IRON-BASED CATHODE MATERIAL FOR SODIUM-ION BATTERY, PREPARATION METHOD THEREOF, AND CORRESPONDING SODIUM-ION FULL BATTERY

Abstract

The present invention discloses an iron-based cathode material for a sodium-ion battery, which comprises a Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material and a carbon-based material embedded into the bulk structure of Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material. The weight percentage of the carbon-based material is ranked between 1% and 10%. The present invention also provides a method for preparing the above-mentioned iron-based cathode material for a sodium-ion battery, and a corresponding sodium-ion full battery using the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F-based cathode material. The Na.sub.3Fe.sub.2(SO.sub.4).sub.3F cathode material ensures desired electrochemical sodium storage performance, involving high specific sodium storage capacity, improved cycle stability and superior rate performance in comparison with that of various pristine Na.sub.xFe.sub.y(SO.sub.4).sub.z materials. The actual operating potential of the reported sodium-ion full battery in the present invention is significantly higher than the output potential of existing commercial sodium-ion full batteries, and the increase in battery energy density is also achieved.

Claims

1. An iron-based cathode material for a sodium-ion battery, comprising a Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material and a carbon-based material embedded into the bulk structure of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material, wherein in the iron-based cathode material for a sodium-ion battery, the weight percentage of the carbon-based material is ranked between 1% and 10%.

2. The iron-based cathode material for a sodium-ion battery according to claim 1, wherein the carbon-based material is selected from the group consisting of carbon nanotubes, carbon fibers, graphene, reduced graphene oxide and amorphous carbon.

3. A method for preparing an iron-based cathode material for a sodium-ion battery according to claim 1, comprising steps of: S1: mixing anhydrous ferrous sulfate, sodium sulfate, sodium fluoride at a molar ratio of 1:2:1 with a carbon-based material, followed by ball milling under a protective atmosphere, and drying the milled mixture to obtain a cathode material precursor; and S2: under a sintering atmosphere, sintering the cathode material precursor at a temperature between 300 and 450° C. for a duration time between 1 and 24 h to obtain the iron-based cathode material for a sodium-ion battery.

4. The method for preparing an iron-based cathode material for a sodium-ion battery according to claim 3, wherein in Step S1, the weight ratio of ball to material during the ball milling is in a range of 0.1-100, a ball milling medium is one of stainless steel balls, ZrO.sub.2 balls and agate balls, and the protective atmosphere is nitrogen gas or argon gas.

5. The method for preparing an iron-based cathode material for a sodium-ion battery according to claim 4, wherein in Step S1, a solvent is added during the ball milling, and the solvent is selected from ethanol, acetone, ethylene glycol, N-methylpyrrolidone or any combination thereof; the speed of ball milling is in a range of 100-1200 r/min, and the time of ball milling is in a range of 1-72 h.

6. The method for preparing an iron-based cathode material for a sodium-ion battery according to claim 3, wherein in Step S1, the drying is performed in vacuum or under nitrogen or argon atmosphere, the drying temperature is in a range of 80-120° C., and the drying time is in a range of 1-24 h.

7. The method for preparing an iron-based cathode material for a sodium-ion battery according to claim 3, wherein in Step S2, the sintering atmosphere is nitrogen or argon.

8. A sodium-ion full battery, comprising a positive electrode prepared from the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/C cathode material according to claim 1, a conductive carbon material and a binder.

9. A method for preparing a sodium-ion full battery according to claim 8, comprising steps of: (1) mixing the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/C cathode material, a conductive carbon material and a binder in a solvent, followed by coating the resulting mixture on an aluminum foil as a current collector, and drying to obtain a cathode plate; (2) mixing a hard carbon anode material, a conductive carbon material and a binder in a solvent, followed by coating the resulting mixture on a copper foil as a current collector, and drying to obtain an anode plate; and (3) assembling the cathode plate and the anode plate with a separator, a spacer, a spring within cathode and anode shells, followed by adding an electrolyte and sealing to obtain the sodium-ion full battery.

