阅读说明：本技术 钠离子电池用正极材料及其制备方法 (Positive electrode material for sodium ion battery and preparation method thereof ) 是由 刘倩 林文光 郭永胜 梁成都 于 2019-08-28 设计创作，主要内容包括：本发明涉及钠离子电池用正极材料及其制备方法,所述钠离子电池用正极材料包含卤代磷酸钠盐/碳复合物,所述卤代磷酸钠盐/碳复合物具有如下分子式：Na_2M1_hM2_k(PO_4)X/C,其中M1和M2各自独立地为选自Ti、V、Cr、Mn、Fe、Co、Ni、Cu、Zn、Ga、Sr、Y、Nb、Mo、Sn、Ba和W的过渡金属离子；h为0到1,k为0到1,且h+k＝1；X是选自F、Cl和Br的卤离子,并且其中所述正极材料在12MPa压力下具有在10Ω·cm至5000Ω·cm的范围内的粉末电阻率,优选在20Ω·cm至2000Ω·cm的范围内。本发明还涉及一种钠离子电池,包括正极、负极、隔离膜和电解液,其中,所述正极中的活性物质包括本发明的正极材料。(The invention relates to a positive electrode material for a sodium-ion battery and a preparation method thereof, wherein the positive electrode material for the sodium-ion battery comprises a halogenated sodium phosphate salt/carbon compound, and the halogenated sodium phosphate salt/carbon compound has the following molecular formula: na (Na) 2 M1 h M2 k (PO 4 ) X/C, wherein M1 and M2 are each independently transition metal ions selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Nb, Mo, Sn, Ba, and W; h is 0 to 1, k is 0 to 1, and h + k is 1; x is a halide ion selected from F, Cl and Br, and wherein the positive electrode material has a powder resistivity in the range of 10 to 5000 Ω -cm, preferably in the range of 20 to 2000 Ω -cm, under a pressure of 12 MPa. The invention also relates toThe sodium ion battery comprises a positive electrode, a negative electrode, a separation film and electrolyte, wherein an active substance in the positive electrode comprises the positive electrode material.)

1. A positive electrode material for a sodium-ion battery comprising a sodium halophosphate/carbon complex, wherein the sodium halophosphate salt/carbon complex has the formula:

Na₂M1_hM2_k(PO₄)X/C

wherein M1 and M2 are each independently a transition metal ion selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Nb, Mo, Sn, Ba, and W; h is 0 to 1, k is 0 to 1, and h + k is 1; x is a halide ion selected from F, Cl and Br, and

wherein the positive electrode material has a powder resistivity in the range of 10 Ω · cm to 5000 Ω · cm, preferably in the range of 20 Ω · cm to 2000 Ω · cm, under a pressure of 12 MPa.

2. The positive electrode material for sodium-ion batteries according to claim 1, characterized in that the grain size of the positive electrode material is in the range of 0.01 to 20 micrometers, preferably in the range of 0.05 to 5 micrometers.

3. The positive electrode material for a sodium-ion battery according to claim 1, characterized in that the positive electrode material has a carbon content in the range of 0.05 wt% to 15 wt%.

4. The positive electrode material for sodium-ion batteries according to claim 1, characterized in that the carbon of the positive electrode material is derived from a carbon source comprising inorganic carbon, organic carbon or a combination thereof.

5. The positive electrode material for sodium-ion batteries according to claim 4, wherein the organic carbon is selected from one or more of glucose, fructose, sucrose, maltose, starch, cellulose, citric acid, ascorbic acid, glutamic acid, polypyrrole, polyaniline, polythiophene, polyethylenedioxythiophene, polystyrene sulfonate, polyphenylene sulfide, and derivatives thereof.

6. The positive electrode material for sodium-ion batteries according to claim 4, wherein the inorganic carbon is selected from one or more of acetylene black, conductive carbon black, conductive graphite, Ketjen black, carbon nanotubes, carbon nanoribbons, carbon fibers, graphene, and carbon dots.

