Secondary battery, preparation method thereof and electric equipment

Abstract

The invention discloses a secondary battery and a preparation method thereof and electric equipment, the secondary battery comprises a positive pole piece, the positive pole piece comprises a positive pole current collector and a positive pole film layer located on at least one surface of the positive pole current collector, and the positive pole film layer comprises a positive pole active material for providing active metal ions; the negative electrode plate comprises a negative electrode current collector; the lithium ion battery comprises a positive pole piece, a negative pole piece and an isolating membrane, the isolating membrane is located between the positive pole piece and the negative pole piece, the isolating membrane comprises a first organic membrane with a plurality of first pores, the peak pore diameter of the plurality of first pores is D1, and D1 is smaller than or equal to 30nm.

Description

Secondary batteries and their preparation methods, and electrical equipment

[0001] This application relates to the field of batteries, specifically to secondary batteries and their preparation methods and electrical devices.

[0002] Secondary batteries are not only used in energy storage systems for hydropower, thermal power, wind power, and solar power plants, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. During the charging and discharging process, metal dendrites generated in secondary batteries can easily pierce the separator, connecting the positive and negative electrodes, leading to short circuits and reducing the battery's cycle life and storage life.

[0003]

[0004] A first aspect of this application provides a secondary battery, comprising: a positive electrode, the positive electrode including a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer including a positive active material providing active metal ions; a negative electrode, the negative electrode including a negative current collector; and a separator, the separator being located between the positive electrode and the negative electrode, the separator including a first organic film having a plurality of first pores, the peak pore size of the plurality of first pores being D1, wherein D1 satisfies: D1≤30nm. This reduces the probability of metal dendrites entering the first pores and piercing the separator, reduces the risk of short circuit between the positive and negative electrodes, and thereby improves the cycle life and storage life of the secondary battery.

[0005] According to some embodiments of this application, the tortuosity of the first organic membrane is τ1, wherein τ1 satisfies: τ1≥4.

[0006] According to some embodiments of this application, 4≤τ1≤4.8.

[0007] Therefore, by keeping the tortuosity of the first pore within the above range, the probability of metal dendrites piercing the separator is further reduced, the risk of short circuit between the positive and negative electrodes is reduced, and the cycle life and storage life of the battery are improved.

[0008] According to some embodiments of this application, the average pore size of the plurality of first pores is D2, wherein D2 satisfies: 11nm ≤ D2 ≤ 22nm. Therefore, by keeping the average pore size within the above range, the probability of metal dendrites piercing the separator is reduced, the risk of short circuit between the positive and negative electrodes is reduced, thereby improving the cycle life and storage life of the battery.

[0009] According to some embodiments of this application, the first organic membrane comprises a first polyolefin resin. This improves the insulation performance of the first organic membrane.

[0010] According to some embodiments of this application, the thermal shrinkage rate of the first organic film is 3%-5%. This reduces the probability of the separator deforming due to heat, lowers the risk of short circuits between the positive and negative electrodes, and improves the cycle life and storage life of the battery.

[0011] According to some embodiments of this application, the thickness of the first organic film is 'a', and it satisfies 7μm≤a≤22μm. This increases the thickness of the separator, reducing the risk of metal dendrites piercing the separator.

[0012] According to some embodiments of this application, the separator further includes a second organic film located on at least one surface of the first organic film. The second organic film includes a plurality of second pores, the peak pore size of which is D3, and satisfies D1≤D3. Thus, through a layering effect between the first and second organic films, the risk of metal dendrites piercing the separator can be further reduced.

[0013] According to some embodiments of this application, the ratio of the thickness of the first organic film to the total thickness of the separator is greater than or equal to 0.4. This increases the proportion of the first organic film in the separator, increases the separator's resistance to metal dendrites, and reduces the risk of metal dendrites piercing the separator.

[0014] According to some embodiments of this application, the second organic membrane comprises a second polyolefin resin. This improves the insulation properties of the second organic membrane.

