Sodium ion battery and preparation method thereof

Abstract

The invention discloses a sodium ion battery and a preparation method thereof, the sodium ion battery comprises an electrolyte and an electrode assembly, the electrode assembly comprises a negative electrode and a positive electrode; the electrolyte comprises a fluorine-containing sodium salt, a fluorine-containing potassium salt and an organic solvent, the negative electrode comprises a negative electrode active material layer, the negative electrode active material layer comprises a negative electrode active material and an additive, and the additive is aluminum hydroxide. According to the sodium ion battery provided by the invention, the potassium salt in the electrolyte and the hydroxyl alumina in the negative electrode act together, the surface of the hydroxyl alumina pre-adsorbs potassium ions to form an adsorption layer, and the adsorption layer and anions existing in the electrolyte form a double-electrode layer, so that the interface reaction is accelerated, and the crystallinity of NaF is further improved; and thus, the cycle life and the rate capability of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a sodium ion battery and a method for preparing the sodium ion battery. Background Art

[0002] During the operation of sodium-ion batteries, the properties of the solid electrolyte interface (SEI) membrane play a critical role in battery performance. The SEI membrane effectively prevents the continued reaction between the electrolyte and the anode material, thereby ensuring the stability and cycle life of the battery. As a key component of the SEI membrane, the degree of crystallinity of NaF has a significant impact on the ionic conductivity, mechanical stability, and chemical stability of the SEI membrane. NaF with high crystallinity can reduce the internal resistance of the SEI membrane, enabling rapid sodium ion transport, while also strengthening the mechanical strength of the SEI membrane and inhibiting its rupture and reconstruction during charge and discharge, ultimately improving the battery's cycle performance and rate capability.

[0003] The current method for regulating the crystallinity of NaF in the SEI film includes introducing sodium salts containing fluorine elements. Although it can affect the formation of the SEI film and the crystallinity of NaF to a certain extent, the improvement of NaF crystallinity is limited and it cannot effectively improve the cycling and storage performance of sodium-ion batteries. Summary of the Invention

[0004] To address the problem that the prior art cannot effectively improve the crystallinity of NaF in the SEI film of a sodium ion battery, a sodium ion battery and a method for preparing the sodium ion battery are provided.

[0005] The technical solutions adopted by the present invention to solve the above technical problems are as follows: In one aspect, the present invention provides a sodium ion battery comprising an electrolyte and an electrode assembly, wherein the electrode assembly comprises a negative electrode and a positive electrode; The electrolyte includes a sodium salt, a potassium salt and an organic solvent containing fluorine elements. The negative electrode includes a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material and an additive. The additive is aluminum oxyhydroxide.

[0006] Optionally, the potassium salt includes one or more of KPF6, KTFSI, KFSI, KBOB, KBF4, KClO4, KCF3SO3 and KAc.

[0007] Optionally, the sodium salt includes one or more of NaPF6, NaBF4, NaCF3SO3, NaTFSI, and NaFSI.

[0008] Optionally, the organic solvent includes but is not limited to one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene glycol dimethyl ether (DME), dioxolane (DOL), and acetonitrile (AN).

[0009] Optionally, the aluminum oxyhydroxide includes one or more of α-aluminum oxyhydroxide, γ-aluminum oxyhydroxide, and β-aluminum oxyhydroxide.

[0010] Optionally, in the electrolyte of the sodium ion battery, the mass percentage of the potassium salt is 0.1%-0.5%.

[0011] Optionally, the mass percentage of the potassium salt to the aluminum oxyhydroxide is (0.3%-1.5%): (0.33%-1.3%).

[0012] Optionally, the particle size of the aluminum oxyhydroxide is 5-20 nm.

[0013] Optionally, the mass percentage of the aluminum oxyhydroxide accounts for 0.05%-0.2% of the mass percentage of the negative electrode active material.

[0014] Optionally, in the slurry used to form the active material layer, the mass percentage of the negative electrode active material is 80%-98%.

