Positive electrode material and preparation method thereof, positive plate and sodium ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a positive electrode material and a preparation method thereof, a positive plate and a sodium ion battery. The positive electrode material comprises a base material, a first coating layer and a second coating layer, the bulk phase of the matrix material is NVP doped with M ions and Ce ions, and the M ions comprise at least one of Mn ions, Cu ions and Zn ions; the surface of the base material is coated with the first coating layer in situ, and the first coating layer comprises cerium dioxide; the surface, away from the base material, of the first coating layer is coated with the second coating layer in situ, and the second coating layer comprises a carbon material. According to the positive electrode material, the solid-phase diffusion kinetics and capacity of the material can be improved, the stability of a crystal structure can be effectively improved, the ginger-Taylor effect is inhibited, the coating layer is not prone to falling off from the surface of the positive electrode material, and the cycling stability and conductivity of the material are improved.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and specifically relates to a cathode material, a preparation method thereof, a cathode sheet, and a sodium-ion battery. Background Art

[0002] In recent years, sodium-ion batteries (SIBs) have attracted much attention due to the abundant sodium resources in nature and low cost. In addition, SIBs and LIBs (lithium-ion batteries) have similar electrochemical mechanisms. Therefore, studying cheaper SIBs is of great significance for meeting the development of large-scale energy storage. It should be noted that limited by the relatively large sodium ion radius (rNa + = 0.097 nm, rLi + = 0.068 nm) and the huge volume change during the insertion and extraction of Na + from the electrode material, the structural stability and cycle life of SIBs are generally lower than those of LIBs. Therefore, developing a cathode material with a three-dimensional channel and a stable framework as the main body to achieve fast and stable sodium ion transport is crucial for improving the electrochemical performance and kinetic characteristics of SIBs. Currently, many sodium-ion cathode materials have been widely studied. Among them, Na3V2(PO4)3 (NVP) with a NASICON-type structure has high working voltage, high specific capacity, high structural stability and other advantages due to the "inductive effect" of the polyanion group. Therefore, this type of phosphate material is a very promising electrode material for sodium-ion batteries. However, the low conductivity of NVP seriously affects its electrochemical performance, thus limiting the application of this material. Therefore, carbon coating, reducing particle size, and cation doping are usually used to improve the electrochemical performance of NVP materials.

[0003] An ideal coating strategy needs to consider the ionic conductivity and electronic conductivity of the coating layer, as well as the compatibility with the electrolyte, in addition to meeting the requirements of isolating the cathode material from the electrolyte, suppressing interfacial side reactions, suppressing the dissolution of transition metals, suppressing gas generation, and improving the structure. The carbon coating layer can well improve the conductivity of the material, enhance the rate performance of the material, and enhance the charge and discharge depth to reduce the polarization effect. However, the carbon coating layer has an unstable binding condition and is prone to fragmentation during long-term cycling due to the expansion and contraction of the NVP inside the bulk phase, accelerating the failure and aging behavior of the material. The carbon coating layer is easily detached from the surface of the cathode material, exposing the internal cathode material, reducing the conductivity of the material, and also causing the electrolyte and the material to come into direct contact, resulting in electrolyte failure, and further accelerating the failure of the battery cell. Summary of the Invention

[0004] In view of this, the present invention provides a cathode material, a preparation method thereof, a cathode sheet and a sodium-ion battery. The cathode material can improve the solid-phase diffusion kinetics and capacity of the material, effectively improve the crystal structure stability, inhibit the Jahn-Teller effect, the coating layer is not easily detached from the surface of the cathode material, and increase the cycle stability and conductivity of the material.

[0005] In order to achieve the above-mentioned invention object, the present invention provides the following technical solutions:

[0006] In the first aspect, the present invention provides a preparation method of a cathode material, comprising the following steps:

[0007] S1, mixing a vanadium source, a carbon source and a good solvent, and performing a first heating and stirring treatment to obtain a solution A; mixing the solution A, an M source, a sodium source and a phosphate precursor, and performing a second heating and stirring treatment to obtain a solution B; the M source includes at least one of a manganese source, a copper source and a zinc source;

[0008] Mixing a cerium source and a poor solvent to obtain a solution C;

[0009] S2, using the anti-solvent method, mixing the solution B and the solution C, and performing a third heating and stirring treatment to obtain a solution D; the solution D includes M ions, Ce 3+ doped Na3V2(PO4)3 precursor and a carbon source;

[0010] S3, using the spray freeze-drying method, drying the solution D to obtain a precursor E; the precursor E includes M ions, Ce 3+ doped Na3V2(PO4)3 precursor, and a carbon source coated on its surface;

[0011] S4, sintering the precursor E in an inert gas atmosphere to obtain the cathode material.

[0012] In an embodiment of the present invention, the vanadium source includes at least one of NH4VO3, V2O3, VO2, V2O4, V2O5, H4V2O 10 , V(OH)3, Na3VO4, Na4V2O7.

[0013] In an embodiment of the present invention, the carbon source includes at least one of citric acid, glucose, asphalt, polydopamine, resorcinol, formaldehyde, starch, sucrose, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, tannic acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, anthracene, aniline.

