Negative active material and preparation method thereof, negative plate and sodium ion battery

Abstract

The invention discloses a negative electrode active material and a preparation method thereof, a negative electrode plate and a sodium ion battery, the negative electrode active material comprises Na2Ti3O7 loaded nitrogen-doped carbon nanofiber, and Na2Ti3O7 is loaded on the surface of the nitrogen-doped carbon nanofiber. The negative electrode active material comprises the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber, and the Na2Ti3O7 is loaded on the surface of the nitrogen-doped carbon nanofiber, so that the conductivity of the material is greatly improved, the polarization is obviously reduced, and the material can have relatively high capacity even in a low-temperature environment; besides, the negative electrode active material has relatively high sodium intercalation potential, can prevent the growth of sodium dendrites, and remarkably improves the discharge and safety performance of the battery at low temperature as the negative electrode active material.

Description

Technical Field

[0001] The present application relates to the technical field of negative electrode materials, and in particular to a negative electrode active material and a preparation method thereof, a negative electrode sheet and a sodium ion battery. Background Art

[0002] As an ideal candidate for the next generation of large-scale energy storage devices, sodium-ion batteries (SIBs) have attracted widespread attention due to their low cost. Sodium-ion batteries have a similar energy storage mechanism to lithium-ion batteries, and metallic sodium resources are abundant. They have broad application prospects in large-scale grid energy storage, low-speed electric vehicles and other fields. SIBs also have advantages over LIBs in low temperature and fast charging, so they also have good application prospects in special and complex environments. The hard carbon commonly used in the negative electrode of SIBs has good sodium storage performance, but its relative to Na + The low-potential discharge platform of / Na is between 0 and 0.1 V, which makes it easier for sodium dendrites to form in low-temperature environments, further causing safety issues. Summary of the Invention

[0003] The present invention provides a negative electrode active material and a preparation method thereof, a negative electrode sheet and a sodium ion battery, so as to solve the technical problem in the prior art that sodium ion batteries are prone to form sodium dendrites in a low temperature environment, thereby causing safety problems.

[0004] To achieve the above object, the technical solution provided by the present invention is as follows: In a first aspect of the present invention, a negative electrode active material is provided, comprising nitrogen-doped carbon nanofibers supported by Na2Ti3O7, wherein the Na2Ti3O7 is supported on the surface of the nitrogen-doped carbon nanofibers.

[0005] Furthermore, the mass ratio of the Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 0.5 to 4:1.

[0006] Furthermore, the particle size of the nitrogen-doped carbon nanofibers loaded with Na2Ti3O7 is 20 nm to 100 nm.

[0007] A second aspect of the present invention provides a method for preparing the above-mentioned negative electrode active material, comprising the following steps: S1, dissolving a carbon nanofiber precursor in a solvent, adding a titanate precursor and a weak acid, stirring, transferring the mixture to a syringe, obtaining electrospun fibers by an electrospinning method, baking the electrospun fibers, transferring the fibers to a tube furnace, and carbonizing the fibers under a nitrogen atmosphere to obtain titanium dioxide-supported nitrogen-doped carbon nanofibers; S2, adding the titanium dioxide-loaded nitrogen-doped carbon nanofibers obtained in step S1 into an alkaline solution and transferring the mixture to a reactor for hydrothermal reaction to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor; S3. calcining the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step S2 to obtain the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber.

[0008] Furthermore, in step S1, the carbon nanofiber precursor is selected from one or more of polyvinyl pyrrolidone, polyacrylonitrile, and polyimide.

[0009] Furthermore, in step S1, the titanate precursor is selected from one or more of tetrabutyl titanate and tetraisopropyl titanate.

[0010] Furthermore, in the step S1, the baking temperature of the electrospinning is 100°C to 250°C, and the carbonization temperature in a nitrogen atmosphere is 500°C to 700°C.

[0011] Furthermore, in step S2, the temperature of the hydrothermal reaction is 140°C to 180°C.

[0012] Furthermore, in step S3, the calcination temperature is 500°C to 600°C.

[0013] Furthermore, the mass ratio of the titanate precursor to the carbon nanofiber precursor is 1 to 9:1.

[0014] According to a third aspect of the present invention, a negative electrode sheet is provided, comprising the above-mentioned negative electrode active material or the negative electrode active material prepared by the above-mentioned preparation method.

[0015] Furthermore, the negative electrode sheet includes a negative electrode current collector and a coating formed by a negative electrode slurry coated on the negative electrode current collector, and the coating is formed on at least one side of the negative electrode current collector; the negative electrode slurry includes the negative electrode active material.

[0016] In a fourth aspect of the present invention, a sodium ion battery is provided, comprising a positive electrode sheet, a separator and the above-mentioned negative electrode sheet, wherein the separator is located between the positive electrode sheet and the negative electrode sheet.

