Sodium secondary battery cell, battery, and electric apparatus

Abstract

Provided in the present application are a sodium secondary battery cell, a battery and an electric apparatus. The sodium secondary battery cell comprises a casing and an electrode assembly. The electrode assembly is arranged in the casing. During one charging process, when the sodium secondary battery cell reaches full charge, the group margin of the sodium secondary battery cell in the width direction is A1; and before the sodium secondary battery cell reaches full charge, the maximum value of the group margin of the sodium secondary battery cell in the width direction is A2, wherein A1≤A2. The present application can solve the problem of it being difficult to balance the expansion force and energy density of a battery, and thus the energy density of the battery can be improved, and the expansion force does not increase or increases minimally.

Description

Sodium secondary battery cell, battery, and electric device Cross-reference to related applications

[0001] This application claims priority to Chinese Patent Application CN202410666909.4, filed on May 27, 2024, entitled “Sodium secondary battery cell, battery, and electric device”, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the technical field of batteries, and particularly relates to a sodium secondary battery cell, a battery, and an electric device. BACKGROUND

[0003] In the design of a battery, various performances, such as energy density, safety, swelling force, etc., need to be considered comprehensively. In order to improve the energy density of the battery, as much electrode active material as possible needs to be put into the limited battery shell. However, during the charging and discharging process of the battery, the phenomenon of electrode assembly swelling and transmitting swelling force to the outside often occurs. If the swelling force generated by the electrode assembly to the outside is too large, it will squeeze the various components in the battery, thereby causing the performance of the battery to decrease and safety hazards to be generated. Moreover, the more electrode active material put into the battery shell, the more obvious this swelling phenomenon is. Therefore, the swelling force and the energy density of the battery are often difficult to be considered together, which has caused certain limitations to the development of the battery. SUMMARY

[0004] In view of the above problems, the present application provides a sodium secondary battery cell, a battery, and an electric device, which can solve the problem that the swelling force and the energy density of the battery are difficult to be considered together, can improve the energy density of the battery, and the swelling force does not increase or increases very little.

[0005] In a first aspect, the present application provides a sodium secondary battery cell, which comprises a shell and an electrode assembly, the electrode assembly being arranged in the shell; during a first charging process, when the sodium secondary battery cell reaches full charge, the width direction group margin of the sodium secondary battery cell is A1; before the sodium secondary battery cell reaches full charge, the maximum value of the width direction group margin of the sodium secondary battery cell is A2, which satisfies A1≤A2.

[0006] According to the sodium secondary battery cell in the embodiments of the present application, during the first charging process, the maximum width direction group margin is A2 before the sodium secondary battery cell reaches full charging, the width direction group margin is A1 when the sodium secondary battery cell reaches full charging, and A1≤A2. Before reaching full charging, the sodium secondary battery cell has the maximum width direction group margin, and after the sodium secondary battery cell reaches the maximum group margin during the charging process, if the sodium secondary battery cell is continuously charged, the overall trend of the width direction group margin of the sodium secondary battery cell is to decrease or basically remain unchanged. Therefore, the maximum group margin before full charging can be set to a value as large as possible under the premise of safety, so that as much electrode active material as possible can be placed inside the shell of the battery cell, and the energy density of the sodium secondary battery cell is improved.

[0007] Moreover, the group margin generally has a positive correlation with the expansion force of the sodium secondary battery cell, and the greater the group margin, the greater the expansion force of the sodium secondary battery cell. After the expansion force of the sodium secondary battery cell reaches the maximum value, if the sodium secondary battery cell is continuously charged, the overall change trend of the expansion force is to decrease or basically remain unchanged, rather than continuously increase. Therefore, by configuring the group margin of the sodium secondary battery cell by using the scheme, the energy density can be improved, and the expansion force does not increase or increases very little.

[0008] In some embodiments, the sodium secondary battery cell is a fresh cell, and the fresh cell has a charge-discharge cycle number less than or equal to 100, and the width direction group margin A1 when full charging is 94% to 96%.

[0009] When the sodium secondary battery cell reaches full charging, A1 satisfying the above range can take into account the energy density and safety; because the expansion force of the sodium secondary battery cell is low, more active substances can be arranged in the shell under the premise of safety, so that the energy density is improved, and the expansion force does not increase or increases very little.

[0010] In some embodiments, the sodium secondary battery cell has a charge-discharge cycle number less than or equal to 100, and A2 is 98% to 103%.

[0011] Before the fresh sodium secondary battery cell reaches full charging, A2 satisfies the above range, the expansion force of the sodium secondary battery cell is relatively large, but is still within an acceptable range of safety performance.

[0012] In some embodiments, the design minimum voltage of the sodium secondary battery cell is not lower than 1.5 V, and the design maximum voltage of the sodium secondary battery cell is not higher than 4.2 V.

[0013] When the design voltage of the sodium secondary battery cell is 1.5 V to 4.2 V, the expansion force of the sodium secondary battery cell reaches the maximum value before the sodium secondary battery cell reaches full charging during the same cycle process, and the expansion force of the sodium secondary battery cell decreases when the sodium secondary battery cell is full charged.

[0014] In some embodiments, the sodium secondary battery cell has a cycle number of charge and discharge of less than or equal to 100, and a maximum value of the group margin in the width direction of the sodium secondary battery cell is less than or equal to 98% when the state of charge of the sodium secondary battery cell is greater than 65%.

[0015] At this time, the expansion force of the sodium secondary battery cell can be ensured to be small, and the energy density of the sodium secondary battery cell can be ensured to be higher than when the expansion force is the largest.

[0016] In some embodiments, the sodium secondary battery cell has a cycle number of charge and discharge of less than or equal to 100, and a maximum value of the group margin in the width direction of the sodium secondary battery cell is greater than 98% and less than or equal to 103% when the state of charge of the sodium secondary battery cell is 40% to 65%.

[0017] The group margin in the width direction of the sodium secondary battery cell is configured to be close to or equal to 100% at a state of charge of 45% to 65%, and then gradually decreases as the state of charge increases. Therefore, by using the configuration method of the group margin, the sodium secondary battery cell can have a high group margin as much as possible, and the energy density can be improved, and the expansion force of the sodium secondary battery cell does not increase significantly as the state of charge increases.

[0018] In some embodiments, when the sodium secondary battery cell is cycled more than 200 times, the group margin in the width direction of the sodium secondary battery cell is 99.5% to 102% when the sodium secondary battery cell is fully charged.

[0019] When the sodium secondary battery cell is fully charged, the group margin in the width direction of the sodium secondary battery cell satisfies the above range. Considering the shrinkage of the positive electrode and the thickness increase of the negative electrode SEI film after more than 200 cycles, the expansion force of the sodium secondary battery cell (hereinafter referred to as battery expansion force) can be significantly delayed, and high energy density and long life can be achieved.

[0020] In some embodiments, the electrode assembly includes a positive electrode sheet and a negative electrode sheet, and a ratio of a thickness of the positive electrode sheet to a thickness of the negative electrode sheet is 0.65 to 0.75.

[0021] The ratio of the thickness of the positive electrode sheet to the thickness of the negative electrode sheet satisfies the above range, which on the one hand reduces the risk of metal ion precipitation, and on the other hand avoids the thickness of the positive and negative electrode sheets being too thick, which wastes volume.

[0022] In some embodiments, the electrode assembly includes a negative electrode sheet, and the negative electrode sheet includes a negative electrode current collector, and the negative electrode current collector is an aluminum foil, and a thickness of the aluminum foil is 10 μm to 12 μm.

[0023] The negative electrode current collector is made of aluminum foil. The negative electrode will not undergo an alloying reaction with the aluminum-based current collector. By reducing the local current density of the aluminum-based current collector in the negative electrode of the sodium secondary battery cell, the growth of sodium dendrites and volume expansion can be suppressed. The thickness of the aluminum foil satisfies the above relationship. On the one hand, the processing technology is simple and easy to manufacture. On the other hand, the negative electrode sheet has a small thickness and occupies less space.

[0024] In some embodiments, during a single charge, the group margin of the sodium secondary battery cell exhibits a trend of first increasing and then decreasing. That is, as the state of charge of the sodium secondary battery cell increases from 0% to 100%, the group margin of the sodium secondary battery cell first increases and then decreases, and the corresponding expansion force also first increases and then decreases. Therefore, the growth rate of the expansion force of the sodium secondary battery cell after full charge is relatively small compared to the expansion force before charging.

[0025] In some embodiments, the operating voltage range of the sodium secondary battery cell is 1.5V to 4.2V. In this embodiment, the sodium secondary battery cell, after reaching its maximum capacity during charging, will not continue to expand. Therefore, it can continue to be charged at a higher voltage without needing to stop charging before or when the maximum capacity is reached. This allows the positive electrode active material in the sodium secondary battery cell to continue releasing metal ions for electrochemical reactions, which is beneficial for obtaining higher energy density.

[0026] In some embodiments, when the state of charge of the sodium secondary battery cell is 45% to 65%, the rebound rate of the positive electrode contained in the sodium secondary battery cell is 5% to 8%. When the state of charge of the sodium secondary battery cell is greater than 65%, the rebound rate of the positive electrode contained in the sodium secondary battery cell is less than or equal to 5%.

[0027] When the state of charge is 45% to 65%, the positive electrode has a high rebound rate. However, as the state of charge increases, the rebound rate of the positive electrode decreases. This is beneficial for making the full charge margin of the sodium secondary battery cell smaller than the maximum margin before full charge, which is beneficial for improving the energy density of the sodium secondary battery cell and reducing the expansion force of the sodium secondary battery cell under high margin.

[0028] In some embodiments, when the state of charge of the sodium secondary battery cell is 80% to 95%, the rebound rate of the positive electrode contained in the sodium secondary battery cell is 2% to 4.5%. When the state of charge of the sodium secondary battery cell is 95% to 100%, the rebound rate of the positive electrode contained in the sodium secondary battery cell is 0% to 1.9%.

[0029] When the charge is close to full charge, the rebound rate of the positive electrode gradually decreases as the state of charge increases. This allows the full charge margin of a sodium secondary battery cell to be less than the maximum margin before full charge, which is beneficial to improving the energy density of the sodium secondary battery cell and reducing the expansion force of the battery cell under high margin.

[0030] In some embodiments, the electrode assembly includes a positive electrode sheet containing a positive electrode active material. When the sodium secondary battery cell is fully charged, the cell length of the positive electrode active material in at least one crystal axis direction is less than the maximum cell length in that crystal axis direction before full charging. By using a positive electrode active material with this cell characteristic in the positive electrode sheet, the rebound rate of the positive electrode sheet under full charging can be lower than the rebound rate before full charging. This results in the full-charge group margin of the sodium secondary battery cell being lower than the maximum group margin before full charging, which is beneficial for improving the energy density of the sodium secondary battery cell and reducing the expansion force of the sodium secondary battery cell under high group margin.

[0031] In some embodiments, the electrode assembly includes a positive electrode sheet, the positive electrode sheet includes a positive active material, and the positive active material includes a nickel-manganese-iron-based sodium oxide.

[0032] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide is Na. q Ni x Mn y Fe z M p O2, where 0 < q ≤ 1, 0 < x, 0 < y, 0 < z, 0 ≤ p, x + y + z ≥ 0.81, x + y + z + p ≤ 1, and M includes one or more of V, Cr, Zn, Al, Sc, Sn, Sb, Zr, Nb, Ti, Mg, Cu, Ru, and Ir. Layered oxide Na q Ni x Mn y Fe z M p The c-axis unit cell length of O2 initially increases and then decreases with increasing sodium removal. Using Na... q Ni x Mn y Fe z M p O2 serves as the positive electrode active material; during charging, Na... q Ni x Mn y Fe z M p O2 undergoes a sodium removal reaction, and as the charging voltage gradually increases, or as the state of charge of the sodium secondary battery cells gradually increases, Na... q Ni x Mny Fe z M p The amount of sodium removed from O2 also increases, then Na q Ni x Mn y Fe z M p The c-axis cell length of O2 first increases and then decreases. Under full charge, the cathode material of the above molecular formula has a c-axis cell length that is smaller than the maximum cell length in the direction before full charge. This structural characteristic makes Na... q Ni x Mn y Fe z M p During the charging process, O2 exhibits a macroscopic phenomenon of first expanding and then contracting, which in turn causes the rebound rate of the positive electrode to also exhibit the characteristic of first expanding and then contracting. This results in the full charge margin of a sodium secondary battery cell being less than the maximum margin before full charge, which is beneficial to improving the energy density of the sodium secondary battery cell and reducing the expansion force of the battery cell under high margin conditions.

