Sodium secondary battery and electric device

Abstract

The invention provides a sodium secondary battery and an electric device. The sodium secondary battery comprises a positive pole piece, a negative pole piece and an isolating membrane arranged between the positive pole piece and the negative pole piece, the negative pole piece comprises a negative current collector or comprises the negative current collector and a sodium metal layer arranged on the surface of the negative current collector, the isolating membrane comprises a first membrane layer, and the first membrane layer comprises a second membrane layer. The pore diameter peak value of the air holes of the first film layer is less than 35 nm, and the pore diameter peak value is the pore diameter corresponding to the highest peak in a pore diameter distribution curve. According to the invention, dendritic crystals can be prevented from puncturing the isolating membrane, and the safety performance of the sodium secondary battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a sodium secondary battery and an electric device. BACKGROUND

[0002] In recent years, with the application range of secondary batteries becoming more and more extensive, secondary batteries are widely used in energy storage power supply systems such as hydroelectric, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, etc.

[0003] As a typical secondary battery, the anode-free sodium metal battery has the advantages of abundant raw material resources and low cost compared with the lithium secondary battery, and is increasingly concerned. However, in the anode-free sodium metal battery, the problem of sodium dendrite growth and penetration of the separator membrane is particularly prominent, which seriously affects the safety performance of the battery. SUMMARY

[0004] The present application is made in view of the above-mentioned problems, and aims to provide a sodium secondary battery and an electric device, which can inhibit sodium dendrite penetration of the separator membrane and has excellent safety performance.

[0005] To achieve the above-mentioned purpose, the first aspect of the present application provides a sodium secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and a separator membrane arranged between the positive electrode sheet and the negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector, or comprising a negative electrode current collector and a sodium metal layer arranged on the surface of the negative electrode current collector, the separator membrane comprising a first base film, the pore size peak value of the air permeable hole of the first base film being 35 nm or less, the pore size peak value being the pore size corresponding to the highest peak in the pore size distribution curve. In the present application, by making the pore size peak value of the air permeable hole of the first base film of the separator membrane 35 nm or less, the size restriction effect is utilized to inhibit the growth of dendrites in the thickness direction of the separator membrane, change the growth direction, thereby reducing the risk of short circuit caused by sodium dendrite penetration of the separator membrane, and improving the safety of the battery.

[0006] In some embodiments, the first base film comprises a pore structure formed by a fibrous substance. In this way, the integrity of the structure of the separator membrane is better maintained when subjected to external force applied by sodium dendrites, thereby reducing the probability of being penetrated.

[0007] In some embodiments, the first base film comprises at least one polymeric base film, and the polymeric base film comprises a polyethylene film, a polypropylene film, a polyimide film, and a polytetrafluoroethylene film. In this way, it is helpful to further reduce the probability of the separator membrane being penetrated.

[0008] In some embodiments, the pore size distribution of the surface of the first base film is 60 nm to 200 nm. In this way, it is helpful to further inhibit the formation of sodium dendrites.

[0009] In some embodiments, in the first base film, the size distribution of the pore size of the air permeable pores is 2 nm to 40 nm. Thereby, it is helpful to further reduce the formation of sodium dendrites.

[0010] In some embodiments, in the first base film, the size distribution of the pore size of the air permeable pores is 15 nm to 38 nm. Thereby, it is helpful to further reduce the formation of sodium dendrites. In some embodiments, the peak value of the pore size of the air permeable pores of the first base film is 5 nm to 30 nm. By making the peak value of the pore size of the air permeable pores of the first base film in the above range, the pore size of the first base film is small and uniform, which is helpful to reduce the local deposition of sodium ions on the surface of the negative electrode, to inhibit the formation of sodium dendrites, and to reduce the probability of sodium ions being deposited in the air permeable pores becoming "precipitated ions", to increase the number of active sodium ions in the secondary battery, and to further improve the capacity retention rate of the secondary battery.

[0011] In some embodiments, the peak value of the pore size of the air permeable pores of the first base film is 10 nm to 25 nm. By making the peak value of the pore size of the air permeable pores of the first base film in the above range, it is helpful to further reduce the local deposition of sodium ions on the surface of the negative electrode, to inhibit the formation of sodium dendrites.

[0012] In some embodiments, the proportion of the number of air permeable pores with a pore size of 40 nm to 45 nm to the total number of air permeable pores of the first base film is less than 5%. By making the proportion of the number of pores with a pore size of 40 nm to 45 nm in the above range, it is possible to reduce the risk of the isolation film being penetrated by sodium dendrites.

[0013] In some embodiments, the proportion of the number of air permeable pores with a pore size of 40 nm to 45 nm to the total number of air permeable pores of the first base film is less than or equal to 1%. By making the proportion of the number of pores with a pore size of 40 nm to 45 nm in the above range, it is possible to further reduce the risk of the isolation film being penetrated by sodium dendrites.

[0014] In some embodiments, the average pore size of the air permeable pores of the first base film is 30 nm or less. The average pore size of the air permeable pores of the first base film is in the above range, reflecting that there are more small-pore-size pores in the first base film, further reducing the risk of sodium dendrites penetrating the isolation film.

[0015] In some embodiments, the maximum pore size of the air permeable pores of the first base film is 40 nm or less. By making the maximum pore size of the air permeable pores of the first base film in the above range, it is helpful to reduce the local deposition of sodium ions on the surface of the negative electrode, to inhibit the formation of sodium dendrites, and to further reduce the probability of sodium dendrites penetrating the isolation film.

[0016] In some embodiments, the isolation film further comprises a second base film disposed on at least one side of the first base film. The second base film is disposed on at least one side of the first base film. By disposing the second base film, a multi-layer barrier is formed against the growth of sodium dendrites, enhancing the barrier effect and inhibiting the penetration of sodium dendrites through the isolation film.

[0017] In some embodiments, the peak pore size of the air permeable pores of the second base film is 36 nm to 100 nm. By setting the peak pore size of the air permeable pores of the second base film within the above range, it is beneficial to reduce the probability of the isolation film being penetrated.

[0018] In some embodiments, the peak pore size of the air permeable pores of the second base film is 36 nm to 45 nm. By setting the peak pore size of the air permeable pores of the second base film within the above range, it is beneficial to further reduce the probability of the isolation film being penetrated.

[0019] In some embodiments, the isolation film further comprises a coating layer disposed on at least one side of the first base film. The coating layer can protect the first base film from chemical or electrolyte erosion, thereby improving its chemical stability in harsh environments and further improving the performance of the isolation film.

[0020] In some embodiments, the pore size distribution of the surface of the coating layer is 50 nm to 100 nm. Thus, it is helpful to further inhibit the formation of sodium dendrites.

[0021] In some embodiments, the coating layer comprises nanocellulose and inorganic fillers, and the volume median particle size Dv50 of the inorganic fillers is 100 nm to 500 nm. When the volume median particle size of the inorganic fillers is controlled within the given range, the energy density of the battery can be further improved under the premise of better safety performance of the isolation film.

[0022] In some embodiments, the nanocellulose comprises a modification group comprising at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group. In this implementation, the nanocellulose with a specific modification group (such as at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group) is contained in the coating layer, which on the one hand can effectively improve the heat stability of the isolation film, thereby improving the thermal safety performance of the battery and further improving the safety of the battery during use; on the other hand, the nanocellulose with a modification group in the coating layer itself has a certain rigidity, which can ensure the bonding force between the first base film while also guaranteeing ion conduction efficiency and improving the resistance of the isolation film to external pressure, thereby improving the energy density and thermal stability of the battery. At the same time, the presence of the modification group reduces the proportion of hydroxyl groups in the raw material, thus reducing the viscosity of the coating layer slurry, thereby reducing the difficulty of slurry coating and improving the uniformity of coating, and further improving the production efficiency of the isolation film.

