A positive electrode material, a positive electrode sheet, a sodium-ion battery and an electric device

Abstract

The application provides a positive electrode material, a positive electrode sheet, a battery and an electric device, and relates to the technical field of batteries.The structure of the positive electrode material is a core-shell structure, the inner core of which is a layered oxide and the outer shell layer of which is a polyanion material.The layered oxide is a polycrystalline particle.The size of the sodium iron pyrophosphate phosphate crystal in at least one dimension is 2-20 nm, and the size of the sodium iron pyrophosphate phosphate crystal in at least one dimension is 500 nm-1 mu m.The positive electrode material provided by the application is coated on the surface of the layered oxide particle by using a polyanion material which can intercalate into the crystal gap of the polycrystalline particle, so as to inhibit the lattice distortion of the O3 phase layered oxide, and meanwhile, the charge and discharge capacity and the rate performance of the layered oxide are optimized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode material, a positive electrode sheet, a sodium-ion battery and an electric device. BACKGROUND

[0002] Layered oxides have become one of the current research and development mainstreams of positive electrode materials of sodium-ion batteries due to their high guest capacity and high compaction density. However, due to the large ionic radius of sodium ions, expansion is prone to occur in the adsorption and desorption cycle process, which leads to the decrease of the cycle performance and the service life of the layered oxides. Therefore, the prior art proposes to use a coating layer to inhibit the expansion of the layered oxides, so as to improve the cycle performance of the layered oxides. However, the conventional coating layer, such as a carbon coating layer, has an adverse effect on the charge and discharge capacity of the layered oxides. SUMMARY

[0003] The present application aims to provide a positive electrode material, a positive electrode sheet, a battery and an electric device, which can improve the cycle stability of the layered oxide positive electrode material while maintaining its good ionic conductivity and charge and discharge capacity.

[0004] One of the purposes of the present application is also to provide a positive electrode sheet.

[0005] Another purpose of the present application is also to provide a sodium-ion battery.

[0006] One of the purposes of the present application is also to provide an electric device.

[0007] In a first aspect, the present application provides a positive electrode material, comprising:

[0008] a core, a secondary particle of a layered oxide;

[0009] an outer shell layer formed of sodium iron pyrophosphite phosphate crystals; the size of the sodium iron pyrophosphite phosphate crystals in at least one dimension is 2 nm to 20 nm, and the size of the sodium iron pyrophosphite phosphate crystals in at least one dimension is 200 nm to 1 μm.

[0010] Further, in some embodiments of the present application, the sodium iron pyrophosphite phosphate is partially intercalated into the core.

[0011] Further, in some embodiments of the present application, the volume expansion of the positive electrode material after full charge or full discharge or 2000 cycles is not higher than 0.5%.

[0012] Further, in some embodiments of the present application, the ionic conductivity of the positive electrode material is not less than 1.0 x 10 -4 S / cm.

[0013] Further, in some embodiments of the present application, the sodium iron pyrophosphate crystal is a sodium iron pyrophosphate whisker or a needle-shaped sodium iron pyrophosphate crystal or a flaky sodium iron pyrophosphate crystal.

[0014] Further, in some embodiments of the present application, the sodium iron pyrophosphate accounts for 1% to 20% of the positive electrode material in terms of mass fraction.

[0015] Further, in some embodiments of the present application, the size of the core is 5 μm to 40 μm, and the thickness of the shell layer is 200 nm to 1 μm.

[0016] Further, in some embodiments of the present application, the chemical structure of the layered oxide is: Na x Ni y Fe z Mn a M (1-y-z-a) O2;

[0017] wherein, the value range of x is 0.9 to 1.0, the value range of y is 0.1 to 0.5, the value range of z is 0.1 to 0.5, and the value range of a is 0.1 to 0.5.

[0018] In a second aspect, the present application further provides a positive electrode sheet, comprising the positive electrode material of the first aspect.

[0019] In a third aspect, the present application further provides a sodium ion battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator; the separator and the electrolyte are arranged between the positive electrode sheet and the negative electrode sheet.

[0020] The positive electrode sheet is the positive electrode sheet of the second aspect.

[0021] In a fourth aspect, the present application further provides a battery module, comprising the sodium ion battery of the third aspect or comprising the positive electrode sheet of the second aspect or the positive electrode material of the first aspect.

