathode material of the present invention; Intensity in the figure represents intensity, and Bindingenergy in the figure represents binding energy.

[0045] Figure 5 The charge and discharge curves of the sodium ion button cells assembled with the positive electrode material samples prepared in Example 1 and Comparative Example 1 of the carbon-coated and metal-doped mixed phosphate sodium positive electrode material of the present invention at a rate of 0.1C, with an operating voltage of 1.5-4.1V vs.Na+ / Na; in the figure, Voltage represents voltage and Capacity represents capacitance.

[0046] Figure 6 The cycle curve of the sodium ion button battery assembled with the samples prepared in Example 1 and Comparative Example 1 of the carbon-coated and metal-doped mixed sodium phosphate positive electrode material of the present invention as positive electrode materials. In the figure, Capacity represents specific capacity, and Cycle number represents the number of cycles. DETAILED DESCRIPTION

[0047] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0048] Example 1

[0049] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material in an embodiment of the present invention, the general structural formula of the mixed sodium phosphate positive electrode material is Na 4-x M 3-x-y A y (PO4) 2-x P2O7@C, where 0≤x<2, 0.01≤y≤0.2, @ represents the coating layer, C represents the coated carbon material forming a carbon coating layer; M is Fe, and A is Ti;

[0050] The raw materials for forming the carbon coating layer are organic carbon source cellulose and inorganic carbon source, the organic carbon source cellulose is ethyl cellulose, and the inorganic carbon source is graphene. According to the percentage of the total content of carbon element in the carbon source to the total mass of the product after sintering (the same below), the carbon coating layer accounts for 14.2% of the weight of the mixed sodium phosphate positive electrode material;

[0051] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 76.4 at. % of the carbon coating layer material.

[0052] The mass ratio of organic carbon source to inorganic carbon source is 20:1.

[0053] The carbon-coated and metal-doped mixed sodium phosphate cathode material of this embodiment is a polycrystalline material with a D50 of 4.5 to 5.5 μm.

[0054] The preparation method of the carbon-coated and metal-doped mixed sodium phosphate positive electrode material of this embodiment comprises the following steps:

[0055] (1) According to the stoichiometric ratio, ferrous oxalate, titanium dioxide and sodium dihydrogen phosphate were mixed at a molar ratio of Na:Fe:Ti:P of 4:2.95:0.05:4, and the total mass was M1. The carbon source premixed with organic carbon source cellulose and inorganic carbon source was taken, and the mass was recorded as M2. The mass ratio of M2 to M1 was 20:100. Ethanol was added as a dispersing solvent to prepare a mixture with a solid content of 30%, and the mixture was ball milled and sand milled. The ball milling time is set to 30min, the ball mill speed is set to 600rmp, the ball-to-material ratio is 20:1, and the diameter of the zirconia ball is 0.1mm; the sand milling speed is set to 1000rpm, and the particle size of the solid in the slurry after sand milling is D50≤500μm, D100≤1.3μm; the prepared mixed slurry is spray dried, the spray drying inlet temperature is set to 230℃, the outlet temperature is set to 100℃, the spray rate is 15L / h, and the carrier gas is selected as nitrogen. After removing the solvent, the precursor material before sintering with uniform element dispersion is obtained;

[0056] (2) The precursor material in step (1) is subjected to a two-step calcination treatment in a nitrogen protective atmosphere, namely: heating to 350°C at a heating rate of 5°C / min, sintering at a constant temperature for 2h, then heating to 500°C at a heating rate of 5°C / min, sintering at a constant temperature for 8h, and then cooling to room temperature at a cooling rate of 5°C / min, and crushing and grading by a jet mill to obtain a carbon-coated and metal-doped mixed sodium phosphate cathode material.

[0057] The carbon-coated and metal-doped mixed sodium phosphate cathode material of this embodiment is denoted as Na4Fe 2.95 Ti 0.05 (PO4)2P2O7@0.2C-20. In the formula, "0.2" represents the mass ratio of the composite carbon source to the raw material, and "20" represents the mass ratio of the organic carbon source to the inorganic carbon source. Mass multiple of carbon source.That is, in the general formula, x=0, y=0.05.

[0058] The carbon-coated and metal-doped mixed sodium phosphate positive electrode material of this embodiment is a polycrystalline material that is aggregated and has a D50 of about 5 μm.

[0059] Example 2

[0060] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material of an embodiment of the present invention, the difference between this embodiment and embodiment 1 is that M is Fe and A is Mg.