[0023] In some embodiments, the average diameter of the nanocellulose is less than or equal to 40 nm. When the average diameter of the nanocellulose is within the above given range, the heat resistance of the separator film can be further improved, and the heat shrinkage of the separator film can be reduced.

[0024] In some embodiments, the content of the nanocellulose in the coating layer is greater than or equal to 5 wt%. When the content of the nanocellulose in the coating layer is within the above given range, the coating slurry containing the nanocellulose can have a more suitable viscosity, which is more conducive to coating; and it is more conducive to the nanocellulose itself or other components in the coating layer to build a stable space network structure, which can further increase the ion conduction channel of the separator film, improve the resistance to external extrusion and voltage breakdown capacity, and thus the thermal safety of the battery can be further improved.

[0025] In some embodiments, the thickness of the coating layer on one side of the first base film is less than or equal to 3 μm. When the thickness of the coating layer is within the given range, the energy density can be further improved on the premise of ensuring the safety performance of the battery.

[0026] In some embodiments, the thickness of the first base film accounts for more than 30% of the thickness of the separator film. By making the thickness of the first base film account for more than 30% within the above range, the thicker first base film can provide better barrier effect, further inhibiting the sodium dendrite from penetrating the separator film.

[0027] In some embodiments, the thickness of the first base film is 3 μm to 30 μm. By making the thickness of the first base film within the above range, a better barrier effect can be provided, and the sodium dendrite can be better inhibited from penetrating the separator film.

[0028] In some embodiments, the air permeability of the separator film is 350 s / 100 mL or less. When the air permeability of the separator film is within the above range, the risk of short circuit of the battery can be reduced.

[0029] In some embodiments, the first base film is arranged on the side of the second base film close to the negative electrode tab. In this way, the sodium dendrite can be more effectively inhibited from penetrating the separator film.

[0030] In some embodiments, the electrolyte includes a solvent and a sodium salt dissolved in the solvent, the solvent includes an ether solvent, the ether solvent includes a first ether solvent and a second ether solvent, the first ether solvent includes 2-4 carbon atoms, the second ether solvent includes R1-(O-R3)n-O-R2, R1 and R2 each independently include a linear or branched alkyl group with 1-6 carbon atoms, R3 includes a linear or branched alkylene group with 2-5 carbon atoms, 2≤n≤5, and the volume percentage of the first ether solvent in the total volume of the electrolyte is 4%-45%. By selecting the above electrolyte, the dissolution of the sodium metal layer can be reduced.

[0031] In some embodiments, the positive electrode tab includes a positive electrode active material, the positive electrode active material includes sodium iron pyrophosphate, Na a Ni b Fe c Mn d M e O f , wherein 0.85≤a≤1, 0≤b≤0.3, 0≤c≤0.4, 0≤d≤0.4, 0.02≤e≤0.1, 1.8≤f≤2, and b+c+d+e+f>0; M includes at least one of Zn, V, Cr, Al, Sc, Sn, Sb, Zr, Nb, Ti, Mg, Cu, Ru, Ir, and Ca. In the battery using the above positive electrode active material, the main component of the sodium dendrite is sodium metal. Due to the low Young's modulus of sodium metal, the growth path of the sodium dendrite is more likely to change when it contacts the battery separator, thereby to some extent alleviating the risk of the dendrite penetrating the separator.

[0032] In some embodiments, the ratio of the sum of the thickness of the sodium metal layer and the unoccupied space in the sodium secondary battery to the total thickness of the sodium metal layer in the thickness direction of the sodium metal layer is 1-2 when the sodium secondary battery is in a full charge state. In this way, the unoccupied space in the sodium secondary battery can serve as a Na free growth space. The energy barrier that Na needs to overcome to grow into the gas permeable hole of the separator is higher than the energy barrier that Na needs to overcome to grow in the height direction, which helps Na to grow horizontally rather than into the separator, thereby further reducing the probability of the separator being penetrated.

[0033] In some embodiments, when the state of charge of the sodium secondary battery is 80%-90%, the average surface current of the sodium secondary battery is ≤7 mA / cm 2 By controlling the average surface current of the sodium secondary battery within the above range, the generation of sodium dendrites can be inhibited, thereby reducing the probability of the sodium dendrites penetrating the separator. The second aspect of the present application provides an electric device including the sodium secondary battery of the first aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1A is a schematic view of a separator film according to an embodiment of the present application.

[0035] Figure 1B is a schematic view of a separator film according to another embodiment of the present application.

[0036] Figure 1C is a schematic view of a separator film according to still another embodiment of the present application.

[0037] Figure 2 is a schematic view of a battery cell according to an embodiment of the present application.

[0038] Figure 3 is an exploded view of a battery cell according to an embodiment of the present application. Figure 2

[0039] Figure 4 is a schematic view of a battery module according to an embodiment of the present application.

[0040] Figure 5 is a schematic view of a battery pack according to an embodiment of the present application.

[0041] Figure 6 is an exploded view of a battery pack according to an embodiment of the present application. Figure 5

[0042] Figure 7 is a schematic view of an electric device using a sodium secondary battery as a power source according to an embodiment of the present application.

[0043] Figure 8 is a pore size distribution curve of the separator film according to Example 1 of the present application and the separator film according to Comparative Example 1, which was obtained by testing in a test environment having a temperature of 25°C and a humidity of 2% RH.

[0044] Figure 9 is a surface SEM photograph of the separator film according to Example 1 of the present application.

[0045] Figure 10 is a surface SEM photograph of the separator film according to Comparative Example 1 of the present application.

[0046] BRIEF DESCRIPTION OF DRAWINGS

[0047] 10 first base film; 20 second base film; 30 coating layer; 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 case; 52 electrode assembly; 53 top cap assembly. DETAILED DESCRIPTION

[0048] ​​Hereinafter, specific embodiments of the sodium secondary battery and the electric device of the present application will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to sufficiently understand the present application, and are not intended to limit the subject matter recited in the claims.

[0049] The ranges disclosed herein are defined by their lower and upper limits. Ranges can be defined by selecting either the lower or the upper limit. The selected lower and upper limits define the range. Ranges defined by their lower and upper limits can be either inclusive or exclusive of the end values. Ranges can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if a range is listed as 60-120 and 80-110, it is understood that a range of 60-110 and 80-120 is also contemplated. Furthermore, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" indicates a shorthand way of describing each and every intervening real number with the set of "a-b", wherein "a" and "b" are real numbers. For instance, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shorthand way of describing these numerical combinations. Also, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0051] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0052] If not specifically stated, all steps of the present application can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method further comprises step (c), which means that step (c) can be added to the method in any sequence. For example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0053] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0054] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0055] Currently, in traditional sodium-ion batteries, sodium ions can be embedded within the negative electrode active material through the electrolyte, and the electrolyte typically uses ester solvents that can dissolve sodium metal, thus making it difficult for sodium dendrites to form at the negative electrode. In contrast, in negative electrode-less sodium batteries, the negative electrode does not contain a traditional negative electrode active material layer. During charging, sodium ions are reduced on the surface of the negative electrode current collector, depositing to form a sodium metal layer. This leads to a particularly prominent problem of sodium dendrite growth penetrating the separator in negative electrode-less sodium metal batteries, severely impacting battery safety performance.

[0056] In view of the above, this application proposes a sodium secondary battery and an electrical device. The invention and its optional embodiments will be described in more detail below.

[0057] The first aspect of this application provides a sodium secondary battery, comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The negative electrode includes a negative current collector and / or a sodium metal layer disposed on the surface of the negative current collector. The separator includes a first base film, wherein the peak pore size of the permeable pores in the first base film is below 35 nm, and the peak pore size is the pore size corresponding to the highest peak in the pore size distribution curve.

