Sodium-ion secondary battery, preparation method thereof, and electric device

Abstract

The application provides a sodium-ion secondary battery and a preparation method and an electric device thereof, and relates to the field of batteries. The sodium-ion secondary battery adds a sodium supplement agent in a negative electrode sheet, so that the irreversible release in reverse formation can compensate for the loss of active sodium ions in the SEI film formation process, effectively improving the energy density of the sodium-ion secondary battery. By controlling the relative size of the Dv50 particle size of the negative electrode active material and the Dv50 particle size of the sodium supplement agent before reverse formation, not only is the decomposition of the sodium supplement agent beneficial to improving the sodium supplement effect, but also, since pores are formed after the decomposition of the sodium supplement agent, controlling the relative size of the Dv50 particle sizes of the two is beneficial to controlling the negative electrode active material layer after formation to have a suitable porosity, improving the kinetic performance of the negative electrode and controlling the interface electrochemical reaction, and effectively improving the cycle performance of the sodium-ion secondary battery.

Description

Technical Field

[0001] This application relates to the field of batteries, and more specifically, to a sodium-ion secondary battery, its preparation method, and an electrical device thereof. Background Technology

[0002] Currently, hard carbon is the primary anode material used in sodium-ion batteries. It possesses high sodium storage capacity, low sodium storage potential, and excellent cycle stability, making it the most promising anode material for sodium-ion batteries. During the first charge cycle, the SEI film formed on the anode surface consumes some of the sodium ions released from the cathode, reducing the battery's reversible capacity.

[0003] Adding sodium can increase the sodium content in the entire battery system, thereby improving the energy density of sodium-ion rechargeable batteries. However, existing sodium addition methods still do not achieve a completely satisfactory improvement in cycle life.

[0004] Therefore, optimizing sodium supplementation to further improve the cycle life of sodium-ion secondary batteries has become an important research topic. Summary of the Invention

[0005] This application is made in view of the above-mentioned issues, and its purpose is to provide a sodium-ion secondary battery, a method for preparing the same, and an electrical device thereof.

[0006] To achieve the above objectives, this application provides a sodium-ion secondary battery, its preparation method, and an electrical device thereof, which can effectively improve the cycle performance while improving the energy density of the sodium-ion secondary battery.

[0007] The first aspect of this application provides a sodium-ion secondary battery, which includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative current collector including aluminum, the negative active material layer including a negative active material and a sodium supplementer, and the negative active material including hard carbon.

[0008] Among them, the volumetric particle size distribution Dv50 of the sodium supplement before formation is smaller than that of the negative electrode active material.

[0009] The sodium-ion secondary battery provided in this application uses aluminum as the negative electrode current collector. Therefore, by adding a sodium-replenishing agent to the negative electrode sheet, its irreversible release during reverse formation can compensate for the active sodium ions lost during SEI film formation, effectively improving the energy density of the sodium-ion secondary battery. By controlling the relative size of the Dv50 particle size of the negative electrode active material and the Dv50 particle size of the sodium-replenishing agent before reverse formation, it is not only beneficial to the decomposition of the sodium-replenishing agent, reducing post-formation residue and improving the sodium-replenishing effect, but also to ensure that the negative electrode active material layer after formation has a suitable porosity. The greater the porosity of the negative electrode active material layer... The more pathways the electrolyte penetrates and wets the negative electrode active material layer, the more ion pathways are formed. This reduces the impact of concentration polarization caused by sodium ions in the electrolyte on the kinetics of the electrochemical device, lowers impedance, and improves cycle performance. Therefore, if the porosity is too low, the ion pathways in the negative electrode film layer are insufficient to offset the impact of sodium ion concentration polarization on the electrochemical device, resulting in excessive impedance and poor cycle performance. If the porosity is relatively high, too many sodium ions will be consumed during the formation of the solid electrolyte membrane, leading to a low sodium ion content in the electrolyte, which reduces the electrolyte conductivity and cycle performance. Therefore, controlling the appropriate porosity is beneficial for improving the kinetic performance of the negative electrode and controlling interfacial electrochemical reactions, thereby improving the cycle performance of sodium-ion secondary batteries.

[0010] In any embodiment, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 10 μm, and the volumetric particle size distribution Dv50 of the sodium supplement before formation is 2 μm to 5 μm. Controlling the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation within the above ranges, on the one hand, the negative electrode active material layer has both good compaction density and good processing performance, which is beneficial to improving the energy density of sodium-ion secondary batteries; on the other hand, it is beneficial to make the negative electrode active material layer after formation have suitable porosity, improve the kinetic performance of the negative electrode and control the interfacial electrochemical reaction, thereby improving the cycle performance of sodium-ion secondary batteries.

[0011] In any embodiment, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 8 μm. Controlling the volumetric particle size distribution Dv50 of the negative electrode active material within the above range is beneficial for the negative electrode active material layer to have a higher compaction density, thereby improving the energy density of the battery.

[0012] In any embodiment, the difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation is 1 μm to 5 μm. By controlling the particle size difference between the two within the above-mentioned reasonable range, it is beneficial to improve the compaction density of the negative electrode sheet, while ensuring that the negative electrode active material layer after formation has a suitable porosity, thereby improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thus improving the cycle performance of the sodium-ion secondary battery.

[0013] In any embodiment, before formation, the mass content of the sodium supplement in the negative electrode active material layer is 1% to 10%, and after formation, the mass content of the sodium supplement in the negative electrode active material layer is ≤0.007%. By maintaining a mass content of 1% to 10% of the sodium supplement in the negative electrode active material layer before formation, its irreversible release during reverse formation can compensate for the active sodium ions lost during SEI film formation, effectively improving the energy density of the sodium-ion secondary battery. Simultaneously, by controlling the relative particle size of the negative electrode active material and the sodium supplement Dv50 particle size, as well as the mass content of the sodium supplement, the negative electrode active material layer after formation has suitable porosity, improving the cycle performance of the sodium-ion secondary battery. Furthermore, the extremely low sodium supplement content after formation indicates that the sodium supplement can be fully decomposed during the first charge after formation, leaving very low residue, thus suppressing cycle performance deterioration caused by high residue levels.

[0014] In any embodiment, the sodium replenishing agent includes one or any two of sodium nitrite, sodium squartzate, sodium sulfide, and sodium oxalate. During the reverse conversion process for sodium replenishment, the aforementioned sodium replenishing agent exhibits a low decomposition voltage and good sodium replenishment effect, which is beneficial for improving the energy density and safety of sodium-ion secondary batteries.