[0061] According to theoretical calculation results, the carbon coating layer accounts for 28.9% of the weight of the mixed sodium phosphate positive electrode material;

[0062] In the preparation method: the differences include:

[0063] Iron phosphate, magnesium oxide, sodium dihydrogen phosphate and sodium bicarbonate were weighed according to the molar ratio of Na:Fe:Mg:P of 4:2.95:0.05:4.

[0064] The mass ratio of M2 to M1 is 50:100.

[0065] In this example, a carbon-coated and metal-doped mixed sodium phosphate cathode material was prepared, denoted as Na4Fe 2.95 Mg 0.05 (PO4)2P2O7@0.5C-20. In the formula, "0.5" represents the mass ratio of the composite carbon source to the raw material, and "20" represents the mass multiple of the organic carbon source to the inorganic carbon source. That is, in the general formula, x=0, y=0.05.

[0066] The high-resolution X-ray photoelectron spectroscopy of C 1s revealed that the sp 2 The carbon material with hybrid carbon structure accounts for 70.2 at. % of the carbon coating layer material.

[0067] Example 3

[0068] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material of an embodiment of the present invention, the difference between this embodiment and embodiment 1 is as follows:

[0069] In the preparation method:

[0070] Ferrous oxalate, titanium dioxide and sodium dihydrogen phosphate were weighed according to the molar ratio of Na:Fe:Ti:P of 3:1.97:0.03:3. The mass ratio of M2 to M1 was 20:100.

[0071] In this example, a carbon-coated and metal-doped mixed sodium phosphate cathode material was prepared, denoted as Na3Fe 1.97 Ti0.03 (PO4)P2O7@0.2C-20. That is, in the general formula, x=1, y=0.03.

[0072] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 75.3 at. % of the carbon coating layer material.

[0073] Example 4

[0074] As a carbon-coated and metal-doped mixed sodium phosphate cathode material of an embodiment of the present invention, the difference between this embodiment and embodiment 1 is that M is Fe and A is Zr.

[0075] In the preparation method:

[0076] (a) Ferric nitrate, zirconium oxide, ammonium dihydrogen phosphate and sodium carbonate were weighed respectively according to the molar ratio of Na:Fe:Zr:P of 4:2.99:0.01:4; the weight ratio of the organic carbon source cellulose and the inorganic carbon source premixed carbon source was 10:1; the mass ratio of M2 to M1 was 50:100.

[0077] (b) The precursor material in step (1) is subjected to a two-step calcination treatment in a nitrogen protective atmosphere, i.e., the temperature is increased to 350°C at a heating rate of 2°C / min, and the material is sintered at a constant temperature for 4 hours, then the temperature is increased to 550°C at a heating rate of 5°C / min, and the material is sintered at a constant temperature for 6 hours, and then the material is cooled to room temperature at a cooling rate of 5°C / min, and the material is crushed and graded by a jet mill to obtain a carbon-coated and metal-doped mixed sodium phosphate cathode material.

[0078] The carbon-coated and metal-doped mixed sodium phosphate cathode material of this embodiment is denoted as Na4Fe 2.99 Zr 0.01 (PO4)2P2O7@0.5C-10. In the formula, "0.5" represents the mass ratio of the composite carbon source to the raw material, and "10" represents the mass ratio of the organic carbon source to the inorganic carbon source. Mass multiple of carbon source. That is, in the general formula, x=0, y=0.01.

[0079] The high-resolution X-ray photoelectron spectroscopy of C 1s revealed that the sp 2 The carbon material with hybrid carbon structure accounts for 72.5 at. % of the carbon coating layer material.

[0080] The carbon-coated and metal-doped mixed sodium phosphate positive electrode material of this embodiment is a polycrystalline material that is aggregated and has a D50 of about 5 μm.

[0081] Example 5

[0082] As a carbon-coated and metal-doped mixed sodium phosphate cathode material according to an embodiment of the present invention, the only difference between this embodiment and embodiment 1 is that ethyl cellulose is replaced by hydroxyethyl cellulose.

[0083] The high-resolution X-ray photoelectron spectroscopy of C 1s revealed that the sp 2 The carbon material with hybrid carbon structure accounts for 74.9at.% of the carbon coating layer material.

[0084] Example 6

[0085] As a carbon-coated and metal-doped mixed sodium phosphate cathode material of an embodiment of the present invention, the only difference between this embodiment and Example 1 is that carbon nanotubes are used to replace graphene as an inorganic carbon source.

[0086] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 67.4 at. % of the carbon coating layer material.

[0087] Example 7

[0088] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material in an embodiment of the present invention, the only difference between this embodiment and Example 1 is that the molar ratio of Na:Fe:Ti:P is 3.8:2.75:0.05:3.8; that is, in the general formula, x=0.2, y=0.05.

