Sodium ion battery

Abstract

The invention relates to a sodium ion battery, and belongs to sodium ion batteries. The battery cell of the sodium-ion battery comprises a positive plate, a negative plate and an electrolyte, the active material of the positive plate is carbon-coated aluminum-doped sodium ferric sulfate; the chemical formula of the aluminum-doped sodium ferric sulfate is Na < 2 + 2y > Fe < 2-x-y > Al < x > (SO4) 3, x is more than or equal to 0.01 and less than or equal to 0.05, and y is more than or equal to 0.1 and less than or equal to The particle size of the carbon-coated aluminum-doped sodium ferric sulfate ranges from 50 nm to 100 nm; the weight ratio of a carbon coating layer in the carbon-coated aluminum-doped sodium ferric sulfate is 1%-5%; the active material of the negative plate is carbon-coated sodium titanate; the specific surface area of the carbon-coated sodium titanate is 20 m < 2 > / g to 25 m < 2 > / g; the weight ratio of a carbon coating layer in the carbon-coated sodium titanate is 0.5%-3%. Volume expansion is jointly inhibited through structural stability of positive and negative electrode materials, and the cycle life is prolonged.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and in particular relates to a sodium ion battery. Background Art

[0002] The current sodium-ion battery cathode material system mainly focuses on three categories: polyanionic compounds, layered metal oxides, and Prussian blue analogs. Among them, polyanionic compounds such as Na3V2(PO4)3 have the advantage of NASICON framework structure, but the vanadium element they contain is highly toxic, resulting in greater environmental risks, and their intrinsic electronic conductivity is low (usually <10 -6 S / cm), which limits the improvement of its rate performance. Layered metal oxides such as NaNiO2 undergo irreversible phase transitions during the charge and discharge process, which leads to voltage decay. The typical cycle decay rate is greater than 0.3mV / week, which makes the stability of the actual working voltage platform poor. Although Prussian blue analogues have an open framework structure and a high theoretical capacity (about 170mAh / g), it is difficult to avoid [Fe(CN)6] vacancy defects and lattice water residue problems (usually the water content is greater than 5wt%) during the preparation process. These problems seriously weaken the structural stability and thermal safety of the material.

[0003] In terms of negative electrode material systems, although commercial hard carbon materials have a high specific capacity (about 300mAh / g), they will produce anisotropic volume expansion during the insertion / extraction process of sodium ions. The expansion rate of the first cycle exceeds 15%, which can easily lead to the collapse of the electrode structure, and then the contact between the active material and the current collector fails. In particular, when hard carbon negative electrode materials and Prussian blue positive electrode materials form a full battery system, the interface compatibility problem is particularly prominent. In conventional carbonate-based electrolytes, the system is prone to irreversible electrolyte decomposition reactions, resulting in reduced coulombic efficiency (first efficiency less than 85%) and shortened cycle life (100-week capacity retention rate less than 70%). Although titanium-based materials such as Na2Ti3O7 exhibit excellent structural stability and low sodium storage potential (about 0.3V vs. Na + / Na), but its theoretical specific capacity is low (about 155mAh / g), which limits the energy density of the whole battery. To achieve a breakthrough in energy density, it is necessary to match it with a high working voltage cathode (>3.5V vs.Na + / Na), which further increases the design difficulty and cost of the battery system.

[0004] Therefore, it is still an urgent problem to be solved by constructing a matching system of positive and negative active materials and combining it with interface engineering optimization to break through the energy density bottleneck of existing material combinations and achieve long-term cycle stability of sodium-ion batteries. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a sodium ion battery, in which carbon-coated aluminum-doped sodium ferric sulfate is used as the active material of the positive electrode sheet, and carbon-coated sodium titanate is used as the active material of the negative electrode sheet. The two are used in combination to form a high-voltage, high-cycle stability system.