[0058] In their research, the inventors disassembled and analyzed numerous short-circuited sodium-ion batteries without a negative electrode. They discovered that, except for the negative electrode side, sodium dendrites primarily deposited and grew within the pores of the separator, eventually piercing it. Furthermore, the deposition size of metallic sodium dendrites within the separator was mostly between 40 nm and 100 nm. This resulted in the existing separator having excessively large pore sizes and a large number of large pores, making it easy for deposited sodium dendrites to penetrate the separator, leading to battery failure. Additionally, the inventors found that sodium metal has a lower modulus than lithium metal and is more prone to yielding under stress; therefore, limiting the pore size of the separator can effectively block sodium dendrite growth. Based on these findings, this application utilizes the size limitation effect by setting the peak pore size of the vent holes in the first base film of the separator to below 35 nm, thereby inhibiting dendrite growth in the thickness direction of the separator and altering its growth direction. This reduces the risk of short circuits caused by sodium dendrites penetrating the separator, improving battery safety.

[0059] Separating membrane

[0060] In this application, the separator includes a first base film, which is a membrane layer with permeable pores. Here, permeable pores refer to holes that allow sodium ions to permeate through the base film, serving as channels for sodium ion transport between the positive and negative electrode plates. In some embodiments, the separator may comprise a single first base film. In some embodiments, the separator may comprise multiple layers of the first base film. When the first base film is multilayered, the materials of each layer can be the same or different, without particular limitation.

[0061] In this application, the peak pore size of the venting pores can be determined in the following way. The separator to be tested can be a pre-prepared separator or a separator obtained by disassembling a battery. The latter will be used as an example to illustrate the testing process. Specifically, the separator is obtained by disassembling the battery. The disassembled separator is placed in a petri dish and soaked in DME for 30 minutes. Afterward, it is removed and rinsed with DME until there are no obvious foreign objects on the surface. This soaking-rinsing process is repeated three times, followed by air drying in a glove box for 6 hours. After obtaining the separator material, the pore size distribution curve is determined using a pore size analyzer (model: PMIPorometer). In some embodiments, GB / T 21650.2-2008 can also be referenced to test the pore size of the venting pores of the first base membrane. Then, using PMIPorometer software, a pore size distribution curve is plotted based on the test results. Furthermore, the peak pore size of the venting pores can be read from this pore size distribution curve. In this application, the peak pore size of the vent pores of the first base film is ≤35nm. For example, the peak pore size of the first base film is 35nm, 30nm, 27nm, 24nm, 21nm, 18nm, 15nm, 12nm, 9nm, 6nm, 3nm, 2nm or any value within a range of two such values.

[0062] In some embodiments, the peak pore size of the pores in the first base film is between 5 nm and 30 nm. By ensuring that the peak pore size of the pores in the first base film is within this range, the pore size of the first base film is small and uniform. This helps to reduce the local deposition of sodium ions on the negative electrode surface and inhibit the formation of sodium dendrites. It also helps to reduce the probability of sodium ions depositing in the pores and becoming "precipitated ions," thereby increasing the number of active sodium ions in the secondary battery and improving the capacity retention rate of the secondary battery. Optionally, the peak pore size of the pores in the first base film is between 10 nm and 25 nm.

[0063] In some embodiments, the pore size distribution in the first base film is from 2 nm to 40 nm. Optionally, it is from 15 nm to 38 nm. This helps to further suppress the formation of sodium dendrites.

[0064] In this application, the pore size distribution of the venting pores can be determined as follows: The separator to be tested can be a pre-prepared separator or a separator obtained by disassembling a battery. The latter will be used as an example to illustrate the testing process. Specifically, the separator is obtained by disassembling the battery. The disassembled separator is placed in a petri dish and soaked in DME for 30 minutes. Then, it is removed and rinsed with DME until there are no obvious foreign objects on the surface. This soaking-rinsing process is repeated three times, followed by air drying in a glove box for 6 hours. Then, a pore size meter (model: PMIPorometer) can be used, referring to GB / T 21650.2-2008, to test the pore size of the base membrane. Using PMIPorometer software, the pore size distribution of the venting pores is generated based on the test results.

[0065] In some embodiments, the proportion of pores with a diameter of 40 nm to 45 nm is less than 5% relative to the total number of pores in the first base film. For example, the proportion is 4.5%, 4%, 3%, 2%, 1%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, 0.1%, 0%, or any value within a range of two such values. By ensuring that the proportion of pores with a diameter of 40 nm to 45 nm is within the above range, the risk of the separator being penetrated by sodium dendrites can be further reduced. Optionally, the proportion is ≤1%, further optionally, the proportion is ≤0.5%, and even more optionally, the proportion is 0.

[0066] In this application, the proportion of pores with a diameter of 40nm to 45nm can be determined by the following method: the separator to be tested can be a pre-prepared separator or a separator obtained by disassembling a battery. The latter is used as an example to illustrate the testing process. Specifically, the separator is obtained by disassembling the battery. The disassembled separator is placed in a petri dish and soaked in DME for 30 minutes. Then, it is removed and rinsed with DME until there are no obvious foreign objects on the surface. This soaking-rinsing process is repeated three times, followed by air drying in a glove box for 6 hours. Then, a pore size analyzer (model: PMIPorometer) can be used, referring to GB / T21650.2-2008, to test the pore size of the base film. Using PMIPorometer software, based on the test results, the proportion of pores with a diameter of 40nm-45nm is statistically determined.

[0067] In some embodiments, the average pore size of the pores in the first base film is less than 30 nm. For example, the average pore size of the pores in the first base film is a value between 30 nm, 27 nm, 24 nm, 21 nm, 18 nm, 15 nm, 12 nm, 9 nm, 6 nm, 3 nm, 2 nm, or any two of these values. An average pore size within the above range reflects a greater number of small-diameter channels within the first base film, further reducing the risk of sodium dendrites penetrating the separator. Optionally, the average pore size of the pores in the first base film is from 5 nm to 0 nm; more preferably, the average pore size is from 10 nm to 25 nm.

[0068] For the average pore size, a pore size meter (model: PMIPorometer) can be used, referring to GB / T21650.2-2008, to test the pore size of the air pores of the base film; using PMIPorometer software, the average pore size of the air pores of the base film can be determined based on the test results.

[0069] In some embodiments, the maximum pore size of the pores in the first base film is 40 nm or less. For example, the maximum pore size of the pores in the first base film is a value within the range of 40 nm, 39 nm, 36 nm, 35 nm, 30 nm, 27 nm, 24 nm, 21 nm, 18 nm, 15 nm, 12 nm, 9 nm, 6 nm, 3 nm, or any two of these values. By ensuring that the maximum pore size of the pores in the first base film is within the above range, it helps to reduce the local deposition of sodium ions on the negative electrode surface and suppress the formation of sodium dendrites, thus further reducing the probability of sodium dendrites penetrating the separator. Optionally, the maximum pore size of the pores in the first base film is 35 nm or less.

[0070] In this application, the maximum pore size of the venting pores can be determined in the following way: the separator to be tested can be a pre-prepared separator or a separator obtained by disassembling the battery. The latter will be used as an example to illustrate the testing process. Specifically, the battery is disassembled to obtain the separator. The disassembled separator is placed in a petri dish and soaked in DME for 30 minutes. Then, it is removed and rinsed with DME until there are no obvious foreign objects on the surface. This soaking-rinsing process is repeated three times, followed by air drying in a glove box for 6 hours. Then, a pore size meter (model: PMIPorometer) can be used, referring to GB / T 21650.2-2008, to test the pore size of the base membrane's venting pores. Using PMIPorometer software, based on the test results, the maximum pore size of the base membrane's venting pores is determined.