[0017] The negative electrode active material provided by the present invention includes nitrogen-doped carbon nanofibers loaded with Na2Ti3O7, which loads Na2Ti3O7 onto the surface of the nitrogen-doped carbon nanofibers, greatly improving the electrical conductivity of the material and significantly reducing polarization, so that the material can have a higher capacity even in a low-temperature environment; in addition, the above-mentioned negative electrode active material has a higher sodium insertion potential, which can prevent the growth of sodium dendrites. As a negative electrode active material, it significantly improves the discharge and safety performance of the battery at low temperatures. DETAILED DESCRIPTION

[0018] In order to help those skilled in the art better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without making any creative efforts shall fall within the scope of protection of this application.

[0019] In a first aspect of an embodiment of the present application, a negative electrode active material is provided, comprising nitrogen-doped carbon nanofibers loaded with Na2Ti3O7, wherein Na2Ti3O7 is loaded on the surface of the nitrogen-doped carbon nanofibers.

[0020] Na2Ti3O7 has a lower working voltage (average potential of 0.3V vs. Na + / Na), and it is not easy to deposit sodium, which has excellent potential for realizing low-temperature resistant and highly safe sodium-ion batteries, but the Na2Ti3O7 material has the problem of low electrical conductivity. The negative electrode active material of the embodiment of the present application loads Na2Ti3O7 on the surface of nitrogen-doped carbon nanofibers, which greatly improves the electrical conductivity of the material and significantly reduces polarization, so that the material can have a high capacity even in a low-temperature environment; in addition, the above-mentioned negative electrode active material has a high sodium insertion potential, which can prevent the growth of sodium dendrites. As a negative electrode active material, it significantly improves the discharge and safety performance of the battery at low temperatures.

[0021] Specifically, the mass ratio of Na2Ti3O7 to nitrogen-doped carbon nanofibers in the negative electrode active material is 0.5 to 4: 1. The particle size of the nitrogen-doped carbon nanofibers supported by Na2Ti3O7 is 20 nm to 100 nm.

[0022] A second aspect of the embodiments of the present application provides a method for preparing the above-mentioned negative electrode active material, comprising the following steps: S1, dissolving a carbon nanofiber precursor in a solvent, adding a titanate precursor and a weak acid, stirring, and transferring the mixture to a needle tube, using an electrospinning method to obtain electrospun fibers, baking the electrospun fibers, transferring the fibers to a tube furnace, and carbonizing them under a nitrogen atmosphere to obtain titanium dioxide-supported nitrogen-doped carbon nanofibers; S2, adding the titanium dioxide-loaded nitrogen-doped carbon nanofibers obtained in step S1 to an alkaline solution and transferring the mixture to a reactor for hydrothermal reaction to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor; S3. calcining the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step S2 to obtain Na2Ti3O7-loaded nitrogen-doped carbon nanofibers.

[0023] In the method for preparing the negative electrode active material in the embodiments of this application, step S1 utilizes electrospinning to prepare nitrogen-doped carbon nanofibers loaded with titanium dioxide. This method has the following advantages: 1. Controllable structure. Electrospinning allows precise control of the nanofiber morphology, porosity, and orientation by adjusting parameters such as voltage, injection rate, solution viscosity, and temperature, resulting in a three-dimensional network structure with a high specific surface area, enhancing electrolyte wetting and ion transport efficiency. Furthermore, the titanate precursor and carbon nanofiber precursor are uniformly mixed in the solution, resulting in uniform and continuous TiO2-C precursor composite fibers after electrospinning, thus avoiding the particle agglomeration problem encountered in traditional methods. 2. Process versatility. Doping elements or functional materials (such as MOFs and graphene) can be flexibly introduced to produce multifunctional composite fibers in a single step. Furthermore, the mature electrospinning equipment technology is suitable for continuous production, providing a foundation for industrial applications. 3. Particle aggregation is suppressed, allowing for the construction of a conductive network. The titanate precursor is carbonized during the subsequent calcination process to form a carbon fiber network that provides a continuous electron transmission channel, reduces the electrode impedance, and the nanofibers can inhibit the aggregation of TiO2 particles and alleviate volume expansion.

[0024] In addition, after electrospinning and baking, carbonization is carried out in a nitrogen atmosphere. The N2 atmosphere isolates oxygen and prevents the carbon layer from oxidative decomposition at high temperature (C + O2→ CO2), which can ensure the integrity of the carbon structure. In the N2 atmosphere, N-doped C nanofibers can be carried out to effectively improve the electronic conductivity of the material. Anatase phase TiO2: In the N2 atmosphere, a certain calcination temperature and time can inhibit the transformation of TiO2 to rutile phase and maintain the high reactivity of anatase. Ti-OC bond formation: TiO2 and the surface of nitrogen-doped carbon nanofibers are chemically bonded, which can significantly reduce the interfacial charge transfer resistance R ct .

