A sodium-ion battery and electrical device

Abstract

This application provides a sodium-ion battery and an electrical device, relating to the field of sodium-ion batteries. The sodium-ion battery satisfies the following relationship: K≤2000; where: a is the aspect ratio of the positive electrode active material; D 50 D, the positive electrode active material 50 T is the thickness of the diaphragm; ε is the porosity of the diaphragm; r is the low-temperature viscosity ratio of the electrolyte, where r satisfies r=η ‑20 / η 25 η 25 η is the viscosity of the electrolyte at 25°C. ‑20 This refers to the viscosity of the electrolyte at a temperature of -20℃. By precisely controlling the sheet morphology, aspect ratio, and particle size distribution of the layered oxide cathode material, matching the design of the membrane thickness and porosity, and synergistically regulating the electrolyte viscosity, the interfacial side reactions are suppressed while shortening the sodium ion solid-phase diffusion path and increasing the liquid-phase transport rate. This achieves a synergistic improvement in high capacity retention at low temperatures and long cycle life at room temperature / high temperature.

Description

Technical Field

[0001] This application relates to the field of sodium-ion batteries, and more particularly to a sodium-ion battery and an electrical device. Background Technology

[0002] Sodium-ion batteries have shown great promise for large-scale energy storage and low-speed electric vehicles due to their abundant resources and low cost. However, sodium ions have a relatively large radius (approximately 1.02 Å), and their diffusion kinetics in cathode materials and electrolytes are inherently weaker than those of lithium ions. Especially at low temperatures, the electrolyte viscosity increases, the ionic conductivity drops sharply, and the electrode interface impedance increases significantly, leading to rapid capacity decay and deterioration of rate performance, which severely restricts their practical application.

[0003] On the positive electrode side, the O3 layered oxide NaMO2 positive electrode material has the advantages of high specific capacity and high compact density, making it one of the most promising material systems for industrialization. The particle morphology of this type of material is typically plate-like, with its (003) crystal plane corresponding to the transition metal layer. This plane is a non-sodium ion diffusion active crystal plane, while sodium ions mainly intercalate and deintercalate along the thickness direction (i.e., perpendicular to the (003) crystal plane). In existing technologies, such as Chinese patent CN119944043A, only the overall particle size or specific surface area is considered, neglecting the precise control of the width-to-thickness ratio (the ratio of width to thickness) of the plate-like crystals. When the width-to-thickness ratio is too large, even if the overall particle size is small, the actual diffusion path of sodium ions is still significantly lengthened, with particularly pronounced kinetic degradation at low temperatures.

[0004] Furthermore, the particle size distribution of the cathode material has a dual impact on battery performance: excessively large particles prolong the diffusion distance of sodium ions in the bulk phase, deteriorating low-temperature performance; while excessively small particles shorten the diffusion path and improve low-temperature kinetics, they increase the specific surface area, exacerbating interfacial side reactions between the electrode material and the electrolyte during room temperature or high-temperature cycling, accelerating cycle capacity decay, and simultaneously reducing electrode compaction density, sacrificing battery energy density. Current technologies lack a systematic approach to finely optimize particle morphology and size distribution to balance low-temperature performance and cycle life.

[0005] On the membrane side, the membrane thickness and porosity directly affect the electrolyte absorption rate and the tortuosity of the ion transport path. Traditional polyolefin membranes (such as PP and PE) exhibit significant pore shrinkage and decreased ionic conductivity at low temperatures, and lack a synergistic design approach that matches the morphology and particle size of the cathode material.

[0006] On the electrolyte side, the viscosity of the electrolyte directly affects the migration rate of sodium ions in the liquid phase. At low temperatures, the electrolyte viscosity increases sharply, and the ionic conductivity decreases significantly, becoming one of the main bottlenecks for the low-temperature performance of batteries. In existing technologies, electrolyte formulation design is often independent of electrode materials and membrane structure, lacking systematic and synergistic optimization across the entire chain.