[0015] According to some embodiments of this application, the secondary battery satisfies one or two of the following conditions: the first polyolefin resin includes polyethylene resin; the second polyolefin resin includes one or two of polyethylene resin or polypropylene resin.

[0016] According to some embodiments of this application, the first organic film is disposed close to the positive electrode, and the second organic film is disposed close to the negative electrode. Therefore, when the secondary battery is a negative electrode-less secondary battery, the peak pore size of the second pores on the second organic film is relatively large. When a metal layer is deposited on the negative electrode current collector, some metal can be deposited within the second pores. This alleviates the volume expansion of the secondary battery through the second organic film, and reduces the risk of metal dendrites penetrating the separator through the first organic film.

[0017] According to some embodiments of this application, after the first charge and discharge, a metal layer is deposited in situ on the negative electrode current collector of the secondary battery, and the metal layer includes a metal element corresponding to the active metal ions. This improves the energy density of the secondary battery.

[0018] According to some embodiments of this application, the negative electrode current collector comprises copper foil or aluminum foil. Therefore, the secondary battery can be a negative electrode-free secondary battery, thereby improving the energy density of the negative electrode-free secondary battery.

[0019] According to some embodiments of this application, the negative electrode further includes a conductive layer located on at least one surface of the negative current collector. This improves the ionic conductivity of the negative electrode.

[0020] According to some embodiments of this application, the secondary battery satisfies one or two of the following conditions: the thickness of the negative electrode current collector is 3μm-20μm; the thickness of the conductive layer is 1μm-20μm.

[0021] According to some embodiments of this application, the conductive layer includes a conductive agent and an adhesive. Based on the total mass of the conductive layer, the conductive agent accounts for 1%-50% of the mass, and the adhesive accounts for 50%-99% of the mass. This reduces the risk of the conductive layer peeling off.

[0022] According to some embodiments of this application, the secondary battery satisfies one or more of the following conditions: the coating weight of the positive electrode film is 150 mg / 1540.25 mm. 2 -450mg / 1540.25mm 2 The compaction density of the positive electrode film is 1.6 g / cm³. 2 -2.2g / cm 2 The positive electrode active material includes Na. q Fe y P m O n and Na x-a A a V w-b M b (PO4) 2-2c (DO4) 2c F z-d Q d One or two of the following are given: 3.9≤q≤4.2, 2.8≤y≤3.1, 3.8≤m<4, 14.5≤n≤15.5; A represents an alkali metal element replacing Na; M represents a metal element replacing V; D represents a dopant replacing P; Q represents a dopant replacing F; 3.5≤x≤4.5, 0≤a≤0.15x, 0.8≤w≤1.1, 0≤b≤0.3w, 0≤c≤0.15, 0.8≤z≤1.1, 0≤d≤0.2z. This improves the energy density of the secondary battery.

[0023] According to some embodiments of this application, element A includes one or both of K and Li, element M includes one or more of Fe, Cr, Al, Sc, Ga, In, Ti, Zr, Mn, Zn, Ni, Cu, and Co, element D includes one or both of Si and S, and element Q includes one or both of Cl and O.

[0024] A second aspect of this application provides a method for preparing a secondary battery. The method includes preparing a positive electrode sheet comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer comprising a positive active material providing active metal ions; preparing a negative electrode sheet comprising a negative current collector; preparing a separator membrane comprising a first organic membrane having a plurality of first pores, the peak pore size of the plurality of first pores being D1, wherein D1 satisfies: D1 ≤ 30 nm; and assembling the positive electrode sheet, the negative electrode sheet, and the separator membrane into a secondary battery. This reduces the risk of metal dendrites piercing the separator membrane and improves the cycle life and storage life of the secondary battery.