[0022] The application provides a positive electrode material, which is a composite positive electrode material in a core-shell structure, wherein a polycrystal structure of a layered oxide is used as an inner core, and a poly-anion material of needle-shaped or sheet-shaped structure of sodium iron pyrophosphate phosphate is used as an outer shell layer. The needle-shaped or sheet-shaped structure is partially intercalated into the crystal gap between the polycrystal structure of the inner core in the coating process, so as to inhibit the lattice strain of the layered oxide and improve the cycle stability of the positive electrode material. In addition, the positive electrode material provided by the application after being coated with the poly-anion material of the specific morphology and specific material not only does not decrease in the discharge capacity, but also shows more excellent discharge capacity, which can reach 184 mAh / g or even higher, and the ionic conductivity is also better. It can be seen that the positive electrode material provided by the application not only inhibits the lattice distortion of the sodium ion layered oxide of O3 phase, but also has good charge and discharge capacity and ionic conductivity, and in addition, has good capacity retention rate at high rate, which provides support for the application of the positive electrode material of the sodium ion fast-charging battery. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the application, the drawings needed to be used in the description of the specific embodiments or prior art will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0024] Figure 1 The scanning electron microscope image of the positive electrode material provided in Example 1 of the application;

[0025] Figure 2 The scanning electron microscope image of the positive electrode material provided in Example 1 of the application;

[0026] Figure 3 The interface electron microscope image of the positive electrode material provided in Example 1 of the application;

[0027] Figure 4 The XRD image of the positive electrode material provided in Example 1 of the application. DETAILED DESCRIPTION

[0028] The technical solutions of the application will be described in detail below with reference to the embodiments. Obviously, the described embodiments are some of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0029] In the description of the present application, it needs to be understood that the terms "first", "second", are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0030] The following disclosure provides many different embodiments or examples for implementing different structures of the present application. In order to simplify the disclosure of the present application, the components and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present application. In addition, the present application can repeatedly refer to numbers and / or letters in different examples, and such repetition is for the purpose of simplification and clarity, which itself does not indicate the relationship between the various embodiments and / or settings discussed. In addition, the present application provides examples of various specific processes and materials, but those skilled in the art can realize the application of other processes and / or the use of other materials.

[0031] The defect of poor cycle performance of O3 phase in sodium ion positive electrode material is mainly due to the fact that the size of sodium ion is larger than that of lithium ion, which can easily cause the expansion of layered oxide during continuous adsorption and desorption, and then destroy the structure of layered oxide. To this end, the main solution of the prior art is to use a coating layer to inhibit the expansion of layered oxide, thereby improving the stability of layered oxide during the cycle process. However, due to the poor conductivity of the coating material to sodium ions, the charge and discharge capacity of the layered oxide is usually affected. To this end, the applicant proposes a new positive electrode material, which uses a specific structure of sodium iron pyrophosphate phosphate to coat the secondary particles of O3 phase layered oxide, and uses the specific structure of the coating to partially insert between the crystal interfaces of the secondary particles of O3 phase layered oxide during coating, so that the obtained positive electrode material not only has good cycle performance, but also has optimized charge and discharge capacity and rate performance, which improves the defect that the material coating layer in the prior art is difficult to simultaneously have optimized cycle performance, improved charge and discharge capacity and optimized rate performance. Specifically, the positive electrode material provided by the present application comprises:

[0032] The core is a secondary particle of layered oxide;

[0033] The shell layer is formed by sodium iron pyrophosphate phosphate crystals; the size of the sodium iron pyrophosphate phosphate crystals in at least one dimension is 2-20 nm, and the size of the sodium iron pyrophosphate phosphate crystals in at least one dimension is 200 nm-1 μm.

[0034] It should be noted that the "secondary particles" in the present application are polycrystalline particles, i.e. the inner core of the positive electrode material provided in the present application is a polycrystalline material of layered oxide, and there is an interface between each crystal thereof. The shell layer is a continuous or discontinuous shell layer grown on the surface of the inner core. It is preferably a continuous shell layer. It should be noted that the continuous shell layer in the present application refers to a layer structure in which the sodium iron pyrophosphate crystal particles are not island-shaped distributed, but are in a net-shaped shell or mutually connected sheet-shaped shell or form a complete spherical or spherical shell. It should be noted that the crystal size of the layered oxide of the inner core in the present application is 100 nm-1 μm, preferably 400 nm-500 nm.

[0035] The size of the sodium iron pyrophosphate crystal in at least one dimension is 2-20 nm, and the size of the sodium iron pyrophosphate crystal in at least one dimension is 200 nm-1 μm. That is, the sodium iron pyrophosphate crystal in the positive electrode material provided in the present application is in the form of sodium iron pyrophosphate whisker or needle-shaped sodium iron pyrophosphate crystal or sheet-shaped sodium iron pyrophosphate crystal.

[0036] In some embodiments, the sodium iron pyrophosphate crystal of the shell layer can also be a secondary particle formed by the growth / combination of primary particles in sequence, and the particle size of the sodium iron pyrophosphate single crystal in the long diameter direction thereof is 50 nm-500 nm, preferably 100-200 nm. The particle size of the sodium iron pyrophosphate crystal of the shell layer in the short diameter direction thereof is usually equal to the particle size of the single crystal in the short diameter direction thereof.

[0037] In the present application, the needle-shaped structure refers to a structure in which the size of the crystal in the long direction is much larger than the size of the crystal in the width and height directions, such as a rod-shaped or needle-shaped or whisker-shaped crystal with a length-diameter ratio not less than 10; the sheet-shaped structure in the present application refers to a structure in which the size of the crystal in the thickness direction is much smaller than the size of the crystal in the length and width directions, such as a sheet-shaped crystal with a thickness-length ratio and a thickness-width ratio both not less than 8.