[0015] Optionally, the positive electrode includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material; In the positive electrode active material layer, the mass percentage of the positive electrode active material is 80%-98%.

[0016] Optionally, the method for preparing a sodium ion battery includes the following operations: Dissolve the sodium salt in an organic solvent, then add the potassium salt and dissolve it to obtain an electrolyte; Aluminum oxyhydroxide is mixed with the negative electrode active material, dispersed evenly, coated on the surface of the negative electrode foil, and dried to obtain the negative electrode; coating the positive electrode active material on the surface of the positive electrode foil and drying it to obtain the positive electrode; Assemble the positive electrode and the negative electrode to obtain a sodium ion battery.

[0017] Optionally, the preparation of aluminum oxyhydroxide includes the following operations: Dispersing aluminum oxide in deionized water to obtain a dispersion; The dispersion is placed in a reaction device, subjected to heat treatment, and centrifuged to obtain aluminum oxyhydroxide.

[0018] Optionally, the heat treatment temperature is 160-200° C., and the heat treatment time is 8-16 hours.

[0019] The beneficial effects of the present invention are: The sodium ion battery provided by the present invention comprises an electrolyte comprising a sodium salt, a potassium salt, and an organic solvent, and the negative electrode of the sodium ion battery comprises aluminum oxyhydroxide. The potassium ions in the potassium salt in the electrolyte interact with the sodium ions in the sodium salt. The difference in radius between potassium ions and sodium ions is utilized, and the charge density of potassium ions is lower than that of sodium ions. After being embedded in the crystal lattice, local static electricity is reduced, promoting the orderly arrangement of NaF crystals. The solvation energy of potassium ions is lower than that of sodium ions, making them more easily desolvated and preferentially adsorbed on the electrode surface, thereby reducing the interfacial energy and promoting the merging and densification of NaF grains. That is, the NaF crystallization kinetics are optimized from the two aspects of charge density regulation and interfacial adsorption effect. In addition, the hydroxylated aluminum oxide acts as a heterogeneous nucleation site, and its surface groups combine with fluoride ions to induce the preferential growth of NaF along the crystal plane, thereby enhancing the performance of the SEI film and ensuring the stability and cycle life of the sodium ion battery. Furthermore, the surface of the aluminum oxyhydroxide pre-adsorbs potassium ions to form an adsorption layer, which forms a double layer with the anions present in the electrolyte, which is conducive to accelerating the interfacial reaction and further improving the crystallinity of NaF, thereby improving the cycle life and rate performance of the battery. DETAILED DESCRIPTION

[0020] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0021] The present invention provides a sodium ion battery comprising an electrolyte and an electrode assembly, wherein the electrode assembly comprises a negative electrode and a positive electrode; The electrolyte includes a sodium salt, a potassium salt and an organic solvent containing fluorine elements. The negative electrode includes a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material and an additive. The additive is aluminum oxyhydroxide.

[0022] Specifically, in the sodium ion battery provided by the present invention, the electrolyte includes a sodium salt, a potassium salt, and an organic solvent, and the negative electrode of the sodium ion battery includes aluminum oxyhydroxide. The potassium ions in the potassium salt in the electrolyte interact with the sodium ions in the sodium salt. The difference in radius between potassium ions and sodium ions is utilized. The charge density of potassium ions is lower than that of sodium ions. After being embedded in the crystal lattice, local static electricity is reduced, promoting the orderly arrangement of NaF crystals. The solvation energy of potassium ions is lower than that of sodium ions. They are more easily desolvated and preferentially adsorbed on the electrode surface, reducing the interfacial energy and promoting the merging and densification of NaF grains. That is, the NaF crystallization kinetics are optimized from the two aspects of charge density regulation and interfacial adsorption effect. In addition, the hydroxylated aluminum oxide acts as a heterogeneous nucleation site. Its surface groups combine with fluoride ions to induce the preferential growth of NaF along the crystal plane, enhance the performance of the SEI film, and ensure the stability and cycle life of the sodium ion battery. Furthermore, the surface of the aluminum oxyhydroxide pre-adsorbs potassium ions to form an adsorption layer, which forms a double layer with the anions present in the electrolyte, which is conducive to accelerating the interfacial reaction and further improving the crystallinity of NaF, thereby improving the cycle life and rate performance of the battery.

