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

Abstract

The invention relates to a positive active material and a preparation method thereof, a positive pole piece, a sodium ion battery and a device, the positive active material comprises an active material body, the general formula of the active material body is Na4Fe3-x-yZrxMy (PO4) 2 (P2O7), M is at least one of Ti, Zn, V, Mn, Co and Al, x is more than or equal to 0.01 and less than or equal to 0.05, and y is more than or equal to 0.01 and less than or equal to 0.05. According to the positive electrode active material, through co-doping of Fe, Zr and M metal ions, lattice defects are increased, lattice distortion is reduced, and electronic conductivity is improved, so that the positive electrode active material shows excellent rate capability and cycling stability under the condition of high rate, and has the advantages of high capacity and high capacity retention rate.

Description

Technical Field

[0001] The present application relates to the technical field of electrode materials, and in particular to a positive electrode active material and a preparation method thereof, a positive electrode sheet, a sodium ion battery and a device. Background Art

[0002] Lithium-ion batteries are widely used in all aspects of daily life. However, the scarcity and uneven distribution of lithium resources have forced people to look for alternatives to lithium-ion batteries. Sodium-ion batteries have similar working principles to lithium-ion batteries and are low-cost and easy to obtain raw materials. They have received widespread attention in recent years. Among the many sodium-ion battery positive electrode materials, NACICON-type positive electrode materials have excellent cycle stability, but have poor rate performance. Therefore, how to provide a sodium-ion positive electrode material that can exhibit excellent rate performance under high rate conditions has become a technical problem that needs to be solved urgently. Summary of the invention

[0003] Based on this, it is necessary to provide a positive electrode active material and a preparation method thereof, a positive electrode plate, a sodium ion battery and a device that can improve the rate performance.

[0004] In one aspect of the present application, a positive electrode active material is provided, wherein the positive electrode active material comprises an active material body, wherein the general formula of the active material body is Na4Fe 3-x-y Zr x M y (PO4)2(P2O7), wherein M is at least one of Ti, Zn, V, Mn, Co and Al, 0.01≤x≤0.05, 0.01≤y≤0.05.

[0005] The above-mentioned positive electrode active material increases the ion diffusion channel during the charge and discharge process by co-doping with Fe, Zr and M metal ions, promotes carrier diffusion, and improves the electronic conductivity of the positive electrode active material; and the co-doping with Fe, Zr and M metal ions also increases lattice defects, reduces lattice distortion, and inhibits phase change, making the positive electrode active material not only more conducive to carrier embedding, but also during the embedding and extraction process, the crystal structure is not prone to irreversible phase change or collapse, thereby improving the cycle stability of the positive electrode active material, and ultimately making the positive electrode active material exhibit excellent rate performance and cycle stability under high rate conditions, and has the advantages of large capacity and high capacity retention rate.

[0006] In some embodiments, the positive electrode active material further includes a carbon coating layer, which is disposed on the outer surface of the active material body, and the coating amount of C is 3% to 5% of the mass of the positive electrode active material.

[0007] In some embodiments, M includes at least one of Ti and Al.

[0008] In some embodiments, the general formula of the active material body includes Na4Fe 2.95 Zr 0.02 Ti 0.03 (PO4)2(P2O7),Na4Fe 2.94 Zr 0.01 Zn 0.05 (PO4)2(P2O7) and Na4Fe 2.93 Zr 0.02 Co 0.05 At least one of (PO4)2(P2O7).

[0009] In some embodiments, the median particle size of the positive electrode active material is 3µm~7µm.

[0010] In some embodiments, the positive electrode active material is an orthorhombic system, Pn21a space group; and / or, the positive electrode active material is a block polycrystalline structure.

[0011] The second aspect of the present application provides a method for preparing the positive electrode active material as described in the first aspect, comprising the following steps:

[0012] Dissolving a sodium source, an iron source, a zirconium source, a metal M source and a phosphorus source with a complexing agent in a solvent according to the stoichiometric molar ratio of the positive electrode active material and blending them to obtain a mixed solution;

[0013] Drying the mixed solution to obtain a mixed powder;

[0014] The mixed powder is sintered to obtain the positive electrode active material.

