Positive electrode material, preparation method thereof and battery

Abstract

The invention relates to the field of batteries, in particular to a positive electrode material, a preparation method thereof and a battery. The positive electrode material comprises an inner core and a coating layer coating the surface of the inner core, the chemical general formula of the inner core is Na3V < 1.925-x-y > Mn < x > Al < y > Mg < 0.075 > (PO4) 3, x is smaller than or equal to 0.4, and y is smaller than or equal to 0.3; and the coating layer comprises a carbon coating layer. The positive electrode material provided by the invention comprises a manganese, aluminum and magnesium co-doped NVP inner core and a carbon coating layer coating the surface of the inner core, al < 3 + >, Mn < 2 + > and Mg < 2 + > are synergistically introduced into a Na3V2 (PO4) 3 crystal lattice, and the outer layer of the Na3V2 (PO4) 3 crystal lattice is coated with a carbon layer, so that crystal structure regulation and control and collaborative optimization of electrochemical kinetics can be realized, and the working voltage and low-temperature sodium storage capacity of the material are effectively improved while the structural stability is maintained, so that the application requirements of energy storage and low-temperature starting equipment in a cold region are met.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a positive electrode material, a preparation method thereof and a battery. BACKGROUND

[0002] In high-cold regions, such as northern winter, plateau region, etc., the environmental temperature often drops to-20℃ or even below-40℃. The new energy resources such as wind energy and solar energy in such regions are rich, but the power generation process has obvious intermittency and volatility, and a high-performance energy storage system is needed to realize peak shaving and frequency modulation to ensure the stable operation of the power system. In addition, the communication base station, border defense facility and emergency power supply system in high-cold regions also have higher reliability requirements for energy storage devices in low-temperature environments.

[0003] However, the existing energy storage technology generally has performance decay problems under low-temperature conditions. For example, the capacity of lead-acid batteries decays significantly at low temperatures, and the shell is prone to thermal expansion and cold contraction, resulting in damage; the charging and discharging efficiency of lithium ion batteries drops sharply below-10℃, and they cannot work normally, which seriously restricts their application in cold region energy storage field.

[0004] Sodium ion batteries are considered as a potential solution for cold region energy storage and backup power supply due to their wide operating temperature range (-40℃~80℃), abundant sodium resources and low cost. Among them, sodium vanadium phosphate [Na3V2(PO4)3, NVP] positive electrode material has a three-dimensional NASICON structure, has high structural stability and fast Na + migration channels. However, NVP still has two main problems: a low operating voltage platform (about 3.4V) limits its energy density; under low-temperature conditions (-20℃ or below), the Na + diffusion dynamics are significantly weakened, resulting in severe capacity decay, increased polarization and decreased rate performance, which makes it difficult to meet the use requirements of cold region energy storage systems.

[0005] To improve the electrochemical performance of NVP, researchers have tried to use metal ion doping to regulate the crystal structure and electrochemical behavior. However, the current doping modification methods can improve some performance, but often have the problem of uneven function improvement, and cannot balance the crystal structure stability and high voltage characteristics and low temperature dynamics performance.

[0006] Therefore, how to use collaborative doping means to maintain the stability of the NVP crystal structure while improving its high voltage characteristics and low temperature dynamics performance has become a key technical problem in the current research of sodium ion battery positive electrode materials. SUMMARY

[0007] The application provides a positive electrode material, a preparation method thereof and a battery, so as to solve the problem that the existing doped modified NVP cannot improve high-voltage characteristics and low-temperature kinetic performance while maintaining the stability of the NVP crystal structure.

[0008] To solve the above technical problems, the application provides the following technical solutions. The application provides a positive electrode material, which comprises a core and a coating layer covering the surface of the core. The chemical formula of the core is Na3V 1.925-x-y Mn x Al y Mg 0.075 (PO4)3, wherein x≤0.4 and y≤0.3. The coating layer comprises a carbon coating layer.

[0009] In some specific embodiments, the content of the carbon coating layer is 4wt%-7wt% based on the mass of the positive electrode material.

[0010] In some specific embodiments, the porosity of the positive electrode material is 20%-30%.

[0011] In some specific embodiments, the pore size of the pores in the positive electrode material is 10-20 μm.

[0012] In some specific embodiments, the positive electrode material is R a rhombohedral phase structure of the space group c.

[0013] The second aspect of the application also provides a preparation method of the above positive electrode material, comprising the following steps. Mixing a sodium source, a vanadium source, a phosphorus source, a magnesium source, an aluminum source, a manganese source, a carbon source and a solvent to obtain a precursor slurry; Drying the precursor slurry to obtain a precursor material; Calcining the precursor material to obtain a positive electrode material.

[0014] In some specific embodiments, the molar ratio of sodium in the sodium source, vanadium in the vanadium source, phosphorus in the phosphorus source, magnesium in the magnesium source, aluminum in the aluminum source, manganese in the manganese source and carbon in the carbon source is 3: (1.125-1.925):3:0.075:(0-0.5):(0-0.3):(6-9).