[0089] In this example, a carbon-coated and metal-doped mixed sodium phosphate cathode material was prepared, denoted as Na 3.8 Fe 2.75 Mg 0.05 (PO4) 1.8 P2O7@0.5C-20.

[0090] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 75.7 at. % of the carbon coating layer material.

[0091] Example 8

[0092] As a carbon-coated and metal-doped mixed sodium phosphate cathode material in an embodiment of the present invention, the only difference between this embodiment and Example 1 is that the molar ratio of Na:Fe:Ti:P is 3.5:2.45:0.05:3.5; that is, in the general formula, x=0.5, y=0.05.

[0093] In this example, a carbon-coated and metal-doped mixed sodium phosphate cathode material was prepared, denoted as Na 3.5 Fe 2.45 Mg 0.05 (PO4) 1.5 P2O7@0.5C-20.

[0094] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material of the hybrid carbon structure accounts for 75.0 at. % of the carbon coating layer material.

[0095] Example 9

[0096] As a carbon-coated and metal-doped mixed sodium phosphate cathode material in an embodiment of the present invention, the only difference between this embodiment and Example 1 is that the molar ratio of Na:Fe:Ti:P is 3.4:2.35:0.05:3.4; that is, in the general formula, x=0.6, y=0.05.

[0097] In this example, a carbon-coated and metal-doped mixed sodium phosphate cathode material was prepared, denoted as Na 3.4 Fe 2.35 Mg 0.05 (PO4) 1.4 P2O7@0.5C-20.

[0098] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 74.8 at. % of the carbon coating layer material.

[0099] Example 10

[0100] As a carbon-coated and metal-doped mixed sodium phosphate cathode material in an embodiment of the present invention, the only difference between this embodiment and Example 1 is that the molar ratio of Na:Fe:Ti:P is 3.2:2.15:0.05:3.2; that is, in the general formula, x=0.8, y=0.05.

[0101] In this example, a carbon-coated and metal-doped mixed sodium phosphate cathode material was prepared, denoted as Na 3.2 Fe 2.15 Mg 0.05 (PO4) 1.2 P2O7@0.5C-20.

[0102] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 75.2 at. % of the carbon coating layer material.

[0103] Embodiment 11

[0104] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material of an embodiment of the present invention, the only difference between this embodiment and embodiment 1 is that: the carbon material with sp2 hybridized carbon structure in the carbon coating layer accounts for 77.5% of the carbon coating layer material according to the high-resolution X-ray photoelectron energy spectrum of C1s;

[0105] The mass ratio of organic carbon source to inorganic carbon source is 10:1.

[0106] Comparative Example 1

[0107] As a carbon-coated and metal-doped mixed sodium phosphate cathode material as a comparative example of the present invention, the only difference between this comparative example and Example 1 is that no organic carbon source cellulose is added, the amount of raw materials remains unchanged, and the mass of the organic carbon source cellulose is supplemented by the inorganic carbon source graphene.

[0108] The material of this comparative example is recorded as Na4Fe 2.95 Ti 0.05 (PO4)2P2O7@0.2C.

[0109] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 78.4 at. % of the carbon coating layer material.

[0110] Comparative Example 2

[0111] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material as a comparative example of the present invention, the only difference between this comparative example and Example 1 is that no inorganic carbon source graphene is added, the amount of raw materials remains unchanged, and the mass of the inorganic carbon source is supplemented by the organic carbon source cellulose.

[0112] The material of this comparative example is recorded as Na4Fe 2.95 Ti 0.05 (PO4)2P2O7@0.2C.

[0113] The high-resolution X-ray photoelectron spectroscopy of C1s revealed that sp 2 The carbon material with hybrid carbon structure accounts for 63.9 at. % of the carbon coating layer material.

[0114] Comparative Example 3

[0115] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material of the comparative example of the present invention, the only difference between this comparative example and Example 1 is that the metal of the A element is not doped, that is, Ti is not doped, and y=0.

[0116] Ferrous oxalate, sodium dihydrogen phosphate and sodium phosphate were weighed according to the molar ratio of Na:Fe:P of 4:3:4. The amounts of other raw materials and the steps remained unchanged.

[0117] The material of this comparative example is recorded as Na4Fe3(PO4)2P2O7@0.2C-20.