10. The method for preparing a sodium-ion full battery according to claim 9, wherein: the conductive carbon material is acetylene black or Super P; the binder is polyvinylidene fluoride, and the solvent is N-methylpyrrolidone; the weight ratio of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/C cathode material, the conductive carbon material and the binder is 8:1:1; and the weight ratio of the hard carbon anode material, the conductive carbon material and the binder is 7:2:1; and the electrolyte comprises sodium perchlorate as a solute with a concentration of 1 mol/L, and ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1 as a solvent, in which 5 wt. % of vinylene carbonate is added as an additive.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 shows the electron cloud distribution of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material; FIG. 2 is an SEM image of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% material; FIG. 3 is an HRTEM image of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% material;

[0039] FIG. 4 shows charge/discharge curves of a coin cell prepared in Example 2 in different cycles at a current density of 0.1 C (1 C=120 mA g.sup.−1);

[0040] FIG. 5 shows charge/discharge curves in the second cycle of the coin cell prepared in Example 2 at different current densities;

[0041] FIG. 6 shows the capacity retention curve and the coulombic efficiency of the coin cell prepared in Example 2 at a current density of 2 C for prolonged cycles;

[0042] FIG. 7 shows the comparison of the rate performance of the Na.sub.6Fe.sub.5(SO.sub.4).sub.8 material (NFS) prepared in Chinese Patent Publication No. CN108682827A and the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material (NFSF) prepared in the present invention;

[0043] FIG. 8 is an SEM image of the hard carbon anode material in Example 3; and

[0044] FIG. 9 is a charge/discharge curve of a sodium-ion full battery prepared in Example 3 at a current density of 0.5 C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] The present invention will be further described below referred to the accompanying drawings and specific examples, so that those skilled in the art can better understand and implement the present invention; however, the present invention is not limited thereto.

[0046] In the following examples, the terms involving SEM, HRTEM and CNF used herein are all professional terms in the art. The SEM refers to scanning electron microscope, HRTEM refers to high resolution transmission electron microscope, and CNF refers to carbon nanofibers.

EXAMPLE 1

Preparation of Na.SUB.3.Fe.SUB.2.(SO.SUB.4.).SUB.3.F/CNF Cathode Material for Sodium-Ion Battery

[0047] 1. Ferrous sulfate heptahydrate was dried in an oven at 200° C. for 10 h to obtain anhydrous ferrous sulfate.

[0048] 2. 0.4675 g of sodium sulfate, 1.00 g of anhydrous ferrous sulfate, 0.1379 g of sodium fluoride and 0.0803 g of (5 wt%) carbon fibers were weighed, and added to a 50 mL zirconia jar. 34 g of zirconia balls was added, and the ball-to-material ratio was set to 20:1. Argon was fed for protection, and the material was ball milled at a rotation speed of 200 r/min and a revolution speed of 500 r/min for 6 h.

[0049] 3. The resulting ball-milled composite precursor was transferred to a tube furnace, thermally treated under an argon atmosphere, and calcinated at 350° C. for 5 h. The calcined product was ground into powder to obtain a composite material containing 5% carbon fibers, which was designated as Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% cathode material.

[0050] FIG. 1 shows the electron cloud distribution of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material. It can be seen from FIG. 1 that the incorporation of F.sup.− ion makes the electron cloud distribution between Fe and Fe, and Fe and O atoms more uniform, which improves the interaction between atoms, effectively stabilizes the crystal structure of the material, and inhibits the oxidation of Fe element as well as the formation of impurity phases during the preparation of the material, and thus is conducive to improving the sodium storage capacity, cycling stability and rate performance of the cathode material for sodium-ion batteries.

[0051] FIG. 2 is an SEM image of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% cathode material. It can be seen from FIG. 2 that the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% cathode material is in the form of micron-scale bulk particles, in which the carbon fibers are clearly wound between the particles to form a micro-nano structure with resembling ribbon-wound particles.

[0052] FIG. 3 is an HRTEM image of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% cathode material. It can be seen from FIG. 3 that the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material shows high crystallinity, and the carbon fibers are graphitized and tightly embedded into the bulk structure of Na.sub.3Fe.sub.2(SO.sub.4).sub.3F particle.