[0025] According to some embodiments of this application, a method for preparing the separator includes: melting and extruding a first polyolefin resin with an alkane mixture, extracting the alkane mixture, and performing a first stretching to form the first organic membrane. The alkane mixture comprises alkanes with 16-31 carbon atoms, and the tensile force during the first stretching is 10 MPa-50 MPa. This reduces the peak pore size and average pore size of the first organic membrane, lowering the risk of metal dendrites piercing the separator.

[0026] According to some embodiments of this application, the method further includes: melting and extruding a second polyolefin resin with the alkane mixture; extracting the alkane mixture; and performing a second stretching to form a second organic film, wherein the tensile force during the second stretching is 10 MPa-60 MPa; forming the second organic film on at least one surface of the first organic film, wherein the second organic film includes a plurality of second pores, the peak pore size of the plurality of second pores being D3, and satisfying D1≤D3. Thus, through a layering effect, the risk of metal dendrites piercing the separator can be further reduced between the first and second organic films.

[0027] According to some embodiments of this application, the method satisfies one or two of the following conditions: based on the total mass of the first polyolefin resin and the alkane mixture, the mass percentage of the first polyolefin resin is 95%-99%, and the mass percentage of the alkane mixture is 1%-5%; based on the total mass of the second polyolefin resin and the alkane mixture, the mass percentage of the second polyolefin resin is 97.5%-99%, and the mass percentage of the alkane mixture is 1%-2.5%. Thus, in the process of preparing the first organic membrane, increasing the mass percentage of the alkane mixture reduces the peak pore size of the first pores on the first organic membrane, thereby reducing the probability of metal dendrites piercing the separator membrane; by forming a second organic membrane on at least one surface of the first organic membrane, the thickness of the separator membrane is further increased, further reducing the probability of metal dendrites piercing the separator membrane.

[0028] The third aspect of this application provides an electrical device, including a secondary battery provided in the first aspect of this application or a secondary battery prepared by the method provided in the second aspect of this application.

[0029] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application.

[0030] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0031] Figure 1 is a schematic diagram of a battery according to one embodiment of this application.

[0032] Figure 2 is an exploded view of the battery according to one embodiment of this application shown in Figure 1.

[0033] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0034] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0035] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0036] Figure 6 is a schematic diagram of an electrical device in which a battery is used as a power source according to an embodiment of this application.

[0037] Explanation of reference numerals in the attached figures:

[0038] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery; 51 Housing; 52 Electrode assembly; 53 Cover plate.

[0039] The embodiments of the technical solution of this application are described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0040] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0041] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0042] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0043] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0044] Currently, judging from market trends, battery applications are becoming increasingly widespread. Batteries are not only used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, but also extensively in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With the continuous expansion of battery applications, market demand is also constantly increasing.

[0045] During the charging and discharging process of secondary batteries, when conditions such as large surface roughness of the positive and negative electrodes, high current density, or over-discharge occur, metal dendrites are prone to form on the electrode surface (e.g., sodium dendrites on the negative electrode surface of sodium secondary batteries, and lithium dendrites on the negative electrode surface of lithium secondary batteries). These metal dendrites can easily pierce the separator, connecting the positive and negative electrodes and causing a short circuit, thus affecting the cycle life and storage life of the secondary battery. Related technologies can reduce the probability of metal dendrites piercing the separator by forming a dense coating on the separator surface. To improve the density of the coating, the material forming the dense coating typically has a small particle size. To improve the adhesion between materials, more binder needs to be added to the dense coating. For batteries without a negative electrode, the metal layer formed by the negative electrode deposition (e.g., a sodium metal layer) has strong reducing properties and easily reacts with binders (e.g., polyvinylpyrrolidone, PVDF), consuming the sodium metal and leading to a decrease in the content of active metal ions, thereby reducing the battery's cycle life and storage life.

[0046] This application proposes a secondary battery. The separator of the secondary battery includes a first organic membrane, which can be directly used as a base membrane. The first organic membrane has multiple first pores. By reducing the peak pore size of the first pores, the probability of metal dendrites entering the first pores and passing through the separator, causing a short circuit between the positive and negative electrodes, can be reduced. Under the condition that there are no other dense coatings (such as ceramic layers) on the separator, the risk of short circuit between the positive and negative electrodes can also be reduced, thereby improving the cycle life and storage life of the battery.