[0007] Therefore, there is an urgent need to provide a sodium-ion battery to solve the above problems. Summary of the Invention

[0008] The purpose of this application is to provide a sodium-ion battery and an electrical device to solve the above-mentioned problems.

[0009] To achieve the above objectives, the first aspect of this application provides a sodium-ion battery, including a cell, wherein the cell includes a positive electrode, a separator, an electrolyte, and a negative electrode. The positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material is in sheet form; The sodium-ion battery satisfies the following relationship: K≤2000; Where: a is the aspect ratio of the positive electrode active material; D 50 D is the positive electrode active material 50 T is the thickness of the diaphragm; ε is the porosity of the diaphragm; r is the low-temperature viscosity ratio of the electrolyte, where r satisfies r=η -20 / η 25 η 25 η is the viscosity of the electrolyte at a temperature of 25°C. -20 The viscosity of the electrolyte at a temperature of -20°C.

[0010] Optionally, the sodium-ion battery satisfies at least one of the following conditions: (1) The value of a is 3-20; (2) The D 50 It is 3-12 μm.

[0011] Optionally, T is 4-30 μm.

[0012] Optionally, the ε is 35%-65%.

[0013] Optionally, r is 2-5.

[0014] Optionally, the positive electrode active material comprises an O3 layered transition metal oxide.

[0015] Optionally, the volume percentage of the positive electrode active material with a particle size ≤1μm is ≤5%.

[0016] Optionally, the volume percentage of the positive electrode active material with a particle size ≥20μm is ≤3%.

[0017] Optionally, the span of the positive electrode active material is 0.45-1.5.

[0018] Optionally, η 25 ≤ 8.0 mPa·s.

[0019] A second aspect of this application provides an electrical device including the aforementioned sodium-ion battery.

[0020] Compared with the prior art, the beneficial effects of this application include: The sodium-ion battery provided in this application achieves a synergistic improvement in high capacity retention at low temperatures and long cycle life at room temperature / high temperature by precisely controlling the aspect ratio and particle size distribution of the layered oxide cathode material, matching the thickness and porosity of the separator, and synergistically regulating the viscosity of the electrolyte. This shortens the solid-phase diffusion path of sodium ions, increases the liquid-phase transport rate, and suppresses interfacial side reactions. This application also introduces a four-dimensional synergistic coefficient K for the first time, which integrates the anisotropic morphology (aspect ratio a) and solid-phase diffusion scale (particle size D) of the cathode material. 50 The design incorporates a correlation between the liquid phase transport capability of the separator (thickness T / porosity ε) and the low-temperature viscosity characteristics of the electrolyte (low-temperature viscosity ratio r). This coefficient comprehensively reflects the overall transport impedance of sodium ions as they escape from the cathode particles, pass through the solid phase, cross the separator, and migrate in the electrolyte. When the cathode material has a large aspect ratio or large particles (long solid-phase diffusion path), the transport impedance is compensated by selecting a thinner separator with higher porosity and a lower viscosity electrolyte. Conversely, when the aspect ratio is small or the particles are small (short solid-phase diffusion path), a slightly thicker separator with lower porosity and a slightly higher viscosity electrolyte are permissible to ensure mechanical strength, safety, and cost control. This application's four-dimensional collaborative design breaks through the limitations of traditional single or two-dimensional optimization, achieving systematic optimization of the ion transport path throughout the entire battery.

[0021] The electrical equipment provided in this application has a long service life and excellent electrochemical performance. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation on the scope of this application.

[0023] Figure 1 SEM image of the layered transition metal oxide cathode material provided in Example 1. Detailed Implementation

[0024] First, the solution provided in this application will be explained in more detail as follows: The first aspect of this application provides a sodium-ion battery, including a cell, wherein the cell includes a positive electrode, a separator, an electrolyte, and a negative electrode; The positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material is in sheet form; The sodium-ion battery satisfies the following relationship: K≤2000; Where: a is the aspect ratio of the positive electrode active material; D 50 D is the positive electrode active material 50 T is the thickness of the diaphragm; ε is the porosity of the diaphragm; r is the low-temperature viscosity ratio of the electrolyte, where r satisfies r=η -20 / η 25 , where η 25 η is the viscosity of the electrolyte at a temperature of 25°C. -20 The viscosity of the electrolyte at a temperature of -20°C.

