Electrolyte, sodium ion battery and electrical device

Abstract

The present application relates to an electrolyte, a sodium ion battery, and an electrical device. The electrolyte comprises a sodium salt additive; the sodium salt additive comprises a first additive and a second additive; the first additive is a sodium sulfonate additive; the second additive is a fluorine-containing sodium salt additive; in the electrolyte, the mass fraction of the first additive is less than the mass fraction of the second additive.

Description

Electrolyte, sodium-ion battery and electric device

[0001] The present application claims priority to the Chinese patent application No. 2024115473035, filed on October 31, 2024, and entitled "Electrolyte, sodium-ion battery and electric device", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of batteries, in particular to an electrolyte, a sodium-ion battery and an electric device. BACKGROUND

[0003] With the development of battery technology, in order to meet the demand for higher energy density, the traditional negative electrode sheet is limited in application due to the volume expansion of silicon-based materials in the active material layer. Therefore, the anode-free battery system has become a new research direction. In the anode-free system, the negative electrode is mainly a conductive layer coated current collector, and the sodium ions released from the positive electrode during the charging process of the battery are deposited on the conductive coating to form a sodium metal negative electrode. Since there is no active material layer, the mass and volume of the negative electrode can be greatly reduced to improve the energy density of the battery. However, the conductive coating has poor compactness and rough surface, and the adhesion of sodium on the conductive coating is poor, thereby reducing the first charge-discharge efficiency of the battery. SUMMARY

[0004] In view of this, the present application provides an electrolyte, a sodium-ion battery and an electric device. The electrolyte improves the deposition compactness of sodium metal on the surface of the negative electrode sheet and improves the first charge-discharge efficiency and cycle performance of the sodium-ion battery.

[0005] The present application provides an electrolyte, which comprises a sodium salt additive, the sodium salt additive comprises a first additive and a second additive, the first additive is a sodium sulfonate additive, and the second additive is a fluorine-containing sodium salt additive. In the electrolyte, the mass fraction of the first additive is less than the mass fraction of the second additive.

[0006] The present application provides a sodium-ion battery, which comprises an electrode assembly and the electrolyte provided by the present application. The electrode assembly comprises a negative electrode sheet, the negative electrode sheet comprises a current collector layer and a conductive layer which are stacked, and the conductive layer comprises a carbon material. The electrolyte is used to infiltrate at least part of the electrode assembly.

[0007] The present application also provides an electric device, which comprises a device body and the sodium-ion battery provided by the present application. The sodium-ion battery is used to supply power to the device body. BRIEF DESCRIPTION OF DRAWINGS

[0008] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings described in the following embodiments are some of the embodiments of the present application, and all other drawings obtained by those of ordinary skill in the art without creative effort based on these drawings also belong to the protection scope of the present application.

[0009] Fig. 1 is a schematic diagram of the cross-sectional structure of a sodium ion battery according to an embodiment of the present application;

[0010] Fig. 2 is a schematic diagram of the cross-sectional structure of a negative electrode sheet according to an embodiment of the present application;

[0011] Fig. 3 is a comparison diagram of the electrolyte conductivity of Example 2, Comparative Example 1, Comparative Example 3, Comparative Example 5 and Comparative Example 6 in the initial state;

[0012] Fig. 4 is a comparison diagram of the sodium deposition of the negative electrode sheet of Example 2, Comparative Example 1, Comparative Example 3, Comparative Example 5 and Comparative Example 6 in the sodium ion battery formation and in the full charge state;

[0013] Fig. 5 is a comparison diagram of the peeling of the conductive layer of the negative electrode sheet of Example 2, Comparative Example 1, Comparative Example 3, Comparative Example 5 and Comparative Example 6 in the sodium ion battery cycle and in the full discharge state;

[0014] Fig. 6 is a scanning electron microscope spectrum of the negative electrode sheet of Example 2 in the initial state;

[0015] Fig. 7 is a scanning electron microscope spectrum of the negative electrode sheet of Example 2 in the full discharge state of the sodium ion battery;

[0016] Fig. 8 is a circuit block diagram of an electrical equipment according to an embodiment of the present application;

[0017] Fig. 9 is a schematic diagram of the structure of an electrical equipment according to an embodiment of the present application.

[0018] Legend of reference signs: 100-sodium ion battery, 110-electrolyte, 120-electrode assembly, 121-negative electrode sheet, 1211-current collector layer, 1212-conductive layer, 122-positive electrode sheet, 123-separator, 200-electrical equipment, 210-equipment body. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, and not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort also belong to the protection scope of the present application.

[0020] The terms "first", "second", and the like in the description and in the claims of the present application and above-described drawings are used to distinguish different objects, and are not used to describe a particular order. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device including a series of steps or units is not limited to the listed steps or units, but can optionally further include steps or units not listed, or can optionally further include other steps or units inherent to the process, method, product, or device.

[0021] Reference herein to "an embodiment" or "an implementation" means that a particular feature, structure, or characteristic described in connection with the embodiment or implementation can be included in at least one embodiment of the application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor does it necessarily exclude other embodiments. It is expressly understood that the embodiments described herein can be combined with each other.

[0022] With the development of battery technology, in order to meet the demand for higher energy density, the traditional negative electrode sheet is limited in application due to the volume expansion of silicon-based materials in the active material layer. Therefore, the anode-free battery system has become a new research direction. In the anode-free system, the negative electrode is mainly a conductive layer coated current collector. During the charging process of the battery, the sodium ions released from the positive electrode are deposited on the conductive coating to form a sodium metal negative electrode. However, the conductive coating has poor compactness and rough surface. When sodium is deposited on the conductive coating, a small amount of sodium penetrates the conductive coating and is embedded in the conductive coating or penetrates between the conductive coating and the current collector layer. After deposition or peeling, the conductive coating expands, the adhesion between the conductive coating and the current collector layer decreases, the conductive coating falls off, which affects the further deposition of sodium, and ultimately makes the adhesion of sodium on the conductive coating poor, thereby reducing the first charge-discharge efficiency of the battery.

[0023] Referring to FIG. 1, the present application provides an electrolyte 110, the electrolyte 110 comprises a sodium salt additive, the sodium salt additive comprises a first additive and a second additive, the first additive is a sodium sulfonate additive, and the second additive is a fluorine-containing sodium salt additive. In the electrolyte 110, the mass fraction of the first additive is less than the mass fraction of the second additive.

[0024] Understandably, in the electrolyte 110, the mass fraction of the first additive is the ratio of the mass of the first additive to the mass of the electrolyte 110; and the mass fraction of the second additive is the ratio of the mass of the second additive to the mass of the electrolyte 110.

[0025] Understandably, the electrolyte 110 in this embodiment is applied to a sodium-ion battery 100. The sodium-ion battery 100 further includes an electrode assembly 120, which includes a negative electrode 121. The negative electrode 121 includes a current collector layer 1211 and a conductive layer 1212 stacked together. During the charging process of the sodium-ion battery 100, sodium ions in the electrolyte 110 are reduced to sodium atoms on the surface of the negative electrode 121 and form a sodium metal layer. As the charging voltage gradually increases, a second additive forms an SEI film (Solid Electrolyte Interface Membrane) at the interface between the negative electrode 121 and the electrolyte 110. When the sodium-ion battery 100 is fully charged, the sodium ions are deposited on the surface of the negative electrode 121, and the second additive continues to form an SEI film to isolate the sodium metal layer and the electrolyte 110.

[0026] Understandably, the first additive includes sulfonic acid groups. The first additive is composed of small-molecule sulfur-containing organic compounds and other functional groups, so that the first additive is conducive to the formation of crystal nuclei, refinement of crystal grains, and dense distribution of crystal nuclei, so as to promote the uniform, dense and smooth deposition of sodium atoms on the surface of the negative electrode 121.

[0027] Understandably, the second additive is a sodium fluoride salt. The fluorine atoms in the second additive form CF bonds with carbon. The bond energy of the CF bond is stronger than the single bonds formed by other elements with carbon. Therefore, the second additive is difficult to reduce, resulting in the second additive having a lower reduction potential compared to other components in the electrolyte 110.

[0028] In this embodiment, the electrolyte 110 includes a sodium salt additive. When the electrolyte 110 is applied to the sodium-ion battery 100, the sodium salt additive is used to induce sodium ions in the electrolyte 110 to deposit on the surface of the negative electrode 121 and form a sodium metal layer. It also improves the density of sodium metal deposition on the surface of the negative electrode 121, enhances the adhesion performance between the sodium metal layer and the negative electrode 121, thereby reducing the probability of sodium metal layer detachment caused by the expansion of the conductive layer 1212 of the negative electrode 121. This facilitates the continued deposition of sodium ions on the surface of the negative electrode 121 during subsequent charge and discharge processes of the sodium-ion battery 100, thereby improving the first charge and discharge efficiency and cycle performance of the sodium-ion battery 100.

[0029] Specifically, the first additive is a sodium sulfonate salt additive. The sulfonic acid group in the first additive has strong electrophilicity, giving it a strong adsorption effect. The first additive can be adsorbed on protrusions, active sites, or special crystal surfaces with high surface tension on the negative electrode 121. Furthermore, the sulfonate anion in the first additive captures sodium ions and, through the electrophilicity of the sulfonic acid group, reduces sodium ions to sodium atoms, thus inducing the migration of sodium atoms on the surface of the negative electrode 121. Sodium atoms migrate to the depressions on the surface of the negative electrode 121 and enter the lattice of the sodium metal layer, resulting in a dense and flat deposition layer of sodium atoms on the surface of the negative electrode 121, which is tightly bonded to the negative electrode 121. This improves the adhesion performance between the sodium metal layer and the negative electrode 121. When the sodium-ion battery 100 discharges, compared to sodium atoms partially deposited inside the negative electrode 121 and detached from it, the sodium atoms provided by this solution directly detach from the sodium metal layer, reducing the probability of the conductive layer 1212 of the negative electrode 121 expanding and detaching due to sodium atom detachment. Furthermore, the second additive contains fluorine atoms, which have a strong electron-withdrawing effect. The electron-withdrawing effect of fluorine helps to distribute negative charges, thereby reducing the lattice energy of the second additive, promoting the dissolution of the fluorine-containing sodium salt in organic solvents, and simultaneously capturing sodium ions detached from the positive electrode 122, inducing sodium deposition on the surface of the negative electrode 121. Furthermore, the second additive is a sodium fluoride salt additive. Compared with other components in the electrolyte 110, the second additive has a lower reduction potential, so that when the voltage of the sodium-ion battery 100 reaches the reduction potential and before the first additive induces sodium ion deposition, the second additive preferentially undergoes a reduction reaction compared with other components in the electrolyte 110, and forms a sodium fluoride-containing SEI film (Solid Electrolyte Interface Membrane) on the surface of the negative electrode 121. The sodium fluoride-containing SEI film is dense and stable, which can effectively reduce the internal resistance of the sodium-ion battery 100. After the sodium atom deposition is completed, the second additive continues to form and repair the SEI film, so that when the sodium-ion battery 100 is in a fully charged state, the SEI film is dense and intact, effectively isolating the sodium metal layer and the electrolyte 110, suppressing the side reaction between sodium and the electrolyte 110, which is beneficial to reduce the loss of metallic sodium and maintain the activity of the metallic sodium layer. Furthermore, both the first additive and the second additive contain a large amount of sodium ions, and the sodium ions exist in the electrolyte 110 in a free state, which facilitates the replenishment of sodium ions to the electrolyte 110, increases the number of sodium ions in the sodium-ion battery 100, and improves the first charge and discharge efficiency of the sodium-ion battery 100.