[0014] In an embodiment of the present invention, the manganese source includes at least one of manganese acetate, manganese oxalate, manganese chloride, manganese nitrate, manganese sulfate, manganese bromide, manganese oxide, manganese hydroxide, manganese cyanide, manganese fatty acid.

[0015] In the embodiments of the present invention, the copper source includes at least one of copper acetate, copper oxalate, copper chloride, copper nitrate, copper sulfate, copper bromide, copper oxide, copper hydroxide, copper cyanide, and copper fatty acid.

[0016] In the embodiments of the present invention, the zinc source includes at least one of zinc acetate, zinc oxalate, zinc chloride, zinc nitrate, zinc sulfate, zinc bromide, zinc oxide, zinc hydroxide, zinc cyanide, and zinc fatty acid.

[0017] In the embodiments of the present invention, the sodium source includes at least one of sodium acetate, sodium oxalate, sodium citrate, and sodium carboxymethyl cellulose.

[0018] In the embodiments of the present invention, the phosphate precursor includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0019] In the embodiments of the present invention, the cerium source includes at least one of cerium acetate, cerium oxalate, cerium nitrate, cerium sulfate, cerium oxide, and cerium hydroxide.

[0020] In the embodiments of the present invention, the good solvent includes water.

[0021] In the embodiments of the present invention, the poor solvent includes at least one of N,N-dimethylformamide and N,N-dimethylacetamide.

[0022] Preferably, the molar ratio of the vanadium source, the carbon source, the M source, the sodium source, the phosphate precursor, and the cerium source is (0.5 - 1.5):(1 - 5):(0.5 - 4):(1 - 5):(0.3 - 5):(0.05 - 0.15).

[0023] Preferably, the molar volume ratio of the vanadium source to the good solvent is 0.2 - 1 mol / L.

[0024] Preferably, the molar volume ratio of the cerium source to the poor solvent is 0.01 - 0.05 mol / L.

[0025] Preferably, the temperature of the first heating and stirring treatment is 30 - 100 °C, and the time of the first heating and stirring treatment is 0.1 - 6 h.

[0026] Preferably, the temperature of the second heating and stirring treatment is 30 - 100 °C, and the time of the second heating and stirring treatment is 0.1 - 6 h.

[0027] Preferably, the temperature of the third heating and stirring treatment is 30 - 100 °C, and the time of the third heating and stirring treatment is 0.1 - 6 h.

[0028] Preferably, the temperature of the drying treatment is 100 - 110 °C.

[0029] Preferably, the sintering temperature is 300 to 900 °C, and the sintering time is 0.1 to 16 h.

[0030] In a second aspect, the present invention provides a positive electrode material, which includes:

[0031] (a) A matrix material, the bulk phase of the matrix material is M ions, Ce 3+ doped Na3V2(PO4)3; M ions include Mn 2+ , Cu 2+ , Zn 2+ at least one of;

[0032] (b) A first coating layer, the first coating layer is in-situ coated on the surface of the matrix material, and the first coating layer includes cerium dioxide;

[0033] (c) A second coating layer, the second coating layer is in-situ coated on the surface of the first coating layer away from the matrix material, and the second coating layer includes a carbon material.

[0034] In the embodiment of the present invention, the chemical formula of the matrix material is Na 4-x M 1-x Ce x V(PO4)3, where 0.01 ≤ X ≤ 0.2.

[0035] Preferably, the mass ratio of the first coating layer to the matrix material is (0.5 to 2):100.

[0036] Preferably, the mass ratio of the second coating layer to the matrix material is (0.05 to 1):100.

[0037] In a third aspect, the present invention provides a positive electrode sheet, which includes the positive electrode material prepared by the above preparation method, and / or the above positive electrode material.

[0038] In a fourth aspect, the present invention provides a sodium ion battery, which includes the above positive electrode sheet.

[0039] Compared with the prior art, the beneficial effects of the present invention are:

[0040] The positive electrode material of the present invention includes a matrix material, a first coating layer and a second coating layer. Among them, the bulk phase of the matrix material is M ions, Ce 3+ doped Na3V2(PO4)3; M ions include Mn 2+ , Cu 2+ , Zn 2+ at least one of; the first coating layer includes cerium dioxide; the second coating layer includes a carbon material;

[0041] (1) Improve the solid-state diffusion kinetics of the material: The bulk phase of the matrix material includes M ions (Mn 2+ , Cu 2+ , Zn 2+ ) and Ce 3+ . By replacing some V 3+ with M ions and Ce 3+ with smaller ionic radii between the material lattices, since the M ions, Ce 3+ , and V 3+ have the same number of electron layers, but the M ions and Ce 3+ have a larger number of protons inside the atomic nucleus, lattice distortion can be formed inside the material. Moreover, doping and modification with cerium (Ce) can reduce the sodium ion migration energy barrier, reduce the diffusion resistance of Na + inside the material, and increase the diffusion kinetics;

[0042] (2) Enhance the capacity of the material: Na3V2(PO 4)3 as the cathode material for sodium-ion batteries, 1 mol of the compound contains 3 mol of Na + . Using at least one of the divalent ions Mn 2+ , Cu 2+ , Zn 2+ to perform atomic doping on the bulk phase of the material and replace V 3+ , it is necessary to supplement the same molecular weight of Na + to maintain charge balance and form Na 4-x M 1-x Ce x V(PO4)3, thereby increasing the capacity;