[0025] It should be noted that the thickness is denoted as h, the width as d, and the width-to-thickness ratio a = d / h. The width d corresponds to the in-plane dimension of the (003) crystal plane of the material. The (003) crystal plane corresponds to the close-packed plane of the transition metal layer and is a non-sodium ion diffusion active crystal plane. The thickness h corresponds to the length of the diffusion channel of sodium ions between layers. The low-temperature performance of sodium-ion batteries is mainly limited by the total impedance of sodium ions along the entire transport path, which can be decomposed into three series impedance components: 1) Positive electrode solid-phase diffusion impedance Rsolid: the impedance of sodium ions diffusing from the bulk phase to the particle surface inside the layered oxide particles; 2) Separator liquid-phase transport impedance Rseparator: the impedance of sodium ions passing through the separator in the electrolyte; 3) Electrolyte liquid-phase diffusion impedance Relectrolyte: the migration impedance of sodium ions in the electrolyte within the pores of the positive and negative electrode plates. 1: For the solid-state diffusion impedance Rsolid, sodium ions in layered oxide cathode materials mainly undergo interlayer insertion / extraction along the thickness direction (i.e., perpendicular to the (003) crystal plane). The solid-state diffusion time constant satisfies: Where h is the thickness of the flaky particles, and D Na+ Let be the diffusion coefficient of sodium ions in the solid phase; however, in actual materials, the particle size is not a single value, but follows a certain distribution. The relationship between diffusion resistance and characteristic particle size can be expressed as: , where d charFor plate-like morphologies, since the (003) crystal plane is an inactive plane, sodium ions can only enter and exit the particle from the thickness direction. Therefore, the characteristic diffusion distance is the thickness h, not the width d. Simultaneously, the bulk diffusion path is also related to the volume average particle size D. 50 Positive correlation, D 50 The larger the value, the longer the average distance that sodium ions travel from the interior of the particle to the surface. Therefore, the solid-phase diffusion impedance of the positive electrode can be expressed as: , among which, S surface Where a is the active specific surface area, and a = d / h is the aspect ratio; when a is larger, it means that the particles are wider at the same thickness, the active specific surface area is relatively smaller, and the unit area needs to carry a larger ion flux, thus increasing the equivalent impedance; the core finding of this application is that the aspect ratio a and the particle size D50 have a product coupling effect on the solid-phase diffusion impedance—even if D50 is small, if a is too large, the solid-phase diffusion path is still longer. 2: Membrane liquid phase transport impedance Rseparator: The impedance of the membrane to ion transport can be described by the MacMullin number, i.e. Where T is the membrane thickness, which directly affects the ion transport distance; ε is the porosity, which determines the electrolyte volume fraction; k is the electrolyte ionic conductivity; and τ is the electrolyte ionic conductivity. sep The tortuosity of the diaphragm is usually related to porosity. Since there is an intrinsic relationship between tortuosity and porosity, the diaphragm impedance can be simplified as: Where β>1; In this application, to simplify engineering applications, β=1 is taken as a first-order approximation, i.e., Rseparator=T / ε; 3: Electrolyte diffusion resistance (Relectrolyte): The migration rate of sodium ions in the electrolyte is described by the Stokes-Einstein equation. The ion diffusion coefficient is inversely proportional to the electrolyte viscosity η, and the liquid phase diffusion impedance satisfies Relectrolyte = 1 / m = η, where m is the ionic conductivity. At low temperatures, the electrolyte viscosity increases significantly, which is one of the main factors leading to battery performance degradation. To characterize the low-temperature performance of the electrolyte, the low-temperature viscosity ratio is defined as: r = η. -20 / η 25 This ratio reflects the degree of viscosity increase of the electrolyte at low temperatures; the smaller the value of r, the better the low-temperature performance of the electrolyte.