[0015] In some specific embodiments, the amount ratio of the vanadium source to the solvent is 1.125-1.925 mol:20-40 mL.

[0016] In some embodiments, the sodium source comprises a sodium salt.

[0017] In some embodiments, the vanadium source comprises a vanadium oxide.

[0018] In some embodiments, the phosphorus source comprises a phosphate.

[0019] In some embodiments, the magnesium source comprises a magnesium salt.

[0020] In some embodiments, the aluminum source comprises an aluminum salt.

[0021] In some embodiments, the manganese source comprises a manganese salt.

[0022] In some embodiments, the carbon source comprises citric acid.

[0023] In some embodiments, the solvent comprises water and / or ethanol.

[0024] In some embodiments, the drying temperature is 80-120℃, and the drying time is 10-16h.

[0025] In some embodiments, the calcination is a staged calcination.

[0026] In some embodiments, the first stage calcination in the staged calcination has a temperature increasing rate of 3-5℃ / min, a final temperature of 300-350℃, a holding time at the final temperature of 4-6h, and a mixed gas of a first reducing gas and a first inert gas as the atmosphere.

[0027] In some embodiments, the first reducing gas and the first inert gas have a volume ratio of 1-5:95-99. In some embodiments, the second stage calcination in the staged calcination has a temperature increasing rate of 3-5℃ / min, a final temperature of 800-900℃, a holding time at the final temperature of 8-10h, and a mixed gas of a second reducing gas and a second inert gas as the atmosphere.

[0028] In some embodiments, the second reducing gas and the second inert gas have a volume ratio of 1-5:95-99.

[0029] In some embodiments, the first reducing gas and the second reducing gas each independently comprises hydrogen.

[0030] The third aspect of the present application also provides a battery comprising a cathode, wherein the cathode comprises the above-mentioned cathode material or the cathode material prepared according to the above-mentioned preparation method.

[0031] Compared with the prior art, the present application has the following beneficial effects: (1) The positive electrode material provided by the present application comprises a NVP core co-doped with manganese, aluminum and magnesium, and a carbon coating layer coated on the surface of the core. By simultaneously introducing Al 3+ , Mn 2+ and Mg 2+ into the Na3V2(PO4)3 lattice, and coating a carbon layer on the outer layer, the crystal structure can be controlled and the electrochemical dynamics can be optimized simultaneously, the working voltage and the low-temperature sodium storage capacity of the material can be effectively improved while the structural stability is maintained, thereby meeting the application requirements of cold energy storage and low-temperature starting equipment.

[0032] (2) The preparation method of the positive electrode material provided by the present application can be realized by a conventional liquid phase-drying-calcination method, and is suitable for practical applications such as cold energy storage systems, low-temperature electric vehicles and distributed grid regulation equipment, and is conducive to industrial production and popularization. BRIEF DESCRIPTION OF DRAWINGS

[0033] The above and other objects, features and advantages of the present application will be apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings, in which: Figure 1 Fig. 1 shows the XRD pattern of the positive electrode material prepared in Example 1 and Comparative Example 1; Figure 2 Fig. 2 shows the XRD-Rietveld pattern of the positive electrode material prepared in Example 1; Figure 3 Fig. 3 shows the scanning electron microscope SEM images of the positive electrode materials prepared in Example 1 and Comparative Example 1, wherein, Figure 3 Fig. 3(a) is a scanning electron microscope SEM image of the positive electrode material NVP / C in Comparative Example 1 with a scale of 1 μm, Figure 3 Fig. 3(b) is a scanning electron microscope SEM image of the positive electrode material NVMgMnAlP / C in Example 1 with a scale of 2 μm, Figure 3 Fig. 3(c) is a scanning electron microscope SEM image of the positive electrode material NVMgMnAlP / C in Example 1 with a scale of 1 μm; Figure 4 Fig. 4 shows the scanning electron microscope EDS image of the positive electrode material prepared in Example 1; Figure 5 Fig. 5 shows the transmission electron microscope image of the positive electrode material prepared in Example 1; Figure 6 Fig. 6 shows the initial charge-discharge curve of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1 at 25℃ and 0.1C; Figure 7The figure shows the constant current cycling performance of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1 at 25℃, 50C; Figure 8 The figure shows the initial charge-discharge curve of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1 at -20℃, 0.1C; Figure 9 The figure shows the constant current cycling performance of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1 at -20℃, 10C; Figure 10 The figure shows the cyclic voltammetry performance of the battery composed of the positive electrode material prepared in Comparative Example 1 at 0.1mV S -1 ; Figure 11 The figure shows the cyclic voltammetry performance of the battery composed of the positive electrode material prepared in Example 1 at 0.1mV S -1 ; Figure 12 The figure shows the AC impedance spectrogram of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0034] The present application will be described in detail below through specific embodiments, and it can be understood by those skilled in the art that the specific embodiments below are only for illustrative purposes, and do not limit the scope of the present application in any way. In addition, in the following embodiments, unless otherwise specified, the reagents and equipment used are commercially available. If the specific processing conditions and processing methods are not explicitly described in the following embodiments, the processing can be carried out using the conditions and methods known in the art.