[0043] (3) Suppress the Jahn-Teller effect of the material: In a symmetric non-linear molecule, if the ground state of the system has multiple degenerate energy levels, spontaneous distortion will occur to eliminate the degeneracy. This degeneracy elimination can often reduce the symmetry and energy of the system. Therefore, the Jahn-Teller effect usually occurs spontaneously. Currently, most views believe that the Jahn-Teller distortion has an adverse effect on the performance of the material, such as being unfavorable to structural stability or promoting the dissolution of transition metals, etc. In the present invention, Ce 3+ is an inactive metal element, and its valence state is different from that of M ions such as Mn 2+ . It can reduce the structural changes caused by irreversible phase transitions and suppress the Jahn-Teller effect. The introduction of Ce 3+ can effectively improve the crystal structure stability. Due to the good structural stability of the Na 4-x M 1-x Ce x V(PO4)3 (NMCVP) cathode, and it can further suppress the dissolution of M ions. The M metal is a reactive metal, which can increase the reversibility and structural stability of the active material;

[0044] (4) Increase the cycling stability and conductivity of the material: During the sintering process, oxygen in the cerium source forms O2 under high temperature, and some cerium ions form a cerium dioxide coating layer on the surface of the bulk phase under the action of O2. Cerium dioxide is an electrochemically active compound that improves the conductivity of the electrode. Since the cerium dioxide coating layer is formed in-situ, it is not easily detached during long cycling. This structure can reduce the electrolyte failure caused by the direct contact between the material surface and the electrolyte, reduce the polarization of the electrode material, increase the conductivity of the material while increasing the cycling stability;

[0045] (5) In the preparation method of the cathode material of the present invention, the precursor Na 4-x M 1-x Ce x V(PO4)3 (NMCVP) is prepared by the antisolvent method. Among them, the good solvent (such as water) has a high solubility for the cerium source, while the poor solvent (N,N-dimethylformamide DMF or N,N-dimethylacetamide DMA) has a low solubility in the good solvent. Solution C (containing the cerium source and the poor solvent) is slowly added to solution B (containing the vanadium source, carbon source, M source, sodium source, phosphate precursor and good solvent), and the precursor can be crystallized and precipitated from the good solvent; inducing the morphology of Na 4-x M 1-x Ce x V(PO4)3 grows directionally from an irregular bulk to a uniform hierarchical nanosheet self-assembled structure, and the NMCVP crystal grows along the

[100] direction, thereby exposing a larger (001) crystal plane, promoting the migration of sodium ions in the NMCVP framework and increasing the reaction kinetics. Description of the Drawings

[0046] Figure 1 SEM images of sodium vanadium phosphate and the cathode material of the present invention, where

[0047] (a) SEM image of sodium vanadium phosphate (Na3V2(PO4)3);

[0048] (b) SEM image of the cathode material of the present invention (Na 3.9 Mn 0.9 Ce 0.1 V(PO4)3@CeO2 / C). Detailed Embodiments

[0049] The present invention discloses a cathode material, a preparation method thereof, a cathode sheet and a sodium ion battery. Those skilled in the art can draw on the content of this article and appropriately improve the process parameters to achieve it. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments. Those related can obviously make changes or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0050] In the description of the present invention, it should be noted that the terms "first", "second", etc. are only used for descriptive purposes and do not indicate or imply relative importance.

[0051] In the description of the present invention, the list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A, B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0052] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range or individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0053] If there is no special indication, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0054] If there is no special indication, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0055] If there is no special indication, the "including" and "comprising" mentioned in the present application mean open-ended or can also be closed-ended. For example, the "including" and "comprising" can mean that other components not listed can also be included or comprised, or it can only include or comprise the listed components.

[0056] Specifically, the present invention adopts the following technical solutions:

[0057] In a first aspect, the present invention provides a method for preparing a cathode material, comprising the following steps:

[0058] S1, Mix a vanadium source, a carbon source, and a good solvent, and perform a first heating and stirring treatment to obtain solution A; mix solution A, an M source, a sodium source, and a phosphate precursor, and perform a second heating and stirring treatment to obtain solution B; the M source includes at least one of a manganese source, a copper source, and a zinc source;

[0059] Mix a cerium source and a poor solvent to obtain solution C;

[0060] S2, Using the anti-solvent method, mix solution B and solution C, and perform a third heating and stirring treatment to obtain solution D; solution D includes M ions, Ce 3+ doped Na3V2(PO4)3 precursor and a carbon source;

[0061] S3, Using the spray freeze-drying method, dry solution D to obtain precursor E; precursor E includes M ions, Ce 3+ doped Na3V2(PO4)3 precursor, and a carbon source coated on its surface;

[0062] S4, In an inert gas atmosphere, sinter precursor E to obtain the cathode material.