[0026] Derivation of the synergy coefficient: The total ion transport impedance of a battery can be considered as the series sum of the three components mentioned above, i.e., R total = R solid + R separator + R electrolyte. Substituting the proportional relationship of each component: R total = (a × D50) + T / ε + η. The above sum can be converted into a product form, and its physical meaning is: the relative rate of change of the total impedance is equal to the algebraic sum of the rates of change of each parameter. Taking the logarithm of the total impedance: ln Rtotal = ln((a × D50 × T × r)) / ε, i.e., R = ((a × D50 × T × r)) / ε. The smaller the four-dimensional synergistic K value, the lower the total path transport impedance of sodium ions from the inside of the positive electrode particles to passing through the separator, and the better the low-temperature performance.

[0027] It should also be noted that an excessively high K value (>2000) can lead to severe degradation of certain performance characteristics. The essence of this application lies in defining this optimal balance range and providing an engineering method to achieve this balance. For example, the coefficient 'a' related to the positive electrode may increase the actual diffusion distance if the particles are too thick; or the particles may become equiaxed, resulting in insufficient specific surface area. A surge in fine particles (a significant increase in V < 1 μm) exacerbates interfacial side reactions and drastically reduces cycle life. An excessively low R value requires a large amount of low-viscosity solvent (such as DME, DOL), resulting in poor oxidation resistance of the electrolyte, insufficient high-voltage stability, and particularly poor cycling performance at room temperature and high temperature.

[0028] In some embodiments, the sodium-ion battery satisfies at least one of the following conditions: (1) The value of a is 3-20; Optionally, a can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or any value between 3 and 20; It should be noted that the thickness h of the layered oxide cathode material represents the length of the sodium ion diffusion channel, and the width d corresponds to the size of the inactive (003) crystal plane. By controlling the width-to-thickness ratio a (d / h) within a reasonable range, the diffusion distance of sodium ions in the solid phase is effectively limited, thus avoiding the low-temperature kinetic degradation caused by unilaterally pursuing large-size sheet morphology to improve the room temperature / high temperature cycling capacity retention rate. (2) The D 50 It is 3-12 μm.

[0029] Optionally, D 50 It can be any value between 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm or 3-12μm.

[0030] It is important to note that by controlling D 50Within the 3-12 μm range, avoid excessively large particles that would increase the bulk diffusion distance, and ensure rapid sodium ion insertion / extraction at low temperatures.

[0031] In some embodiments, T is 4-30 μm.

[0032] Optionally, T can be any value between 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, or 4-30μm.

[0033] In some embodiments, ε is 35%-65%.

[0034] Optionally, ε can be any value between 35%, 40%, 45%, 50%, 55%, 60%, or 35-60%.

[0035] In some embodiments, r is 2-5.

[0036] Optionally, r can be any value between 2, 3, 4, 5, or 2-5.

[0037] It is important to note that by controlling the viscosity of the electrolyte at 25℃ and -20℃ and the low-temperature viscosity ratio r, the migration rate of sodium ions in the liquid phase at low temperatures can be ensured. Low-viscosity electrolytes (especially those with low r values) can significantly reduce liquid phase transport impedance, forming a synergistic effect with the cathode material and the separator, and further improving low-temperature performance.

[0038] In some embodiments, the positive electrode active material comprises an O3 layered transition metal oxide.

[0039] In some embodiments, the volume percentage of the positive electrode active material with a particle size ≤1μm is ≤5%.

[0040] Optionally, the volume percentage of positive electrode active material with a particle size ≤1μm can be any value of 1%, 2%, 3%, 4%, 5% or ≤5%.