[0035] The present application provides a positive electrode material, which comprises an inner core and a coating layer coated on the surface of the inner core; The chemical formula of the inner core is Na3V 1.925-x-y Mn x Al y Mg 0.075 (PO4)3, wherein x≤0.4, y≤0.3; The coating layer comprises a carbon coating layer.

[0036] The positive electrode material (Na3V 1.925-x-y Mn x Al y Mg 0.075 (PO4)3 / C, referred to as NVMgMnAlP / C) of the present application comprises a Mn, Al and Mg co-doped NVP inner core and a carbon coating layer coated on the surface of the inner core; the Al 3+ , Mg 2+ and Mn 2+The synergistic doping and carbon coating of NVP significantly improves the overall electrochemical performance of the material: first, the introduction of Al 3+ can stabilize the lattice structure and increase the V-O bond energy, increasing the average discharge voltage from about 3.40V to about 3.55V, thereby effectively increasing the energy density; second, the doping of Mn 2+ enhances the electronic conductivity and reduces the ion migration impedance, improving the kinetic behavior at low temperatures, while the incorporation of Mg 2+ further optimizes the lattice stability, inhibiting excessive fluctuations in V 3+ / V 4+ valence state, both of which synergistically improve the reversible capacity and rate performance of the material at-20°C or even lower temperatures; in addition, the combined doping of the three effectively suppresses lattice distortion and volume expansion during cycling, delaying structural collapse while maintaining structural stability, effectively improving the working voltage and low-temperature sodium storage capacity of the material, thereby significantly improving the cycle stability and rate performance; at the same time, Mg 2+ can fine-tune the stress distribution of the crystal structure, combined with the electronic conduction path provided by Mn 2+ , which helps to maintain the stability of the carbon coating layer and the crystal surface interface, reduces the polarization impedance, and further improves the electrochemical performance; finally, this material system has good applicability and is suitable for large-scale promotion.

[0037] As a preferred, the chemical formula of the core is Na3V 1.925-x-y Mn x Al y Mg 0.075 (PO4)3, wherein 0.1≤x≤0.3, 0.1≤y≤0.2; In some embodiments, the chemical formula of the core can be Na3V 1.525 Mn 0.1 Al 0.3 Mg 0.075 (PO4)3, Na3V 1.725 Mn 0.1 Al 0.1 Mg 0.075 (PO4)3, Na3V 1.525 Mn 0.2 Al 0.2 Mg 0.075 (PO4)3, Na3V 1.325 Mn 0.3 Al 0.3 Mg 0.075 (PO4)3, Na3V 1.225 Mn 0.3 Al 0.4 Mg 0.075 (PO4)3.

[0038] In some embodiments, the content of the carbon coating layer is 4wt%-7wt% based on the mass of the positive electrode material. For example, the content of the carbon coating layer can be 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt% and 7wt% based on the mass of the positive electrode material.

[0039] In some embodiments, the porosity of the positive electrode material is 20%-30%. For example, the porosity of the positive electrode material can be 20%, 22%, 23%, 25%, 28%, 29% and 30%.

[0040] In some embodiments, the pore diameter of the pores in the positive electrode material is 10-20μm. For example, the pore diameter of the pores in the positive electrode material can be 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 18μm, 19μm and 20μm.

[0041] In some specific embodiments, the positive electrode material is R The rhombohedral phase structure of the space group c.

[0042] In the present application, the crystal form of the positive electrode material is consistent with that of sodium vanadium phosphate, which indicates that the introduction of aluminum, manganese and magnesium does not affect the crystal structure of the material and there is no impurity The present application also provides a preparation method of the above positive electrode material, comprising the following steps: Mixing a sodium source, a vanadium source, a phosphorus source, a magnesium source, an aluminum source, a manganese source, a carbon source and a solvent to obtain a precursor slurry; Drying the precursor slurry to obtain a precursor material; Calcining the precursor material to obtain a positive electrode material.

[0043] The preparation process of the positive electrode material is simple, which can be realized by a conventional liquid phase-drying-calcination method, and is suitable for practical applications such as cold region energy storage systems, low-temperature electric vehicles and distributed power grid regulation equipment, and is conducive to industrial production and popularization.