[0071] In some embodiments, the thickness of the first base film accounts for 30% or more of the thickness of the separator. For example, the thickness percentage of the first base film is 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, or any value within a range of two such values. By ensuring that the thickness percentage of the first base film is within the aforementioned range, a thicker first base film can provide a better barrier effect, further suppressing sodium dendrite penetration into the separator. Optionally, the thickness percentage of the first base film is 50% or more.

[0072] In some embodiments, the thickness of the first base film is from 3 μm to 30 μm, for example, the thickness of the first base film is 3 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 20 μm, 25 μm, 30 μm, or a value within a range of any two values. By making the thickness of the first base film within the above range, a better barrier effect can be provided, and sodium dendrites can be better suppressed from penetrating the separator. Optionally, the thickness of the first base film is from 5 μm to 25 μm.

[0073] In some embodiments, the first base film includes a porous structure formed by fibrous material. This allows the separator to better maintain its structural integrity when subjected to external forces applied by sodium dendrites, thereby reducing the likelihood of it being penetrated.

[0074] In some embodiments, the first base film comprises at least one layer of a polymer base film, including a polyethylene film, a polypropylene film, a polyimide film, and a polytetrafluoroethylene film. This helps reduce the likelihood of the separator being penetrated. In some embodiments, the first base film comprises at least one layer of a polyethylene film. The polyethylene film has good flexibility, a cross-pore structure, and uniform mechanical strength, allowing the separator to better maintain its structural integrity when subjected to external forces applied by sodium dendrites, thereby reducing the likelihood of penetration.

[0075] In some embodiments, the pore size distribution on the surface of the first base film is from 60 nm to 200 nm. This helps to further suppress the formation of sodium dendrites. Optionally, the pore size distribution on the surface of the first base film is from 70 nm to 180 nm.

[0076] In this application, the pore size distribution of the surface of the first base film can be determined as follows: Specifically, the separator to be tested can be a prepared separator or a separator disassembled from a battery. The latter will be used as an example. First, the separator is obtained by disassembling the battery. The disassembled separator is placed in a petri dish and soaked in DME for 30 minutes. Then, it is removed and rinsed with DME until there are no obvious foreign objects on the surface. This soaking-rinsing process is repeated three times, followed by air drying in a glove box for 6 hours. Then, the surface of the first base film can be observed using a scanning electron microscope (ZEISS SEM). The surface of the separator is then randomly selected and photographed using a ZEISS SEM (magnification of 30,000x). SEM dimensional calibration is used to determine the pore size of the first base film surface. Nano Measurer software is then used to determine the pore size distribution of the first base film surface.

[0077] Figure 1A This is a schematic diagram of a separator membrane according to one embodiment of this application. In some embodiments, the separator membrane only includes a first base membrane 10.

[0078] Figure 1B This is a schematic diagram of a separator according to an embodiment of this application. It can be seen that in some embodiments, the separator includes a first base film 10 and a second base film 20 disposed on at least one side of the first base film 10. By providing the second base film, a multilayer barrier is formed against the growth of sodium dendrites, enhancing the barrier effect and inhibiting sodium dendrites from penetrating the separator. Optionally, the peak pore size of the pores in the second base film is between 36 nm and 100 nm. For example, the peak pore size of the pores in the second base film is a value between 36 nm, 37 nm, 38 nm, 39 nm, 40 nm, 41 nm, 42 nm, 43 nm, 44 nm, 45 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any two of these values. Optionally, the peak pore size of the pores in the second base film is between 36 nm and 45 nm.

[0079] Figure 1B This is merely an illustrative example of a second base film disposed on one side of a first base film. In one implementation, the second base film may be disposed between two layers of the first base film. In another implementation, the first base film may be disposed between two layers of the second base film.

[0080] In some embodiments, the first base film is disposed on the side of the second base film closer to the negative electrode sheet. This allows for more effective suppression of sodium dendrites penetrating the separator.

[0081] This application does not impose any particular limitation on the type of the second base film; any known base film with good chemical and mechanical stability can be selected. In some embodiments, the material of the second base film may include at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride.

[0082] In one implementation, the separator further includes a coating disposed on at least one side of the first base membrane. The coating can protect the first base membrane from corrosion by chemicals or electrolytes, thereby improving its chemical stability in harsh environments and further enhancing the performance of the separator.

[0083] Figure 1C This is a schematic diagram of an isolation membrane according to one embodiment of the present application. In one implementation, the isolation membrane includes a first base film 10, a second base film 20, and a coating 30, wherein the coating 30 is disposed on the side of the first base film 10 away from the second base film 20.

[0084] In one implementation, the separator includes a first base film, a second base film, and a coating. The coating is disposed between the first base film and the second base film. In another implementation, the separator includes only a first base film and a coating, with the coating disposed on the surface of the first base film.

[0085] In some embodiments, the pore size distribution on the surface of the coating is between 50 nm and 100 nm. This helps to further suppress the formation of sodium dendrites.

[0086] In this application, the pore size distribution of the surface of the coating can be determined by the following method, which can be found in the method for determining the pore size distribution of the surface of the first base film.

[0087] In some embodiments, the coating comprises nanocellulose and inorganic fillers, wherein the median volumetric particle size (Dv50) of the inorganic fillers is between 100 nm and 500 nm. When the median volumetric particle size of the inorganic fillers is controlled within the given range, it can be ensured that the separator further improves the energy density of the battery while maintaining good safety performance.

[0088] For example, inorganic fillers include at least one of boehmite (γ-AlOOH), barium sulfate (BaSO4), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), silicon oxide SiOx (0<x≤2), tin dioxide (SnO2), titanium oxide (TiO2), calcium oxide (CaO), zinc oxide (ZnO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), nickel oxide (NiO), hafnium dioxide (HfO2), cerium oxide (CeO2), zirconium titanate (ZrTiO3), barium titanate (BaTiO3), magnesium fluoride (MgF2), Pb(Zr,Ti)O3 (PZT), calcium carbonate (CaCO4), alumina (e.g., Al2O3), and alumina-doped modified materials (e.g., alumina doped with Na or Li).

[0089] In some embodiments, the nanocellulose includes modifying groups, which include at least one selected from sulfonic acid groups, boric acid groups, and phosphate groups. In this implementation, the coating contains nanocellulose with specific modifying groups (e.g., at least one from sulfonic acid groups, boric acid groups, and phosphate groups). On the one hand, this effectively improves the thermal stability of the separator, thereby enhancing the thermal safety performance of the battery and thus improving the safety of the battery during use. On the other hand, the nanocellulose with modified groups in the coating itself has a certain rigidity, which ensures adhesion to the base film while also guaranteeing ion conduction efficiency and improving the separator's resistance to external extrusion, thereby increasing the battery's energy density and thermal stability. Simultaneously, the presence of modified groups reduces the proportion of hydroxyl groups in the raw materials, thus reducing the viscosity of the coating slurry, thereby reducing the coating difficulty and improving the coating uniformity, and ultimately increasing the production efficiency of the separator.

[0090] In some embodiments, the average diameter of the nanocellulose is less than or equal to 40 nm. When the average diameter of the nanocellulose is within the range given above, the heat resistance stability of the separator can be further improved, and the thermal shrinkage rate of the separator can be reduced.

[0091] In some embodiments, the content of the nanocellulose in the coating is greater than or equal to 5 wt%. When the content of nanocellulose in the coating is within the range given above, it can ensure that the coating slurry containing the nanocellulose has a more suitable viscosity, which is more conducive to coating; and it is more conducive to the nanocellulose itself or to the formation of a stable spatial network structure with other components in the coating, which can further increase the channels for ion conduction in the separator, improve the resistance to external extrusion and voltage breakdown, and thus further improve the thermal safety of the battery.