[0030] In this embodiment, when the electrolyte 110 is applied to the sodium-ion battery 100 and the sodium-ion battery 100 is in a charging state, the first additive has strong electrophilicity, and the second additive has strong electron-withdrawing properties. The first additive and the second additive together induce and capture sodium ions released from the positive electrode 122. After the first additive reduces the sodium ions to sodium atoms, it induces the sodium atoms to form a dense and uniform sodium metal layer on the surface of the negative electrode 121. Furthermore, as the charging voltage of the sodium-ion battery 100 increases, the second additive preferentially undergoes a reduction reaction compared to other components in the electrolyte 110 and forms an SEI film on the surface of the negative electrode 121. Finally, a complete SEI film is formed on the outer periphery of the sodium metal layer, effectively isolating the sodium metal layer and the electrolyte 110, suppressing the side reactions between sodium and the electrolyte 110, which helps to reduce the loss of metallic sodium and maintain the activity of the sodium metal layer. Furthermore, during the charging process of the sodium-ion battery 100, sodium atoms are deposited on the surface of the negative electrode 121. During the discharging process of the sodium-ion battery 100, sodium atoms are released from the surface of the negative electrode 121 and form sodium ions. In other words, the formation of the SEI film and the deposition of the sodium metal layer occur cyclically during each charge and discharge cycle of the sodium-ion battery 100. In the electrolyte 110, the mass fraction of the first additive is less than the mass fraction of the second additive, which facilitates the formation of an SEI film containing sodium fluoride on the surface of the negative electrode 121 by the second additive, thereby isolating the sodium metal layer from the electrolyte 110, maintaining the activity of the sodium metal layer, improving the utilization rate of sodium during the charge and discharge cycle, and ultimately improving the first charge and discharge efficiency and cycle performance of the sodium-ion battery 100.

[0031] Optionally, in some embodiments, the mass fraction of the first additive ranges from 0.002% to 0.02%.

[0032] Specifically, the mass fraction of the first additive can be, but is not limited to, 0.002%, 0.003%, 0.005%, 0.008%, 0.009%, 0.01%, 0.012%, 0.014%, 0.015%, 0.017%, 0.018%, 0.019%, and 0.02%.

[0033] In this embodiment, when the mass fraction of the first additive is within the range of 0.002% to 0.02%, the mass fraction of the first additive in the electrolyte 110 is within a reasonable range, and therefore the number of sulfonic acid groups in the first additive in the electrolyte 110 is within a reasonable range. On one hand, the first additive can fully exert its effect of inducing sodium ions to deposit on the surface of the negative electrode 121, so that during the charging process of the sodium-ion battery 100, sodium ions form a dense and uniform sodium metal layer on the surface of the negative electrode 121, thereby improving the first charge-discharge efficiency and cycle performance of the sodium-ion battery 100. On the other hand, it can avoid excessive loss of sodium ions due to an excessive number of sulfonic acid groups, further improving the first charge-discharge efficiency of the sodium-ion battery 100. When the mass fraction of the first additive is too high, the number of sulfonic acid groups in the electrolyte 110 is excessive, which may cause the first additive to adsorb too many sodium ions and reduce more sodium ions to sodium atoms, resulting in the loss of sodium ions in the electrolyte 110. When the electrolyte 110 is used in a sodium-ion battery 100, it may reduce the initial charge-discharge efficiency of the sodium-ion battery 100. When the mass fraction of the first additive is too low, the number of sulfonic acid groups in the electrolyte 110 is too small, making it difficult to effectively induce sodium ions in the electrolyte 110 to form a dense and uniform sodium metal layer on the surface of the negative electrode 121. If sodium atoms are unevenly deposited on the surface of the negative electrode 121 and some sodium atoms are embedded inside the negative electrode 121, during the discharge process, the sodium atoms may damage the structure of the negative electrode 121 and cause the conductive layer 1212 of the negative electrode 121 to fall off, which reduces the performance of the negative electrode 121 and shortens the cycle life of the sodium-ion battery 100.

[0034] Optionally, in some embodiments, the mass fraction of the second additive ranges from 0.1% to 1.0%.

[0035] Specifically, the mass fraction of the second additive can be, but is not limited to, 0.1%, 0.12%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.5%, 0.6%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, and 1.0%.

[0036] In this embodiment, when the mass fraction of the second additive is within the range of 0.1% to 1.0%, the mass fraction of the second additive is within a reasonable range. On one hand, during the charging process of the sodium-ion battery 100, the second additive can preferentially undergo a reduction reaction with other substances in the electrolyte 110 and form a dense and stable SEI film on the surface of the negative electrode 121. When the sodium-ion battery 100 is fully charged and the sodium metal layer is deposited, a complete SEI film can be formed on the outer periphery of the sodium metal layer to effectively isolate the sodium metal layer and the electrolyte 110, suppress the side reactions between sodium and the electrolyte 110, reduce the loss of metallic sodium and maintain the activity of the metallic sodium layer, improve the utilization rate of sodium in the charge-discharge cycle, and ultimately improve the first charge-discharge efficiency and cycle performance of the sodium-ion battery 100. On the other hand, it can avoid the second additive forming an excessively thick SEI film on the surface of the negative electrode 121, which would increase the impedance of the sodium-ion battery 100, thereby enabling the sodium-ion battery 100 to have a higher first charge-discharge efficiency. When the mass fraction of the second additive is too high, during the charging process of the sodium-ion battery 100, the thickness of the SEI film formed on the surface of the negative electrode 121 by the reaction of the second additive is too large, increasing the impedance of sodium ions in the electrolyte 110 in the sodium-ion battery 100, thereby reducing the first charge-discharge efficiency of the sodium-ion battery 100. When the mass fraction of the second additive is too low, during the charging process of the sodium-ion battery 100, the electron-withdrawing effect of the second additive is weakened during the deposition of the sodium metal layer, reducing the efficiency of the second additive in inducing and capturing sodium ion deposition. This results in insufficiently dense deposition of sodium on the surface of the negative electrode 121, reducing the adhesion performance of the sodium metal layer deposited on the surface of the negative electrode 121 to the negative electrode 121, and ultimately reducing the first charge-discharge efficiency of the sodium-ion battery 100 when the electrolyte 110 is used. Furthermore, when the sodium-ion battery 100 is fully charged and the sodium metal layer is deposited, the second additive is unlikely to form a complete SEI film on the surface of the sodium metal layer. This may cause the sodium metal layer to partially contact the electrolyte 110 and undergo side reactions with the electrolyte 110, increasing the loss of metallic sodium, reducing the utilization rate of sodium in the charge-discharge cycle, and ultimately reducing the first charge-discharge efficiency and cycle performance of the sodium-ion battery 100.

[0037] In some embodiments, the mass ratio α of the first additive to the second additive satisfies the range: 1 / 500 ≤ α ≤ 1 / 20.

[0038] Specifically, the value of α can be, but is not limited to, 1 / 500, 1 / 480, 1 / 450, 1 / 400, 1 / 350, 1 / 320, 1 / 300, 1 / 250, 1 / 230, 1 / 200, 1 / 180, 1 / 150, 1 / 130, 1 / 100, 1 / 80, 1 / 50 and 1 / 20, etc.

[0039] In this embodiment, when the mass ratio α of the first additive to the second additive satisfies the range of 1 / 500 ≤ α ≤ 1 / 20, the mass of both the first additive and the second additive in the electrolyte 110 is within a reasonable range, and the number of sulfonic acid groups in the first additive and the number of fluorine-containing groups in the second additive are both within a reasonable range in the electrolyte 110. On one hand, the first additive can fully exert its effect of inducing sodium ions to deposit on the surface of the negative electrode 121, so that during the charging process of the sodium-ion battery 100, sodium ions form a dense and uniform sodium metal layer on the surface of the negative electrode 121, improving the initial charge-discharge efficiency and cycle performance of the sodium-ion battery 100. On the other hand, during the charging process of the sodium-ion battery 100, the second additive can preferentially undergo a reduction reaction with other substances in the electrolyte 110 and form a dense and stable SEI film on the surface of the negative electrode 121. When the sodium-ion battery 100 is fully charged, the SEI film is dense and complete, effectively isolating the sodium metal layer and the electrolyte 110, suppressing side reactions between sodium and the electrolyte 110, which helps reduce sodium metal loss and maintain the activity of the sodium metal layer. In addition, it can avoid excessive sodium ion loss due to an excessive number of sulfonic acid groups, and it can also avoid the second additive forming an excessively thick SEI film on the surface of the negative electrode 121, which would increase the impedance of the sodium-ion battery 100. This results in the sodium-ion battery 100 having a higher first charge-discharge efficiency when the electrolyte 110 is used. When the mass ratio α of the first additive to the second additive is too large, the mass fraction of the first additive in the electrolyte 110 is too large or the mass fraction of the second additive is too small. If the mass fraction of the first additive is too high, it may adsorb excessive sodium ions and reduce more sodium ions to sodium atoms, resulting in sodium ion loss in the electrolyte 110. If the mass fraction of the second additive is too low, during the charging process of the sodium-ion battery 100, it is difficult for the second additive to form an SEI film on the surface of the negative electrode 121. The sodium metal layer may partially contact the electrolyte 110 and undergo side reactions with it, increasing sodium loss, reducing sodium utilization during charge-discharge cycles, and ultimately reducing the initial charge-discharge efficiency and cycle performance of the sodium-ion battery 100. When the mass ratio α of the first additive to the second additive is too low, the mass fraction of the first additive in the electrolyte 110 is too low or the mass fraction of the second additive is too high. If the mass fraction of the first additive is too low, it is difficult for the first additive to effectively induce sodium ions in the electrolyte 110 to form a dense and uniform sodium metal layer on the surface of the negative electrode 121.If the mass fraction of the second additive is too high, the thickness of the SEI film formed on the surface of the negative electrode 121 during the charging process of the sodium-ion battery 100 will be too large. This will increase the difficulty for sodium ions in the electrolyte 110 to enter or exit the negative electrode 121 through the SEI film, increase the impedance of sodium ions in the sodium-ion battery 100, and ultimately reduce the initial charge-discharge efficiency and cycle performance of the sodium-ion battery 100.