[0038] Preferably, the sodium iron pyrophosphate crystal in the positive electrode material provided in the present application is in the form of sodium iron pyrophosphate whisker.

[0039] The positive electrode material provided in the application not only has good cycle performance, but also has more optimal charge and discharge capacity and rate performance. The possible reason is that the positive electrode material provided in the application is coated with needle-shaped or flaky or whisker-shaped sodium iron pyrophosphophosphate crystals on the surface. When the needle-shaped or flaky or whisker-shaped sodium iron pyrophosphophosphate crystals grow on the surface of the polycrystalline core, part of the crystals invades between the layered oxide crystals, forming an intercalation structure, which can not only inhibit the lattice distortion of the O3 phase layered oxide, but also form a three-dimensional ion channel embedded in the core. At the same time, sodium iron pyrophosphophosphate can preferentially adsorb Na + in the high voltage region (>3.8V), reduce the lattice strain of the O3 phase, and thus achieve the cycle performance of the positive electrode material, especially in the high voltage region (>3.8V), while maintaining or even improving the charge and discharge capacity and rate performance. The applicant has also found that the charge and discharge capacity of the positive electrode material provided in the application, especially at high potential, can exhibit a performance close to the full potential capacity.

[0040] In some embodiments, the volume expansion of the positive electrode material after full charge or full discharge or 2000 cycles is not higher than 0.5%. That is, when the positive electrode material provided in the application is applied in a sodium ion battery, the volume expansion rate during the charging process is not higher than 0.5%, the volume change rate during the discharging process is also not higher than 0.5%, and the volume expansion rate after 2000 cycles is also not higher than 0.5%. As can be seen, the positive electrode material provided in the application has good stability and good cycle stability.

[0041] In some embodiments, the ionic conductivity of the positive electrode material is not less than 1.0x10 -4 S / cm, and can even reach 5.3x10 -4 S / cm.

[0042] In some embodiments, the sodium iron pyrophosphophosphate crystals are needle-shaped structures with an aspect ratio of 20-30; or

[0043] The sodium iron pyrophosphophosphate crystals are flaky structures with a thickness of 2-20 nm; or

[0044] The sodium iron pyrophosphophosphate crystals are whisker-shaped structures with an aspect ratio of 20-50.

[0045] In some embodiments, the sodium iron pyrophosphate phosphate accounts for 1% to 20%, preferably 5% to 12% of the positive electrode material by mass fraction. The sodium iron pyrophosphate phosphate should not be too high or too low in the positive electrode material. Too high sodium iron pyrophosphate phosphate content will result in too high shell thickness, thus lengthening the ion diffusion path and reducing the rate capability. Too low sodium iron pyrophosphate phosphate content will result in too low shell coverage, making it difficult to form a relatively continuous shell, thus resulting in insufficient inhibition of the volume expansion of the core and possible direct contact of the electrolyte with O3, increasing the side reaction and reducing the cycle performance.

[0046] In some embodiments, the size of the core is 5 μm to 40 μm, preferably 5 μm to 15 μm. The thickness of the shell is 200 nm to 1 μm.

[0047] In some embodiments, the chemical formula of the layered oxide is: x Ni y Fe z Mn a M (1-y-z-a) O2;

[0048] wherein x is 0.9 to 1.0, y is 0.1 to 0.5, z is 0.1 to 0.5, and a is 0.1 to 0.5.

[0049] In some embodiments, the chemical formula of the sodium iron pyrophosphate phosphate is Na4Fe3(PO4)2(P2O7).

[0050] To enable those skilled in the art to better implement the present solution, a preparation method of the positive electrode material is also provided in the present application, comprising the following steps:

[0051] (1) Preparation of layered oxide precursor

[0052] The core raw materials such as sodium source, iron source, nickel source, manganese source and dopant are provided in a molar ratio of 1:0.4:0.3:0.2:0.1, the core raw materials are mixed with the first solution and the dispersion liquid, and the Zeta potential is adjusted to +35 mV or more by adding the dispersion liquid, then grinded, spray dried to obtain layered oxide precursor particles;

[0053] (2) Preparation of sodium iron pyrophosphate phosphate precursor

[0054] The shell raw materials are provided in a molar ratio of 4:3:4 of sodium source, iron source and phosphorus source, and the shell raw materials are stirred with the second solution in a water bath at 40 to 90°C for 4 to 24 hours to obtain a transparent sol-like sodium iron pyrophosphate phosphate precursor;

[0055] (3) mixing the sodium iron phosphate pyrophosphate precursor and the layered oxide precursor particles according to a mass ratio of 1:10-50, ultrasonic dispersion for 30-180 min, and concentration at 50-80℃ to increase the weight of the sodium iron phosphate pyrophosphate precursor by 5-8%, so that the sodium iron phosphate pyrophosphate precursor is uniformly attached to the surface of the layered oxide precursor particles, to obtain a composite material;

[0056] (4) sintering

[0057] vacuum drying the composite material for 4-24 h, heating to 400-450℃ in an environment of (91.5-95%) protective gas-(5%-8.5%) oxygen, and sintering for 4-12 h, adjusting the sintering atmosphere to a second sintering atmosphere, sintering at 600-900℃ for 4-12 h, and annealing at 200-300℃ for 2-8 h, to obtain the positive electrode material in the present application.