7. A method for preparing the positive electrode material for sodium-ion batteries according to any one of claims 1 to 6, characterized in that the positive electrode material is obtained by:

i) fully mixing materials for forming the halogenated sodium phosphate salt/carbon compound in the presence of a solvent to form uniform powder; and

ii) heat-treating the uniform powder at a temperature of 500 ℃ to 700 ℃ under an inert atmosphere, thereby forming the positive electrode material.

8. The method for producing a positive electrode material for a sodium-ion battery according to claim 7, wherein the mixing is performed using a ball mill, a rod mill, a centrifugal mill, a stirring mill, a jet mill, a sand mill, and a Raymond film.

9. The method for preparing a positive electrode material for a sodium-ion battery according to claim 7, wherein the solvent is one or more selected from deionized water, methanol, ethanol, acetone, isopropanol, n-hexanol, dimethylformamide, ethylene glycol, and diethylene glycol.

10. A sodium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein an active material in the positive electrode comprises the positive electrode material according to any one of claims 1 to 6.

Technical Field

The invention relates to a positive electrode material for a sodium ion battery, in particular to a positive electrode material for a sodium ion battery of a halogenated sodium phosphate/carbon compound and a preparation method thereof. The invention also relates to a sodium-ion battery made of the positive electrode material for the sodium-ion battery.

Background

Since the commercialization of lithium ion batteries, lithium ion batteries have rapidly become the first choice for energy storage devices of devices such as computers, electric tools, digital cameras, and the like, due to their advantages of high energy density, long cycle life, high safety, and the like. In recent years, with the rapid rise of the electric vehicle market, lithium ion batteries have been more widely used. However, with the wide application of lithium ion batteries, the problems of uneven distribution of lithium resources, relative shortage of resources, and the like are gradually highlighted.

Compared with lithium, sodium resources are widely distributed and abundant, and have the advantages of resources and cost. Sodium ion batteries developed on the basis of sodium are expected to replace part of the market of lithium ion batteries due to the advantages of low manufacturing cost, good safety and the like, and become powerful competitors of next-generation batteries. The positive electrode material of the sodium-ion battery is a main factor influencing the performance of the sodium-ion battery. Among the widely studied cathode materials such as oxides, fluorides, sulfides, phosphates, pyrophosphates, metal organic frameworks/metal hexacyanides, and organic compounds, the sodium fluorophosphate cathode material having a layered structure has attracted much attention due to its high theoretical capacity, low cost, and good cycle stability, and thus has a great potential as a cathode material for sodium ion batteries. However, such positive electrode materials for sodium ion batteries still cannot meet the existing market demands, particularly in terms of electrochemical and kinetic properties.

Therefore, in the battery industry, there is a need to develop a positive electrode material that combines good electrochemical performance and good kinetic performance of a sodium ion battery.

Disclosure of Invention

In view of the above, the present invention provides a positive electrode material for a sodium ion battery, comprising a sodium halophosphate/carbon complex having the following molecular formula:

Na₂M1_hM2_k(PO₄)X/C

wherein M1 and M2 are each independently a transition metal ion selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Nb, Mo, Sn, Ba, and W; h is 0 to 1, k is 0 to 1, and h + k is 1; x is a halide ion selected from F, Cl and Br, and

wherein the positive electrode material has a powder resistivity in the range of 10 Ω · cm to 5000 Ω · cm, preferably in the range of 20 Ω · cm to 2000 Ω · cm, under a pressure of 12 MPa.

Preferably, the grain size D of the positive electrode material is in the range of 0.01 to 20 micrometers, preferably in the range of 0.05 to 5 micrometers.

Preferably, the cathode material has a carbon content in the range of 0.05 wt% to 15 wt%, preferably in the range of 0.5 wt% to 5 wt%.