[0015] In any embodiment, the negative electrode active material layer further includes a conductive agent and a binder, comprising, by mass percentage, 88%–95% negative electrode active material, 1%–3% conductive agent, and 1%–3% binder. Controlling the addition amounts of negative electrode active material, sodium replenishing agent, conductive agent, and binder within a reasonable range helps the negative electrode active material layer achieve a higher capacity. Furthermore, reasonable levels of conductive agent and sodium replenishing agent also facilitate the utilization of the sodium replenishing agent's capacity, improve sodium replenishment efficiency, and enhance the battery's energy density and cycle performance.

[0016] In any embodiment, the porosity of the formed negative electrode active material layer is 42% to 55%. The formed negative electrode active material layer has a suitable porosity, which is beneficial for improving the cycle performance of the sodium-ion secondary battery.

[0017] In any embodiment, the sodium-ion secondary battery includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes one or more of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

[0018] In any embodiment, the sodium transition metal oxide includes at least one of sodium iron pyrophosphate and doped sodium iron pyrophosphate, wherein the sodium iron pyrophosphate includes (a1) and / or (a2):

[0019] (a1)Na x Fe y (PO4)2P2O7, 3.5≤x≤4.5, 2.75≤y≤3.5;

[0020] (a2)Na x Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0021] The aforementioned sodium iron pyrophosphate has good electrochemical performance and rate performance, but its energy density is low. By supplementing sodium at the negative electrode, the energy density and cycle performance of the sodium iron pyrophosphate secondary battery can be effectively improved.

[0022] A second aspect of this application also provides an electrical device comprising the sodium-ion secondary battery provided in the first aspect of this application.

[0023] A third aspect of this application provides a method for preparing a sodium-ion secondary battery, comprising:

[0024] A sodium-ion battery to be formed is obtained. The sodium-ion battery to be formed includes a negative electrode sheet, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative current collector includes aluminum, the negative active material layer includes a negative active material and a sodium supplementer, the negative active material includes hard carbon, the volume particle size distribution Dv50 of the sodium supplementer is smaller than the volume particle size distribution Dv50 of the negative active material, and the mass content of the sodium supplementer in the negative active material layer is 1% to 10%.

[0025] The sodium-ion battery to be formed is formed, and the mass content of sodium replenishing agent in the negative electrode active material layer after formation is ≤0.007%.

[0026] In the preparation method provided in this application, the mass content of the sodium supplement agent in the negative electrode active material layer before formation is 1% to 10%. This allows it to irreversibly release during reverse formation, compensating for the active sodium ions lost during SEI film formation and effectively improving the energy density of the sodium-ion secondary battery. By controlling the volume particle size distribution Dv50 of the sodium supplement agent before formation to be smaller than that of the negative electrode active material, on the one hand, its combination with the negative electrode active material is conducive to the formation of a conductive network in the negative electrode active material layer and reduces contact resistance. On the other hand, it is conducive to the sodium supplement agent being fully decomposed during the first charge after formation, reducing residues and ensuring that the sodium supplement agent content after formation is ≤0.007%. This reduces the deterioration of cycle performance caused by residues. Moreover, the small voids generated in situ after decomposition reduce the deterioration of cycle performance caused by structural damage to the negative electrode active material layer due to decomposition. It also helps to ensure that the negative electrode active material layer after formation has a suitable porosity. As the porosity of the negative electrode active material layer increases, the pathways for electrolyte to penetrate and wet the layer also increase. This results in more ion pathways, which helps reduce the impact of concentration polarization caused by sodium ions in the electrolyte on the kinetics of the electrochemical device, lowering impedance and improving cycle performance. Conversely, if the porosity is too low, the ion pathways in the negative electrode film are insufficient to counteract the effects of sodium ion concentration polarization, leading to excessive impedance and poor cycle performance. If the porosity is too high, excessive sodium ions will be consumed during the formation of the SEI film, resulting in a low sodium ion content in the electrolyte, which reduces electrolyte conductivity and cycle performance. Therefore, an appropriate porosity is beneficial for improving the kinetic performance of the negative electrode and controlling interfacial electrochemical reactions, thereby enhancing the cycle performance of sodium-ion secondary batteries.

[0027] In any embodiment, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 10 μm; or, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 8 μm. By controlling the volumetric particle size distribution Dv50 of the negative electrode active material within the above range, the negative electrode active material layer achieves both good compaction density and good processability.

[0028] In any embodiment, the volumetric particle size distribution Dv50 of the sodium-ion battery to be formed is 2 μm to 5 μm. Based on controlling the volumetric particle size distribution Dv50 of the negative electrode active material, further controlling the volumetric particle size distribution Dv50 of the sodium-ion battery before formation within the above range is beneficial. On the one hand, this ensures that the negative electrode active material layer has both good compaction density and good processing performance, which is conducive to improving the energy density of the sodium-ion secondary battery. On the other hand, it helps to ensure that the negative electrode active material layer after formation has suitable porosity, improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thereby improving the cycle performance of the sodium-ion secondary battery.

[0029] In any embodiment, in the sodium-ion battery to be formed, the difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement is 1 μm to 5 μm. By controlling the particle size difference between the two within the above-mentioned reasonable range, it is beneficial to improve the compaction density of the negative electrode sheet, while ensuring that the formed negative electrode active material layer has a suitable porosity, thereby improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thus improving the cycle performance of the sodium-ion secondary battery.

[0030] In any embodiment, the sodium replenishing agent includes one or any two of sodium nitrite, sodium squartzate, sodium sulfide, and sodium oxalate. During the reverse conversion process for sodium replenishment, the aforementioned sodium replenishing agent exhibits a low decomposition voltage and good sodium replenishment effect, which is beneficial for improving the energy density and safety of sodium-ion secondary batteries.

[0031] In any embodiment, the formation includes: first, reversing the power supply, and charging at a constant current to the cutoff voltage, the cutoff voltage being greater than the oxidative decomposition potential of the sodium supplement, and the cutoff voltage being -1.8V to -3V.

[0032] In any embodiment, the porosity of the negative electrode active material layer in the sodium-ion secondary battery is 42% to 55%. Controlling the porosity of the negative electrode active material layer in the sodium-ion secondary battery within the above range is beneficial to improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thereby improving the cycle performance of the sodium-ion secondary battery.