[0092] In some embodiments, the thickness of the coating located on one side of the first base film is less than or equal to 3 μm. When the coating thickness is within the given range, the energy density can be further improved while ensuring battery safety performance.

[0093] In some embodiments, the coating includes an organic coating. An organic coating is made of a polymer compound. The adhesive layer includes, but is not limited to, polyurethane films, polyethylene films, and silicone films. Arranging an adhesive layer on the surface of the base film helps to further improve the toughness of the separator, reduces the probability of the separator being penetrated, and further enhances the durability of the separator.

[0094] In some embodiments, the air permeability of the separator is ≤350 s / 100 mL. For example, the air permeability of the separator is a value within the range of 350 s / 100 mL, 340 s / 100 mL, 330 s / 100 mL, 320 s / 100 mL, 310 s / 100 mL, 300 s / 100 mL, 290 s / 100 mL, 280 s / 100 mL, 270 s / 100 mL, 260 s / 100 mL, 250 s / 100 mL, 240 s / 100 mL, 230 s / 100 mL, 220 s / 100 mL, 210 s / 100 mL, 200 s / 100 mL, or any two of these values. Air permeability within this range helps reduce the risk of battery short circuits. Optionally, the air permeability of the separator is from 250 s / 100 mL to 320 s / 100 mL, and more preferably, the air permeability of the separator is from 250 s / 100 mL to 300 s / 100 mL.

[0095] In this application, the air permeability of the separator can be tested in the following way: the separator to be tested can be a pre-prepared separator or a separator obtained by disassembling the battery. The latter will be used as an example to illustrate the testing process. Specifically, the separator is obtained by disassembling the battery. The disassembled separator is placed in a petri dish and soaked in DME for 30 minutes. It is then removed and rinsed with DME until there are no obvious foreign objects on the surface. This soaking and rinsing process is repeated three times, followed by air drying in a glove box for 6 hours. Afterwards, the air permeability of the separator is tested according to GB / T 458-2008.

[0096] In addition, the method for preparing the separator membrane of this application may include the following steps.

[0097] Step (1): Mix the pore-forming agent (white oil) and the precursor (polyethylene resin) at a mass ratio of 0.5-5:100.

[0098] Step (2): The mixture is heated and melted, then cooled to separate the phases. It is then pressed into a casting, and then annealed under bidirectional micro-tension at a temperature of 100°C to 150°C.

[0099] Step (3): The casting is oriented and stretched longitudinally or bidirectionally to obtain the first base film of this application.

[0100] When stretched longitudinally, the longitudinal stretch ratio is 5-50 times.

[0101] When biaxially stretched, the longitudinal stretch ratio is 5-50 times, and the transverse stretch ratio is 5-50 times.

[0102] In some embodiments, the mass ratio of the pore-forming agent to the precursor in step (1) is 0.5-85:100, for example, 0.5:100, 10:100, 20:100, 30:100, 40:100, 80:100, 85:100 or any range of two values.

[0103] In some implementations, the annealing time for step (2) is 10s-60s.

[0104] In some embodiments, the pore-forming agent remaining on / inside the first base film can be extracted with a solvent after step (3). Alternatively, the pore-forming agent can be dried and evaporated after step (3). This application does not specifically limit the solvent used; any solvent known in the art for extracting pore-forming agents may be used.

[0105] Furthermore, the coating can be prepared using coating preparation methods known in the art, and this application does not impose any further limitations on it.

[0106] A sodium-ion secondary battery also includes a positive electrode, a negative electrode, and an electrolyte. During charging and discharging, sodium ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. A separator is placed between the positive and negative electrodes, primarily to prevent short circuits while allowing ions to pass through.

[0107] Positive electrode sheet

[0108] 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 the positive electrode active material of the first aspect of this application.

[0109] 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.

[0110] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0111] In some embodiments, the positive electrode sheet includes a positive electrode active material, which includes sodium iron pyrophosphate or Na. a Ni b Fe c Mn d M e O f Wherein, 0.85≤a≤1, 0≤b≤0.3, 0≤c≤0.4, 0≤d≤0.4, 0.02≤e≤0.1, 1.8≤f≤2, and b+c+d+e+f>0; M includes at least one of Zn, V, Cr, Al, Sc, Sn, Sb, Zr, Nb, Ti, Mg, Cu, Ru, Ir, and Ca. In the battery using the above-mentioned positive electrode active material, the main component of sodium dendrites is sodium metal. Due to the low Young's modulus of sodium metal, the growth path of sodium dendrites is more easily altered when they come into contact with the battery separator, thereby mitigating the risk of dendrites penetrating the separator to some extent.

[0112] In some embodiments, the positive electrode active material may be a known positive electrode active material for sodium secondary batteries. As an example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc. The positive electrode active material may include, but is not limited to, NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, Na... 0.67 MO2 (M includes at least two of Fe, Co, Cr, Mn, Ni, V, Ti, and Mo), NaMO2 (M includes at least two of Fe, Co, Ni, V, Ti, and Mo), NaFePO4, NaMnPO4, NaCoPO4, Na4Fe3(PO4)2O7, Na3V2(PO4)2F3, Na3V2(PO4)3, Prussian blue, Prussian white, sodium iron pyrophosphate, and one or more of their respective modified compounds.

[0113] In some embodiments, when the sodium secondary battery is fully charged, the ratio of the unfilled dimension within the sodium secondary battery to the sum of the thicknesses of all sodium metal layers in the thickness direction is 1 to 2, for example, a value between 1, 1.2, 1.4, 1.6, 1.8, 2, or any two of these values. Optionally, it is 1.05 to 1.5. With this design, the unfilled space within the sodium secondary battery can serve as a free growth space for Na. The energy barrier that Na needs to overcome to grow into the pores of the separator is higher than the energy barrier overcome in the height direction of free Na growth, which helps Na to grow laterally during deposition rather than growing into the separator, thereby further reducing the probability of the separator being penetrated.

[0114] In this application, "fully charged" means that the state of charge of a single battery cell reaches 100% or is charged to the highest cutoff voltage of the sodium secondary battery cell.

[0115] In some embodiments, when the state-of-charge percentage of the sodium secondary battery is 80% to 90%, the average surface current of the sodium secondary battery is ≤7 mA / cm². 2 For example, the average surface current is 7 mA / cm. 2 6mA / cm 2 5mA / cm 2 4mA / cm 2 3mA / cm 2 Or the value between any two values ​​within a range. Optionally, the average surface current is ≤3.5 mA / cm². 2 By controlling the average surface current within the above range, it is beneficial to suppress the formation of sodium dendrites, thereby reducing the probability of sodium dendrites penetrating the separator.

[0116] During the charging and discharging process of a battery, sodium (Na) undergoes insertion / extraction and consumption, resulting in varying molar Na content at different discharge states. In the examples of positive electrode active materials in this application, the molar Na content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Na content changes after charge-discharge cycles.

[0117] In the examples of positive electrode active materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] Negative electrode sheet

[0122] In this application, the negative electrode sheet does not include a negative electrode active material. In one embodiment, the negative electrode sheet includes a negative electrode current collector, during which sodium ions on the negative electrode side are reduced and deposited on the negative electrode current collector to form a sodium metal layer. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a sodium metal layer disposed on the surface of the negative electrode current collector.

[0123] 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.

[0124] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0125] In some embodiments, a coating may also be applied to the surface of the negative electrode current collector. The coating may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), and carboxymethyl chitosan (CMCS). The coating may also optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The coating may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)). In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned conductive agent, binder, and any other components in a solvent (e.g., deionized water) to form a slurry; coating the slurry onto the negative electrode current collector to form a coating; and then obtaining the negative electrode sheet through processes such as drying and cold pressing.