In one embodiment of the present invention, the positive electrode material is obtained by: i) mixing the materials for forming the sodium halophosphate/carbon complex in the presence of a solvent to form a uniform powder; and ii) heat-treating the uniform powder at a temperature of 500 ℃ to 700 ℃ in an inert atmosphere, thereby forming the positive electrode material.

In another aspect, the invention further provides a sodium ion battery, which comprises a positive electrode, a negative electrode, a separation film and an electrolyte, wherein the active material in the positive electrode comprises the positive electrode material for the sodium ion battery.

The positive electrode material for sodium ion batteries according to the present invention has appropriate powder conductivity while exhibiting excellent capacity and cycle performance. The inventors have surprisingly found that sodium halophosphate salts having a particular grain size coated with an amount of carbon exhibit excellent powder resistivity, and thus sodium ion batteries formed from such materials exhibit excellent capacity and cycling performance. The inventor of the invention further surprisingly discovers that the positive electrode material for the sodium-ion battery can be obtained by adopting a solid-phase reaction method, and the solid-phase reaction method has the advantages of simple operation and strong practicability, so that the positive electrode material has wider industrial prospect.

The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

Definition of

The use of quantitative terms in describing the invention, and not in the context of the claims, should be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.

Where a method is described as including or comprising a particular process step, it is contemplated that alternative process steps not explicitly specified are not excluded from the method, and that the method may also consist of or consist of the process step involved.

For the sake of brevity, only some numerical ranges are explicitly disclosed herein. However, any lower limit may be combined with any upper limit to form ranges not explicitly recited; and any lower limit may be combined with any other lower limit to form a range not explicitly recited, and similarly any upper limit may be combined with any other upper limit to form a range not explicitly recited. Also, although not explicitly recited, each point or individual value between endpoints of a range is encompassed within the range. Thus, each point or individual value can form a range not explicitly recited as its own lower or upper limit in combination with any other point or individual value or in combination with other lower or upper limits.

In the context of the present invention, "material for forming a sodium halophosphate salt/carbon complex" includes precursors and a source of carbon for forming the sodium halophosphate salt.

In the context of the present invention, "precursors for forming a sodium halophosphate salt" refers to one or more compounds capable of forming a sodium halophosphate salt by an oxidation reaction, such as a sintering process, including, but not limited to, sodium precursors, phosphates, halogen precursors, and transition metal precursors.

In the context of the present invention, "carbon source" refers to a raw material for forming carbon elements, including, but not limited to, inorganic carbon, organic carbon, or a combination thereof.

When used in the context of a positive electrode material for a sodium ion battery, the term "powder resistivity" is a parameter used to characterize the conductive properties of the positive electrode material itself, which is different from the resistivity of the positive electrode sheet. Typically, the powder resistivity is measured after the positive electrode material is molded into a sheet, for example, a 3mm sheet, using a tester such as a four-probe apparatus.

The term "specific first discharge capacity" when used in the context of a positive electrode material for a sodium ion battery refers to the first discharge capacity of a button cell formed by assembling the material as a positive electrode active material with a sodium sheet as a negative electrode and a 1mol/L solution of ethylene carbonate and propylene carbonate of sodium perchlorate (in a volume ratio of 1:1) as an electrolyte, which is an effective parameter for measuring the electrical properties of the material per unit weight.

The term "2C specific discharge capacity" when used in the context of a positive electrode material for a sodium ion battery refers to the discharge capacity at 2C of a button cell formed by assembling the material as a positive electrode active material with a sodium sheet as a negative electrode and a 1mol/L solution of sodium perchlorate in ethylene carbonate and propylene carbonate (in a 1:1 volume ratio) as an electrolyte, which is an effective parameter for measuring the electrical properties of the material per unit weight.

The terms "preferred" and "preferably" refer to embodiments of the invention that may provide certain benefits under certain circumstances. However, other embodiments may be preferred, under the same or other circumstances. In addition, recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

Drawings

Fig. 1 is a first charge-discharge curve of a button cell made of the positive electrode material for a sodium ion battery prepared in example 3 of the present invention.