[0047] The secondary battery proposed in this application can be used in electrical devices that use secondary batteries as a power source or in various energy storage systems that use secondary batteries as energy storage elements. Electrical devices can include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0048] The first aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, and a separator. The positive electrode includes a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer comprising a positive active material providing active metal ions. The negative electrode includes a negative current collector. The separator is located between the positive electrode and the negative electrode, the separator comprising a first organic membrane having a plurality of first pores, the peak pore size of the plurality of first pores being D1, wherein D1 satisfies: D1≤30nm.

[0049] The separator membrane for the secondary battery proposed in this application can directly use the first organic membrane as the base membrane. Without other dense coatings, by reducing the peak pore size of the first pore, the probability of metal dendrites piercing the separator membrane can be reduced, thereby reducing the risk of short circuit between the positive and negative electrodes and improving the cycle life and storage life of the battery.

[0050] In this application, the peak pore size refers to the pore size distribution of 99% of the first pores. The peak pore size D1 can be tested using a capillary flow aperture meter (CFP). The specific testing process is as follows: ① The first pores of the separator to be tested are completely wetted and filled with liquid. Due to capillary action, a positive pressure is formed in the first pores. The liquid can be ethanol. ② The separator is placed in a sealed tank, and the liquid is squeezed out of the first pores by applying gas pressure. ③ Based on the relative relationship between the pressure applied when the liquid in the first pores is completely squeezed out and the diameter of the first pores, the peak pore size of the separator can be obtained according to the Laplace equation. The Laplace equation is as follows: D1=-(4γCosθ / p)×100%, where d is the peak pore size of the first pore, P is the pressure, γ is the surface tension of the liquid, and θ is the contact angle between the separator and the liquid.

[0051] As an example, D1 can be 5nm, 10nm, 15nm, 18nm, 20nm, 22nm, 24nm, 26nm, 28nm or 30nm, or a range of any of the above values.

[0052] According to some embodiments of this application, the tortuosity of the first organic membrane is τ1, where τ1 satisfies: τ1≥4. For example, it can be 4, 4.2, 4.4, 4.6, 4.8, or 5.0, or a range of any of the above values. Therefore, when the tortuosity of the first organic membrane is within the above range, the degree of curvature of the first pore is greater, which can reduce the probability of metal dendrites passing through the first pore and causing a short circuit between the positive and negative electrodes. According to some specific embodiments of this application, 4≤τ1≤4.8.

[0053] In this application, the tortuosity of the first organic membrane refers to the ratio of the actual path traveled by the electrolyte in the first organic membrane to the thickness of the first organic membrane.

[0054] The permeability of a separator membrane can be characterized by the amount of gas passing through it under certain time and pressure. It primarily reflects the ease with which active metal ions permeate the membrane, and is generally related to the membrane's porosity and the tortuosity of the first pore. The tortuosity of the first pore has the greatest impact on permeability; an increase in tortuosity leads to a quadratic decrease in permeability. Therefore, the permeability of the separator membrane can be tested using a pressure drop meter, and the tortuosity of the membrane can then be calculated. The calculation formula is: τ=(2 / 3rε.v.ΔP / (R) gas.d.Ps))1 / 2, where r is the average pore diameter of the first pore, in nm; R gas ε is the gas constant; M is the molecular weight of air, in g / mol; ΔP is the pressure difference, in Pa; ε is the porosity of the first organic membrane, in %; d is the thickness of the first organic membrane, in μm; v is the average velocity of molecular motion, in m / s; Ps is the standard atmospheric pressure, in Pa.