[0058] The sodium source in step (1) can be at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate. The iron source in step (1) can be at least one of ferric nitrate, diiron trioxide, iron phosphate, ferrous sulfate, ferrous oxalate dihydrate, and elemental iron. The nickel source in step (1) can be at least one of nickel hydroxide, nickel nitrate, and nickel sulfate. The manganese source in step (1) can be at least one of manganese oxide, dimanganese trioxide, manganese hydroxide, and manganese sulfate. The dopant in step (1) can be at least one of copper, vanadium, titanium, and aluminum. The first solution can be at least one of deionized water, ethylene glycol, N-methyl pyrrolidone, and ethanol. The dispersion can be at least one of polyacrylic acid, ethylene glycol, and polyvinyl alcohol.

[0059] The sodium source in step (2) can be at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate. The iron source in step (1) can be at least one of ferric nitrate, diiron trioxide, iron phosphate, ferrous sulfate, ferrous oxalate dihydrate, and elemental iron. The second solution can be at least one of deionized water, ethylene glycol, N-methyl pyrrolidone, and ethanol.

[0060] Preferably, the sodium source in step (1) can be the same as in step (2), so as to reduce the introduction of more impurities.

[0061] The second sintering atmosphere is a protective gas such as nitrogen, argon, and helium, or a mixed gas environment of a protective gas and carbon dioxide and water vapor. Preferably, the mixed gas environment of a protective gas and carbon dioxide and water vapor, more preferably, the mass ratio of the carbon dioxide in the second sintering atmosphere is 3.2%-5.5%, and the mass ratio of the gaseous water in the second sintering atmosphere is 1.0%-2.2 wt%.

[0062] In a second aspect, the application further provides a positive electrode sheet, comprising the positive electrode material of the first aspect.

[0063] In a third aspect, the application further provides a sodium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator; the separator and the electrolyte are arranged between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet is the positive electrode sheet of the second aspect.

[0064] The electrolyte can be a liquid electrolyte, a solid electrolyte or other electrolytes. The separator can be a polyolefin separator or a polyolefin separator provided with a ceramic coating. The negative electrode sheet can comprise a current collector and a coating layer coated on the surface of the current collector. The current collector can comprise a metal foil such as an aluminum foil, a copper foil or the like. The coating layer can comprise a negative electrode active material, a negative electrode binder and a negative electrode conductive agent. The negative electrode active material can be a carbon material or other material capable of deintercalating sodium ions. Similarly, the positive electrode sheet can also comprise a current collector and a coating layer coated on the surface of the current collector. The coating layer can comprise the positive electrode material and a binder as described above.

[0065] In a fourth aspect, the application further provides a battery device, comprising the sodium-ion battery of the third aspect or the positive electrode sheet of the second aspect or the positive electrode material of the first aspect.

[0066] The power-consuming device provided by the application comprises a battery module composed of a plurality of batteries to provide power for the device. The power-consuming device can be, for example, a mobile communication device (such as a mobile phone), a computer (such as a notebook computer, an IPAD or the like), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric truck or the like), an electric train, a ship and a satellite, an energy storage system or the like.

[0067] In order to better understand the innovations of the application, the technical solutions of the application are described in further detail below in combination with embodiments. The embodiments of the application described in detail below are exemplary and are used to explain the application, and cannot be understood as a limitation of the application.

[0068] Embodiment 1

[0069] The embodiment provides a positive electrode material, and a preparation method of the positive electrode material is as follows:

[0070] (1) Preparation of layered oxide precursor

[0071] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide are provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide and an ethanol solution are added to a planetary ball mill, an ammonium polyacrylate dispersant is added, the Zeta potential is adjusted to +40 mV or higher, the rotation speed is 400 rpm, and grinding is performed for 24 hours to obtain a wet mill base; the wet mill base is spray dried under the following conditions: an air inlet temperature of 220-250°C, an air outlet temperature of 80-100°C, an atomization pressure of 0.3-0.5 MPa, a nozzle diameter of 0.5 mm, and a peristaltic pump control feed rate of 10 mL / min to obtain a spherical layered oxide precursor with an average particle size of D50: 8 μm.