[0033] In some embodiments, the total mass percentage of Ni, Mn, and Fe in the nickel-manganese-iron-based sodium-containing oxide is 40%–50%, optionally 42%–46%. At this ratio, Na… q Ni x Mn y Fe z M p The c-axis cell length of O2 shows a trend of first increasing and then decreasing with the increase of sodium removal.

[0034] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1≤x≤0.5, and optionally, 0.2≤x≤0.4.

[0035] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1≤y≤0.5, and optionally, 0.2≤y≤0.4.

[0036] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1≤z≤0.5.

[0037] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.2≤z≤0.3.

[0038] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.8≤x+y+z≤1, and optionally, 0.9≤x+y+z≤1.

[0039] In some embodiments, the electrode assembly includes a negative electrode sheet comprising a negative electrode active material with a porosity of 40%–70%, optionally 45%–55%. The abundant porosity of the negative electrode active material facilitates thorough wetting of the electrolyte through these pores, thereby enabling the effective transport of active ions from the sodium secondary battery cell between the positive and negative electrode sheets via the electrolyte, improving ion transport efficiency and enhancing the electrochemical performance of the sodium secondary battery cell. Simultaneously, the presence of numerous pores also provides a buffering effect, reducing the volume expansion of the negative electrode sheet when active ions are embedded within it.

[0040] In some embodiments, the electrode assembly includes a negative electrode sheet containing a negative electrode active material, the negative electrode active material having an average pore size of 1 nm to 30 nm, optionally 1 nm to 30 nm. The nanopores facilitate the entry of electrolyte into the negative electrode active material through capillary forces, thereby improving the electrolyte wetting performance of the negative electrode sheet.

[0041] In some embodiments, the electrode assembly includes a negative electrode sheet, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes hard carbon.

[0042] Hard carbon has low expansion force. Using it as a negative electrode active material can help reduce the expansion force of the negative electrode during charging and discharging, thereby reducing the expansion force of sodium secondary battery cells.

[0043] Secondly, this application provides a battery comprising the sodium secondary battery cell described in the first aspect.

[0044] The sodium secondary battery cell of this application embodiment combines high group margin, high energy density, and low expansion force. Therefore, the battery containing this sodium secondary battery cell also exhibits the characteristics of high group margin, high energy density, and low expansion force. Moreover, since the sodium secondary battery cell has low expansion force, it does not cause the top shell to expand or deform when fully charged, and the battery composed of this sodium secondary battery cell has high reliability.

[0045] In some embodiments, the number of sodium secondary battery cells is multiple, and the multiple sodium secondary battery cells are arranged along the width direction of the sodium secondary battery cells; the battery also includes a heat insulation pad, which is disposed between two adjacent sodium secondary battery cells.

[0046] Multiple sodium-ion battery cells are arranged along the width of the battery, facilitating their assembly. The inclusion of heat-insulating pads reduces heat transfer between adjacent cells, resulting in higher battery reliability.

[0047] In some embodiments, the thickness of the heat insulation pad is 0.5 mm to 4 mm.

[0048] The thickness of the heat insulation pad satisfies the above relationship, which on the one hand provides good heat insulation effect, and on the other hand occupies less assembly space.

[0049] In some embodiments, the capacity of the sodium secondary battery cell is 50Ah to 150Ah, and the thickness of the heat insulation pad is 0.4mm to 1.0mm; or the capacity of the sodium secondary battery cell is 151Ah to 250Ah, and the thickness of the heat insulation pad is 1.1mm to 1.9mm.

[0050] The thickness of the heat insulation pad varies depending on the capacity of the sodium secondary battery cell. When the capacity of the sodium secondary battery cell is low (e.g., greater than or equal to 50Ah, less than or equal to 150Ah), the thickness of the heat insulation pad can be thinner (e.g., greater than or equal to 0.4mm, less than or equal to 1.0mm); when the capacity of the sodium secondary battery cell is high (e.g., greater than or equal to 151Ah, less than or equal to 250Ah), the thickness of the heat insulation pad can be thicker (e.g., greater than or equal to 1.1mm, less than or equal to 1.9mm). The thickness of the heat insulation pad satisfies the above relationship based on the capacity of the sodium secondary battery cell, thus meeting the heat insulation requirements while occupying less space.

[0051] In some embodiments, the heat insulation pad is made of aerogel, ceramic, porous vacuum silica, or asbestos.

[0052] Aerogel, ceramic, porous vacuum silica or asbestos all have good thermal insulation properties.

[0053] Thirdly, this application also provides an electrical device comprising a sodium secondary battery cell as described in the first aspect, or comprising a battery as described in the second aspect.

[0054] The battery in this application embodiment can be used in electrical devices that use batteries as a power source, or in various energy storage systems that use batteries as energy storage elements, to provide electrical energy. The battery described above has the advantages of high capacity margin, high energy density, and low expansion force. Therefore, applying this battery to various electrical devices can provide strong power to the devices, improve their safety, and enhance the user experience of various electrical devices. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0056] Figure 1 is a schematic diagram of the structure of a vehicle according to one embodiment of this application;

[0057] Figure 2 is a schematic diagram of a battery according to an embodiment of this application;

[0058] Figure 3 is an exploded view of a sodium secondary battery cell according to an embodiment of this application;

[0059] Figure 4 is a cross-sectional view of a sodium secondary battery cell according to an embodiment of this application;

[0060] Figure 5 is a cross-sectional view of a sodium secondary battery cell in an expanded state according to an embodiment of this application;

[0061] Figure 6 shows the relationship between the voltage and state of charge (SOC) of a sodium secondary battery cell in the range of 1.5 to 4.2V in one embodiment of this application.

[0062] Figure 7 shows Na in one embodiment of this application. q Ni x Mn y Fe z M p The relationship curve between the c-axis unit cell length of O2 and the Na content;

[0063] Figure 8 shows Na in one embodiment of this application. q Ni x Mn y Fe z M p Schematic diagram of the changes in the unit cell structure during the O2 desodiumization process;

[0064] Figure 9 is a schematic diagram of a battery according to an embodiment of this application;

[0065] Figure 10 shows the variation curves of expansion force and voltage of a sodium secondary battery cell in Example 1 of this application under the same charge-discharge cycle in the voltage range of 2.0 to 3.95V.

[0066] Figure 11 shows the expansion force variation curves of the sodium secondary battery cell in Example 1 of this application during charge-discharge cycles in the voltage ranges of 1.5V to 3.65V and 1.5V to 4.0V.

[0067] The accompanying drawings are not drawn to scale.

[0068] Marking Explanation: 100-Battery; 20-Box; 21-First Sub-Box; 22-Second Sub-Box; 30-Sodium Secondary Battery Cell; 31-Shell; 311-First Wall; 312-Second Wall; 313-Third Wall; 314-End Cap; 315-Shell; 316-Bottom Wall; 32-Electrode Assembly; 33-Electrode Terminal; 40-Heat Insulation Pad; 200-Controller; 300-Motor; 1000-Vehicle; X-Width direction of Sodium Secondary Battery Cell; Y-Length direction of Sodium Secondary Battery Cell; Z-Height direction of Sodium Secondary Battery Cell. Detailed Implementation

[0069] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0070] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0071] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0072] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0073] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0074] In the description of the embodiments of this application, the term "at least one" refers to one or more, and "more than one" refers to two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of a single item or a plurality of items. For example, "at least one of a, b or c", or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single item or multiple items.

[0075] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0076] The mass of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the masses of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass mentioned in the embodiments of this application can be a mass unit known in the chemical industry, such as μg, mg, g, or kg.

[0077] To improve battery energy density, as much electrode active material as possible needs to be placed within the limited space of the battery casing. However, during battery charging and discharging, the insertion or extraction of active metal ions in the electrode active material causes its volume to expand, leading to electrode rebound and increasing the size of the electrode assembly (bare cell), thus transmitting expansion forces externally. Excessive expansion forces generated by the bare cell can compress various components within the battery, resulting in decreased battery performance and potential safety hazards.

[0078] Therefore, in battery cell design, sufficient gaps are typically required between the bare cell and the casing, a design feature known as a group margin. This allows for expansion space of the bare cell, mitigating the negative impacts of expansion. A smaller group margin results in a larger gap between the bare cell and the casing, while a larger group margin results in a smaller gap. While a small group margin effectively reduces the risks associated with bare cell expansion and facilitates casing, excessive space reduces the amount of electrode active material per unit mass or volume, leading to lower energy density. Conversely, a high group margin achieves high energy density, but the reduced internal space makes the battery more susceptible to expansion during charging, generating excessive expansion forces that cause the battery to bulge, ultimately affecting battery performance and reliability. Therefore, balancing battery expansion force and energy density is often difficult.

[0079] To improve battery energy density while minimizing battery expansion force and mitigating structural damage during charging and discharging, this application presents a sodium secondary battery cell comprising a casing and electrode assemblies. The electrode assemblies are housed within the casing. During a single charge, when the sodium secondary battery cell reaches full charge, its width group margin is A1. Before reaching full charge, the maximum width group margin is A2, satisfying A1 ≤ A2. Using this sodium secondary battery cell, it achieves the highest width group margin before full charge, while the full charge width group margin is configured to be less than or equal to this highest margin. Therefore, during charging, once the sodium secondary battery cell reaches this highest width group margin, it can continue charging to achieve higher energy density without increasing expansion force.

[0080] The sodium secondary battery cell with high energy density and low expansion force provided in this application embodiment can be assembled into a battery and further applied to the manufacture of various electrical devices.

[0081] The present application is further illustrated below with reference to embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0082] This application provides an electrical device that includes a sodium secondary battery cell or a battery.

[0083] The battery in this application embodiment can be used in electrical devices that use batteries as a power source, or in various energy storage systems that use batteries as energy storage elements, to provide electrical energy. The battery described above has the advantages of high capacity margin, high energy density, and low expansion force. Therefore, applying this battery to various electrical devices can provide strong power to the devices, improve their safety, and enhance the user experience of various electrical devices.

[0084] Electrical devices can include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0085] For ease of explanation, the following embodiments will use a vehicle as an example of an electrical device according to an embodiment of this application.

[0086] Figure 1 is a schematic diagram of the structure of a vehicle as an example. The vehicle 1000 can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc.

[0087] The vehicle 1000 has a battery 100 installed inside it. The battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The battery 100 can be used to power the vehicle 1000. For example, the battery 100 can be used as the operating power source for the vehicle 1000's electrical system, such as to meet the power requirements for starting, navigation, and operation of the vehicle 1000.

[0088] The vehicle 1000 may also include a controller 200 and a motor 300. The controller 200 is used to control the battery 100 to supply power to the motor 300, for example, for the power needs of the vehicle 1000 during startup, navigation and driving.