[0118] Comparative Example 4

[0119] As a carbon-coated and metal-doped mixed sodium phosphate positive electrode material of the comparative example of the present invention, the only difference between this comparative example and Example 1 is:

[0120] Without adding doping agent, ferrous oxalate and sodium dihydrogen phosphate were weighed in a Na:Fe:P molar ratio of 3:2:3. The amounts of other raw materials and the steps remained unchanged.

[0121] The material of this comparative example is recorded as Na3Fe2(PO4)P2O7@0.2C-20.

[0122] Experimental methods

[0123] 1. Material Characterization

[0124] 1. X-ray diffraction was performed on the samples prepared in Example 1 and Comparative Example 1 of the present invention. The experimental results are as follows: Figure 1 As shown. Figure 1 It can only be seen that no characteristic diffraction peak belonging to maricite-NaFePO4 is observed in the range of about 2θ=33°. The battery material of the present invention effectively solves the problem of associated impurities.

[0125] 2. Raman test spectra were performed on the samples prepared in Example 2 of the present invention and Comparative Examples 1 and 2. The experimental results are shown in Figure 2 shown.

[0126] 3. The samples prepared in Example 1 of the present invention were subjected to scanning electron microscopy, and the results are as follows: Figure 3 shown.

[0127] 4. For the samples prepared in Examples 1-10 and Comparative Examples 1-2 of the present invention, high-resolution X-ray photoelectron spectra of C1s were obtained, and peak fitting data processing was performed. Figure 4 The figures are those of Example 1 and Comparative Example 2.

[0128] 2. Battery performance test

[0129] The mixed phosphate material prepared in Examples 1 to 10 and Comparative Examples 1 to 4 was used as the positive electrode material for the sodium ion battery. In a drying room (humidity below 15%), the positive electrode active material, the conductive agent (super-P) and the binder (PVDF) were added to NMP in a mass ratio of 8:1:1 for pulping, and the solid content was controlled at 50%. After the pulping was completed, the slurry was evenly coated on the aluminum foil current collector with the help of an automatic coating machine, and transferred to a vacuum drying oven at 120°C for drying for 12 hours. The material was taken out and naturally cooled to room temperature, and the slices were cut using a manual battery slicer. The cut positive electrode sheets were collected, weighed, and put into a ziplock bag, and placed in a glove box filled with inert gas for standby use. The metal sodium sheet was used as the negative electrode, the GF / A type glass fiber diaphragm was used as the diaphragm, and the electrolyte was added to assemble a CR2032 button battery, which was tested for electrochemical performance after standing. The electrochemical performances were tested using a CT-2001A battery testing system (1C = 129 mA / g).

[0130] The test results are shown in Table 1:

[0131]

[0132]

[0133] As can be seen from Table 1, the capacitance of the embodiment, especially the cycle performance, conductivity and rate performance are better than those of the comparative example. It can be seen that the carbon-coated and metal-doped mixed sodium phosphate cathode material of the present invention is based on sodium phosphate cathode material, adopts Fe, Ni, Mn, Co as metal elements, and selects Mg, Al, Cr, Zr, and Ti to perform trace doping at the metal position in the crystal structure of the material. The structures such as Mg, Al, Cr, Zr, and Ti replace part of Fe, Ni, Mn, and Co. On the basis of not destroying the original structure, the material structure defects caused by the main element defects are used to increase the generation entropy of the doped and modified material, and effectively solve the problem of maricite-NaFePO4 impurity phase that is easy to generate in the original structure of the material. At the same time, the metal-doped mixed sodium phosphate positive electrode material is coated by a cellulose organic carbon source and a graphene and carbon nanotube inorganic carbon source, and the ratio of the organic carbon source to the graphene and carbon nanotube inorganic carbon source is controlled so that the organic carbon source and the inorganic carbon source produce a synergistic effect, so that the carbon coating layer material forms a sp 2 Carbon materials with hybrid carbon structures, sp 2 The carbon material with hybrid carbon structure has high electronic conductivity. 2The carbon material with hybrid carbon structure wrapped in carbon layer can not only achieve higher conductivity and better coating effect of the material, but also ensure the stability of the coating layer structure, improve the material's fatigue resistance and rate performance. At the same time, it also overcomes the problem of expensive cost of graphene-coated materials.

[0134] Compared with comparative example 1, the embodiment has a slight improvement in capacity retention performance, and other performances are basically at the same level. The organic carbon source cellulose and the inorganic carbon source carbon nanotubes or graphene are used for coating, and the proportion of graphene is very low, which can greatly reduce the material cost.