[0040] In some embodiments, the mass ratio α of the first additive to the second additive is in the range of 1 / 150 ≤ α ≤ 1 / 50.

[0041] Specifically, the value of α can be, but is not limited to, 1 / 150, 1 / 130, 1 / 100, 1 / 80, and 1 / 50.

[0042] In some embodiments, in the electrolyte 110, the mass fraction A1 of the sodium salt additive satisfies the range: 0.001% ≤ A1 ≤ 2%.

[0043] Specifically, the mass fraction A1 of the sodium salt additive can be, but is not limited to, 0.001%, 0.002%, 0.005%, 0.008%, 0.01%, 0.015%, 0.02%, 0.04%, 0.06%, 0.08%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2%.

[0044] Understandably, in the electrolyte 110, the mass fraction of the sodium salt additive is the ratio of the mass of the sodium salt additive to the mass of the electrolyte 110.

[0045] In this embodiment, when the mass fraction A1 of the sodium salt additive meets the range of 0.001% ≤ A1 ≤ 2%, the mass fraction of the sodium salt additive is within a reasonable range. The sodium salt additive can fully exert its function of inducing and capturing sodium ions in the electrolyte 110 and forming a sodium metal layer on the surface of the negative electrode 121. This improves the density of sodium metal deposition on the surface of the negative electrode 121, enhances the adhesion performance between the sodium metal layer and the negative electrode 121, and reduces the probability of sodium metal layer detachment caused by the expansion of the conductive layer 1212 of the negative electrode 121. This facilitates the continued deposition of sodium ions on the surface of the negative electrode 121 during subsequent charge and discharge processes of the sodium-ion battery 100, thereby improving the first charge and discharge efficiency and cycle performance of the sodium-ion battery 100. Furthermore, it prevents the SEI film from becoming too thick or excessive sodium ion consumption due to an excessively high mass fraction of the sodium salt additive, thus enabling the sodium-ion battery 100 to have higher first charge and discharge efficiency and cycle performance. When the mass fraction of the sodium salt additive is too high, the sodium salt additive has a stronger effect in inducing and capturing sodium ions in the electrolyte 110 and forming a sodium metal layer on the surface of the negative electrode 121. However, correspondingly, the sodium salt additive may reduce too many sodium ions to sodium atoms, resulting in the loss of sodium ions in the electrolyte 110. Furthermore, when the sodium-ion battery 100 is fully charged, the sodium salt additive forms and repairs the SEI film on the surface of the sodium metal layer. The higher the mass fraction of the sodium salt additive, the thicker the SEI film becomes, which in turn increases the resistance of sodium ions to enter or exit the negative electrode 121 through the SEI film, increasing the internal resistance of the sodium-ion battery 100, thereby reducing the initial charge-discharge efficiency of the sodium-ion battery 100. When the mass fraction of the sodium salt additive is too small, the sodium salt additive is unable to fully exert its effect of inducing sodium ions in the electrolyte 110 to deposit on the surface of the negative electrode 121. Furthermore, due to the unevenness of the conductive layer 1212, the sodium metal is not deposited tightly on the surface of the negative electrode 121, or some sodium metal enters the interior of the negative electrode 121. When the sodium-ion battery 100 is in a discharge state, the process of the sodium metal embedded in the interior of the negative electrode 121 being extracted from the interior of the negative electrode 121 may cause the conductive layer 1212 to expand, increasing the probability of the conductive layer 1212 of the negative electrode 121 peeling off from the current collector layer 1211, which may cause the conductive layer 1212 of the negative electrode 121 to fall off. Furthermore, when the sodium-ion battery 100 is fully charged, the sodium salt additives are also unable to form a complete SEI film on the surface of the sodium metal layer. This may cause the sodium metal layer to partially contact the electrolyte 110 and undergo side reactions with the electrolyte 110, increasing the loss of metallic sodium, reducing the utilization rate of sodium in the charge-discharge cycle, and ultimately reducing the first charge-discharge efficiency and cycle performance of the sodium-ion battery 100.

[0046] In some embodiments, in the electrolyte 110, the mass fraction A1 of the sodium salt additive satisfies the range: 0.01% ≤ A1 ≤ 1.0%.

[0047] Specifically, the mass fraction A1 of the sodium salt additive can be, but is not limited to, 0.01%, 0.03%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, and 1.0%.

[0048] In some embodiments, the first additive is selected from at least one of sodium dodecyl sulfonate, sodium o-benzoyl sulfimide, sodium polydithiopropane sulfonate, sodium 3-mercaptopropane sulfonate, sodium thiopropane sulfonate, and sodium 2,3-dimercaptopropane sulfonate; and the second additive is selected from at least one of sodium difluorooxalate borate, sodium tetrafluoroborate, and sodium difluorophosphate.

[0049] Understandably, sodium o-benzoylsulfonylimide is sodium saccharin.

[0050] In this embodiment, the sodium dodecyl sulfonate, sodium o-benzoyl sulfonylimide, sodium polydithiopropane sulfonate, sodium 3-mercaptopropane sulfonate, sodium thiopropane sulfonate, and sodium 2,3-dimercaptopropane sulfonate in the first additive all include sulfonic acid groups. The sulfonic acid groups have strong electrophilicity, and the sulfonate anions can induce the capture of sodium ions and reduce the sodium ions to sodium atoms, thereby inducing the deposition of sodium atoms on the surface of the negative electrode 121, promoting the formation of sodium atom nuclei, refining the grains, and making the nuclei densely distributed, so as to promote the uniform, dense and smooth deposition of sodium atoms on the surface of the negative electrode 121, and ultimately improve the adhesion performance of the sodium metal layer to the negative electrode 121. In the second additive, sodium fluorooxalate borate, sodium tetrafluoroborate, and sodium difluorophosphate all contain fluorine atoms. On the one hand, the electron-withdrawing effect of fluorine atoms can work synergistically with sulfonic acid groups to induce and capture sodium ions to be densely deposited on the surface of the negative electrode 121. On the other hand, the SEI film containing sodium fluoride formed by the second additive has strong stability and is dense, which can effectively reduce the impedance of the sodium-ion battery 100, so that the sodium-ion battery 100 has good cycle performance.

[0051] In some embodiments, the electrolyte 110 further includes a film-forming additive, wherein the mass ratio β of the sodium salt additive to the film-forming additive satisfies the range: 1 / 1500≤β≤1 / 5.

[0052] Specifically, the value of β can be, but is not limited to, 1 / 1500, 1 / 1200, 1 / 1000, 1 / 800, 1 / 500, 1 / 480, 1 / 450, 1 / 400, 1 / 350, 1 / 320, 1 / 300, 1 / 250, 1 / 230, 1 / 200, 1 / 180, 1 / 150, 1 / 130, 1 / 100, 1 / 80, 1 / 50, 1 / 20, 1 / 10, and 1 / 5.

[0053] Understandably, in the electrolyte 110, the mass fraction of the sodium salt additive is less than the mass fraction of the film-forming additive.

[0054] Understandably, during the charging process of the sodium-ion battery 100, the second additive reaches the reduction potential earlier than the film-forming additive, and the second additive preferentially undergoes a reduction reaction compared to the film-forming additive.

[0055] In this embodiment, the electrolyte 110 further includes a film-forming additive. During the charging process of the sodium-ion battery 100, the second additive preferentially reaches the reduction potential and forms an SEI film on the surface of the negative electrode 121. As the charging voltage increases, the film-forming additive reaches the reduction potential and also undergoes a reduction reaction on the surface of the negative electrode 121, repairing the SEI film to further improve the density and uniformity of the SEI film. This can slow down the loss of sodium ions in the electrolyte 110, thereby improving the cycle performance of the sodium-ion battery 100. When the mass ratio β of the sodium salt additive to the film-forming additive satisfies the range of 1 / 1500≤β≤1 / 5, the mass of both the sodium salt additive and the film-forming additive in the electrolyte 110 is within a reasonable range. On the one hand, the sodium salt additive is used to induce sodium ions in the electrolyte 110 to deposit on the surface of the negative electrode 121 and form a sodium metal layer, and improve the density of sodium metal deposition on the surface of the negative electrode 121, thereby improving the adhesion performance of the sodium metal layer to the negative electrode 121. This reduces the probability of sodium metal layer falling off due to the expansion of the conductive layer 1212 of the negative electrode 121, and facilitates the continued deposition of sodium ions on the surface of the negative electrode 121 during the subsequent charge and discharge process of the sodium-ion battery 100, thereby improving the first charge and discharge efficiency and cycle performance of the sodium-ion battery 100. On the other hand, the film-forming additive can promote the repair of the SEI film, thereby further improving the density and uniformity of the SEI film and slowing down the loss of sodium ions in the electrolyte 110, thus improving the cycle performance of the sodium-ion battery 100. When the mass ratio β of the sodium salt additive to the film-forming additive is too large, the mass fraction of the sodium salt additive is too large or the mass fraction of the film-forming additive is too small. If the mass fraction of the sodium salt additive is too large, the sodium salt additive may reduce too many sodium ions to sodium atoms, thereby increasing the loss of sodium ions in the electrolyte 110 and reducing the utilization rate of sodium ions in the electrolyte 110. If the mass fraction of the film-forming additive is too small, the density and uniformity of the SEI film formed on the surface of the negative electrode 121 may be poor, increasing the internal resistance of the sodium-ion battery 100, thereby reducing the first charge-discharge efficiency and cycle performance of the sodium-ion battery 100.When the mass ratio β of the sodium salt additive to the film-forming additive is too small, the mass fraction of the sodium salt additive is too small, or the mass fraction of the film-forming additive is too large. If the mass fraction of the sodium salt additive is too small, it is difficult for the sodium salt additive to fully exert its effect of inducing sodium ions in the electrolyte 110 to deposit on the surface of the negative electrode 121. The deposition of sodium metal on the surface of the negative electrode 121 is not dense enough, increasing the probability of sodium metal layer peeling off due to expansion of the conductive layer 1212, thus reducing the first charge-discharge efficiency and cycle performance of the sodium-ion battery 100. If the mass fraction of the film-forming additive is too large, the thickness of the SEI film formed on the surface of the negative electrode 121 is too large, increasing the resistance of sodium ions to enter or exit the negative electrode 121 through the SEI film, thus reducing the first charge-discharge efficiency of the sodium-ion battery 100.