[0072] (2) Preparation of sodium iron pyrophosphate phosphate precursor

[0073] Ferrous oxalate dihydrate and sodium pyrophosphate are dissolved in a 0.1 mol / L citric acid solution at a molar ratio of 3:1, and stirred in a water bath at 80°C for 4 hours to form a blue-green transparent sol, thereby obtaining a nanomicelle-shaped sodium iron pyrophosphate phosphate precursor;

[0074] (3) The sodium iron pyrophosphate phosphate precursor is added to the transparent sol at a mass ratio of 1:10, ultrasonic dispersion is performed for 30 minutes, and 50°C reduced pressure concentration is performed until the sol completely coats the particle surface (weight gain of 5-8%); the sodium iron pyrophosphate phosphate precursor is uniformly attached to the layered oxide precursor particle surface to obtain a composite material;

[0075] (4) Sintering

[0076] The composite material is vacuum dried at 100°C for 12 hours; then heated to 400°C, and sintered in a 95% N2+5% O2 atmosphere for 4 hours; then N2 is introduced, the sintering atmosphere is adjusted to a nitrogen environment, heated to 600°C and sintered for 6 hours, and then annealed at 300°C for 2 hours to obtain the positive electrode material in the present application: NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O 2 @1 / 11Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), such as Figures 1-4 as shown.

[0077] Example 2

[0078] The embodiment provides a positive electrode material, and a preparation method of the positive electrode material is as follows:

[0079] (1) Preparation of a layered oxide precursor

[0080] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide are provided in a molar ratio of Na:Ni:Fe:Mn:Cu=1:0.3:0.3:0.3:0.1, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide and an ethanol solution are added to a planetary ball mill, an ammonium polyacrylate dispersant is added, the Zeta potential is adjusted to +40 mV or above, the rotation speed is 400 rpm, and grinding is performed for 24 hours to obtain a wet grinding material; the wet grinding material is spray dried under the conditions of an air inlet temperature of 220-250 DEG C, an air outlet temperature of 80-100 DEG C, an atomization pressure of 0.3-0.5 MPa, a nozzle diameter of 0.5 mm, and a peristaltic pump control feeding rate of 10 mL / min to obtain a spherical layered oxide precursor with an average particle size of D50: 13 μm.

[0081] (2) Preparation of a sodium iron pyrophosphate phosphate precursor

[0082] Ferrous oxalate dihydrate and sodium pyrophosphate are dissolved in a 0.1 mol / L citric acid solution in a molar ratio of 3:1, and stirred in a water bath at 80 DEG C for 4 hours to form a blue-green transparent sol, so that a nano-micellar sodium iron pyrophosphate phosphate precursor is obtained;

[0083] (3) The sodium iron pyrophosphate phosphate precursor is added to the transparent sol in a mass ratio of 1:10, ultrasonic dispersion is performed for 30 minutes, and 50 DEG C reduced pressure concentration is performed until the sol completely coats the particle surface (weight gain of 5-8%); the sodium iron pyrophosphate phosphate precursor is uniformly attached to the surface of the layered oxide precursor particles to obtain a composite material;

[0084] (4) Sintering

[0085] The composite material is vacuum dried at 100 DEG C for 12 hours; then heated to 400 DEG C, sintered in a 95% N2+5% O2 atmosphere for 4 hours, then N2 is introduced, the sintering atmosphere is adjusted to a nitrogen atmosphere, heated to 750 DEG C and sintered for 6 hours, and then annealed at 300 DEG C for 2 hours to obtain the positive electrode material NaNi 0.3 Fe 0.3 Mn 0.3 Cu 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0086] Example 3

[0087] The embodiment provides a positive electrode material, and a preparation method of the positive electrode material is as follows:

[0088] (1) Preparation of layered oxide precursor

[0089] Sodium carbonate, iron phosphate, nickel oxide, manganese sesquioxide and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1, and the sodium carbonate, iron phosphate, nickel oxide, manganese sesquioxide and copper oxide and ethanol solution were added to a planetary ball mill, ammonium polyacrylate dispersant was added, the Zeta potential was adjusted to +40 mV or more, the rotation speed was 400 rpm, and grinding was performed for 24 hours to obtain a wet mill base; the wet mill base was spray dried under the conditions of an air inlet temperature of 220-250°C, an air outlet temperature of 80-100°C, an atomization pressure of 0.3-0.5 MPa, a nozzle diameter of 0.5 mm, and a peristaltic pump control feed rate of 10 mL / min to obtain spherical layered oxide precursors with an average particle size of D50: 12 μm.

[0090] (2) Preparation of sodium iron pyrophosphate precursor

[0091] Sodium carbonate and iron phosphate were dissolved in a 0.1 mol / L citric acid solution at a molar ratio of 1:1.2, stirred in a water bath at 80°C for 4 hours, and a blue-green transparent sol was formed to obtain a nano-micellar sodium iron pyrophosphate precursor;

[0092] (3) The sodium iron pyrophosphate precursor was added to the transparent sol at a mass ratio of 1:10, ultrasonically dispersed for 30 minutes, and concentrated at 50°C under reduced pressure until the sol completely coated the surface of the particles (weight gain of 5-8%); the sodium iron pyrophosphate precursor was uniformly attached to the surface of the layered oxide precursor particles to obtain a composite material;

[0093] (4) Sintering

[0094] The composite material was vacuum dried at 100°C for 12 hours; then heated to 400°C and sintered in a 95% N2+5% O2 atmosphere for 4 hours; then N2+4% CO2+2% H2O was introduced to adjust the sintering atmosphere to a protective gas and a mixed atmosphere of 4% carbon dioxide and 2% water vapor; then heated to 600°C and sintered for 6 hours, and then annealed at 300°C for 2 hours to obtain the positive electrode material NaNi 0.3 Fe 0.3 Mn 0.3 Cu 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7) in the application.