[0015] In some embodiments, the method for preparing the positive electrode active material satisfies at least one of the following:

[0016] (1) The sodium source includes at least one of NaH2PO4 and CH3COONa;

[0017] (2) The iron source includes at least one of Fe(NO3)3, Fe(NO3)2 and FeC2O4;

[0018] (3) The zirconium source includes at least one of Zr(NO3)4 and Zr(CH3COO)4;

[0019] (4) the complexing agent comprises at least one of citric acid and glucose; and / or the molar ratio of the complexing agent to the metal ions in the positive electrode active material is 1.2 to 2:1;

[0020] (5) The metal M source is at least one of acetate, nitrate and oxide of metal M.

[0021] In some embodiments, the sintering temperature is 500°C to 700°C.

[0022] The third aspect of the present application provides a positive electrode slurry, comprising the positive electrode active material as described in the first aspect and the positive electrode active material prepared by the preparation method as described in the second aspect.

[0023] In some embodiments, the positive electrode slurry includes the positive electrode active material, a conductive agent, a binder, and a solvent.

[0024] In some embodiments, the positive electrode slurry satisfies at least one of the following conditions:

[0025] (1) The mass ratio of the positive electrode active material, the conductive agent and the binder is 8-9:0.25-1:0.25-1;

[0026] (2) The conductive agent includes at least one of Super P, graphene, carbon dots, CNT and Ketjen black;

[0027] (3) The binder includes at least one of difluoroethylene, polyvinylidene fluoride and tetrafluoroethylene-hexafluoropropylene copolymer;

[0028] (4) The solid content of the positive electrode slurry is 50% to 60%.

[0029] The fourth aspect of the present application provides a positive electrode plate, comprising a positive electrode collector and a positive electrode active layer, wherein the positive electrode active layer is arranged on at least one side of the positive electrode collector, and the positive electrode active layer comprises the positive electrode active material as described in the first aspect; or the positive electrode active layer is made of the positive electrode slurry as described in the third aspect.

[0030] In some embodiments, the positive electrode sheet satisfies at least one of the following conditions:

[0031] (1) The thickness of the positive electrode plate is 0.2 mm to 0.6 mm;

[0032] (2) The single-sided surface density of the positive electrode sheet is 110 g / m 2 ~160g / m 2 .

[0033] The fifth aspect of the present application provides a sodium ion battery, comprising the positive electrode sheet as described in the fourth aspect.

[0034] The sixth aspect of the present application provides an electrical device comprising the sodium ion battery as described in the fifth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is an electron microscope scan of the positive electrode active material prepared in Example 1.

[0036] Figure 2 The charge and discharge curves of a half-cell assembled with sodium metal as the positive electrode active material of Example 1 at 0.2C.

[0037] Figure 3 The charge and discharge curves of the sodium ion battery cell using the positive electrode active material of Example 1 at 2C. DETAILED DESCRIPTION

[0038] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present application are given in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0040] NASICON type positive electrode active material is a polyanion compound with broad application prospects in sodium ion batteries. Currently, the common NASICON type positive electrode active materials are , , , and These NASICON-type positive electrode active materials have good cycle stability, but have poor rate performance due to poor electronic conductivity.

[0041] Based on this, in the first aspect of the present application, an embodiment provides a positive electrode active material, wherein the positive electrode active material comprises an active material body, and the general formula of the active material body is Na4Fe 3-x-y Zr x M y (PO4)2(P2O7), wherein M is at least one of Ti, Zn, V, Mn, Co and Al, 0.01≤x≤0.05, 0.01≤y≤0.05.

[0042] The above-mentioned positive electrode active material increases the ion diffusion channel during the charge and discharge process by co-doping with Fe, Zr and M metal ions, promotes carrier diffusion, and improves the electronic conductivity of the positive electrode active material; and the co-doping with Fe, Zr and M metal ions also increases lattice defects, reduces lattice distortion, and inhibits phase change, making the positive electrode active material not only more conducive to carrier embedding, but also during the embedding and extraction process, the crystal structure is not prone to irreversible phase change or collapse, thereby improving the cycle stability of the positive electrode active material, and ultimately making the positive electrode active material exhibit excellent rate performance and cycle stability under high rate conditions, and has the advantages of large capacity and high capacity retention rate.

[0043] The positive electrode active material of the present application has a capacity of 90 mAh / g to 110 mAh / g for a metal sodium half-cell in a voltage window of 2V to 4V; at a current density of 2C, the capacity retention rate after 1000 cycles is 90% to 96%, and the lattice distortion during charge and discharge is 7% to 12%.

[0044] As an example, x can be 0.01, 0.02, 0.03, 0.04 and 0.05, or can be within a range consisting of any two of the above values ​​as end values. Preferably, x is 0.01 to 0.03.