EXAMPLE 2

Preparation of Sodium-Ion Button Battery

[0053] 0.8 g of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% cathode material, 0.1 g of a conductive carbon material (acetylene black), and 0.1 g of a binder (polyvinylidene fluoride) were weighed at a weight ratio of 8:1:1, and uniformly dispersed in N-methyl pyrrolidone as a solvent. The resulting mixed slurry was evenly coated on an aluminum foil, and dried under vacuum at 120° C. for 10 h to obtain a cathode plate. A thin metal sodium sheet was used as the counter electrode, and the cathode plate, a separator, the counter electrode, a spacer, and a spring were placed in sequence in a CR2032-type coin cell, and a electrolyte comprising sodium perchlorate as the solute at a concentration of 1 mol/L and propylene carbonate as the solvent was added, and then sealed to obtain a sodium-ion button battery.

[0054] FIGS. 4-6 show the electrochemical performance of the coin cell cycled in a voltage range of 2.0-4.5 V vs. Na.sup.+/Na. FIG. 4 shows charge/discharge curves in different cycles at a current density of 0.1 C. It can be seen from FIG. 4 that the assembled half sodium-ion battery has high cycling stability, resulting in a specific initial discharge capacity of 109 mAh g.sup.−1 in the first cycle, and retained 90 mAh g.sup.−1 after 150 cycles.

[0055] FIG. 5 shows charge/discharge curves in the second cycle at different current densities. It can be seen from FIG. 5 that the assembled half sodium-ion battery has a higher operating voltage and better rate performance. The capacity is obtained at 65 mAh g.sup.−1 at a current density of 20 C.

[0056] FIG. 6 shows the capacity retention curve and the coulombic efficiency at a current density of 2 C. It can be seen from FIG. 6 that the assembled half sodium-ion battery has good cycling stability at a high rate, and the specific discharge capacity after 1200 cycles at a current density of 2 C is still 70 mAh g.sup.−1.

[0057] FIG. 7 compares the rate performance of the Na.sub.6Fe.sub.5(SO.sub.4).sub.8 material prepared in Chinese Patent Publication No. CN108682827A and the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material prepared in the present invention. It can be seen from FIG. 7 that the incorporation of F.sup.− ion can effectively improve the rate performance of such a cathode material. At a current density of 20 C, the specific discharge capacity of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material is still 50 mAh g.sup.−1. After 40 cycles of charging and discharging, the capacity at a current density of 0.1 C still remains at 90 mAh g.sup.−1.

EXAMPLE 3

Preparation of a Sodium-Ion Full Battery

[0058] 1. 0.8 g of the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F/CNF-5% cathode material, 0.1 g of acetylene black as a conductive carbon material, and 0.1 g of polyvinylidene fluoride as a binder were weighed at a weight ratio of 8:1:1, and dispersed in N-methyl pyrrolidone as a solvent. The uniformly mixed slurry was evenly coated on an aluminum foil, and dried under vacuum at 120° C. for 12 h to obtain a cathode plate.

[0059] 2. 0.7 g of hard carbon anode material, 0.2 g of acetylene black as a conductive carbon material, and 0.1 g of polyvinylidene fluoride as a binder were weighed at a weight ratio of 7:2:1, and dispersed in N-methyl pyrrolidone as a solvent. The uniformly mixed slurry was evenly coated on a copper foil, and dried under vacuum at 120° C. for 12 h to obtain an anode plate.

[0060] 3. The cathode plate, a separator, the anode plate, a spacer and a spring were placed in sequence in a CR2032-type coin cell. A electrolyte comprising sodium perchlorate as the solute at a concentration of 1 mol/L, ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1 as the solvent, and 5 wt. % vinylene carbonate as the additive was added, and sealed to obtain a sodium-ion full battery.

[0061] FIG. 8 is an SEM image of the hard carbon anode material. It can be seen from FIG. 8 that the hard carbon material is in the form of micron-scale spherical particles, which are aggregated from nano-scale primary particles.

[0062] FIG. 9 shows charge/discharge curves in different cycles of the full battery at a current density of 0.5 C. It can be seen from FIG. 9 that the assembled full battery has a higher operating voltage and better charge/discharge specific capacity, and the specific discharge capacity at 0.5 C is up to 81 mAh g.sup.−1 in the first cycle.

[0063] The above-described embodiments are merely preferred embodiments for the purpose of fully illustrating the present invention, and the scope of the present invention is not limited thereto. Equivalent substitutions or modifications can be made by those skilled in the art based on the present invention, which are within the scope of the present invention. The scope of the present invention is defined by the appended claims.