Sodium secondary battery and electric device

Abstract

A sodium secondary battery and an electric device. The sodium secondary battery comprises a positive electrode sheet, a negative electrode sheet, and a separator provided between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector, or comprises a negative electrode current collector and a sodium metal layer provided on a surface of the negative electrode current collector. The separator comprises a first film layer, wherein the pore diameter peak of ventilation holes of the first film layer is 35 nm or less, and is the pore diameter corresponding to the highest peak in the pore diameter distribution curve.

Description

Sodium secondary battery and electric device

[0001] Cross Reference to Related Applications

[0002] The present disclosure is based on and claims priority to Chinese Patent Application No. 202411084602.X, filed on August 8, 2024, entitled “Sodium secondary battery and electric device”, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to the technical field of batteries, and in particular to a sodium secondary battery and an electric device. BACKGROUND

[0004] 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 hydropower, thermal power, wind power and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, etc.

[0005] As a typical secondary battery, the anode-free sodium metal battery has the advantages of abundant raw material resources and low cost compared with lithium secondary batteries, 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

[0006] The present disclosure 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 dendrites from penetrating the separator membrane and have excellent safety performance.

[0007] To achieve the above-mentioned purpose, the first aspect of the present disclosure 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 of the air permeable hole of the first base film being 35 nm or less, the pore size peak being the pore size corresponding to the highest peak in the pore size distribution curve. In the present disclosure, by making the pore size peak 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 dendrites penetrating the separator membrane, and improving the safety of the battery.

[0008] 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.

[0009] In some embodiments, the first base film comprises at least one layer of a polymeric base film, and the polymeric base film comprises a polyethylene film, a polypropylene film, a polyimide film, and a polytetrafluoroethylene film. Thereby, it is helpful to further reduce the probability of the isolation film being penetrated.

[0010] In some embodiments, the pore size distribution of the surface of the first base film is 60 nm to 200 nm. Thereby, it is helpful to further reduce the formation of sodium dendrites.

[0011] In some embodiments, the pore size distribution of the air-permeable pores in the first base film is 2 nm to 40 nm. Thereby, it is helpful to further reduce the formation of sodium dendrites.

[0012] In some embodiments, the pore size distribution of the air-permeable pores in the first base film is 15 nm to 38 nm. Thereby, it is helpful to further reduce the formation of sodium dendrites. In some embodiments, the peak pore size of the air-permeable pores of the first base film is 5 nm to 30 nm. By making the peak pore size of the air-permeable pores of the first base film within 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 the sodium ions 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.

[0013] In some embodiments, the peak pore size of the air-permeable pores of the first base film is 10 nm to 25 nm. By making the peak pore size of the air-permeable pores of the first base film within 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.

[0014] In some embodiments, the number of air-permeable pores with a pore size of 40 nm to 45 nm accounts for less than 5% of the total number of air-permeable pores of the first base film. By making the number of pores with a pore size of 40 nm to 45 nm within the above range, the risk of the isolation film being penetrated by sodium dendrites can be reduced.

[0015] In some embodiments, the number of air-permeable pores with a pore size of 40 nm to 45 nm accounts for less than or equal to 1% of the total number of air-permeable pores of the first base film. By making the number of pores with a pore size of 40 nm to 45 nm within the above range, the risk of the isolation film being penetrated by sodium dendrites can be further reduced.

[0016] 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 within the above range reflects that there are more small-pore channels in the first base film, which further reduces the risk of sodium dendrites penetrating the isolation film.

[0017] In some embodiments, the maximum pore size of the air-permeable pores of the first base film is 40 nm or less. By having the maximum pore size of the air-permeable pores of the first base film in the above range, the local deposition of sodium ions on the surface of the negative electrode is reduced, and the formation of sodium dendrites is inhibited, which is conducive to further reducing the probability of sodium dendrites penetrating the separator film.

[0018] In some embodiments, the separator 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 separator film.

[0019] In some embodiments, the peak pore size of the air-permeable pores of the second base film is 36 nm to 100 nm. By having the peak pore size of the air-permeable pores of the second base film in the above range, it is conducive to reducing the probability of the separator film being penetrated.

[0020] In some embodiments, the peak pore size of the air-permeable pores of the second base film is 36 nm to 45 nm. By having the peak pore size of the air-permeable pores of the second base film in the above range, it is conducive to further reducing the probability of the separator film being penetrated.

[0021] In some embodiments, the separator 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 separator film.

[0022] In some embodiments, the pore size distribution of the surface of the coating layer is 50 nm to 100 nm. Thus, it is conducive to further inhibiting the formation of sodium dendrites.

[0023] 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 separator film.

[0024] In some embodiments, the nanocellulose comprises a modification group, and the modification group comprises 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 (e.g., at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group) in the coating can effectively improve the heat stability of the separator 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 itself has a certain rigidity, which can ensure the bonding force between the coating and the first base film, while also ensuring the ion conduction efficiency and improving the extrusion resistance of the separator film, 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, thereby reducing the viscosity of the coating slurry, which reduces the difficulty of coating the slurry and improves the uniformity of the coating, thereby improving the production efficiency of the separator film.

[0025] 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 stability of the separator film can be further improved, and the thermal shrinkage of the separator film can be reduced.

[0026] In some embodiments, the content of the nanocellulose in the coating is greater than or equal to 5 wt%. When the content of the nanocellulose in the coating is within the above given range, the coating slurry containing the nanocellulose can have a more suitable viscosity, which is more conducive to coating. In addition, the nanocellulose itself or in combination with other components in the coating can build a stable space network structure, which can further increase the ion conduction channels of the separator film, improve the extrusion resistance and voltage breakdown resistance, and thereby further improve the thermal safety of the battery.

[0027] In some embodiments, the thickness of the coating on one side of the first base film is less than or equal to 3 μm. When the thickness of the coating is within the given range, the energy density can be further improved while ensuring the safety performance of the battery.

[0028] 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% of the thickness of the separator film, the thicker first base film can provide better barrier action, further inhibiting the penetration of sodium dendrites through the separator film.

[0029] 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 to better inhibit the penetration of sodium dendrites through the separator film.

[0030] In some embodiments, the air permeability of the separator film is 350 s / 100 mL or less. The air permeability of the separator film in the above range is conducive to reducing the risk of short circuit of the battery.

[0031] 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 dendrites can be more effectively inhibited from penetrating the separator film.

[0032] 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 fraction of the first ether solvent is 4%-45% based on the total volume of the electrolyte. By selecting the above electrolyte, the dissolution of the sodium metal layer is reduced.

[0033] In some embodiments, the positive electrode tab includes a positive electrode active material, and 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 comes into contact with the battery separator film, thereby to some extent alleviating the risk of dendrite penetration of the separator film.

[0034] In some embodiments, the ratio of the sum of the size of the un-filled space in the sodium secondary battery and the thickness of the entire sodium metal layer in the thickness direction of the sodium metal layer under full charge is 1-2. In this way, the un-filled space in the sodium secondary battery can be used as a Na free growth space. The energy barrier that needs to be overcome by Na growing into the air permeable hole of the separator film is higher than the height direction energy barrier that needs to be overcome by Na free growth. This is conducive to the lateral growth of Na during deposition, rather than growing into the separator film, thereby further reducing the probability of the separator film being penetrated.