Detailed Description

The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. It is to be understood that the described embodiments are merely illustrative of some, but not all, of the present invention. All technical solutions obtained by ordinary persons skilled in the art through routine modifications or changes based on the embodiments of the present invention fall within the scope of the present invention.

According to the present invention, a positive electrode material for a sodium-ion battery comprises a sodium halophosphate salt/carbon composite having the following formula:

Na₂M1_hM2_kPO₄X/C

wherein M1 and M2 are each independently a transition metal ion selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Nb, Mo, Sn, Ba, and W; h is 0 to 1, k is 0 to 1, and h + k is 1; x is a halide ion selected from F, Cl and Br, and wherein

The positive electrode material has a powder resistivity in a range of 10 Ω -cm to 5000 Ω -cm under a pressure of 12 MPa.

The inventors of the present invention have surprisingly found that the powder resistivity of the positive electrode material is an important parameter affecting the electrochemical performance of sodium ion batteries, in particular the specific discharge capacity and rate performance. Either too low or too high a resistivity is detrimental to the electrochemical performance of the sodium ion battery. In some embodiments of the invention, the positive electrode material has a powder resistivity in a range of 20 Ω · cm to 2000 Ω · cm under a pressure of 12MPa, for example in a range of 30 Ω · cm to 2000 Ω · cm, in a range of 90 Ω · cm to 2000 Ω · cm, in a range of 100 Ω · cm to 2000 Ω · cm, in a range of 120 Ω · cm to 2000 Ω · cm, in a range of 950 Ω · cm to 2000 Ω · cm, in a range of 1600 Ω · cm to 2000 Ω · cm, in a range of 30 Ω · cm to 1600 Ω · cm, in a range of 90 Ω · cm to 1600 Ω · cm, in a range of 100 Ω · cm to 1600 Ω · cm, in a range of 120 Ω · cm to 1600 Ω · cm, in a range of 30 Ω · cm to 950 Ω · cm, in a range of 90 Ω · cm to 950 Ω · cm, in a range of 100 Ω · cm to 950 Ω · cm, in a range of 950 Ω · cm, in the range of 30 to 120 Ω · cm, in the range of 90 to 120 Ω · cm, in the range of 100 to 120 Ω · cm, in the range of 30 to 100 Ω · cm, in the range of 90 to 100 Ω · cm, in the range of 30 to 90 Ω · cm.

In some embodiments of the invention, the average grain size D of the cathode material is within a particular range. The inventors of the present invention found that the average grain size of the positive electrode material is an important parameter affecting the powder resistivity of the material. If the grain size of the anode material is too large, the conductivity and the ion conducting performance of the anode material are poor, so that the electrochemical dynamic performance in the charging and discharging process is poor, the polarization is large, the battery capacity and the coulombic efficiency are low, and the cycle attenuation is fast. If the grain size of the positive electrode material is too small, the crystallinity and chemical composition of the particles are more defective, which also hinders the exertion of the electrochemical performance of the battery. Therefore, preferably, the average grain size D of the cathode material is in the range of 0.01 to 20 micrometers, preferably in the range of 0.05 to 5 micrometers.

The inventors of the present invention found that conventional cathode materials, for example, cathode materials synthesized by hydrothermal or solvent methods disclosed in CN105428649, tend to have too small a grain size and too large a specific surface area. Such materials have a large number of crystallinity and chemical composition defects due to their too small grain size, and thus fail to achieve desired electrochemical properties. In addition, the excessively high specific surface area has extremely serious imbibition phenomenon in the slurry stirring process, which causes low solid content of the slurry, striated pole pieces and low compaction density, and seriously reduces the capacity, coulomb efficiency and overall energy density of the battery. In contrast, the positive electrode material of the present invention has an appropriate grain size and may also have an appropriate specific surface area, thus exhibiting excellent electrochemical properties. Moreover, the anode material can be synthesized by a solid-phase reaction method, and the solid-phase reaction method has the advantages of simple operation and strong implementation, so the anode material has wider industrial prospect.