[0063] In the method for preparing the cathode material of the present invention, solution A, solution B, and solution C are respectively prepared, and the anti-solvent method is used to prepare the precursor Na 4-x M 1-x Ce x V(PO4)3 (NMCVP). Among them, the good solvent (such as water) has a high solubility for the cerium source, while the poor solvent (N,N-dimethylformamide DMF or N,N-dimethylacetamide DMA) has a low solubility in the good solvent and can play an anti-induction role. Slowly adding solution C (containing the cerium source and the poor solvent) to solution B (containing the vanadium source, the carbon source, the M source, the sodium source, the phosphate precursor, and the good solvent) can cause the precursor to crystallize out from the good solvent, inducing the morphology of Na 4-x M 1-x Ce x V(PO4)3 to grow directionally from an irregular block to a uniform multi-level nanosheet self-assembled structure, and the NMCVP crystal grows along the

[100] direction, thereby exposing a larger (001) crystal plane, promoting the migration of sodium ions in the NMCVP framework, increasing the reaction kinetics, and enabling the crystallization of the M ion, Ce 3+ doped Na3V2(PO4)3 precursor from the good solvent. If the components in solution B and solution C are mixed together instead of dissolved step by step, a precursor precipitate will be directly formed, the reaction will be uneven, and the anti-induction effect of the poor solvent cannot be achieved.

[0064] In the preparation method of the present invention, the spray freeze-drying method can dry the material without destroying the morphology of the precursor.

[0065] In step S4 of the preparation method of the present invention, during the sintering treatment, part of the cerium ions in the bulk phase form a cerium dioxide coating layer and a carbon coating layer on the surface of the bulk phase. The reasons for the formation of the cerium dioxide coating layer between the bulk phase and the carbon coating layer include:

[0066] (1) Overall reaction mechanism: Since the cerium source is dissolved in a poor solvent to form a homogeneous solution, when it is mixed with the vanadium source, manganese source, and carbon source solutions, a precursor is formed and precipitated. The carbon source, as a macromolecule, does not participate in the precipitation reaction but is physically adsorbed on the precipitate precursor. During the formation of the precursor, the anti-solvent method is used to induce

[0067] Na 4-x M 1-x Ce x The morphology of V(PO4)3 grows directionally from an irregular block to a uniform hierarchical nanosheet self-assembled structure;

[0068] (2) Since the activation energy required for the reaction to form cerium dioxide is lower than the activation energy required for the reaction to form the oxide of M (manganese dioxide, copper dioxide, or zinc dioxide), cerium dioxide is formed during the sintering process rather than the oxide of M; and the activation energy for the formation of sodium manganese vanadium phosphate is also lower than the activation energy required for the reaction of the oxide of M. Sodium manganese vanadium phosphate is preferentially formed, and no excess components participate in the formation of the oxide of M when the stoichiometric coefficient is appropriate;

[0069] (3) During the spray freeze-drying process, the precursor has been formed. Since the carbon source is a macromolecule, it will be adsorbed on the surface of the precursor, and a carbon layer on the surface of the material is formed after sintering. Therefore, the cerium dioxide coating layer is inside the carbon coating layer.

[0070] In the embodiment of the present invention, the vanadium source includes at least one of NH4VO3, V2O3, VO2, V2O4, V2O5, H4V2O 10 , V(OH)3, Na3VO4, Na4V2O7.

[0071] In the embodiment of the present invention, the carbon source includes at least one of citric acid, glucose, asphalt, polydopamine, resorcinol, formaldehyde, starch, sucrose, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, tannic acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, anthracene, aniline.

[0072] In the embodiment of the present invention, the manganese source includes at least one of manganese acetate, manganese oxalate, manganese chloride, manganese nitrate, manganese sulfate, manganese bromide, manganese oxide, manganese hydroxide, manganese cyanide, manganese fatty acid.

[0073] In an embodiment of the present invention, the copper source includes at least one of copper acetate, copper oxalate, copper chloride, copper nitrate, copper sulfate, copper bromide, copper oxide, copper hydroxide, copper cyanide, and copper fatty acid.

[0074] In an embodiment of the present invention, the zinc source includes at least one of zinc acetate, zinc oxalate, zinc chloride, zinc nitrate, zinc sulfate, zinc bromide, zinc oxide, zinc hydroxide, zinc cyanide, and zinc fatty acid.

[0075] In an embodiment of the present invention, the sodium source includes at least one of sodium acetate, sodium oxalate, sodium citrate, and sodium carboxymethyl cellulose.

[0076] In an embodiment of the present invention, the phosphate precursor includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate.

[0077] In an embodiment of the present invention, the cerium source includes at least one of cerium acetate, cerium oxalate, cerium nitrate, cerium sulfate, cerium oxide, and cerium hydroxide.

[0078] In an embodiment of the present invention, the good solvent includes water.

[0079] In an embodiment of the present invention, the poor solvent includes at least one of N,N-dimethylformamide and N,N-dimethylacetamide.

[0080] Preferably, the molar ratio of the vanadium source, the carbon source, the M source, the sodium source, the phosphate precursor, and the cerium source is (0.5 - 1.5):(1 - 5):(0.5 - 4):(1 - 5):(0.3 - 5):(0.05 - 0.15). Exemplarily, the molar ratio of the vanadium source, the carbon source, the M source, the sodium source, the phosphate precursor, and the cerium source is any value among 0.5:5:1:5:0.3:0.15, 1.5:1:4:1:1.5:0.05, 1:3:0.9:4:3:0.1 or any value within the range value composed of any two of the above values.