[0045] As an example, y can be 0.01, 0.02, 0.03, 0.04 and 0.05, or can be within a range consisting of any two of the above values ​​as end values. Preferably, y is 0.03 to 0.05.

[0046] In some embodiments, the positive electrode active material further includes a carbon coating layer, which is disposed on the outer surface of the active material body, and the coating amount of C is 3% to 5% of the mass of the positive electrode active material. Carbon coating is used to improve the conductivity and structural stability of the positive electrode active material, thereby further improving the electronic conductivity of the positive electrode active material.

[0047] Understandably, the positive electrode active material includes Na4Fe 3-x-y Zr x M y (PO4)2(P2O7) and a carbon conductive layer coated on its outer surface; the carbon coating layer can cover part of the outer surface of the active material body, or can cover the entire outer surface of the active material body.

[0048] In some embodiments, M includes at least one of Ti, Zn, Co, and Al.

[0049] In some embodiments, M includes at least one of Ti and Al.

[0050] When the M metal is Ti, the co-doping of Fe, Zr and Ti can jointly change the band structure of the material. The doping of Fe and Ti mainly adjusts the band gap by changing the electron cloud distribution and oxidation state of the atoms, and the defect energy level formed by the doping of Zr will also intervene. The three metal ions work together to optimize the band structure of the material and form a band distribution that is more conducive to electronic conduction, thereby reducing the energy barrier of electronic transition and further improving electronic conductivity. In addition, the co-doping of Fe, Zr and Ti can also optimize the crystal structure, reduce lattice distortion, improve the structural stability of the positive electrode active material, and inhibit side reactions.

[0051] When the M metal is Al, the different valence changes of Fe can introduce some localized electronic states into the energy band structure of the material. The doping of Zr will form defect energy levels, and the doping of Al will also produce similar defect energy levels due to the differences in atomic size and electronic structure. These impurity energy levels introduced by the three ions of Fe, Zr and Al interact with each other to form a denser and more continuous electron transmission network inside the material, allowing electrons to jump and conduct more smoothly in the material, thereby improving electronic conductivity. Moreover, the co-doping of Fe, Zr and Al can also optimize the crystal structure, reduce lattice distortion, and reduce the resistance to ion migration in the positive electrode active material, improving the ion transmission environment.

[0052] Furthermore, M includes Ti and Al. Co-doping of Fe, Zr, Ti and Al can further reduce the energy barrier of electron transition and the resistance of ion migration, thereby further improving the electronic conductivity of the positive electrode active material.

[0053] In some embodiments, the general formula of the active material body includes Na4Fe 2.95 Zr 0.02 Ti 0.03 (PO4)2(P2O7),Na4Fe 2.94 Zr 0.01 Zn 0.05 (PO4)2(P2O7) and Na4Fe 2.93 Zr 0.02 Co 0.05 (PO4)2(P2O7),Na4Fe 2.95 Zr 0.02 Mn 0.03 (PO4)2(P2O7),Na4Fe 2.95 Zr 0.02 Ti 0.02 Al 0.01 (PO4)2(P2O7),Na4Fe 2.94 Zr 0.01 Zn 0.02 Ti 0.03(PO4)2(P2O7),Na4Fe 2.95 Zr 0.01 Zn 0.01 Mn 0.03 (PO4)2(P2O7) 、Na4Fe 2.93 Zr 0.02 Zn 0.03 Co 0.02 At least one of (PO4)2(P2O7).

[0054] In some embodiments, the general formula of the active material body includes Na4Fe 2.95 Zr 0.02 Ti 0.03 (PO4)2(P2O7),Na4Fe 2.94 Zr 0.01 Zn 0.05 (PO4)2(P2O7) and Na4Fe 2.93 Zr 0.02 Co 0.05 At least one of (PO4)2(P2O7).

[0055] In some embodiments, the general formula of the positive electrode active material includes Na4Fe 2.95 Zr 0.02 Ti 0.03 (PO4)2(P2O7)@C、Na4Fe 2.94 Zr 0.01 Zn 0.05 (PO4)2(P2O7)@C and Na4Fe 2.93 Zr 0.02 Co 0.05 (PO4)2(P2O7)@C、Na4Fe 2.95 Zr 0.02 Mn 0.03 (PO4)2(P2O7)@C、Na4Fe 2.95 Zr 0.02 Ti 0.02 Al 0.01 (PO4)2(P2O7)@C、Na4Fe 2.94 Zr 0.01 Zn 0.02 Ti 0.03 (PO4)2(P2O7) @C、Na4Fe 2.95 Zr 0.01 Zn 0.01 Mn 0.03 (PO4)2(P2O7)@C、Na4Fe 2.93 Zr 0.02 Zn 0.03 Co0.02 At least one of (PO4)2(P2O7) @C.

