A sodium tritiate / hard carbon composite material, its preparation method and application

Abstract

This invention relates to a sodium titanate / hard carbon composite material, its preparation method, and its application, belonging to the field of sodium-ion battery technology. The sodium titanate / hard carbon composite material of this invention comprises sodium titanate nanosheets, hard carbon microspheres, and a carbon conductive layer co-coating the sodium titanate nanosheets and hard carbon microspheres; the sodium titanate nanosheets and hard carbon microspheres are carbonized and reinforced to form a C-N-Ti covalent bond interface under the action of a reinforcing agent; the thickness of the sodium titanate nanosheets is less than 10 nm, and the oxygen defect concentration is 5%-10%; the specific surface area of ​​the hard carbon microspheres is 10 m² / h. 2 / g-50m 2 / g, discharge specific capacity greater than 300mAh / g, I D / I G The particle size (D50) is 0.8–3, and the particle size is 200 nm–500 nm. The initial coulombic efficiency (ICE) of this composite material can reach over 80%, and the capacity retention rate is greater than 90% after 10,000 cycles.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and particularly relates to a sodium trititanate / hard carbon composite material, its preparation method and application. Background Technology

[0002] With the accelerated global energy transition towards a low-carbon structure, the demand for grid-connected storage of renewable energy is exploding, ushering in a critical period of development opportunities for energy storage technology. Sodium-ion batteries, with their high sodium abundance in the Earth's crust (2.75%), moderate electrode potential (-2.71V vs. SHE), and significant cost advantage (only 1 / 600th that of lithium resources), demonstrate irreplaceable application value in large-scale energy storage scenarios such as megawatt-level grid energy storage and smoothing renewable energy output. As the anode material, a core component of the battery, its electrochemical performance directly determines the battery's energy density, cycle life, and safety threshold, becoming a key bottleneck for current breakthroughs in sodium-ion battery technology.

[0003] Among numerous candidate materials, hard carbon anodes have become an early research focus due to their reversible specific capacity of 100 mAh / g-300 mAh / g, high initial efficiency of 75%-85%, and excellent cycling stability (capacity retention >80% after 1000 cycles). However, their sodium ion insertion / extraction potential range is concentrated in the range of 0.1V-0.3V (vs Na). + The sodium content ( / Na) is close to the deposition potential (0V) of metallic sodium. Under fast charging or over-discharging conditions, sodium metal deposition is easily triggered, leading to continuous growth of the SEI film, electrolyte consumption, and short circuit risk, which seriously restricts the safety performance and service life of the battery.

[0004] In recent years, layered sodium titanate (Na2Ti3O7) materials have stood out due to their unique structure. Their layered tunnel structure imparts a 0.3V (vs Na) to sodium ions. + The safe insertion / extraction potential of Na+ fundamentally avoids the risk of sodium deposition. This material has a reversible specific capacity of 180mAh / g-220mAh / g, utilizes abundant ilmenite reserves as raw material, and its preparation process is compatible with existing lithium-ion battery production lines, reducing production costs by more than 30% compared to hard carbon materials. Its 0.78nm interlayer sodium-ion transport channels, combined with Ti+... 4+ / Ti 3+ The reversible redox reaction enables the rapid migration of sodium ions (diffusion coefficient of 10). -10 cm 2 It can maintain 60% capacity output even at a high rate of 5C. However, the material has a low initial efficiency (about 60%-70%) and is accompanied by a charge-discharge phase transition, resulting in insufficient cycle life (<500 cycles).

[0005] To improve the performance defects of single materials, researchers have attempted to combine different materials. For example, patent CN117727899A discloses a composite of hard carbon and lithium titanate (Li4Ti5O). 12 The composite material, lithium titanate, is embedded within the pores of amorphous hard carbon. Due to its small volume deformation, fast ion migration rate, and high sodium intercalation potential, lithium titanate can solve the problems of low sodium ion migration rate and easy sodium precipitation within the amorphous hard carbon. However, this study did not address the sodium ion intercalation / deintercalation issue in lithium titanate, meaning that lithium titanate does not contribute to capacity. Furthermore, current technology has not solved the interfacial compatibility problem between titanium-based materials and hard carbon. Poor interfacial compatibility can hinder electron and ion transport, affecting battery performance.

[0006] Therefore, developing a Na2Ti3O7 / hard carbon composite material suitable for sodium-ion batteries to solve the interfacial compatibility problem in order to achieve high energy density and long cycle life is of great practical significance. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a sodium trititanate / hard carbon composite material, its preparation method, and its applications.