[0033] In any embodiment, the sodium-ion secondary battery includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes one or more of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

[0034] In any embodiment, the sodium transition metal oxide includes at least one of sodium iron pyrophosphate and doped sodium iron pyrophosphate, wherein the sodium iron pyrophosphate includes (a1) and / or (a2):

[0035] (a1)Na x Fe y (PO4)2P2O7, 3.5≤x≤4.5, 2.75≤y≤3.5;

[0036] (a2)Na x Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0037] The aforementioned sodium iron pyrophosphate has good electrochemical performance and rate performance, but its energy density is low. By supplementing sodium at the negative electrode, the energy density and cycle performance of the sodium iron pyrophosphate secondary battery can be effectively improved. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0039] Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0040] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0041] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0042] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0043] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0044] Explanation of reference numerals in the attached figures:

[0045] 1-Battery pack; 2-Upper housing; 3-Lower housing; 4-Battery module; 5-Battery cell; 51-Housing; 52-Electrode assembly; 53-Top cover assembly. Detailed Implementation

[0046] The following detailed description, with appropriate reference to the accompanying drawings, discloses the sodium-ion secondary battery of this application, its preparation method, and embodiments of the power-using device. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0047] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is understood that ranges of 60–110 and 80–120 are also expected. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0048] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0049] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0050] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0051] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0052] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0053] Adding sodium can increase the sodium content in the entire battery system, thereby improving the energy density of sodium-ion rechargeable batteries. However, existing sodium addition methods still do not achieve a completely satisfactory improvement in cycle life.

[0054] Based on this, the first aspect of the present application provides a sodium-ion secondary battery, which includes a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode current collector including aluminum, the negative electrode active material layer including a negative electrode active material and a sodium supplementer, and the negative electrode active material including hard carbon.

[0055] Among them, the volumetric particle size distribution Dv50 of the sodium supplement before formation is smaller than that of the negative electrode active material.

[0056] Formation refers to the initial charging process performed on a sodium-ion secondary battery after electrolyte injection. This process activates the active materials in the battery, thus activating the sodium-ion secondary battery. Simultaneously, the sodium salt undergoes a side reaction with the electrolyte, forming a solid electrolyte interphase (SEI) film on the negative electrode side of the sodium-ion secondary battery.

[0057] The testing methods for the volumetric particle size distribution (Dv50) of the sodium supplement before formation and the volumetric particle size distribution (Dv50) of the negative electrode active material include: disassembling the battery to obtain the positive electrode sheet, then cutting it into 6mm*6mm pieces with ceramic scissors, attaching it to a sample stage coated with paraffin, and ensuring the sample protrudes slightly (<1mm) from the edge of the sample stage. Appropriate polishing time and voltage are set to perform ion cross-section polishing on the electrode end face. The polished sample is then tested using a scanning electron microscope and energy dispersive spectroscopy (SEM) (equipment model Sigma300), according to JY / T010-1996. Sodium supplement (e.g., 200 particles) or negative electrode active material (e.g., 200 particles) are selected from different positions on the electrode sheet, and Avizo image processing analysis is used to obtain the volumetric particle size distribution (Dv50) of the sodium supplement before formation and the volumetric particle size distribution (Dv50) of the negative electrode active material.

[0058] The volumetric particle size distribution Dv50 of the sodium supplement before formation is smaller than that of the negative electrode active material. On the one hand, its combination with the negative electrode active material is conducive to the formation of the conductive network in the negative electrode active material layer and reduces contact resistance. On the other hand, it is conducive to the sodium supplement being fully decomposed during the first charge after formation, reducing residues and reducing the deterioration of cycle performance caused by residues. Moreover, the voids generated in situ after decomposition are small, reducing the deterioration of cycle performance caused by structural damage to the negative electrode active material layer due to decomposition. At the same time, it is also conducive to the negative electrode active material layer after formation having a suitable porosity.

[0059] As the porosity of the negative electrode active material layer increases, the pathways for electrolyte to penetrate and wet the layer also increase. This results in more ion pathways, which helps reduce the impact of concentration polarization caused by sodium ions in the electrolyte on the kinetics of the electrochemical device, lowering impedance and improving cycle performance. Conversely, if the porosity is too low, the ion pathways in the negative electrode film are insufficient to counteract the effects of sodium ion concentration polarization, leading to excessive impedance and poor cycle performance. If the porosity is too high, excessive sodium ions will be consumed during the formation of the SEI film, resulting in a low sodium ion content in the electrolyte, which reduces electrolyte conductivity and cycle performance. Therefore, an appropriate porosity is beneficial for improving the kinetic performance of the negative electrode and controlling interfacial electrochemical reactions, thereby enhancing the cycle performance of sodium-ion secondary batteries.

[0060] In summary, the sodium-ion secondary battery provided in this application, since the negative electrode current collector is aluminum, can compensate for the loss of active sodium ions during SEI film formation by adding a sodium replenishing agent to the negative electrode sheet, which is irreversibly released during reverse formation. This effectively improves the energy density of the sodium-ion secondary battery. By controlling the relative size of the Dv50 particle size of the negative electrode active material and the Dv50 particle size of the sodium replenishing agent before reverse formation, it is not only beneficial to decompose the sodium replenishing agent, reduce residues after formation, and improve the sodium replenishment effect, but also beneficial to ensure that the negative electrode active material layer after formation has a suitable porosity, improve the kinetic performance of the negative electrode, control the interfacial electrochemical reaction, and improve the cycle performance of the sodium-ion secondary battery.

[0061] In some embodiments, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 10 μm, and the volumetric particle size distribution Dv50 of the sodium supplement before formation is 2 μm to 5 μm.

[0062] Controlling the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation within the above range ensures that the negative electrode active material layer has both good compaction density and good processing performance, which is beneficial to improving the energy density of sodium-ion secondary batteries. On the other hand, it also helps to ensure that the negative electrode active material layer after formation has suitable porosity, thereby improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thus improving the cycle performance of sodium-ion secondary batteries.

[0063] For example, the volumetric particle size distribution Dv50 of the sodium supplement before formation is any value of 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm or between any two values.

[0064] For example, the volumetric particle size distribution Dv50 of the negative electrode active material is any value of 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm or between any two values.

[0065] In some embodiments, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 8 μm.

[0066] Controlling the volumetric particle size distribution Dv50 of the negative electrode active material within the aforementioned range is beneficial for achieving a higher compaction density in the negative electrode active material layer, thereby increasing the energy density of the battery. Furthermore, in conjunction with the volumetric particle size distribution Dv50 of the sodium supplement before formation, it helps to ensure that the negative electrode active material layer after formation has suitable porosity, improving the kinetic performance of the negative electrode and controlling interfacial electrochemical reactions, thus enhancing the cycle performance of the sodium-ion secondary battery.