[0126] electrolyte

[0127] The electrolyte plays a role in conducting ions between the positive and negative electrode plates.

[0128] In some embodiments, the electrolyte comprises an ether solvent and a sodium salt dissolved in the ether solvent. Using this electrolyte helps to reduce the dissolution of the sodium metal layer.

[0129] In some embodiments, the electrolyte comprises a solvent and a sodium salt dissolved in the solvent; wherein the solvent comprises an ether solvent, which includes a first ether solvent and a second ether solvent, wherein the first ether solvent has 2-4 carbon atoms, and the second ether solvent comprises R1-(O-R3)nO-R2, wherein R1 and R2 each independently comprise a straight-chain or branched alkyl group having 1-6 carbon atoms, and R3 comprises a straight-chain or branched alkylene group having 2-5 carbon atoms, where 2≤n≤5, and the volume percentage of the first ether solvent is 4%-45% based on the total volume of the electrolyte. Using the above electrolyte helps to reduce the dissolution of the sodium metal layer.

[0130] In some embodiments, the first ether solvent includes ethylene glycol dimethyl ether; and / or the second ether solvent includes one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, triethylene glycol diethyl ether, tetraethylene glycol diethyl ether, pentaethylene glycol diethyl ether, and ethylene glycol dibutyl ether.

[0131] In some embodiments, the mass ratio of the short-chain ether to the long-chain ether is (4-45):(29-90).

[0132] In some embodiments, the sodium salt includes at least one selected from sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluorooxalate borate, sodium di(oxalate borate), sodium difluorodi(oxalate phosphate), and sodium tetrafluorooxalate phosphate. Optionally, the sodium salt includes sodium hexafluorophosphate.

[0133] In some embodiments, the electrolyte may optionally include additives. For example, additives may include those 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.

[0134] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0135] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0136] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0137] 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 2 The example shown is a square-structured battery cell 5.

[0138] In some implementations, refer to Figure 3 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.

[0139] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0140] Figure 4 This is battery module 4, used as an example. (See reference...)Figure 4 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.

[0141] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0142] 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.

[0143] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 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.

[0144] Electrical appliances

[0145] In addition, a second aspect of this application provides an electrical device, which includes the sodium secondary battery provided in the first aspect of this application. The sodium 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.

[0146] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0147] Figure 7 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 sodium-ion batteries for this device, a battery pack or battery module can be used.

[0148] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0149] Example

[0150] 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.

[0151] Example 1

[0152] Preparation of the separating membrane

[0153] Step (1): Mix polyethylene resin and white oil to form a mixture, wherein the mass ratio of polyethylene resin to white oil is 95:5.

[0154] Step (2): The mixture is melted and plasticized at 130°C and extruded through a die to form a casting. Next, it is annealed under bidirectional micro-tension at a temperature of 110°C for 40 seconds.

[0155] Step (3): Perform longitudinal stretching with a stretching ratio of 30 to obtain a first base film with a thickness of 14μm, which is used as the isolation film.

[0156] Example 2

[0157] Step (1): Mix polyethylene resin and white oil to form a mixture, wherein the mass ratio of polyethylene resin to white oil is 9:1.

[0158] Step (2): The mixture is melted and plasticized at 130°C and extruded through a die to form a casting. Next, it is annealed under bidirectional micro-tension at 100°C for 60 seconds.

[0159] Step (3): Perform longitudinal stretching with a stretching ratio of 45 to obtain a first base film with a thickness of 14 μm, which is used as the isolation membrane.

[0160] Example 3

[0161] Step (1): Mix polyethylene resin and white oil to form a mixture, wherein the mass ratio of polyethylene resin to white oil is 91.5:8.5.

[0162] Step (2): The mixture is melted and plasticized at 130°C and extruded through a die to form a casting. Next, it is annealed under bidirectional micro-tension at a temperature of 105°C for 50 seconds.

[0163] Step (3): Perform longitudinal stretching with a stretching ratio of 35 to obtain a first base film with a thickness of 14 μm, which is used as the isolation membrane.

[0164] Example 4

[0165] Step (1): Mix polyethylene resin and white oil to form a mixture, wherein the mass ratio of polyethylene resin to white oil is 97:3.

[0166] Step (2): The mixture is melted and plasticized at 130°C and extruded through a die to form a casting. Next, it is annealed under bidirectional micro-tension at a temperature of 115°C for 30 seconds.

[0167] Step (3): Perform longitudinal stretching with a stretching ratio of 25 to obtain a first base film with a thickness of 14 μm, which is used as the isolation film.

[0168] Example 5

[0169] Step (1): Mix and stir the polyolefin resin and white oil to form a mixture, wherein the mass ratio of polyolefin resin to white oil is 97.5:2.5.

[0170] Step (2): The mixture is melted and plasticized at 130°C and extruded through a die to form a casting. Next, it is annealed under bidirectional micro-tension at a temperature of 115°C for 25 seconds.

[0171] Step (3): Perform longitudinal stretching with a stretching ratio of 15 to obtain a first film layer with a thickness of 14 μm, which is used as a separator.

[0172] Example 6

[0173] Step (4): Mix and stir the polyethylene resin and white oil to form a mixture, wherein the mass ratio of polyethylene resin to white oil is 98:2.

[0174] Step (5): The mixture is melted and plasticized at 130°C and extruded through a die to form a casting. Next, it is annealed under bidirectional micro-tension at 120°C for 20 seconds.

[0175] Step (6): Perform longitudinal stretching with a stretching ratio of 15 to obtain a second base film with a thickness of 7 μm.

[0176] Step (7): After spraying PMMA (polymethyl methacrylate) adhesive onto the first base film obtained in Example 1, it is bonded to the second base film described above. Then, a release film is obtained by hot lamination through a roller.

[0177] Example 7

[0178] A coating was applied to the isolation membrane obtained in Example 6, and the specific method is as follows:

[0179] Coating slurry preparation: The filler (alumina with Dv50 of 200nm), nanocellulose containing modified groups, and water-soluble non-particulate binder (polyacrylate) are mixed evenly in an appropriate amount of solvent (deionized water) at a mass ratio of 79.2:20:0.8 to obtain the coating slurry.

[0180] The coating slurry is applied (sprayed) onto the surface of the first base film away from the second base film, and dried to obtain a release film. The thickness of the single-sided coating is 2 μm, and the weight of the single-sided coating per unit area is 1.0 g / m². 2 .

[0181] The nanocellulose containing the modified groups mentioned above is obtained using the following S1-S3 methods.

[0182] S1: Preparation of cellulose powder:

[0183] After the cotton lint is opened and slag is removed by a cotton opener, it is cooked with a 5wt% NaOH aqueous solution. The cotton then undergoes three separate processes: washing to remove impurities, sodium hypochlorite bleaching, acid washing to remove impurities, washing to remove impurities, water removal, and air drying to obtain cotton cellulose powder with a whiteness of ≥85%.

[0184] S2: Esterification of cellulose:

[0185] 1 kg of cotton cellulose powder obtained in step S1 was mixed with 30 kg of sulfuric acid solution (mass concentration of 60 wt%) and reacted at 60°C for 2 hours. After the reaction was completed, the mixture was washed and filtered three times with water to remove acid and impurities, and nanofiber whiskers with sulfonated groups were obtained.

[0186] S3: Neutralization of cellulose:

[0187] The pH of the sulfonated nanofiber whiskers was first adjusted to neutral using a 10wt% NaOH aqueous solution. Then, they were dispersed by high-speed grinding for 2.5 hours. The mixture was then ground twice more to obtain modified nanocellulose containing modified groups with an average diameter of 35nm, an average length of 500nm, and an aspect ratio of 16.6.