[0008] The first objective of this invention is to provide a sodium titanate / hard carbon composite material, comprising sodium titanate nanosheets, hard carbon microspheres, and a carbon conductive layer that co-coats the sodium titanate nanosheets and hard carbon microspheres; wherein the sodium titanate nanosheets and hard carbon microspheres, under the action of a reinforcing agent, are carbonized to form a CN-Ti covalent bond interface, thereby reducing the interface resistivity and improving the rate performance.

[0009] The mass ratio of sodium titanate nanosheets to hard carbon microspheres is (1:4)-(4:1); the weight percentage of the carbon conductive layer in the sodium titanate / hard carbon composite material is 0.5%-5.0%;

[0010] The amount of the reinforcing agent is 1%-3% of the mass of the sodium trititanate / hard carbon composite material;

[0011] The sodium tritiate nanosheets have a thickness of less than 10 nm and an oxygen defect concentration of 5%-10%.

[0012] The specific surface area of ​​the hard carbon microspheres is 10 m². 2 / g-50m 2 / g, discharge specific capacity greater than 300mAh / g, I D / I G The particle size is 0.8-3, and the particle size D50 is 200nm-500nm;

[0013] The reinforcing agent is selected from polyacrylonitrile and / or polydopamine.

[0014] In one embodiment of the present invention, the material of the carbon conductive layer is selected from graphene and / or carbon nanotubes.

[0015] In one embodiment of the present invention, the preparation of the sodium trititanate nanosheets includes the following steps:

[0016] S1. Titanium dioxide and sodium hydroxide are ball-milled evenly and then sintered to obtain sodium tritiate powder;

[0017] The sodium tritiate powder described in S2 and S1 is etched in hydrochloric acid solution, and then centrifuged, washed, and dried to obtain the sodium tritiate nanosheets.

[0018] In one embodiment of the present invention, in S1, the molar ratio of titanium dioxide to sodium hydroxide is 2:(3.1-3.2), and the excess sodium ions compensate for the volatilization of sodium ions during the sintering process;

[0019] The ball milling solvent is ethanol, the rotation speed is 300rpm-400rpm, and the time is 10h-20h;

[0020] The sintering process involves first heating to 200℃-400℃ at a rate of 1℃ / min-3℃ / min (to remove residual ethanol and moisture); then heating to 700℃-1000℃ at a rate of 3℃ / min-8℃ / min and holding at that temperature for 8h-12h; at the high temperature, NaOH melts (melting point 318℃) as a liquid medium to promote the exfoliation and recombination of the TiO2 layered structure, forming Na2Ti3O7 nanosheets; the reaction equation is: TiO2+3NaOH→Na2Ti3O7+H2O↑+other gaseous byproducts.

[0021] In one embodiment of the present invention, in S2, the concentration of the hydrochloric acid solution is 0.1 mol / L-0.5 mol / L (0.1 mol / L corresponds to approximately 5% oxygen vacancy concentration, and 0.5 mol / L corresponds to approximately 10% oxygen vacancy concentration); through H + Selective attack on oxygen sites on the surface of Na2Ti3O7, partially releasing Na + And form oxygen vacancies; Reaction equation: Na₂Ti₃O₇ + xH₂ + →Na 2- x Ti3O 7-x / 2 +xNa + +x / 2H2O; Additionally, avoid excessive concentrations that could damage the lamellar structure;

[0022] The etching time is 6h-24h; the defect concentration is precisely controlled by adjusting the concentration of hydrochloric acid solution and etching time.

[0023] In one embodiment of the present invention, the preparation of the hard carbon microspheres includes the following steps:

[0024] S1. Under the action of an organic amine catalyst, monophenolic compounds and aldehyde compounds react and are dehydrated to obtain phenolic resin prepolymer;

[0025] S2. The phenolic resin prepolymer described in S1 is dissolved in an alkaline solution, and then spray-dried and carbonized to obtain the hard carbon microspheres.

[0026] In one embodiment of the present invention, in S1, the monophenolic compound is selected from one or more of phenol, o-cresol, m-cresol, p-cresol and bisphenol A;

[0027] The aldehyde compounds are selected from formaldehyde and / or furfural;

[0028] The organic amine catalyst is selected from one or more of hexamethylenetetramine (HMTA), ethylenediamine, and triethanolamine;

[0029] The molar ratio of the monophenolic compounds to the aldehyde compounds is 1:(1.5-4);

[0030] The amount of the organic amine catalyst used is 1.0%-2.0% of the mass of the monophenol compound;

[0031] The reaction is carried out at a temperature of 70℃-120℃ for 2h-5h.

[0032] In one embodiment of the present invention, in S2, the alkaline solution is selected from one or more of sodium hydroxide solution, potassium hydroxide solution, ammonia water, tetramethylammonium hydroxide solution and sodium carbonate solution; the concentration of the alkaline solution is 0.9 mol / L-1.1 mol / L;

[0033] The spray dryer has an inlet air temperature of 240℃-260℃, an outlet air temperature of 80℃-100℃, and a rotation speed of 800rpm-2000rpm.