[0025] In the embodiment of the present application, step S2 uses a hydrothermal reaction to prepare a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor. The reaction conditions are mild, the temperature is usually below 200°C, and the pressure is self-generated, which avoids the oxidation or structural damage of the carbon layer that may be caused by the high-temperature solid-phase method, while promoting the embedding of sodium ions and the formation of a layered structure. During the hydrothermal process, the carbon / nitrogen-doped structural fiber skeleton can be completely retained, and only local chemical conversion occurs. In addition, the hydrothermal reaction is well controllable. By adjusting the molar ratio of Na to Ti and the reaction time, the product stoichiometric ratio and crystallinity can be precisely controlled.

[0026] In the examples of the present application, after the hydrothermal reaction, the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor is subjected to high-temperature calcination. High temperature promotes Na2Ti3O7 grain growth, sharpens XRD peaks, and improves crystallinity. Residual TiO2 or unreacted sodium salt is further converted into the target product, significantly reducing the intensity of the XRD miscellaneous peaks. High temperature can further optimize the carbon layer structure of the material and improve conductivity. Excess sodium source can also be dynamically adjusted during calcination to obtain the final product.

[0027] In some embodiments, in step S1 , the carbon nanofiber precursor is selected from one or more of polyvinyl pyrrolidone, polyacrylonitrile, and polyimide.

[0028] In the embodiments of the present application, when selecting carbon nanofiber precursors, the polymer chains need to form continuous and uniform fibers during the electrospinning process; the carbon yield needs to be high, and the carbon layer conductivity and structural stability need to be good; it needs to be evenly mixed with the titanium source to avoid phase separation or reaction; and it needs to support doping or porous structure design, so the above three precursors were selected.

[0029] In some embodiments, in step S1, the titanate precursor is selected from one or more of tetrabutyl titanate and tetraisopropyl titanate. In the embodiments of the present application, the titanium source is a key precursor for the generation of TiO2 and must meet the following conditions: 1. Hydrolysis controllability. Condition: The hydrolysis rate of the titanium source must be moderate to avoid too fast gelation or too slow reaction and insufficient reaction. 2. Solvent compatibility. Condition: The titanium source must be compatible with the solvent and the carbon nanofiber precursor to avoid phase separation. 3. Product crystal form control. Condition: The generation of anatase or rutile phase TiO2 after calcination must match the target application. 4. Process safety. Condition: Avoid using highly toxic or corrosive titanium sources.

[0030] Specifically, titanates (such as tetrabutyl titanate, TBT) have a controllable hydrolysis rate in acidic or weakly polar solvents, making them suitable for electrospinning. Tetrabutyl titanate is readily soluble in ethanol and N,N-dimethylformamide (DMF) and mixes well with polyvinylpyrrolidone (PVP) and polyacrylonitrile (PAN). Titanates (such as tetrabutyl titanate, TBT) calcined at 600-700°C form an anatase phase (suitable for ion storage). Tetrabutyl titanate (TBT) is relatively safe to handle due to its low toxicity.

[0031] In some embodiments, in step S1, the baking temperature of the electrospinning is 100°C to 250°C, and the carbonization temperature in a nitrogen atmosphere is 500°C to 700°C. In the embodiment of the present application, the electrospinning voltage is set to 15kV to 20kV, and the distance between the needle head and the aluminum foil current collector is 10cm to 20cm. The electrospinning is then baked to remove residual solvent and organic matter, evaporate the residual solvent, decompose some of the organic matter in the carbon nanofiber precursor, and prevent the fibers from melting or sticking during high-temperature carbonization; the titanate precursor molecular chains undergo partial dehydrogenation and cross-linking in an oxidizing atmosphere to form a stable carbon skeleton precursor, enhance the mechanical strength of the fibers, and prevent the fibers from melting during subsequent high-temperature carbonization. The pre-fibering temperature is the core process parameter for synthesizing TiO2@C / N nanofibers by electrospinning. Too high a temperature will cause the carbon layer to be destroyed and the crystal form to be mixed, resulting in decreased performance; too low a temperature will result in more residual organic matter and insufficient TiO2 crystallinity. After baking, it is transferred to a tube furnace for carbonization. If the carbonization temperature is too low, the carbonization is incomplete, the conductivity is low, and the nitrogen doping amount is small; if the carbonization temperature is too high, it is highly graphitized (ID / IG≈0.8) and the conductivity is excellent (10 -1 S / cm), but the carbon layer may shrink excessively, causing the nanofibers to break; nitrogen doping is lost, and nitrogen atoms escape at high temperatures; anatase and rutile phases coexist, and the proportion of rutile phase increases with increasing temperature, which hinders ion diffusion.

[0032] In some embodiments, in step S2, the temperature of the hydrothermal reaction is 140°C to 180°C.