[0188] Example 8

[0189] A coating was applied to the isolation membrane obtained in Example 6, and the specific method is as follows:

[0190] Coating slurry preparation: The filler (alumina with Dv50 of 500nm), nanocellulose containing modified groups, and water-soluble non-particulate binder (polyacrylate) are mixed evenly in an appropriate amount of solvent (deionized water) at a mass ratio of 75:20:5 to obtain the coating slurry.

[0191] The coating slurry is applied to the surface of the first base film away from the second base film, and after drying, a release film is obtained. The thickness of the single-sided coating is 2 μm, and the weight of the single-sided coating per unit area is 1.0 g / m². 2 .

[0192] The nanocellulose containing the modified groups mentioned above is obtained using the following S1-S3 methods.

[0193] S1: Preparation of cellulose powder:

[0194] After the cotton lint is opened and slag is removed by a cotton opener, it is cooked with a 5wt% NaOH aqueous solution. The cotton then undergoes three separate processes: washing to remove impurities, sodium hypochlorite bleaching, acid washing to remove impurities, washing to remove impurities, water removal, and air drying to obtain cotton cellulose powder with a whiteness of ≥85%.

[0195] S2: Esterification of cellulose:

[0196] 1 kg of cotton cellulose powder obtained in step S1 was mixed with 30 kg of sulfuric acid solution (mass concentration of 60 wt%) and reacted at 60°C for 2 hours. After the reaction was completed, the mixture was washed and filtered three times with water to remove acid and impurities, and nanofiber whiskers with sulfonated groups were obtained.

[0197] S3: Neutralization of cellulose:

[0198] The pH of the sulfonated nanofiber whiskers was first adjusted to neutral using a 10wt% NaOH aqueous solution. Then, they were dispersed by high-speed grinding for 2.5 hours. The mixture was then ground twice more to obtain modified nanocellulose containing modified groups with an average diameter of 35nm, an average length of 500nm, and an aspect ratio of 16.6.

[0199] Comparative Example 1

[0200] Step (1): Mix polyethylene resin and white oil to form a mixture, wherein the mass ratio of polyethylene resin to white oil is 98:2.

[0201] Step (2): The mixture is melted and plasticized at 130°C and extruded through a die to form a casting. Next, it is annealed under bidirectional micro-tension at 120°C for 20 seconds.

[0202] Step (3): Perform longitudinal stretching with a stretching ratio of 15 to obtain a second base film with a thickness of 14μm, which is used as an isolation membrane.

[0203] Test of the separator film

[0204] (1) Determination of the peak pore size of the ventilator

[0205] Using a pore size meter (model: PMIPorometer) and referring to GB / T 21650.2-2008, the pore size of the vent pores of the first or second base film was tested to obtain the pore size distribution curve of the first or second base film. Using PMIPorometer software, the pore size corresponding to the peak value was read from the curve based on the test results to obtain the pore size peak value.

[0206] For example, the pore size distribution curves of the pores in the separator (first base membrane) prepared in Example 1 and the separator (second base membrane) prepared in Comparative Example 1 were tested, and the pore size distribution curves of the pores were obtained, such as... Figure 8 As shown in the figure. The horizontal axis of the pore size distribution curve represents the pore size of the air pores in the separator, and the vertical axis represents the percentage of air pores. From Figure 8 The data shows that the peak pore size of the pores in the separator of Example 1 is 26 nm. The peak pore size of the pores in the separator of Comparative Example 1 is 36 nm.

[0207] Data for the first base film in other embodiments and comparative examples are shown in Table 1-1. Data for the second base film in other embodiments and comparative examples are shown in Table 1-2.

[0208] (2) The proportion of pores with a diameter of 40nm-45nm

[0209] A pore size meter (model: PMIPorometer) was used to test the pore size of the first base film according to GB / T 21650.2-2008. Based on the test results, the percentage of pores with a pore size of 40nm-45nm was statistically analyzed using PMIPorometer software. The statistical results are shown in Table 1-1.

[0210] (3) Determination of the maximum pore diameter of the vent.

[0211] The pore size of the first base film was measured using a PMIPorometer (model: PMIPorometer) in accordance with GB / T 21650.2-2008. Based on the test results, the maximum pore size of the first base film was determined using PMIPorometer software. The test results are shown in Table 1-1.

[0212] (4) Determination of pore size distribution on the surface

[0213] The separator prepared in Example 1 was placed in a petri dish and soaked in DME for 30 minutes. Afterward, it was removed and rinsed with DME until no obvious foreign matter was visible on the surface. This soaking-rinsing process was repeated three times. The membrane was then air-dried in a glove box for 6 hours. Then, the surface of the separator was randomly selected and photographed using a scanning electron microscope (ZEISS SEM) at 30,000x magnification. Samples were randomly taken from the obtained photographs to obtain the following results: Figure 9 The photo shown. (By...) Figure 9 Using the scale in the figure, SEM dimensional calibration was used to determine that the pore size distribution of the surface of the separator prepared in Example 1 was 71.13 nm-181.7 nm.

[0214] The pore size distribution of the surface of the separator prepared in Comparative Example 1 was tested using the same method as in Example 1, and the test results are as follows: Figure 10 As shown in Table 1-1, the pore size distribution on the surface of the separator prepared in Comparative Example 1 ranges from 82.59 nm to 319.9 nm.

[0215]

[0216] Table 1-2

[0217]

[0218] In Tables 1-1 and 1-2, " / " indicates that no related items have been added.

[0219] Battery manufacturing

[0220] (1) Preparation of positive electrode sheet

[0221] The positive electrode active material (sodium iron pyrophosphate), conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were thoroughly mixed in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a weight ratio of 90:5:5 to form a uniform positive electrode slurry. The positive electrode slurry was then coated onto the surface of the positive electrode current collector aluminum foil at a coating weight of 20 mg / cm². 2 After drying and cold pressing, a positive electrode sheet is obtained.

[0222] (2) Preparation of negative electrode sheet

[0223] Carbon nanotubes (CNTs) and sodium carboxymethyl cellulose (CMC) were thoroughly mixed in an appropriate amount of solvent (deionized water) at a weight ratio of 50:50 to form an interface modification layer slurry. The interface modification layer slurry was coated on the surface of the copper foil of the negative electrode current collector to a thickness of 5 μm. After drying and cold pressing, the negative electrode sheet was obtained.

[0224] (3) Preparation of electrolyte

[0225] Fully dried NaPF6 was dissolved in a mixed solvent of ethylene glycol dimethyl ether (DME) and diethylene glycol dimethyl ether (DEGDME) in a volume ratio of 3:7 to prepare an electrolyte with a NaPF6 concentration of 1 mol / L.

[0226] (4) Battery manufacturing

[0227] The positive electrode, separator (Examples 1-8, Comparative Example 1), and negative electrode are stacked in sequence to obtain a coin cell.

[0228] Battery performance test

[0229] (1) Cyclic performance test

[0230] The coin cell battery was charged at a constant current of 1C to 3.65V under a constant temperature environment of 25℃, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, and then discharged at a constant current of 1C to 1.5V. A charge-discharge test was performed (and the first 1C discharge capacity was recorded as C0). This charge-discharge cycle was repeated until 100 cycles, and the discharge capacity after 100 cycles was recorded as C. n .

[0231] Capacity retention rate = discharge capacity after 100 cycles (C) n ) / First-cycle discharge capacity (C0).

[0232] (2) Determination of short circuit during the loop

[0233] The battery is considered to be short-circuited if it exhibits at least one of the following characteristics.

[0234] The voltage cannot be raised to the target cutoff voltage of 3.65V during charging.

[0235] The current cannot reach 0.05C during the 3.65V constant voltage stage, or the constant voltage stage lasts longer than normal.

[0236] After charging is completed, the voltage drop after 30 minutes of rest is greater than 40mV.