Preferably, the specific surface area of the cathode material may be 0.01m²/g-30m²In the range of/g, preferably in the range of 1m²/g-20m²In the range of/g.

In some embodiments of the invention, the carbon content of the positive electrode material is within a specific range. The inventors of the present invention found that the carbon content of the positive electrode material is also an important parameter affecting the powder resistivity of the material. If the carbon content of the cathode material is too high, it will adversely affect the crystallinity of the sodium halophosphate salt crystals in the in situ synthesis of the sodium halophosphate salt/carbon composite and the uniformity of the distribution of carbon in the composite. Moreover, because carbon has stronger liquid absorption, the viscosity of the slurry can be greatly improved by excessive carbon, so that the compacted density of the prepared pole piece is too high, and the distribution of the active substance film layer of the pole piece is not uniform. If the carbon content of the cathode material is too low, the aim of improving the electrochemical activity of the material, the initial capacity and the rate performance of the battery cannot be fulfilled. Therefore, preferably, the cathode material has a carbon content in the range of 0.05 wt% to 15 wt%, preferably 0.5 wt% to 5 wt%, calculated on the amount of carbon source dosed in the preparation of the cathode material containing the sodium halophosphate salt/carbon complex.

In the cathode material according to the present invention, the kind of carbon is not particularly limited and may be selected according to actual needs. In particular, the carbon may be an inorganic conductive carbon, or may be derived from an organic carbon. The inorganic conductive carbon can be selected from one or more of acetylene black, conductive carbon black, conductive graphite, Ketjen black, carbon nanotubes, carbon nanoribbons, carbon fibers, graphene and carbon dots. The organic carbon can be selected from one or more of glucose, fructose, sucrose, maltose, starch, cellulose and other carbohydrates, citric acid, ascorbic acid, glutamic acid, polypyrrole, polyaniline, polythiophene, polyethylene dioxythiophene, polystyrene sulfonate, polyphenylene sulfide and derivatives thereof. However, in consideration of production cost and conductivity, etc., it is preferable that the carbon of the positive electrode active material be derived from glucose, sucrose, ketjen black, polypyrrole, or any combination thereof, and preferably, be derived from ketjen black.

Preferably, the morphology of the primary particles of the cathode material is sheet-like, spherical or polyhedral.

Preferably, the tap density of the cathode material is 0.5g/cm³To 2.5g/cm³In the range of (1), preferably 0.7g/cm³To 2.0g/cm³Range of (1)And (4) the following steps.

Preferably, the compacted density of the cathode material under the pressure of 8 tons is 1.5g/cm³To 3.5g/cm³In the range of (1), preferably 2.0g/cm³To 3.0g/cm³Within the range of (1).

In some embodiments of the invention, the positive electrode material is obtained by:

i) fully mixing materials for forming the halogenated sodium phosphate salt/carbon compound in the presence of a solvent to form uniform powder; and

ii) heat-treating the uniform powder at a temperature of 500 ℃ to 700 ℃ under an inert atmosphere, thereby forming the positive electrode material.

The inventors of the present invention have found that, in the preparation of the positive electrode material of the present invention, step i) of sufficiently mixing the materials for forming the sodium halophosphate salt/carbon complex is important for obtaining a positive electrode material having an appropriate specific resistance. Preferably, the material is mixed sufficiently that when three or more samples are tested in different regions of the mixed material using X-ray photoelectron spectroscopy (XPS), the results obtained show that the composition and amount of each sample does not differ by more than 5%, preferably by more than 2%, more preferably by more than 1%, still more preferably by more than 0.5%, even more preferably by more than 0.1%. More importantly, under the mixing condition, the crystallization speed of the cathode material is controllable, and the cathode material with proper grain size can be obtained. In the above step i), the mixing is performed by using a ball mill, a rod mill, a centrifugal mill, a stirring mill, a jet mill, a sand mill or a Raymond film. In one embodiment of the present invention, the mixing may be performed using a laboratory scale high energy ball mill or a planetary ball mill. In other embodiments of the present invention, the mixing may be performed using an industrial scale agitator mill or a jet mill.