[0023] In some embodiments, the potassium salt comprises one or more of KPF6, KTFSI, KFSI, KBOB, KBF4, KClO4, KCF3SO3 and KAc.

[0024] Specifically, the charge density of potassium ions in the potassium salt is lower than that of sodium ions. After being embedded in the crystal lattice, the local electrostatic exclusion is reduced, which can promote the orderly arrangement of NaF crystals. In addition, the solvation energy of potassium ions is lower than that of sodium ions. They are easier to desolvate and preferentially adsorb on the electrode surface, reducing the interfacial energy, which is conducive to promoting the merging and densification of NaF grains. That is, the potassium ions in the potassium salt optimize the crystallization kinetics of NaF in sodium ion batteries through charge density regulation and interfacial adsorption effect.

[0025] In some embodiments, the sodium salt includes one or more of NaPF6, NaBF4, NaCF3SO3, NaTFSI, and NaFSI.

[0026] Specifically, in the electrolyte of sodium ion batteries, NaPF6 Anions can participate in the formation of SEI film on the electrode surface, which helps to form a more stable SEI film. At the same time, NaPF6 has good ionic conductivity and can provide sufficient sodium ions in the electrolyte to ensure the rapid transmission of sodium ions during the charge and discharge process of the battery, thereby improving the battery's rate performance.

[0027] In some embodiments, the organic solvent includes one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene glycol dimethyl ether (DME), dioxolane (DOL), and acetonitrile (AN).

[0028] Specifically, in a preferred embodiment, the organic solvent is selected from one or more of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0029] In some embodiments, the aluminum oxyhydroxide includes one or more of α-aluminum oxyhydroxide, γ-aluminum oxyhydroxide, and β-aluminum oxyhydroxide.

[0030] Specifically, the crystal structure and surface properties of aluminum oxyhydroxide enable it to provide heterogeneous nucleation sites for NaF in the negative active material layer of sodium-ion batteries, which in turn helps to reduce the energy barrier for NaF nucleation and promotes the preferential crystallization of NaF at specific locations, making the growth of NaF more orderly and improving its crystallinity, thereby enhancing the performance of the battery.

[0031] On the one hand, the hydroxyl groups (-OH) on the surface of aluminum oxide hydroxide can interact with fluoride ions, thereby guiding NaF to grow preferentially along the crystal plane, making the NaF crystal grow more significantly in a specific direction, optimizing the crystal orientation of NaF, which is beneficial to the transmission and diffusion of sodium ions in the electrode material, reducing the resistance during the battery charge and discharge process, and improving the battery's charge and discharge efficiency and rate performance.

[0032] On the other hand, the surface of aluminum oxide hydroxide can adsorb potassium ions to form an adsorption layer, which constitutes a double layer with the anions in the electrolyte. The existence of the double layer can effectively regulate the electric field distribution on the electrode surface, accelerate the interfacial reaction between the electrode and the electrolyte, and help to increase the battery's charge and discharge speed, shorten the battery's charging time, and enhance the battery's stability under high-rate charge and discharge conditions.

[0033] Through these effects, aluminum oxyhydroxide improves the crystal quality of NaF and the electrode interface properties, thereby enhancing the stability of sodium-ion batteries during cyclic charge and discharge. This reduces battery performance degradation caused by imperfect NaF crystallization or poor interfacial reactions, extending the battery's cycle life and reducing the rate of capacity decay during battery use.

[0034] In some embodiments, the mass percentage of the potassium salt in the electrolyte of the sodium ion battery is 0.1%-0.5%.