[0033] In the embodiment of the present application, the hydrothermal reaction temperature is too low, the reaction rate is slow, and the sodium ion (Na + Insufficient driving force for embedding the titania structure may result in an incomplete reaction; the resulting Na₂Ti₃Oₐ is primarily amorphous or has low crystallinity; unreacted TiO₂ particles may adhere to the nanofiber surface; the hydrothermal environment may slightly damage the carbon layer, resulting in decreased conductivity. Furthermore, the specific capacity is reduced due to incomplete conversion of the active material; and the amorphous structure is prone to irreversible volume expansion during charge and discharge.

[0034] In the examples of this application, excessively high hydrothermal reaction temperatures accelerate the reaction, potentially leading to excessive structural growth or byproduct formation; the formation of impurity phases, which reduces material purity; high temperature and pressure leading to fiber breakage or carbon layer oxidation; and excessive crystallization narrowing interlayer channels, hindering ion diffusion. Furthermore, the formation of impurity phases and structural defects can easily lead to structural collapse, exacerbating capacity degradation; and increased impedance can hinder ion transport.

[0035] In other embodiments, in step S3, the calcination temperature is 500°C to 600°C. In the embodiments of the present application, 500-600°C is the optimal temperature range for synthesizing Na2Ti3O7@C / N nanofibers. Its core advantages lie in the optimization of the carbon layer: balancing carbonization and nitrogen doping to provide high conductivity and surface activity; crystal control to form high-purity layered Na2Ti3O7 and open ion diffusion channels; chemical stability to inhibit sodium volatilization and phase change, maintaining an ideal stoichiometric ratio; process feasibility, low equipment requirements, and controllable energy consumption and safety.

[0036] Specifically, the mass ratio of the titanate precursor to the carbon nanofiber precursor is 1 to 9:1.

[0037] In the embodiment of the present application, selecting the mass ratio of the titanate precursor to the carbon nanofiber precursor within the above range can improve the low-temperature performance of the final material and enable normal use even at an ultra-low temperature of -60°C.

[0038] In the embodiments of the present application, the relationship between the mass ratio and the fiber morphology is as follows: the titanate precursor is used as the inorganic phase and the carbon precursor is used as the organic phase, and the mass ratio of the two directly affects the diameter and uniformity of the fiber. Generally, when the proportion of the carbon precursor is higher, the fiber diameter is thinner and the surface is smoother, but the TiO2 content is low and the capacity will be limited. If the titanate ratio increases, the formation of TiO2 particles will be promoted, which may cause the fiber diameter to increase or agglomerate. In addition, the performance-oriented optimization rule: if used as a negative electrode material for sodium ion batteries, the conductive network of the carbon matrix is ​​crucial. At this time, the proportion of the carbon precursor needs to be appropriately increased, but at the same time, attention should be paid to balancing the conductivity and capacity of the material. The optimal selection of the mass ratio of the titanate precursor and the carbon nanofiber precursor requires comprehensive consideration of: uniform fiber morphology, continuous carbon layer, and avoidance of local insufficient conductivity; TiO2 and carbon work synergistically to improve the conductivity of the material and ensure good capacity performance.

[0039] According to a third aspect of the embodiments of the present application, a negative electrode sheet is provided, comprising the above-mentioned negative electrode active material or the negative electrode active material prepared by the above-mentioned preparation method.

[0040] In the embodiment of the present application, the negative electrode active material loads Na2Ti3O7 on the surface of nitrogen-doped carbon nanofibers and is used for the negative electrode sheet, which can significantly improve the discharge and safety performance of the battery at low temperatures.

[0041] Specifically, the negative electrode sheet includes a negative electrode current collector and a coating formed of a negative electrode slurry applied to the negative electrode current collector. The coating is formed on at least one side of the negative electrode current collector. The negative electrode slurry includes a negative electrode active material. In addition, the negative electrode slurry also includes conductive carbon (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR).

[0042] In a fourth aspect of the embodiments of the present application, a sodium ion battery is provided, comprising a positive electrode sheet, a separator and the above-mentioned negative electrode sheet, wherein the separator is located between the positive electrode sheet and the negative electrode sheet.

[0043] In the embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode coating applied to the surface of the positive electrode current collector. The positive electrode active material forming the positive electrode coating includes one or more of a layered oxide, Prussian blue, and a polyanionic compound. The sodium-ion battery of the embodiments of the present application has a high capacity in low-temperature environments, can prevent the growth of sodium dendrites, and also has excellent safety performance in low-temperature environments.

[0044] The specific implementation of this application is further described below in conjunction with specific examples, but the implementation and protection of this application are not limited thereto. It should be noted that if there are any processes not specifically described below, they can be implemented or understood by those skilled in the art with reference to the prior art. If the manufacturer of the reagents or instruments used is not indicated, they are deemed to be conventional products that can be purchased commercially.

[0045] Example 1 The negative electrode active material includes nitrogen-doped carbon nanofibers supported by Na2Ti3O7, wherein the Na2Ti3O7 is supported on the surface of the nitrogen-doped carbon nanofibers. The mass ratio of Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 3:1. The particle size of the nitrogen-doped carbon nanofibers supported by Na2Ti3O7 is 40 nm.