[0056] In some embodiments, the median particle size of the positive electrode active material is 3µm to 7µm. At this median particle size, the diffusion path of sodium ions in the active positive electrode material can be shortened, the diffusion speed can be accelerated, and the battery charging and discharging efficiency can be higher; and at this median particle size, the volume change of the positive electrode active material during the charging and discharging process is relatively uniform, which can reduce particle breakage, pulverization and structural damage caused by volume expansion and contraction, thereby further improving rate performance and cycle stability.

[0057] As an example, the median particle size of the positive electrode active material may be 3µm, 3.5µm, 4µm, 4.5µm, 5µm, 5.5µm, 6µm, 6.5µm, 7µm, or may be within a range consisting of any two of the above point values ​​as end values. The median particle size of the positive electrode active material is preferably 6µm to 7µm.

[0058] In some embodiments, the positive electrode active material is an orthorhombic system, Pn21a space group.

[0059] In some embodiments, the positive electrode active material is in a bulk polycrystalline structure.

[0060] The second aspect of the present application provides a method for preparing the positive electrode active material as described in the first aspect, comprising the following steps:

[0061] Dissolving a sodium source, an iron source, a zirconium source, a metal M source and a phosphorus source with a complexing agent in a solvent according to the stoichiometric molar ratio of the positive electrode active material and blending them to obtain a mixed solution;

[0062] The mixed liquid is dried to obtain mixed powder.

[0063] The mixed powder is sintered to obtain the positive electrode active material.

[0064] In some embodiments, sodium source, iron source, zirconium source, metal M source, phosphorus source and carbon source are dissolved in a solvent and blended with a complexing agent according to the stoichiometric molar ratio of the positive electrode active material to obtain a mixed solution. That is, a carbon coating layer is coated on the surface of the active material body.

[0065] Furthermore, the carbon source includes at least one of citric acid and sucrose.

[0066] In some embodiments, the sodium source includes at least one of NaH2PO4 and CH3COONa.

[0067] In some embodiments, the iron source includes at least one of Fe(NO3)3, Fe(NO3)2 and FeC2O4.

[0068] In some embodiments, the zirconium source includes at least one of Zr(NO3)4 and Zr(CH3COO)4.

[0069] In some embodiments, the complexing agent includes at least one of citric acid and glucose.

[0070] In some embodiments, the molar ratio of the complexing agent to the metal ions in the positive electrode active material is 1.2 to 2:1.

[0071] In some embodiments, the metal M source is at least one of an acetate, a nitrate and an oxide containing metal M.

[0072] In some embodiments, the phosphorus source includes phosphate.

[0073] Furthermore, the phosphorus source is sodium phosphate.

[0074] In some embodiments, the solvent is deionized water or ethanol.

[0075] In some embodiments, the drying process is performed by spray drying using a spray dryer.

[0076] In some embodiments, the sintering treatment is high-temperature sintering at a temperature of 500° C. to 700° C. under protective gas conditions.

[0077] The third aspect of the present application provides a positive electrode slurry, comprising the positive electrode active material as described in the first aspect and the positive electrode active material prepared by the preparation method as described in the second aspect.

[0078] In some embodiments, the positive electrode slurry includes the positive electrode active material, a conductive agent, a binder, and a solvent.

[0079] In some embodiments, the mass ratio of the positive electrode active material, the conductive agent and the binder is 8-9: 0.25-1: 0.25-1. At this ratio, the electronic conductivity and structural stability of the positive electrode active material are further improved.

[0080] The mass ratio of the positive electrode active material, the conductive agent and the binder is preferably 8.5-9:0.25-0.6:0.25-0.6.

[0081] In some embodiments, the conductive agent includes at least one of Super P, graphene, carbon dots, CNTs and Ketjen black.

[0082] In some embodiments, the binder includes at least one of difluoroethylene, polyvinylidene fluoride and tetrafluoroethylene-hexafluoropropylene copolymer.

[0083] In some embodiments, the solid content of the positive electrode slurry is 50% to 60%.