[0067] For example, the volumetric particle size distribution Dv50 of the negative electrode active material is any value among 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, and 8.0 μm, or between any two values.

[0068] In some embodiments, the difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation is 1 μm to 5 μm.

[0069] By controlling the particle size difference between the two within the above-mentioned reasonable range, it is beneficial to improve the compaction density of the negative electrode sheet, while ensuring that the formed negative electrode active material layer has a suitable porosity, thereby improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thus improving the cycle performance of the sodium-ion secondary battery.

[0070] For example, the difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation is any value of 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or between any two values.

[0071] In some embodiments, before formation, the mass content of sodium supplement in the negative electrode active material layer is 1% to 10%, and after formation, the mass content of sodium supplement in the negative electrode active material layer is ≤0.007%.

[0072] By adding sodium supplementer to the negative electrode active material layer at a mass content of 1% to 10% before formation, its irreversible release during reverse formation can compensate for the active sodium ions lost during SEI film formation, effectively improving the energy density of sodium-ion secondary batteries. At the same time, by controlling the relative particle size of the negative electrode active material and the sodium supplementer Dv50, as well as the mass content of the sodium supplementer, the negative electrode active material layer after formation can have a suitable porosity. Therefore, controlling the appropriate porosity is beneficial to improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thereby improving the cycle performance of sodium-ion secondary batteries.

[0073] The sodium replenishment content after formation is extremely low, which in turn indicates that the sodium replenishment can be fully decomposed during the first charge after formation, leaving very low residue and inhibiting the deterioration of cycle performance caused by high residue.

[0074] For example, before formation, the mass content of sodium supplement in the negative electrode active material layer is any one of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or between any two values.

[0075] For example, after formation, the mass content of the sodium supplement in the negative electrode active material layer is any one of 0%, 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, or between any two values.

[0076] In some embodiments, the sodium supplement includes one or any two of sodium nitrite, sodium squartzate, sodium sulfide, and sodium oxalate.

[0077] The sodium replenishing agent described above has a low decomposition voltage and good sodium replenishment effect during the reverse formation sodium replenishment process, which is beneficial to improving the energy density and safety of sodium-ion secondary batteries.

[0078] For example, the sodium supplement is sodium nitrite, sodium squartz, sodium sulfide or sodium oxalate, or a mixture of sodium nitrite and sodium squartz, or a mixture of sodium squartz and sodium sulfide.

[0079] In some embodiments, the negative electrode active material layer further includes a conductive agent and a binder, wherein the negative electrode active material comprises 88% to 95% by mass, the conductive agent comprises 1% to 3% by mass, and the binder comprises 1% to 3% by mass.

[0080] Controlling the addition of negative electrode active material, sodium replenisher, conductive agent, and binder within a reasonable range helps the negative electrode active material layer to have a higher capacity. Furthermore, reasonable content of conductive agent and sodium replenisher is also conducive to the utilization of sodium replenisher capacity, improving sodium replenishment efficiency, and enhancing the energy density and cycle performance of the battery.

[0081] For example, the adhesive includes 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).

[0082] For example, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0083] Exemplarily, the negative electrode active material includes, but is not limited to, at least one of alloy-based materials and carbon-based negative electrode materials. The alloy-based material includes, but is not limited to, at least one of elemental tin, tin oxides, and tin alloys. The carbon-based negative electrode material includes, but is not limited to, at least one of artificial graphite, natural graphite, soft carbon, and hard carbon; however, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for sodium-ion secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0084] In some embodiments, the porosity of the formed negative electrode active material layer is 42% to 55%. A suitable porosity in the formed negative electrode active material layer is beneficial for improving the cycle performance of the sodium-ion secondary battery. Exemplarily, the porosity of the formed negative electrode active material layer is any value of 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, or 55%, or between any two values.

[0085] In some embodiments, the sodium-ion secondary battery includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes one or more of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

[0086] Among them, polyanionic compounds can be those containing sodium ions, transition metal ions, or tetrahedral (YO4) ions. 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 the valence state of (YO4). n- valence state.

[0087] Exemplarily, the polyanionic compound can be Na 4+x R 3-y P 4-m O 15 / C; where 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.

[0088] Exemplarily, 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 , where 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, 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.

[0089] In sodium transition metal oxides, the transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, NaM0, where M is one or several of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x ≤ 1.

[0090] Prussian blue compounds can be a class of compounds having sodium ions, transition metal ions, and cyanide ions (CN - ). Among them, the transition metal can 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 Meb 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。

[0091] It should be noted that sodium-ion secondary batteries experience sodium intercalation / deintercalation and consumption during charging and discharging, resulting in varying molar Na content at different discharge states. In the chemical formula of the positive electrode active material in this application, the molar Na content represents the initial state of the material, i.e., the state before material addition. As the positive electrode active material is applied to the battery system, the molar Na content changes after charge-discharge cycles. The molar O content is only a theoretical value; lattice oxygen release leads to changes in the molar oxygen content, causing fluctuations in the actual molar O content.

[0092] In some embodiments, the sodium transition metal oxide includes at least one of sodium iron pyrophosphate and doped sodium iron pyrophosphate, wherein the sodium iron pyrophosphate comprises (a1) and / or (a2):

[0093] (a1)Na x Fe y (PO4)2P2O7, 3.5≤x≤4.5, 2.75≤y≤3.25;

[0094] (a2)Na x Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0095] The aforementioned sodium iron pyrophosphate has good electrochemical performance and rate performance, but its energy density is low. By supplementing sodium at the negative electrode, the energy density and cycle performance of the sodium iron pyrophosphate secondary battery can be effectively improved.

[0096] Doped sodium iron pyrophosphate, wherein the doping element includes, but is not limited to, one or more of Mn, Co, Ni, Mg, Cu, Zn, Zr, Ti, V, and Nb. For example, the doped sodium iron pyrophosphate is Na₄Fe₂O₃. 3-2x V x (PO4)2P2O7, where x takes values ​​in the range of 0 < x < 0.5. For example, the doped sodium iron pyrophosphate is Na. x Fe y Nb m (PO4) n (P2O7); where x is 2-8, y is 1.5-6, m is 0.05-0.3, and n is 1-4.

[0097] Optionally, the positive electrode active material also includes a carbon coating layer on the surface of sodium iron pyrophosphate / doped sodium iron pyrophosphate. The introduction of carbon coating improves the conductivity of sodium iron pyrophosphate and enhances the electrochemical performance of the sodium-ion secondary battery.