[0089] In some embodiments of this application, the battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0090] Please refer to Figure 2, which is a schematic diagram of a battery according to an embodiment of this application. The battery 100 includes a housing 20 and a sodium secondary battery cell 30, the sodium secondary battery cell 30 being housed within the housing 20. The housing 20 provides a space for housing the sodium secondary battery cell 30, and the housing 20 can adopt various structures. In some embodiments, the housing 20 may include a first sub-housing 21 and a second sub-housing 22, the first sub-housing 21 and the second sub-housing 22 overlapping each other, and the first sub-housing 21 and the second sub-housing 22 together define a space for housing the sodium secondary battery cell 30. The second sub-box 22 can be a hollow structure with one end open, and the first sub-box 21 can be a plate-like structure. The first sub-box 21 covers the opening side of the second sub-box 22 so that the first sub-box 21 and the second sub-box 22 together define the accommodating space. Alternatively, the first sub-box 21 and the second sub-box 22 can both be hollow structures with one side open, and the opening side of the first sub-box 21 covers the opening side of the second sub-box 22.

[0091] In battery 100, multiple sodium secondary battery cells 30 can be connected in series, parallel, or in a mixed configuration. A mixed configuration means that multiple sodium secondary battery cells 30 are connected in both series and parallel. Battery 100 may also include other structures, such as a busbar for electrically connecting multiple sodium secondary battery cells 30.

[0092] Please refer to Figures 3 to 5. Figure 3 is an exploded view of a sodium secondary battery cell according to an embodiment of this application, Figure 4 is a cross-sectional view of a sodium secondary battery cell according to an embodiment of this application, and Figure 5 is a cross-sectional view of a sodium secondary battery cell according to an embodiment of this application in an expanded state. A first aspect of this application provides a sodium secondary battery cell 30, which includes a housing 31 and an electrode assembly 32. The electrode assembly 32 is disposed within the housing 31.

[0093] During a single charge, when the sodium secondary battery cell 30 reaches full charge, the width direction group margin of the sodium secondary battery cell 30 is A1; before the sodium secondary battery cell 30 reaches full charge, the maximum value of the width direction group margin of the sodium secondary battery cell 30 is A2, satisfying that A1≤A2.

[0094] "Fully charged" means that the state of charge of a sodium secondary battery cell reaches 100% or is charged to the highest cutoff voltage of the sodium secondary battery cell.

[0095] "Before reaching full charge" means that the state of charge of a sodium secondary battery cell has not reached 100% or has not been charged to the highest cutoff voltage of the sodium secondary battery cell.

[0096] "One-time charging process" refers to the charging process of a sodium secondary battery cell from any low state of charge to full charge, which can be the charging process from 0% SOC constant current charging to 100% SOC.

[0097] The "maximum value of the group margin in the width direction" is the maximum value on the fitted curve of the group margin in the width direction as a function of the state of charge.

[0098] "Sodium secondary battery cell" can be a fresh battery cell (fresh battery cell) or a battery cell that has been recycled.

[0099] The housing 31 is used to provide a receiving cavity for accommodating the electrode assembly 32.

[0100] The outer casing 31 includes two first walls 311 arranged opposite each other along the width direction X of the sodium secondary battery cell, two second walls 312 arranged opposite each other along the length direction Y of the sodium secondary battery cell, and two third walls 313 arranged opposite each other along the height direction Z of the sodium secondary battery cell. The width direction X, the length direction Y, and the height direction Z of the sodium secondary battery cell are perpendicular to each other.

[0101] The sodium secondary battery cell 30 also includes an electrode terminal 33, which is disposed on the third wall 313. The electrode terminal 33 is electrically connected to the tabs of the electrode assembly 32 and the conductors on the outside of the sodium secondary battery cell 30.

[0102] Along the width direction X of the sodium secondary battery cell, the electrode assembly 32 is located between two first walls 311. The thickness direction of the electrode assembly 32 is parallel to the width direction X of the sodium secondary battery cell. The first wall 311 can be the large surface of the sodium secondary battery cell 30.

[0103] Two first walls 311 and two second walls 312 form a receiving space for accommodating the electrode assembly 32.

[0104] The inner surface of the second wall 312 refers to the surface of the second wall 312 facing the electrode assembly 32.

[0105] When the electrode assembly 32 has a stacked structure, the thickness direction of the electrode assembly 32 refers to the stacking direction of the electrode sheets. When the electrode assembly 32 has a wound structure, the electrode assembly 32 is flat and includes a flat region and a bent region. The thickness direction of the electrode assembly 32 is the stacking direction of the electrode sheets in the flat region, and the thickness direction of the electrode assembly 32 is perpendicular to the winding axis direction of the electrode assembly 32.

[0106] Referring to Figures 4 and 5, along the width direction X of the sodium secondary battery cell, the maximum dimension of the electrode assembly 32 is defined as W1, and the dimension of the inner surface of the second wall 312 is defined as W2. The dimension of the inner surface of the second wall 312 can be understood as the distance between the inner surfaces of the two first walls 311 along the width direction X of the sodium secondary battery cell before the sodium secondary battery cell 30 is charged; that is, the dimension of the housing cavity of the outer casing 31 along the width direction X of the sodium secondary battery cell before the outer casing 31 expands and deforms.

[0107] Along the width direction X of the sodium secondary battery cell, the ratio of the maximum dimension of the electrode assembly 32 to the dimension of the inner surface of the second wall 312 can be called the width direction group margin of the sodium secondary battery cell 30 (hereinafter referred to as the width direction group margin).

[0108] In some embodiments, the housing 31 further includes an end cap 314 and a housing 315. The housing 315 has an opening and includes two first walls 311, two second walls 312, and a bottom wall 316. The two first walls 311 and the two second walls 312 surround the bottom wall 316. The end cap 314 and the bottom wall 316 are disposed opposite each other in the height direction Z of the battery cell. The end cap 314 and the bottom wall 316 are two third walls 313. The end cap 314 closes the opening.

[0109] In some embodiments, the sodium secondary battery cell 30 further includes an electrode terminal 33, which may be disposed on the end cap 314.

[0110] In some embodiments, the electrode assembly 32 has a flat structure, and the sodium secondary battery cell 30 is a square, hard-shell battery cell.

[0111] The electrode assembly 32 includes a positive electrode, a negative electrode, and a separator. The positive electrode, negative electrode, and separator can be formed into the electrode assembly 32 through a winding process or a stacking process. One or more electrode assemblies 32 are encapsulated within a receiving cavity. When multiple electrode assemblies 32 are encapsulated within the receiving cavity, the maximum size of the electrode assembly 32 along its thickness direction refers to the maximum size of all electrode assemblies 32 as a whole in the thickness direction of the electrode assembly. Electrolyte is immersed in the electrode assembly 32.

[0112] It should be noted that when the second wall 312 and the first wall 311 are connected by an arc transition, the dimension of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell includes the dimension of the arc. For example, the dimension of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell can be the dimension of the inner surfaces of the two first walls 311 in the width direction X of the sodium secondary battery cell before the electrode assembly 32 is assembled into the housing 315, where the inner surface of the first wall 311 refers to the surface of the first wall 311 facing the electrode assembly 32.

[0113] In some embodiments, the method for measuring the maximum size of the electrode assembly 32 in the width direction X of the sodium secondary battery cell is as follows: at 25°C, the sodium secondary battery cell 30 is scanned along the height direction Z of the sodium secondary battery cell using a nanoVoxel-5000 CT system to obtain a tomographic image of the electrode assembly 32, and the maximum size of the tomographic image in the width direction X of the sodium secondary battery cell is measured.

[0114] For a sodium secondary battery cell 30, the dimension of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell is a constant value. During the charge and discharge cycle of the sodium secondary battery cell 30, the dimension of the inner surface of the second wall 312 in the width direction X of the battery cell remains unchanged.

[0115] One method for measuring the dimension of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell is as follows: at 25°C, the sodium secondary battery cell 30 is scanned along the height direction Z of the sodium secondary battery cell using a nanoVoxel-5000 CT system to obtain a tomographic image of the outer shell 31. On the obtained tomographic image, the dimension of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell is measured.

[0116] Another method for measuring the dimension of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell is as follows: First, measure the width of the sodium secondary battery cell 30 with a micrometer. On the outer surface of the first wall 311, take a point on the first wall 311 that is 3mm away from the bottom wall 316 in the height direction Z of the sodium secondary battery cell and 3mm away from the second wall 312 in the length direction Y of the sodium secondary battery cell as the measurement point. The distance between these two measurement points on the first wall 311 in the width direction X of the sodium secondary battery cell is the width of the sodium secondary battery cell 30. Then, disassemble the sodium secondary battery cell 30 and measure the thickness of the two first walls 311 with a micrometer. Subtract the thickness of the two first walls 311 from the width of the sodium secondary battery cell 30 to obtain the dimension of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell.

[0117] The width group margin of the sodium secondary battery cell 30 corresponds to the expansion force of the sodium secondary battery cell 30. During the same charging process, the maximum value of the width group margin of the sodium secondary battery cell 30 can be reflected by the width group margin of the sodium secondary battery cell 30 when the expansion force is at its maximum value. The method for testing the maximum value of the width group margin of the sodium secondary battery cell 30 before it reaches full charge is as follows: At 25°C, firstly, select multiple sodium secondary battery cells 30 of the same specification (multiple cells can be selected from one battery 100); secondly, place one of the sodium secondary battery cells 30 between two parallel steel plates, and install a pressure sensor on the side of one of the steel plates facing the corresponding first wall 311. Move the steel plate until it contacts the corresponding first wall 311 and fix the positions of the two steel plates. At this time, the pressure sensor is clamped between the first wall 311 and the corresponding steel plate. The steel plates do not apply force to the sodium secondary battery cell 30. Charge the sodium secondary battery cell 30 to full charge at a constant current rate of 0.5C, and obtain the state of charge corresponding to the maximum pressure value; thirdly, charge another sodium secondary battery cell 30 of the same specification to the above-mentioned state of charge, stop charging, and along the height direction Z of the sodium secondary battery cell using a nanoVoxel-5000... The CT system performs a CT scan on the sodium secondary battery cell 30 to obtain the dimensions of the electrode assembly 32 in the width direction X of the sodium secondary battery cell and the dimensions of the inner surface of the second wall 312 in the width direction X of the sodium secondary battery cell. The maximum value of the width direction group margin of the sodium secondary battery cell 30 before full charge is calculated.

[0118] Among them, the sodium secondary battery cell 30, also known as the cell, is the most basic unit of the battery 100.

[0119] Meanwhile, during charging and discharging, the volume change of the electrode assembly 32 leads to differences in the width group margin under different states of charge (SOC) or different voltages. That is, the instantaneous width group margin of the sodium secondary battery cell 30 changes. For example, the width group margin in a fully discharged state (0% SOC) is typically different from the width group margin (full charge group margin) in a fully charged state (100% SOC). In this embodiment, unless otherwise specified, the width group margin of the sodium secondary battery cell 30 refers to the instantaneous group margin of the sodium secondary battery cell 30. For example, the full charge group margin refers to the width group margin of the sodium secondary battery cell 30 in a fully charged state, and the x% SOC group margin refers to the width group margin when the SOC is x%.

[0120] According to the sodium secondary battery cell 30 of this application embodiment, before the sodium secondary battery cell 30 reaches full charge, the maximum group margin in the width direction is A2, and when the sodium secondary battery cell 30 reaches full charge, the group margin in the width direction is A1, where A1≤A2. Before reaching full charge, the sodium secondary battery cell 30 has a maximum group margin in the width direction. After the sodium secondary battery cell 30 reaches this maximum group margin during charging, if the sodium secondary battery cell 30 continues to be charged, the overall trend of the group margin in the width direction of the sodium secondary battery cell 30 is to decrease or remain basically unchanged. Therefore, by utilizing the above principle, the maximum group margin before full charge can be set to a value as large as possible while meeting safety requirements, thereby allowing as much electrode active material as possible to be placed in the casing 315 of the sodium secondary battery cell 30, thus increasing the energy density of the sodium secondary battery cell 30.