[0084] The present application also provides a conductive coating, the components of which include the positive electrode active material, conductive agent and binder as described in the first aspect; or, the conductive coating is made of the positive electrode slurry as described in the third aspect.

[0085] The fourth aspect of the present application provides a positive electrode plate, comprising a positive electrode collector and a positive electrode active layer, wherein the positive electrode active layer is arranged on at least one side of the positive electrode collector, and the positive electrode active layer comprises the positive electrode active material as described in the first aspect; or the positive electrode active layer is made of the positive electrode slurry as described in the third aspect.

[0086] In some of the embodiments, the positive electrode sheet is prepared by uniformly coating the positive electrode slurry on at least one side of the positive electrode current collector, and drying the positive electrode slurry to form a positive electrode active layer to prepare the positive electrode sheet.

[0087] In some embodiments, the positive electrode current collector is aluminum foil.

[0088] In some embodiments, the thickness of the positive electrode plate is 0.2 mm to 0.6 mm. The thickness of the positive electrode plate is the overall thickness of the positive electrode active layer and the positive electrode current collector. Preferably, it is 0.2 mm to 0.4 mm.

[0089] In some embodiments, the single-sided surface density of the positive electrode active layer is 110 g / m 2 ~160g / m 2 .

[0090] The fifth aspect of the present application provides a sodium ion battery, comprising the positive electrode sheet as described in the fourth aspect.

[0091] The above-mentioned positive electrode active material increases the ion diffusion channel during the charge and discharge process by co-doping with Fe, Zr and M metal ions, promotes the diffusion of sodium ions, and improves the electronic conductivity of the positive electrode active material; and the co-doping with Fe, Zr and M metal ions also increases lattice defects, reduces lattice distortion, and inhibits phase change, so that the positive electrode active material is not only more conducive to the embedding of sodium ions, but also the crystal structure is not prone to irreversible phase change or collapse during the embedding and extraction of sodium ions, thereby improving the cycle stability of the positive electrode active material; at the same time, carbon coating is used to improve the conductivity and structural stability of the positive electrode active material, thereby further improving the electronic conductivity of the positive electrode active material, and ultimately the sodium ion battery exhibits excellent rate performance and cycle stability under high rate conditions, and has the advantages of large capacity and high capacity retention rate.

[0092] In some embodiments, the sodium ion battery includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the positive electrode sheet and the negative electrode sheet are arranged opposite to each other, and the separator is located between the positive electrode sheet and the negative electrode sheet.

[0093] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode active layer, and the negative electrode active layer is disposed on at least one side of the negative electrode current collector.

[0094] In some embodiments, the negative electrode current collector is an aluminum-plastic film.

[0095] The preparation process of the negative electrode sheet is to uniformly apply the negative electrode slurry to at least one side of the negative electrode collector, form a negative electrode active layer after drying, and obtain the negative electrode sheet.

[0096] In some embodiments, the negative electrode active layer includes a negative electrode active material, a conductive agent, a binder, and a thickener.

[0097] In some embodiments, the negative electrode active material is a carbon-based active material.

[0098] Furthermore, the negative electrode active material is hard carbon.

[0099] In some embodiments, the conductive agent is at least one of conductive carbon black, CNT, carbon dots, graphene and Ketjen black.

[0100] In some embodiments, the binder is at least one of divinylidene fluoride, polyvinylidene fluoride and styrene-butadiene rubber.

[0101] In some embodiments, the thickening agent is carboxymethyl cellulose.

[0102] In some of the embodiments, in the negative electrode slurry, the mass ratio of the negative electrode active material, the conductive agent, the binder and the thickener is 8.5~9.5:0.25~0.4:0.1~0.4:0.1~0.4.

[0103] In some embodiments, the solid content of the negative electrode slurry is 30% to 50%.

[0104] In some embodiments, the single surface density of the negative electrode active layer is 40 g / m 2 ~80g / m 2 . Select a suitable negative electrode surface density so that the N / P ratio of the battery cell is 1:1.05~1.09.

[0105] In some of the embodiments, the diaphragm is at least one of a glass fiber diaphragm, a PE diaphragm, a PP diaphragm, a PP diaphragm coated with alumina on one side, and a PP diaphragm coated with alumina on both sides.

[0106] In some of the embodiments, the electrolyte is a 1 M ~ 1.5 M NaPF6 or NaClO4 solution.

[0107] In some embodiments, the solvent used in the electrolyte includes at least one of EC, DEC, DMC, EMC and DME.