[0098] A second aspect of this application provides an electrical device including the sodium-ion secondary battery provided in the first aspect of this application. A third aspect of this application provides a method for preparing a sodium-ion secondary battery, comprising:

[0099] A sodium-ion battery to be formed is obtained. The sodium-ion battery to be formed includes a negative electrode sheet, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative current collector includes aluminum, the negative active material layer includes a negative active material and a sodium supplementer, the negative active material includes hard carbon, the volume particle size distribution Dv50 of the sodium supplementer is smaller than the volume particle size distribution Dv50 of the negative active material, and the mass content of the sodium supplementer in the negative active material layer is 1% to 10%.

[0100] The sodium-ion battery to be formed is formed, and the mass content of sodium replenishing agent in the negative electrode active material layer after formation is ≤0.007%.

[0101] It is understandable that a sodium-ion battery awaiting formation refers to a sodium-ion secondary battery that needs to be formed after electrolyte injection. Formation refers to the first charging process performed on a sodium-ion secondary battery after electrolyte injection. This process can activate the active materials in the secondary battery, thereby activating the sodium-ion secondary battery. At the same time, the sodium salt undergoes a side reaction with the electrolyte, forming a solid electrolyte interphase (SEI) film on the negative electrode side of the sodium-ion secondary battery.

[0102] It is understandable that, since the sodium replenisher is at the negative electrode, in order to make up for the loss of active sodium ions during the formation of the SEI film by the irreversible release of the sodium replenisher during the formation process, the formation should be reversed. That is, the first charge should be reversed by first connecting the power supply to perform reverse formation. Since the current collector at the negative electrode is aluminum and aluminum does not form an alloy with sodium, the method of first connecting the power supply to perform reverse formation can not only decompose the sodium replenisher to replenish sodium, but also prevent aluminum from dissolving.

[0103] For example, before formation, the mass content of sodium supplement in the negative electrode active material layer is any one of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or between any two values.

[0104] Understandably, ideally, the sodium replenisher has a mass content of 0 in the negative electrode active material layer after formation. However, in reality, there is a residue. But since the residue is ≤0.007%, the residue is very low and therefore does not affect the subsequent cycle performance of the battery.

[0105] For example, after formation, the mass content of the sodium supplement in the negative electrode active material layer is any one of 0%, 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, or between any two values.

[0106] In the preparation method provided in this application, the mass content of the sodium supplement agent in the negative electrode active material layer before formation is 1% to 10%. This allows it to irreversibly release during reverse formation, compensating for the active sodium ions lost during SEI film formation and effectively improving the energy density of the sodium-ion secondary battery. By controlling the volume particle size distribution Dv50 of the sodium supplement agent before formation to be smaller than that of the negative electrode active material, on the one hand, its combination with the negative electrode active material is conducive to the formation of a conductive network in the negative electrode active material layer and reduces contact resistance. On the other hand, it is conducive to the sodium supplement agent being fully decomposed during the first charge after formation, reducing residues and ensuring that the sodium supplement agent content after formation is ≤0.007%. This reduces the deterioration of cycle performance caused by residues. Moreover, the small voids generated in situ after decomposition reduce the deterioration of cycle performance caused by structural damage to the negative electrode active material layer due to decomposition. It also helps to ensure that the negative electrode active material layer after formation has a suitable porosity. As the porosity of the negative electrode active material layer increases, the pathways for electrolyte to penetrate and wet the layer also increase. This results in more ion pathways, which helps reduce the impact of concentration polarization caused by sodium ions in the electrolyte on the kinetics of the electrochemical device, lowering impedance and improving cycle performance. Conversely, if the porosity is too low, the ion pathways in the negative electrode film are insufficient to counteract the effects of sodium ion concentration polarization, leading to excessive impedance and poor cycle performance. If the porosity is too high, excessive sodium ions will be consumed during the formation of the SEI film, resulting in a low sodium ion content in the electrolyte, which reduces electrolyte conductivity and cycle performance. Therefore, an appropriate porosity is beneficial for improving the kinetic performance of the negative electrode and controlling interfacial electrochemical reactions, thereby enhancing the cycle performance of sodium-ion secondary batteries.

[0107] In some embodiments, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 10 μm; or, the volumetric particle size distribution Dv50 of the negative electrode active material is 4 μm to 8 μm.

[0108] By controlling the volumetric particle size distribution Dv50 of the negative electrode active material within the above range, the negative electrode active material layer achieves both good compaction density and good processing performance.

[0109] For example, the volumetric particle size distribution Dv50 of the negative electrode active material is any value of 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm or between any two values.

[0110] In some embodiments, in the sodium-ion battery to be formed, the volumetric particle size distribution Dv50 of the sodium supplement is 2 μm to 5 μm.

[0111] Based on controlling the volumetric particle size distribution Dv50 of the negative electrode active material, further controlling the volumetric particle size distribution Dv50 of the sodium supplement before formation within the above range will, on the one hand, ensure that the negative electrode active material layer has both good compaction density and good processing performance, which is beneficial to improving the energy density of sodium-ion secondary batteries. On the other hand, it will help to ensure that the negative electrode active material layer after formation has suitable porosity, improve the kinetic performance of the negative electrode, control the interfacial electrochemical reaction, and improve the cycle performance of sodium-ion secondary batteries.

[0112] For example, the volumetric particle size distribution Dv50 of the sodium supplement before formation is any value of 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm or between any two values.

[0113] In some embodiments, the difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation is 1 μm to 5 μm.

[0114] By controlling the particle size difference between the two within the above-mentioned reasonable range, it is beneficial to improve the compaction density of the negative electrode sheet, while ensuring that the formed negative electrode active material layer has a suitable porosity, thereby improving the kinetic performance of the negative electrode and controlling the interfacial electrochemical reaction, thus improving the cycle performance of the sodium-ion secondary battery.

[0115] For example, the difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation is any value of 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or between any two values.

[0116] In some embodiments, the sodium supplement includes one or any two of sodium nitrite, sodium squartzate, sodium sulfide, and sodium oxalate.

[0117] The sodium replenishing agent described above has a low decomposition voltage and good sodium replenishment effect during the reverse formation sodium replenishment process, which is beneficial to improving the energy density and safety of sodium-ion secondary batteries.

[0118] In some embodiments, the formation process includes: first, reversing the power supply and charging at a constant current to a cutoff voltage, the cutoff voltage being greater than the oxidative decomposition potential of the sodium supplement, and the cutoff voltage being -1.8V to -3V.