[0006] The present invention aims to provide a sodium ion battery, wherein the battery cell of the sodium ion battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte;

[0007] The active material of the positive electrode is carbon-coated aluminum-doped sodium ferric sulfate; the chemical formula of the aluminum-doped sodium ferric sulfate is Na 2+2y Fe 2-x-y Al x (SO4)3, wherein 0.01≤x≤0.05, 0.1≤y≤1; the particle size of the carbon-coated aluminum-doped sodium ferric sulfate is 50nm-100nm; the weight proportion of the carbon coating layer in the carbon-coated aluminum-doped sodium ferric sulfate is 1%-5%;

[0008] The active material of the negative electrode sheet is carbon-coated sodium titanate; the specific surface area of the carbon-coated sodium titanate is 20m 2 / g-25m 2 / g; the weight proportion of the carbon coating layer in the carbon-coated sodium titanate is 0.5%-3%.

[0009] In one embodiment of the present invention, the preparation of the carbon-coated aluminum-doped sodium ferric sulfate comprises the following steps: uniformly mixing a sodium source, an iron source, an aluminum source, a sulfur source and an organic antioxidant, and subjecting the mixture to a hydrothermal reaction and calcination to obtain the carbon-coated aluminum-doped sodium ferric sulfate; the organic antioxidant is obtained by compounding glucose and polyethylene glycol in a mass ratio of (2-4):1.

[0010] In one embodiment of the present invention, the sodium source is selected from one or more of sodium sulfate, sodium carbonate, sodium hydroxide, sodium citrate and sodium acetate;

[0011] The iron source is selected from ferrous sulfate and / or ferrous oxalate;

[0012] The aluminum source is selected from aluminum sulfate and / or aluminum isopropoxide;

[0013] The sulfur source is selected from one or more of sodium sulfate, ferrous sulfate, aluminum sulfate and thiourea.

[0014] In one embodiment of the present invention, the pH of the hydrothermal reaction is 3-4, the temperature is 100° C.-120° C., and the time is 22 h-26 h;

[0015] The calcination is carried out under a protective atmosphere at 200° C.-450° C. for 1.5 h-2.5 h.

[0016] Furthermore, the pH regulator is dilute sulfuric acid;

[0017] In one embodiment of the present invention, the carbon-coated aluminum-doped sodium ferric sulfate is prepared by a low-temperature hydrothermal method (100°C-120°C) in combination with glucose / polyethylene glycol organic antioxidants, which can reduce energy consumption while obtaining carbon-coated aluminum-doped sodium ferric sulfate with smaller particle size and higher performance; the crystal nucleation rate at low temperature is greater than the growth rate, and nano-scale particles are easily generated; and the energy consumption is low and the equipment requirements are low: the use of high-temperature and high-pressure reactors is avoided, which is safer. In addition, the dual reduction-carbonization function of glucose / polyethylene glycol is used to achieve Fe 2+ Protection and in situ growth of carbon layers.

[0018] In one embodiment of the present invention, glucose has strong reducing properties and can convert Fe 3+ Reduction to Fe 2+ . During the roasting process, glucose will generate amorphous carbon, which has a medium conductivity and general coating uniformity. Due to the low molecular weight of glucose, there is no steric hindrance effect, so it has no dispersing effect. At a carbonization temperature of 200℃-400℃, a dense carbon layer will be generated. Polyethylene glycol only relies on terminal hydroxyl groups for reduction reaction, so its reducing ability is limited. During the roasting process, polyethylene glycol will form a porous carbon skeleton. Although its conductivity is low, it has rich ion diffusion channels. Since polyethylene glycol is a long-chain polymer, it can be adsorbed on the surface of the particles, effectively inhibiting particle agglomeration, thereby exhibiting a strong dispersing effect. Polyethylene glycol decomposes at 200℃-300℃, and a porous structure remains. By compounding glucose and polyethylene glycol, polyethylene glycol uses the steric hindrance effect to disperse the particles, allowing glucose molecules to contact Fe more evenly. 2+ , thereby improving the reduction efficiency (oxidation loss rate is less than 5%). Glucose itself has strong reducing properties, which can make up for the lack of reducing ability of polyethylene glycol. In addition, glucose can provide a continuous conductive network (resistivity is less than 100Ω·cm), and the pores derived from polyethylene glycol can promote the infiltration of the electrolyte. The thickness of the carbon layer of this composite system is controllable, and it has good mechanical flexibility and ion diffusion rate. At low temperatures (less than 80°C), the solubility of glucose is low (about 30g / 100mL water) and it is easy to crystallize. Polyethylene glycol increases the solubility of glucose through hydrogen bonding, so that its solubility after compounding is increased to 50g / 100mL, thereby ensuring the uniformity of the reaction.