[0041] In some embodiments, the volume percentage of the positive electrode active material with a particle size ≥20μm is ≤3%.

[0042] Optionally, the volume percentage of positive electrode active material with a particle size ≥20μm can be any value of 1%, 2%, 3% or ≤3%.

[0043] It is important to note that by strictly limiting the content of fine particles (V<1μm≤5%) and coarse particles (V≥20μm≤3%), the exacerbation of interfacial side reactions caused by excessive specific surface area is avoided (improving cycle stability), while ensuring the electrode compaction density (improving energy density). Precise control of particle size distribution achieves synergistic optimization of low-temperature performance and cycle life, overcoming the technical challenge of balancing these two aspects in traditional designs.

[0044] In some embodiments, the span of the positive electrode active material is 0.45-1.5.

[0045] Optionally, the span of the positive electrode active material can be any value between 0.45, 0.5, 1, 1.5, or 0.45-1.5.

[0046] In some embodiments, η 25 ≤ 8.0 mPa·s.

[0047] Optionally, η 25 It can be any value of 1 mPa·s, 2 mPa·s, 3 mPa·s, 4 mPa·s, 5 mPa·s, 6 mPa·s, 7 mPa·s, 8 mPa·s or ≤ 8.0 mPa·s.

[0048] A second aspect of this application provides an electrical device including the aforementioned sodium-ion battery.

[0049] The implementation schemes of this application will be described in detail below with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only for illustrating this application and should not be regarded as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0050] Example 1 The first aspect of this application provides a sodium-ion battery and a method for preparing the same, including a battery cell comprising a positive electrode, a separator, an electrolyte, and a negative electrode. The specific preparation steps are as follows: S1: Preparation of the positive electrode: Step 1: Mix nickel sulfate, manganese sulfate, and ferric sulfate solutions in a molar ratio of Ni:Fe:Mn = 33:30:37 until homogeneous. Add 2 mol / L ammonia and sodium hydroxide solution to neutralize the mixture. Perform a co-precipitation reaction under a nitrogen atmosphere with a purity of 99.5% to obtain the hydroxide precursor Ni. 0.33 Fe 0.3 Mn 0.37(OH)₂ was used to filter and wash the obtained hydroxide precursor, which was then dried at 150°C for 5 hours. The precursor, sodium carbonate, and other additives were mixed and sintered in an oxygen-containing atmosphere (96% molar oxygen content) at the sintering temperatures and times shown in Table 1. After cooling to room temperature, the material was crushed and sieved to obtain a layered transition metal oxide cathode material with a sheet-like morphology, an average thickness h = 1.5 μm, an average width d = 4.5 μm, and a width-to-thickness ratio a = 3. Particle size distribution: D 50 =4.2μm, Span=1.0, V<1μm=2.5%, V≥20μm=0.5%; SEM images of the layered transition metal oxide cathode material are shown below. Figure 1 As shown, the thickness h and width d indicate the diffusion path of sodium ions along the thickness direction; Step 2: Using the layered transition metal oxide obtained in Step 1 as the positive electrode active material, the positive electrode active material, conductive agent SP, CNT, and binder PVDF are mixed in a mass ratio of 94:1.5:1.5:3. Solvent NMP is added, and the mixture is stirred under vacuum until the system is homogeneous, obtaining a positive electrode slurry with a solid content of 64%. The positive electrode slurry is uniformly coated onto the two opposite surfaces of the positive electrode current collector aluminum foil, transferred to an oven via a coating machine conveyor belt for drying, and then cold-pressed to form positive electrode active material layers on the two opposite surfaces of the positive electrode current collector. Finally, the foil is cut to obtain the positive electrode sheet. The coating compaction density of the positive electrode sheet is shown in Table 1. S2: Preparation of negative electrode sheet: Using commercially available hard carbon negative electrode material, the negative electrode active material, carbon nanotubes, conductive agent SP, and binder SBR are mixed in a mass ratio of 93:2:2.5:2.5. Deionized water is added as a solvent and the mixture is dispersed evenly to obtain a negative electrode slurry. The negative electrode slurry is evenly coated on the two opposite surfaces of the negative electrode current collector aluminum foil, and then transferred to an oven for further drying. After rolling, the negative electrode active material layers formed on the two opposite surfaces of the negative electrode current collector are formed respectively. Finally, the negative electrode sheet is cut to obtain a negative electrode sheet. S3: Electrolyte Preparation: Prepare the electrolyte by dissolving 1.0 mol / L NaPF6 in EC-EMC-PC (EC, EMC, PC volume ratio 1:2:1), and adding 2% FEC additive. The viscosity η of this electrolyte at 25℃ was tested. 25 =4.8 mPa·s, viscosity η at -20℃ -20 =14.4 mPa·s, low-temperature viscosity ratio r=3.0; S4: Selection of diaphragm: Polypropylene ceramic-coated diaphragm with thickness T=12μm and porosity ε=60% is selected; S5: Stack the positive electrode sheet, separator, and negative electrode sheet obtained in the above steps in sequence, so that the separator is between the positive electrode sheet and the negative electrode sheet to play a role in isolation, and then wind them to obtain a bare cell; place the bare cell in an aluminum-plastic film shell, dry it, inject electrolyte, and obtain a sodium-ion battery after vacuum sealing, standing, formation, shaping and other processes.