[0095] Example 4

[0096] In this example, the mass ratio of the layered oxide precursor particles and the sodium iron pyrophosphate precursor was adjusted to 1:8, and the remaining steps were the same as in Example 1 to obtain the positive electrode material NaNi 0.4Fe 0.3 Mn 0.2 Cu 0.1 O2@1 / 9Na4Fe3(PO4)2(P2O7)。

[0097] Example 5

[0098] Compared with Example 1, the mixing mass ratio of the layered oxide precursor particles and the sodium iron phosphate precursor is adjusted to 1:20, and the remaining steps are the same as those of Example 1, to obtain a positive electrode material: NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2@1 / 20Na4Fe3(PO4)2(P2O7)。

[0099] Comparative Example 1

[0100] (1) Preparation of a layered oxide precursor

[0101] 1 mol of a sodium source, 0.3 mol of an iron source, 0.4 mass of a nickel source, 0.2 mol of a manganese source, and 0.1 mol of a copper source are provided, the sodium source, the iron source, the nickel source, the manganese source, and the copper source being the same as in the example, and the remaining steps being the same as those of step (1) in Example 1, to obtain layered oxide precursor particles;

[0102] (2) Preparation of a Na iron phosphate precursor

[0103] 1 mol of a sodium source, 1 mol of an iron source, and 1 mol of a phosphorus source are provided, and a process gel is prepared

[0104] (3) The sodium iron phosphate precursor and the layered oxide precursor are mixed in a mass ratio of 1:10, and ultrasonic dispersion is performed for 30 minutes; the sodium iron phosphate precursor is uniformly attached to the surface of the layered oxide precursor particles, to obtain a composite material;

[0105] (4) Sintering

[0106] The composite material is vacuum dried at 100°C for 12 hours; then the temperature is raised to 400°C, and sintering is performed for 4 hours; then the temperature is raised to 600°C, and sintering is performed for 6 hours; and then the temperature is raised to 300°C, and annealing is performed for 2 hours, wherein the sintering atmosphere is the same as that of Example 1, to obtain a positive electrode material in the present application: NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2@1 / 11Na iron phosphate.

[0107] Comparative Example 2

[0108] Comparative Example 1: The composite material was vacuum dried at 100°C for 12 hours in the sintering step; then heated to 300°C and sintered for 4 hours, then heated to 550°C and sintered for 6 hours, and annealed at 200°C for 2 hours, wherein the sintering atmosphere was the same as in Example 1, and the other steps were the same as in Example 1, to obtain a positive electrode material:

[0109] NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2@1 / 10Na4Fe3(PO4)2(P2O7)·H2O.

[0110] Comparative Example 3: The composite material was vacuum dried at 100°C for 12 hours in the sintering step; then heated to 300°C and sintered for 4 hours, then heated to 500°C and sintered for 6 hours, and annealed at 200°C for 2 hours, wherein the sintering atmosphere was the same as in Example 1, and the other steps were the same as in Example 1, to obtain a positive electrode material: 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2@1 / 10Na4Fe3(PO4)2(P2O7)·H2O.

[0111] Comparative Example 4

[0112] Comparative Example 4: The step (3) used in Comparative Example 4 was mixing 1g of the sodium iron phosphate pyrophosphate precursor and 100g of the layered oxide precursor particles according to a mass ratio of 1:100, and the other steps were the same as in Example 1, to obtain a positive electrode material:

[0113] NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2@1 / 101Na4Fe3(PO4)2(P2O7)·H2O.

[0114] Comparative Example 5

[0115] Comparative Example 5: The step (3) used in Comparative Example 5 was mixing 1g of the sodium iron phosphate pyrophosphate precursor and 5g of the layered oxide precursor particles according to a mass ratio of 1:20, and the other steps were the same as in Example 1, to obtain a positive electrode material:

[0116] NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2@1 / 21Na4Fe3(PO4)2(P2O7)·H2O.

[0117] Comparative Example 6

[0118] (1) Preparation of layered oxide precursor

[0119] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide and ethanol solution were added to a planetary ball mill, and the remaining steps were the same as steps (1) in Example 1.

[0120] (2) The layered oxide precursor was mixed with glucose in a mass ratio of 1:10 and ground; the glucose was uniformly attached to the surface of the layered oxide precursor particles to obtain a composite material;

[0121] (3) Sintering

[0122] The composite material was vacuum dried at 100°C for 12 hours; then the temperature was raised to 400°C and sintered for 4 hours, and then the temperature was raised to 600°C and sintered for 6 hours, and then annealed at 300°C for 2 hours, wherein the sintering atmosphere was the same as in Example 1, to obtain the positive electrode material NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2 / C

[0123] Comparative Example 7

[0124] (1) Preparation of layered oxide precursor

[0125] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide and ethanol solution were added to a planetary ball mill, and the remaining steps were the same as steps (1) in Example 1.