[0035] Specifically, the potassium salt dissociates into potassium ions in the electrolyte. The present application has verified through preliminary experiments that when the potassium salt mass percentage is 0.1%-0.5%, it can effectively improve the ion conductivity without significantly increasing the viscosity of the electrolyte, ensuring the rapid migration of sodium ions in the electrolyte, which helps to improve the charge and discharge efficiency of the battery; if the potassium salt content is too high, it will cause the ionic strength of the electrolyte to be too high, causing the interaction between ions to be enhanced, which is not conducive to improving the migration rate of ions. If the content is too low, it cannot fully exert its optimization effect on ionic conductivity and interface performance; In a specific embodiment, the mass percentage of the potassium salt can be 0.1%, 0.2%, 0.3%, 0.4% or 0.5%.

[0036] In some embodiments, the mass percentage of the aluminum oxyhydroxide accounts for 0.05%-0.2% of the mass percentage of the negative electrode active material.

[0037] Aluminum oxyhydroxide can provide heterogeneous nucleation sites for the crystallization of NaF. Similarly, when the mass percentage of aluminum oxyhydroxide accounts for 0.05%-0.2% of the mass percentage of the negative electrode active material, the hydroxyl groups on its surface can fully interact with the fluoride ions in the electrolyte to improve the crystallinity of NaF. Within this content range, a better heterogeneous nucleation effect can be achieved with a smaller dosage, making the NaF crystal growth more orderly, which is beneficial to the transmission of sodium ions and the improvement of battery performance.

[0038] Aluminum oxide hydroxide can adsorb potassium ions on its surface to form an adsorption layer, and form a double layer with the anions in the electrolyte. This double layer can accelerate the interfacial reaction and promote the rapid exchange of sodium ions between the electrode and the electrolyte. However, if the content is too low, a sufficiently effective double layer cannot be formed, and it is difficult to fully play the role of accelerating the interfacial reaction; if the content is too high, it may cause problems such as agglomeration, affecting its dispersion and uniformity in the electrolyte, and thus affecting its role, and may even have a negative impact on other performance of the battery.

[0039] The mass percentage of the aluminum oxyhydroxide may be 0.05%, 0.1%, 0.15% or 0.2%.

[0040] In some embodiments, the mass percentage of the potassium salt to the aluminum oxyhydroxide is (0.3-1.5): (0.33-1.3).

[0041] Specifically, the mass percentages of the potassium salt and the aluminum oxyhydroxide are set within the above range, and the potassium salt and aluminum oxyhydroxide can better synergize to improve the charge and discharge efficiency, cycle stability and rate performance of the sodium ion battery; the potassium salt can increase the ionic conductivity of the electrolyte and optimize the electrode / electrolyte interface performance, while the aluminum oxyhydroxide can provide heterogeneous nucleation sites, induce the preferential growth of NaF and form a double layer to accelerate the interface reaction.

[0042] In some embodiments, the aluminum oxyhydroxide has a particle size of 5-20 nm.

[0043] Specifically, aluminum oxide hydroxide within this particle size range has a larger specific surface area and can provide more surface active sites. On the one hand, it is conducive to full contact and interaction with fluoride ions in the electrolyte, more effectively inducing the preferential growth of NaF along the crystal plane. On the other hand, the larger specific surface area also increases the contact area with potassium ions, facilitating the pre-adsorption of more potassium ions, forming a more stable and effective double-layer structure, thereby accelerating interfacial reactions and improving battery performance. In addition, the particle size of 5-20 nm has good dispersibility in the electrolyte and can be evenly distributed in the electrolyte, avoiding agglomeration due to excessive particle size. The uniform dispersion ensures the consistency of the role played by aluminum oxide hydroxide in the entire battery system, making the performance of each region inside the battery more stable and uniform, which is conducive to improving the overall performance and cycle stability of the battery. Aluminum oxide hydroxide in this particle size range is well compatible with the electrode materials of sodium ion batteries, can be more easily adsorbed on the electrode surface, and form a good interface bond with the electrode material, thereby more effectively playing its role in providing heterogeneous nucleation sites and improving interface performance, promoting the transmission and reaction of sodium ions between the electrode and the electrolyte, and improving the battery's charge and discharge efficiency and rate performance.