[0051] The sodium-ion battery described above satisfies the following relationship: ; Where: a is the aspect ratio of the positive electrode active material; D 50 D, the positive electrode active material 50 T is the thickness of the diaphragm; ε is the porosity of the diaphragm; r is the low-temperature viscosity ratio of the electrolyte, where r satisfies r=η -20 / η 25 , where η 25 η is the viscosity of the electrolyte at 25°C. -20 The viscosity of the electrolyte at a temperature of -20℃.

[0052] Example 2 The difference from Example 1 is as follows: Electrolyte formulation adjustment: 1.0 mol / L NaPF6 is dissolved in EC-EMC-DMC (the volume ratio of EC, EMC, and DMC is 1:1:2), and 2% FEC is added, η 25 =3.5 mPa·s, η -20 =8.75 mPa·s, r=2.5.

[0053] Example 3 The difference from Example 1 is as follows: the diaphragm is a non-woven fabric diaphragm with T=10μm and ε=65%; the electrolyte formulation is adjusted as follows: a low-viscosity ether electrolyte is used, 1.0 mol / L NaFSI is dissolved in DME-DOL (DME and DOL volume ratio is 1:1), and 2% FEC is added, η 25 =2.2 mPa·s, η -20 =5.5 mPa·s, r=2.5.

[0054] Example 4 The difference from Example 1 is: the average thickness of the cathode material particles is h = 1.2 μm, the average width is d = 6.0 μm, and the aspect ratio is a = 5; particle size distribution: D 50 =5.0μm, Span=1.1, V<1μm=2.0%, V≥20μm=0.8%; the diaphragm has a thickness T=16μm and a porosity ε=55%; electrolyte formulation adjustment: a low-viscosity ether electrolyte is used: 1.0 mol / L NaFSI dissolved in DME-DOL (DME, DOL volume ratio is 1:1), with the addition of 2% FEC, η 25=2.2 mPa·s, η -20 =5.5 mPa·s, r=2.5.

[0055] Example 5 The difference from Example 1 is: the average thickness of the cathode material particles is h = 0.53 μm, the average width is d = 3.7 μm, and the aspect ratio is a = 7; particle size distribution: D 50 =6.0μm, Span=1.2, V<1μm=1.5%, V≥20μm=1.2%; The diaphragm has a thickness T=6μm and a porosity ε=60%; Electrolyte formulation adjustment: Use a low-viscosity ether electrolyte: 1.0 mol / L NaPF6 dissolved in EC-EMC-DMC (EC, EMC, and DMC volume ratio 1:1:2), with the addition of 2% FEC, η 25 =3.5 mPa·s, η -20 =9.1 mPa·s, r=2.6.