[0044] In some embodiments, the molar ratio of sodium element in the sodium source, vanadium element in the vanadium source, phosphorus element in the phosphorus source, magnesium element in the magnesium source, aluminum element in the aluminum source, manganese element in the manganese source, and carbon element in the carbon source is 3: (1.125~1.925):3:0.075:(0~0.5):(0~0.3):(6~9). In the present application, the molar ratio of sodium element in the sodium source, vanadium element in the vanadium source, phosphorus element in the phosphorus source, magnesium element in the magnesium source, aluminum element in the aluminum source, manganese element in the manganese source, and carbon element in the carbon source is preferably 3: (1.125~1.725):3:0.075:(0.1~0.5):(0.1~0.3):(6~9), further preferably 3: (1.225~1.725):3:0.075:(0.1~0.4):(0.1~0.3):(6~9), and more preferably 3: (1.325~1.525):3:0.075:(0.2~0.3):(0.2~0.3):(6~9).

[0045] In the present application, by regulating the amount ratio of raw materials within the above range, it is helpful to realize stable crystal structure, accelerated ion and charge transfer, and improved cycle performance. If the doping amount of the magnesium source is too much, it will cause the cell distortion to be intensified, Na + If the doping amount of the magnesium source is too much, it will cause the excessive Al-O bond in the crystal structure to result in reduced conductivity, which seriously affects the charge transfer efficiency. If the doping amount of the aluminum source is too little, it will cause the structural stability to be insufficiently improved, and it is difficult to improve the structure collapse problem in the high-voltage region. If the doping amount of the manganese source is too much, it will cause the generation of heterogeneous phases or cause the electronic structure to be disordered, which in turn causes the capacity decay to be intensified or the initial coulombic efficiency to be reduced. If the doping amount of the manganese source is too little, it will cause the electronic conductivity to be unable to be effectively improved, and it is also difficult to play its synergistic enhancement effect on the reversible capacity.

[0046] In some embodiments, the amount ratio of the vanadium source and the solvent is 1.125~1.925 mol:20~40 mL. As an example, the amount ratio of the vanadium source and the solvent can be 1.125 mol:20 mL, 1.3 mol:23 mL, 1.5 mol:25 mL, 1.7 mol:28 mL, 1.9 mol:32 mL, 1.925 mol:35 mL, 1.925 mol:40 mL, 1.925 mol:30 mL, 1.925 mol:25 mL, and 1.925 mol:20 mL, etc.

[0047] In some embodiments, the sodium source includes but is not limited to sodium salt. As preferred, the sodium salt includes but is not limited to sodium carbonate.

[0048] In some embodiments, the vanadium source includes, but is not limited to, vanadium oxide. Preferably, the vanadium oxide includes, but is not limited to, vanadium pentoxide.

[0049] In some embodiments, the phosphorus source includes, but is not limited to, phosphate. Preferably, the phosphate includes, but is not limited to, ammonium dihydrogen phosphate.

[0050] In some embodiments, the magnesium source includes, but is not limited to, magnesium salt. Preferably, the magnesium salt includes, but is not limited to, magnesium nitrate.

[0051] In some embodiments, the aluminum source includes, but is not limited to, aluminum salt; preferably, the aluminum salt includes, but is not limited to, aluminum nitrate.

[0052] In some embodiments, the manganese source includes, but is not limited to, manganese salt; preferably, the manganese salt includes, but is not limited to, manganese acetate.

[0053] In some embodiments, the carbon source includes, but is not limited to, citric acid.

[0054] In some embodiments, the solvent includes, but is not limited to, water and / or ethanol.

[0055] In some embodiments, the drying temperature is 80-120℃, and the drying time is 10-16h. For example, the drying temperature can be any one of 80℃, 82℃, 85℃, 87℃, 90℃, 92℃, 95℃, 98℃, 100℃, 105℃, 108℃, 110℃, 115℃ and 120℃, or a range value between any two of them; the drying time can be any one of 10h, 12h, 12.5h, 13h, 13.5h, 14h, 14.5h, 15h, 15.5h and 16h, or a range value between any two of them.

[0056] In some embodiments, the calcination is a staged calcination.

[0057] In some embodiments, the first stage calcination in the staged calcination has the following conditions: the temperature rising rate is 3-5℃ / min, the final temperature is 300-350℃, the holding time at the final temperature is 4-6h, and the atmosphere is a mixture of the first reducing gas and the first inert gas. For example, in the first stage calcination, the temperature rising rate can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min and 5℃ / min, etc., the final temperature can be 300℃, 310℃, 320℃, 330℃, 340℃ and 350℃, etc., and the holding time at the final temperature can be 4h, 4.5h, 5h, 5.5h and 6h, etc.

[0058] In some embodiments, the conditions of the second stage calcination in the staged calcination: the heating rate is 3-5 ℃ / min, the final temperature is 800-900 ℃, the holding time at the final temperature is 8-10 h, and the atmosphere is a mixed gas of the second reducing gas and the second inert gas. For example, in the second stage calcination, the heating rate can be 3 ℃ / min, 3.5 ℃ / min, 4 ℃ / min, 4.5 ℃ / min, 5 ℃ / min, or the like, the final temperature can be 800 ℃, 810 ℃, 820 ℃, 830 ℃, 840 ℃, 850 ℃, 860 ℃, 870 ℃, 880 ℃, 890 ℃, or 900 ℃, and the holding time at the final temperature can be 8 h, 8.5 h, 9 h, 9.5 h, or 10 h, or the like.