[0046] The method for preparing the above-mentioned negative electrode active material comprises the following steps: 1. Dissolve 2g of polyvinyl pyrrolidone in 20mL of anhydrous ethanol and stir magnetically for 1h. Then add 18g of tetrabutyl titanate and 4mL of acetic acid and continue stirring for 12h. Transfer the mixed solution to a syringe with a capillary tip. Set the electrospinning voltage to 20kV and the distance between the syringe head and the aluminum foil current collector to 10cm. Bake the obtained electrospun fiber at 100℃ for 2h, then transfer it to a tube furnace and carbonize it at 700℃ in a N2 atmosphere for 3h to finally obtain TiO2-loaded nitrogen-doped carbon nanofibers.

[0047] 2. The TiO2-loaded nitrogen-doped carbon nanofibers obtained above were added to a NaOH solution and stirred evenly for 2 hours. The mixture was then transferred to a high-temperature reactor and hydrothermally reacted at 140°C for 12 hours to obtain a sheet. The sheet was then washed six times with deionized water and anhydrous ethanol and dried to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor.

[0048] 3. Place the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step 2 in a tubular furnace and calcine it at 600°C for 6 hours in an argon atmosphere to obtain Na2Ti3O7-loaded nitrogen-doped carbon nanofibers.

[0049] The Na2Ti3O7-loaded nitrogen-doped carbon nanofibers prepared in this example are used to prepare sodium ion batteries. The main steps include: (1) Using propylene carbonate (PC) and ethyl methyl carbonate (EMC) in a volume ratio of 8:2 as an ester solvent, sodium bis(fluorosulfonyl)imide is dissolved in the ester solvent at a concentration of 1 mol / L, and then fluoroethylene carbonate (FEC) with a mass fraction of 5% is added as an additive. After complete dissolution and uniform mixing, a sodium ion battery electrolyte is obtained.

[0050] (2) Na4Fe3(PO4)2P2O7:SP:carbon nanotubes:polyvinylidene fluoride were mixed and stirred in a mass ratio of 94:2:0.5:3.5 to obtain a positive electrode slurry, and the positive electrode slurry was coated on the surface of the positive electrode carbon-coated aluminum foil to form a positive electrode coating to obtain a positive electrode sheet; wherein the single-sided coating density of the positive electrode coating was 18 mg / cm 2 .

[0051] (3) According to the mass ratio of modified Na2Ti3O7-loaded nitrogen-doped carbon nanofibers: conductive carbon black: water-based carbon nanotubes: sodium carboxymethyl cellulose: styrene-butadiene rubber: sodium ion conductor of 93:1.5:0.5:3:1.5:0.5, the mixture was evenly stirred to obtain a negative electrode slurry, and the negative electrode slurry was coated on the surface of the negative electrode aluminum foil to form a negative electrode coating to obtain a negative electrode sheet; wherein the single-side coating density of the negative electrode coating was 12 mg / cm 2 .

[0052] (4) Using 3C (ceramic layer) + 9PE (polyethylene) + 3C (ceramic layer) double-sided ceramic coating as the battery separator, the positive electrode sheet, separator and negative electrode sheet are stacked in sequence, wound and fixed, and then sealed with aluminum plastic film. After drying, the electrode sheet is injected with electrolyte after the water content is qualified, and then the degassing bag is used for secondary sealing to obtain the low-temperature resistant sodium ion battery.

[0053] Example 2 The negative electrode active material includes nitrogen-doped carbon nanofibers supported by Na2Ti3O7, wherein the Na2Ti3O7 is supported on the surface of the nitrogen-doped carbon nanofibers. The mass ratio of Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 2:1. The particle size of the nitrogen-doped carbon nanofibers supported by Na2Ti3O7 is 30 nm.

[0054] The method for preparing the above-mentioned negative electrode active material comprises the following steps: 1. Dissolve 2g of polyvinyl pyrrolidone in 20mL of anhydrous ethanol and stir magnetically for 1h. Then add 12g of tetrabutyl titanate and 4mL of acetic acid and continue stirring for 12h. Transfer the mixed solution to a syringe with a capillary tip. Set the electrospinning voltage to 15kV and the distance between the syringe head and the aluminum foil current collector to 20cm. Bake the obtained electrospun fibers at 250℃ for 1h, then transfer them to a tube furnace and carbonize them at 700℃ in a N2 atmosphere for 3h to finally obtain TiO2-loaded nitrogen-doped carbon nanofibers.

[0055] 2. The TiO2-loaded nitrogen-doped carbon nanofibers obtained above were added to a NaOH solution and stirred evenly for 2 hours. The mixture was then transferred to a high-temperature reactor and hydrothermally reacted at 180°C for 8 hours to obtain a sheet. The sheet was then washed six times with deionized water and anhydrous ethanol and dried to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor.