[0044] In some embodiments, the particle size of the aluminum oxyhydroxide can be 5 nm, 8 nm, 10 nm, 15 nm, or 5 nm, or any value in the range of 5-20 nm.

[0045] In some embodiments, the mass of the aluminum oxyhydroxide accounts for 0.05%-0.2% of the mass of the negative electrode active material.

[0046] Aluminum oxyhydroxide can serve as a heterogeneous nucleation site, reducing the energy barrier for NaF nucleation and promoting its preferential crystallization at specific locations. By limiting the mass ratio of aluminum oxyhydroxide to the mass of the negative electrode active material to 0.05%-0.2%, its surface hydroxyl groups can fully interact with the fluoride ions in the electrolyte, guiding the preferential growth of NaF along the crystal plane, optimizing the crystal orientation, making the NaF crystal more orderly, and improving the crystallinity, which is beneficial for sodium ion transport and improves the battery's charge and discharge efficiency and rate performance. If the proportion of aluminum hydroxide is too low, an effective double layer cannot be formed, and it is difficult to fully exert its role in accelerating the interfacial reaction. If the proportion is too high, it is easy to agglomerate, affecting the dispersion and uniformity in the electrolyte, and may have a negative impact on other battery performance. Controlling it at 0.05%-0.2% can ensure that aluminum hydroxide is evenly dispersed in the negative electrode active material, stably play the role of providing heterogeneous nucleation sites and improving interfacial performance, thereby improving the overall performance and cycle stability of the battery.

[0047] In some embodiments, in the slurry used to form the active material layer, the mass percentage of the negative electrode active material is 80%-98%.

[0048] Specifically, the negative electrode active material content in the range of 80%-98% is conducive to the subsequent coating process operation, and at the same time can ensure that the formed active material layer has good electrochemical properties; if the negative electrode active material content is too low, it may lead to insufficient active material, affecting the charge and discharge capacity of the battery; if the content is too high, the viscosity, fluidity and other properties of the slurry may deteriorate, which is not conducive to uniform coating, and thus affects the quality of the active material layer and the consistency of the battery.

[0049] In some embodiments, the positive electrode includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material; In the positive electrode active material layer, the mass percentage of the positive electrode active material is 80%-98%.

[0050] Positive electrode active materials are a key component of batteries, responsible for electrochemical reactions, energy storage, and energy release. A higher percentage by mass means more active material participates in the charge and discharge process within the same volume or mass of the positive electrode active material layer, enabling more energy storage and release. This increases the battery's energy density and helps meet the battery life requirements of devices. Reducing the proportion of inactive ingredients like binders and conductive agents reduces energy loss during transmission and improves the battery's charge and discharge efficiency.

[0051] Specifically, when the mass percentage of the positive electrode active material is 92%-96%, it can work well with the positive electrode conductor and the positive electrode binder to improve the electrical performance of the sodium ion battery.

[0052] In some embodiments, the method for preparing a sodium ion battery comprises the following operations: Dissolve the sodium salt in an organic solvent, then add the potassium salt and dissolve it to obtain an electrolyte; Aluminum oxyhydroxide is mixed with the negative electrode active material, dispersed evenly, coated on the surface of the negative electrode foil, and dried to obtain the negative electrode; coating the positive electrode active material on the surface of the positive electrode foil and drying it to obtain the positive electrode; Assemble the positive electrode and the negative electrode to obtain a sodium ion battery.

[0053] Specifically, sodium salt is dissolved in an organic solvent to form a uniform solution. The sodium salt is a key component in providing sodium ions in the electrolyte, while the organic solvent acts as a medium for the transport of sodium ions. The addition of potassium salt can optimize the performance of the electrolyte (increasing its ionic conductivity) and improve the electrode / electrolyte interface.