In step i), the material used to form the sodium halophosphate salt/carbon complex comprises a precursor. The precursors include sodium precursors, halogen precursors (e.g., fluorine precursors), phosphates, and other metal precursors. As an example, the phosphate may be selected from one or more of ammonium phosphate, diammonium phosphate, monoammonium phosphate, sodium phosphate, disodium phosphate, monosodium phosphate, potassium phosphate, dipotassium phosphate, monopotassium phosphate, phosphoric acid. As an example, the fluorine precursor may be selected from one or more of ammonia fluoride, lithium fluoride, sodium fluoride, potassium fluoride, and hydrogen fluoride. As an example, the sodium precursor may be selected from one or more of sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium fluoride. As an example, the other metal precursor includes a manganese source, an iron source, or a combination thereof, wherein the manganese source may be selected from one or more of manganese nitrate, manganese chloride, manganese sulfate, manganese acetate, manganese oxalate, manganese carbonate, and manganese hydroxide, and the iron source may be selected from one or more of ferrous nitrate, ferric nitrate, ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, ferrous oxalate, ferrous acetate, and ferric acetate.

In step i), the material used to form the sodium halophosphate salt/carbon complex comprises a source of carbon. The carbon source may be inorganic carbon, organic carbon, or a mixture thereof. As an example, the inorganic carbon may be selected from one or more of acetylene black, conductive carbon black, conductive graphite, ketjen black, carbon nanotubes, carbon nanoribbons, carbon fibers, graphene, carbon dots. The organic carbon may be selected from one or more of glucose, fructose, sucrose, maltose, starch, cellulose, citric acid, ascorbic acid, glutamic acid, polypyrrole, polyaniline, polythiophene, polyethylenedioxythiophene, polystyrene sulfonate, polyphenylene sulfide, and derivatives thereof.

In step i), the material used to form the sodium halophosphate salt/carbon complex may also optionally include a complexing agent. The complexing agent may be selected from one or more of ammonia, ammonium carbonate, urea, hexamethylenetetramine, ethylenediaminetetraacetic acid, citric acid and ascorbic acid.

In step i), the solvent is selected from one or more of deionized water, methanol, ethanol, acetone, isopropanol, n-hexanol, dimethylformamide, ethylene glycol, diethylene glycol.

The inventors of the present invention have found that in the preparation of the positive electrode material of the present invention, step ii) of heat-treating the uniform powder at a temperature of 500 ℃ to 700 ℃ under an inert atmosphere is important for obtaining a positive electrode material having an appropriate specific resistance. At such heating temperatures, the formed carbon can be uniformly distributed in the positive electrode material, and the crystal grains of the positive electrode material are not damaged. In contrast to conventional solid phase methods (e.g., the high temperature solid phase method disclosed in CN 1948138A), the heat treatment step of the present invention needs to be performed at a relatively low temperature (e.g., 500 ℃ to 700 ℃), which not only reduces energy consumption, but also avoids the problems of uneven distribution of the carbon material and damage to the crystal form of the sodium halophosphate salt, which are caused by excessive sintering of the carbon source during the heat treatment, and thus causes an excessively high resistivity of the positive electrode active material.

Optionally, the preparation of the cathode material of the present invention further comprises the steps of washing and drying the cathode material obtained in step ii).

In one embodiment of the present invention, the positive active material has a first discharge voltage plateau at 2.95V to 3.15V and a second discharge voltage plateau at 2.75V to 2.95V (reference voltage of positive material to metallic sodium); the positive active material has a discharge capacity at the first discharge voltage plateau of Q1, the positive active material has a discharge capacity at the second discharge voltage plateau of Q2, the positive active material has a full discharge capacity of Q, and Q1, Q2 and Q satisfy: 50% ≦ (Q1+ Q2)/QX100% ≦ 95%, preferably 70% ≦ (Q1+ Q2)/QX100% ≦ 90%. Moreover, most of the capacity can be exerted in the plateau part, so that the sodium ion battery manufactured by using the positive electrode active material can be charged and discharged in a wider electrochemical window, and the capacity and the energy density of the battery are not influenced.