[0108] In some of the embodiments, FEC with a mass content of 2% to 5% is added to the electrolyte as an electrolyte additive.

[0109] The sixth aspect of the present application provides an electrical device comprising the sodium ion battery as described in the sixth aspect.

[0110] Electrical devices include but are not limited to watches, wireless headphones, digital products, smart cards, remote controls, auto parts and electronic products.

[0111] The following are specific embodiments.

[0112] Example 1

[0113] Example 1 provides a positive electrode active material, the general formula of which is Na4Fe 2.95 Zr 0.02 Ti 0.03 (PO4)2(P2O7)@C, i.e., active material Na4Fe 2.95 Zr 0.02 Ti 0.03 The outer surface of (PO4)2(P2O7) is provided with a carbon coating.

[0114] The preparation method of the positive electrode active material of Example 1 is as follows:

[0115] Sodium phosphate, ferric nitrate, zirconium oxide and titanium oxide were weighed and dissolved in deionized water according to the stoichiometric ratio to prepare a suspension. Citric acid was used as both a carbon source and a complexing agent. The molar ratio of citric acid to metal ions was 1.55:1. Citric acid was added to the suspension and stirred for 0.5 h. The suspension was transferred to a spray dryer for spray drying. The spray drying inlet temperature was 110-120 °C and the outlet temperature was 80-90 °C. A yellow powder was obtained. After collection and grinding, it was calcined at 550 °C for 10 h in a mixed atmosphere of hydrogen and argon (a mixture of 5% by mass hydrogen and 95% by mass argon) to obtain the positive electrode active material Na4Fe 2.95 Zr 0.02 Ti 0.03 (PO4)2(P2O7)@C, the positive electrode active material is in the form of black powder with a median particle size of 5μm.

[0116] The preparation method of the positive electrode sheet of Example 1 is as follows:

[0117] The positive electrode active material, the conductive agent Super P and the binder PVDF (polyvinylidene fluoride) were weighed in a mass ratio of 8:1:1, wherein PVDF was prepared in advance into a PVDF NMP solution (N-methylpyrrolidone) with a mass fraction of 4%. After the positive electrode active material, Super P and PVDF were mixed, NMP was added to make the solid content of the slurry 45% to prepare the positive electrode slurry. The positive electrode slurry was evenly coated on the aluminum foil with a surface density of 115g / m 2 , and the positive electrode sheet is obtained after drying and cold pressing.

[0118] Example 2

[0119] Example 2 provides a positive electrode active material, the general formula of which is Na4Fe 2.94 Zr 0.01 Zn 0.05 (PO4)2(P2O7)@C, i.e., active material Na4Fe 2.94 Zr 0.01 Zn 0.05 The outer surface of (PO4)2(P2O7) is provided with a carbon coating.

[0120] The preparation method of the positive electrode active material of Example 2 is basically the same as that of Example 1, except that zinc acetate is used as the zinc source, sucrose is used as the carbon source, the amount of sucrose added is 3% of the theoretical mass of the positive electrode active material, and the molar ratio of citric acid to metal ions is 1.2:1.

[0121] The positive electrode active material is in the form of black powder with a median particle size of 7 μm.

[0122] The preparation method of the positive electrode sheet of Example 2 is basically the same as that of Example 1, except that the mass ratio of the positive electrode active material, the conductive agent and the binder is 9:0.5:0.5, and the conductive agent is Ketjen black.

[0123] Example 3

[0124] Example 3 provides a positive electrode active material, the general formula of which is Na4Fe 2.93 Zr 0.02 Co 0.05 (PO4)2(P2O7)@C, i.e., active material Na4Fe 2.93 Zr 0.02 Co 0.05 The outer surface of (PO4)2(P2O7)@C is provided with a carbon coating layer.

[0125] The preparation method of the positive electrode active material of Example 3 is basically the same as that of Example 1, except that the cobalt source is cobalt acetate and the calcination temperature is 600°C.

[0126] The positive electrode active material is in the form of black powder with a median particle size of 5 μm.

[0127] The preparation method of the positive electrode sheet of Example 3 is basically the same as that of Example 1, except that the mass ratio of the positive electrode active material to the conductive agent and the binder is 8.5:0.25:0.5.

[0128] Example 4

[0129] Example 4 provides a positive electrode active material, the general formula of which is Na4Fe 2.95 Zr 0.02 Ti 0.02 Al 0.01 (PO4)2(P2O7)@C.