[0056] In some embodiments, the mass ratio β of the sodium salt additive to the film-forming additive satisfies the range: 1 / 800 ≤ β ≤ 1 / 100.

[0057] In some embodiments, in the electrolyte 110, the mass fraction A2 of the film-forming additive satisfies the range: 0.1% ≤ A2 ≤ 10%.

[0058] Specifically, the mass fraction A2 of the film-forming additive can be, but is not limited to, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 2%, 2.5%, 3%, 3.5%, 4%, 5%, 6%, 7%, 8%, 9%, 9.5%, and 10%.

[0059] Understandably, in the electrolyte 110, the mass fraction of the film-forming additive is the ratio of the mass of the film-forming additive to the mass of the electrolyte 110.

[0060] In this embodiment, when the mass fraction A2 of the film-forming additive meets the range of 0.1% ≤ A2 ≤ 10%, the mass fraction of the film-forming additive is within a reasonable range. The film-forming additive, in conjunction with the second additive, forms a complete, dense, uniform, and appropriately thick SEI film on the surface of the negative electrode 121. This not only isolates the sodium metal layer from the electrolyte 110, preventing the sodium metal layer from contacting the electrolyte 110 and undergoing side reactions with the electrolyte 110, thus slowing down the loss of metallic sodium and improving the utilization rate of sodium during charge-discharge cycles, but also avoids increasing the impedance of sodium ions entering or leaving the negative electrode through the SEI film. As a result, when the electrolyte 110 is applied to the sodium-ion battery 100, the sodium-ion battery 100 has a high initial charge-discharge efficiency and good cycle performance. When the mass fraction of the film-forming additive is too high, the thickness of the SEI film formed on the surface of the negative electrode 121 will be too large, increasing the resistance of sodium ions entering or leaving the negative electrode 121 through the SEI film and reducing the initial charge-discharge efficiency of the sodium-ion battery 100. When the mass fraction of the film-forming additive is too low, the density and uniformity of the SEI film formed on the surface of the negative electrode 121 may be poor, increasing the internal resistance of the sodium-ion battery 100, thereby reducing the initial charge-discharge efficiency and cycle performance of the sodium-ion battery 100.

[0061] In some embodiments, the mass fraction A2 of the film-forming additive satisfies the range: 0.5% ≤ A2 ≤ 5%. Specifically, the value of the mass fraction A2 of the film-forming additive can be, but is not limited to, 0.5%, 0.8%, 1%, 1.2%, 2%, 2.5%, 3%, 3.5%, 4%, and 5%.

[0062] Optionally, the film-forming additive is selected from at least one of fluoroethylene carbonate (FEC), ethylene sulfate (DTD), 4-methyl ethylene sulfate (MDTD), 4-propyl ethylene sulfate (PDTD), diethyl sulfite (DES), 1,3-propanesulfonate lactone (PS), ethylene sulfite (ES), propenyl-1,3-sulfonate lactone (PST), and 1,3-propanediol cyclosulfite (PCS).

[0063] Optionally, the electrolyte 110 further includes a solvent used to dissolve the sodium salt additive and the film-forming additive, wherein the solvent is an ether solvent.

[0064] In this embodiment, the solvent is an ether solvent. Compared to ester solvents, the ether solvent provided in this embodiment has lower viscosity, higher dielectric constant, and higher solubility. The ether solvent has high compatibility with sodium metal, thereby reducing the decomposition of the solvent during charging and discharging, resulting in better stability of the interface between the negative electrode 121 and the electrolyte 110. However, correspondingly, the ether solvent has poor oxidation resistance and easily forms a passivation film on the surface of the sodium metal layer, consuming a small amount of solvent. Furthermore, during sodium ion deposition on the negative electrode 121, the presence of defects and microscopic roughness in the negative electrode 121 leads to a higher concentration of sodium ions at localized defect sites. On one hand, these defect sites easily become nuclei, inducing sodium ions to form sodium clusters, which gradually grow into needle-like whiskers, causing micro-short circuits in the battery and accelerating the decomposition of the ether-based electrolyte 110. On the other hand, the loosely deposited sodium layer results in a larger specific surface area, contacting more electrolyte 110, and significantly increasing the amount of solvent consumed to form the passivation film. This makes it difficult to guarantee the quality of the passivation film. The loose sodium layer also increases the amount of reversible sodium ions consumed during the formation of the SEI film, further reducing the battery's initial efficiency. Therefore, it is necessary to add a first additive and a second additive to the electrolyte 110 to comprehensively improve the performance of the solvent. Specifically, the electrophilic nature of the sulfonic acid group in the first additive weakens the interaction between the solvent and sodium ions, lowering the intercalation / deintercalation barrier of sodium ions at the interface between the negative electrode 121 and the electrolyte 110. This facilitates the solvation and desolvation of sodium ions, improves the stability of the interface between the negative electrode 121 and the electrolyte 110, and makes it easier for sodium ions to deposit on the surface of the negative electrode 121. Furthermore, during the charging process of the sodium-ion battery 100, the second additive preferentially reduces the solvent to inhibit the decomposition of ether solvents, slowing down the formation of peroxides from ether solvents and, to some extent, repairing the rough surface of the negative electrode 121 to reduce the formation of sodium clusters and reduce the consumption of reversible sodium ions. In this embodiment, the first additive, the second additive, and the ether solvent work together to give the electrolyte 110 better performance.

[0065] Optionally, in the electrolyte 110, the mass fraction of the solvent is in the range of 60% to 95%. Specifically, the mass fraction of the solvent can be, but is not limited to, 60%, 65%, 68%, 70%, 72%, 75%, 80%, 85%, 90%, and 95%.

[0066] Optionally, the ether solvent includes chain ether solvents and cyclic ether solvents, wherein the chain ether solvent has four or more carbon atoms, and the cyclic ether solvent has three or more carbon atoms.

[0067] In this embodiment, the ether solvent includes chain ether solvents and cyclic ether solvents. When the number of atoms in the chain ether solvent is greater than or equal to four, the chain ether solvent has better reduction stability and oxidation stability, which can improve the stability of the solvent and avoid the solvent being decomposed during charging and discharging to generate side reactions, thereby improving the stability of the interface between the negative electrode 121 and the electrolyte 110. Moreover, the chain ether solvent reacts synergistically with the second additive and the film-forming additive to form a suitable, dense, and stable SEI film at the interface between the negative electrode 121 and the electrolyte 110. When the number of carbon atoms in the cyclic ether solvent is greater than or equal to three, the cyclic ether solvent has higher ionic conductivity, which facilitates the transport of sodium ions and improves the efficiency of sodium ion deposition on the surface of the negative electrode 121, which is beneficial to improving the density of the sodium metal layer deposited on the surface of the negative electrode 121. Furthermore, the combination of the chain ether solvent and the cyclic ether solvent gives the solvent better low-temperature performance, thereby improving the low-temperature performance of the electrolyte 110.

[0068] Optionally, the number of carbon atoms in the chain ether solvent ranges from four to ten, specifically, the number of carbon atoms in the chain ether solvent is one of four, five, six, seven, eight, nine, and ten.

[0069] Optionally, the number of carbon atoms in the cyclic ether solvent ranges from three to five, specifically, the number of carbon atoms in the cyclic ether solvent is one of three, four, or five.

[0070] Optionally, in the electrolyte 110, the mass fraction A3 of the chain ether solvent satisfies the range: 30% ≤ A3 ≤ 60%; and the mass fraction A4 of the cyclic ether solvent satisfies the range: 20% ≤ A4 ≤ 50%.

[0071] Specifically, the mass fraction A3 of the chain ether solvent can be, but is not limited to, 30%, 32%, 35%, 40%, 42%, 45%, 48%, 50%, 52%, 56%, 58%, and 60%.

[0072] Specifically, the mass fraction A4 of the cyclic ether solvent can be, but is not limited to, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 40%, 42%, 45%, and 50%.

[0073] In the electrolyte 110 provided in this embodiment, the mass fractions of both the chain ether solvent and the cyclic ether solvent are within a reasonable range, ensuring good reduction and oxidation stability of the solvent and preventing decomposition and side reactions during charging and discharging. Furthermore, the solvent reacts synergistically with the second additive and the film-forming additive to form a suitable, dense, and stable SEI film at the interface between the negative electrode 121 and the electrolyte 110. This reduces the consumption of reversible sodium ions in the electrolyte 110, facilitates sodium ion transport, and improves the efficiency of sodium ion deposition on the surface of the negative electrode 121, thereby enhancing the density of the sodium metal layer deposited on the surface of the negative electrode 121. In addition, the solvent exhibits good low-temperature performance.

[0074] Optionally, the solvent is selected from at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and crown ethers.

[0075] Optionally, the electrolyte 110 further includes a sodium salt, which provides a greater amount of sodium ions to the electrolyte 110, and the solvent is used to dissolve the sodium salt.

[0076] Optionally, the concentration of the sodium salt ranges from 0.01 mol / L to 10 mol / L. Specifically, the concentration of the sodium salt can be, but is not limited to, 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, 2.5 mol / L, 3 mol / L, 3.2 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, and 10 mol / L.

[0077] In some embodiments, the concentration of the sodium salt ranges from 0.1 mol / L to 3 mol / L. Specifically, the concentration of the sodium salt can be, but is not limited to, 0.1 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, 2.5 mol / L, and 3 mol / L.

[0078] Optionally, the sodium salt is selected from at least one of sodium perchlorate, sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, sodium bis(oxalatoborate) and sodium bis(trifluoromethanesulfonyl)imide.

[0079] Optionally, the conductivity γ of the electrolyte 110 satisfies the range: 2.0 mS / cm ≤ γ ≤ 12.0 mS / cm.

[0080] Specifically, the conductivity γ of the electrolyte 110 can be, but is not limited to, 2.0 mS / cm, 2.5 mS / cm, 3.0 mS / cm, 3.5 mS / cm, 4.0 mS / cm, 4.5 mS / cm, 5 mS / cm, 6 mS / cm, 7 mS / cm, 8 mS / cm, 9 mS / cm, 10 mS / cm, 11 mS / cm, and 12 mS / cm.