[0121] Group margin is generally positively correlated with the expansion force of the sodium secondary battery cell 30; the larger the group margin, the greater the expansion force of the sodium secondary battery cell 30. In this application, after the expansion force of the sodium secondary battery cell 30 reaches its maximum value, continued charging results in a general decreasing or essentially unchanged trend in the expansion force, without further increasing it. Therefore, configuring the group margin of the sodium secondary battery cell 30 using this scheme can improve energy density while the expansion force does not increase or increases only slightly, thus balancing energy density improvement with safety performance in the later stages of cycling.

[0122] In some embodiments, the sodium secondary battery cell 30 is a fresh cell, the charge-discharge cycle number of the sodium secondary battery cell 30 is less than or equal to 100, and A1 is 94% to 96%.

[0123] For example, A1 can be any one of the point values ​​of 94%, 94.5%, 95%, 95.5%, and 96%, or a range between any two point values.

[0124] When a fresh sodium secondary battery cell 30 is fully charged, A1 meets the above range, which can balance energy density and safety. Since the expansion force of the sodium secondary battery cell 30 is low, more active materials can be placed inside the casing 31 under the premise of meeting safety requirements, which can improve energy density while the expansion force does not increase or increases very little.

[0125] In some embodiments, the charge-discharge cycle number of the sodium secondary battery cell 30 is less than or equal to 100, and A2 is 98% to 103%.

[0126] For example, A2 can be any one of the point values ​​of 98%, 98.5%, 99%, 99.5%, 100%, 100.5%, 101%, 101.5%, 102%, 102.5%, 103%, or a range between any two point values.

[0127] Before the fresh sodium secondary battery cell 30 reaches full charge, A2 meets the above range. The expansion force of the sodium secondary battery cell 30 is relatively large, but it is still within the acceptable range of safety performance.

[0128] In some embodiments, the sodium secondary battery cell 30 is designed with a minimum voltage of not less than 1.5V and a maximum voltage of not more than 4.2V.

[0129] In some embodiments, the sodium secondary battery cell 30 is designed with a minimum voltage of 1.5V and a maximum voltage of 4.2V.

[0130] Optionally, the sodium secondary battery cell 30 is designed with a minimum voltage of 2.0V and a maximum voltage of 4.1V.

[0131] For example, the minimum design voltage of the sodium secondary battery cell 30 can be any one of 1.5V, 1.6V, 1.7V, 1.8V, 1.9V, 2.0V, and 2.1V, or a range between any two of these values. The maximum design voltage of the sodium secondary battery cell 30 can be any one of 3.8V, 3.9V, 4.0V, 4.1V, and 4.2V, or a range between any two of these values.

[0132] The design voltage is the minimum discharge voltage and the maximum recharge voltage specified for the battery in the electrical device (such as a vehicle). For example, the minimum design voltage of the sodium secondary battery cell 30 is 1.5V, and the minimum discharge voltage of the sodium secondary battery cell 30 is 1.5V; the maximum design voltage of the sodium secondary battery cell 30 is 4.2V, and the maximum recharge voltage of the sodium secondary battery cell 30 is 4.2V.

[0133] When the minimum design voltage of the sodium secondary battery cell 30 is 1.5V and the maximum design voltage of the sodium secondary battery cell 30 is 4.2V, the expansion force of the sodium secondary battery cell 30 is the greatest before it reaches full charge during the same charge-discharge cycle. As charging continues, the expansion force decreases when the sodium secondary battery cell 30 is fully charged.

[0134] In some embodiments, the number of charge-discharge cycles of the sodium secondary battery cell 30 is less than or equal to 100, and when the state of charge of the sodium secondary battery cell 30 is greater than 65%, the maximum value of the width direction group margin of the sodium secondary battery cell 30 is ≤98%.

[0135] For example, if the number of charge-discharge cycles of the sodium secondary battery cell 30 is less than or equal to 100, and the state of charge of the sodium secondary battery cell 30 is greater than 65%, the maximum value of the width group margin of the sodium secondary battery cell 30 can be any one of 96%, 96.5%, 97%, 97.5%, 98%, or a range between any two of these values.

[0136] In some of the above-mentioned schemes, meeting the above conditions can ensure that the expansion force of the sodium secondary battery cell 30 is small and the energy density of the sodium secondary battery cell 30 is high, thus balancing safety performance and energy density improvement.

[0137] In some embodiments, the number of charge-discharge cycles of the sodium secondary battery cell 30 is less than or equal to 100, and when the state of charge of the sodium secondary battery cell 30 is 40% to 65%, the maximum value of the width direction group margin of the sodium secondary battery cell 30 is configured to be 98% to 103%.

[0138] For example, if the charge-discharge cycle number of the sodium secondary battery cell 30 is less than or equal to 100, and the state of charge of the sodium secondary battery cell 30 is 40% to 65%, the maximum value of the width group margin of the sodium secondary battery cell 30 can be any one of 98%, 98.5%, 99%, 99.5%, 100%, 100.5%, 101%, 101.5%, 102%, 102.5%, and 103%, or a range between any two of these values.

[0139] In this application, the group margin in the width direction of the sodium secondary battery cell 30 at a state of charge of 45% to 65% is configured to be close to or equal to 100%. This allows the sodium secondary battery cell 30 to have a high group margin as much as possible, thereby increasing the energy density. At the same time, the expansion force of the sodium secondary battery cell 30 does not increase significantly with the increase of the state of charge.

[0140] The state of charge (SOC) of the sodium secondary battery cell 30 can be directly measured using relevant analytical instruments, such as an in-situ expansion analyzer or an electrochemical workstation, or by referring to GB / T 31486-2015. For example, the SOC testing method is as follows: First, estimate the total discharge capacity A of the battery according to the lithium battery rated capacity test standard in GB / T 31486-2015; then, after charging the battery to SOC B, discharge it to 0% SOC to release capacity b (charge and discharge conditions refer to GB / T 31486-2015); calculate the battery SOC B = b / A * 100%.

[0141] In some embodiments, when the sodium secondary battery cell 30 has been charged and discharged more than 200 times, the width group margin of the sodium secondary battery cell 30 is 99.5% to 102% when the sodium secondary battery cell 30 is fully charged.

[0142] Optionally, the width group margin of the sodium secondary battery cell 30 is 99.5% to 101%.

[0143] For example, when the sodium secondary battery cell 30 has been charged and discharged more than 200 times, the width group margin of the sodium secondary battery cell 30 in the fully charged state can be any one of 99.5%, 100%, 100.5%, 101%, 101.5%, 102%, or a range between any two of these values.

[0144] The width group margin of sodium secondary battery cell 30 meets the above range. Considering the shrinkage of the positive electrode when fully charged and the increase in the thickness of the negative electrode SEI film after more than 200 cycles, the growth of battery expansion force can be significantly delayed while taking into account high energy density and lifespan.

[0145] In some embodiments, the electrode assembly 32 includes a positive electrode and a negative electrode, wherein the thickness ratio of the positive electrode to the negative electrode is 0.65 to 0.75.

[0146] For example, the ratio of the thickness of the positive electrode to the thickness of the negative electrode can be any one of the following values: 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, or a range between any two values.

[0147] The ratio of the thickness of the positive electrode to the thickness of the negative electrode should meet the above range. This reduces the risk of metal ion precipitation and avoids excessive thickness of the positive and negative electrodes, which would waste volume.

[0148] In some embodiments, the electrode assembly 32 includes a negative electrode sheet, which includes a negative current collector. The negative current collector is an aluminum foil with a thickness of 10 μm to 12 μm.

[0149] For example, the thickness of the aluminum foil can be any one of the following values, or a range between any two values: 10μm, 10.1μm, 10.2μm, 10.3μm, 10.4μm, 10.5μm, 10.6μm, 10.7μm, 10.8μm, 10.9μm, 11μm, 11.1μm, 11.2μm, 11.3μm, 11.4μm, 11.5μm, 11.6μm, 11.7μm, 11.8μm, 11.9μm, and 12μm.

[0150] The negative electrode current collector is aluminum foil (which can be considered an aluminum-based current collector). The negative electrode will not undergo an alloying reaction with the aluminum-based current collector. By reducing the local current density of the aluminum-based current collector in the negative electrode of the sodium secondary battery cell, sodium dendrite growth and volume expansion can be suppressed. The thickness of the aluminum foil satisfies the above relationship, resulting in a relatively thin negative electrode sheet that occupies less space.

[0151] In some embodiments, during a single charge, the group margin in the width direction of the sodium secondary battery cell 30 exhibits a trend of first increasing and then decreasing. That is, as the SOC of the sodium secondary battery cell 30 increases from 0% to 100%, the group margin of the sodium secondary battery cell 30 first increases and then decreases, and the corresponding expansion force also first increases and then decreases. Therefore, the growth rate of the expansion force of the sodium secondary battery cell 30 at full charge is smaller compared to the expansion force before reaching full charge.

[0152] In some embodiments, the operating voltage range of the sodium secondary battery cell 30 is 1.5V to 4.2V, optionally 1.5V to 4.0V, and even more optionally 2.0V to 4.0V. In the embodiments of this application, the sodium secondary battery cell 30, after reaching its maximum width group margin during charging, essentially stops expanding. Therefore, the sodium secondary battery cell 30 can continue to be charged at a higher voltage, and charging will not stop before or at the point of reaching the maximum width group margin. This allows the positive electrode active material in the sodium secondary battery cell 30 to continue releasing metal ions for electrochemical reactions, which is beneficial for obtaining higher energy density.

[0153] The operating voltage of sodium secondary battery cell 30 is the voltage during charging and discharging.

[0154] As an example, under this operating voltage, the SOC and operating voltage have the corresponding relationship shown in Figure 6. For example, 45% to 65% SOC corresponds to a voltage of 3V to 3.4V, 80% to 95% SOC corresponds to a voltage of 3.55V to 3.75V, and 100% SOC corresponds to 4.2V.

[0155] In some embodiments, during a single charge, the rebound rate of the positive electrode sheet contained in the sodium secondary battery cell 30 exhibits a trend of first increasing and then decreasing. The rebound rate of the positive electrode sheet refers to the percentage increase in the thickness of the positive electrode sheet, i.e., the rebound rate of the positive electrode sheet = (positive electrode sheet thickness after charging - initial positive electrode sheet thickness) / initial positive electrode sheet thickness * 100%. The thickness of the positive electrode sheet can be measured using a micrometer.

[0156] The expansion of the sodium secondary battery cell 30 mainly originates from the thickness expansion of the electrode sheets. During the first charge, the rebound rate of the positive electrode sheet shows a trend of first increasing and then decreasing. That is, as the state of charge of the sodium secondary battery cell 30 increases from 0% to 100%, the rebound rate of the positive electrode sheet first increases and then decreases. The expansion of the negative electrode sheet increases continuously with the increase of the state of charge, and the corresponding expansion force of the sodium secondary battery cell 30 also shows a trend of first increasing and then decreasing. Therefore, the growth rate of the expansion force of the sodium secondary battery cell 30 at full charge compared to the expansion force before charging is relatively small.

[0157] In some embodiments, when the state of charge of the sodium secondary battery cell 30 is 45% to 65%, the rebound rate of the positive electrode is 5% to 8%, optionally 5% to 6%; for example, it can be any one of 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8% or a range between any two of these values.

[0158] When the state of charge of sodium secondary battery cell 30 is greater than 65%, the rebound rate of the positive electrode is less than or equal to 5%.

[0159] When the state of charge is 45% to 65%, the positive electrode has a high rebound rate. However, as the state of charge increases, the rebound rate of the positive electrode decreases. This is beneficial for making the full-charge width group margin of the sodium secondary battery cell 30 smaller than the maximum width group margin before full charge. This is beneficial for improving the energy density of the sodium secondary battery cell 30 and reducing the expansion force of the sodium secondary battery cell 30 under high group margin.

[0160] In some embodiments, when the state of charge of the sodium secondary battery cell 30 is 80% to 95%, the rebound rate of the positive electrode is 2% to 4.5%, optionally 2% to 3%; for example, it can be any one of 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or a range between any two of these values.