[0035] In some embodiments, the average surface current of the sodium secondary battery is ≤ 7 mA / cm2when the state of charge percentage of the sodium secondary battery is 80% to 90%. 2 By controlling the average surface current of the sodium secondary battery in the above range, it is advantageous to suppress the generation of sodium dendrites, thereby reducing the probability of sodium dendrites penetrating the separator. The second aspect of the present disclosure provides an electric device including the sodium secondary battery of the first aspect of the present disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1A is a schematic view of a separator according to an embodiment of the present disclosure.

[0037] FIG. 1B is a schematic view of a separator according to another embodiment of the present disclosure.

[0038] FIG. 1C is a schematic view of a separator according to still another embodiment of the present disclosure.

[0039] FIG. 2 is a schematic view of a battery cell according to an embodiment of the present disclosure.

[0040] FIG. 3 is an exploded view of the battery cell according to an embodiment of the present disclosure shown in FIG. 2.

[0041] FIG. 4 is a schematic view of a battery module according to an embodiment of the present disclosure.

[0042] FIG. 5 is a schematic view of a battery pack according to an embodiment of the present disclosure.

[0043] FIG. 6 is an exploded view of the battery pack according to an embodiment of the present disclosure shown in FIG. 5.

[0044] FIG. 7 is a schematic view of an electric device using the sodium secondary battery according to an embodiment of the present disclosure as a power source.

[0045] FIG. 8 is a pore size distribution curve of the separator according to Example 1 of the present disclosure and the separator according to Comparative Example 1, which were tested in a test environment with a temperature of 25°C and a humidity of 2% RH.

[0046] FIG. 9 is a surface SEM photograph of the separator according to Example 1 of the present disclosure.

[0047] FIG. 10 is a surface SEM photograph of the separator according to Comparative Example 1 of the present disclosure.

[0048] BRIEF DESCRIPTION OF DRAWINGS DETAILED DESCRIPTION

[0049] Hereinafter, embodiments of the sodium secondary battery and the electric device of the present disclosure are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed description is omitted. For example, there are cases where detailed description of matters known well, repeated description of substantially the same structure 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 drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter recited in the claims.

[0050] The "ranges" disclosed in the present disclosure are defined in the form of lower and upper limits, and a given range is defined by selecting one lower limit and one upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The ranges defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present disclosure, unless otherwise stated, a numerical range "a-b" represents a shorthand manner of describing each and every numerical value that is contained in the range between "a" and "b," wherein "a" and "b" are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0" and "5" have been listed herein, and "0-5" is merely a shorthand manner of describing each and every numerical value that is contained in the range between "0" and "5." In addition, when it is stated that a parameter is 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, and the like.

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

[0052] If not specifically stated, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions.

[0053] If not specifically stated, all steps of the present disclosure can be performed in sequence or randomly, and preferably in sequence. For example, a method includes steps (a) and (b) indicates that the method can include steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, it is mentioned that the method can further include step (c), which indicates that step (c) can be added to the method in any order, for example, the method can include steps (a), (b), and (c), or steps (a), (c), and (b), or steps (c), (a), and (b), and the like.

[0054] The terms used in the present disclosure have the meanings commonly understood by those skilled in the art if not specifically defined.

[0055] The values of the parameters mentioned in the present disclosure can be measured by various test methods commonly used in the art if not specifically defined, for example, can be measured according to the test methods given in the present disclosure.

[0056] Currently, in the traditional sodium ion battery, sodium ions can be inserted into the negative electrode active material through the electrolyte, and the electrolyte usually uses an ester solvent that can dissolve sodium metal, so it is generally not easy to generate sodium dendrites at the negative electrode. In contrast, in the negative electrode-free sodium battery, the negative electrode plate does not contain a traditional negative electrode active material layer, and during the charging process, sodium ions are reduced on the surface of the negative electrode current collector and deposited to form a sodium metal layer. This leads to the problem that sodium dendrites grow and penetrate the separator membrane in the negative electrode-free sodium metal battery, which seriously affects the safety performance of the battery.

[0057] In view of the above, the present disclosure proposes a sodium secondary battery and a power utilization device. The present disclosure and the optional embodiments are described in more detail below.

[0058] The first aspect of the present disclosure provides a sodium secondary battery, which comprises a positive electrode plate, a negative electrode plate, and a separator membrane arranged between the positive electrode plate and the negative electrode plate, the negative electrode plate comprises a negative electrode current collector and / or a sodium metal layer arranged on the surface of the negative electrode current collector, and the separator membrane comprises a first base film, the pore size peak of the gas permeable hole in the first base film is 35 nm or less, and the pore size peak is the pore size corresponding to the highest peak in the pore size distribution curve.

[0059] In the research, the inventors disassembled and analyzed a large number of short-circuited negative electrode-free sodium batteries and found that in the negative electrode-free sodium battery, in addition to the negative electrode side, sodium dendrites mainly deposit and grow in the pores of the separator membrane, and then pierce the separator membrane. Moreover, the deposition size of the metal sodium dendrites in the separator membrane is mostly between 40 nm and 100 nm. This leads to the problem that when the existing separator membrane is used in the negative electrode-free sodium battery, due to the excessively large pore size of the existing separator membrane and the large number of large-size pores, the sodium dendrites deposited in the separator membrane can easily penetrate the separator membrane, leading to battery failure. In addition, the inventors have found that the modulus of sodium metal is lower than that of lithium metal, and it is more likely to yield under stress, so the restriction of the pore size of the separator membrane can block the sodium dendrites. Based on the above findings, in the present disclosure, the pore size peak of the gas permeable hole of the first base film of the separator membrane is 35 nm or less, the size restriction is used to inhibit the growth of the dendrites in the thickness direction of the separator membrane, the growth direction is changed, and the risk of short circuit caused by the penetration of the sodium dendrites through the separator membrane is reduced, thereby improving the safety of the battery.

[0060] Separator membrane

[0061] In the present disclosure, the separator film comprises a first base film, which is a film layer with air permeable pores. In the present disclosure, the air permeable pores refer to pores that allow sodium ions to penetrate the base film. These air permeable pores can serve as channels for sodium ions to transport between the positive electrode and the negative electrode. In some embodiments, the separator film can comprise one layer of the first base film. In some embodiments, the separator film can comprise multiple layers of the first base film. When the first base film is multi-layered, the materials of each layer can be the same or different, and there is no particular limitation.