[0081] In the electrolyte 110 provided in this embodiment, the first additive and the second additive jointly induce and capture sodium ions released from the positive electrode 122. After the first additive reduces the sodium ions to sodium atoms, it induces the sodium atoms to form a dense and uniform sodium metal layer on the surface of the negative electrode 121. Furthermore, as the charging voltage of the sodium-ion battery 100 increases, the second additive preferentially undergoes a reduction reaction compared to other components in the electrolyte 110 and forms an SEI film on the surface of the negative electrode 121. Finally, a complete SEI film is formed on the outer periphery of the sodium metal layer, effectively isolating the sodium metal layer from the electrolyte 110, suppressing side reactions between sodium and the electrolyte 110, and helping to reduce sodium loss and maintain the activity of the sodium metal layer. Furthermore, the second additive and the film-forming additive work together to form a complete SEI film on the surface of the negative electrode 121, thereby mitigating the side reactions of the electrolyte 110 and ultimately enabling the electrolyte 110 to have better conductivity, thereby improving the first charge and discharge efficiency of the sodium-ion battery 100 when the electrolyte 110 is applied.

[0082] Please refer to Figure 2. This application provides a sodium-ion battery 100, which includes an electrode assembly 120 and an electrolyte 110 provided in this application. The electrode assembly 120 includes a negative electrode 121, which includes a current collector layer 1211 and a conductive layer 1212 stacked together. The conductive layer 1212 includes a carbon material. The electrolyte 110 is used to wet at least a portion of the electrode assembly 120.

[0083] Optionally, in some embodiments, the carbon material is activated carbon.

[0084] Understandably, the charging process of the sodium-ion battery 100 is divided into three stages. In the first stage, sodium ions in the electrolyte 110 are reduced to sodium atoms on the surface of the negative electrode 121 and form a sodium metal layer. In the second stage, as the charging voltage gradually increases, the second additive forms an SEI film (Solid Electrolyte Interface Membrane) at the interface between the negative electrode 121 and the electrolyte 110. At this time, sodium atoms are deposited on the surface of the negative electrode 121, that is, the SEI film is in a broken state. In the third stage, the sodium-ion battery 100 is in a fully charged state, the sodium metal layer stops depositing, and the second additive and film-forming additive continue to form a dense, uniform and complete SEI film on the outer periphery of the sodium metal layer to isolate the sodium metal layer and the electrolyte 110.

[0085] In this embodiment, the negative electrode 121 includes a current collector layer 1211 and a conductive layer 1212 stacked together. The conductive layer 1212 is used to improve the conductivity of the negative electrode 121. When the electrolyte 110 wets at least part of the electrode assembly 120, the sodium salt additive induces sodium ions in the electrolyte 110 to deposit on the surface of the conductive layer 1212 and form a dense sodium metal layer. This reduces the probability of sodium entering the conductive layer 1212 or between the current collector layer 1211 and the conductive layer 1212, improves the density of sodium metal deposition on the surface of the negative electrode 121, and enhances the adhesion performance between the sodium metal layer and the negative electrode 121. This reduces the probability of sodium metal layer detachment caused by the expansion of the conductive layer 1212 of the negative electrode 121, facilitating the continued deposition of sodium ions on the surface of the negative electrode 121 during subsequent charge and discharge processes of the sodium-ion battery 100. Consequently, the sodium-ion battery 100 has higher initial charge and discharge efficiency and better cycle performance.

[0086] Optionally, the current collector layer 1211 is selected from copper foil, aluminum foil, titanium foil, stainless steel foil, graphite carbon paper, conductive agent-coated aluminum foil, conductive agent-coated copper foil, conductive agent-coated titanium foil, and conductive agent-coated stainless steel foil, preferably conductive agent-coated aluminum foil, conductive agent-coated copper foil, or conductive agent-coated stainless steel foil. The conductive agent is selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0087] Optionally, the electrode assembly 120 further includes a positive electrode 122 and a separator 123, wherein the positive electrode 122, the separator 123 and the negative electrode 121 are stacked in sequence, the positive electrode 122 includes a positive electrode material, which is selected from at least one of sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium iron phosphate, sodium iron pyrophosphate and composite sodium iron phosphate, and the separator 123 is selected from one of polypropylene separator 123 and polyimide film.

[0088] In some embodiments, the thickness d1 of the conductive layer 1212 satisfies the range: 10μm≤d1≤50μm.

[0089] Specifically, the thickness d1 of the conductive layer 1212 can be, but is not limited to, 10μm, 12μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm and 50μm.

[0090] Understandably, the thickness of the conductive layer 1212 can be measured using a micrometer.

[0091] In this embodiment, when the thickness d1 of the conductive layer 1212 meets the range of 10μm≤d1≤50μm, the thickness of the conductive layer 1212 is within a reasonable range, so that when the sodium-ion battery 100 is charged, sodium atoms can be deposited on the surface of the conductive layer 1212, reducing the probability of sodium atoms entering the interior of the conductive layer 1212 or between the conductive layer 1212 and the current collector layer 1211. The conductive layer 1212 and the sodium salt additive work together to improve the performance of sodium metal layer deposition on the surface of the negative electrode 121, and improve the bonding performance between the sodium metal layer and the conductive layer 1212, thereby reducing the probability of sodium metal layer falling off due to the expansion of the conductive layer 1212 of the negative electrode 121. This facilitates the continued deposition of sodium ions on the surface of the negative electrode 121 during subsequent charge and discharge processes of the sodium-ion battery 100, thereby improving the first charge and discharge efficiency and cycle performance of the sodium-ion battery 100. When the thickness of the conductive layer 1212 is too large, the adhesion between the conductive layer 1212 and the current collector layer 1211 is poor, making it easy for the conductive layer 1212 and the sodium metal layer to fall off from the current collector layer 1211. Furthermore, during the discharge process of the sodium-ion battery 100, the expansion of the conductive layer 1212 is more pronounced, which also reduces the deposition performance of the sodium metal layer on the surface of the negative electrode 121, thereby reducing the initial charge and discharge efficiency of the sodium-ion battery 100. When the thickness of the conductive layer 1212 is too small, the conductive layer 1212 is too thin. During the charging process of the sodium-ion battery 100, sodium ions in the electrolyte 110 are reduced to sodium atoms and can easily enter the conductive layer 1212 or pass through the conductive layer 1212 into the gap between the conductive layer 1212 and the current collector layer 1211. During the discharge process of the sodium-ion battery 100, the probability of the conductive layer 1212 expanding and falling off due to the deintercalation of sodium atoms is increased, and the adhesion performance between the sodium metal layer and the negative electrode 121 is reduced.

[0092] In some embodiments, the thickness d1 of the conductive layer 1212 satisfies the range: 10μm≤d1≤30μm. Specifically, the value of the thickness d1 of the conductive layer 1212 can be, but is not limited to, 10μm, 12μm, 15μm, 18μm, 20μm, 22μm, 25μm, 28μm, and 30μm.

[0093] In some embodiments, on the current collector layer 1211, the coating surface density g of the conductive layer 1212 satisfies the range of 5 g / m². 2 ≤g≤100g / m 2 .

[0094] Specifically, the coating surface density g of the conductive layer 1212 can be, but is not limited to, 5 g / m². 2 10g / m 2 15g / m 2 20g / m 2 25g / m 2 30g / m 2 35g / m 2 40g / m 2 45g / m 2 50g / m 2 55g / m 2 60g / m 2 65g / m 2 70g / m 2 75g / m 2 80g / m 2 85g / m 2 90g / m 2 95g / m 2 and 100g / m 2 wait.

[0095] Understandably, the coating density of the conductive layer 1212 is the coating mass per unit area of ​​the current collector layer 1211. The coating density of the conductive layer 1212 is calculated by the ratio of the mass of the conductive coating to the area of ​​the current collector layer 1211.

[0096] Understandably, the coating surface density of the conductive layer 1212 can characterize the "foil leakage rate" of the negative electrode 121. The greater the coating surface density of the conductive layer 1212, the lower the foil leakage rate of the negative electrode 121. The smaller the coating surface density of the conductive layer 1212, the higher the foil exposure rate of the negative electrode 121.

[0097] In this embodiment, when the coating surface density g of the conductive layer 1212 meets the range of 5 g / m 2 ≤g≤100g / m2 When the coating density g of the conductive layer 1212 is within a reasonable range, during the charging process of the sodium-ion battery 100, sodium atoms can be deposited on the surface of the conductive layer 1212, reducing the probability of sodium atoms entering the interior of the conductive layer 1212 or between the conductive layer 1212 and the current collector layer 1211. The conductive layer 1212 and the sodium salt additive work together to improve the performance of sodium metal layer deposition on the surface of the negative electrode 121, and enhance the bonding performance between the sodium metal layer and the conductive layer 1212, thereby reducing the probability of sodium metal layer falling off due to the expansion of the conductive layer 1212 of the negative electrode 121. This facilitates the continued deposition of sodium ions on the surface of the negative electrode 121 during subsequent charging and discharging processes of the sodium-ion battery 100, thereby improving the first charge and discharge efficiency and cycle performance of the sodium-ion battery 100. When the coating density of the conductive layer 1212 is too high, if some sodium metal enters the gap between the conductive layer 1212 and the current collector layer 1211, it increases the difficulty for the sodium metal to detach from the conductive layer 1212 during the discharge process of the sodium-ion battery 100. This makes the conductive layer 1212 more likely to peel off from the current collector layer 1211, reducing the performance of the negative electrode 121. When the coating density of the conductive layer 1212 is too low, the conductive layer 1212 is difficult to cover the current collector layer 1211, and the exposed foil rate of the negative electrode 121 is too high. This means that when the sodium-ion battery 100 is in a charging state, most of the sodium metal may be directly deposited on the surface of the current collector layer 1211, increasing the probability of the conductive layer 1212 peeling off from the current collector layer 1211, which is not conducive to the dense deposition of sodium metal on the surface of the negative electrode 121.

[0098] In some embodiments, the coating surface density g of the conductive layer 1212 satisfies the range of 2 g / m. 2 ≤g≤50g / m 2 Specifically, the coating surface density g of the conductive layer 1212 can be, but is not limited to, 2 g / m². 2 5g / m 2 10g / m 2 15g / m 2 20g / m 2 30g / m 2 40g / m 2 and 50g / m 2 wait.