[0081] Preferably, the molar volume ratio of the vanadium source to the good solvent is 0.2 - 1 mol / L. Exemplarily, the molar volume ratio of the vanadium source to the good solvent is any value among 0.2 mol / L, 0.4 mol / L, 0.6 mol / L, 0.8 mol / L, 1 mol / L or any value within the range value composed of any two of the above values.

[0082] Preferably, the molar volume ratio of the cerium source to the poor solvent is 0.01 - 0.05 mol / L. Exemplarily, the molar volume ratio of the cerium source to the poor solvent is any value among 0.01 mol / L, 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, 0.05 mol / L or any value within the range value composed of any two of the above values.

[0083] Preferably, the temperature of the first heating and stirring treatment is 30 to 100 °C, and the time of the first heating and stirring treatment is 0.1 to 6 h. Exemplarily, the temperature of the first heating and stirring treatment is any value among 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C or any value within the range value composed of any two of the above values, and the time of the first heating and stirring treatment is any value among 0.1 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value within the range value composed of any two of the above values.

[0084] Preferably, the temperature of the second heating and stirring treatment is 30 to 100 °C, and the time of the second heating and stirring treatment is 0.1 to 6 h. Exemplarily, the temperature of the second heating and stirring treatment is any value among 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C or any value within the range value composed of any two of the above values, and the time of the second heating and stirring treatment is any value among 0.1 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value within the range value composed of any two of the above values.

[0085] Preferably, the temperature of the third heating and stirring treatment is 30 to 100 °C, and the time of the third heating and stirring treatment is 0.1 to 6 h. Exemplarily, the temperature of the third heating and stirring treatment is any value among 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C or any value within the range value composed of any two of the above values, and the time of the third heating and stirring treatment is any value among 0.1 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value within the range value composed of any two of the above values.

[0086] Preferably, the temperature of the drying treatment is 100 to 110 °C. Exemplarily, the temperature of the drying treatment is any value among 100 °C, 102 °C, 104 °C, 106 °C, 108 °C, 110 °C or any value within the range value composed of any two of the above values.

[0087] Preferably, the temperature of the sintering treatment is 300 to 900 °C, and the time of the sintering treatment is 0.1 to 16 h. Exemplarily, the temperature of the sintering treatment is any value among 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C or any value within the range value composed of any two of the above values, and the time of the sintering treatment is any value among 0.1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h or any value within the range value composed of any two of the above values.

[0088] Second aspect, the present invention provides a cathode material, which comprises:

[0089] (a) A matrix material, the bulk phase of the matrix material is M ions, Ce 3+ doped Na3V2(PO4)3; M ions include Mn 2+ , Cu 2+ , Zn 2+ at least one of;

[0090] (b) A first coating layer, the first coating layer is in-situ coated on the surface of the matrix material, and the first coating layer includes cerium dioxide;

[0091] (c) A second coating layer, the second coating layer is in-situ coated on the surface of the first coating layer away from the matrix material, and the second coating layer includes a carbon material.

[0092] In the embodiment of the present invention, the chemical formula of the matrix material is Na 4-x M 1-x Ce x V(PO4)3, where 0.01 ≤ X ≤ 0.2. Exemplarily, the value of X is any value among 0.01, 0.05, 0.1, 0.15, 0.2 or any value within the range value composed of any two of the above values.

[0093] In the cathode material of the present invention, the ionic radii of M ions and Ce ions are smaller than that of V ions, the number of electron layers of M ions, Ce ions, and V ions is the same, the number of protons inside the atomic nucleus of M ions and Ce ions is greater than that of V ions, and there is lattice distortion inside the matrix material. The bulk phase of the matrix material includes M ions (Mn 2+ , Cu 2+ , Zn 2+ ) and Ce 3+ , replacing part of V 3+ with M ions and Ce 3+ with smaller ionic radii between the material lattices. Since the number of electron layers of M ions, Ce 3+ , V 3+ is the same, but the number of protons inside the atomic nucleus of M ions and Ce 3+ is greater, lattice distortion can be formed inside the material, and doping modification with cerium element (Ce) can reduce the sodium ion migration energy barrier, reduce the diffusion resistance of Na + inside the material, and increase the diffusion kinetics.

[0094] In the cathode material of the present invention, Na3V2(PO 4)3 as the cathode material of a sodium ion battery, 1 mol of the compound contains 3 mol of Na + , using divalent ions Mn 2+ , Cu2+ , Zn 2+ At least one of them is used for atomic doping of the material bulk phase to replace V 3+ , and an equivalent molecular weight of Na needs to be added + to maintain charge balance and form Na 4-x M 1-x Ce x V(PO4)3, thereby increasing the capacity.

[0095] In the cathode material of the present invention, Ce 3+ is an inactive metal element, and its valence state is different from that of M ions such as Mn 2+ . It can reduce the structural changes caused by irreversible phase transitions and inhibit the Jahn-Teller effect. The introduction of Ce 3+ can effectively improve the crystal structure stability. Due to Na 4-x M 1-x Ce x The good structural stability of the V(PO4)3(NMCVP) cathode can further inhibit the dissolution of M ions. M metal is a reactive metal, which can increase the reversibility and structural stability of the active material.