In view of the above effects, another aspect of the present invention provides a sodium ion battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, where the positive electrode material of the sodium ion battery is the positive electrode material for the sodium ion battery according to the above technical solution.

The preparation method of the sodium ion battery is not particularly limited, and the technical scheme of preparing the positive electrode material into the sodium ion battery, which is well known to those skilled in the art, is adopted.

In one embodiment of the invention, a button cell is prepared by:

1. preparation of positive pole piece

Fully stirring and mixing the positive electrode active material, conductive carbon and a binding agent polyvinylidene fluoride (PVDF) in a proper amount of N-methyl pyrrolidone (NMP) solvent according to a weight ratio of 7:2:1 to form uniform positive electrode slurry; the slurry is coated on a positive electrode current collector carbon-coated Al foil, and is dried and then punched into a small wafer with the diameter of 14 mm.

2. Preparation of the electrolyte

The ethylene carbonate with the same volume is dissolved in the propylene carbonate, and then a proper amount of sodium perchlorate is uniformly dissolved in the mixed solvent for standby.

3. Negative pole piece: and selecting a metal sodium sheet.

4. And (3) isolation film: glass fiber or nonwoven fabric can be used without special selection.

5. Preparing a button battery:

and stacking the positive plate, the isolating film and the negative plate in sequence to enable the isolating film to be positioned between the positive plate and the negative plate to play an isolating role, and injecting the prepared electrolyte into the battery core to finish the preparation of the buckle battery.

Examples

In order to facilitate understanding of the present invention, the present invention will be described below by way of examples. It should be understood by those skilled in the art that the examples are only for the purpose of facilitating understanding of the present invention and should not be construed as specifically limiting the present invention.

Test method

Resistivity of powder

Drying the anode material powder, weighing a proper amount of powder, and then measuring the powder resistivity of the sample by using a powder resistivity tester and an ST2722 type digital four-probe instrument according to GB/T30835-2014 (carbon composite lithium iron phosphate anode material for lithium ion batteries).

Carbon content

The carbon content in the powder was measured using a carbon content analyzer, equipment model HCS-140, according to the determination of the total carbon and sulfur content of the steel using a high frequency induction furnace post combustion infrared absorption method (conventional method) GBT 20123-2006.

Grain size

The crystal structure and grain size of the positive electrode active material can be measured by an X-ray powder diffractometer, for example, a Brucker D8A _ A25X-ray diffractometer from Brucker AxS, Germany, using CuKa rays as a radiation source and wavelength of the raysThe angle range of the scanning 2 theta is 10-90 DEG, and the scanning speed is 4 DEG/min. After obtaining the diffraction pattern of the powder, the diffraction peak with appropriate diffraction intensity and appropriate 2 theta angular position is selected, and the grain size is calculated according to the Scherrer formula.

Specific capacity of initial discharge

Charging to 4.5V at 1.8-4.5V according to 0.1C, then charging to a constant voltage of less than or equal to 0.05mA at 4.5V, standing for 2min, recording the charging capacity as C0, and then discharging to 1.8V according to 0.1C, wherein the discharging capacity at the moment is the first discharging specific capacity and is recorded as D0.

Specific discharge capacity under 2C

Charging to 4.5V at 1.8-4.5V according to 2C, then charging to the current of less than or equal to 0.05mA at 4.5V under constant voltage, standing for 2min, recording the charging capacity as C0, then discharging to 1.8V according to 2C, and recording the discharging capacity as 2C discharging specific capacity as Dn.