[0054] Aluminum oxyhydroxide is mixed with the negative electrode active material. Aluminum oxyhydroxide provides heterogeneous nucleation sites, which can improve the performance of the negative electrode after mixing with the negative electrode active material.

[0055] The evenly mixed materials are coated on the surface of the negative electrode foil so that the active materials can adhere to the current collector to form a negative electrode structure; the drying process removes the solvent introduced during the coating process, so that the negative electrode material solidifies and is tightly bonded to the foil.

[0056] In some embodiments, the preparation of aluminum oxyhydroxide comprises the following operations: Dispersing aluminum oxide in deionized water to obtain a dispersion; The dispersion is placed in a reaction device, subjected to heat treatment, and centrifuged to obtain aluminum oxyhydroxide.

[0057] Specifically, aluminum oxide is placed in deionized water for dispersion operation to finally obtain a dispersion liquid. The purpose of this operation is to make the aluminum oxide evenly distributed in the deionized water.

[0058] The dispersion is transferred to a reaction device for heat treatment. During the heat treatment process, alumina reacts chemically with water, and the hydroxyl groups (-OH) in the water molecules adhere to the surface of alumina, thereby achieving hydroxylation of alumina.

[0059] After the heat treatment is completed, the product is separated by centrifugation, and the hydroxylated alumina will precipitate to obtain the aluminum oxyhydroxide product.

[0060] In some embodiments, the heat treatment temperature is 160-200° C., and the heat treatment time is 8-16 hours.

[0061] In a specific operation, the heat treatment temperature is 180° C. and the heat treatment time is 12 h.

[0062] The present invention is further described below with reference to the following examples.

[0063] Table 1 Example 1 This example is used to illustrate the preparation method of the sodium ion battery disclosed in the present invention, which includes the following steps: Preparation of electrolyte: In an argon glove box (H2O <0.1ppm, O2 <0.1ppm), NaPF6 was dissolved in a mixed organic solvent of EC:DEC (1:1 vol%), 0.1wt% of KPF6 was added, and the mixture was magnetically stirred for 18 hours (800rpm, 25°C) to completely dissolve the mixture and ensure that the potassium salt was evenly dispersed, thereby obtaining the electrolyte; Preparation of electrode assembly: Al2O3 with a particle size of 20 nm was dispersed in deionized water and ultrasonically treated for 30 minutes. The suspension was then transferred to an autoclave and hydrothermally reacted at 180°C for 12 hours to generate hydroxyl groups (-OH) on the surface. The solid was then centrifuged, washed three times with alcohol, and dried in a vacuum at 60°C to obtain aluminum oxyhydroxide. The negative electrode active material, negative electrode conductive agent and negative electrode binder (94:3:3) were mixed, and 0.05wt% of aluminum oxyhydroxide was added. The mixture was ultrasonically dispersed for 0.5 hours to obtain a negative electrode slurry. The obtained slurry was then evenly coated on a copper foil and dried at 60°C for 24 hours. Subsequently, the negative electrode was obtained by roller pressing and slitting. The positive electrode active material, positive electrode conductive agent and positive electrode binder were mixed in a weight ratio of 94:3:3, mixed at 2000 rpm, and the slurry was evenly coated on aluminum foil and vacuum dried at 80°C overnight. Subsequently, the positive electrode was obtained by roller pressing and slitting. Place the positive electrode, separator, and negative electrode in order, with the separator placed between the positive and negative electrodes to act as an isolation layer. Then, a lamination process is used to prepare a bare cell. The bare battery cell is placed in an aluminum-plastic film for packaging, and the preparation of the sodium ion battery is completed through vacuum packaging, static standing, formation, and shaping processes.

[0064] Example 2-16 Examples 2-16 are used to illustrate the preparation method of the sodium ion battery disclosed in the present invention, which includes most of the operating steps in Example 1, except that: The potassium salt, aluminum oxyhydroxide, aluminum oxyhydroxide particle size, and the mass percentage of potassium salt and aluminum oxyhydroxide shown in Examples 2-16 in Table 1 were used.