[0130] The preparation method of the positive electrode active material of Example 4 is substantially the same as that of Example 1, except that aluminum nitrate is added as an aluminum source, the calcination temperature is increased to 600° C., and the calcination time is 10 h.

[0131] The positive electrode active material is in the form of black powder with a median particle size of 6.5 μm.

[0132] The preparation method of the positive electrode sheet of Example 4 is basically the same as that of Example 1, except that the solid content of the slurry is adjusted to 50%, and the single-side surface density of the electrode sheet is adjusted to 120 g / m 2

[0133] Comparative Example 1

[0134] Comparative Example 1 is substantially the same as Example 1, except that the zirconium source, titanium source and carbon source are omitted during the preparation of the positive electrode active material, and the prepared positive electrode active material is Na4Fe4 (PO4)2 (P2O7).

[0135] Comparative Example 2

[0136] Comparative Example 2 is substantially the same as Example 1, except that the zirconium source is omitted during the preparation of the positive electrode active material.

[0137] Comparative Example 3

[0138] Comparative Example 3 is substantially the same as Example 1, except that the titanium source is omitted during the preparation of the positive electrode active material.

[0139] The positive electrode sheets made of the positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 3 were assembled with the metal sodium negative electrode into 2032 button half-cells, and the electrochemical performance was tested. The positive electrode sheets of Examples 1 to 4 and Comparative Examples 1 to 3 were made into sodium ion battery cells according to the following method, and the electrochemical performance was tested:

[0140] The negative electrode active material hard carbon, conductive agent Super P, binder SBR and thickener CMC were mixed in a mass ratio of 9.4:0.2:0.1:0.1 to prepare a slurry with a solid content of 55%. After stirring evenly, the slurry was evenly applied on the aluminum foil with a single-sided surface density of 55g / m 2 , after drying and roller pressing, a negative electrode sheet is obtained;

[0141] The positive electrode sheets of Examples 1 to 4 and Comparative Examples 1 to 3, the negative electrode sheets and the PP diaphragm (polypropylene diaphragm) are stacked and wound in a conventional manner to pack batteries, and the injection operation is performed in a glove box with a water and oxygen content of less than 0.5 ppm, and an electrolyte 1M NaPF6 EC:DEC=1:1 is added, that is, the concentration of the electrolyte salt in the mixed solvent of EC and DEC is 1M, and the volume ratio of EC and DEC is 1:1, and a sodium ion battery cell is obtained after encapsulation.

[0142] The test results are shown in Table 1. The test conditions for each performance test item are relative humidity ≤ 80% and relative atmospheric pressure 86KPa~106KPa.

[0143] Table 1

[0144]

[0145] As can be seen from Table 1 above, the sodium ion battery cell using Na4Fe4 (PO4)2 (P2O7) as the positive electrode active material in Comparative Example 1 has a capacity retention rate of 20% at 500 cycles and a 2C capacity of 963 mAh. The positive electrode active materials in Examples 1 to 4 utilize the synergy of multiple metal elements and the carbon coating effect, so that the capacity retention rate of the sodium ion battery cell at 500 cycles can reach 88%, and the 2C capacity can reach 1030 mAh. Examples 1 to 4 show a rate performance that is significantly better than that of Comparative Examples 1 to 3. It can be seen from Comparative Examples 1 to 3 that the lack of any of the factors of Zr and metal M cannot achieve the technical effect of the present application.

[0146] It can be seen from Examples 1 and 4 that when metal M includes Zn or metal M includes Ti and Al, the rate performance of the sodium ion battery is particularly outstanding. It can be seen from Example 2 that when the mass ratio of the positive electrode active material, the conductive agent and the binder is 8.5~9:0.25~0.6:0.25~0.6, the electrical performance of the sodium ion battery is better. It can be seen from the examples that when the median particle size of the positive electrode active material is 6µm~7µm, the electronic conductivity of the positive electrode active material is higher, and the rate performance is better under high rate conditions.

[0147] Among them, the electron microscope scanning image of the positive electrode active material of Example 1 is as follows Figure 1 As shown, its morphology is a blocky polycrystalline structure with a grain size of about 5 μm. The charge and discharge curve of the metal sodium assembled half-cell of the positive electrode active material of Example 1 at 0.2C is shown in Figure 2 As shown, the first cycle discharge capacity is about 102 mAh / g, the first cycle coulomb efficiency is 85%, and the charge and discharge curve of the sodium ion battery cell of the positive electrode active material of Example 1 at 2C is as follows Figure 3 As shown in the figure, the discharge capacity is 95 mAh / g at 0.2C, and the capacity retention rate is 93% compared with 0.2C.