[0019] In one embodiment of the present invention, the preparation of the carbon-coated sodium titanate comprises the following steps:

[0020] S1. Ball-milling a sodium source, a titanium source, and a dispersant in a ball mill until uniform, and pre-calcining to obtain an intermediate;

[0021] S2. The intermediate described in S1 and the organic carbon source are uniformly ball-milled in a ball mill, and then calcined to obtain carbon-coated sodium titanate; the particle size D90 after ball milling is less than 0.8 μm.

[0022] In one embodiment of the present invention, in S1, the sodium source is selected from sodium carbonate and / or sodium nitrate;

[0023] The titanium source is selected from titanium dioxide and / or tetrabutyl titanate;

[0024] The dispersant is selected from one or more of stearic acid, sodium lauryl sulfate (SDS) and polyacrylic acid (PAA);

[0025] The amount of the dispersant used is 1%-2% of the total mass of the sodium source and the titanium source.

[0026] In one embodiment of the present invention, in S2, the organic carbon source is selected from one or more of sucrose, glucose, polyacrylic acid and polyethylene glycol;

[0027] The amount of the organic carbon source is 5%-20% of the mass of the intermediate.

[0028] In one embodiment of the present invention, the solvent for ball milling is selected from anhydrous ethanol and / or isopropanol, the ball-to-material ratio is 10:1, the rotation speed is 280 rpm-320 rpm, and the time is 6 h-12 h;

[0029] The pre-calcination temperature is 400-600°C and the time is 2h-4h;

[0030] The calcination is carried out under a protective atmosphere at 800° C.-900° C. for 3 h-8 h.

[0031] In one embodiment of the present invention, the carbon-coated sodium titanate is prepared by dispersant-assisted ball milling and a two-step calcination process, which significantly increases the surface area of the carbon-coated sodium titanate. The high specific surface area is conducive to increasing the ion diffusion rate.

[0032] In one embodiment of the present invention, the electrolyte consists of sodium hexafluorophosphate, fluoroethylene carbonate (FEC) and a solvent;

[0033] The solvent is selected from one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate (EC), dimethyl carbonate (DMC) and diethylene glycol dimethyl ether.

[0034] In one embodiment of the present invention, the concentration of sodium hexafluorophosphate in the electrolyte is 0.9 mol / L-1.1 mol / L, and the concentration of fluoroethylene carbonate is 1 wt %-5 wt %.

[0035] The technical solution of the present invention has the following advantages over the prior art:

[0036] (1) The active material of the positive electrode of the sodium ion battery described in the present invention is carbon-coated aluminum-doped sodium ferric sulfate, which has an Alluaudite crystal structure and a three-dimensional sodium ion diffusion channel, which can achieve rapid sodium ion migration. In addition, the material has high voltage, and its high voltage characteristic comes from SO4 2- The strong electronegativity and Fe 2+ / Fe 3+ The redox couple has high activity, the voltage platform can reach 3.8V, and the theoretical capacity is 120mAh / g. 3+ (ionic radius ) partially replaces Fe 3+ (ionic radius ), which causes lattice contraction and forms a local stress field, thereby widening the sodium ion diffusion channel and greatly improving the ion mobility. The active material of the negative electrode is carbon-coated sodium titanate, and the sodium insertion potential is 0.3V-0.5V (vs.Na + / Na), when combined with carbon-coated aluminum-doped sodium ferric sulfate (such as 3.8V), the operating voltage of the full battery can reach 3.3V-3.5V, significantly improving the energy density.

[0037] (2) The sodium ion battery described in the present invention suppresses volume expansion through the structural stability (zero strain characteristics) of the positive and negative electrode materials, thereby extending the cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0039] Figure 1 This is an SEM image of the carbon-coated aluminum-doped sodium ferric sulfate of Example 1 in Test Example 2 of the present invention;

[0040] Figure 2 This is an SEM image of the carbon-coated sodium titanate of Example 1 in Test Example 2 of the present invention;

[0041] Figure 3 This is a charge and discharge curve of a half-cell made of carbon-coated sodium titanate in Example 1 of Test Example 3 of the present invention at a rate of 0.1C;

[0042] Figure 4 This is a charge and discharge curve of a half-cell made of carbon-coated aluminum-doped sodium ferric sulfate in Example 1 of Test Example 3 of the present invention at a rate of 0.1C;

[0043] Figure 5This is a charge and discharge curve diagram of the square shell battery of Example 1 in Test Example 3 of the present invention at a rate of 0.1C;

[0044] Figure 6 This is the long cycle performance of the sodium ion battery of Example 1 in Test Example 4 of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. It should be understood that the specific embodiments are only used to illustrate the present invention and are not intended to limit the present invention.