[0065] Comparative Examples 1-3 Comparative Examples 1-3 are used to illustrate the preparation method of the sodium ion battery disclosed in the present invention, which includes most of the operating steps in Example 1, except that: The potassium salt, aluminum oxyhydroxide, aluminum oxyhydroxide particle size, and the mass percentage of potassium salt and the mass percentage of aluminum oxyhydroxide shown in Comparative Examples 1-3 in Table 1 were used.

[0066] Performance Testing The following performance tests were performed on Examples 1-16 and Comparative Examples 1-3 prepared above: XRD test: Scanning range 20°-80°, scanning rate 5° / min, find the NaF characteristic peak at 2θ=43° in the spectrum, and calculate its half-height width.

[0067] Young's modulus: Use a nanoindenter to fix the electrode sample on the test bench and perform an indentation test to obtain the indentation depth-load curve. By analyzing the curve, the Young's modulus of each test point is calculated. Finally, the results of multiple test points are averaged to obtain the Young's modulus of the sample.

[0068] Sodium ion diffusion coefficient: Using an electrochemical workstation, connect the assembled sodium-ion battery to the workstation and perform electrochemical impedance spectroscopy (EIS) testing to obtain the battery's impedance spectrum. Fit the impedance spectrum using an equivalent circuit model to obtain parameters related to the Warburg impedance. The sodium ion diffusion coefficient is then calculated using the Nernst-Einstein equation and related formulas.

[0069] Battery Capacity Retention: Using a battery testing system, place the assembled sodium-ion battery in a suitable constant temperature and humidity environment. Charge and Discharge Testing: First, perform an initial charge and discharge test on the battery at a current density of 0.1C, recording the initial charge and discharge capacity and coulombic efficiency. Then, perform a 1C charge / 1C discharge test for up to 1000 cycles, recording the data. Finally, perform a charge and discharge test at a 5C rate, recording the discharge capacity and calculating the 5C capacity retention rate.

[0070] The test results are entered in Table 2.

[0071] Table 2 It can be seen from the test data in Table 1 that compared with Comparative Example 1-3, the test data of Example 1-16 are better than those of Comparative Example 1-3. The reason is that the sodium ion battery prepared in Example 1-16 has aluminum oxyhydroxide and potassium salt, and the surface of aluminum oxyhydroxide pre-adsorbs potassium ions to form an adsorption layer, which forms a double electric layer with the anions in the electrolyte, accelerates the interfacial reaction, and improves the crystallinity of NaF. On the other hand, in the sodium ion battery of the present application, in the electrolyte, the potassium ions in the potassium salt interact with the sodium ions in the sodium salt, and the difference in radius between potassium ions and sodium ions is used to promote the orderly arrangement of NaF crystals. In the sodium ion battery prepared in Comparative Example 1, potassium salt and aluminum oxyhydroxide are not added, aluminum oxyhydroxide is not added in Comparative Example 2, and potassium salt is not added in Comparative Example 3. Compared with Examples 1-16, the sodium ion batteries prepared in Comparative Examples 1-3 cannot achieve the synergistic effect of potassium salt and aluminum oxyhydroxide to achieve the effect of improving the crystallinity of NaF in its SEI film, improving the cycle performance and rate performance; Comparing Example 5 and Examples 13-14, it can be seen that different potassium salts are selected in Example 5 and Examples 13-14. From the test results, it can be seen that the battery performance is different. When the potassium salt is KPF6, as in Example 5, the various performance test results of the battery are better. This is because the potassium ions dissociated from different potassium salts have different effects on the optimization of NaF crystallization kinetics. Compared with Examples 13-14, KPF6 is more conducive to promoting NaF grain merging and densification, enhancing SEI film performance, and thus improving the electrochemical performance of the battery; By comparing Example 5 and Examples 15-16, it can be seen that the battery performance varies depending on the type of hydroxyalumina; γ-hydroxyalumina performs better in improving battery performance. For example, when γ-hydroxyalumina is used in Example 5, the various performance indicators of the battery are better than those of Example 15 (α-hydroxyalumina) and Example 16 (β-hydroxyalumina). This is because different types of hydroxyalumina have different crystal structures and surface properties. γ-hydroxyalumina can better provide heterogeneous nucleation sites for NaF, interact more fully with fluoride ions, optimize NaF crystal orientation, accelerate interfacial reactions, and improve battery performance.