[0034] The carbonization process is divided into three stages: the first stage involves heating to 200℃-300℃ in an air or oxygen atmosphere at a rate of 1℃ / min-5℃ / min and holding for 1h-5h (to improve thermal stability and prevent the microspheres from melting and deforming during carbonization); the second stage involves heating to 600℃-800℃ in a protective atmosphere at a rate of 2℃ / min-10℃ / min and holding for 1h-2h (to remove light components and form the initial carbon skeleton); and the third stage involves heating to 1100℃-1400℃ at a rate of 3℃ / min-5℃ / min and holding for 1h-3h (to form a hard carbon microsphere structure).

[0035] The second objective of this invention is to provide a method for preparing the sodium tritiate / hard carbon composite material, comprising the following steps: ball milling sodium tritiate nanosheets, hard carbon microspheres, reinforcing agent and carbon conductive material, and obtaining the sodium tritiate / hard carbon composite material by carbonization reinforcement; wherein the carbonization reinforcement is carried out under a protective atmosphere at 600℃-1000℃ for 5h-15h.

[0036] Furthermore, when polyacrylonitrile is used as a reinforcing agent, the carbonization and reinforcing temperature is 800℃-1000℃, and when polydopamine is used as a reinforcing agent, the carbonization and reinforcing temperature is 600℃-800℃.

[0037] In one embodiment of the present invention, the ball mill rotates at a speed of 200 rpm to 500 rpm for a time of 5 h to 10 h.

[0038] A third objective of this invention is to provide an application of the aforementioned sodium tritiate / hard carbon composite material in sodium-ion batteries, for constructing sodium-ion batteries with high energy density and long cycle life.

[0039] The technical solution of the present invention has the following advantages compared with the prior art:

[0040] (1) The sodium titanate / hard carbon composite material of the present invention compensates for the shortcomings of hard carbon microspheres by the long lifespan of sodium titanate nanosheets, while the high first-efficiency of sodium titanate nanosheets improves the problem of low first-efficiency of hard carbon microspheres, significantly improving the overall performance of the composite material. Furthermore, the nanosheet structure and oxygen defect characteristics of sodium titanate can shorten the sodium ion diffusion path and regulate the electronic structure of the material, thereby improving electrochemical activity; the porous structure of hard carbon is conducive to the storage and transport of sodium ions, further optimizing the electrochemical performance of the material. In addition, hard carbon provides high specific capacity (about 300 mAh / g) through micropore filling and surface adsorption, while sodium titanate contributes stable capacity (about 150 mAh / g) through intercalation reaction. The combination of the two achieves capacity superposition, taking into account different sodium storage mechanisms.

[0041] (2) The sodium tritiate nanosheets in the sodium tritiate / hard carbon composite material of the present invention have a low sodium storage potential (approximately 0.3V vs. Na). + The potential of the electrode is typically higher (0.1V-1.0V), while that of hard carbon microspheres is typically higher. The combination of the two can broaden the operating voltage range of the electrode and improve the overall energy density.

[0042] (3) The sodium titanate nanosheets in the sodium titanate / hard carbon composite material of the present invention have high structural stability and small volume change, which can effectively suppress electrode pulverization; the flexible structure of the hard carbon microspheres further buffers the volume expansion and significantly improves the cycle life.

[0043] (4) The conductivity (approximately 1 S / cm) of the hard carbon microspheres in the sodium tritiate / hard carbon composite material described in this invention is significantly better than that of sodium tritiate nanosheets (approximately 10 S / cm). -5 The mixture (S / cm) enhances the overall conductivity of the electrode and accelerates charge transfer.

[0044] (5) The high sodium storage site density of the hard carbon microspheres in the sodium titanate / hard carbon composite material of the present invention can reduce the local current density, reduce the risk of sodium precipitation in the low potential region (close to the sodium deposition potential 0V) of the sodium titanate nanosheets, thereby inhibiting dendrite growth and improving safety. At the same time, the high thermal stability of the sodium titanate nanosheets can compensate for the thermal risk generated when the hard carbon microspheres precipitate sodium.