[0046] In the present invention, unless otherwise defined, technical and scientific terms used herein have the same meanings as commonly understood by one skilled in the art to which the present invention belongs.

[0047] In the present invention, unless stated otherwise, the term "and / or" used in the present invention includes any and all combinations of one or more of the associated listed items.

[0048] In the present invention, unless otherwise stated, the experimental methods used in the examples of the present invention are conventional methods unless otherwise stated, and the materials, reagents, etc. used are all commercially available unless otherwise stated.

[0049] In the present invention, unless otherwise specified, the separator used in the embodiments of the present invention is a glass fiber membrane.

[0050] Example 1

[0051] The sodium ion battery and its preparation method of this embodiment specifically include the following steps:

[0052] S1. Preparation of carbon-coated aluminum-doped sodium ferric sulfate: According to Na3Fe 1.48 Al 0.02 Sodium sulfate, ferrous sulfate and aluminum sulfate were weighed in a stoichiometric ratio of (SO4)3, and glucose (10wt% of all raw materials) and PEG-2000 (3wt%) were added and stirred evenly. Dilute sulfuric acid was added to adjust the pH value to about 3.5. The mixture was hydrothermally reacted at 110°C for 24h, and then annealed at 400°C in a nitrogen atmosphere for 2h to obtain carbon-coated aluminum-doped sodium ferric sulfate with a carbon coating layer accounting for 3% by weight.

[0053] S2. Preparation of carbon-coated sodium titanate: rutile titanium dioxide and sodium carbonate were weighed according to the stoichiometric ratio of Na2Ti3O7, stearic acid (1.5 wt% of the total mass of the sodium source and the titanium source) was added and premixed, and then transferred to a ball mill for ball milling with a ball-to-material ratio of 10:1, a rotation speed of 300 rpm, a time of 12 h, and anhydrous ethanol as the solvent; after ball milling, the mixture was pre-calcined at 500°C for 2 h to remove organic residues and initially form a Na-Ti-O intermediate phase; the obtained material was crushed, and sucrose (10 wt% of the Na-Ti-O intermediate phase) was added, and the ball milling was repeated for 6 h until D90 was less than 0.5 μm; finally, the mixture was calcined at 850°C for 5 h in a nitrogen atmosphere to obtain a carbon-coated sodium titanate with a carbon coating layer accounting for 1.5% by weight;

[0054] S3. Preparation of pole pieces: Carbon-coated aluminum-doped sodium ferric sulfate was used as the positive electrode active material, carbon-coated sodium titanate was used as the negative electrode active material, acetylene black was used as the conductive agent, and PVDF was used as the binder. The active material:conductive agent:binder were added to the solvent NMP in a mass ratio of 92:5:3 to obtain a positive and negative electrode slurry with a binder concentration of 5 wt%. The positive and negative electrode slurries were applied to the surface of 6 μm aluminum foil by manual coating, and dried to obtain positive and negative electrode pieces; wherein, the active material loading of the positive electrode piece was 10 mg / cm 2 , the active material loading of the negative electrode is 8 mg / cm 2 .

[0055] S4. Preparation of electrolyte: Sodium hexafluorophosphate and fluoroethylene carbonate were dissolved in a solvent (the volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) was 1:1) to obtain an electrolyte with a sodium hexafluorophosphate concentration of 1 mol / L and a fluoroethylene carbonate concentration of 2 wt%.

[0056] S5. Assembly of sodium ion battery: Arrange the negative electrode sheet, diaphragm, positive electrode sheet and isolation membrane in order, assemble them into a square shell battery, and inject electrolyte.

[0057] Example 2

[0058] The same as Example 1, except that: in the preparation process of carbon-coated aluminum-doped sodium ferric sulfate, aluminum-doped sodium ferric sulfate Na3Fe 1.48 Al 0.02 (SO4)3(x=0.02,y=0.5) is replaced by Na 2.2 Fe 1.89 Al 0.01 (SO4)3(x=0.01, y=0.1).