[0126] (2) Sintering

[0127] The spherical sodium iron pyrophosphate precursor is vacuum dried at 100°C for 12 hours; then heated to 400°C and sintered for 4 hours, and then heated to 600°C and sintered for 6 hours, and then annealed at 300°C for 2 hours, wherein the sintering atmosphere is the same as in Example 1, to obtain an O3 layered oxide.

[0128] (3) FeSO4.7H2O and Na4P2O7 are dissolved in a 0.1 mol / L citric acid solution at a molar ratio of 3:1, and stirred in a water bath at 80°C for 4 hours to form a blue-green transparent sol, thereby obtaining a nanometer micellar sodium iron pyrophosphate precursor; the sodium iron pyrophosphate precursor is concentrated and dried, and then sintered at 400°C for 4 hours, and then heated to 600°C and sintered for 6 hours, and then annealed at 300°C for 2 hours, wherein the sintering atmosphere is the same as in Example 1, to obtain sodium iron pyrophosphate.

[0129] (4) The O3 layered oxide and the sodium iron pyrophosphate obtained in steps (3) and (4) are mixed at a mass ratio of 1:10, and sintered at 400°C in a nitrogen atmosphere for 6 hours to form an O3 coated sodium iron pyrophosphate composite positive electrode material.

[0130] Comparative Example 8

[0131] (1) Preparation of layered oxide precursor

[0132] Carbonate sodium, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide, and copper oxide are provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1, and the carbonate sodium, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide, and copper oxide and an ethanol solution are added to a planetary ball mill, an ammonium polyacrylate dispersant is added, the Zeta potential is adjusted to +40 mV or more, the rotation speed is 400 rpm, and the wet milling is carried out for 24 hours to obtain a wet mill base; the wet mill base is spray dried under the conditions of an inlet air temperature of 220-250°C, an outlet air temperature of 80-100°C, an atomization pressure of 0.3-0.5 MPa, a nozzle diameter of 0.5 mm, and a peristaltic pump control feeding rate of 10 mL / min to obtain a spherical layered oxide precursor with an average particle size of D50: 8 μm.

[0133] (2) Sintering

[0134] The spherical sodium iron pyrophosphate precursor is vacuum dried at 100°C for 12 hours; then heated to 400°C and sintered for 4 hours, and then heated to 600°C and sintered for 6 hours, and then annealed at 300°C for 2 hours, wherein the sintering atmosphere is the same as in Example 1, to obtain an O3 layered oxide: NaNi 0.4 Fe 0.3 Mn 0.2 Cu 0.1 O2.

[0135] In order to verify the excellent performance of the positive electrode material provided in the present application, the applicant prepared a corresponding sodium ion battery using the positive electrode material obtained in Examples 1-5 and Comparative Examples 1-8 as the positive electrode active material, and the specific preparation is as follows:

[0136] (1) Preparation of positive electrode sheet: the positive electrode material, conductive carbon black, polyvinylidene fluoride, and nanometer carbon tube were mixed, stirred, and kneaded in a mass ratio of 95:1:3:1, N-methyl pyrrolidone solvent was added, and the slurry was adjusted to a positive electrode slurry with a solid content of 60% and a viscosity of 6000 mPa·s; using a transfer coating method, the negative electrode slurry was transferred and coated onto a 15 μm thick carbon-coated aluminum foil; the coated electrode sheet was rolled to a compaction density of 3 mg / cm3; and the rolled electrode sheet was die-cut into an electrode sheet with a length of 48 mm and a width of 38 mm for standby use.

[0137] (2) Preparation of negative electrode sheet: the hard carbon material, conductive carbon black, carboxymethyl cellulose, and butadiene-styrene rubber were mixed, stirred, and kneaded in a mass ratio of 95:1:3:1, N-methyl pyrrolidone solvent was added, and the slurry was adjusted to a negative electrode slurry with a solid content of 60% and a viscosity of 6000 mPa·s; using a transfer coating method, the negative electrode slurry was transferred and coated onto a 15 μm thick carbon-coated aluminum foil; the coated electrode sheet was rolled to a compaction density of 3 mg / cm 2 ; and the rolled electrode sheet was die-cut into an electrode sheet with a length of 48 mm and a width of 38 mm for standby use.

[0138] (3) Preparation of sodium ion battery: the striped positive electrode sheet and negative electrode sheet were laminated on a laminator, the separator was a PP / PE / PP three-layer material, a soft package battery was formed, electrolyte (composition: carbonate solvent and 1M sodium hexafluorophosphate) was injected, and the sodium ion battery was obtained after packaging, drying, formation, and capacity distribution.