[0045] (6) The sodium tritiate / hard carbon composite material of the present invention solves the interfacial compatibility problem between sodium tritiate and hard carbon through chemical bonding reinforcement and carbon conductive layer modification, giving the composite material excellent cycle life and high energy density. The initial coulombic efficiency (ICE) of this composite material can reach more than 80%, and the capacity retention rate after 10,000 cycles is greater than 90%, which is significantly better than single materials and existing composite materials. Attached Figure Description

[0046] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein:

[0047] Figure 1 This is a SEM image of the Na2Ti3O7 nanosheets from Example 1 of Test Example 1 of this invention;

[0048] Figure 2 This is a SEM image of the hard carbon microspheres from Example 1 in Test Example 3 of the present invention;

[0049] Figure 3 This is a charge-discharge curve of a battery made of hard carbon microspheres from Test Example 3 (Example 1) of the present invention at a rate of 0.1C. Detailed Implementation

[0050] 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 and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0051] In this invention, unless otherwise stated, the oxygen defect concentration involved in the embodiments of this invention is obtained by X-ray photoelectron spectroscopy (XPS) measurement.

[0052] Example 1

[0053] The sodium tritiate / hard carbon composite material and its preparation method of the present invention specifically include the following steps:

[0054] Preparation of Si and Na2Ti3O7 nanosheets:

[0055] S11. Anatase TiO2 (purity ≥99.9%, particle size 50nm-100nm) and NaOH particles were added to the ball mill jar of a planetary ball mill at a molar ratio of 2:3.1. Ethanol was used as the dispersion medium and the solid content was 30%. The mixture was ball milled at 350 rpm for 12 hours to obtain a mixture.

[0056] S12. The mixture is transferred to a crucible and sintered in a tube furnace. The temperature is first increased to 300°C at a rate of 2°C / min, then increased to 800°C at a rate of 5°C / min and held for 10 hours. The mixture is then naturally cooled to room temperature to obtain Na2Ti3O7 powder.

[0057] S13. Disperse Na2Ti3O7 powder in 0.2mol / L hydrochloric acid solution (solid-liquid ratio 1:100), stir magnetically at room temperature (25℃) for 8h, centrifuge at 8000rpm for 5min, wash with deionized water until neutral (pH about 7), and finally vacuum dry at 65℃ for 12h to obtain Na2Ti3O7 nanosheets with an oxygen defect concentration of about 6%.

[0058] S2. Preparation of hard carbon microspheres:

[0059] S21. Phenol and formaldehyde are mixed in a molar ratio of 1:2, and hexamethylenetetramine (1.6% of the mass of phenol) is added. The mixture is stirred at a constant temperature of 100°C for 3 hours. After the reaction is completed, the phenolic resin prepolymer is dehydrated.

[0060] S22. Dissolve the phenolic resin prepolymer in a 1 mol / L NaOH solution to form a phenolic resin salt solution with a solid content of 40%.

[0061] S23. Phenolic resin salt solution is rapidly dried by spray drying to remove moisture. The inlet air temperature is 250℃, the outlet air temperature is 90℃, and the rotation speed is 1200rpm to obtain phenolic resin salt microsphere precursor.

[0062] The S24 phenolic resin salt microsphere precursor was first heated to 250℃ in air at a rate of 2℃ / min and held for 3 hours; then, nitrogen atmosphere was introduced, and the temperature was increased to 700℃ at a rate of 5℃ / min and held for 1.5 hours; the temperature was then increased to 1300℃ at a rate of 4℃ / min and held for 2 hours. Finally, impurities were removed by washing with dilute nitric acid to obtain hard carbon microspheres; the specific surface area was approximately 26 m² / g by BET analysis. 2 / g.

[0063] Preparation of S3 sodium tritiate / hard carbon composite material:

[0064] Na2Ti3O7 nanosheets and hard carbon microspheres were mixed at a mass ratio of 1:1. During ball milling, polyacrylonitrile (1.5% of the mass of the sodium titanate / hard carbon composite material) and graphene (2.0% of the mass of the sodium titanate / hard carbon composite material) were introduced. After ball milling at 300 rpm for 6 h, the mixture was carbonized and reinforced at 850 °C for 8 h under a nitrogen atmosphere to form CN-Ti covalent bonds. A carbon conductive layer was then coated on the surface of both the sodium titanate nanosheets and hard carbon microspheres, resulting in a sodium titanate / hard carbon composite material with a carbon conductive layer accounting for 2.0% of the weight.

[0065] Example 2

[0066] The basic structure is the same as in Example 1, except that the mass ratio of Na2Ti3O7 nanosheets to hard carbon microspheres is 1:4.

[0067] Example 3

[0068] The basic structure is the same as in Example 1, except that the mass ratio of Na2Ti3O7 nanosheets to hard carbon microspheres is 4:1.

[0069] Comparative Example 1

[0070] The process is basically the same as in Example 1, except that hard carbon microspheres are not used for composite formation; instead, only Na2Ti3O7 nanosheets are used.

[0071] Comparative Example 2

[0072] The basic structure is the same as in Example 1, except that Na2Ti3O7 nanosheets are not used for composite formation; instead, only hard carbon microspheres are used.