[0062] In the present disclosure, the pore size peak of the air permeable pores can be determined in the following manner. The separator film to be tested can be a prepared separator film or a separator film obtained by disassembling a battery. Hereinafter, the testing process is described by way of example using the latter. Specifically, the separator film is obtained by disassembling a battery, and the disassembled separator film is placed in a petri dish and soaked in DME for 30 min. Then, the separator film is rinsed with DME until no obvious foreign matter is present on the surface. The soaking and rinsing steps are repeated three times, and the separator film is then dried in a glove box for 6 h. After obtaining the separator film material, the pore size distribution curve is determined using a pore size tester (model: PMI Porometer). In some embodiments, the pore size of the air permeable pores of the first base film can be tested in accordance with GB / T 21650.2-2008. Then, based on the test results, the pore size distribution curve is plotted using the PMI Porometer software. Further, the pore size peak of the air permeable pores can be read from the pore size distribution curve. In the present disclosure, the pore size peak of the air permeable pores of the first base film is ≤ 35 nm, for example, the pore size peak of the first base film is 35 nm, 30 nm, 27 nm, 24 nm, 21 nm, 18 nm, 15 nm, 12 nm, 9 nm, 6 nm, 3 nm, 2 nm, or a value within a range defined by any two of these values.

[0063] In some embodiments, the pore size peak of the air permeable pores of the first base film is 5 nm to 30 nm. By setting the pore size peak of the air permeable pores of the first base film within the above range, the pore size of the first base film is small and uniform, which helps to reduce the local deposition of sodium ions on the negative electrode surface, inhibit the formation of sodium dendrites, and reduce the probability of sodium ions deposited in the air permeable pores 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. Alternatively, the pore size peak of the air permeable pores of the first base film is 10 nm to 25 nm.

[0064] In some embodiments, the size distribution of the pore size of the air permeable pores in the first base film is 2 nm to 40 nm. Alternatively, it is 15 nm to 38 nm. In this way, it is further helpful to inhibit the formation of sodium dendrites.

[0065] In the present disclosure, the size distribution of the pore size of the air-through pores can be determined as follows: the isolation film to be tested can be a prepared isolation film or an isolation film obtained by disassembling a battery. Hereinafter, the testing process is described by taking the latter as an example. Specifically, the isolation film is obtained by disassembling a battery, and the disassembled isolation film is placed in a petri dish and soaked in DME for 30 min. Then, the surface is rinsed with DME until no obvious foreign matter is present on the surface. The soaking and rinsing are repeated for another 3 times, and the film is dried in a glove box for 6 h. Then, a pore size tester (model: PMI Porometer) can be used to test the pore size of the air-through pores of the base film according to GB / T 21650.2-2008. Based on the test results, the size distribution of the pore size of the air-through pores is generated by using the PMI Porometer software.

[0066] In some embodiments, the number of pores having a pore size of 40 nm to 45 nm accounts for less than 5% of the total number of air-through pores in the first base film, for example, the number accounts for 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 a value between any two of the above values. By making the number of pores having a pore size of 40 nm to 45 nm account for a value within the above range, the risk of the isolation film being penetrated by sodium dendrites can be further reduced. Alternatively, the number accounts for ≤1%, further alternatively, the number accounts for ≤0.5%, and further alternatively, the number accounts for 0.

[0067] In the present disclosure, the number of pores having a pore size of 40 nm to 45 nm can be determined as follows: the isolation film to be tested can be a prepared isolation film or an isolation film obtained by disassembling a battery. Hereinafter, the testing process is described by taking the latter as an example. Specifically, the isolation film is obtained by disassembling a battery, and the disassembled isolation film is placed in a petri dish and soaked in DME for 30 min. Then, the surface is rinsed with DME until no obvious foreign matter is present on the surface. The soaking and rinsing are repeated for another 3 times, and the film is dried in a glove box for 6 h. Then, a pore size tester (model: PMI Porometer) can be used to test the pore size of the air-through pores of the base film according to GB / T 21650.2-2008. Based on the test results, the number of pores having a pore size of 40 nm to 45 nm is counted by using the PMI Porometer software.

[0068] In some embodiments, the average pore size of the air-through pores of the first base film is 30 nm or less. For example, the average pore size of the air-through pores of the first base film is 30 nm, 27 nm, 24 nm, 21 nm, 18 nm, 15 nm, 12 nm, 9 nm, 6 nm, 3 nm, 2 nm, or a value between any two of the recited values. Having the average pore size of the air-through pores of the first base film in the above range reflects that the first base film has more small-pore-size pores, which further reduces the risk of sodium dendrites penetrating the separator film. Alternatively, the average pore size of the air-through pores of the first base film is 5 nm to 0 nm, and further alternatively, the average pore size of the air-through pores of the first base film is 10 nm to 25 nm.

[0069] For the average pore size, a pore size tester (Model: PMI Porometer) can be used to test the pore size of the air-through pores of the base film, according to GB / T 21650.2-2008. Based on the test results, the average pore size of the air-through pores of the base film can be determined using the PMI Porometer software.

[0070] In some embodiments, the maximum pore size of the air-through pores of the first base film is 40 nm or less, for example, the maximum pore size of the air-through pores of the first base film is 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 a value between any two of the recited values. By having the maximum pore size of the air-through pores of the first base film in the above range, it helps to reduce the local deposition of sodium ions on the surface of the negative electrode and inhibit the formation of sodium dendrites, which is conducive to further reducing the probability of sodium dendrites penetrating the separator film. Alternatively, the maximum pore size of the air-through pores of the first base film is 35 nm or less.

[0071] In the present disclosure, the maximum pore size of the air-through pores can be determined as follows: the separator film to be tested can be a prepared separator film or a separator film obtained by disassembling a battery. Hereinafter, the test process is described by taking the latter as an example. Specifically, the battery is disassembled to obtain the separator film, and the disassembled separator film is placed in a petri dish and soaked in DME for 30 min. Then, the separator film is rinsed with DME until no obvious foreign matter is present on the surface. The soaking and rinsing are repeated for another 3 times, and the separator film is dried in a glove box for 6 h. Then, a pore size tester (Model: PMI Porometer) can be used to test the pore size of the air-through pores of the base film, according to GB / T 21650.2-2008. Based on the test results, the maximum pore size of the air-through pores of the base film can be determined using the PMI Porometer software.

[0072] In some embodiments, the thickness of the first base film accounts for 30% or more of the thickness of the isolation film. For example, the thickness of the first base film accounts for 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, or a value between any two of the foregoing. By having the thickness of the first base film account for a value in the foregoing range, the thicker first base film can provide a better barrier, further inhibiting the sodium dendrites from penetrating the isolation film. Optionally, the thickness of the first base film accounts for 50% or more.

[0073] In some embodiments, the thickness of the first base film is 3 pm to 30 pm, for example, the thickness of the first base film is 3 pm, 5 pm, 6 pm, 7 pm, 8 pm, 9 pm, 10 pm, 20 pm, 25 pm, 30 pm, or a value between any two of the foregoing. By having the thickness of the first base film account for a value in the foregoing range, the thicker first base film can provide a better barrier, further inhibiting the sodium dendrites from penetrating the isolation film. Optionally, the thickness of the first base film is 5 pm to 25 pm.