[0161] When the state of charge of the sodium secondary battery cell 30 is 95% to 100%, the rebound rate of the positive electrode is 0% to 1.9%, optionally 0% to 1.5%, for example, it can be any one of 0%, 0.5%, 1%, 1.5%, 1.9%, or a range between any two of these values.

[0162] When the charge is close to full charge, the electrode rebound rate gradually decreases as the state of charge increases. This allows the full charge width margin of the sodium secondary battery cell 30 to be smaller than the maximum width margin before full charge, which is beneficial to improving the energy density of the sodium secondary battery cell 30 and reducing the expansion force of the sodium secondary battery cell 30 under high width margin.

[0163] In some embodiments, the electrode assembly 32 includes a positive electrode sheet containing a positive electrode active material. When the sodium secondary battery cell 30 is fully charged, the cell length of the positive electrode active material in at least one crystal axis direction is less than the maximum cell length in that crystal axis direction before full charging. The cell length along the crystal axis direction can represent the size of the cell, which is the distance from a given atom to the same atom at the same position and direction in adjacent cells. It is an important lattice parameter (lattice parameters typically include the cell lengths along the three crystal axis directions a, b, and c, and the angles between the three crystal axes). Optionally, in the embodiments of this application, when the sodium secondary battery cell 30 is fully charged, the cell length in the c-axis direction of the positive electrode active material is less than the maximum cell length in that direction before the sodium secondary battery cell 30 reaches a fully charged state.

[0164] By employing a positive electrode active material with this structural characteristic in the sodium secondary battery cell 30, the rebound rate of the positive electrode sheet under full charge can be reduced compared to the rebound rate before full charge. This results in the full charge group margin of the sodium secondary battery cell 30 being lower than the maximum group margin before full charge, which is beneficial for improving the energy density of the sodium secondary battery cell 30 and reducing the expansion force of the sodium secondary battery cell 30 under high group margin. This phenomenon is speculated to occur because during the phase reconstruction process of the positive electrode active material, as cycling progresses, the crystal structure of the reconstructed phase in the mixed phase, after being extracted to the maximum interlayer spacing, is not easily reversed back to the initial phase. Therefore, the gradual increase in the proportion of the reconstructed phase leads to an increase in expansion force. Furthermore, as the operating voltage continues to increase, some phases collapse, leading to expansion and contraction.

[0165] In some embodiments, electrode assembly 32 includes a positive electrode sheet, which includes a positive active material, including a nickel-manganese-iron-based sodium oxide.

[0166] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide is Na. q Ni x Mn y Fe z M p O2, where 0 < q ≤ 1, 0 < x, 0 < y, 0 < z, 0 ≤ p, x + y + z ≥ 0.81, x + y + z + p ≤ 1, and M includes one or more of V, Cr, Zn, Al, Sc, Sn, Sb, Zr, Nb, Ti, Mg, Cu, Ru, and Ir.

[0167] Layered oxide Na q Ni x Mn y Fe z M pWith increasing sodium removal, the c-axis cell length of O2 initially increases and then decreases. Using the aforementioned layered oxide as the positive electrode active material, during a single charge, Na... q Ni x Mn y Fe z M p O2 undergoes a sodium removal reaction, and as the charging voltage gradually increases, or as the state of charge of the sodium secondary battery cell 30 gradually increases, Na... q Ni x Mn y Fe z M p The amount of sodium removed from O2 also increases, then Na q Ni x Mn y Fe z M p The c-axis unit cell length of O2 first increases and then decreases, as shown in Figures 7 and 8. In Figure 7, the vertical axis represents the c-axis unit cell length, which increases from smallest to largest according to the direction of the arrows. Figure 7 shows the effect of increasing sodium removal, i.e., Na... q Ni x Mn y Fe z M p As the Na content of O2 decreases, its c-axis cell length first increases and then decreases. Figure 8 shows the Na content (i.e., the subscript q of Na) of 0.89. q Ni x Mn y Fe z M p The change in cell structure during the process of removing sodium from O2 (Na0.89) to a Na content of 0.11 (Na0.11). This cell structure characteristic leads to the change in Na... q Ni x Mn y Fe z M p During the charging process, O2 exhibits a macroscopic phenomenon of first expanding and then contracting, which in turn causes the rebound rate of the positive electrode to also exhibit the characteristic of first expanding and then contracting. Consequently, the full charge margin of the sodium secondary battery cell 30 is less than the maximum margin before full charge, which is beneficial to improving the energy density of the sodium secondary battery cell 30 and reducing the expansion force of the sodium secondary battery cell 30 under high margin.

[0168] In some embodiments, the total mass percentage of Ni, Mn, and Fe in the nickel-manganese-iron-based sodium-containing oxide is 40%–50%, optionally 42%–46%, for example, it can be any one or a range between any two of 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, and 50%. At this ratio, Na… q Ni x Mn y Fe z M p The c-axis cell length of O2 shows a trend of first increasing and then decreasing with the increase of sodium removal.

[0169] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1 ≤ q ≤ 1. Optionally, 0.2 ≤ q ≤ 0.9, and even more preferably, 0.4 ≤ q ≤ 0.6. For example, q can be, but is not limited to, any one of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or a range between any two of these values. q can reflect the Na content in the positive electrode active material. With a suitable Na content, the positive electrode active material can provide active Na during the charge-discharge process of the sodium secondary battery cell. + This promotes the electrochemical reaction of the sodium secondary battery cell 30, which is beneficial to improving the energy density of the sodium secondary battery cell 30.

[0170] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1 ≤ x ≤ 0.5. Optionally, 0.2 ≤ x ≤ 0.4. For example, x can be any one of 0.1, 0.2, 0.3, 0.4, 0.5, or a range between any two of these values.

[0171] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1 ≤ y ≤ 0.5. Optionally, 0.2 ≤ y ≤ 0.4. For example, y can be any one of the values ​​of 0.1, 0.2, 0.3, 0.4, and 0.5, or a range between any two of these values.

[0172] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium oxide satisfies 0.1≤z≤0.5, for example, z can be any one of 0.1, 0.2, 0.3, 0.4, 0.5 or a range between any two of these values.

[0173] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium oxide satisfies 0.2≤z≤0.3.

[0174] The molar proportion of Fe in the entire cathode material is 0.2-0.3%. Excessive Fe doping can lead to the migration of the transition metal layer, causing irreversible changes in the crystal structure and deteriorating the lifespan of the battery cell. Insufficient Fe can prevent the material from effectively suppressing phase transitions, resulting in a decrease in the 30-hour power performance of sodium secondary battery cells.

[0175] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.8 ≤ x + y + z ≤ 1. Optionally, 0.9 ≤ x + y + z ≤ 1. For example, x + y + z can be any one of 0.8, 0.85, 0.9, 0.95, 1, or a range between any two of these values.

[0176] In some embodiments, the general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0 ≤ p ≤ 0.2. Optionally, 0 ≤ p ≤ 0.1. For example, p can be any one of the values ​​of 0, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, or a range between any two values.

[0177] In some embodiments, when the state of charge of the sodium secondary battery cell 30 is 0%, Na q Ni x Mn y Fe z M p The c-axis unit cell length of O2 is c1. For example, it could be The range between any one or both of the point values ​​in the range;

[0178] When the state of charge of sodium secondary battery cell 30 is 50%–60%, Na q Ni x Mn y Fe z M p The c-axis unit cell length of O2 is c2. For example, it could be The range between any one or both of the point values ​​in the range;

[0179] When the state of charge of sodium secondary battery cell 30 is 95%–100%, Na q Ni x Mn y Fe z M p The c-axis unit cell length of O2 is c3. For example, it could be The range between any one or both of the point values ​​in the range; and c1 < c2, c3 < c2.

[0180] The cell length can be determined by measuring the interplanar spacing of the material using X-ray diffraction combined with Bragg's equation, and then calculated using the formula relating interplanar spacing to cell length. As the state of charge increases, Na... q Ni x Mn y Fe z M p The c-axis cell length of O2 first increases and then decreases, which is beneficial to control the full charge group margin of sodium secondary battery cell 30 to be less than the maximum group margin before full charge, which is beneficial to improve the energy density of sodium secondary battery cell 30 and reduce the expansion force of sodium secondary battery cell 30 under high group margin.

[0181] In some embodiments, the electrode assembly 32 includes a negative electrode sheet containing a negative electrode active material. The porosity of the negative electrode active material is 40% to 70%, optionally 45% to 55%, for example, it can be any one of 40%, 42%, 45%, 46%, 48%, 50%, 52%, 54%, 55%, 56%, 58%, 60%, 62%, 64%, 65%, 66%, 68%, 70%, or a range between any two values. The porosity of the negative electrode active material can be obtained by gas adsorption. The abundant pores of the negative electrode active material facilitate the full wetting of it by the electrolyte, thereby enabling the active ions in the sodium secondary battery cell 30 to be effectively transported between the positive and negative electrode sheets through the electrolyte, improving ion transport efficiency and enhancing the electrochemical performance of the sodium secondary battery cell 30. Meanwhile, the presence of more pores can also buffer the volume expansion of the negative electrode sheet when active ions are embedded in the negative electrode active material and volume expansion occurs.

[0182] In some embodiments, the electrode assembly 32 includes a negative electrode sheet containing a negative electrode active material. The average pore size of the negative electrode active material is 1 nm to 30 nm, optionally 1 nm to 10 nm, for example, any one or a range between any two of the following values: 1 nm, 2 nm, 4 nm, 6 nm, 8 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm, 22 nm, 24 nm, 26 nm, 28 nm, and 30 nm. The average pore size of the negative electrode active material can be obtained by gas adsorption. The nanopores facilitate the entry of electrolyte into the interior of the negative electrode active material through capillary forces, thereby improving the electrolyte wetting performance of the negative electrode sheet.

[0183] In some embodiments, the electrode assembly 32 includes a negative electrode sheet, the negative electrode sheet includes a negative electrode active material, and the negative electrode active material includes hard carbon.

[0184] Hard carbon has low expansion force. Using it as a negative electrode active material can help reduce the expansion force of the negative electrode during charging and discharging, thereby reducing the expansion force of the sodium secondary battery cell 30.

[0185] For more detailed technical features of each component in the sodium secondary battery cell 30, please refer to the following:

[0186] 1. Electrode assembly

[0187] The electrode assembly 32 is an important component of the sodium secondary battery cell 30, and it typically includes a positive electrode, a negative electrode, and a separator. When the secondary battery is a solid-state battery, the electrode assembly may include a positive electrode, a solid electrolyte, and a negative electrode.

[0188] [Positive electrode plate]

[0189] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0190] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0191] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0192] In some embodiments, the battery cell is a sodium-ion battery, and the positive electrode active material can be a positive electrode active material known in the art for use in sodium-ion batteries. As an example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc., and other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries can also be used. For example, as an optional technical solution in this application, the transition metal in the sodium transition metal oxide can be at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, Na. x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, 0 <x≤1。

[0193] As an optional technical approach in this application, the polyanionic compound can be a compound containing sodium ions, transition metal ions, or a tetrahedral (YO4) structure. n- A class of compounds with anionic units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents (YO4). n- The valence state. Polyanionic compounds can also have sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds containing anionic units and halide anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si, and n represents (YO4). n- The valence state; the halogen can be at least one of F, Cl, and Br. Polyanionic compounds can also have sodium ions, tetrahedral (YO4) valence states. n- Anionic unit, polyhedral unit (ZO) y ) m+ And a class of compounds with optional halide anions. Y can be at least one of P, S, and Si, and n represents (YO4). n- Valence state: Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; m represents (ZO) y ) m+ The valence state; the halogen can be at least one of F, Cl, and Br. Examples of polyanionic compounds include NaFePO4, Na3V2(PO4)3, NaM'PO4F (M' is one or more of V, Fe, Mn, and Ni), and Na3(VO4)2(PO4)3. y )2(PO4)2F 3-2y At least one of (0≤y≤1).