[0096] In the cathode material of the present invention, cerium dioxide is an electrochemically active compound, which improves the conductivity of the electrode. Since the cerium dioxide coating layer is in-situ generated, the cerium dioxide coating layer is not easy to fall off during long-term cycling. This structure can reduce the electrolyte failure caused by the direct contact between the material surface and the electrolyte, reduce the polarization of the electrode material, and increase the conductivity of the material while increasing the cycling stability.

[0097] Preferably, the mass ratio of the first coating layer to the matrix material is (0.5-2):100. Exemplarily, the mass ratio of the first coating layer to the matrix material is any value among 0.5:100, 1:100, 1.5:100, 2:100 or any value within the range value composed of any two of the above values.

[0098] Preferably, the mass ratio of the second coating layer to the matrix material is (0.05-1):100. Exemplarily, the mass ratio of the second coating layer to the matrix material is any value among 0.05:100, 0.1:100, 0.2:100, 0.4:100, 0.6:100, 0.8:100, 1:100 or any value within the range value composed of any two of the above values.

[0099] In the third aspect, the present invention provides a cathode sheet, which includes the cathode material prepared by the above preparation method, and / or the above cathode material.

[0100] In the fourth aspect, the present invention provides a sodium ion battery, which includes the above cathode sheet.

[0101] In the embodiments of the present invention, the battery structure includes, but is not limited to, button cells, soft-pack batteries, cylindrical batteries, and the like.

[0102] This application places no special restrictions on the negative electrode sheet, separator, and electrolyte in the battery. Those skilled in the art can select according to actual needs as long as the purpose of this application can be achieved.

[0103] The reagents, instruments, materials, etc. used in the present invention can all be obtained through commercial channels.

[0104] The present invention will be further described below in conjunction with embodiments:

[0105] Example 1:

[0106] First, 0.01 mol of NH4VO3 and 0.03 mol of citric acid were dissolved in 20 mL of water and stirred at 70 °C for 30 minutes to form a blue solution (Solution A); subsequently, 0.009 mol of (CH3COO)2Mn·4H2O, 0.04 mol of CH3COONa, and 0.03 mol of NH4H2PO4 were added to Solution A and stirred at 70 °C for 30 minutes to form Solution B;

[0107] 0.001 mol of (CH3COO)3Ce·H2O was dissolved in 20 mL of N,N-dimethylformamide (DMF) and stirred at 70 °C for 30 minutes to form Solution C;

[0108] Solution C was slowly added to Solution B, and finally a 100 mL mixed solution D was prepared. After complete dissolution, the above mixed solution D was spray-dried (at 108 °C) to obtain a powdery precursor E.

[0109] Finally, the powdery precursor E was calcined in an argon atmosphere at 700 °C for 10 hours to obtain a three-dimensional spherical Na 3.9 Mn 0.9 Ce 0.1 V(PO4)3@CeO2 / C (NMCVP@CeO2 / C) cathode material, and its SEM image is shown in Figure 1 (b).

[0110] Through Figure 1 (a) of the SEM image, it can be seen that the surface of the undoped sodium vanadium phosphate sample is smooth;

[0111] Through Figure 1 (b) of the cathode material (Na 3.9 Mn 0.9 Ce 0.1From the SEM image of V(PO4)3@CeO2 / C, it can be seen that after the oxide doping and coating, the surface of the cathode material is covered with an oxide coating layer. The introduction of cerium ions partially forms a cerium dioxide coating layer during the sintering process. At the same time, the surface coating of the electrochemically active compound improves the conductivity of the electrode. Since it is in-situ generated, the cerium dioxide coating layer is not easily detached during long-term cycling. This structure can reduce the electrolyte failure caused by the direct contact between the material surface and the electrolyte, reduce the polarization of the electrode material, increase the conductivity of the material, and increase the cycling stability.

[0112] Example 2:

[0113] The only difference between this example and Example 1 is that the amount of citric acid is 0.02 mol.

[0114] Example 3:

[0115] The only difference between this example and Example 1 is that the amount of citric acid is 0.04 mol.

[0116] Example 4:

[0117] The only difference between this example and Example 1 is that the amount of (CH3COO)3Ce·H2O is 0.00075 mol.

[0118] Example 5:

[0119] The only difference between this example and Example 1 is that the amount of (CH3COO)3Ce·H2O is 0.00125 mol.

[0120] Comparative Example 1:

[0121] The difference between this comparative example and Example 1 is that no citric acid is added, and the chemical formula of the cathode material is Na 3.9 Mn 0.9 Ce 0.1 V(PO4)3@CeO2.

[0122] Comparative Example 2:

[0123] The difference between this comparative example and Example 1 is that no (CH3COO)3Ce·H2O is added, and the chemical formula of the cathode material is Na4MnV(PO4)3 / C.

[0124] Comparative Example 3:

[0125] The difference between this comparative example and Example 1 is that no (CH3COO)3Ce·H2O and (CH3COO)2Mn·4H2O are added, and the chemical formula of the cathode material is Na3V2(PO4)3 / C.