[0072] By comparing Example 5 with Examples 11-12, it can be seen that Example 5 (particle size 10 nm), Example 11 (particle size 5 nm), and Example 12 (particle size 20 nm) have better performance indicators than Example 12. The particle size of hydroxy aluminum oxide in Example 12 is relatively large, and it is easy to agglomerate, which affects its function.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A sodium ion battery, characterized in that The electrolyte comprises an electrolyte and an electrode assembly, wherein the electrode assembly comprises a negative electrode and a positive electrode; The electrolyte includes a sodium salt, a potassium salt and an organic solvent containing fluorine elements. The negative electrode includes a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material and an additive. The additive is aluminum oxyhydroxide.

2. The sodium ion battery according to claim 1, characterized in that The potassium salt includes one or more of KPF6, KTFSI, KFSI, KBOB, KBF4, KClO4, KCF3SO3 and KAc.

3. The sodium ion battery according to claim 1, characterized in that The sodium salt includes one or more of NaPF6, NaBF4, NaCF3SO3, NaTFSI, and NaFSI.

4. The sodium ion battery according to claim 1, characterized in that The organic solvent includes, but is not limited to, one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene glycol dimethyl ether, dioxolane, and acetonitrile.

5. The sodium ion battery according to claim 1, characterized in that The aluminum oxyhydroxide includes one or more of α-aluminum oxyhydroxide, γ-aluminum oxyhydroxide, and β-aluminum oxyhydroxide.

6. The sodium ion battery according to claim 1, characterized in that In the electrolyte of the sodium ion battery, the mass percentage of the potassium salt is 0.1%-0.5%.

7. The sodium ion battery according to claim 1, characterized in that In the electrolyte of the sodium ion battery, the mass percentage of the potassium salt and the aluminum oxyhydroxide is (0.3-1.5): (0.33-1.3).

8. The sodium ion battery according to claim 1, characterized in that The particle size of the aluminum oxyhydroxide is 5-20 nm.

9. The sodium ion battery according to claim 1, characterized in that The mass percentage of the aluminum oxyhydroxide accounts for 0.05%-0.2% of the mass of the negative electrode active material.

10. The sodium ion battery according to claim 1, characterized in that In the slurry used to form the active material layer, the mass percentage of the negative electrode active material is 80%-98%.

11. The sodium ion battery according to claim 1, characterized in that The positive electrode includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material; In the positive electrode active material layer, the mass percentage of the positive electrode active material is 80%-98%.

12. The method for preparing a sodium ion battery according to any one of claims 1 to 11, characterized in that: The following operations are included: Dissolve the sodium salt in an organic solvent, then add the potassium salt and dissolve it to obtain an electrolyte; Aluminum oxyhydroxide is mixed with the negative electrode active material, dispersed evenly, coated on the surface of the negative electrode foil, and dried to obtain the negative electrode; coating the positive electrode active material on the surface of the positive electrode foil and drying it to obtain the positive electrode; Assemble the positive electrode and the negative electrode to obtain a sodium ion battery.

13. The method for preparing a sodium ion battery according to claim 12, wherein: The preparation of the aluminum oxyhydroxide comprises the following operations: Dispersing aluminum oxide in deionized water to obtain a dispersion; The dispersion is placed in a reaction device, subjected to heat treatment, and centrifuged to obtain aluminum oxyhydroxide.

14. The method for preparing a sodium ion battery according to claim 13, wherein: The heat treatment temperature is 160-200° C., and the heat treatment time is 8-16 hours.

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