[0237] (3) Testing of the average surface current of the battery

[0238] The coin cell battery was charged to 3.65V at a constant current of 1C under a constant temperature environment of 25°C. The average surface current of the battery during this constant current charging process, from 2.92V (80% SOC) to 3.29V (90% SOC), was calculated to be 2mA / cm². 2 .

[0239] In a fully charged sodium secondary battery, the ratio of the unfilled dimension to the sum of the thicknesses of the entire sodium metal layer in the direction of sodium metal layer thickness is 1.5.

[0240] Table 2

[0241] Serial number Capacity retention rate (%) Short circuit case Example 1 87％ No short circuit occurred for 100 cycles Example 2 95％ No short circuit occurred for 100 cycles Example 3 92％ No short circuit occurred for 100 cycles Example 4 85％ No short circuit occurred for 100 cycles Example 5 80％ No short circuit occurred for 100 cycles Example 6 88％ No short circuit occurred for 100 cycles Example 7 93％ No short circuit occurred for 100 cycles Example 8 90％ No short circuit occurred for 100 cycles Comparative Example 1 NA Short circuit for 20 cycles

[0242] In Table 2, “NA” indicates that the battery short-circuited before reaching 100 cycles.

[0243] The data in Tables 1-1, 1-2, and 2 show that, compared to Comparative Example 1 (where the peak pore size of the separator is 36 nm), the peak pore size of the pores in the first base film of Examples 1-8 is below 35 nm, indicating that the secondary batteries of Examples 1-8 have improved safety performance. It should be noted that this application is not limited to the above 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, are also included in the scope of this application without departing from the spirit of this application.

Claims

1. A sodium secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The negative electrode sheet includes a negative electrode current collector, or includes the negative electrode current collector and a sodium metal layer disposed on the surface of the negative electrode current collector. The isolation membrane includes a first base membrane, wherein the peak pore size of the pores of the first base membrane is below 35 nm, and the peak pore size is the pore size corresponding to the highest peak in the pore size distribution curve.

2. The sodium secondary battery according to claim 1, characterized in that, The first base membrane includes a porous structure formed by fibrous material.

3. The sodium secondary battery according to claim 1 or 2, characterized in that, The first base film includes at least one polymer base film, which includes a polyethylene film, a polypropylene film, a polyimide film, and a polytetrafluoroethylene film.

4. The sodium secondary battery according to claim 3, characterized in that, The pore size distribution on the surface of the first base film is from 60 nm to 200 nm.

5. The sodium secondary battery according to any one of claims 1 to 4, characterized in that, In the first base film, the pore size distribution of the pores ranges from 2 nm to 40 nm.

6. The sodium secondary battery according to claim 5, characterized in that, In the first base film, the pore size distribution of the pores is from 15 nm to 38 nm.

7. The sodium secondary battery according to any one of claims 1 to 6, characterized in that, The peak pore size of the pores in the first base film is between 5 nm and 30 nm.

8. The sodium secondary battery according to any one of claims 1 to 7, characterized in that, The peak pore size of the pores in the first base film is between 10 nm and 25 nm.

9. The sodium secondary battery according to any one of claims 1 to 8, characterized in that, The number of pores with a diameter of 40nm to 45nm accounts for less than 5% of the total number of pores in the first base film.

10. The sodium secondary battery according to any one of claims 1 to 9, characterized in that, The proportion of pores with a diameter of 40nm to 45nm is less than or equal to 1% relative to the total number of pores in the first base film.

11. The sodium secondary battery according to any one of claims 1 to 10, characterized in that, The average pore size of the pores in the first base film is less than 30 nm.

12. The sodium secondary battery according to any one of claims 1 to 11, characterized in that, The maximum pore size of the pores in the first base film is less than 40 nm.

13. The sodium secondary battery according to any one of claims 1 to 12, characterized in that, The isolation membrane further includes a second base membrane disposed on at least one side of the first base membrane.

14. The sodium secondary battery according to claim 13, characterized in that, The peak pore size of the pores in the second base film is 36 nm to 100 nm.

15. The sodium secondary battery according to claim 14, characterized in that, The peak pore size of the pores in the second base film is 36 nm to 45 nm.

16. The sodium secondary battery according to any one of claims 1 to 15, characterized in that, The isolation membrane further includes a coating disposed on at least one side of the first base membrane.

17. The sodium secondary battery according to claim 16, characterized in that, The pore size distribution on the surface of the coating is from 50 nm to 100 nm.

18. The sodium secondary battery according to claim 16 or 17, characterized in that, The coating comprises nanocellulose and inorganic fillers, wherein the volume median particle size Dv50 of the inorganic fillers is 100 nm to 500 nm.

19. The sodium secondary battery according to claim 18, characterized in that, The nanocellulose includes a modifying group, which includes at least one of a sulfonic acid group, a boric acid group, and a phosphate group.

20. The sodium secondary battery according to claim 18 or 19, characterized in that, The average diameter of the nanocellulose is less than or equal to 40 nm.

21. The sodium secondary battery according to any one of claims 18 to 20, characterized in that, The content of the nanocellulose in the coating is greater than or equal to 5 wt%.

22. The sodium secondary battery according to any one of claims 16 to 21, characterized in that, The thickness of the coating located on one side of the first base film is less than or equal to 3 μm.

23. The sodium secondary battery according to any one of claims 1 to 22, characterized in that, The thickness of the first base film accounts for more than 30% of the thickness of the separator.

24. The sodium secondary battery according to any one of claims 1 to 23, characterized in that, The thickness of the first base film is 3μm-30μm.

25. The sodium secondary battery according to any one of claims 1 to 24, characterized in that, The air permeability of the isolation membrane is below 350s / 100mL.

26. The sodium secondary battery according to any one of claims 1 to 25, characterized in that, The first base film is disposed on the side of the second base film closer to the negative electrode sheet.

27. The sodium secondary battery according to any one of claims 1 to 26, characterized in that, The electrolyte comprises a solvent and a sodium salt dissolved in the solvent. The solvent includes an ether solvent, which further includes a first ether solvent and a second ether solvent. The first ether solvent has 2 to 4 carbon atoms, and the second ether solvent comprises R1-(O-R3)nO-R2, wherein R1 and R2 each independently comprise a straight-chain or branched alkyl group having 1 to 6 carbon atoms, and R3 comprises a straight-chain or branched alkylene group having 2 to 5 carbon atoms, where 2 ≤ n ≤ 5. Based on the total volume of the electrolyte, the volume percentage of the first ether solvent is 4% to 45%.

28. The sodium secondary battery according to any one of claims 1 to 27, characterized in that, The positive electrode sheet includes a positive electrode active material, which includes sodium iron pyrophosphate or Na. a Ni b Fe c Mn d M e O f Wherein, 0.85≤a≤1, 0≤b≤0.3, 0≤c≤0.4, 0≤d≤0.4, 0.02≤e≤0.1, 1.8≤f≤2, and b+c+d+e+f>0; M includes at least one of Zn, V, Cr, Al, Sc, Sn, Sb, Zr, Nb, Ti, Mg, Cu, Ru, Ir, and Ca.

29. The sodium secondary battery according to any one of claims 1 to 28, characterized in that, When the sodium secondary battery is fully charged, the ratio of the unfilled dimension within the sodium secondary battery to the sum of the thicknesses of all sodium metal layers in the direction of sodium metal layer thickness is 1-2.

30. The sodium secondary battery according to any one of claims 1 to 29, characterized in that, When the state-of-charge percentage of the sodium secondary battery is 80% to 90%, the average surface current of the sodium secondary battery is ≤7 mA / cm². 2 .

31. An electrical device, characterized in that, The sodium secondary battery includes any one of claims 1 to 30.

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