[0059] In the present application, the staged calcination can make the particle size distribution uniform. In the rapid heating process, the material can form a stable crystal phase, resulting in a complete crystal structure of the material, which is beneficial to improve the electrochemical performance.

[0060] In some embodiments, the volume ratio of the first reducing gas to the first inert gas is 1-5:95-99. For example, the volume ratio of the first reducing gas to the first inert gas can be 1:99, 2:98, 3:97, 4:96, or 5:95, or the like.

[0061] In some embodiments, the volume ratio of the second reducing gas to the second inert gas is 1-5:95-99. For example, the volume ratio of the second reducing gas to the second inert gas can be 1:99, 2:98, 3:97, 4:96, or 5:95, or the like.

[0062] In some embodiments, the first reducing gas and the second reducing gas each independently comprises hydrogen.

[0063] In the present application, the inert atmosphere refers to an atmosphere that is inert to the reaction (i.e., does not have chemical reactivity) to avoid oxidation of the reaction raw materials, intermediates, or products to produce byproducts. In some embodiments, the inert atmosphere includes but is not limited to a nitrogen atmosphere and / or a noble gas atmosphere (such as argon, or the like).

[0064] In the present application, the mixed atmosphere containing a reducing atmosphere can provide a slightly reducing environment, which can significantly inhibit the oxidation of doped elements such as Al, Mn, Mg, and promote the effective solid solution of the doped elements in the NVP lattice. At the same time, the reducing environment can help to form partial V 3+ / V 4+ The mixed valence structure can improve the electronic conductivity and the phase purity, thereby achieving more excellent rate performance and cycle stability.

[0065] The third aspect of the present application also provides a battery comprising a positive electrode, wherein the positive electrode comprises the positive electrode material described above or prepared according to the preparation method described above.

[0066] The technical solutions in the present application will be described clearly and completely below in combination with the drawings and examples. The examples in the present application are only for illustration, and all other examples obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0067] Example 1 Preparation of the positive electrode material, comprising the following steps: (1) dispersing 0.15 mol of sodium carbonate, 0.07625 mol of vanadium pentoxide, 0.3 mol of ammonium dihydrogen phosphate, 0.0075 mol of magnesium nitrate hexahydrate, 0.03 mol of aluminum nitrate nonahydrate, 0.01 mol of manganese acetate tetrahydrate and 0.1 mol of citric acid monohydrate into 40 mL of water, mixing and stirring in a ball magnetic stirrer at a speed of 100 r·min -1 for 3 h to obtain a yellow liquid precursor slurry; (2) drying the precursor slurry in a forced air drying oven at 80℃ for 10 h, then grinding in a corundum mortar for 10 min and passing through a 400-mesh sieve to obtain a precursor material; (3) transferring the precursor material to a tube furnace, and performing two-stage sintering in the tube furnace under the protection of a mixed gas of hydrogen and argon (the volume content of hydrogen is 5%) with a temperature rising rate of 5℃ / min to 350℃, keeping the temperature at 350℃ for 8 h, then with a temperature rising rate of 5℃ / min to 800℃, keeping the temperature at 800℃ for 8 h, then cooling to room temperature, and grinding and passing through a 400-mesh sieve to obtain a positive electrode material Na3V 1.525 Mn 0.1 Al 0.3 Mg 0.075 (PO4)3 / C, denoted as NVMgMnAlP / C; the content of the carbon coating layer is 4.9wt%.

[0068] Example 2 Preparation of the positive electrode material, which is different from example 1 in that the amount of vanadium pentoxide is 0.08625 mol, the amount of aluminum nitrate nonahydrate is 0.01 mol, and the amount of manganese acetate tetrahydrate is 0.01 mol.

[0069] The rest is the same as example 1.

[0070] Example 3 The preparation of the positive electrode material is different from that of Example 1 in that the amount of vanadium pentoxide is 0.07625 mol, the amount of aluminum nitrate nonahydrate is 0.02 mol, and the amount of manganese acetate tetrahydrate is 0.02 mol.

[0071] The rest is the same as that in Example 1.

[0072] Example 4 The preparation of the positive electrode material is different from that of Example 1 in that the amount of vanadium pentoxide is 0.06625 mol, the amount of aluminum nitrate nonahydrate is 0.03 mol, and the amount of manganese acetate tetrahydrate is 0.03 mol.

[0073] The rest is the same as that in Example 1.

[0074] Example 5 The preparation of the positive electrode material is different from that of Example 1 in that the amount of vanadium pentoxide is 0.06125 mol, the amount of aluminum nitrate nonahydrate is 0.04 mol, and the amount of manganese acetate tetrahydrate is 0.03 mol.