[0194] As an optional technical approach in this application, the polyanionic compound can be Na... x-a A a V y-b M b (PO4) 2-2c (DO4) 2c F z-d Q d, wherein the A element represents an alkali metal element that dopes and replaces the Na element, the M element represents a metal element that replaces the V element, the D element represents a doping element that replaces the P element, the Q element represents a doping element that replaces the F element, the D element includes at least one of Si and S, and the Q element includes at least one of Cl and O; 3.5 ≤ x ≤ 4.5, 0 ≤ a ≤ 0.15x, 0.8 ≤ y ≤ 1.1, 0 ≤ b ≤ 0.3y, 0 ≤ c ≤ 0.15, 0.8 ≤ z ≤ 1.1, 0 ≤ d ≤ 0.2z. Optionally, the A element includes at least one of K and Li; the M element includes at least one of Fe, Cr, Al, Sc, Ga, In, Ti, Zr, Mn, Zn, Ni, Cu, and Co.

[0195] As an optional technical solution of the present application, the polyanionic compound may be Na x R y (PO4)2P2O7, wherein, x = 3.5 - 4.5, y = 2.75 - 3.25, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb.

[0196] As an optional technical solution of the present application, the polyanionic compound may be Na 4+x R 3-y P 4-m O 15 / C; wherein, 0 < x < 0.5, 0 < y ≤ 0.5, 0 < m ≤ 0.2, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb.

[0197] The Prussian blue compound may be a type of compound having sodium ions, transition metal ions, and cyanide ions (CN-). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The Prussian blue compound is, for example, Na a Me b Me’ c (CN)6, wherein Me and Me’ are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a ≤ 2, 0 < b < 1, 0 < c < 1.

[0198] In some embodiments, the positive electrode active material includes at least one of a sodium-containing layered oxide, a polyanionic sodium compound, and a Prussian blue sodium compound.

[0199] In some embodiments, the sodium-containing layered oxide is an iron-manganese layered oxide, specifically including at least one of nickel-iron-manganese layered oxide and copper-iron-manganese layered oxide.

[0200] During the charging and discharging process, active ions (Na+) are intercalated and deintercalated, resulting in a different molar content of Na at different discharge states. In the examples of positive electrode active materials in this application, the molar content of Na refers to the initial state of the material, i.e., the state before feeding. When the positive electrode active material is applied to the battery system, the molar content of Na changes after charge-discharge cycles.

[0201] In the examples of positive electrode active materials in this application, the molar content of oxygen is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of oxygen will fluctuate.

[0202] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0203] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0204] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0205] [Negative electrode plate]

[0206] The negative electrode includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector.

[0207] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0208] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil or copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0209] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may be any negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0210] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0211] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0212] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0213] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0214] In other embodiments, the current collector of the negative electrode sheet typically includes a current collector body and a base coating. The base coating can be disposed on at least one side of the current collector body. The base coating basically does not contain negative electrode active material, and may include a small amount of carbon material. However, the carbon material forms a thin coating and cannot function as a negative electrode active material. In this embodiment, the negative electrode sheet can be an electrode sheet without a negative electrode active material layer. For a negative electrode sheet without a negative electrode active material layer, when the current collector of the negative electrode sheet does not contain a base coating, the film layer can be disposed on the surface of at least one side of the current collector; when the current collector of the negative electrode sheet includes a base coating, the film layer can be disposed on the surface of the base coating away from the current collector.

[0215] In some embodiments, the film layer may further include a binder for fixing the additive to the negative electrode sheet. The type of binder is not particularly limited, and those skilled in the art can choose flexibly according to actual needs.

[0216] [Electrolytes]

[0217] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0218] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0219] In some embodiments, the battery cell is a sodium-ion battery, and the electrolyte salt may be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium difluorosulfonamide, sodium ditrifluoromethanesulfonamide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluorooxalate borate, sodium dioxalate borate, sodium difluorodioxalate phosphate, and sodium tetrafluorooxalate phosphate.

[0220] In some embodiments, the battery cell is a lithium-ion battery, and the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0221] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0222] In some embodiments, the solvent of the electrolyte in the sodium-ion battery includes carbonates, which include at least one of ethylene carbonate, propylene carbonate, or fluoroethylene carbonate.

[0223] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0224] [Isolation membrane]

[0225] In some embodiments, the battery cell also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0226] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0227] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0228] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0229] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0230] This application also provides a battery 100, including the sodium secondary battery cell 30 described above.

[0231] The sodium secondary battery cell 30 of this application embodiment combines high group margin, high energy density, and low expansion force. Therefore, the battery 100 including the sodium secondary battery cell 30 also exhibits the characteristics of high group margin, high energy density, and low expansion force. Moreover, since the sodium secondary battery cell 30 has low expansion force, it does not cause the top shell to expand or deform when fully charged. Therefore, the battery 100 composed of the sodium secondary battery cell 30 can have high reliability.

[0232] Please refer to Figure 9, which is a schematic diagram of a battery according to one embodiment of this application. In some embodiments, there are multiple sodium secondary battery cells 30, and the multiple sodium secondary battery cells 30 are arranged along the width direction X of the sodium secondary battery cells; the battery 100 also includes a heat insulation pad 40, which is disposed between two adjacent sodium secondary battery cells 30.

[0233] The width direction X of a sodium secondary battery cell can be parallel to the length direction of the battery 100.

[0234] Multiple sodium secondary battery cells 30 are arranged along the width direction X of the sodium secondary battery cells, which facilitates the assembly of the sodium secondary battery cells 30. The heat insulation pad 40 can help reduce heat transfer between two adjacent sodium secondary battery cells 30, so that the battery 100 has high reliability.

[0235] In some embodiments, the thickness of the heat insulation pad 40 is 0.5 mm to 4 mm.

[0236] The thickness of the heat insulation pad 40 can be any one of the following values ​​or a range between any two values: 0.5mm, 0.75mm, 1mm, 1.25mm, 1.5mm, 1.75mm, 2mm, 2.25mm, 2.5mm, 2.75mm, 3mm, 3.25mm, 3.5mm, 3.75mm, and 4mm.

[0237] The thickness of the heat insulation pad 40 satisfies the above relationship, which on the one hand provides good heat insulation effect, and on the other hand occupies less assembly space.

[0238] In some embodiments, the capacity of the sodium secondary battery cell 30 is 50Ah to 150Ah, and the thickness of the heat insulation pad 40 is 0.4mm to 1.0mm; or, the capacity of the sodium secondary battery cell 30 is 151Ah to 250Ah, and the thickness of the heat insulation pad 40 is 1.1mm to 1.9mm.

[0239] The thickness of the heat insulation pad 40 varies depending on the capacity of the sodium secondary battery cell 30. When the capacity of the sodium secondary battery cell 30 is low (e.g., greater than or equal to 50Ah, less than or equal to 150Ah), the thickness of the heat insulation pad 40 can be thinner (e.g., greater than or equal to 0.4mm, less than or equal to 1.0mm); when the capacity of the sodium secondary battery cell 30 is high (e.g., greater than 150Ah, less than or equal to 250Ah), the thickness of the heat insulation pad 40 can be thicker (e.g., greater than or equal to 1.1mm, less than or equal to 1.9mm). The thickness of the heat insulation pad 40 satisfies the above relationship according to the capacity of the sodium secondary battery cell 30, thus meeting the heat insulation requirements while occupying less space.

[0240] In some embodiments, the heat insulation pad 40 is made of aerogel, ceramic, porous vacuum silicone, or asbestos.

[0241] Aerogel, ceramic, porous vacuum silica or asbestos all have good thermal insulation properties.

[0242] This application also provides an electrical device, which includes the above-mentioned sodium secondary battery cell 30, or the above-mentioned battery 100.

[0243] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0244] Example 1

[0245] This embodiment provides a sodium secondary battery cell, which includes a positive electrode, a negative electrode, a separator, and an electrolyte. The active material in the positive electrode is Na, with a molar percentage of Ni, Fe, and Mn of 45.5%. 0.96 Ni 0.22 Mn 0.33 Fe 0.33 Zn 0.08 O2, the active material of the negative electrode is hard carbon with a porosity of 50.6% and an average pore size of 40nm.

[0246] This sodium-based secondary battery cell can be prepared using the following method:

[0247] (1) Positive electrode plate

[0248] The positive electrode active material (Na) 0.96 Ni 0.22 Mn 0.33 Fe 0.33 Zn 0.08 O2), conductive agent (conductive carbon black), and binder (polyvinylidene fluoride, PVDF) are thoroughly mixed in a solvent (N-methylpyrrolidone, NMP) to obtain a positive electrode slurry. The mass ratio of the positive electrode active material, solvent, conductive agent, and binder is 95:35:3:2. The positive electrode slurry is coated onto both surfaces of a current collector (aluminum foil), with a single-sided area of ​​1540.25 mm². 2 The single-sided loading of the positive electrode active material on the current collector was 300 mg; after drying and cold pressing, a compacted density of 3.1 g / cm³ was obtained. 3 The positive electrode sheet.

[0249] (2) Negative electrode plate

[0250] The negative electrode active material (hard carbon), conductive agent (conductive carbon black), and binder (carboxymethyl cellulose, CMC) were thoroughly mixed in deionized water to obtain a negative electrode slurry. The mass ratio of the negative electrode active material, deionized water, conductive agent, and binder was 95:50:2:3. The negative electrode slurry was then coated onto a current collector (copper foil), with a single-sided area of ​​1540.25 mm². 2 The material was dried and cold-pressed to obtain a compacted density of 0.95 g / cm³. 3 The negative electrode sheet.

[0251] (3) Electrolyte

[0252] A 1 mol / L NaPF6 solution was used as the electrolyte, which was a mixed solvent of ethylene carbonate EC: propylene carbonate PC: fluoroethylene carbonate FEC = 47.5: 47.5: 5 (volume ratio).

[0253] (4) Separating membrane

[0254] Polyethylene film is used as the separation membrane.

[0255] (5) Sodium secondary battery cell

[0256] The positive electrode sheet, polyethylene film, and negative electrode sheet are stacked in sequence and wound to form a square electrode assembly (bare cell). The electrode assembly is then pressed into a flat shape. The bare cell is placed into a square aluminum shell, electrolyte is added, and the shell is sealed to produce a square-shell sodium secondary battery cell. It is then formed and left to stand.

[0257] To ensure that the bare cell has sufficient expansion space in the square aluminum casing, the initial value of the width direction group margin of the sodium secondary battery cell is set to 92% in this embodiment.

[0258] Example 2

[0259] This embodiment provides a sodium secondary battery cell, which differs from Embodiment 1 in that: the positive electrode active material is Na with a molar mass ratio of Ni, Fe, and Mn of 45.8%. 0.87 Ni 0.20 Mn 0.36 Fe 0.28 Cu 0.06 O2.

[0260] Example 3

[0261] This embodiment provides a sodium secondary battery cell, which differs from Embodiment 1 in that: the positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 45.86%. 0.92 Ni 0.2 Mn 0.3 Fe 0.3 O2.

[0262] Within the operating voltage range of 1.5V to 4.2V, several different cutoff voltages were selected (the voltage at which a sodium secondary battery cell is charged to its highest point within the operating voltage range is called the cutoff voltage, as shown in the table below: 3.55V, 3.6V, 3.65V, 3.7V, 3.75V, 3.8V, 3.85V, 3.9V, 3.95V, 4.0V, 4.1V, 4.2V), and charge-discharge tests were performed on the sodium secondary battery cells of Examples 1 to 3 at different cutoff voltages.