[0056] 3. Place the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step 2 in a tubular furnace and calcine it at 600°C for 6 hours in an argon atmosphere to obtain Na2Ti3O7-loaded nitrogen-doped carbon nanofibers.

[0057] The method for preparing the sodium ion battery of this embodiment is basically the same as that of Example 1, except that the electrolyte solvent is propylene carbonate (abbreviated as PC) and ethyl methyl carbonate (abbreviated as EMC) in a volume ratio of 7.5:2.5 as an ester solvent.

[0058] Example 3 The negative electrode active material includes nitrogen-doped carbon nanofibers supported by Na2Ti3O7, wherein the Na2Ti3O7 is supported on the surface of the nitrogen-doped carbon nanofibers. The mass ratio of Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 1:1. The particle size of the nitrogen-doped carbon nanofibers supported by Na2Ti3O7 is 25 nm.

[0059] The method for preparing the above-mentioned negative electrode active material comprises the following steps: 1. Dissolve 2g of polyvinyl pyrrolidone in 20mL of anhydrous ethanol and stir magnetically for 1h. Then add 6g of tetrabutyl titanate and 4mL of acetic acid and continue stirring for 12h. Transfer the mixed solution to a syringe with a capillary tip. Set the electrospinning voltage to 15kV and the distance between the syringe head and the aluminum foil current collector to 20cm. Bake the obtained electrospun fibers at 250℃ for 1h, then transfer them to a tube furnace and carbonize them at 700℃ in a N2 atmosphere for 3h to finally obtain TiO2-loaded nitrogen-doped carbon nanofibers.

[0060] 2. The TiO2-loaded nitrogen-doped carbon nanofibers obtained above were added to a NaOH solution and stirred evenly for 2 hours. The mixture was then transferred to a high-temperature reactor and hydrothermally reacted at 180°C for 8 hours to obtain a sheet. The sheet was then washed six times with deionized water and anhydrous ethanol and dried to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor.

[0061] 3. Place the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step 2 in a tubular furnace and calcine it at 600°C for 6 hours in an argon atmosphere to obtain Na2Ti3O7-loaded nitrogen-doped carbon nanofibers.

[0062] The sodium ion battery preparation method of this embodiment is basically the same as that of Example 1, except that the electrolyte solvent is propylene carbonate (abbreviated as PC) and ethyl methyl carbonate (abbreviated as EMC) in a volume ratio of 7:3 as an ester solvent.

[0063] Example 4 The negative electrode active material includes nitrogen-doped carbon nanofibers supported by Na2Ti3O7, wherein the Na2Ti3O7 is supported on the surface of the nitrogen-doped carbon nanofibers, and the mass ratio of Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 2:1.

[0064] The method for preparing the above-mentioned negative electrode active material comprises the following steps: 1. Dissolve 3.6 g of urea and 2.4 g of polyacrylonitrile (PAN) in 30 mL of N,N-dimethylformamide (DMF) solvent and stir magnetically at 50°C for 6 h. Then add 14.4 g of tetraisopropyl titanate (TTIP) and continue stirring at 50°C for 2 h. Transfer the resulting homogeneous solution to a syringe with a capillary tip. Set the electrospinning voltage to 20 kV and the distance between the syringe tip and the aluminum foil current collector to 10 cm. The resulting electrospun fibers are baked at 100°C for 2 h and then transferred to a tube furnace for carbonization at 700°C in a N2 atmosphere for 3 h to obtain TiO2-loaded nitrogen-doped carbon nanofibers.

[0065] 2. The TiO2-loaded nitrogen-doped carbon nanofibers obtained above were added to a NaOH solution and stirred evenly for 2 hours. The mixture was then transferred to a high-temperature reactor and hydrothermally reacted at 140°C for 12 hours to obtain a sheet. The sheet was then washed six times with deionized water and anhydrous ethanol and dried to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor.

[0066] 3. Place the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step 2 in a tubular furnace and calcine it at 600°C for 6 hours in an argon atmosphere to obtain Na2Ti3O7-loaded nitrogen-doped carbon nanofibers.

[0067] The preparation method of the sodium ion battery in this embodiment is basically the same as that in Example 1, with the only difference being that the active material in the positive electrode slurry is Na2Fe2(SO4)3.

[0068] Example 5 The negative electrode active material includes nitrogen-doped carbon nanofibers supported by Na2Ti3O7, wherein the Na2Ti3O7 is supported on the surface of the nitrogen-doped carbon nanofibers, and the mass ratio of the Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 1:1.