[0075] The rest is the same as that in Example 1.

[0076] Comparative Example 1 The preparation of the positive electrode material is different from that of Example 1 in that no magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, and manganese acetate tetrahydrate are added, and the obtained positive electrode material is Na3V2(PO4)3 / C, which is denoted as NVP / C.

[0077] The rest is the same as that in Example 1.

[0078] Comparative Example 2 The preparation of the positive electrode material is different from that of Example 1 in that no aluminum nitrate nonahydrate is added, the amount of vanadium pentoxide is 0.09125 mol, and the amount of manganese acetate tetrahydrate is 0.01 mol.

[0079] The rest is the same as that in Example 1.

[0080] Comparative Example 3 The preparation of the positive electrode material is different from that of Example 1 in that the amount of vanadium pentoxide is 0.08125 mol, the amount of aluminum nitrate nonahydrate added is 0.03 mol, and no manganese acetate tetrahydrate is added.

[0081] The rest is the same as that in Example 1.

[0082] Comparative Example 4 The preparation of the positive electrode material is different from that of Example 1 in that the mixed atmosphere of hydrogen and argon is replaced by pure argon.

[0083] The rest is the same as that in Example 1.

[0084] Comparative Example 5 The preparation of the positive electrode material is different from that of Example 1 in that magnesium titanium aluminum is used for doping, and the specific implementation is that no manganese acetate tetrahydrate is added, the addition amount of aluminum nitrate nonahydrate is 0.03 mol, and the addition amount of tetrabutyl titanate is 0.01 mol.

[0085] The rest is the same as that of Example 1.

[0086] Comparative Example 6 The preparation of the positive electrode material is different from that of Example 1 in that magnesium iron aluminum is used for doping, and the specific implementation is that no manganese acetate tetrahydrate is added, the addition amount of aluminum nitrate nonahydrate is 0.03 mol, and the addition amount of iron nitrate nonahydrate is 0.01 mol.

[0087] The rest is the same as that of Example 1.

[0088] Battery assembly and performance test 1. Battery assembly The positive electrode materials prepared in the examples and comparative examples are respectively used as the positive electrode materials of sodium ion batteries, and conductive agents (acetylene black) and binders (polyvinylidene fluoride) are added in a ratio of 7:2:1 into methylpyrrolidone solvents to mix and obtain slurries; the slurries are coated on metal aluminum current collectors to obtain positive electrodes; the assembly of button cells is completed in an argon glove box, metal sodium is used as a counter electrode, and glass fiber is used as a separator. Finally, two kinds of electrolytes are injected: the electrolyte used for normal temperature test is a 1M NaClO4 ethylene carbonate (EC) / propylene carbonate (PC) (1:1, v / v) solution containing 5wt% fluoroethylene carbonate (FEC) additive; the electrolyte used for low temperature test is a 0.5M NaPF6 DIGLYME solution.

[0089] 2. Performance test (1) The positive electrode materials prepared in Example 1 and Comparative Example 1 are characterized, and the results are shown in Table 1. Figures 1-5

[0090] Figure 1 The XRD pattern of the positive electrode material prepared in Example 1 and Comparative Example 1 is shown in Figure 1. As can be seen from Figure 1, the positive electrode material NVMgMnAlP / C in Example 1 and the positive electrode material NVP / C in Comparative Example 1 are consistent with all diffraction peak positions of standard card JCPDS #53-0018, and all belong to rhombohedral phase structure of R Figure 1 c space group and no impurity peak is detected.

[0091] Figure 2 The XRD-Rietveld pattern of the positive electrode material prepared in Example 1 is shown in Figure 2. As can be seen from Figure 2, the positive electrode material NVMgMnAlP / C in Example 1 is consistent with all diffraction peak positions of standard card JCPDS #53-0018, and all belong to rhombohedral phase structure of R Figure 2 ​​The results of the refinement show that the introduction of aluminum, manganese, and magnesium did not affect the crystal structure of the material and there were no impurities.