[0074] In some embodiments, the first base film includes a porous structure formed by a fibrous material. As such, the isolation film is more likely to maintain its structural integrity when subjected to an external force exerted by the sodium dendrites, thereby reducing the likelihood of being penetrated.

[0075] In some embodiments, the first base film includes at least one polymeric base film, which includes a polyethylene film, a polypropylene film, a polyimide film, and a polytetrafluoroethylene film. As such, the likelihood of the isolation film being penetrated is reduced. In some embodiments, the first base film includes at least one polyethylene film. The polyethylene film has good flexibility, and has a cross-linked porous structure and uniform mechanical strength, such that the isolation film is more likely to maintain its structural integrity when subjected to an external force exerted by the sodium dendrites, thereby reducing the likelihood of being penetrated.

[0076] In some embodiments, the pore size distribution of the surface of the first base film is 60 nm to 200 nm. As such, the formation of sodium dendrites is further inhibited. Optionally, the pore size distribution of the surface of the first base film is 70 nm to 180 nm.

[0077] In the present disclosure, the pore size distribution of the surface of the first base film can be determined as follows: specifically, the separator film to be tested can be a prepared separator film or a separator film disassembled from a battery. The latter is specifically described. First, the separator film is disassembled from the battery, and the disassembled separator film is placed in a surface dish and soaked in DME for 30 min. Then, the surface is rinsed with DME until no obvious foreign matter is present on the surface. The soaking and rinsing are repeated again for 3 times, and the separator film is dried in a glove box for 6 h. Then, the surface of the first base film is observed using a scanning electron microscope (ZEISS SEM). Then, the surface of the separator film is randomly selected and photographed using a scanning electron microscope (ZEISS SEM) with a magnification of 30000 times. The pore size of the surface of the first base film is determined using SEM size calibration, and the pore size distribution of the surface of the first base film is determined using Nano Measurer software.

[0078] FIG. 1A is a schematic diagram of a separator film according to an embodiment of the present disclosure. In some embodiments, the separator film only includes a first base film 10.

[0079] FIG. 1B is a schematic diagram of a separator film according to an embodiment of the present disclosure. As can be seen, in some embodiments, the separator film includes a first base film 10 and a second base film 20 arranged on at least one side of the first base film 10. By arranging the second base film, a multi-layer barrier is formed against the growth of sodium dendrites, the blocking effect is enhanced, and the penetration of sodium dendrites through the separator film is inhibited. Optionally, the pore size peak of the air permeable hole of the second base film is 36 nm to 100 nm. For example, the pore size peak of the air permeable hole of the second base film is 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 a value between any two of the above values. Optionally, the pore size peak of the air permeable hole of the second base film is 36 nm to 45 nm.

[0080] FIG. 1B is only an example showing an example in which the second base film is arranged on one side of the first base film. In an embodiment, the second base film can be arranged between two layers of the first base film. In an embodiment, the first base film can be arranged between two layers of the second base film.

[0081] 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 penetration of sodium dendrites through the separator film can be more effectively inhibited.

[0082] The second base film is not particularly limited in the present disclosure, and any known base film having good chemical stability and mechanical stability can be used. In some embodiments, the material of the second base film can include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride.

[0083] In an implementation, the isolation film further includes 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.

[0084] FIG. 1C is a schematic diagram of an isolation film according to an embodiment of the present disclosure. In an implementation, the isolation film includes a first base film 10, a second base film 20, and a coating layer 30, wherein the coating layer 30 is disposed on a side of the first base film 10 away from the second base film 20.

[0085] In an implementation, the isolation film includes a first base film, a second base film, and a coating layer. The coating layer is arranged between the first base film and the second base film. In an implementation, the isolation film includes only the first base film and the coating layer, and the coating layer is arranged on the surface of the first base film.

[0086] In some embodiments, the pore size distribution of the surface of the coating layer is 50 nm to 100 nm. In this way, it is helpful to further inhibit the formation of sodium dendrites.

[0087] In the present disclosure, the pore size distribution of the surface of the coating layer can be determined by the following method, which can refer to the method for determining the pore size distribution of the surface of the first base film.

[0088] In some embodiments, the coating layer includes 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.

[0089] Exemplary inorganic fillers include at least one of boehmite (γ-AlOOH), barium sulfate (BaSO4), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), silicon oxide compound SiOx(0

[0090] In some embodiments, the nanocellulose includes a modification group, and the modification group includes at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group. In this implementation, the nanocellulose with the specific modification group (e.g., at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group) in the coating layer can effectively improve the heat resistance of the separator 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 the modification group in the coating layer itself has a certain rigidity, which can ensure the bonding force between the coating layer and the base film, and also can ensure the ion conduction efficiency and improve the extrusion resistance of the separator film, 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, so the viscosity of the coating slurry can be reduced, thereby reducing the difficulty of coating the slurry and improving the uniformity of the coating, and further improving the production efficiency of the separator film.

[0091] 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 thermal shrinkage of the separator film can be reduced.

[0092] 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. In addition, the nanocellulose itself or in combination with other components in the coating layer can build a stable space network structure, which can further increase the ion conduction channels of the separator film, improve the extrusion resistance and voltage breakdown resistance, and thereby further improve the thermal safety of the battery.

[0093] In some embodiments, the thickness of the coating layer on one side of the first base film is less than or equal to 3 pm. When the thickness of the coating layer is within the given range, the energy density can be further improved while ensuring the safety performance of the battery.

[0094] In some embodiments, the coating layer comprises an organic coating layer. The organic coating layer refers to being made of a high molecular compound. The adhesive layer includes but is not limited to polyurethane film, polyethylene film, and silicone film. Arranging the adhesive layer on the surface of the base film is conducive to further improving the toughness of the separator film, reducing the probability of the separator film being penetrated, and further improving the durability of the separator film.

[0095] In some embodiments, the air permeability of the separator film is less than or equal to 350 s / 100 mL, for example, the air permeability of the separator film is 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 a value within a range between any two of the above values. The air permeability of the separator film within the above range is conducive to reducing the risk of battery short circuit. Optionally, the air permeability of the separator film is 250 s / 100 mL to 320 s / 100 mL, and further optionally, the air permeability of the separator film is 250 s / 100 mL to 300 s / 100 mL.

[0096] In the present disclosure, the air permeability of the separator film can be tested by the following method: the separator film to be tested can be a prepared separator film or a separator film obtained by disassembling a battery. Hereinafter, the testing process is described by taking the latter as an example. Specifically, the separator film is obtained by disassembling a battery, and the disassembled separator film is placed in a petri dish and soaked in DME for 30 min. The DME is flushed out until there is no obvious foreign matter on the surface, and the soaking-flushing process is repeated for 3 times. The petri dish is dried in a glove box for 6 h. Then, the air permeability of the separator film is tested according to GB / T 458-2008.

[0097] In addition, for the preparation method of the separator film of the present disclosure, the following method can be used, which comprises the following steps.

[0098] Step (1): The pore-forming agent (white oil) is mixed with the precursor (polyethylene resin) at a mass ratio of 0.5-5:100.