[0119] Since the negative electrode current collector is aluminum and aluminum does not form an alloy with sodium, a reverse formation method is adopted, which involves first reversing the power supply. This not only allows the sodium replenishing agent to decompose and replenish sodium, but also prevents aluminum from dissolving. Simultaneously, controlling the cutoff voltage within the aforementioned range is beneficial for obtaining suitable porosity and improving the cycle performance and energy density of the secondary battery.

[0120] For example, the cutoff voltage is any value of -1.8V, -2.0V, -2.2V, -2.3V, -2.5V, -2.8V, -3.0V or between any two values.

[0121] In some embodiments, the porosity of the negative electrode active material layer in the sodium-ion secondary battery is 42% to 55%.

[0122] The higher the porosity of the negative electrode active material layer, the more pathways the electrolyte can penetrate and wet the layer, resulting in more ion pathways. This reduces the impact of concentration polarization caused by sodium ions in the electrolyte on the kinetics of the electrochemical device, lowers impedance, and improves cycle performance. Conversely, if the porosity is too low, the ion pathways in the negative electrode film layer are insufficient to counteract the effects of sodium ion concentration polarization on the electrochemical device, leading to excessive impedance and poor cycle performance. If the porosity is relatively high, excessive sodium ions will be consumed during the formation of the solid electrolyte membrane, resulting in a low sodium ion content in the electrolyte, which reduces the electrolyte conductivity and cycle performance. Therefore, controlling the porosity of the negative electrode active material layer in sodium-ion secondary batteries within the aforementioned range is beneficial for improving the kinetic performance of the negative electrode and controlling interfacial electrochemical reactions, thereby enhancing the cycle performance of the sodium-ion secondary battery.

[0123] For example, the porosity of the formed negative electrode active material layer is any one of 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, or between any two values.

[0124] In some embodiments, the sodium-ion secondary battery includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes one or more of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

[0125] Among them, polyanionic compounds can be those containing sodium ions, transition metal ions, or tetrahedral (YO4) ions. 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 price state.

[0126] Exemplarily, the polyanionic compound can 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.

[0127] Exemplarily, 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, 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.

[0128] In the sodium transition metal oxide, the transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, NaM0, where M is one or several of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x ≤ 1.

[0129] The Prussian blue compound can be a type of compound having sodium ions, transition metal ions, and cyanide ions (CN - ). Among them, the transition metal can 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, where 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.

[0130] It should be noted that sodium-ion secondary batteries experience sodium intercalation / deintercalation and consumption during charging and discharging, resulting in varying molar Na content at different discharge states. In the chemical formula of the positive electrode active material in this application, the molar Na content represents the initial state of the material, i.e., the state before material addition. As the positive electrode active material is applied to the battery system, the molar Na content changes after charge-discharge cycles. The molar O content is only a theoretical value; lattice oxygen release leads to changes in the molar oxygen content, causing fluctuations in the actual molar O content.

[0131] In some embodiments, the sodium transition metal oxide includes at least one of sodium iron pyrophosphate and doped sodium iron pyrophosphate, wherein the sodium iron pyrophosphate comprises (a1) and / or (a2):

[0132] (a1)Na x Fe y (PO4)2P2O7, 3.5≤x≤4.5, 2.75≤y≤3.25;

[0133] (a2)Na x Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0134] The aforementioned sodium iron pyrophosphate has good electrochemical performance and rate performance, but its energy density is low. By supplementing sodium at the negative electrode, the energy density and cycle performance of the sodium iron pyrophosphate secondary battery can be effectively improved.

[0135] Doped sodium iron pyrophosphate, wherein the doping element includes, but is not limited to, one or more of Mn, Co, Ni, Mg, Cu, Zn, Zr, Ti, V, and Nb. For example, the doped sodium iron pyrophosphate is Na₄Fe₂O₃. 3-2x V x (PO4)2P2O7, where x takes values ​​in the range of 0 < x < 0.5. For example, the doped sodium iron pyrophosphate is Na. x Fe y Nb m (PO4) n (P2O7); where x is 2-8, y is 1.5-6, m is 0.05-0.3, and n is 1-4.

[0136] Optionally, the positive electrode active material also includes a carbon coating layer on the surface of sodium iron pyrophosphate / doped sodium iron pyrophosphate. The introduction of carbon coating improves the conductivity of sodium iron pyrophosphate and enhances the electrochemical performance of the sodium-ion secondary battery.

[0137] In addition, the sodium-ion secondary battery and power-consuming device of this application will be described below with appropriate reference to the accompanying drawings.

[0138] Typically, a sodium-ion secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0139] [Positive electrode plate]

[0140] 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.

[0141] 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.

[0142] 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 aluminum metal on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0143] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0144] 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.

[0145] 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.

[0146] [Negative electrode plate]

[0147] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector.

[0148] 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.

[0149] In some embodiments, the negative electrode film layer includes a negative electrode active material, optionally a binder, and optionally a conductive agent. The specific material selection can be referred to the sodium-ion secondary battery provided in the first aspect of this application.

[0150] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0151] 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.

[0152] In other embodiments, the current collector of the negative electrode sheet may typically include a current collector body and a base coating. The base coating may be disposed on at least one side of the current collector body. The base coating basically does not contain negative electrode active material, but may include a small amount of carbon material. However, the carbon material forms a thin coating and cannot play the role of negative electrode active material. When the current collector of the negative electrode sheet includes a base coating, the negative electrode active material layer may be disposed on the surface of the base coating away from the current collector.

[0153] Electrolyte

[0154] The electrolyte acts as a conductor of ions between the positive and negative electrodes. The electrolyte can be selected based on existing sodium-ion secondary batteries.

[0155] In some implementations, the electrolyte comprises an organic solvent and a sodium salt.

[0156] For example, the sodium salt includes, but is not limited to, at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3 or Na(CH3)C6H4SO3.

[0157] For example, the organic solvents include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), or diethyl sulfone (ESE).

[0158] 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.

[0159] For example, the additive may include, but is not limited to, at least one of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), propylene sulfate, vinyl sulfite (ES), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), sulfonate cyclic quaternary ammonium salts, succinic anhydride, succinic anhydride (SN), adiponitrile (AND), tris(trimethylsilane) phosphate (TMSP), or tris(trimethylsilane) borate (TMSB).

[0160] [Isolation membrane]

[0161] In some embodiments, the sodium-ion secondary battery 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.

[0162] 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.

[0163] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0164] In some embodiments, the sodium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0165] In some embodiments, the outer packaging of a sodium-ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of a sodium-ion secondary battery can also be a soft pack, such as a pouch. The soft pack can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0166] In this application, a sodium-ion secondary battery can refer to a single battery cell, or it can refer to a single physical module comprising multiple battery cells to provide higher voltage and capacity, and it can take the form of a battery pack, battery module, etc.