[0099] In some embodiments, when the sodium-ion battery 100 is fully charged, the sodium-ion battery 100 further includes a deposition layer, the deposition layer being disposed on the side of the conductive layer 1212 opposite to the current collector layer 1211, the deposition layer comprising sodium, and the deposition thickness d2 of the deposition layer satisfying the range: 10μm≤d2≤30μm.

[0100] Specifically, the deposition thickness d2 of the deposition layer can be, but is not limited to, 10μm, 11μm, 12μm, 15μm, 16μm, 18μm, 20μm, 22μm, 24μm, 25μm, 26μm, 28μm, 29μm and 30μm.

[0101] Understandably, the deposited layer is a sodium metal layer.

[0102] Optionally, the test conditions for the deposition thickness of the deposited layer are as follows: the sodium-ion battery 100 is formed at 25°C, 0.1C, 2.5V to 3.5V, and 100% SOC. Here, charging the sodium-ion battery 100 at 25°C, 0.1C, 2.5V to 3.5V, and 100% SOC means that the sodium-ion battery 100 is charged at a temperature of 25°C and at a current of one-tenth of its capacity. The voltage range during the formation process is 2.5V to 3.5V. SOC is an abbreviation for "State of Charge," representing the remaining charge or state of charge of the sodium-ion battery 100, used to describe the amount of charge already charged relative to the total battery capacity. 100% SOC indicates that the sodium-ion battery 100 is in a fully charged state.

[0103] In this embodiment, when the deposition thickness d2 of the deposited layer meets the range of 10μm≤d2≤30μm, the deposition thickness of the deposited layer is within a reasonable range. The deposited layer maintains good adhesion to the negative electrode 121 and has good density. During the discharge process of the sodium-ion battery 100, sodium in the deposited layer can be converted into sodium ions and detached, resulting in high initial charge-discharge efficiency, good cycle performance, and high energy density for the sodium-ion battery 100. When the deposition thickness of the deposited layer is too large, the mass of the deposited layer is too large, making it easy to detach from the conductive layer 1212 as a whole, thereby reducing the initial charge-discharge efficiency of the sodium-ion battery 100. When the deposition thickness of the deposited layer is too small, the amount of sodium metal deposited in the deposited layer is too small, resulting in low energy density of the sodium-ion battery 100.

[0104] In some embodiments, when the sodium-ion battery 100 is fully charged, the sodium metal shedding rate σ1 is ≤ 0.1%; wherein, the sodium metal shedding rate σ1 = m1 / (m1+m2)×100%, m1 is the mass of the shed sodium metal, and m2 is the mass of the sodium metal remaining in the deposition layer.

[0105] Specifically, the value of the sodium detachment rate σ1 can be, but is not limited to, 0.001%, 0.002%, 0.005%, 0.008%, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, and 0.1%.

[0106] Optionally, the test conditions for the sodium detachment rate are as follows: after the sodium-ion battery 100 is formed at 25°C, 0.1C, 2.5V to 3.5V, and 100% SOC, the sodium-ion battery 100 is disassembled. The mass of detached sodium m1 is the mass of sodium detached from the deposited layer during the disassembly process, and m1+m2 is the mass of sodium in the deposited layer before disassembly.

[0107] In this embodiment, when the sodium-ion battery 100 is fully charged, the sodium detachment rate σ1 in the deposited layer is ≤0.1%, which is extremely low. Therefore, sodium exhibits good deposition and adhesion properties on the surface of the negative electrode 121. The negative electrode 121, the sodium salt additive, and the film-forming additive work together to improve the density of sodium metal deposition on the surface of the negative electrode 121, enhancing the adhesion between the sodium metal layer and the negative electrode 121. This reduces the probability of sodium metal layer detachment caused by the expansion of the conductive layer 1212 of the negative electrode 121, facilitating continued deposition of sodium ions on the surface of the negative electrode 121 during subsequent charge and discharge processes. Consequently, the sodium-ion battery 100 exhibits high initial charge / discharge efficiency and good cycle performance.

[0108] In some embodiments, when the sodium-ion battery 100 is in a fully discharged state, the shedding rate σ2 of the conductive layer 1212 is ≤0.01%; wherein, the shedding rate σ2 of the conductive layer 1212 is =m3 / (m3+m4)×100%, where m3 is the weight of the portion of the conductive layer 1212 that has detached, and m4 is the weight of the portion of the conductive layer 1212 that has not detached.

[0109] Understandably, when the sodium-ion battery 100 is in a fully discharged state, the charge in the sodium-ion battery 100 is close to zero, and it is in a 0% SOC state. During the discharge process, the sodium metal layer is converted into sodium ions and falls off from the negative electrode, and the surface of the negative electrode 121 no longer has a sodium metal layer deposited.

[0110] Specifically, the value of the shedding rate σ2 of the conductive layer 1212 can be, but is not limited to, 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, and 0.01%.

[0111] Optionally, the test conditions for the shedding rate of the conductive layer 1212 are as follows: after the sodium-ion battery 100 is cycle-charged and discharged five times under the conditions of 25°C, 0.1C, and 2.5V to 3.5V, the sodium-ion battery 100 is in a fully discharged state and then disassembled. The weight m3 of the part of the conductive layer 1212 that has fallen off is the mass of the part of the conductive layer 1212 that has fallen off during the disassembly process, and m3+m4 is the mass of the negative electrode 121 minus the mass of the current collector layer 1211.

[0112] In this embodiment, when the sodium-ion battery 100 is fully charged, the shedding rate σ2 of the conductive layer 1212 is ≤0.01%, which is extremely low. Therefore, the conductive layer 1212 and the current collector layer 1211 have excellent bonding performance. This is because sodium has good deposition and adhesion performance on the surface of the negative electrode 121. The negative electrode 121, the sodium salt additive, and the film-forming additive work together to improve the density of sodium metal deposition on the surface of the negative electrode 121, enhance the adhesion performance between the sodium metal layer and the negative electrode 121, thereby reducing the probability of sodium metal layer shedding due to the expansion of the conductive layer 1212 of the negative electrode 121. This facilitates the continued deposition of sodium ions on the surface of the negative electrode 121 during subsequent charge and discharge processes of the sodium-ion battery 100, resulting in the sodium-ion battery 100 having high initial charge and discharge efficiency and good cycle performance.

[0113] The technical solution of this application will be further described below with reference to several embodiments:

[0114] Examples 1 to 14, Comparative Examples 1 to 8:

[0115] Preparation of sodium-ion battery 100:

[0116] 1. Preparation of positive electrode 122:

[0117] The positive electrode active material sodium iron pyrophosphate, conductive carbon black (SP), and binder polyvinylidene fluoride (PVDF) are dispersed in the solvent N-methylpyrrolidone (NMP) and mixed evenly to obtain a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector aluminum foil; after drying, cold pressing, slitting, and cutting, positive electrode sheet 122 is obtained.

[0118] 2. Preparation of negative electrode plate 121:

[0119] Activated carbon and the binder polyvinylidene fluoride (PVDF) are dispersed in the solvent N-methylpyrrolidone (NMP) and mixed to form a conductive paste. The conductive paste is coated on the negative electrode current collector aluminum foil. After drying, cold pressing, slitting and cutting, a stacked current collector layer 1211 and a conductive layer 1212 are obtained, which are used as the negative electrode sheet 121.

[0120] 3. Preparation of electrolyte 110:

[0121] The ether solvent was thoroughly mixed in an argon-atmosphere glove box with a moisture content ≤0.01ppm. Then, the sodium salt (sodium hexafluorophosphate) was dissolved in the mixed solvent and stirred until completely and uniformly dissolved. Finally, the sodium salt additive and film-forming additive were added and mixed evenly to obtain the electrolyte 110 of Examples 1 to 14 and Comparative Examples 1 to 8. The solvent types and mixing mass ratios, the type and mass fraction of the first additive, and the type and mass fraction of the second additive in the electrolyte 110 of Examples 1 to 14 and Comparative Examples 1 to 8 are shown in Table 1.

[0122] 4. Preparation of diaphragm 123:

[0123] A polyethylene film is used as the diaphragm 123.

[0124] 5. Assembly of sodium-ion battery 100:

[0125] Using a stacking process, the prepared positive electrode 122, separator 123, and negative electrode 121 are sequentially fabricated into a stacked bare cell. After welding the tabs, the bare cell is assembled in an aluminum-plastic film. After vacuum baking, the prepared electrolyte 110 is injected, and the cell is encapsulated to finally obtain a negative electrode-free sodium-ion battery 100, which yields implementation batteries 1 to 14 and control batteries 1 to 8. Implementation battery 1 uses the electrolyte 110 of Example 1, implementation battery 2 uses the electrolyte 110 of Example 2, control battery 1 uses the electrolyte 110 of Comparative Example 1, control battery 2 uses the electrolyte 110 of Comparative Example 2, and so on.

[0126] Table 1: Component parameters of sodium-ion batteries 100 of Examples 1 to 14 and Comparative Examples 1 to 8.

[0127] In Table 2, in the column for solvent type and mixing mass ratio, diethylene glycol dimethyl ether: tetrahydrofuran = 2:1 indicates that the solvent types include diethylene glycol dimethyl ether and tetrahydrofuran, and the mass ratio of diethylene glycol dimethyl ether to tetrahydrofuran is 2:1; diethylene glycol dimethyl ether: ethylene glycol diethyl ether: tetrahydrofuran = 2:2:1 indicates that the solvent types include diethylene glycol dimethyl ether, ethylene glycol diethyl ether, and tetrahydrofuran, and the mass ratio of diethylene glycol dimethyl ether to ethylene glycol diethyl ether to tetrahydrofuran is 2:2:1, and so on.

[0128] Performance testing of sodium-ion batteries 100:

[0129] The experimental batteries 1 to 14 and the control batteries 1 to 8 underwent standing, formation, shaping, and capacity testing. The electrolyte 110 had an injection coefficient of 6.58 g / Ah, and the battery's rated capacity was 0.280 Ah. The battery voltage window was 2.5 to 3.5V. Formation involved charging to 3.5V at 0.1C; capacity testing involved discharging to 2.5V at 0.1C for 3 cycles, followed by a final discharge to 1.0V at 0.1C.

[0130] 1. Calculation of the initial charge / discharge efficiency of a sodium-ion battery (100%):

[0131] The initial charge-discharge efficiency of sodium-ion battery 100 = initial discharge capacity / initial charge capacity × 100%. The initial charge-discharge efficiencies of experimental batteries 1 to 14 and comparative batteries 1 to 8 are shown in Table 2.