[0099] Step (2): The mixture is heated and melted, and then cooled for phase separation. Then, the mixture is pressed into a cast sheet, and then annealed at a temperature of 100-150°C under bidirectional micro-tension.

[0100] Step (3): Oriented stretching of the cast sheet in longitudinal direction or biaxial direction to obtain the first base film of the present disclosure.

[0101] In the longitudinal stretching, the longitudinal stretching ratio is 5-50 times.

[0102] In the biaxial stretching, the longitudinal stretching ratio is 5-50 times and the transverse stretching ratio is 5-50 times.

[0103] 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 value between the range consisting of any two of the values.

[0104] In some embodiments, the annealing treatment time in step (2) is 10s-60s.

[0105] In some embodiments, the pore-forming agent remaining on the surface / inside of the first base film can be extracted with a solvent after step (3). The pore-forming agent can also be evaporated by drying after step (3). The solvent is not specifically limited in the present disclosure and any solvent known in the art for extracting the pore-forming agent can be used.

[0106] In addition, the coating preparation method can use any coating preparation method known in the art, which is not specifically limited in the present disclosure.

[0107] The sodium secondary battery further comprises a positive electrode sheet, a negative electrode sheet, and an electrolyte. During the charging and discharging of the battery, sodium ions are inserted into and extracted from the positive electrode sheet and the negative electrode sheet. The electrolyte serves to conduct ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly serving to prevent short circuit between the positive and negative electrodes, while allowing ions to pass through.

[0108] The positive electrode sheet

[0109] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer comprises the positive electrode active material of the first aspect of the present disclosure.

[0110] For example, the positive electrode current collector has two opposite surfaces in the thickness direction of itself, and the positive electrode film layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector.

[0111] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0112] In some embodiments, the positive electrode tab includes a positive active material, and the positive active material 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, Ca. In a battery employing the above positive active material, the main component of the sodium dendrite is sodium metal, and due to the low Young's modulus of the sodium metal, the growth path of the sodium dendrite is more likely to change when it comes into contact with the battery separator, thereby to some extent alleviating the risk of dendrite penetration through the separator.

[0113] In some embodiments, the positive active material can employ a positive active material for a sodium secondary battery known in the art. As an example, the positive active material can include a sodium transition metal oxide, a polyanion compound, a Prussian blue compound, etc. The positive active material can include, but is not limited to, one or more of NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, Na 0.67 MO2(M includes at least two of Fe, Co, Cr, Mn, Ni, V, Ti, Mo), NaMO2(M includes at least two of Fe, Co, Ni, V, Ti, Mo), NaFePO4, NaMnPO4, NaCoPO4, Na4Fe3(PO4)2O7, Na3V2(PO4)2F3, Na3V2(PO4)3, Prussian blue, Prussian white, sodium iron pyrophosphate, and respective modified compounds thereof.

[0114] In some embodiments, the ratio of the sum of the size of the unfilled space in the sodium secondary battery and the thickness of the entire sodium metal layer in the thickness direction of the sodium metal layer is 1 to 2, for example, 1, 1.2, 1.4, 1.6, 1.8, 2, or a value between any two of the values, when the sodium secondary battery is in a full charge state. Alternatively, the ratio is 1.05 to 1.5. In this way, the unfilled space in the sodium secondary battery can serve as a Na free growth space, and the energy barrier that Na needs to overcome to grow into the gas permeable hole of the separation film is higher than the energy barrier that Na needs to overcome to grow in the height direction of the free growth, which helps Na to grow horizontally rather than into the separation film, thereby further reducing the probability of the separation film being penetrated.

[0115] In the present disclosure, “full charge” refers to the state of charge of the battery cell reaching 100% or being charged to the highest cut-off voltage of the sodium secondary battery cell.

[0116] In some embodiments, the average surface current of the sodium secondary battery is ≤7 mA / cm 2 , for example, 7 mA / cm 2 , 6 mA / cm 2 , 5 mA / cm 2 , 4 mA / cm 2 , 3 mA / cm 2 , or a value between any two of the values, when the state of charge of the sodium secondary battery is 80% to 90%. 2 By controlling the average surface current within the above range, the generation of sodium dendrites can be inhibited, thereby reducing the probability of the sodium dendrites penetrating the separation film.

[0117] The battery will undergo Na deintercalation and consumption during the charging and discharging process, and the molar content of Na is different when the battery is discharged to different states. In the present disclosure, the molar content of Na in the listing of the positive electrode active material is the initial state of the material, i.e., the state before feeding, and the positive electrode active material is applied to the battery system. After charging and discharging cycles, the molar content of Na will change.

[0118] In the present disclosure, the molar content of O in the listing of the positive electrode active material is only the theoretical state value, and the release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0119] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene-fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene-fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0120] 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.

[0121] 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.

[0122] Negative electrode sheet

[0123] In this disclosure, 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.

[0124] 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.

[0125] 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.).

[0126] In some embodiments, the surface of the negative current collector can further be provided with a coating. The coating can optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The coating can further optionally include a conductive agent. The conductive agent can 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 can further optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc. 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 on the negative current collector to form a coating; and then performing processes such as drying, cold pressing, etc., to obtain the negative electrode sheet.

[0127] Electrolyte

[0128] The electrolyte serves to conduct ions between the positive electrode sheet and the negative electrode sheet.

[0129] In some embodiments, the electrolyte includes an ether-based solvent and a sodium salt dissolved in the ether-based solvent. The use of the above-mentioned electrolyte is conducive to reducing the dissolution of the sodium metal layer.

[0130] In some embodiments, the electrolyte includes a solvent and a sodium salt dissolved in the solvent; wherein the solvent includes an ether-based solvent, the ether-based solvent includes a first ether-based solvent and a second ether-based solvent, the first ether-based solvent includes 2-4 carbon atoms, the second ether-based 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 fraction of the first ether-based solvent is 4%-45% based on the total volume of the electrolyte. The use of the above-mentioned electrolyte is conducive to reducing the dissolution of the sodium metal layer.

[0131] In some embodiments, the first ether-based solvent includes one or more of ethylene glycol dimethyl ether, and / or the second ether-based 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.

[0132] In some embodiments, the mass ratio of the short-chain ether to the long-chain ether is (4-45):(29-90).

[0133] In some embodiments, the sodium salt includes at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bisfluorosulfonimide, sodium bis-trifluoromethanesulfonimide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluoroboric oxalate, sodium boric oxalate, sodium difluoroboric dioxalate phosphate, and sodium tetrafluoroboric dioxalate phosphate. Optionally, the sodium salt includes sodium hexafluorophosphate.

[0134] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include an additive that can improve certain performance of the battery, such as an additive that can improve overcharge performance of the battery, an additive that can improve high or low temperature performance of the battery, etc.

[0135] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be formed into an electrode assembly through a winding process or a stacking process.

[0136] In some embodiments, the battery cell can include an outer package. The outer package can be used to encapsulate the electrode assembly and the electrolyte solution described above.