[0139] The above-prepared sodium ion battery was subjected to morphology characterization test, cycle performance test, charge and discharge capacity test, and rate performance test; the specific test methods are as follows:

[0140] (1) Cycle performance test

[0141] The Neware tester was used to test the battery at a rate of 1C / 1C, and the voltage range was 2.5V-4.2V.

[0142] (2) Charge and discharge capacity test

[0143] The Neware tester was used to test the battery at a rate of 0.5C charging and 1C discharging, and the 1C discharge capacity was taken as the standard capacity, and the voltage range was 2.5V-4.2V.

[0144] (3) Rate performance test

[0145] The new Wei tester is used to test the battery cell at 1C / 1C, 1C / 2C, 1C / 5C, 1C / 10C, and the voltage range is 2.5V-4.2V.

[0146] The test results are shown in Table 1.

[0147] From Figures 1-3 It can be seen that the morphology of the coating layer on the surface of the positive electrode material provided by the application is a whisker structure, and part of the whiskers are embedded into the core. Therefore, the positive electrode material provided by the application is a core-shell structure, and the crystal structure of the shell layer is a whisker structure.

[0148] Table 1

[0149]

[0150] From Table 1, it can be seen that the sodium ion battery based on the positive electrode material provided by the application has good charge and discharge capacity, and also has good cycle performance, and its capacity retention rate is also very high at this rate, and its 5C capacity retention rate can reach more than 94%, and even at more than 10C, its capacity retention rate can still reach more than 89%. Therefore, the positive electrode material provided by the application performs extremely excellent in fast charging applications. In addition, its ionic conductivity is also high. Compared with Example 4 and Comparative Example 4, less coating material is used in Comparative Example 4, although its charge and discharge capacity is also high, but its cycle performance decreases obviously, and its capacity retention rate decreases rapidly at high rate, and its ionic conductivity is also low; and in Comparative Example 5, when the coated sodium iron pyrophosphate phosphate is too much, its ionic conductivity does not increase but decreases, and its cycle performance and capacity retention rate at high rate also decrease.

[0151] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the application, but not to limit them; although the application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application.

Claims

1. A positive electrode material, characterized by, Comprise: a core, a secondary particle of a layered oxide; an outer shell layer formed by sodium iron pyrophosphate phosphate crystals; the size of the sodium iron pyrophosphate phosphate crystals in at least one dimension is 2-20 nm; and the size of the sodium iron pyrophosphate phosphate crystals in at least one dimension is 200 nm-1 μm; the sodium iron pyrophosphate phosphate crystals are sodium iron pyrophosphate phosphate whiskers or needle-shaped sodium iron pyrophosphate phosphate crystals or flaky sodium iron pyrophosphate phosphate crystals; The chemical structural formula of the layered oxide is: Na x Ni y Fe z Mn a M (1-y-z-a) O2; x is 0.9-1.0; y is 0.1-0.5; z is 0.1-0.5; and a is 0.1-0.

5. The positive electrode material is prepared by a preparation method comprising the following steps: (1) preparing a layered oxide precursor; (2) preparing a sodium iron pyrophosphate phosphate precursor; (3) mixing the sodium iron pyrophosphate phosphate precursor and the layered oxide precursor particles in a mass ratio of 1:10, ultrasonic dispersion for 30-180 min, and concentrating at 50-80°C to increase the weight of the sodium iron pyrophosphate phosphate precursor by 5-8%, so that the sodium iron pyrophosphate phosphate precursor is uniformly attached to the surface of the layered oxide precursor particles, to obtain a composite material; (4) sintering vacuum drying the composite material for 4-24 h, and heating to 400-450°C in an environment of 91.5-95% protective gas + 5%-8.5% oxygen, and sintering for 4-12 h, adjusting the sintering atmosphere to a second sintering atmosphere, sintering at 600-900°C for 4-12 h, and annealing at 200-300°C for 2-8 h, to obtain a positive electrode material.

2. The positive electrode material of claim 1, wherein, The sodium iron pyrophosphate phosphate is partially intercalated into the core.

3. The positive electrode material of claim 1, wherein, The volume expansion of the positive electrode material after full charging or full discharging or 2000 cycles is not higher than 0.5%.

4. The positive electrode material of claim 1, wherein, The ionic conductivity of the positive electrode material is not less than 1.0 x 10 -4 S / cm.

5. The cathode material of claim 1, wherein, The mass fraction of the sodium iron pyrophosphate phosphate in the positive electrode material is 1%-20%.

6. The cathode material of claim 1, wherein, The size of the core is 5 μm-40 μm; and the thickness of the outer shell layer is 200 nm-1 μm.

7. A positive electrode sheet characterized by comprising: Comprise the positive electrode material of any one of claims 1-6.

8. A sodium-ion battery, characterized in that, Comprise a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator; the separator and the electrolyte are arranged between the positive electrode sheet and the negative electrode sheet; The positive electrode sheet is the positive electrode sheet of claim 7.

9. A battery module, characterized by Comprise the sodium ion battery of claim 8 or the positive electrode sheet of claim 7 or the positive electrode material of any one of claims 1-6.

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