[0092] Figure 3 The image shown is a scanning electron microscope (SEM) image of the cathode material prepared in Example 1 and Comparative Example 1. Figure 3 In the middle (a), there is a scanning electron microscope (SEM) image of the NVP / C cathode material in Comparative Example 1 with a scale bar of 1 μm. Figure 3 Image (b) is a scanning electron microscope (SEM) image of the cathode material NVMgMnAlP / C in Example 1 with a scale bar of 2 μm. Figure 3 Image (c) is a scanning electron microscope (SEM) image of the cathode material NVMgMnAlP / C in Example 1, with a scale bar of 1 μm. Figure 3 It can be seen that the cathode material NVMgMnAlP / C in Example 1 consists of blocky particles of approximately 2 μm in size, and the material surface exhibits a porous morphology. Furthermore... Figure 3 (c) shows that the cathode material prepared in Example 1 has obvious voids, exhibiting a coral-like structure with a porosity of 20%~30% and a pore size of 10~20μm. The main reason is that when the aluminum, manganese, and magnesium co-doped samples are sintered in a 5% H2 / Ar mixed atmosphere, the lattice distortion and defects introduced by the dopant ions provide channels for gas escape, while the slightly reducing atmosphere promotes redox reactions and the decomposition and vaporization of organic residues, which together lead to local expansion between particles and the formation of voids, thus generating a porous coral-like structure. Conversely, in the undoped or pure Ar atmosphere, due to defects and insufficient gas release, the structure tends to be dense, making it difficult to form a porous morphology. In Comparative Example 1, the cathode material NVP / C has an irregular blocky structure with a particle size of 1-3μm, a slightly rough surface, and no porous structure is observed.

[0093] Figure 4 The image shown is a scanning electron microscope (SEM) image (EDS) of the cathode material prepared in Example 1. Figure 4 EDS results show that Mn, Mg, and Al atoms were successfully doped into the NVP material.

[0094] Figure 5 The image shown is a transmission electron microscope (TEM) image of the cathode material prepared in Example 1. (The image is derived from...) Figure 5 It can be seen that the cathode material prepared in Example 1 has obvious pores on its surface and is covered by a thin carbon layer with a thickness of 8~10nm.

[0095] (2) The performance of the batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1 was tested, and the results are as follows: Figures 6-12 As shown.

[0096] Figure 6 The diagram shows the initial charge-discharge curves of batteries composed of the positive electrode materials prepared in Example 1 and Comparative Example 1 at 25°C and 0.1C. Figure 6It can be seen that, compared with Comparative Example 1, the initial discharge specific capacity of the battery composed of the positive electrode material in Example 1 is increased from 98.7 mAh / g to 108.5 mAh / g at room temperature 25℃ and 0.1C current density, and the charge-discharge platform of 4.0V is increased, and the energy density is improved.

[0097] Figure 7 The figure shows the cycle performance at 25℃ and 50C constant current of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1. Figure 7 It can be seen that, the initial discharge specific capacity of the battery composed of the positive electrode material in Example 1 is 100.7 mAh / g at room temperature 25℃ and 50C (5.85 Ah / g) high current density, and the capacity retention rate after 3000 cycles is as high as 82.2%, which has good cycle stability; while the initial discharge specific capacity of the battery composed of the positive electrode material in Comparative Example 1 is 79.8 mAh / g, and the capacity retention rate after 2300 cycles is 52%, which has poor stability.

[0098] Figure 8 The figure shows the initial charge-discharge curve of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1 at-20℃ and 0.1C. Figure 8 It can be seen that, the initial discharge specific capacity of the battery composed of the positive electrode material in Example 1 is 102 mAh / g at room temperature-20℃ and 0.1C current density, which is 94% of the room temperature capacity; while the initial discharge specific capacity of the battery composed of the positive electrode material in Comparative Example 1 is 90 mAh / g at 0.1C current density, which has a lower initial capacity.

[0099] Figure 9 The figure shows the cycle performance at-20℃ and 10C constant current of the battery composed of the positive electrode material prepared in Example 1 and Comparative Example 1. Figure 9 It can be seen that, the initial capacity of the battery composed of the positive electrode material in Example 1 is 82 mAh / g at-20℃ and 10C current density, and the capacity retention rate is close to 100% after 3000 cycles, while the initial capacity of the battery composed of the positive electrode material in Comparative Example 1 is 70 mAh / g at-20℃ and 10C current density, which is lower than that of Example 1.

[0100] From the above Figure 8 and Figure 9 It can be seen that the positive electrode material provided by the present application has high energy density under extremely cold conditions, and has high cycle stability, which further proves the application potential of the positive electrode material provided by the present application under extremely cold conditions.

[0101] Figure 10 The figure shows the cyclic voltammetry performance of the battery composed of the positive electrode material prepared in Comparative Example 1 at 0.1mV S -1 .

[0102] Figure 11The image shows a battery composed of the positive electrode material prepared in Example 1 at 0.1 mV / s. -1 The cyclic volt-ampere performance diagram. (From...) Figures 10-11 It can be seen that, compared with Comparative Example 1, the battery composed of the positive electrode material prepared in Example 1 not only exhibits V at 3.45V. 3+ / V 4+ The redox peaks are present, and Mn is also present. 2+ / Mn 3+ The redox peaks were observed, and a V peak also appeared at 3.96 V. 4 + / V 5+ The redox peaks are consistent with the plateau of the charge-discharge curve. This indicates that Mn, Mg, and Al atoms activate the voltage plateau at around 4V, thereby increasing the energy density.