[0126] Comparative Example 4:

[0127] The difference between this comparative example and Example 1 is that citric acid, (CH3COO)3Ce·H2O, and (CH3COO)2Mn·4H2O were not added, and the chemical formula of the cathode material is Na3V2(PO4)3.

[0128] Battery preparation and performance testing:

[0129] I. Battery preparation:

[0130] Preparation of the negative electrode sheet: Hard carbon material (HC) was used as the negative electrode active material, which was mixed with the conductive agent SP and the binder polyvinylidene fluoride (PVDF), with HC:SP:PVDF = 98:1:1. It was coated on aluminum foil and made into a negative electrode sheet through processes such as drying and pressing. The size of the negative electrode sheet was (100mm×55mm).

[0131] Preparation of the positive electrode sheet: The cathode materials prepared in the examples and comparative examples were used as the positive electrode active material, which was mixed with the conductive agent SP and the binder PVDF, with cathode material:SP:PVDF = 97:2:1. It was coated on aluminum foil and made into a positive electrode sheet through processes such as drying and pressing. The size of the positive electrode sheet was 96mm×51mm.

[0132] Whatman glass fiber separator GF / D was used as the separator. The electrolyte was 1M NaClO4, dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1:1. For the full cell, the capacity ratio of the positive electrode to the negative electrode was 1.2, and the loading of the negative electrode was 9.8mg cm -2 。

[0133] Assembly of the battery: The assembly process was carried out by the soft-pack lamination method. ① Lamination: The positive electrode sheet, glass fiber separator, and negative electrode sheet were laminated in sequence to obtain an electrode core. ② Assembly: The laminated electrode core was hot-pressed and then placed into an aluminum-plastic film for top and side sealing. ③ Injection: The electrolyte was injected into the battery. ④ Encapsulation: The battery after injection was sealed, and the injection coefficient was 3g / Ah. ⑤ Formation: The encapsulated battery was subjected to formation treatment. When comparing the electrochemical performance of different positive electrode sheets, the loading of the negative electrode was basically kept consistent. The charge and discharge cut-off voltages were set at 2.5 - 4.0V, the charge and discharge current was 1A / g, and the test ended after 200 cycles.

[0134] II. Battery performance testing:

[0135] (1) Test method for the initial Coulombic efficiency (first efficiency):

[0136] After the battery is manufactured, it is first charged at a constant current of 0.1C to 3.75V to obtain the formation capacity C0. Then the battery is sorted, charged at a constant current of 0.33C to 4.2V, and then charged at a constant voltage of 4.2V until I≤0.05C to obtain the battery capacity C1 after sorting. After standing for 5 minutes, the battery is discharged at a constant current of 1C to 2.5V to obtain the discharge capacity C2, then discharged at a constant current of 0.1C to 1.0V to obtain the discharge capacity C3, and then discharged at a constant current of 0.01C to 2.5V to obtain the discharge capacity C4; the initial Coulomb efficiency is calculated as (C2 + C3 + C4) / (C0 + C1)×100%.

[0137] (2) 500-cycle capacity retention rate test method:

[0138] 1). Take the battery after sorting and charge it at a constant current of 1C to 4.2V, and then charge it at a constant voltage of 4.2V until I≤0.05C;

[0139] 2). After standing for 5 minutes, discharge the battery at a constant current of 1C to 1.0V to obtain the discharge capacity C1;

[0140] 3). Stand for 30 minutes.

[0141] 4). Repeat steps 1)-3) 499 times to obtain the discharge capacity C500; the 500-cycle capacity retention rate is calculated as (C500) / (C1)×100%.

[0142] (3) 50% SOC, 25°C, 2C, 10s DC internal resistance (DCR) test method:

[0143] 1). Constant current and constant voltage charge at 0.33C to 4.0V;

[0144] 2). Stand for 30 minutes;

[0145] 3). Constant current discharge at 0.33C for 90 minutes to adjust the state of charge to 50% SOC;

[0146] 4). Stand for 30 minutes;

[0147] 5). Discharge at a current of 2C for 10s and record the voltage before and after discharge.

[0148] DCR = (V 放电前 -V 放电末期 ) / current

[0149] Table 1

[0150]

[0151] Examples 1-5: The surface is in-situ coated with an electrochemically active compound, which improves the conductivity of the electrode. Since it is generated in-situ, the cerium dioxide coating layer is not easily detached during long-term cycling. This structure can reduce the electrolyte failure caused by the direct contact between the material surface and the electrolyte, reduce the polarization of the electrode material, increase the conductivity of the material, and increase the cycling stability at the same time.

[0152] Comparative Example 1: Since citric acid was not added, the resulting cathode material could not form a carbon layer, resulting in a decrease in conductivity.

[0153] Comparative Example 2: Since cerium ions were not introduced, a cerium dioxide coating layer could not be partially formed during sintering, reducing the conductivity of the electrode, unable to reduce the electrolyte failure caused by the direct contact between the material surface and the electrolyte, increasing the polarization of the electrode material, and decreasing the conductivity.

[0154] Comparative Example 3: Since manganese and cerium were not added, the Jahn-Teller effect was increased, resulting in a decrease in cycling performance and capacity.