[0059] Example 3

[0060] The same as Example 1, except that: in the preparation process of carbon-coated aluminum-doped sodium ferric sulfate, aluminum-doped sodium ferric sulfate Na3Fe 1.48 Al 0.02 (SO4)3(x=0.02,y=0.5) is replaced by Na 3.6 Fe 1.16 Al 0.04 (SO4)3(x=0.04, y=0.8).

[0061] Example 4

[0062] The same as Example 1, except that: in the preparation process of carbon-coated aluminum-doped sodium ferric sulfate, aluminum-doped sodium ferric sulfate Na3Fe 1.48 Al 0.02 (SO4)3(x=0.02,y=0.5) is replaced by Na 3.4 Fe 1.29 Al 0.01 (SO4)3(x=0.01, y=0.7).

[0063] Comparative Example 1

[0064] The same as Example 1, except that: in the preparation process of carbon-coated aluminum-doped sodium ferric sulfate, aluminum-doped sodium ferric sulfate Na3Fe 1.48 Al 0.02 (SO4)3(x=0.02,y=0.5) is replaced by Na3Fe 1.5 (SO4)3(x=0, y=0.5).

[0065] Comparative Example 2

[0066] The same as Example 1, except that: in the preparation process of carbon-coated aluminum-doped sodium ferric sulfate, aluminum-doped sodium ferric sulfate Na3Fe 1.48 Al 0.02 (SO4)3(x=0.02,y=0.5) is replaced by Na2Fe 1.98 Al 0.02 (SO4)3(x=0.02, y=0).

[0067] Comparative Example 3

[0068] The method is basically the same as Example 1, except that: in the preparation process of carbon-coated sodium titanate, ball milling and pre-calcination are not performed.

[0069] Comparative Example 4

[0070] The method is basically the same as Example 1, except that during the preparation of the negative electrode sheet, the carbon-coated sodium titanate is replaced by hard carbon.

[0071] Test Example 1

[0072] The resistivity R, ion diffusion coefficient, etc. of the positive electrode materials of Examples 1-4 and Comparative Examples 1-2 were tested:

[0073] (1) Resistivity R (Ω·cm) Test: The powder resistivity of the positive electrode materials of Examples 1-4 and Comparative Examples 1-2 was measured using an ST2742C automated powder resistivity tester. When the carbon content was the same, the electronic conductivity of the positive electrode material was directly reflected. The curve between pressure (MPa) and resistivity (Ω·cm) was recorded, and the resistivity at a pressure of 60 MPa was used for comparison between different samples.

[0074] (2) Ion diffusion coefficient test: The ion diffusion coefficients of the positive electrode materials of Examples 1-4 and Comparative Examples 1-2 were tested by the constant current intermittent titration method (GITT).

[0075] Table 1 shows the relevant performance parameters finally measured:

[0076] Table 1

[0077] Sample Resistivity (Ω·cm) <![CDATA[Ionic diffusion coefficient (cm 2 / s)]]> Example 1 1932.3 <![CDATA[3.78×10 -10 ]]> Example 2 1911.2 <![CDATA[3.54×10 -10 ]]> Example 3 1930.0 <![CDATA[3.61×10 -10 ]]> Example 4 1907.8 <![CDATA[3.47×10 -10 ]]> Comparative Example 1 2807.5 <![CDATA[7.89×10 -11 ]]> Comparative Example 2 2934.2 <![CDATA[8.74×10 -11 ]]>

[0078] It can be seen from Table 1 that the resistivity of the positive electrode material of the embodiment is less than 2000Ω·cm, and the ion diffusion coefficient is 10 -10 Power.

[0079] Comparing Example 1 with Comparative Example 1, it can be seen that when Al is not doped, the resistivity and ion diffusion coefficient are both improved. This is mainly because Al 3+ (ionic radius ) partially replaces Fe 3+ (ionic radius ), which causes lattice contraction and lattice distortion to form a local stress field, broaden the sodium ion diffusion channel, and improve ion mobility; Al 3+ Doping leads to Fe 2 + / Fe 3+ The charge imbalance of the redox pair is achieved by adjusting the sodium vacancy concentration (controlled by the y value) to achieve charge compensation, reduce the electron transition barrier, and significantly reduce the resistivity; at the same time, Al 3+ After occupying the Fe site, it reduces the Fe 3+ Side reaction with electrolyte improves cycle stability.