[0167] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 It is a single battery cell with a square structure.

[0168] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0169] In some embodiments, individual battery cells can be assembled into a battery module, and the number of sodium-ion secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0170] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0171] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0172] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0173] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0174] In addition, this application also provides an electrical device, which includes at least one sodium-ion secondary battery (a battery cell, a battery module, or a battery pack) provided in this application. The sodium-ion secondary battery can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0175] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0176] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0177] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use sodium-ion batteries as their power source.

[0178] The following describes embodiments of this application. 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 according to 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.

[0179] I. Preparation of Sodium-ion Secondary Batteries

[0180] Example 1

[0181] Preparation of the positive electrode sheet

[0182] The positive electrode active material carbon-coated Na4Fe3(PO4)2P2O7, the binder polyvinylidene fluoride, and the conductive agent SP are mixed in a weight ratio of 95:2.5:2.5 and dissolved in the solvent N-methylpyrrolidone (NMP) to prepare a positive electrode slurry. The slurry is then coated onto the current collector aluminum foil, dried, and then cold-pressed, trimmed, cut, and slit to produce the positive electrode sheet.

[0183] Preparation of the negative electrode sheet

[0184] The negative electrode active material hard carbon HC, conductive agent SP, binder styrene-butadiene rubber (SBR) and sodium supplement are mixed in a weight ratio of 92:2.5:2.5:3 and dissolved in deionized water to prepare a negative electrode slurry. The slurry is then coated onto the current collector aluminum foil, dried, and then cold-pressed, trimmed, cut, and slit to produce a negative electrode sheet.

[0185] Preparation of Electrolyte

[0186] The electrolyte was prepared in an argon-atmospheric glove box with a water content of <10 ppm. First, sodium salt NaPF6 was added to the solvent (EC and DMC weight ratio of 4:1), followed by the addition of additives to complete the electrolyte preparation. The molar concentration of the sodium salt was 1 mol / L.

[0187]

Isolation Film

[0188] Polyethylene film is used as the separation membrane.

[0189] Preparation of secondary battery cells

[0190] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up to obtain the electrode assembly. The electrode assembly is placed in the casing, dried, and then injected with electrolyte for sodium-ion batteries. After vacuum sealing, settling, formation, and shaping, a sodium-ion secondary battery is obtained.

[0191] In this process, the positive and negative terminals are first reversed (reverse power supply), and then charged in reverse at a constant current of 0.3C until the cutoff voltage is reached. Then, the positive and negative terminals are connected in the positive direction, and charged at a constant current of 0.3C until the corresponding voltage is reached.

[0192] Examples 2-13 and Comparative Examples 1-2

[0193] The sodium-ion secondary batteries of Examples 2-13 and Comparative Examples 1-2 were prepared using the same method as the sodium-ion secondary battery of Example 1, but the composition of the negative electrode and the cutoff voltage during reverse formation were adjusted. The different parameters are detailed in Table 1.

[0194] Table 1 Different parameters

[0195]

[0196]

[0197] II. Battery Performance Testing

[0198] 1. Porosity after formation:

[0199] Referring to GB / T 24586-2009, the following steps were taken to determine the porosity: The formed negative electrode sheet was disassembled, immersed in ethyl methyl carbonate (EMC) for cleaning, and measured using the gas displacement method with an AccuPycⅡ1340 instrument. The calculation formula is as follows: Porosity of negative electrode sheet ρtotal = (V-V0) / Vtotal × 100%, where V0 is the true volume and V is the apparent volume. Porosity of negative electrode active material layer ρ = ρtotal × V / (VV current collector), where V current collector is the current collector volume.

[0200] 2. Energy density and cycle performance cls@90% SOH:

[0201] Under constant temperature conditions of 25°C, the battery is charged at 1C to 3.8V, and then charged at 3.8V at a constant voltage until the current is less than or equal to 0.05mA. After resting for 5 minutes, it is discharged at 1C to 1.5V, and the discharge capacity at this point is recorded as D0. This charge-discharge cycle is repeated until the discharge capacity decreases to 90% of D0. The number of cycles completed at this point is recorded as the cycle life at 90% SOH.

[0202] Wherein, mass energy density = D0 / mass of secondary battery; (unit of mass energy density: Wh / kg), where D0 is the measured discharge capacity of the cell.

[0203] 3. Mass content of sodium supplement in the electrode after reverse formation:

[0204] The battery cell was charged to 3.8V and discharged to 1.5V. Then it was disassembled in an inert atmosphere glove box. The positive and negative electrode plates were removed separately. The powder on the negative electrode plate was scraped off, and 50g was taken out and soaked in 100g of anhydrous ethanol. The mixture was heated to 80℃ and stirred to dissolve for 4h. 100mL of the supernatant was taken and dried under vacuum at 100℃ to obtain powder (by weight W). The powder was sent for XRD and ICP testing.

[0205] The powder was sent for XRD analysis at a scan rate of 1℃ / min from 0-90℃. Peak comparison with standard substances determined the specific structure of the sodium salt. Taking NaNO2 as an example: the powder from the disassembled positive and negative electrode sheets, along with the electrolyte from the battery cell, was sent for ICP analysis to obtain the content values ​​of each element, thus determining the ion content. Combined with the specific XRD results, the sodium salt content was determined. The K content of the negative electrode sheet was greater than 1000 ppm. Simultaneously, the negative electrode sheet was analyzed using XPS, which can detect the presence of N. XRD peak comparison results showed that it was essentially identical to the corresponding PDF card; and the nitrite content in the supernatant of the powder corresponding to the negative electrode sheet was greater than or equal to 100 ppm by mass. The XPS peak spectrum showed the presence of both Na and N peaks, indicating the presence of trace amounts of undecomposed sodium-adding additive NaNO2.

[0206] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0207] The sodium-ion secondary batteries of each embodiment and comparative example were tested according to the above method, and the results are shown in Table 2 below.

[0208] Table 2 Test Results

[0209]

[0210]

[0211] As can be seen from the comparison between Examples 1-12 and Comparative Example 1, the sodium-ion secondary battery provided in this application can effectively improve energy density and cycle performance compared to the one without sodium supplementation.

[0212] As can be seen from Example 1 and Comparative Example 1, the negative electrode of Comparative Example 1 has low energy density and low cycle number because it does not contain sodium supplement.