[0132] 2. Calculation of the sodium metal shedding rate σ1:

[0133] The sodium-ion battery 100, after formation and in a fully charged state, was disassembled in an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm) to observe and calculate the sodium metal shedding rate of the negative electrode 121. The sodium metal shedding rate was calculated by mass, as follows: ① Weighing with an electronic balance: The mass M of the negative electrode 121 in its initial state was measured. 负极 , of which M 负极 The mass of the current collector layer 1211 is the sum of the mass of the conductive layer 1212; during disassembly in the charged state, the sodium metal layer is deposited on the surface of the negative electrode 121, and the sum of the mass of the sodium metal layer and the negative electrode 121 is M. 负极+钠金属层 The mass of metallic sodium that detached during disassembly is M. 钠 Then M 钠 =m1,M 负极+钠金属层 -M 负极 +M 钠 =m1+m2; ② Under full charge, the sodium metal shedding rate = M 钠 / (M 负极+钠金属层 -M 负极 +M 钠 )×100%.

[0134] Table 2 shows the values ​​of sodium shedding rate σ1 of implementation batteries 1 to 14 and comparison batteries 1 to 8 under full charge.

[0135] 3. Calculation of the shedding rate σ2 of conductive layer 1212:

[0136] The sodium-ion battery 100, after capacity testing and complete discharge, was disassembled in an argon atmosphere glove box (H2O < 0.01 ppm, O2 < 0.01 ppm) to observe and calculate the shedding rate of the conductive layer 1212 of the negative electrode 121. The shedding rate of the conductive layer 1212 of the negative electrode 121 was calculated by mass, as follows: ① Weighing with an electronic balance: The mass of the current collector layer 1211 in the initial state was M. 集流体层1211 The mass of the negative electrode 121 is M' 负极 M' 负极 Let M' be the total mass of the current collector layer 1211 and the conductive layer 1212; when the sodium-ion battery 100 is disassembled in a fully discharged state, the weight of the part of the conductive layer 1212 that falls off is M”, then M” = m3, M’ 负极 -M 集流体层1211 =m3+m4; ② Under full load, the shedding rate of conductive layer 1212 σ2=M” / (M' 负极 -M 集流体层1211 )×100%.

[0137] Among them, the values ​​of the shedding rate σ2 of the conductive layer 1212 in the fully discharged state of the implementation batteries 1 to 14 and the comparison batteries 1 to 8 are shown in Table 2.

[0138] Table 2: Performance parameters of implementation batteries 1 to 14 and comparison batteries 1 to 8.

[0139] Please refer to Tables 1 and 2. In Comparative Examples 1 to 8, the electrolyte 110 in Comparative Examples 1 and 8 does not contain sodium salt additives or film-forming additives. The electrolyte 110 in Comparative Examples 2 to 4 includes a first additive. The electrolyte 110 in Comparative Example 5 includes a second additive. The electrolyte 110 in Comparative Example 6 includes a film-forming additive. The electrolyte 110 in Comparative Example 7 includes both a second additive and a film-forming additive. From the performance parameters of Comparative Battery 1 and Comparative Battery 8, it can be seen that when the electrolyte 110 does not contain sodium salt additives... With any additives, the sodium-ion battery 100 has a low initial efficiency, poor adhesion between the sodium metal layer and the negative electrode 121, and severe peeling of the negative electrode conductive layer 1212. In addition, the solvents of the comparative battery 1 and the comparative battery 8 have different compositions, which makes the initial charge and discharge efficiencies of the comparative battery 1 and the comparative battery 8 roughly the same. However, the introduction of ethylene glycol dimethyl ether into the solvent can appropriately improve the deposition performance of the sodium metal layer on the surface of the negative electrode 121 and reduce the peeling of the conductive layer 1212 and the current collector layer 1211 of the negative electrode 121. Based on the performance parameters of comparative batteries 2 to 4, it can be concluded that when the first additive is introduced and its mass fraction is within a reasonable range, the initial charge-discharge efficiency of the sodium-ion battery 100 and the deposition performance of the sodium metal layer on the surface of the negative electrode 121 can be improved within a suitable range, and the peeling of the conductive layer 1212 and the current collector layer 1211 of the negative electrode 121 can be reduced. The degree of improvement is related to the amount of the first additive added, but the degree of improvement is still limited. Based on the data of comparative battery 5, it can be seen that the introduction of the second additive can improve the initial charge-discharge efficiency of the sodium-ion battery 100, but the improvement effect is not obvious. Correspondingly, the second additive can improve the deposition performance of the sodium metal layer on the surface of the negative electrode 121 and reduce the peeling of the conductive layer 1212 and the current collector layer 1211 of the negative electrode 121. Data from comparative batteries 6 and 7 show that, without the addition of sodium salt additives, simply adding film-forming additives does not effectively improve the initial charge-discharge efficiency of the sodium-ion battery 100, nor does it improve the deposition performance of the sodium metal layer on the surface of the negative electrode 121, nor does it improve the bonding performance between the conductive layer 1212 and the current collector layer 1211. Correspondingly, when the second additive is combined with the film-forming additive, both can improve the initial charge-discharge efficiency of the sodium-ion battery 100, but the improvement is not significant. The second additive can also improve the deposition performance of the sodium metal layer on the surface of the negative electrode 121 and reduce the peeling of the conductive layer 1212 and the current collector layer 1211 of the negative electrode 121, corresponding to the performance parameters of comparative battery 5.

[0140] Please refer to Examples 1 to 3 and Examples 5 to 10. As can be seen from the data of Implemented Battery 1 to Implemented Battery 3, under the same conditions, when the mass fraction of the first additive meets a reasonable range, the corresponding sodium-ion battery 100 has a high first charge and discharge efficiency, and the shedding rate σ1 of metallic sodium and the shedding rate σ2 of conductive layer 1212 are both 0. In other words, the first additive can effectively improve the deposition of sodium metal layer on the surface of negative electrode 121 and can effectively prevent conductive layer 1212 from falling off from current collector layer 1211. Similarly, data from embodiments 5 to 7 show that, under the same conditions, when the mass fraction of the first additive meets a reasonable range, the corresponding sodium-ion battery 100 has a higher initial charge-discharge efficiency. However, in the electrolyte 110 of embodiments 5 to 7, the second additive is sodium difluorooxalate borate, while in the electrolyte 110 of embodiments 1 to 3, the second additive is sodium tetrafluoroborate. This indicates that, under the same conditions, sodium difluorooxalate borate is less effective than sodium tetrafluoroborate in improving the deposition density of the sodium metal layer, thus causing the initial charge-discharge efficiency of embodiments 5 to 7 to be lower than that of embodiments 1 to 3. Similarly, data from embodiments 8 to 10 show that when the mass fraction of the first additive is within a reasonable range, the corresponding sodium-ion battery 100 has a high initial charge-discharge efficiency, and the shedding rate σ1 of metallic sodium and the shedding rate σ2 of the conductive layer 1212 are both 0. In other words, the first additive can effectively improve the deposition of the sodium metal layer on the surface of the negative electrode 121 and can effectively prevent the conductive layer 1212 from falling off the current collector layer 1211. In addition, the type of the first additive in embodiments 8 to 10 is sodium saccharin, and the type of the first additive in embodiments 1 to 3 is sodium dodecyl sulfonate. Data from Table 2 shows that, under the same conditions, sodium saccharin has the same effect on improving the deposition density of the sodium metal layer as sodium dodecyl sulfonate, thereby enabling embodiments 1 to 3 and embodiments 8 to 10 to have high initial charge-discharge efficiency.

[0141] Please refer to Examples 2, 4, and Comparative Example 3. Data from Examples 2, 4, and 3 shows that, under the same conditions and with the mass fraction of the first additive within a reasonable range, as long as the mass fraction of the second additive is within a reasonable range, the corresponding sodium-ion battery 100 can maintain a high initial charge-discharge efficiency. Furthermore, the sodium metal shedding rate σ1 and the conductive layer 1212 shedding rate σ2 are both low or zero. Conversely, if the electrolyte 110 only includes the first additive and not the second additive, the initial charge-discharge efficiency of the corresponding sodium-ion battery 100 decreases, and the sodium metal shedding rate σ1 and the conductive layer 1212 shedding rate σ2 are higher. This indicates that when the electrolyte 110 includes both the first and second additives, they have a synergistic effect. The first and second additives jointly induce and capture sodium ions released from the positive electrode 122. After the first additive reduces the sodium ions to sodium atoms, it induces the sodium atoms to form a dense and uniform sodium metal layer on the surface of the negative electrode 121. Furthermore, as the charging voltage of the sodium-ion battery 100 increases, the second additive preferentially undergoes a reduction reaction compared to other components in the electrolyte 110 and forms an SEI film on the surface of the negative electrode 121. Ultimately, a complete SEI film is formed on the outer periphery of the sodium metal layer, effectively isolating the sodium metal layer and the electrolyte 110, suppressing the side reactions between sodium and the electrolyte 110, which helps to reduce the loss of metallic sodium and maintain the activity of the sodium metal layer, improve the utilization rate of sodium in the charge-discharge cycle, and ultimately improve the first charge-discharge efficiency and cycle performance of the sodium-ion battery 100.

[0142] Similarly, please refer to Examples 9 and 11. As can be seen from the data of Examples 9 and 11, when other conditions are the same and the mass fraction of the first additive is within a reasonable range, as long as the mass fraction of the second additive is within a reasonable range, the corresponding sodium-ion battery 100 can maintain a high first charge and discharge efficiency, and the shedding rate σ1 of metallic sodium and the shedding rate σ2 of conductive layer 1212 are both low or 0. This indicates that when the electrolyte 110 includes both a first additive and a second additive, the first additive and the second additive have a synergistic effect. The first additive and the second additive jointly induce and capture sodium ions released from the positive electrode 122. After the first additive reduces the sodium ions to sodium atoms, it induces the sodium atoms to form a dense and uniform sodium metal layer on the surface of the negative electrode 121. The second additive forms an SEI film on the surface of the negative electrode 121 and finally forms a complete SEI film on the outer periphery of the sodium metal layer, so as to effectively isolate the sodium metal layer and the electrolyte 110, suppress the side reaction between sodium and the electrolyte 110, reduce the loss of metallic sodium and maintain the activity of the sodium metal layer, improve the utilization rate of sodium in the charge and discharge cycle, and ultimately improve the first charge and discharge efficiency and cycle performance of the sodium-ion battery 100.