[0080] Comparison of Example 1 and Comparative Example 2 shows that when y=0, the sodium site is fully occupied, the ion migration needs to overcome a higher energy barrier, and the diffusion coefficient decreases. 2+ / Fe 3+Mixed valence state, electron conduction relies on long-range energy band transport, and resistivity increases.

[0081] Test Example 2

[0082] The carbon-coated aluminum-doped sodium ferric sulfate and carbon-coated sodium titanate of Example 1 were characterized by SEM. Figure 1-Figure 2 As shown. Figure 1 It can be seen that the particle size of carbon-coated aluminum-doped sodium ferric sulfate is about 50nm-100nm. This is because carbon-coated aluminum-doped sodium ferric sulfate is prepared by low-temperature hydrothermal method. At low temperature, the crystal nucleation rate is greater than the growth rate, and nano-scale particles are easily generated. Such nano-scale particles provide abundant reactive sites for sodium ion deintercalation and shorten the ion diffusion path, thereby improving the rate performance; small particles can alleviate the volume expansion effect, effectively reduce the risk of particle pulverization, and improve the cycle stability. Figure 2 It can be seen that the average particle size of carbon-coated sodium titanate is approximately 1 μm. Through a two-step calcination process, the electronic conductive network between particles is enhanced. The submicron particles not only ensure sufficient sodium ion diffusion channels, but also avoid the agglomeration problem caused by the high surface energy of nanoparticles. At the same time, the processability and electronic conductivity / ion diffusion rate of the powder are improved.

[0083] Test Example 3

[0084] (1) Specific surface area test: The specific surface areas of the carbon-coated sodium titanate materials of Example 1 and Comparative Example 3 were respectively tested by the nitrogen adsorption-desorption BET method (BET);

[0085] (2) Charge and discharge curve test: The carbon-coated aluminum-doped sodium ferric sulfate, carbon-coated sodium titanate of Example 1 and carbon-coated sodium titanate of Comparative Example 3 were used as active materials, acetylene black was used as a conductive agent, and PVDF was used as a binder. The active material: the conductive agent: the binder were added to the solvent NMP in a mass ratio of 8:1:1 to obtain a slurry with a binder concentration of 5 wt%. The slurry was applied to the surface of a 6 μm aluminum foil by manual coating and dried to obtain an electrode. The electrode was used as a working electrode, the sodium sheet was used as a counter electrode, and the electrolyte was the same as above. A half-cell was assembled and a charge and discharge curve test was performed to obtain the specific capacity of the aluminum-doped sodium ferric sulfate positive electrode active material and the carbon-coated sodium titanate negative electrode active material.

[0086] The square shell battery of Example 1 was subjected to a charge and discharge curve test to obtain the charge and discharge curve and discharge capacity of the square shell battery;

[0087] The charge and discharge curve test is to first charge at 0.5C to a cut-off voltage of 4V, let it stand for 10 minutes, and then discharge at 0.5C to a cut-off voltage of 2V. The sodium ion battery is charged and discharged at a rate of 0.1C.

[0088] Table 2 and Figure 3 Shown are the specific capacity and specific surface area of the carbon-coated sodium titanate of Example 1 and Comparative Example 3:

[0089] Table 2

[0090] Sample Specific capacity (mAh / g) <![CDATA[Specific surface area (m 2 / g)]]> Example 1 180 22.7 Comparative Example 3 137.9 14.8