[0103] Figure 12 The image shows the AC impedance spectra of the batteries composed of the cathode materials prepared in Example 1 and Comparative Example 1. Figure 12 It can be seen that, compared with Comparative Example 1, the battery composed of the cathode material prepared in Example 1 has a higher diffusion rate.

[0104] (3) The following electrochemical performance tests were conducted on the batteries assembled with the cathode materials in Example 1 and Comparative Examples 1-4: The assembled batteries were placed in a 40℃ oven for 12 hours and then subjected to constant current charge-discharge tests. The charge-discharge voltage window was 2.5-4.2V. The specific capacity, rate performance, and long-cycle performance of the sodium-ion battery cathode were measured at room temperature (25℃) and -20℃. The results are shown in Table 1 and Table 2.

[0105] Table 1. Electrochemical performance test results of batteries assembled with cathode materials in Examples 1-5 and Comparative Examples 1-6 at room temperature.

[0106] Table 2. Electrochemical performance test results of batteries assembled with cathode materials in Examples 1-5 and Comparative Examples 1-6 at -20°C and 0.2C.

[0107] From Table 1 and Table 2, in the examples and comparative examples, the cycle life of Example 1 at room temperature and 50C under high current density is higher than that of other comparative examples, and at low temperature-20℃, it has higher initial capacity and energy density, which indicates that the performance of the modification method of the aluminum-manganese-magnesium three-element co-doping provided by the application is higher than that of the single-doping and double-doping modification methods, and the doping amount of the aluminum-manganese-magnesium three elements also affects the performance of the material. Further, it is further indicated that the preparation method of the application adopts specific steps, conditions and parameters to realize overall good interaction to prepare products with more excellent discharge specific capacity and cycle performance, so as to effectively improve the electrochemical performance and low-temperature performance of the products.

[0108] Although the preferred embodiments of the application have been shown and described, it is to be understood that various modifications can be made by those skilled in the art without departing from the spirit and scope of the appended claims.

Claims

1. A positive electrode material, characterized in that, The positive electrode material includes a core and a coating layer covering the surface of the core; The general chemical formula of the core is Na3V. 1.925-x-y Mn x Al y Mg 0.075 (PO4)3, where x≤0.4, y≤0.3; The coating layer includes a carbon coating layer.

2. The cathode material according to claim 1, characterized in that, Based on the mass of the cathode material, the carbon coating content is 4wt%~7wt%; The porosity of the positive electrode material is 20%~30%; The pore size of the positive electrode material is 10~20μm.

3. The cathode material according to claim 1, characterized in that, The positive electrode material is R. Rhombohedral phase structure of space group c.

4. A method for preparing the cathode material according to any one of claims 1 to 3, characterized in that, Includes the following steps: Sodium source, vanadium source, phosphorus source, magnesium source, aluminum source, manganese source, carbon source and solvent are mixed to obtain precursor slurry; The precursor slurry is dried to obtain the precursor material; The precursor material is calcined to obtain the cathode material.

5. The method for preparing the cathode material according to claim 4, characterized in that, The molar ratio of sodium in the sodium source, vanadium in the vanadium source, phosphorus in the phosphorus source, magnesium in the magnesium source, aluminum in the aluminum source, manganese in the manganese source, and carbon in the carbon source is 3:(1.125~1.925):3:0.075:(0~0.5):(0~0.3):(6~9).

6. The method for preparing the cathode material according to claim 4, characterized in that, The ratio of vanadium element in the vanadium source to the solvent is 1.125~1.925mol:20~40mL.

7. The method for preparing the cathode material according to claim 4, characterized in that, The sodium source includes sodium salt; The vanadium source includes vanadium oxide; The phosphorus source includes phosphate; The magnesium source includes magnesium salts; The aluminum source includes aluminum salts; The manganese source includes manganese salts; The carbon source includes citric acid; The solvents include water and / or ethanol.

8. The method for preparing the cathode material according to claim 4, characterized in that, The drying temperature is 80~120℃, and the drying time is 10~16h.

9. The method for preparing the cathode material according to claim 4, characterized in that, The calcination is a staged calcination; The conditions for the first stage of calcination in the staged calcination are as follows: the heating rate is 3~5℃ / min, the final temperature is 300~350℃, the holding time at the final temperature is 4~6h, and the atmosphere is a mixture of the first reducing gas and the first inert gas; the volume ratio of the first reducing gas to the first inert gas is 1~5:95~99. The conditions for the second stage of calcination in the staged calcination are as follows: the heating rate is 3~5℃ / min, the final temperature is 800~900℃, the holding time at the final temperature is 8~10h, and the atmosphere is a mixture of a second reducing gas and a second inert gas; the volume ratio of the second reducing gas to the second inert gas is 1~5:95~99. The first reducing gas and the second reducing gas each independently include hydrogen.

10. A battery comprising a positive electrode, characterized in that the positive electrode comprises the positive electrode material according to any one of claims 1 to 3 or the positive electrode material prepared by the preparation method according to any one of claims 4 to 9.

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