[0137] In some embodiments, the outer package of the battery cell can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the battery cell can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0138] The shape of the battery cell is not particularly limited in the present disclosure, and can be cylindrical, square, or any other arbitrary shape. For example, FIG. 2 is a battery cell 5 of a square structure as an example.

[0139] In some embodiments, referring to FIG. 3, the outer package can include a shell 51 and a top cover assembly 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening in communication with the receiving cavity, and the top cover assembly 53 can be provided on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator film can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte solution is impregnated in the electrode assembly 52. The number of the electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by a person skilled in the art according to specific actual needs.

[0140] In some embodiments, the battery cell can be assembled into a battery module, and the number of the battery cells contained in the battery module can be one or more, which can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0141] FIG. 4 is a battery module 4 as an example. Referring to FIG. 4, in the battery module 4, a plurality of battery cells 5 can be arranged in series along a length direction of the battery module 4. Of course, the plurality of battery cells 5 can be arranged in any other manner. The plurality of battery cells 5 can be fixed by fasteners.

[0142] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of battery cells 5 can be accommodated in the accommodation space.

[0143] In some embodiments, the above-described battery module can be assembled into a battery pack, and the number of battery modules included in the battery pack can be one or more, and the specific number can be selected by a person skilled in the art according to the application and capacity of the battery pack.

[0144] FIGS. 5 and 6 are a battery pack 1 as an example. Referring to FIGS. 5 and 6, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0145] Electric device

[0146] In addition, the second aspect of the present disclosure also provides an electric device, which includes the sodium secondary battery provided by the first aspect of the present disclosure. The sodium secondary battery can be used as a power supply of the electric device, or can be used as an energy storage unit of the electric device. The electric device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0147] As the electric device, the battery cell, the battery module or the battery pack can be selected according to the use requirements thereof.

[0148] FIG. 7 is an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the sodium secondary battery for the electric device, a battery pack or a battery module can be used.

[0149] As another example of the device, it can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and a battery cell can be used as a power supply.

[0150] Embodiments

[0151] Hereinafter, the embodiments of the present disclosure will be described. The embodiments described below are exemplary and are for the purpose of explanation of the present disclosure and should not be understood as a limitation of the present disclosure. In the embodiments, the specific techniques or conditions not mentioned are performed in accordance with the techniques or conditions described in the literature in the field or in accordance with the product manual. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be obtained on the market.

[0152] Example 1

[0153] Preparation of the separation film

[0154] Step (1): The polyethylene resin and white oil were mixed and stirred to form a mixture, wherein the mass ratio of the polyethylene resin to the white oil was 95:5.

[0155] Step (2): The mixture was melt plasticized at a temperature of 130°C, extruded through a die to form a cast sheet, and then subjected to annealing under bidirectional micro tension, with an annealing temperature of 110°C and an annealing time of 40s.

[0156] Step (3): Longitudinal stretching was performed with a stretching ratio of 30, to obtain a first base film with a thickness of 14μm, which was used as the separation film.

[0157] Example 2

[0158] Step (1): The polyethylene resin and white oil were mixed and stirred to form a mixture, wherein the mass ratio of the polyethylene resin to the white oil was 9:1.

[0159] Step (2): The mixture was melt plasticized at a temperature of 130°C, extruded through a die to form a cast sheet, and then subjected to annealing under bidirectional micro tension, with an annealing temperature of 100°C and an annealing time of 60s.

[0160] Step (3): Longitudinal stretching was performed with a stretching ratio of 45, to obtain a first base film with a thickness of 14μm, which was used as the separation film.

[0161] Example 3

[0162] Step (1): The polyethylene resin and white oil were mixed and stirred to form a mixture, wherein the mass ratio of the polyethylene resin to the white oil was 91.5:8.5.

[0163] Step (2): The mixture was melt plasticized at a temperature of 130°C, extruded through a die to form a cast sheet, and then subjected to annealing under bidirectional micro tension, with an annealing temperature of 105°C and an annealing time of 50s.

[0164] Step (3): Longitudinal stretching was performed with a stretching ratio of 35, to obtain a first base film with a thickness of 14μm, which was used as the separation film.

[0165] Example 4

[0166] Step (1) : The polyethylene resin and white oil were mixed and stirred to form a mixture, wherein the mass ratio of the polyethylene resin to the white oil was 97:3.

[0167] Step (2) : The mixture was melt plasticized at a temperature of 130°C, extruded through a die to form a cast sheet, and then subjected to annealing under bidirectional micro tension at an annealing temperature of 115°C for 30 seconds.

[0168] Step (3) : Longitudinal stretching was performed at a stretching ratio of 25 to obtain a first base film having a thickness of 14 μm, which was used as the separator.

[0169] Example 5

[0170] Step (1) : The polyethylene resin and white oil were mixed and stirred to form a mixture, wherein the mass ratio of the polyethylene resin to the white oil was 97.5:2.5.

[0171] Step (2) : The mixture was melt plasticized at a temperature of 130°C, extruded through a die to form a cast sheet, and then subjected to annealing under bidirectional micro tension at an annealing temperature of 115°C for 25 seconds.

[0172] Step (3) : Longitudinal stretching was performed at a stretching ratio of 15 to obtain a first film layer having a thickness of 14 μm, which was used as the separator.

[0173] Example 6

[0174] Step (4) : The polyethylene resin and white oil were mixed and stirred to form a mixture, wherein the mass ratio of the polyethylene resin to the white oil was 98:2.

[0175] Step (5) : The mixture was melt plasticized at a temperature of 130°C, extruded through a die to form a cast sheet, and then subjected to annealing under bidirectional micro tension at an annealing temperature of 120°C for 20 seconds.

[0176] Step (6) : Longitudinal stretching was performed at a stretching ratio of 15 to obtain a second base film having a thickness of 7 μm.

[0177] Step (7) : The first base film obtained in Example 1 was sprayed with PMMA (polymethyl methacrylate) adhesive, and then bonded to the above-mentioned second base film. Then, the separator was obtained by heat lamination.

[0178] Example 7

[0179] A coating layer was applied to the separator obtained in Example 6 by the following method:

[0180] Coating slurry preparation: the filler (aluminum trioxide with Dv50 of 200 nm), the nanocellulose containing modified groups, and the water-soluble non-particulate binder (polyacrylate) were mixed uniformly in a proper amount of solvent (deionized water) at a mass ratio of 79.2:20:0.8 to obtain the coating slurry.

[0181] The coating slurry was coated (sprayed) on the surface of the first base film away from the second base film, and after drying, the isolation film was obtained. The thickness of the single-side coating was 2 μm, and the weight of the single-side coating per unit area was 1.0 g / m 2 .

[0182] The nanocellulose containing modified groups was obtained by S1-S3 as follows.

[0183] S1: Preparation of cellulose powder:

[0184] After the cotton linters were opened and de-residued by an opener, they were digested using a 5 wt% NaOH aqueous solution, sequentially washed to remove impurities 3 times, bleached with sodium hypochlorite, washed to remove impurities, dewatered, and air-dried to obtain cotton cellulose powder with a whiteness of ≥85%.