[0213] As can be seen from Examples 1-5 and Comparative Example 2, the volumetric particle size distribution Dv50 of the sodium supplement before formation is smaller than that of the negative electrode active material. This is beneficial for the decomposition of the sodium supplement, which helps to reduce the mass content of the sodium supplement in the electrode after reverse formation. It also helps to control the porosity of the negative electrode active material layer after formation, thereby increasing the energy density and improving cycle performance.

[0214] As can be seen from Examples 1, 6-9, one or two of sodium nitrite, sodium squartz, and sodium sulfide can be used as sodium replenishing agents, which have a good sodium replenishing effect and are beneficial to improving the energy density and safety of sodium-ion secondary batteries.

[0215] As shown in Examples 1, 10-11, before formation, the mass content of sodium supplement in the negative electrode active material layer is 1%-10%, and after formation, the mass content of sodium supplement in the negative electrode active material layer is ≤0.007%. By keeping the mass content of sodium supplement in the negative electrode active material layer at 1%-10% before formation, its irreversible release during reverse formation can compensate for the active sodium ions lost during SEI film formation, effectively improving the energy density of the sodium-ion secondary battery. At the same time, by controlling the relative particle size of the negative electrode active material and the sodium supplement Dv50 particle size, as well as the mass content of the sodium supplement, the negative electrode active material layer after formation has a suitable porosity, improving the cycle performance of the sodium-ion secondary battery, while the sodium supplement content is extremely low after formation.

[0216] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A sodium-ion secondary battery, characterized in that, The negative electrode includes a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode current collector includes aluminum, the negative electrode active material layer includes a negative electrode active material and a sodium supplementer, and the negative electrode active material includes hard carbon; The volumetric particle size distribution Dv50 of the sodium supplement before formation is smaller than that of the negative electrode active material.

2. The sodium-ion secondary battery according to claim 1, characterized in that, The volumetric particle size distribution Dv50 of the negative electrode active material is 4μm to 10μm, and the volumetric particle size distribution Dv50 of the sodium supplement before formation is 2μm to 5μm.

3. The sodium-ion secondary battery according to claim 2, characterized in that, The volumetric particle size distribution Dv50 of the negative electrode active material is 4μm to 8μm.

4. The sodium-ion secondary battery according to any one of claims 1 to 3, characterized in that, The difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement before formation is 1 μm to 5 μm.

5. The sodium-ion secondary battery according to any one of claims 1 to 4, characterized in that, Before formation, the sodium supplement agent has a mass content of 1% to 10% in the negative electrode active material layer; after formation, the mass content of the sodium supplement agent in the negative electrode active material layer is ≤0.007%.

6. The sodium-ion secondary battery according to any one of claims 1 to 5, characterized in that, The sodium supplement includes one or any two of sodium nitrite, sodium squartzate, sodium sulfide, and sodium oxalate.

7. The sodium-ion secondary battery according to any one of claims 1 to 6, characterized in that, The negative electrode active material layer further includes a conductive agent and a binder, wherein, by mass percentage, the negative electrode active material comprises 88%–95%, the conductive agent comprises 1%–3%, and the binder comprises 1%–3%.

8. The sodium-ion secondary battery according to any one of claims 1 to 7, characterized in that, The porosity of the formed negative electrode active material layer is 42% to 55%.

9. The sodium-ion secondary battery according to any one of claims 1 to 8, characterized in that, The sodium-ion secondary battery includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes one or more of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

10. The sodium-ion secondary battery according to claim 9, characterized in that, The sodium transition metal oxide includes at least one of sodium iron pyrophosphate and doped sodium iron pyrophosphate, wherein the sodium iron pyrophosphate includes (a1) and / or (a2): (a1)Na x Fe y (PO4)2P2O7,3.5≤x≤4.5,2.75≤y≤3.5; (a2) Na x Feb y Q m O n ,3.5≤x≤4.5,2.5≤y≤3.5,3.7<m<4,14.5≤n≤15.5。 11. An electrical appliance, characterized in that, Includes the sodium-ion secondary battery as described in any one of claims 1 to 10.

12. A method for preparing a sodium-ion secondary battery, characterized in that, include: A sodium-ion battery to be formed is obtained, the sodium-ion battery to be formed includes a negative electrode sheet, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative current collector includes aluminum, the negative active material layer includes a negative active material and a sodium supplementer, the negative active material includes hard carbon, the volume particle size distribution Dv50 of the sodium supplementer is smaller than the volume particle size distribution Dv50 of the negative active material, and the mass content of the sodium supplementer in the negative active material layer is 1% to 10%; The sodium-ion battery to be formed is subjected to formation, and after formation, the mass content of the sodium replenishing agent in the negative electrode active material layer is ≤0.007%.

13. The preparation method according to claim 12, characterized in that, The volumetric particle size distribution Dv50 of the negative electrode active material is 4μm to 10μm; or, the volumetric particle size distribution Dv50 of the negative electrode active material is 4μm to 8μm.

14. The preparation method according to claim 12, characterized in that, In the sodium-ion battery to be formed, the volumetric particle size distribution Dv50 of the sodium replenishing agent is 2μm to 5μm.

15. The preparation method according to claim 14, characterized in that, In the sodium-ion battery to be formed, the difference between the volumetric particle size distribution Dv50 of the negative electrode active material and the volumetric particle size distribution Dv50 of the sodium supplement is 1 μm to 5 μm.

16. The preparation method according to any one of claims 12 to 15, characterized in that, The sodium supplement includes one or any two of sodium nitrite, sodium squartzate, sodium sulfide, and sodium oxalate.

17. The preparation method according to any one of claims 12 to 16, characterized in that, The formation process includes: first, reversing the power supply and charging at a constant current to a cutoff voltage, wherein the cutoff voltage is greater than the oxidative decomposition potential of the sodium supplement, and the cutoff voltage is -1.8V to -3V.

18. The sodium-ion secondary battery according to any one of claims 12 to 17, characterized in that, The porosity of the negative electrode active material layer in the sodium-ion secondary battery is 42% to 55%.

19. The sodium-ion secondary battery according to any one of claims 12 to 18, characterized in that, The sodium-ion secondary battery includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes one or more of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

20. The sodium-ion secondary battery according to claim 19, characterized in that, The sodium transition metal oxide includes at least one of sodium iron pyrophosphate and doped sodium iron pyrophosphate, wherein the sodium iron pyrophosphate includes (a1) and / or (a2): (a1)Na x Fe y (PO4)2P2O7,3.5≤x≤4.5,2.75≤y≤3.5; (a2) Na x Feb y Q m O n ,3.5≤x≤4.5,2.5≤y≤3.5,3.7<m<4,14.5≤n≤15.5。

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