[0263] The charge and discharge test procedure is as follows: At 25℃, charge at a constant current of 0.5C to the cutoff voltage, and then charge at a constant voltage of 0.05C at the cutoff voltage. At this time, the sodium secondary battery cell reaches the fully charged state. After that, let it stand for 5 minutes, discharge at a constant current of 0.5C to the voltage of 1.5V, and then let it stand for another 5 minutes to complete one cycle of charge and discharge.

[0264] During the 500th charge-discharge cycle, the sodium secondary battery cells were disassembled and analyzed after being fully charged at different cutoff voltages. The electrode rebound rate and bare cell thickness expansion rate were measured. Please refer to the method attached at the end for the test method. The results are shown in the table below.

[0265] Table 1. Electrode rebound rate and bare cell thickness expansion rate of sodium secondary battery cells in Examples 1-3 at different cutoff voltages

[0266] Meanwhile, the width group margin, cell energy density, and battery expansion force growth rate after 500 cycles of fully charged sodium secondary battery cells under different cutoff voltages were tested. Please refer to the methods attached at the end for the test methods. The results are shown in Table 2 and Figure 10 below.

[0267] Table 2. Width group margin, energy density, and cell expansion force growth rate of fully charged sodium secondary battery cells at different cutoff voltages in Examples 1-3.

[0268] Table 2 shows that the negative electrode rebound rate of sodium secondary battery cells gradually increases with increasing cutoff voltage; however, the positive electrode rebound rate initially increases with increasing cutoff voltage, then decreases, reaching its maximum at approximately 3.65V. Under the combined effect of the positive and negative electrode plates, the bare cell thickness expansion rate also exhibits the same trend as the positive electrode rebound rate with increasing cutoff voltage. That is, when the cutoff voltage is less than or equal to 3.65V, the bare cell thickness expansion rate gradually increases with increasing voltage, but decreases after 3.65V.

[0269] The aforementioned trend in the positive electrode rebound rate is mainly related to the positive electrode active material. Tests revealed that, within the operating voltage range of 1.5V to 4.2V, the c-axis cell length of the positive electrode active material in Example 1 at 1.5V (0% SOC) was... At a voltage of 3.1V–3.3V (50%–60% SOC), the c-axis cell length is At the highest charging voltage of 4.2V (100% SOC, full charge), the c-axis cell length is The c-axis unit cell length of the positive electrode active material exhibits a characteristic of first expanding and then contracting with increasing voltage (or SOC), resulting in the positive electrode rebound rate showing the same trend. Furthermore, under the combined effect of the positive and negative electrode rebound rates, the bare cell thickness expansion rate also shows the same trend as the positive electrode rebound rate.

[0270] Meanwhile, as shown in Table 2, as the cutoff voltage increases, the width group margin of the sodium secondary battery cell first increases. When the cutoff voltage is around 3.65V, the width group margin of the sodium secondary battery cell reaches its maximum value. As the cutoff voltage increases, the width group margin gradually decreases. When the maximum charging voltage is 4.2V, the width group margin decreases to its minimum value. At this point, there is sufficient space between the bare cell and the square aluminum shell in the width direction of the sodium secondary battery cell.

[0271] Referring to Figure 10, this figure shows the changes in expansion force and voltage of the sodium secondary battery cell in Example 1 within the operating voltage range of 1.5V to 3.95V, specifically within the range of 2.0V to 3.95V, during the same charge-discharge cycle. In Figure 10, the curve pointing to the left (arrow) refers to the expansion force on the vertical axis. This curve represents the change in battery expansion force within the 2.0V to 3.95V range during the same charge-discharge cycle. For example, during the period from 0 to 250s, the sodium secondary battery cell is charging, and the battery expansion force first increases and then decreases; during the period from 250s to 550s, the sodium secondary battery cell is discharging, and the battery expansion force first increases and then decreases. In Figure 10, the curve pointing to the right (arrow) refers to the voltage on the vertical axis. This curve represents the change in voltage within the same charge-discharge cycle, where the sodium secondary battery cell charges from 2.0V to 3.95V and then discharges from 3.95V back to 2.0V. As shown in Figure 10, during the charging process from 2.0V to 3.95V, the battery expansion force first increases and then decreases with the increase of voltage.

[0272] Figure 11 shows the original expansion force data for two operating voltage ranges in Example 1. When the sodium secondary battery cell is charged and discharged between 1.5V and 3.65V, the battery expansion force increases rapidly with the increase of the number of charge and discharge cycles. When the sodium secondary battery cell is charged and discharged between 1.5V and 4.0V, the battery expansion force hardly increases. To address this, the cell design can improve the width group margin of the sodium secondary battery cell and place more active material before the electrode assembly is installed in the casing, so that the sodium secondary battery cell has a higher energy density.

[0273] Table 2 shows that the cell's mass energy density gradually increases with increasing cutoff voltage. As the cutoff voltage rises, the positive electrode rebound reaches its peak first, at which point the battery expansion force growth rate initially increases. This growth rate reaches its maximum at approximately 3.75V, and then gradually decreases as the cutoff voltage continues to increase. Similarly, with increasing SOC, the width group margin of the sodium secondary battery cell first increases and then decreases, resulting in a corresponding trend of first increasing and then decreasing battery expansion force growth rate.

[0274] Therefore, based on the rebound rate or expansion rate of the positive electrode and bare cell at different cutoff voltages, the width group margin of the sodium secondary battery cell can be configured as follows: that is, the width group margin of the sodium secondary battery cell at the highest charging voltage is configured to be less than or equal to the highest width group margin before reaching the highest charging voltage. Specifically, according to Table 2, and combined with the correspondence between voltage and SOC during charging (as shown in Figure 6), the width group margin of square sodium secondary battery cells with a voltage of 1.5V to 4.2V at different voltages or SOCs can be configured. After configuring the width group margin in this way, referring to the test results in Table 2, the cell energy density can be increased as much as possible, and the battery expansion force is small at high energy density.

[0275] Example 4

[0276] This embodiment provides a sodium secondary battery cell, which differs from Embodiment 1 in that its positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 45.12%. 0.92 Ni 0.25 Mn 0.30 Fe 0.30 Zn 0.08 O2; at the same time, the full charge group margin of the sodium secondary battery cell in the voltage range of 1.5 to 4.0V is set to: a group margin of 92% in the width direction.

[0277] Example 5

[0278] This embodiment provides a sodium secondary battery cell, which differs from Embodiment 1 in that its positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 45.55%. 0.9 Ni 0.22 Mn 0.32 Fe 0.32 Zn 0.08 O2; at the same time, the full charge group margin of the sodium secondary battery cell in the voltage range of 1.5 to 4.0V is set to: a group margin of 93% in the width direction.

[0279] Example 6

[0280] This embodiment provides a sodium secondary battery cell, which differs from Embodiment 1 in that its positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 45.22%. 0.85 Ni 0.25 Mn 0.30 Fe 0.28 Cu 0.08 O2; at the same time, the full charge group margin of the sodium secondary battery cell in the voltage range of 1.5 to 4.0V is set to: a group margin of 94% in the width direction.

[0281] The sodium secondary battery cells of Examples 4 to 6 were subjected to charge-discharge tests within the operating voltage range of 1.5V to 4.0V. The charge-discharge test procedure was as follows: at 25°C, the cells were charged at a constant current rate of 0.5C to the cutoff voltage, and then charged at a constant voltage rate of 0.1C at the cutoff voltage, at which point the sodium secondary battery cell reached a fully charged state. After that, the cells were left to stand for 5 minutes, and then discharged at a constant current rate of 0.5C to the voltage of 1.5V. After that, the cells were left to stand for 5 minutes to complete one cycle of charge-discharge.

[0282] After 499 charge-discharge cycles, the fully charged sodium secondary battery cells were disassembled and analyzed during the 500th charge-discharge cycle. The relevant performance was measured and compared with that of Example 1 under the same operating voltage. The results are shown in the table below.

[0283] Table 3. Relevant test results of sodium secondary battery cells under full charge in Examples 4 to 6

[0284] Analysis of Tables 2 and 3 shows that, comparing Examples 1 and 4-6, the positive electrode rebound rate is similar during charging and discharging within the same operating voltage range. The width group margin of the fresh cells is designed to be 92%-95%, and the expansion force growth rate is also low, resulting in the sodium secondary battery cells exhibiting high energy density and low expansion force growth rate. Especially within the 94%-95% range, both energy density and low expansion force growth rate are effectively balanced.

[0285] Comparative Example 1

[0286] This comparative example provides a sodium secondary battery cell, which differs from Example 1 in that the positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 35.18%. 0.92 Ni 0.22 Mn 0.3 Fe 0.3 O4.

[0287] Comparative Example 2

[0288] This comparative example provides a sodium secondary battery cell, which differs from Example 1 in that the positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 53.94%. 0.85 Ni 0.38 Mn 0.4 Fe 0.4 Zn 0.06 O2.

[0289] Comparative Example 3

[0290] This comparative example provides a sodium secondary battery cell, which differs from Example 1 in that: the positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 59.53%. 0.81 Ni 0.37 Mn 0.45 Fe 0.5 O2.

[0291] Comparative Example 4

[0292] This comparative example provides a sodium secondary battery cell, which differs from Example 1 in that its positive electrode active material is Na with a molar ratio of Ni, Fe, and Mn of 45.12%. 0.92 Ni 0.25 Mn 0.3 Fe 0.3 Zn 0.08 O2.

[0293] Charge-discharge tests were conducted on sodium secondary battery cells of Comparative Examples 1 to 4 within the operating voltage range of 1.5 to 4.0V.

[0294] The charge-discharge test procedure is as follows: At 25℃, charge at a constant current of 0.5C to the cutoff voltage, and then charge at a constant voltage of 0.1C at the cutoff voltage. At this time, the sodium secondary battery cell reaches the fully charged state. After that, let it stand for 5 minutes, discharge at a constant current of 0.5C to the voltage of 1.5V, and then let it stand for another 5 minutes to complete one cycle of charge and discharge.

[0295] After 499 charge-discharge cycles, the fully charged sodium secondary battery cells were disassembled and analyzed during the 500th charge-discharge cycle. The relevant performance was tested, and the results were compared with those of the sodium secondary battery cells in Example 1 within the same operating voltage range. The results are shown in the table below.

[0296] Table 4. Relevant test results of sodium secondary battery cells in Comparative Examples 1 to 4 under full charge.

[0297] The tests revealed that within the operating voltage range of 1.5 to 4.0V, the positive electrode rebound rate, negative electrode rebound rate, and bare cell thickness expansion rate of Comparative Examples 1 to 3 all increased with increasing voltage, reaching their highest values ​​at full charge. The positive electrode rebound rate (7.2% to 9.3%) and bare cell thickness expansion rate (4.67% to 5.36%) at full charge were significantly higher than the results of Example 1 (positive electrode rebound rate 2.10%, bare cell thickness expansion rate 3.07%) under the same operating voltage range.

[0298] Compared with Example 1, it can be seen that this is mainly because the molar proportions of Ni, Fe, and Mn elements in the positive electrode active material are too small or too large, so the positive electrode active material does not exhibit the characteristic of first increasing and then decreasing the c-axis cell length during charging, but instead shows a monotonic increase. At the same cutoff voltage, the energy density of the cells in Comparative Examples 1 to 3 is significantly lower than that in Example 1, while the battery expansion force is significantly higher than that in Example 1.

[0299] The aforementioned trend results in the width group margin of the sodium secondary battery cells in Comparative Examples 1 to 3 being higher at the highest charging voltage than the corresponding width group margin before reaching that voltage. Therefore, in the design of sodium secondary battery cells, it is necessary to ensure that the width group margin of the sodium secondary battery cells at the highest charging voltage does not exceed 100%. However, this design will result in a lower energy density for the sodium secondary battery cells, while also resulting in a larger expansion force at the highest charging voltage, making it impossible to simultaneously achieve high energy density and low expansion force.