[0073] Test Example 1

[0074] The Na2Ti3O7 nanosheets prepared in Example 1 were characterized, and the results are as follows: Figure 1 As shown. From Figure 1 It can be seen that the thickness of sodium tritiate nanosheets is 1nm-3nm, and thin nanosheets can shorten the Na... + Increase transmission distance and improve the rate performance of composite materials.

[0075] Test Example 2

[0076] Battery assembly:

[0077] Negative electrode sheet: The materials of Examples 1-3 and Comparative Examples 1-2 were used as negative electrode active materials, conductive carbon black as a conductive agent, and polyvinylidene fluoride as a binder. The negative electrode active material, conductive agent, and binder were mixed at a mass ratio of 92:5:3, and N-methylpyrrolidone was added and stirred to form a uniform and stable negative electrode slurry. The negative electrode slurry was then uniformly coated onto the surface of a 6 μm aluminum foil, with a negative electrode active material loading of 2 mg / cm³. 2The negative electrode sheet is obtained by drying.

[0078] Positive electrode sheet: Sodium iron pyrophosphate (NFPP) is used as the positive electrode active material, conductive carbon black as the conductive agent, and polyvinylidene fluoride as the binder. The positive electrode active material, conductive agent, and binder are mixed at a mass ratio of 92:5:3, and N-methylpyrrolidone is added and stirred to form a uniform and stable positive electrode slurry. This slurry is then uniformly coated onto the surface of a 6μm aluminum foil, with a positive electrode active material loading of 2.2 mg / cm³. 2 The positive electrode is obtained by drying.

[0079] Separator: Polypropylene film, 9μm thick.

[0080] Electrolyte: Dissolve sodium hexafluorophosphate in a mixed solvent (EC, DMC and EMC in a volume ratio of 1:1:1) to prepare an electrolyte with a concentration of 1 mol / L.

[0081] Sodium-ion battery assembly: Arrange the negative electrode, separator, positive electrode, and separator in sequence, and use winding as the assembly method.

[0082] Performance testing:

[0083] The assembled sodium-ion battery underwent its first coulombic efficiency and other tests.

[0084] (1) Initial Coulombic Efficiency (ICE): The ratio of discharge capacity to charge capacity during the first charge and discharge of a sodium-ion battery, reflecting the quality of film formation at the electrode interface and the utilization rate of active materials; The sodium-ion battery is placed in a constant temperature chamber and charged at a constant current of 0.1C (C is the current corresponding to the theoretical capacity) to the cutoff voltage of 3.9V. After standing for 30 minutes, it is discharged at the same current to 1.5V, and the initial coulombic efficiency data is recorded.

[0085] (2) Capacity retention rate after 10,000 cycles: First, a 0.1C capacity calibration is performed and the discharge capacity is recorded as C0. Then, a 10C charge-discharge cycle is performed (charged to 3.9V and discharged to 1.5V). A 0.1C capacity calibration is performed every 1,000 cycles, and the capacity retention rate after the 1,000th cycle is recorded. The capacity retention rate R2 = C10 / C0*100% is used to evaluate the cycle stability of the battery after 10,000 cycles based on the discharge capacity C10 of the 10th 0.1C capacity calibration.

[0086] Table 1 shows the final measured performance parameters:

[0087] Table 1

[0088] Example 1 82 95 Example 2 80 93 Example 3 81 94 Comparative Example 1 65 60 Comparative Example 2 78 75

[0089] As shown in Table 1, the sodium tritiate / hard carbon composite material of the examples significantly improves the initial coulombic efficiency and cycle stability of sodium-ion batteries. This is because the stable crystal structure of Na2Ti3O7 nanosheets reduces electrolyte decomposition and inhibits irreversible reactions during the first charge-discharge process; the addition of hard carbon microspheres improves electrode porosity and conductivity, facilitating smoother sodium ion transport; the chemical enhancement and conductive layer coating effect of the composite reduce the aggregation of individual materials, improve the utilization rate of active materials, and thus improve the initial coulombic efficiency. In addition, the low volume expansion of Na2Ti3O7 nanosheets as the main framework enhances the structural stability of the electrode; the hard carbon microspheres provide abundant storage sites for sodium ions; the composite optimizes electron conduction and ion transport pathways, reduces energy loss and active material depletion during cycling, and significantly improves capacity retention.