[0069] The method for preparing the above-mentioned negative electrode active material comprises the following steps: 1. Dissolve 3.6 g of urea and 2.4 g of polyacrylonitrile (PAN) in 30 mL of N,N-dimethylformamide (DMF) solvent and stir magnetically at 50°C for 6 h. Then add 7.2 g of tetraisopropyl titanate (TTIP) and continue stirring at 50°C for 2 h. Transfer the resulting homogeneous solution to a syringe with a capillary tip. Set the electrospinning voltage to 20 kV, and the distance between the syringe tip and the aluminum foil current collector to 10 cm. The resulting electrospun fibers were baked at 100°C for 2 h, then transferred to a tube furnace and carbonized at 700°C in a N2 atmosphere for 3 h to obtain TiO2-loaded nitrogen-doped carbon nanofibers.

[0070] 2. The TiO2-loaded nitrogen-doped carbon nanofibers obtained above were added to a NaOH solution and stirred evenly for 2 hours. The mixture was then transferred to a high-temperature reactor and hydrothermally reacted at 140°C for 12 hours to obtain a sheet. The sheet was then washed six times with deionized water and anhydrous ethanol and dried to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor.

[0071] 3. Place the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step 2 in a tubular furnace and calcine it at 600°C for 6 hours in an argon atmosphere to obtain Na2Ti3O7-loaded nitrogen-doped carbon nanofibers.

[0072] The sodium-ion battery preparation method of this embodiment is substantially the same as that of Example 1, except that the active material in the positive electrode slurry is Na2Fe2(SO4)3. Furthermore, the electrolyte solvent is an ester solvent composed of propylene carbonate (PC) and ethyl methyl carbonate (EMC) in a volume ratio of 7.5:2.5.

[0073] Example 6 The negative electrode active material includes nitrogen-doped carbon nanofibers supported by Na2Ti3O7, wherein the Na2Ti3O7 is supported on the surface of the nitrogen-doped carbon nanofibers, and the mass ratio of Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 0.5:1.

[0074] The method for preparing the above-mentioned negative electrode active material comprises the following steps: 1. Dissolve 3.6 g of urea and 2.4 g of polyacrylonitrile (PAN) in 30 mL of N,N-dimethylformamide (DMF) solvent and stir magnetically at 50°C for 6 h. Then add 3.6 g of tetraisopropyl titanate (TTIP) and continue stirring at 50°C for 2 h. Transfer the resulting homogeneous solution to a syringe with a capillary tip. Set the electrospinning voltage to 20 kV, and the distance between the syringe tip and the aluminum foil current collector to 10 cm. The resulting electrospun product is baked at 100°C for 2 h, then transferred to a tube furnace and carbonized at 700°C in a N2 atmosphere for 3 h to obtain TiO2-loaded nitrogen-doped carbon nanofibers.

[0075] 2. The TiO2-loaded nitrogen-doped carbon nanofibers obtained above were added to a NaOH solution and stirred evenly for 2 hours. The mixture was then transferred to a high-temperature reactor and hydrothermally reacted at 140°C for 12 hours to obtain a sheet. The sheet was then washed six times with deionized water and anhydrous ethanol and dried to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor.

[0076] 3. Place the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step 2 in a tubular furnace and calcine it at 600°C for 6 hours in an argon atmosphere to obtain Na2Ti3O7-loaded nitrogen-doped carbon nanofibers.

[0077] The sodium-ion battery preparation method of this embodiment is substantially the same as that of Example 1, except that the active material in the positive electrode slurry is Na2Fe2(SO4)3. Furthermore, the electrolyte solvent is an ester solvent composed of propylene carbonate (PC) and ethyl methyl carbonate (EMC) in a volume ratio of 7:3.

[0078] Comparative Example 1 In the sodium ion battery of this comparative example, ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 6:2:2 are used as the electrolyte solvent, without any additives. The rest is the same as the sodium ion battery of Example 3.

[0079] Comparative Example 2 In the sodium ion battery of this comparative example, the negative electrode active material is Na2Ti3O7@C material, and the rest is the same as that of the sodium ion battery of Example 3.

[0080] Comparative Example 3 In the sodium ion battery of this comparative example, the negative electrode active material is Na2Ti3O7 material, and the rest is the same as that of the sodium ion battery of Example 3.

[0081] Comparative Example 4 In the sodium ion battery of this comparative example, the negative electrode active material is hard carbon HC material, and the negative electrode coating single-sided coating density is 7 mg / cm 2 , and the rest are the same as the sodium ion battery in Example 3.

[0082] The CB value of the sodium ion battery prepared in the above embodiment and comparative example is 1.15, and the relevant performance test results are shown in Table 1 below.

[0083] Table 1 Electrochemical performance test results of sodium ion batteries of Examples 1 to 6 and Comparative Examples 1 to 4

[0084] It can be seen from Table 1 that the sodium ion batteries in Examples 1-6 can allow normal charge and discharge at low temperatures, and the discharge capacity at -60°C can reach 70% or more of the discharge capacity at 25°C, and the low-temperature performance is good; for example, the low-temperature sodium ion battery of Example 1 can work normally at -60°C, and the charge and discharge capacity at -60°C reaches 75% of the initial discharge capacity (i.e., the discharge capacity at 25°C), and the low-temperature sodium ion battery in Example 4 can also reach 70%, and no sodium precipitation occurs at the fully charged negative electrode; it can be seen that compared with the sodium ion battery in Comparative Example 1 using conventional carbonate solvents as electrolyte solvents and the sodium ion batteries in Comparative Examples 2-4 using Na2Ti3O7@C, conventional Na2Ti3O7 and hard carbon as negative electrode active materials, Examples 1-6 successfully synthesized Na2Ti3O7@C / N nanofiber negative electrodes, which greatly improved the performance of the sodium ion battery in a low-temperature environment of -60°C.