[0155] Comparative Example 4: Since citric acid, manganese, and cerium were not added, the conductivity was poor, the cycling performance was poor, and the capacity could not be increased.

[0156] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a positive electrode material, characterized in that: The steps include: S1, mixing a vanadium source, a carbon source and a good solvent, and subjecting the mixture to a first heating and stirring treatment to obtain a solution A; mixing the solution A, an M source, a sodium source and a phosphate precursor, and subjecting the mixture to a second heating and stirring treatment to obtain a solution B; the M source comprises at least one of a manganese source, a copper source and a zinc source; Mixing a cerium source and a poor solvent to obtain a solution C; S2, using the anti-solvent method, mixing the solution B and the solution C, and subjecting the mixture to a third heating and stirring treatment to obtain a solution D; the solution D includes M ions, Ce 3+ Doped Na3V2(PO4)3 precursor and carbon source; S3, using a spray freeze-drying method, drying the solution D to obtain a precursor E; the precursor E includes M ions, Ce 3+ A doped Na3V2(PO4)3 precursor, and a carbon source coated on the surface thereof; S4, sintering the precursor E in an inert gas atmosphere to obtain a positive electrode material.

2. The preparation method according to claim 1, characterized in that: The vanadium source includes NH4VO3, V2O3, VO2, V2O4, V2O5, H4V2O 10 , V(OH)3, Na3VO4, Na4V2O7; And / or, the carbon source includes at least one of citric acid, glucose, asphalt, polydopamine, resorcinol, formaldehyde, starch, sucrose, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyvinyl pyrrolidone, tannic acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, anthracene, and aniline; And / or, the manganese source includes at least one of manganese acetate, manganese oxalate, manganese chloride, manganese nitrate, manganese sulfate, manganese bromide, manganese oxide, manganese hydroxide, manganese cyanide, and fatty acid manganese; And / or, the copper source includes at least one of copper acetate, copper oxalate, copper chloride, copper nitrate, copper sulfate, copper bromide, copper oxide, copper hydroxide, copper cyanide, and fatty acid copper; And / or, the zinc source includes at least one of zinc acetate, zinc oxalate, zinc chloride, zinc nitrate, zinc sulfate, zinc bromide, zinc oxide, zinc hydroxide, zinc cyanide, and fatty acid zinc; And / or, the sodium source includes at least one of sodium acetate, sodium oxalate, sodium citrate, and sodium carboxymethyl cellulose; and / or, the phosphate precursor includes at least one of diammonium phosphate, diammonium hydrogen phosphate, and ammonium phosphate; And / or, the cerium source includes at least one of cerium acetate, cerium oxalate, cerium nitrate, cerium sulfate, cerium oxide, and cerium hydroxide; and / or, the good solvent comprises water; And / or, the poor solvent includes at least one of N,N-dimethylformamide and N,N-dimethylacetamide.

3. The preparation method according to claim 1, characterized in that: The molar ratio of the vanadium source, the carbon source, the M source, the sodium source, the phosphate precursor, and the cerium source is (0.5-1.5):(1-5):(0.5-4):(1-5):(0.3-5):(0.05-0.15); and / or, the molar volume ratio of the vanadium source to the good solvent is 0.2 to 1 mol / L; And / or, the molar volume ratio of the cerium source to the poor solvent is 0.01 to 0.05 mol / L.

4. The preparation method according to claim 1, characterized in that: The temperature of the first heating and stirring treatment is 30 to 100° C., and the time of the first heating and stirring treatment is 0.1 to 6 hours; and / or, the temperature of the second heating and stirring treatment is 30 to 100° C., and the time of the second heating and stirring treatment is 0.1 to 6 hours; And / or, the temperature of the third heating and stirring treatment is 30-100° C., and the time of the third heating and stirring treatment is 0.1-6 hours; And / or, the drying temperature is 100-110°C; And / or, the sintering temperature is 300-900° C., and the sintering time is 0.1-16 hours.

5. A positive electrode material, characterized in that The positive electrode material comprises: (a) a matrix material, wherein the bulk phase of the matrix material is M ions, Ce 3+ Doped Na3V2(PO4)3; the M ions include Mn 2 + , Cu 2+ 、Zn 2+ At least one of; (b) a first coating layer, wherein the first coating layer is in-situ coated on the surface of the base material, and the first coating layer comprises cerium dioxide; (c) a second coating layer, wherein the second coating layer is in-situ coated on a surface of the first coating layer away from the base material, and the second coating layer comprises a carbon material.

6. The positive electrode material according to claim 5, characterized in that The chemical formula of the matrix material is Na 4-x M 1-x Ce x V(PO4)3, where 0.01≤X≤0.

2.

7. The positive electrode material according to claim 5, characterized in that The mass ratio of the first coating layer to the base material is (0.5-2):

100.

8. The positive electrode material according to claim 5, characterized in that The mass ratio of the second coating layer to the base material is (0.05-1):

100.

9. A positive electrode sheet, characterized in that: The positive electrode sheet comprises the positive electrode material prepared by the preparation method according to any one of claims 1 to 4, and / or the positive electrode material according to any one of claims 5 to 8.

10. A sodium ion battery, characterized in that: The sodium ion battery comprises the positive electrode sheet according to claim 9.

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