[0091] From Table 2 and Figure 3 It can be seen that the half-cell made of carbon-coated sodium titanate in Example 1 has a specific capacity of 180 mAh / g at a rate of 0.1 C and a specific surface area of 22.7 m 2 / g. However, the half-cell made of carbon-coated sodium titanate in Comparative Example 3 has a specific capacity of only 137.9 mAh / g at a rate of 0.1C, and a specific surface area of only 14.8 m 2 This is because the carbon-coated sodium titanate in Example 1 first undergoes a single ball milling and low-temperature pre-calcination to form a Na-Ti-O phase and pre-decompose organic residues such as stearic acid, preventing the formation of carbon impurities during high-temperature calcination. Subsequently, a secondary ball milling process refines the particles to the submicron level, facilitating the high-temperature solid-phase reaction kinetics. High-temperature calcination optimizes grain growth, resulting in highly crystalline Na2Ti3O7. Comparative Example 3, on the other hand, skips the single ball milling and low-temperature pre-calcination process, and the TiO2 / Na2CO3 reaction directly forms micron-sized aggregates, extending the sodium ion diffusion path. Furthermore, the localized accumulation of the carbon layer results in uneven carbon coating, hindering electron transport.

[0092] Figure 3-Figure 4 The charge and discharge curves of the half-cell made of carbon-coated sodium titanate and carbon-coated aluminum-doped sodium ferric sulfate in Example 1 at a rate of 0.1C are shown. Figure 3 It can be seen that the discharge voltage of the negative electrode active material is 0.43V and the specific capacity is 180mAh / g; Figure 4 It can be seen that the discharge voltage of the positive electrode active material is 3.88V, the specific capacity is about 100mAh / g, and the material has a high and stable redox potential. According to the half-cell data, the full-cell discharge voltage has a higher discharge voltage, about 3.45V, which is consistent with the Figure 5 The charge and discharge curves of the square shell battery of Example 1 are consistent. Based on the total mass of the positive and negative active materials, the battery energy density reaches 120Wh / kg-125Wh / kg. However, due to the use of sodium titanate as the negative electrode, which is significantly higher than the deposition potential of metallic sodium (0V), the sodium precipitation on the negative electrode surface is fundamentally avoided, and the battery has good cycle performance. 3+ The matching design of the positive electrode is synergistically regulated by the vacancy / vacancy, realizing the unification of the high voltage platform (≥3.4V) of the whole battery and the zero sodium precipitation characteristics, breaking the trade-off between energy density and cycle life of traditional sodium-ion batteries.

[0093] Test Example 4

[0094] The sodium ion batteries of Example 1 and Comparative Example 4 were tested for energy density, cycle stability, etc.;

[0095] (1) Initial discharge specific capacity: refer to the above test.

[0096] (2) Energy density (based on active material): At standard temperature (25±2°C), perform constant current charge and discharge cycle test (0.5C rate) to the cutoff voltage, record the voltage and current data during the discharge process, calculate the integrated discharge capacity and the average voltage during the discharge process, and divide the product of the integrated discharge capacity and the average voltage by the mass of the active material to obtain the energy density (Wh / kg).

[0097] (3) Cyclic stability test: first perform 0.1C capacity calibration and record the discharge capacity as C0, then use 10C charge and discharge (charge to 3.9V, discharge to 1.5V), perform 0.1C capacity calibration once every 1000 cycles, and record the capacity retention rate at the 1000th cycle. The discharge capacity C0 after the 10th 0.1C capacity calibration is 10 , capacity retention rate R=C 10 / C0*100%, to evaluate the cycling stability of sodium-ion batteries for 8000 cycles;

[0098] Table 3 and Figure 6 The final measured performance parameters are shown:

[0099] Table 3

[0100] Sample Example 1 Comparative Example 4 First discharge specific capacity (mAh / g) 122.5(0.5C) 118.3(0.5C) Energy density (Wh / kg) 124.7 105.2 8000 cycle capacity retention rate 98.3％ 0％

[0101] From Table 3 and Figure 6 It can be seen that the sodium ion battery of Example 1 uses carbon-coated aluminum-doped sodium ferric sulfate as the positive electrode material and carbon-coated sodium titanate prepared by dispersant-assisted ball milling and two-step calcination process as the negative electrode material. Due to the synergistic effect of the high ionic conductivity and electronic conductivity of the positive electrode material, the high sodium embedding potential of the negative electrode material, the structural stability of the positive and negative electrode materials, and the stress buffering effect of the nano / submicron particles, the cycle life of the battery is greatly extended, and the capacity retention rate after 8000 cycles is ≥95%. The sodium ion battery of Comparative Example 4 uses hard carbon as the negative electrode, and the sodium storage potential of the hard carbon is close to 0V (vs. Na / Na + ), which easily leads to the appearance of sodium dendrites, causing the rupture and reconstruction of the SEI film, consuming sodium ions, and causing rapid capacity decay.