[0056] Example 6 The difference from Example 1 is that the cathode material achieves a small aspect ratio sheet morphology through process control, without strict fine particle removal, retaining some fine particles. The average thickness h = 0.8 μm, average width d = 3.2 μm, and aspect ratio a = 4; particle size distribution: D 50 =5.4μm, Span=1.3, V<1μm=4.5%, V≥20μm=0.2%; The diaphragm has a T=12 μm and an ε=60% thickness. Electrolyte formulation: 1.0 mol / L NaPF6 dissolved in EC-EMC (EC to EMC volume ratio 3:7), with the addition of 3% FEC, η 25 =4.0 mPa·s, η -20 =12.8 mPa·s, r=3.2.

[0057] Example 7 The difference from Example 6 is that the V of the positive electrode material is less than 1 μm and equal to 8.0%.

[0058] Diaphragm: T=6μm, ε=65%, Electrolyte: r=4.

[0059] Example 8 The difference from Example 1 is that the content of fine particles in the positive electrode material is different, V<1μm=12%.

[0060] Example 9 The difference from Example 1 is that the electrolyte uses a different formulation: 1.0 mol / L NaPF6 dissolved in EC-DEC (volume ratio 1:1), η 25 =8.5 mPa·s, η -20 =42.5 mPa·s, r=5.0.

[0061] Comparative Example 1 The difference from Example 1 is that the a=8 of the positive electrode material and D 50 =10 μm, V≥20μm=6%; the membrane's T=12μm, ε=60%.

[0062] Comparative Example 2 The difference from Example 1 is that the a=26 of the positive electrode material.

[0063] Comparative Example 3 The difference from Example 1 is that the D of the positive electrode material... 50 =16 μm.

[0064] Comparative Example 4 The difference from Example 1 is that the membrane has a T=40μm.

[0065] Comparative Example 5 The difference from Example 1 is that the diaphragm has an ε of 21%.

[0066] Comparative Example 6 The difference from Example 1 is that the r of the electrolyte is 8.

[0067] Comparative Example 7 The difference from Example 1 is that the parameters of the positive electrode material are: average thickness h = 0.65 μm, average width d = 6 μm, aspect ratio a = 9.23, D50 = 8 μm, Span = 1.1, V < 1 μm = 2.5%, V ≥ 20 μm = 0.8%; The relevant parameters of the sodium-ion batteries in the above embodiments and comparative examples are shown in Table 1.

[0068] Table 1 Relevant Parameters

[0069] The sodium-ion batteries of the examples and comparative examples were subjected to electrochemical performance tests, and the test methods are as follows: (1) Cycle life test method at 45°C: The sodium-ion batteries prepared in the examples and comparative examples were subjected to cycle tests at 45°C according to the following procedure: ① Charge at a constant current rate of 1C to 3.8V, and then charge at a constant voltage until the current drops to 0.05C; ② Let stand for 5 minutes; ③ Discharge to 1.5V at a 1C rate; ④ Let stand for 5 minutes; Perform cycle tests according to steps ① to ④ until the capacity of the sodium-ion battery is less than 80% of the initial capacity, and record the number of cycles.

[0070] (2) 25℃ cycle life test method: The cell test environment temperature is 25℃, and the remaining steps are the same as the 45℃ cycle life test method.

[0071] (3) -20℃ cycle life test method: After placing the battery cell in an environment with a temperature of -20℃ for 24 hours, ① charge it to 3.2V at a constant current of 1C, and then charge it to 3.8V at a constant current of 0.3C until the current drops to 0.05C; ② Let stand for 5 minutes; ③ Discharge to 1.5V at a rate of 0.7C; ④ Let stand for 5 minutes; Perform 200 cycles of testing following steps ① to ④.

[0072] The results of the above performance tests are shown in Table 2.