[0300] Compared to Example 1, Comparative Example 4 increased the width group margin at full charge by 4.0V, but its battery expansion force growth rate reached 407%, which is about 14.5 times that of Example 1. Excessive expansion force will lead to battery performance degradation and safety hazards, and it is also impossible to achieve high energy density and low expansion force at the same time.

[0301] Based on the above embodiments and comparative examples, it can be seen that by specifically setting the width group margin of the sodium secondary battery cell—that is, setting the width group margin at the highest charging voltage to be less than or equal to the highest width group margin before reaching the highest charging voltage—and before reaching the full charge state at the highest charging voltage, the maximum width group margin before full charge can be set to the largest possible value while ensuring safety, thereby allowing as much electrode active material as possible to be placed in the casing of the sodium secondary battery cell, thus improving the energy density of the sodium secondary battery cell. Furthermore, by using electrode active materials with specific cell properties (such as positive electrode active materials), the positive electrode rebound rate and bare cell thickness expansion rate of the sodium secondary battery cell first increase and then decrease with increasing voltage or SOC during charging and discharging, which helps to achieve the aforementioned width group margin configuration and is beneficial for simultaneously obtaining high energy density and low expansion force.

[0302] Appendix:

[0303] The test methods for the relevant performance in Tables 1 to 4 are as follows:

[0304] (1) Positive electrode rebound rate, negative electrode rebound rate

[0305] At 25°C, multiple sodium secondary battery cells of the same specification were taken, and one of the sodium secondary battery cells was disassembled to obtain the initial electrode thickness (positive or negative electrode). The other sodium secondary battery cells of the same specification were charged with different dielectric voltage ranges (charging conditions refer to the above embodiment section). After full charging, the sodium secondary battery cells were disassembled, and the thickness of the electrode portion corresponding to the first wall was measured with a micrometer to obtain the electrode thickness after charging. The electrode rebound rate was calculated according to the formula: (electrode thickness after charging - initial electrode thickness) / initial electrode thickness * 100%.

[0306] (2) Bare cell thickness expansion rate

[0307] Before charging at 25℃, the sodium secondary battery cells were scanned along their height direction using a nanoVoxel-5000 CT system to obtain tomographic images of the bare cells. The maximum dimension of the bare cells in the width direction of the sodium secondary battery cells was measured to obtain the initial bare cell thickness. The sodium secondary battery cells were then charged, and the cells were analyzed after full charging at different cutoff voltages. The bare cell thickness was calculated using the formula: Bare Cell Thickness Expansion Rate = (Bare Cell Thickness After Charging - Initial Bare Cell Thickness) / Initial Bare Cell Thickness * 100%.

[0308] (3) Width direction group margin

[0309] At 25℃, the sodium secondary battery cell was scanned along its height direction using a nanoVoxel-5000 CT system to obtain tomographic images. On these images, the maximum dimension W1 of the electrode assembly in the width direction of the sodium secondary battery cell was measured, and the dimension W2 of the inner surface of the second wall in the width direction was also measured. The width group margin was calculated using the formula: W1 / W2*100%. For measurements of the width group margin under different charging states, the sodium secondary battery cell was charged to the corresponding voltage according to the charging conditions, charging was stopped, and a CT scan was performed on the cell to obtain W1 and W2, from which the width group margin was calculated.

[0310] (4) Cell mass energy density

[0311] At 25℃, charge at a constant current of 0.5C to the cutoff voltage, then charge at a constant voltage at the cutoff voltage to a current of 0.1C; then let stand for 5 minutes, discharge at a constant current of 0.5C to a voltage of 1.5V, and let stand for another 5 minutes. Record the discharge energy at this time.

[0312] The sodium secondary battery cells were weighed using an electronic balance.

[0313] Mass energy density (Wh / kg) = Battery energy (mAh) × Voltage (V) / Battery mass (kg).

[0314] (5) Battery expansion force growth rate

[0315] At 25℃, a sodium secondary battery cell is placed between two parallel steel plates. The distance between the two steel plates is adjusted so that each plate contacts one of the two first walls of the sodium secondary battery cell. A pressure sensor is installed between one steel plate and its corresponding first wall. The cell is charged at a constant current rate of 0.5C to the cutoff voltage, and then charged at a constant voltage rate at the cutoff voltage until the current reaches 0.05C, at which point the sodium secondary battery cell is fully charged. After resting for 5 minutes, it is discharged at a constant current rate of 0.5C until the voltage reaches 1.5V, and then rested for another 5 minutes, completing one charge-discharge cycle. The initial expansion force of the sodium secondary battery cell before charging and the expansion force after charging are obtained using the pressure sensor. The battery expansion force growth rate is calculated as (expansion force after charging - initial expansion force) / initial expansion force * 100%.

[0316] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A sodium secondary battery cell, characterized in that, include: shell; Electrode assembly, disposed within the housing; During a single charge, when the sodium secondary battery cell reaches full charge, the width group margin of the sodium secondary battery cell is A1; before the sodium secondary battery cell reaches full charge, the maximum value of the width group margin of the sodium secondary battery cell is A2, satisfying that A1≤A2.

2. The sodium secondary battery cell according to claim 1, characterized in that, The charge-discharge cycle number of the sodium secondary battery cell is less than or equal to 100, and A1 is 94% to 96%.

3. The sodium secondary battery cell according to claim 1 or 2, characterized in that, The charge-discharge cycle number of the sodium secondary battery cell is less than or equal to 100, and A2 is 98% to 103%.

4. The sodium secondary battery cell according to any one of claims 1 to 3, characterized in that, The minimum design voltage of the sodium secondary battery cell is not lower than 1.5V, and the maximum design voltage of the sodium secondary battery cell is not higher than 4.2V.

5. The sodium secondary battery cell according to any one of claims 1 to 4, characterized in that, The sodium secondary battery cell has a charge-discharge cycle count of less than or equal to 100, and when the state of charge of the sodium secondary battery cell is greater than 65%, the maximum value of the width group margin of the sodium secondary battery cell is less than or equal to 98%.

6. The sodium secondary battery cell according to any one of claims 1 to 5, characterized in that, The sodium secondary battery cell has a charge-discharge cycle count of less than or equal to 100, and when the state of charge of the sodium secondary battery cell is 40% to 65%, the maximum value of the width direction group margin of the sodium secondary battery cell is greater than 98% and less than or equal to 103%.

7. The sodium secondary battery cell according to any one of claims 1 to 6, characterized in that, When the sodium secondary battery cell has undergone more than 200 charge-discharge cycles, the width group margin of the sodium secondary battery cell in its fully charged state is 99.5% to 102%.

8. The sodium secondary battery cell according to any one of claims 1 to 7, characterized in that, The electrode assembly includes a positive electrode and a negative electrode, wherein the thickness ratio of the positive electrode to the negative electrode is 0.65 to 0.

75.

9. The sodium secondary battery cell according to any one of claims 1 to 8, characterized in that, The electrode assembly includes a negative electrode sheet, the negative electrode sheet includes a negative current collector, the negative current collector is an aluminum foil, and the thickness of the aluminum foil is 5μm to 15μm.

10. The sodium secondary battery cell according to any one of claims 1 to 9, characterized in that, During a single charge, the width group margin of the sodium secondary battery cell exhibits a trend of first increasing and then decreasing.

11. The sodium secondary battery cell according to any one of claims 1 to 10, characterized in that, The operating voltage range of the sodium secondary battery cell is 1.5V to 4.2V.

12. The sodium secondary battery cell according to any one of claims 1 to 11, characterized in that, When the state of charge of the sodium secondary battery cell is 45% to 65%, the rebound rate of the positive electrode in the sodium secondary battery cell is 5% to 8%. When the state of charge of the sodium secondary battery cell is greater than 65%, the rebound rate of the positive electrode in the sodium secondary battery cell is less than or equal to 5%.

13. The sodium secondary battery cell according to any one of claims 1 to 12, characterized in that, When the state of charge of the sodium secondary battery cell is 80% to 95%, the rebound rate of the positive electrode sheet contained in the sodium secondary battery cell is 2% to 4.5%. When the state of charge of the sodium secondary battery cell is 95% to 100%, the rebound rate of the positive electrode sheet contained in the sodium secondary battery cell is 0% to 1.9%.

14. The sodium secondary battery cell according to any one of claims 1 to 13, characterized in that, The electrode assembly includes a positive electrode sheet, which contains a positive electrode active material; when the sodium secondary battery cell is fully charged, the cell length of the positive electrode active material in at least one crystal axis direction is less than the maximum cell length in the same crystal axis direction before full charge.

15. The sodium secondary battery cell according to any one of claims 1 to 14, characterized in that, The electrode assembly includes a positive electrode sheet, the positive electrode sheet includes a positive active material, and the positive active material includes a nickel-manganese-iron-based sodium oxide.

16. The sodium secondary battery cell according to claim 15, characterized in that, The general chemical formula of the nickel-manganese-iron-based sodium-containing oxide is Na. q Ni x Mn y Fe z M p O2, wherein 0 < q ≤ 1, 0 < x, 0 < y, 0 < z, 0 ≤ p, x + y + z ≥ 0.81, x + y + z + p ≤ 1, and M includes one or more of V, Cr, Zn, Al, Sc, Sn, Sb, Zr, Nb, Ti, Mg, Cu, Ru, and Ir.

17. The sodium secondary battery cell according to claim 16, characterized in that, In the nickel-manganese-iron-based sodium oxide, the total mass percentage of the three elements Ni, Mn, and Fe is 40% to 50%.

18. The sodium secondary battery cell according to claim 16 or 17, characterized in that, The general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1≤x≤0.

5.

19. The sodium secondary battery cell according to any one of claims 16 to 18, characterized in that, The general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1≤z≤0.

5.

20. The sodium secondary battery cell according to any one of claims 16 to 19, characterized in that, The general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.2≤z≤0.

3.

21. The sodium secondary battery cell according to any one of claims 16 to 20, characterized in that, The general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.1≤y≤0.

5.

22. The sodium secondary battery cell according to any one of claims 16 to 21, characterized in that, The general chemical formula of the nickel-manganese-iron-based sodium-containing oxide satisfies 0.8≤x+y+z≤1.

23. The sodium secondary battery cell according to any one of claims 1 to 22, characterized in that, The electrode assembly includes a negative electrode sheet, which contains a negative electrode active material with a porosity of 40% to 70%.

24. The sodium secondary battery cell according to any one of claims 1 to 23, characterized in that, The electrode assembly includes a negative electrode sheet, which contains a negative electrode active material, and the average pore size of the negative electrode active material is 1 to 30 nm.

25. The sodium secondary battery cell according to any one of claims 1 to 24, characterized in that, The electrode assembly includes a negative electrode sheet, the negative electrode sheet contains a negative electrode active material, and the negative electrode active material includes hard carbon.

26. A battery, characterized in that, Includes the sodium secondary battery cell according to any one of claims 1 to 25.

27. The battery according to claim 26, characterized in that, The number of sodium secondary battery cells is multiple, and the multiple sodium secondary battery cells are arranged along the width direction of the sodium secondary battery cells. The battery also includes a heat insulation pad, which is disposed between two adjacent battery cells.

28. The battery according to claim 27, characterized in that, The thickness of the heat insulation pad is 0.5mm to 4mm.

29. The battery according to claim 27 or 28, characterized in that, The capacity of the sodium secondary battery cell is 50Ah to 150Ah, and the thickness of the heat insulation pad is 0.4mm to 1.0mm; or The capacity of the sodium secondary battery cell is 151Ah to 250Ah, and the thickness of the heat insulation pad is 1.1mm to 1.9mm.

30. The battery according to any one of claims 27 to 29, characterized in that, The heat insulation pad is made of aerogel, ceramic, porous vacuum silicone, or asbestos.

31. An electrical device, characterized in that, It includes the sodium secondary battery cell according to any one of claims 1 to 25, or the battery according to any one of claims 26 to 30.

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