[0102] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A sodium ion battery, characterized in that The sodium ion battery cell comprises a positive electrode sheet, a negative electrode sheet and an electrolyte; The active material of the positive electrode is carbon-coated aluminum-doped sodium ferric sulfate; the chemical formula of the aluminum-doped sodium ferric sulfate is Na 2+2y Fe 2-x-y Al x (SO4)3, wherein 0.01≤x≤0.05, 0.1≤y≤1; the particle size of the carbon-coated aluminum-doped sodium ferric sulfate is 50nm-100nm; the weight proportion of the carbon coating layer in the carbon-coated aluminum-doped sodium ferric sulfate is 1%-5%; The active material of the negative electrode sheet is carbon-coated sodium titanate; the specific surface area of the carbon-coated sodium titanate is 20m 2 / g-25m 2 / g; the weight proportion of the carbon coating layer in the carbon-coated sodium titanate is 0.5%-3%.

2. The sodium ion battery according to claim 1, characterized in that The preparation of the carbon-coated aluminum-doped sodium ferric sulfate comprises the following steps: uniformly mixing a sodium source, an iron source, an aluminum source, a sulfur source, and an organic antioxidant, subjecting the mixture to a hydrothermal reaction and calcining to obtain the carbon-coated aluminum-doped sodium ferric sulfate; the organic antioxidant is obtained by compounding glucose and polyethylene glycol in a mass ratio of (2-4):

1.

3. The sodium ion battery according to claim 2, characterized in that The sodium source is selected from one or more of sodium sulfate, sodium carbonate, sodium hydroxide, sodium citrate and sodium acetate; The iron source is selected from ferrous sulfate and / or ferrous oxalate; The aluminum source is selected from aluminum sulfate and / or aluminum isopropoxide; The sulfur source is selected from one or more of sodium sulfate, ferrous sulfate, aluminum sulfate and thiourea.

4. The sodium ion battery according to claim 2, characterized in that The pH of the hydrothermal reaction is 3-4, the temperature is 100°C-120°C, and the time is 22h-26h; The calcination is carried out under a protective atmosphere at 200° C.-450° C. for 1.5 h-2.5 h.

5. The sodium ion battery according to claim 1, characterized in that The preparation of the carbon-coated sodium titanate comprises the following steps: S1. Ball-milling a sodium source, a titanium source, and a dispersant in a ball mill until uniform, and pre-calcining to obtain an intermediate; S2. The intermediate described in S1 and the organic carbon source are uniformly ball-milled in a ball mill, and then calcined to obtain carbon-coated sodium titanate; the particle size D90 after ball milling is less than 0.8 μm.

6. The sodium ion battery according to claim 5, characterized in that In S1, the sodium source is selected from sodium carbonate and / or sodium nitrate; The titanium source is selected from titanium dioxide and / or tetrabutyl titanate; The dispersant is selected from one or more of stearic acid, sodium lauryl sulfate and polyacrylic acid; The amount of the dispersant used is 1%-2% of the total mass of the sodium source and the titanium source.

7. The sodium ion battery according to claim 5, characterized in that In S2, the organic carbon source is selected from one or more of sucrose, glucose, polyacrylic acid and polyethylene glycol; The amount of the organic carbon source is 5%-20% of the mass of the intermediate.

8. The sodium ion battery according to claim 5, characterized in that The solvent of the ball milling is selected from anhydrous ethanol and / or isopropanol, the ball-to-material ratio is 10:1, the rotation speed is 280 rpm-320 rpm, and the time is 6 h-12 h; The pre-calcination temperature is 400-600°C and the time is 2h-4h; The calcination is carried out under a protective atmosphere at 800° C.-900° C. for 3 h-8 h.

9. The sodium ion battery according to claim 1, characterized in that The electrolyte consists of sodium hexafluorophosphate, fluoroethylene carbonate and a solvent; The solvent is selected from one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate and diethylene glycol dimethyl ether.

10. The sodium ion battery according to claim 9, characterized in that The concentration of sodium hexafluorophosphate in the electrolyte is 0.9 mol / L-1.1 mol / L, and the concentration of fluoroethylene carbonate is 1 wt %-5 wt %.

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