[0090] Analysis of Examples 1-3 shows that Example 1 has the highest initial efficiency (82%), while Examples 2 (80%) and 3 (81%) are slightly lower. This is because when the mass ratio of Na2Ti3O7 nanosheets to hard carbon microspheres is 1:1, the two materials are mixed most uniformly, forming an optimal conductive network and sodium storage structure, making the insertion and extraction of sodium ions more efficient and minimizing irreversible reactions during the first charge and discharge process. When the proportion of hard carbon microspheres is too high (Example 2) or the proportion of Na2Ti3O7 nanosheets is too high (Example 3), it can lead to agglomeration of one of the materials, affecting the transport of electrons and ions, thus slightly reducing the initial efficiency, but still greater than 80%. The capacity retention rate of Example 1 is 95%, Example 3 is 94%, and Example 2 is 93%. Although the difference is not significant, it can be seen that when the proportion of Na2Ti3O7 nanosheets is relatively high (Examples 1 and 3), the capacity retention rate is slightly higher. This is because the structural stability of Na2Ti3O7 nanosheets has a more significant impact on long-term cycling performance. A higher proportion of Na2Ti3O7 nanosheets can better suppress the volume change of hard carbon microspheres, maintain the stability of the electrode structure, and thus maintain a higher capacity during long-term cycling.

[0091] Comparing Example 1 and Comparative Example 1, it can be seen that when only Na2Ti3O7 nanosheets are used as the negative electrode active material, the initial efficiency is only 65%. This is because a single Na2Ti3O7 nanosheet undergoes many irreversible reactions during the first charge-discharge process, such as material phase transitions, leading to significant capacity loss. Furthermore, the capacity retention rate is only 60%. This is because, although the Na2Ti3O7 nanosheets exhibit good structural stability during long-term cycling, their sodium storage capacity is limited. Additionally, during cycling, interfacial reactions between the electrode material and the electrolyte gradually accumulate, resulting in the loss of active material and an increase in electrode impedance.

[0092] Comparing Example 1 and Comparative Example 2, it can be seen that when only hard carbon microspheres are used as the negative electrode active material, the initial efficiency is only 78%. This is because hard carbon microspheres have a large specific surface area and abundant pore structure, which can provide more sodium storage sites. However, there are some sodium ions in the hard carbon microspheres that are difficult to completely intercalate or deintercalate, and the SEI film formed during the first charge and discharge also consumes some sodium ions, thus affecting the initial efficiency. In addition, the capacity retention rate is also only 75%. This is because the structure of hard carbon microspheres will expand and contract to a certain extent during cycling. After long-term cycling, the stability of the structure decreases, resulting in a weakening of the sodium ion storage capacity.

[0093] Test Example 3

[0094] Based on Example 1, the effects of different spray drying speeds and maximum carbonization temperatures on the particle size D50, specific surface area, and I of hard carbon microspheres were investigated. D / I G The influence of hard carbon microspheres as the active material on the electrical performance of half-cells; wherein, the particle size D50 was obtained by SEM characterization, I D / I G Found by small-angle X-ray scattering (SAXS) measurements;

[0095] Assembly of the half-cell: Hard carbon microspheres prepared in different groups were used as active materials, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder. The active materials, conductive agents, and binders were added to the solvent N-methylpyrrolidone in a mass ratio of 8:1:1 to obtain a slurry with a binder concentration of 5wt%. The slurry was coated onto the surface of 6μm aluminum foil by hand and dried to obtain the electrode. The electrode was used as the working electrode, the sodium sheet as the counter electrode, and the electrolyte was the same as above to assemble the half-cell.

[0096] (1) Specific capacity test: The test is conducted within the voltage range of 0-2V to test the specific capacity of the battery at 0.1C;

[0097] (2) First Coulomb efficiency test: Refer to test example 2;

[0098] Table 2 and Figures 2-3 The following are the final measured performance parameters:

[0099] Table 2

[0100]

[0101] From Table 2 and Figures 2-3 It can be seen that the particle size and disorder level of the hard carbon microspheres are I D / I G The specific surface area, specific capacity, and initial coulombic efficiency are affected by the spray drying speed and the maximum carbonization temperature, while the particle size and disorder of the hard carbon microspheres are affected by these factors. D / IG .

[0102] As the rotational speed increases, the particle size decreases. At the same carbonization temperature, the smaller particle size increases the specific surface area of ​​the hard carbon microspheres, exposing more sodium-storing active sites (such as edge carbon atoms and microporous structures), thereby improving the specific capacity. For example, hard carbon microspheres prepared at 1200 rpm (particle size of 230 nm) have a specific capacity of 310 mAh / g, which is significantly higher than that of coarse-particle materials prepared at low rotational speeds (hard carbon microspheres prepared at 800 rpm have a particle size of 445 nm and a specific capacity of 305 mAh / g). However, when the particle size is too small (the particle size of hard carbon microspheres prepared at 3000 rpm is 178 nm), although the number of active sites increases, it also leads to an increase in the electrode / electrolyte interface area. This increases the amount of electrolyte decomposition required for SEI film formation during the first charge and discharge, consuming more sodium ions, and theoretically, the initial efficiency tends to decrease.