[0085] At the same time, the discharge capacity of Example 3 in a -60°C environment can reach 84% of the discharge capacity in a 25°C environment. Compared with the sodium ion batteries with full charge and negative electrode sodium precipitation and low discharge capacity at -60°C in Comparative Examples 1-4, the low-temperature performance of the low-temperature sodium ion battery in Example 3 is greatly improved; the discharge capacity of Example 5 in a -60°C environment can reach 79% of the discharge capacity in a 25°C environment. Compared with the sodium ion batteries with extremely low discharge capacity at -60°C in Comparative Examples 1-4, the low-temperature performance of the low-temperature sodium ion battery in Example 5 is greatly improved; at the same time, by comparing Example 3 with Examples 4-6, the low-temperature performance of the Na2Ti3O7@C / N nanofiber negative electrode synthesized in Example 3 is more advantageous. The low-temperature sodium-ion battery in this application uses a polyanionic compound as the positive electrode active material, modified Na2Ti3O7@C / N nanofibers as the negative electrode active material, a mixture of propylene carbonate and ethyl methyl carbonate as the solvent of the electrolyte, and fluoroethylene carbonate (FEC) additives, which greatly improves the low-temperature performance of the low-temperature sodium-ion battery, can meet the charging and discharging requirements of -60°C, and is reliable and safe.

[0086] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A negative electrode active material, characterized in that: The invention comprises Na2Ti3O7-loaded nitrogen-doped carbon nanofibers, wherein the Na2Ti3O7 is loaded on the surface of the nitrogen-doped carbon nanofibers.

2. The negative electrode active material according to claim 1, characterized in that The mass ratio of the Na2Ti3O7 to the nitrogen-doped carbon nanofibers is 0.5-4:

1.

3. The negative electrode active material according to claim 1, characterized in that The particle size of the nitrogen-doped carbon nanofibers loaded with Na2Ti3O7 is 20nm-100nm.

4. The method for preparing the negative electrode active material according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1, dissolving a carbon nanofiber precursor in a solvent, adding a titanate precursor and a weak acid, stirring, transferring the mixture to a syringe, obtaining electrospun fibers by an electrospinning method, baking the electrospun fibers, transferring the fibers to a tube furnace, and carbonizing the fibers under a nitrogen atmosphere to obtain titanium dioxide-supported nitrogen-doped carbon nanofibers; S2, adding the titanium dioxide-loaded nitrogen-doped carbon nanofibers obtained in step S1 into an alkaline solution and transferring the mixture to a reactor for hydrothermal reaction to obtain a Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor; S3. calcining the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber precursor obtained in step S2 to obtain the Na2Ti3O7-loaded nitrogen-doped carbon nanofiber.

5. The method for preparing the negative electrode active material according to claim 4, characterized in that: In step S1, the carbon nanofiber precursor is selected from one or more of polyvinyl pyrrolidone, polyacrylonitrile, and polyimide; and / or, In the step S1, the titanate precursor is selected from one or more of tetrabutyl titanate and tetraisopropyl titanate.

6. The method for preparing the negative electrode active material according to claim 4, characterized in that: In the step S1, the baking temperature of the electrospinning is 100° C. to 250° C., and the carbonization temperature in a nitrogen atmosphere is 500° C. to 700° C.; and / or, In step S2, the temperature of the hydrothermal reaction is 140° C. to 180° C.; and / or, In the step S3, the calcination temperature is 500°C to 600°C.

7. The method for preparing the negative electrode active material according to claim 4, characterized in that: The mass ratio of the titanate precursor to the carbon nanofiber precursor is 1 to 9:

1.

8. A negative electrode sheet, characterized in that: The invention comprises the negative electrode active material according to any one of claims 1 to 3 or the negative electrode active material prepared by the preparation method according to any one of claims 4 to 7.

9. The negative electrode sheet according to claim 8, characterized in that: The negative electrode sheet includes a negative electrode current collector and a coating formed by a negative electrode slurry coated on the negative electrode current collector, wherein the coating is formed on at least one side of the negative electrode current collector; the negative electrode slurry includes the negative electrode active material.

10. A sodium ion battery, characterized in that: The invention comprises a positive electrode sheet, a separator and the negative electrode sheet according to claim 8 or 9, wherein the separator is located between the positive electrode sheet and the negative electrode sheet.

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