[0143] Please refer to Examples 12 to 14. Data from Examples 12 and 13 shows that when the electrolyte 110 includes a first additive and a second additive, and the ratio of the first additive to the second additive is appropriate, the introduction of film-forming additives has no effect on the initial charge-discharge efficiency of the sodium-ion battery 100, the deposition of metallic sodium, or the stripping of the conductive layer 1212. This further demonstrates that the film-forming additives are mainly used during formation to form a stable SEI film on the negative electrode 121. Data from Examples 12 and 14 shows that when the type and mass fraction of the first additive, the type and mass fraction of the second additive, and the type and mass fraction of the film-forming additive are the same, changing the solvent type and mixing mass ratio still results in a high initial charge-discharge efficiency for the sodium-ion battery 100.

[0144] Further, please refer to Examples 2, 1, 3, 5, and 6, and also refer to Figure 3. As shown in Figure 3, the electrolyte 110 in Comparative Example 1 contains no additives, has a conductivity of approximately 6.02 mS / cm, and the initial charge-discharge efficiency of the sodium-ion battery 100 is approximately 40.58%. The electrolyte 110 in Comparative Example 3 includes a first additive, has a conductivity of 6.21 mS / cm, and its corresponding initial charge-discharge efficiency is 54.27%. The electrolyte 110 in Comparative Example 5 includes a second additive, has a conductivity of 6.28 mS / cm, and its corresponding initial charge-discharge efficiency of the sodium-ion battery 100 is 51.36%. Therefore, the addition of the first and second additives is beneficial to improving the conductivity of the electrolyte 110, thereby improving the initial charge-discharge efficiency of the corresponding sodium-ion battery 100. Comparative Example 6's electrolyte 110 includes a film-forming additive. The conductivity of electrolyte 110 is 6.05 mS / cm, corresponding to a sodium-ion battery 100 efficiency of 40.49%. This indicates that without the addition of the first and second additives, the film-forming additives have no effect on improving the conductivity of electrolyte 110 or the initial charge-discharge efficiency of sodium ions. In Example 2, the electrolyte 110 includes both the first and second additives, and the mass fractions of both are within reasonable ranges. This results in an electrolyte 110 conductivity of 6.62 mS / cm and an initial efficiency of 85.98%, further verifying that the first and second additives can improve the conductivity of electrolyte 110, thereby increasing the initial charge-discharge efficiency of the corresponding sodium-ion battery 100.

[0145] Further, please refer to Examples 2, 1, 3, 5, and 6, and also to Figures 4 and 5. As shown in Figure 4, when the first additive and the second additive are used in combination, the sodium metal deposition on the negative electrode 121 is uniform and dense when the sodium-ion battery 100 is fully charged, with no shedding. Please refer to the illustrations in Example 2 for details. When one of the first additive, the second additive, and the film-forming additive is introduced alone, the sodium metal layer on the negative electrode 121 shows varying degrees of shedding when the sodium-ion battery 100 is fully charged, with severe adhesion to the separator 123. However, the conductive layer 1212 of the negative electrode 121 does not peel off. Please refer to the illustrations in Examples 1, 3, 5, and 6 for details. As shown in Figure 5, when the first additive and the second additive are used in combination, when the sodium-ion battery 100 is in a fully discharged state, there is no sodium metal layer residue on the negative electrode 121, and the conductive layer 1212 of the negative electrode 121 remains intact without any peeling. Please refer to the illustration in Example 2 for details. When one of the first additive, the second additive, and the film-forming additive is introduced alone, when the sodium-ion battery 100 is in a fully discharged state, the negative electrode 121 shows varying degrees of sodium metal layer residue, and the conductive layer 1212 of the negative electrode 121 partially peels off. Please refer to the illustrations in Comparative Examples 1, 3, 5, and 6 for details. As shown in Figures 4 and 5, when the electrolyte 110 includes the first additive and the second additive, the first additive and the second additive have a synergistic effect, which can effectively improve the deposition of the sodium metal layer on the surface of the negative electrode 121 and improve the bonding performance between the conductive layer 1212 and the current collector layer 1211.

[0146] Further, please refer to Example 2, Comparative Example 1, Comparative Example 3, Comparative Example 5 and Comparative Example 6, and also refer to Figures 6 and 7. By comparing Figures 6 and 7, it can be seen that in the initial state, the granular distribution on the surface of the negative electrode 121 of Example 2 is the conductive layer 1212 coated on the current collector, and the black area at the particle connection is aluminum foil, i.e., the current collector layer 1211. The conductive layer 1212 on the entire surface of the negative electrode 121 is not uniformly and densely distributed. When the sodium-ion battery 100 is in a fully discharged state, the negative electrode 121 exhibits the same morphology as in the initial state, indicating that adding the first additive and the second additive to the electrolyte 110 of Example 2 can improve the problem of the conductive layer 1212 of the negative electrode 121 falling off. When no additive is introduced, or when only one of the first additive, the second additive, and the film-forming additive is introduced, the conductive layer 1212 of the negative electrode sheet 121 exhibits poor adhesion due to volume expansion, resulting in varying degrees of peeling and exposing the current collector layer 1211. Consequently, the peeling rates of the conductive layer 1212 and the sodium metal in Comparative Examples 1, 3, 5, and 6 are all higher than those in Example 2. Furthermore, data from Comparative Examples 2 to 4 show that introducing different proportions of the first additive alone somewhat improves the peeling of the conductive layer 1212, but the overall situation remains quite serious.

[0147] Please refer to Figures 8 and 9. This application also provides an electrical device 200, which includes a device body 210 and a sodium-ion battery 100 provided in this application. The sodium-ion battery 100 is used to supply power to the device body 210.

[0148] Understandably, the sodium-ion battery 100 is electrically connected to the device body 210.

[0149] In this embodiment, the sodium-ion battery 100 has high initial charge-discharge efficiency and cycle performance, which enables the sodium-ion battery 100 to provide stable power to the device body 210, thereby improving the user experience.

[0150] Optionally, the electrical device 200 in this embodiment can be, but is not limited to, portable electronic devices such as mobile phones, tablets, laptops, desktop computers, smart bracelets, smartwatches, e-readers, and game consoles. It can also be a vehicle such as a car, truck, sedan, van, freight train, high-speed train, or electric vehicle. Furthermore, it can be various household appliances. In the embodiment shown in Figure 9 of this application, the electrical device 200 is an energy storage battery cabinet.

[0151] It is understood that the electrical device 200 described in this embodiment is merely one form of the electrical device 200 used in the sodium-ion battery 100, and should not be construed as a limitation on the electrical device 200 provided in this application, nor should it be construed as a limitation on the electrical device 200 provided in various embodiments of this application.

[0152] In this application, the terms "embodiment" and "implementation" mean that a specific feature, structure, or characteristic described in connection with an embodiment can be included in at least one embodiment of this application. The appearance of these phrases in various locations throughout the specification does not necessarily refer to the same embodiment, nor are they independent or alternative embodiments mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this application can be combined with other embodiments. Furthermore, it should be understood that the features, structures, or characteristics described in the various embodiments of this application can be arbitrarily combined to form yet another embodiment that does not depart from the spirit and scope of the technical solution of this application, provided there is no contradiction between them.

[0153] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. An electrolyte, wherein, The electrolyte includes a sodium salt additive, which includes a first additive and a second additive. The first additive is a sodium sulfonate additive, and the second additive is a fluorine-containing sodium salt additive. In the electrolyte, the mass fraction of the first additive is less than the mass fraction of the second additive.

2. The electrolyte according to claim 1, wherein, The mass ratio α of the first additive to the second additive satisfies the range: 1 / 500≤α≤1 / 20.

3. The electrolyte according to claim 1, wherein, In the electrolyte, the mass fraction A1 of the sodium salt additive satisfies the range: 0.001% ≤ A1 ≤ 2%.

4. The electrolyte according to claim 1, wherein, The first additive is selected from at least one of sodium dodecyl sulfonate, sodium o-benzoyl sulfimide, sodium polydithiopropane sulfonate, sodium 3-mercaptopropane sulfonate, sodium thiopropane sulfonate, and sodium 2,3-dimercaptopropane sulfonate; the second additive is selected from at least one of sodium difluorooxalate borate, sodium tetrafluoroborate, and sodium difluorophosphate.

5. The electrolyte according to any one of claims 1 to 4, wherein, The electrolyte also includes a film-forming additive, and the mass ratio β of the sodium salt additive to the film-forming additive satisfies the range: 1 / 1500≤β≤1 / 5.

6. The electrolyte according to claim 5, wherein, In the electrolyte, the mass fraction A2 of the film-forming additive satisfies the range: 0.1% ≤ A2 ≤ 10%.

7. A sodium-ion battery, wherein, The sodium-ion battery includes: An electrode assembly, comprising a negative electrode sheet, the negative electrode sheet comprising a current collector layer and a conductive layer stacked thereon, the conductive layer comprising a carbon material; and The electrolyte according to any one of claims 1 to 6, wherein the electrolyte is used to wet at least a portion of the electrode assembly.

8. The sodium-ion battery according to claim 7, wherein, The thickness d1 of the conductive layer satisfies the range: 10μm≤d1≤50μm; and / or, On the current collector layer, the surface density g of the conductive layer coating satisfies the range of 5 g / m². 2 ≤g≤100g / m 2 .

9. The sodium-ion battery according to claim 8, wherein, In the fully charged state, the sodium-ion battery further includes a deposition layer, which is disposed on the side of the conductive layer away from the current collector layer. The deposition layer includes metallic sodium, and the deposition thickness d2 of the deposition layer satisfies the range: 10μm≤d2≤30μm.

10. The sodium-ion battery according to claim 9, wherein, When the sodium-ion battery is fully charged, the sodium shedding rate σ1 is ≤ 0.1%; wherein, the sodium shedding rate σ1 = m1 / (m1+m2)×100%, m1 is the mass of the shed sodium, and m2 is the mass of the sodium remaining in the deposition layer.

11. The sodium-ion battery according to claim 7, wherein, In the fully discharged state, the shedding rate of the conductive layer σ2 of the sodium-ion battery is ≤0.01%; wherein, the shedding rate of the conductive layer σ2=m3 / (m3+m4)×100%, where m3 is the weight of the part of the conductive layer that has detached, and m4 is the weight of the part of the conductive layer that has not detached.

12. An electrical appliance, wherein, The electrical equipment includes: Equipment body; The sodium-ion battery according to any one of claims 7 to 11, wherein the sodium-ion battery is used to power the device body.

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