[0185] S2: Esterification of cellulose:

[0186] The cotton cellulose powder 1 kg obtained in step S1 was mixed with a sulfuric acid solution (60 wt% in mass concentration) 30 kg, and the reaction was carried out at a temperature of 60°C for 2 hours. After the reaction was completed, the product was washed with water 3 times, filtered, and de-acidified and de-impurified to obtain nanofiber whiskers with sulfonated groups.

[0187] S3: Neutralization of cellulose:

[0188] The nanofiber whiskers with sulfonated groups were first adjusted to neutral pH with a 10 wt% NaOH aqueous solution, then dispersed by high-speed treatment with a grinder for 2.5 hours, and then treated twice with a grinder to obtain the modified nanocellulose containing modified groups, which had an average diameter of 35 nm, an average length of 500 nm, and an aspect ratio of 16.6.

[0189] Example 8

[0190] The coating was coated on the isolation film obtained in Example 6, and the specific method was as follows:

[0191] Coating slurry preparation: the filler (aluminum trioxide with Dv50 of 500 nm), the nanocellulose containing modified groups, and the water-soluble non-particulate binder (polyacrylate) were mixed uniformly in a proper amount of solvent (deionized water) at a mass ratio of 75:20:5 to obtain the coating slurry.

[0192] The coating slurry is coated on the surface of the first base film away from the second base film, and after drying, an isolation film is obtained. The thickness of the single-side coating is 2 μm, and the weight of the single-side coating per unit area is 1.0 g / m 2 .

[0193] The above modified nanocellulose containing a modified group is obtained by S1-S3.

[0194] S1: Preparation of cellulose powder:

[0195] After the cotton linters are opened and cleaned by an opener, the cotton linters are cooked by using a 5wt% NaOH aqueous solution, sequentially subjected to three times of washing and impurity removal, sodium hypochlorite bleaching, acid washing and impurity removal, washing and impurity removal, water removal, and air flow drying, and a cotton cellulose powder with a whiteness of ≥85% is obtained.

[0196] S2: Esterification of cellulose:

[0197] The cotton cellulose powder obtained in step S1, 1 kg, is mixed with a sulfuric acid solution (60wt% in mass concentration), 30 kg, and reacted at a temperature of 60°C for 2 hours. After the reaction is completed, the product is washed with water three times, filtered, and acid and impurities are removed, to obtain nanofiber whiskers with sulfonated groups.

[0198] S3: Neutralization of cellulose:

[0199] The nanofiber whiskers with sulfonated groups are first adjusted to neutral pH by using a 10wt% NaOH aqueous solution, and then dispersed by high-speed processing with a grinder for 2.5 hours, and processed twice with the grinder, to obtain modified nanocellulose containing a modified group, which has an average diameter of 35 nm, an average length of 500 nm, and an aspect ratio of 16.6.

[0200] Comparative Example 1

[0201] Step (1): Polyethylene resin and white oil are mixed and stirred to form a mixture, wherein the mass ratio of polyethylene resin to white oil is 98:2.

[0202] Step (2): The mixture is melt plasticized at a temperature of 130°C, extruded through a die to form a cast sheet, and then subjected to annealing under bidirectional micro-tension, with an annealing temperature of 120°C and an annealing time of 20s.

[0203] Step (3): Longitudinal stretching is performed with a stretching ratio of 15, to obtain a second base film with a thickness of 14 μm, which is used as an isolation film.

[0204] Test of the isolation film

[0205] (1) Measurement of pore size peak of air permeable pores

[0206] The pore size distribution curve of the first base film or the second base film was obtained by testing the pore size of the air permeable pores of the first base film or the second base film by a pore size tester (model: PMI Porometer) according to GB / T 21650.2-2008. The peak value corresponding to the pore size was read from the curve based on the test results by PMI Porometer software, and the peak value of the pore size was obtained.

[0207] For example, the air permeable pores of the separation film (first base film) prepared in Example 1 and the air permeable pores of the separation film (second base film) prepared in Comparative Example 1 were tested, and the pore size distribution curve of the air permeable pores was obtained, as shown in FIG. 8. The abscissa in the pore size distribution curve is the pore size of the air permeable pores in the separation film, and the ordinate is the proportion of the number of air permeable pores. It can be read from FIG. 8 that the peak value of the pore size of the air permeable pores in the separation film of Example 1 is 26 nm. The peak value of the pore size of the air permeable pores in the separation film of Comparative Example 1 is 36 nm.

[0208] The first base film data of other examples and comparative examples are shown in Table 1-1. The second base film data of other examples and comparative examples are shown in Table 1-2.

[0209] (2) The proportion of the number of air permeable pores with a pore size of 40 nm-45 nm

[0210] The pore size of the air permeable pores of the first base film was tested by a pore size tester (model: PMI Porometer) according to GB / T 21650.2-2008, and the proportion of the number of air permeable pores with a pore size of 40 nm-45 nm was counted based on the test results by PMI Porometer software. The counting results are shown in Table 1-1.

[0211] (3) Determination of the maximum pore size of the air permeable pores

[0212] The pore size of the first base film was tested by a pore size tester (model: PMI Porometer) according to GB / T 21650.2-2008. The maximum pore size of the first base film was determined based on the test results by PMI Porometer software. The test results are shown in Table 1-1.

[0213] (4) Determination of the pore size distribution of the surface

[0214] The isolation film prepared in Example 1 was placed in a watch glass and soaked in DME for 30 min, and then rinsed with DME until no obvious foreign matter was observed on the surface. The soaking and rinsing were repeated three times, and then the isolation film was dried in a glove box for 6 h. Then, the surface of the isolation film was randomly selected and photographed by a scanning electron microscope (SEM) (ZEISS SEM) with a magnification of 30,000 times. Random sampling was performed on the obtained photographs to obtain the photographs shown in FIG. 9. By using the SEM size calibration according to the scale in FIG. 9, it was determined that the pore size distribution of the surface of the isolation film prepared in Example 1 was 71.13 nm-181.7 nm.

[0215] The pore size distribution of the surface of the isolation film prepared in Comparative Example 1 was tested in the same manner as in Example 1, and the test results are shown in FIG. 10. It can be seen that the pore size distribution of the surface of the isolation film prepared in Comparative Example 1 was 82.59 nm-319.9 nm.

[0216] Table 1-1

[0217] Table 1-2

[0218] In Table 1-1 and Table 1-2, " / " indicates that the relevant item is not added.

[0219] Preparation of a battery

[0220] (1) Preparation of a positive electrode sheet

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

[0222] (2) Preparation of a negative electrode sheet

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

[0224] (3) Preparation of an electrolyte

[0225] A 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) Preparation of the battery

[0227] The positive electrode sheet, the separator film (Examples 1-8, Comparative Example 1), and the negative electrode sheet were stacked in order to obtain a button cell.