[0073] Table 2 Performance Tests

[0074] analyze: Comparing Examples 1 and 8, it can be seen that although Example 8 has a lower K value (540) and excellent low-temperature performance, the cycle retention rate at 45°C is only 61.2% due to the excessive fine particles in the positive electrode (V<1μm=12%), and the interfacial side reactions are severe. The embodiments of this application, while ensuring low-temperature performance, achieve a significant improvement in cycle life by controlling the fine particle content.

[0075] Comparing Examples 1 and 9, it can be seen that the cathode and separator parameters are similar in both. However, Example 1 uses a low-viscosity electrolyte (r=3.0), while Example 9 uses a high-viscosity electrolyte (r=5.0), with K values ​​of 756 and 1260, respectively. The -20℃ retention rate of Comparative Example 2 (75.3%) is significantly lower than that of Example 1 (83.8%), verifying the key influence of electrolyte viscosity on low-temperature performance and the necessity of incorporating r into the co-design.

[0076] Example 3, targeting applications at even lower temperatures down to -40°C, employs a small aspect ratio (a=3) and a small particle size (D). 50 It features an ultra-thin high-porosity membrane (T=6μm, ε=45%) and an ultra-low viscosity electrolyte (r=2.5), with a K value as low as 420 and a -20℃ retention rate of 89.1%, demonstrating excellent low-temperature performance.

[0077] When the K value exceeds 2000 (Comparative Example 2 and Comparative Example 3), the low-temperature performance decreases significantly; when the K value is low but the particle size distribution is out of control (Comparative Example 1), the cycle life is impaired. Therefore, only embodiments that simultaneously meet the requirements of four-dimensional collaborative design can achieve the optimal balance between low-temperature performance and cycle life.

[0078] Comparing Example 1 and Comparative Example 1, it can be seen that when the thickness and particle size of the cathode material are increased simultaneously, the ion diffusion path of sodium ions is lengthened, the K value increases significantly, and the low-temperature performance of the battery cell decreases sharply.

[0079] Comparative Examples 2-6 illustrate that when a parameter in the formula exceeds its limit, the resulting deterioration in specificity cannot be fully compensated by other parameters.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit 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. Such 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.

[0081] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, any of the claimed embodiments can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of this application and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. A sodium-ion battery, characterized in that, The battery cell includes a positive electrode, a separator, an electrolyte, and a negative electrode. The positive electrode sheet includes a positive electrode active material layer, which comprises a positive electrode active material, and the positive electrode active material is in sheet form. The sodium-ion battery satisfies the following relationship: ， K ≤2000； Where: a is the aspect ratio of the positive electrode active material; D 50 D is the positive electrode active material 50 T is the thickness of the diaphragm; ε is the porosity of the diaphragm; r is the low-temperature viscosity ratio of the electrolyte, where r satisfies r=η -20 / η 25 η 25 η is the viscosity of the electrolyte at a temperature of 25°C. -20 The viscosity of the electrolyte at a temperature of -20°C.

2. The sodium-ion battery according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The value of a is 3-20; (2) The D 50 It is 3-12 μm.

3. The sodium-ion battery according to claim 1, characterized in that, The T value is 4-30 μm.

4. The sodium-ion battery according to claim 1, characterized in that, The ε is 35%-65%.

5. The sodium-ion battery according to claim 1, characterized in that, The value of r is 2-5.

6. The sodium-ion battery according to claim 1, characterized in that, The positive electrode active material includes O3 layered transition metal oxides.

7. The sodium-ion battery according to claim 1, characterized in that, The volume percentage of the positive electrode active material with a particle size ≤1μm is ≤5%.

8. The sodium-ion battery according to claim 1, characterized in that, The span of the positive electrode active material is 0.45-1.

5.

9. The sodium-ion battery according to any one of claims 1-8, characterized in that, or 25 ≤ 8.0 mPa·s.

10. An electrical appliance, characterized in that, Including the sodium-ion battery according to any one of claims 1-9.

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