[0148] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0149] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A positive electrode active material, characterized in that: The positive electrode active material includes an active material body, and the general formula of the active material body is Na4Fe 3-x-y Zr x M y (PO4)2(P2O7), wherein M is at least one of Ti, Zn, V, Mn, Co and Al, 0.01≤x≤0.05, 0.01≤y≤0.

05.

2. The positive electrode active material according to claim 1, characterized in that The positive electrode active material further includes a carbon coating layer, which is disposed on the outer surface of the active material body, and the coating amount of C is 3% to 5% of the mass of the positive electrode active material.

3. The positive electrode active material according to claim 1, characterized in that M includes at least one of Ti and Al.

4. The positive electrode active material according to claim 1, characterized in that The general formula of the active material body includes Na4Fe 2.95 Zr 0.02 Ti 0.03 (PO4)2(P2O7),Na4Fe 2.94 Zr 0.01 Zn 0.05 (PO4)2(P2O7) and Na4Fe 2.93 Zr 0.02 Co 0.05 At least one of (PO4)2(P2O7).

5. The positive electrode active material according to any one of claims 1 to 4, characterized in that The median particle size of the positive electrode active material is 3µm~7µm.

6. The positive electrode active material according to any one of claims 1 to 4, characterized in that The positive electrode active material is of orthorhombic system and Pn21a space group; and / or, the positive electrode active material is of blocky polycrystalline structure.

7. The method for preparing a positive electrode active material according to any one of claims 1 to 6, characterized in that: The following steps are involved: Dissolving a sodium source, an iron source, a zirconium source, a metal M source and a phosphorus source with a complexing agent in a solvent according to the stoichiometric molar ratio of the positive electrode active material and blending them to obtain a mixed solution; Drying the mixed solution to obtain a mixed powder; The mixed powder is sintered to obtain the positive electrode active material.

8. The method for preparing a positive electrode active material according to claim 7, characterized in that: At least one of the following is met: (1) The sodium source includes at least one of NaH2PO4 and CH3COONa; (2) The iron source includes at least one of Fe(NO3)3, Fe(NO3)2 and FeC2O4; (3) The zirconium source includes at least one of Zr(NO3)4 and Zr(CH3COO)4; (4) the complexing agent comprises at least one of citric acid and glucose; and / or the molar ratio of the complexing agent to the metal ions in the positive electrode active material is 1.2 to 2:1; (5) The metal M source is at least one of acetate, nitrate and oxide of metal M.

9. The method for preparing a positive electrode active material according to claim 7, characterized in that: The sintering temperature is 500°C to 700°C.

10. A positive electrode slurry, characterized in that: The invention comprises the positive electrode active material as claimed in any one of claims 1 to 6 and the positive electrode active material prepared by the preparation method as claimed in any one of claims 7 to 9.

11. The positive electrode slurry according to claim 10, characterized in that: The positive electrode slurry includes the positive electrode active material, a conductive agent, a binder and a solvent.

12. The positive electrode slurry according to claim 11, characterized in that At least one of the following conditions is met: (1) The mass ratio of the positive electrode active material, the conductive agent and the binder is 8-9:0.25-1:0.25-1; (2) The conductive agent includes at least one of Super P, graphene, carbon dots, CNT and Ketjen black; (3) The binder includes at least one of difluoroethylene, polyvinylidene fluoride and tetrafluoroethylene-hexafluoropropylene copolymer; (4) The solid content of the positive electrode slurry is 25% to 50%.

13. A positive electrode plate, characterized in that: The invention comprises a positive electrode current collector and a positive electrode active layer, wherein the positive electrode active layer is arranged on at least one side of the positive electrode current collector, and the positive electrode active layer comprises the positive electrode active material according to any one of claims 1 to 6; or the positive electrode active layer is made of the positive electrode slurry according to any one of claims 10 to 12.

14. The positive electrode sheet according to claim 13, characterized in that: At least one of the following conditions is met: (1) The thickness of the positive electrode plate is 0.2 mm to 0.6 mm; (2) The single-sided surface density of the positive electrode sheet is 110 g / m 2 ~160g / m 2 .

15. A sodium ion battery, characterized in that: Comprising the positive electrode sheet as described in any one of claims 13 to 14.

16. An electrical device, characterized in that: Comprising the sodium ion battery as claimed in claim 15.

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