[0135] By comparing Example 1 and Examples 7-10, it can be seen that by adjusting the ratio of phosphate and pyrophosphate configurations in the carbon-coated and metal-doped mixed sodium phosphate cathode material, it is found that when 0.5≤x≤0.8, the controlled ratio of phosphate and pyrophosphate configurations makes the carbon-coated and metal-doped mixed sodium phosphate cathode material have higher conductivity and better cycle stability.

[0136] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A carbon-coated and metal-doped mixed sodium phosphate cathode material, characterized in that: The general structural formula of the mixed sodium phosphate positive electrode material is Na 4-x M 3-x-y A y (PO4) 2-x P2O7@C, wherein M is Fe, A is one or more of Mg, Zr, and Ti, 0≤x<2, 0.01≤y≤0.2, @C represents the coating layer, and C represents the coated carbon material forming a carbon coating layer; The raw materials for forming the carbon coating layer are organic carbon source cellulose and inorganic carbon source, the inorganic carbon source is graphene, and the carbon coating layer accounts for 1% to 15% of the weight of the mixed sodium phosphate positive electrode material; the mass ratio of the organic carbon source to the inorganic carbon source is (10 to 25): 1; The organic carbon source cellulose is one or more of hydroxypropyl methylcellulose, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, and ethyl cellulose; sp in carbon coating 2 The carbon material of the hybrid carbon structure accounts for 70.2at.% to 85at.% of the carbon coating layer material; The preparation method of the carbon-coated and metal-doped mixed sodium phosphate positive electrode material comprises the following steps: (1) mixing sodium source material powder, M source material powder, A source material powder, and phosphorus source material powder, with the mass being recorded as M1, taking a carbon source premixed with an organic carbon source cellulose and an inorganic carbon source, with the mass being recorded as M2, mixing all the materials, and performing a first sintering in an inert gas atmosphere, heating the mixture to 200-400° C. at a heating rate of 2-10° C. / min, and sintering at a constant temperature for 1-6 hours; (2) performing a second sintering in an inert gas atmosphere, heating the material to 500-550° C. at a heating rate of 2-10° C. / min, and sintering the material at a constant temperature for 2-15 hours; and then cooling the material to below 30° C. at a cooling rate of 2-10° C. / min to obtain the carbon-coated and metal-doped mixed sodium phosphate cathode material.

2. The carbon-coated and metal-doped mixed sodium phosphate cathode material according to claim 1, characterized in that: 0≤x≤1。 3. The carbon-coated and metal-doped mixed sodium phosphate cathode material according to claim 1, characterized in that: The mass ratio of the organic carbon source to the inorganic carbon source is (10-20):

1.

4. The carbon-coated and metal-doped mixed sodium phosphate cathode material according to claim 1, characterized in that: 0.01≤y≤0.1。 5. The carbon-coated and metal-doped mixed sodium phosphate cathode material according to claim 1, characterized in that: The carbon-coated and metal-doped mixed sodium phosphate positive electrode material is a polycrystalline material with a D50 of 2.0 to 8.5 μm.

6. The method for preparing the carbon-coated and metal-doped mixed sodium phosphate cathode material according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: (1) Mix the sodium source material powder, M source material powder, A source material powder, and phosphorus source material powder, and record the mass as M1. Take the carbon source premixed with the organic carbon source cellulose and the inorganic carbon source, and record the mass as M2. Mix all the materials, and perform the first sintering in an inert gas atmosphere, heating to 200-400° C. at a heating rate of 2-10° C. / min, and sinter at a constant temperature for 1-6 h; (2) performing a second sintering in an inert gas atmosphere, heating the material to 500-550° C. at a heating rate of 2-10° C. / min, and sintering the material at a constant temperature for 2-15 hours; and then cooling the material to below 30° C. at a cooling rate of 2-10° C. / min to obtain the carbon-coated and metal-doped mixed sodium phosphate cathode material.

7. The method for preparing the carbon-coated and metal-doped mixed sodium phosphate cathode material according to claim 6, characterized in that: The sodium source is one of sodium dihydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate, sodium bicarbonate, sodium acetate and sodium citrate; the M source material powder is one or more of nitrates, oxalates, oxides and phosphates of the corresponding elements; the A source material powder is a metal salt or oxide of the corresponding element; the phosphorus source is ammonium phosphate, ammonium dihydrogen phosphate or diammonium hydrogen phosphate; the particle size range of the sodium source material powder, M source material powder, A source material powder, phosphorus source material powder, organic carbon source cellulose and inorganic carbon source after grinding and before sintering is D50≤1.2μm and D100≤5μm.

8. The method for preparing the carbon-coated and metal-doped mixed sodium phosphate cathode material according to claim 6, characterized in that: The ratio of M2 to M1 is (6~55):100.

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