Preparation of positive electrode active material

Weighing sodium fluoride, ferrous oxalate and sodium dihydrogen phosphate according to a stoichiometric ratio to ensure that Na: fe: p: the molar ratio of F is 2:1:1: 1. And then mixed well with a certain amount of carbon source and ethanol to make the carbon content as shown in table 1 below. And (3) carrying out heat treatment on the mixed powder under the protection of argon atmosphere, controlling the temperature of the heat treatment within the range of 500-800 ℃, controlling the time of the heat treatment within 5-30 hours, and controlling the flow of argon within 10-500 ml/min. Then, the heat-treated product is washed with a proper amount of deionized water and filtered, dried in a vacuum drying oven at 100 ℃, and then crushed, sieved and classified to obtain the cathode active material of the invention.

As a control, the positive active materials of comparative examples 1 to 3 were prepared by the above-described method except for the difference in carbon content, in which comparative example 1 contained no carbon, comparative example 2 contained less than 0.05 wt% of carbon, and comparative example 3 contained more than 15 wt% of carbon.

Also, as a control, the cathode active materials of comparative examples 4 to 5 were prepared using the above-described method except that the cathode active material having a grain size different from that of the present invention was sieved, wherein comparative example 4 had a grain size of less than 0.01 μm and comparative example 5 had a grain size of more than 20 μm.

The powder resistivity, carbon content and crystal grain size of the positive electrode active material of the invention and the positive electrode active material for control were measured according to the above test section, and the results are summarized in table 1 below.

Manufacture of sodium ion batteries

1. Preparation of positive pole piece

The positive active materials of the invention and the positive active materials for comparison are fully stirred and mixed with conductive carbon and polyvinylidene fluoride (PVDF) as a binder in a proper amount of N-methyl pyrrolidone (NMP) solvent according to the weight ratio of 7:2:1 to form uniform positive slurry; the slurry is coated on a positive electrode current collector carbon-coated Al foil, and is dried and then punched into a small wafer with the diameter of 14 mm.

2. Preparation of the electrolyte

Ethylene carbonate with the same volume is dissolved in propylene carbonate, and then a proper amount of sodium perchlorate is uniformly dissolved in a mixed solvent to form 1mol/L electrolyte for later use.

3. Negative pole piece: and selecting a metal sodium sheet.

4. And (3) isolation film: glass fiber is selected.

5. Preparing a button battery:

and stacking the positive plate, the isolating film and the negative plate in sequence to enable the isolating film to be positioned between the positive plate and the negative plate to play an isolating role, and injecting the prepared electrolyte into the battery core to finish the preparation of the buckle battery.

The specific first discharge capacity and specific discharge capacity at 2C of the button cell made above were determined according to the test section above and the results are summarized in table 1 below. Fig. 1 shows the first charge and discharge curves of a button cell made with the positive active material of example 3.

As can be seen from the data in table 1, the carbon content and the grain size of the positive active material in the form of the sodium halophosphate salt/C complex have a significant effect on the powder resistivity of the positive active material, which in turn further affects the electrochemical performance, in particular, the specific discharge capacity and the rate capability of the sodium ion battery.

Because the intrinsic electronic conductivity of the anode material is lower, when the grain size of the material is larger, the transmission path of electrons in the material grain is longer, so that the resistivity of the material and a pole piece is higher, and the rate performance of the battery is poorer; when the crystal diameter of the material is smaller, the transmission path of electrons in active material particles is shorter, and the electrons can be diffused to the surface of the material more quickly and contact with a medium with better conductivity, so that the conductivity of the material and a pole piece is improved, and the rate performance of the sodium-ion battery is also better.

The positive electrode material has good structure, chemical and electrochemical stability, and can not generate irreversible phase change, irreversible distortion or collapse of the structure in the circulating process, thereby forming a good main body frame capable of reversibly deintercalating sodium ions, and ensuring the capacity exertion and excellent circulating performance of the sodium ion battery.

While the invention has been described with reference to a number of embodiments and examples, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope and spirit of the invention as disclosed herein.