[0103] As the carbonization temperature increases, the degree of disorder I... D / I G While decreasing the numerical value and increasing the degree of ordering improves electronic conductivity, the closed-pore structure reduces the effective pores for reversible sodium ion insertion, and I D / I G A decrease in the specific capacity indicates a reduction in sodium-storing active sites (such as disordered carbon edges and defects), thus the specific capacity decreases with increasing temperature. At high temperatures, the ordered structure of the hard carbon microspheres facilitates electrolyte decomposition reactions at the thermodynamically stable graphitized interface, resulting in a more uniform and dense SEI film composition and reduced irreversible sodium consumption. Simultaneously, increased conductivity accelerates sodium ion migration and reduces polarization losses during the initial charge-discharge process, thus the initial efficiency slightly increases with increasing temperature. Above 1500℃, the specific capacity is less than 300 mAh / g; below 1000℃, due to increased disorder, the initial efficiency is less than 70%.

[0104] Test Example 4

[0105] Based on Example 1, the effects of composite materials prepared with different oxygen defect concentrations of Na2Ti3O7 nanosheets (0.1 mol / L hydrochloric acid solution corresponds to approximately 5% oxygen defect concentration, and 0.5 mol / L hydrochloric acid solution corresponds to approximately 10% oxygen defect concentration), reinforcing agents, and carbon conductive layers as active materials on the electrical performance of sodium-ion batteries were investigated; wherein, the assembly of sodium-ion batteries and the testing of initial coulombic efficiency and capacity retention after 10,000 cycles were conducted in accordance with Test Example 2;

[0106] 10C rate capacity retention rate: First, perform 1C capacity calibration and record the discharge capacity as C1. Then, use 10C charge and discharge and record the discharge capacity as C10 (charge to 3.9V and discharge to 1.5V). 10C rate capacity retention rate R1 = C10 / C1 * 100%.

[0107] Table 3 shows the final measured performance parameters:

[0108] Table 3

[0109]

[0110] As shown in Table 3, the data from Groups 1-4 show that an increase in oxygen defect concentration leads to an increase in adsorption sites, but more oxygen defects result in a slight increase in the thickness of the SEI film (Group 3), and the initial coulombic efficiency decreases from 82.7% (Group 1) to 81.7% (Group 3). Group 1 has the highest capacity retention rate after 10,000 cycles (95.1%), which is because moderate oxygen defects can buffer volume expansion.

[0111] The data from Groups 1 and 5 show that without surface modification with a carbon conductive layer, the initial coulombic efficiency of the sodium-ion battery decreased from 82.7% (Group 1) to 72.9% (Group 5), and the capacity retention rate after 10,000 cycles decreased from 95.1% (Group 1) to 82.5% (Group 5). This is because after modifying the carbon conductive layer, the conductive network shortens the sodium ion migration path, reduces polarization loss, and improves the sodium storage efficiency per unit mass of active material.

[0112] The data from Groups 1 and 6 show that without the introduction of polyacrylonitrile during ball milling, the capacity retention of the sodium-ion battery at 10C rate decreased from 91.9% (Group 1) to 77.4% (Group 6). This is because the introduction of polyacrylonitrile during ball milling allows for the formation of CN-Ti covalent bonds after carbonization. These covalent bond networks replace traditional physical adsorption, constructing efficient electron transport channels. Furthermore, after 10,000 cycles, the capacity retention decreased from 95.1% (Group 1) to 80.6% (Group 6). This is because the bonding effect inhibits Na... + Lattice distortion caused by insertion / extraction.

[0113] In summary, this invention, through the synergistic effect of oxygen defect concentration and bonding enhancement, provides sufficient active sites while stabilizing the defect interface through covalent bonds, effectively avoiding structural collapse caused by high defect concentration, thus achieving a balance between initial coulombic efficiency and cycling performance. Simultaneously, the complementary synergistic mechanism of the carbon conductive layer and bonding enhancement addresses the bulk conductivity issue through the carbon conductive layer and improves interfacial bonding through bonding enhancement. These two mechanisms respectively act on the electron transport path and interfacial stability, jointly constructing a dual optimization system of "bulk phase-interface." They are closely related and indispensable; for example, without any modification, the performance of group 7 exhibits a comprehensive deterioration.