[0228] Battery performance test

[0229] (1) Cycle performance test

[0230] The button cell was charged at 1C constant current to 3.65V, then charged at 3.65V constant voltage until the current dropped to 0.05C, and then discharged at 1C constant current to 1.5V at 25°C in a constant temperature environment. The charge-discharge test was performed (and the first 1C discharge capacity was recorded as C0). This was repeated for 100 cycles, and the discharge capacity after 100 cycles was obtained, denoted as C100. n .

[0231] The capacity retention rate = discharge capacity after 100 cycles (C100) / discharge capacity of the first cycle (C0). n

[0232] (2) Determination of short circuit during cycling

[0233] If the battery has at least one of the following, it is determined that the battery has a short circuit.

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

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

[0236] After charging, the voltage drops by more than 40mV after 30min.

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

[0238] The button cell was charged at 1C constant current to 3.65V at 25°C in a constant temperature environment. The average surface current of the battery during the above constant current charging process from 2.92V (80% SOC) to 3.29V (90% SOC) was calculated. 2 .

[0239] The ratio of the size of the sodium secondary battery not filled to the sum of the thickness of the entire sodium metal layer in the thickness direction of the sodium metal layer of the sodium secondary battery in the fully charged state is 1.5. ​

[0240] Table 2

[0241] "NA" in Table 2 means that the battery short-circuited before the number of cycles reached 100 cycles.

[0242] As can be seen from the data in Table 1-1, Table 1-2 and Table 2, the secondary batteries of Examples 1-8 have improved safety performance, because the peak pore size of the air permeable pores of the first base film in the separator of Comparative Example 1 (the peak pore size of the pores of the separator is 36 nm) and Examples 1-8 is 35 nm or less. It should be noted that the present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration and exerting the same effects as the technical idea within the scope of the technical solutions of the present disclosure are all included in the technical scope of the present disclosure. Furthermore, within the scope of the gist of the present disclosure, other modes obtained by applying various modifications that can be thought of by those skilled in the art to the embodiments or by combining some of the constituent elements of the embodiments are also included in the scope of the present disclosure.

Claims

1. A sodium secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector, or comprising the negative electrode current collector and a sodium metal layer disposed on a surface of the negative electrode current collector, the separator comprising a first base film, the first base film having a peak value of pore diameter of 35 nm or less, the peak value of pore diameter being a pore diameter corresponding to a highest peak in a pore diameter distribution curve.

2. The sodium secondary battery according to claim 1, wherein the first base film comprising a pore structure formed by a fibrous substance.

3. The sodium secondary battery according to claim 1 or 2, wherein, the first base film comprising at least one polymer base film, the polymer base film comprising a polyethylene film, a polypropylene film, a polyimide film, and a polytetrafluoroethylene film.

4. The sodium secondary battery according to claim 3, wherein, a pore diameter size distribution of a surface of the first base film is 60 nm to 200 nm.

5. The sodium secondary battery according to any one of claims 1 to 4, wherein, in the first base film, a size distribution of pore diameters of the air-permeable pores is 2 nm to 40 nm.

6. The sodium secondary battery according to claim 5, wherein in the first base film, a size distribution of pore diameters of the air-permeable pores is 15 nm to 38 nm.

7. The sodium secondary battery according to any one of claims 1 to 6, wherein a peak value of pore diameters of the air-permeable pores of the first base film is 5 nm to 30 nm.

8. The sodium secondary battery according to any one of claims 1 to 7, wherein, a peak value of pore diameters of the air-permeable pores of the first base film is 10 nm to 25 nm.

9. The sodium secondary battery according to any one of claims 1 to 8, wherein, a ratio of a number of air-permeable pores having a pore diameter of 40 nm to 45 nm to a total number of the air-permeable pores of the first base film is less than 5%.

10. The sodium secondary battery according to any one of claims 1 to 9, wherein a ratio of a number of air-permeable pores having a pore diameter of 40 nm to 45 nm to a total number of the air-permeable pores of the first base film is less than or equal to 1%.

11. The sodium secondary battery according to any one of claims 1 to 10, wherein an average pore diameter of the air-permeable pores of the first base film is 30 nm or less.

12. The sodium secondary battery according to any one of claims 1 to 11, wherein, a maximum pore diameter of the air-permeable pores of the first base film is 40 nm or less.

13. The sodium secondary battery according to any one of claims 1 to 12, wherein the separator further comprises a second base film disposed on at least one side of the first base film.

14. The sodium secondary battery according to claim 13, wherein, a peak value of pore diameters of the air-permeable pores of the second base film is 36 nm to 100 nm.

15. The sodium secondary battery according to claim 14, wherein, a peak value of pore diameters of the air-permeable pores of the second base film is 36 nm to 45 nm.

16. The sodium secondary battery according to any one of claims 1 to 15, wherein, the separator further comprises a coating layer disposed on at least one side of the first base film.

17. The sodium secondary battery according to claim 16, wherein, a pore diameter size distribution of a surface of the coating layer is 50 nm to 100 nm.

18. The sodium secondary battery according to claim 16 or 17, wherein, the coating layer comprises nanocellulose and inorganic fillers, the inorganic fillers having a volume median particle diameter Dv50 of 100 nm to 500 nm.

19. The sodium secondary battery according to claim 18, wherein, the nanocellulose comprises a modifying group, the modifying group comprising at least one of a sulfonic acid group, a boric acid group, and a phosphoric acid group.

20. The sodium secondary battery according to claim 18 or 19, wherein, an 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, wherein, a content of the nanocellulose in the coating layer is greater than or equal to 5 wt%.

22. The sodium secondary battery according to any one of claims 16 to 21, wherein, a thickness of the coating layer 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, wherein, a ratio of a thickness of the first base film to a thickness of the separator is 30% or more.

24. The sodium secondary battery according to any one of claims 1 to 23, wherein, the first base film has a thickness of 3 µm to 30 µm.

25. The sodium secondary battery according to any one of claims 1 to 24, wherein, the separator has an air permeability of 350 s / 100 mL or less.

26. The sodium secondary battery according to any one of claims 1 to 25, wherein, the first base film is disposed on a 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, wherein, The electrolyte comprises a solvent and a sodium salt dissolved in the solvent, the solvent comprises an ether solvent, the ether solvent comprises a first ether solvent and a second ether solvent, the first ether solvent comprises 2-4 carbon atoms, the second ether solvent comprises R1-(O-R3)n-O-R2, wherein R1 and R2 independently comprise a linear or branched alkyl group with 1-6 carbon atoms, R3 comprises a linear or branched alkylene group with 2-5 carbon atoms, 2≤n≤5, and the volume ratio of the first ether solvent is 4-45% based on the total volume of the electrolyte.

28. The sodium secondary battery according to any one of claims 1 to 27, wherein, The positive electrode sheet includes a positive electrode active material, the positive electrode active material including 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, Ca.

29. The sodium secondary battery according to any one of claims 1 to 28, wherein, The sodium secondary battery has a ratio of 1-2 between the sum of the size of the sodium secondary battery that is not filled and the thickness of the entire sodium metal layer in the thickness direction of the sodium metal layer under a full charge state.

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

31. An electric device comprising the sodium secondary battery of any one of claims 1-30.

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