[0114] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A sodium trititanate / hard carbon composite material, characterized in that, It includes sodium titanate nanosheets, hard carbon microspheres, and a carbon conductive layer that co-coats the sodium titanate nanosheets and hard carbon microspheres; the sodium titanate nanosheets and hard carbon microspheres are carbonized and reinforced under the action of a reinforcing agent to form a CN-Ti covalent bond interface; The mass ratio of sodium titanate nanosheets to hard carbon microspheres is (1:4)-(4:1); the weight percentage of the carbon conductive layer in the sodium titanate / hard carbon composite material is 0.5%-5.0%; The amount of the reinforcing agent is 1%-3% of the mass of the sodium trititanate / hard carbon composite material; The sodium tritiate nanosheets have a thickness of less than 10 nm and an oxygen defect concentration of 5%-10%. The specific surface area of ​​the hard carbon microspheres is 10 m². 2 / g-50m 2 / g, discharge specific capacity greater than 300mAh / g, I D / I G The particle size is 0.8-3, and the particle size D50 is 200nm-500nm; The reinforcing agent is selected from polyacrylonitrile and / or polydopamine.

2. The sodium trititanate / hard carbon composite material according to claim 1, characterized in that, The material of the carbon conductive layer is selected from graphene and / or carbon nanotubes.

3. The sodium trititanium / hard carbon composite material according to claim 1, characterized in that, The preparation of the sodium trititanate nanosheets includes the following steps: S1. Titanium dioxide and sodium hydroxide are ball-milled evenly and then sintered to obtain sodium tritiate powder; The sodium tritiate powder described in S2 and S1 is etched in hydrochloric acid solution, and then centrifuged, washed, and dried to obtain the sodium tritiate nanosheets.

4. The sodium trititanium / hard carbon composite material according to claim 3, characterized in that, In S1, the molar ratio of titanium dioxide to sodium hydroxide is 2:(3.1-3.2); The ball milling solvent is ethanol, the rotation speed is 300rpm-400rpm, and the time is 10h-20h; The sintering process involves first heating to 200℃-400℃ at a rate of 1℃ / min-3℃ / min; then heating to 700℃-1000℃ at a rate of 3℃ / min-8℃ / min, and holding at that temperature for 8h-12h.

5. The sodium trititanate / hard carbon composite material according to claim 3, characterized in that, In S2, the concentration of the hydrochloric acid solution is 0.1 mol / L to 0.5 mol / L; The etching time is 6h-24h.

6. The sodium trititanium / hard carbon composite material according to claim 1, characterized in that, The preparation of the hard carbon microspheres includes the following steps: S1. Under the action of an organic amine catalyst, monophenolic compounds and aldehyde compounds react and are dehydrated to obtain phenolic resin prepolymer; S2. The phenolic resin prepolymer described in S1 is dissolved in an alkaline solution, and then spray-dried and carbonized to obtain the hard carbon microspheres.

7. The sodium trititanate / hard carbon composite material according to claim 6, characterized in that, In S1, the monophenolic compound is selected from one or more of phenol, o-cresol, m-cresol, p-cresol, and bisphenol A; The aldehyde compounds are selected from formaldehyde and / or furfural; The organic amine catalyst is selected from one or more of hexamethylenetetramine, ethylenediamine, and triethanolamine; The molar ratio of the monophenolic compounds to the aldehyde compounds is 1:(1.5-4); The amount of the organic amine catalyst used is 1.0%-2.0% of the mass of the monophenol compound; The reaction is carried out at a temperature of 70℃-120℃ for 2h-5h.

8. The sodium trititanate / hard carbon composite material according to claim 6, characterized in that, In S2, the alkaline solution is selected from one or more of sodium hydroxide solution, potassium hydroxide solution, ammonia water, tetramethylammonium hydroxide solution, and sodium carbonate solution; the concentration of the alkaline solution is 0.9 mol / L-1.1 mol / L. The spray dryer has an inlet air temperature of 240℃-260℃, an outlet air temperature of 80℃-100℃, and a rotation speed of 800rpm-2000rpm. The carbonization process is divided into three stages; the first stage is to heat the material to 200℃-300℃ in an air or oxygen atmosphere at a rate of 1℃ / min-5℃ / min and hold it at that temperature for 1h-5h. The second stage involves heating to 600℃-800℃ at a rate of 2℃ / min-10℃ / min under a protective atmosphere, and holding at that temperature for 1h-2h. The third stage involves heating to 1100℃-1400℃ at a rate of 3℃ / min-5℃ / min, and holding at that temperature for 1h-3h.

9. The method for preparing the sodium trititanium / hard carbon composite material according to any one of claims 1-8, characterized in that, Includes the following steps: Sodium tritiate nanosheets, hard carbon microspheres, reinforcing agents and carbon conductive materials were ball-milled and then reinforced by carbonization to obtain the sodium tritiate / hard carbon composite material. The carbonization enhancement is carried out under a protective atmosphere at 600℃-1000℃ for 5-15 hours.

10. The application of the sodium trititanium / hard carbon composite material as described in any one of claims 1-8 in sodium-ion batteries.

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