[0055] According to some embodiments of this application, the average pore size of the plurality of first pores is D2, wherein D2 satisfies 11nm ≤ D2 ≤ 22nm. For example, it can be 22nm, 21nm, 20nm, 19nm, 16nm, 14nm, 12nm, or 11nm, or a range of any of the above values. Thus, by reducing the average pore size of the first pores, the probability of metal dendrites passing through the first pores and causing a short circuit between the positive and negative electrodes is reduced.

[0056] In this application, the average pore size refers to the pore size distribution of 50% of the first pores. The test method for the average pore size is the same as that for the peak pore size, and the test result is a normal distribution curve, from which the average pore size data can be directly read.

[0057] According to some embodiments of this application, the first organic membrane comprises a first polyolefin resin.

[0058] According to some specific embodiments of this application, the first polyolefin resin includes polyethylene resin.

[0059] According to some embodiments of this application, the thermal shrinkage rate of the first organic film can be 3%-5%. For example, it can be 3%, 3.5%, 4%, 4.5%, or 5%, or any range of the above values. This reduces the thermal shrinkage rate of the separator, lowering the probability of short-circuiting between the positive and negative electrodes due to separator shrinkage.

[0060] In this application, the test method for the heat shrinkage rate of the release liner is as follows: Sample preparation: Cut the release liner into strips of 15mm × 130mm. Punch holes at both ends of the sample to mount it onto the instrument; the straight-line distance between the two holes is 100mm. Test conditions: Laboratory environment of 23℃ and 50% RH. Test procedure: Clamp the samples onto the fixtures of the FST-02, ensuring the samples are flat. Set the heat shrinkage temperature, and the equipment begins to heat up. When the temperature inside the test chamber reaches the set temperature, send the sample into the test chamber. The sample shrinks due to heat, and the force sensor and displacement sensor of the equipment begin to accurately measure the real-time shrinkage force and shrinkage rate. If the maximum shrinkage force occurs between 15s and 30s, record the maximum shrinkage force and shrinkage rate; otherwise, adjust the heat shrinkage temperature setting and repeat the test.

[0061] According to some embodiments of this application, the separator is composed of the first organic membrane. That is, the separator only includes the first organic membrane, and no other coatings (such as a ceramic layer) are provided on the separator. This avoids introducing binders into the separator, reduces the probability of binders reacting with active metals, and improves the cycle life and storage life of the secondary battery.

[0062] According to some embodiments of this application, the thickness of the first organic film can be 'a', and satisfies 7μm≤a≤22μm. That is, when the separator only includes the first organic film, the thickness of the separator can be 'a', and satisfies 7μm≤a≤22μm. For example, it can be 7μm, 10μm, 13μm, 17μm, 20μm, or 22μm, or it can be any range of the above values.

[0063] In this application, the thickness of the separator membrane can be tested using a thickness gauge. The thickness gauge is used to place its probe on the surface of the separator membrane, and the measurement results are recorded. The above steps are repeated to measure the separator membrane thickness at multiple locations, and the average value is calculated as the separator membrane thickness.

[0064] According to some embodiments of this application, the separator further includes a second organic film located on at least one surface of the first organic film. The second organic film includes a plurality of second pores, the peak pore size of which is D3, and satisfies D1≤D3. On the one hand, the first organic film reduces the risk of metal dendrites penetrating the first organic film through its smaller peak pore size; on the other hand, the first and second organic films reduce the probability of metal dendrites penetrating the separator through a layering effect, thereby improving the cycle life and storage life of the secondary battery.

[0065] According to some embodiments of this application, the tortuosity of the second organic membrane is τ2, where τ2 satisfies: τ1 > τ2. That is, the second organic membrane can be an organic membrane with a small tortuosity. In this case, the probability of metal dendrites penetrating the separator membrane can be reduced through the layering effect and the small peak pore size of the first organic membrane.

[0066] According to some embodiments of this application, when the separator includes a first organic film and a second organic film, the ratio of the thickness of the first organic film to the total thickness of the separator is greater than or equal to 0.4. For example, it can be 0.4, 0.5, 0.6, or 0.7, or any range of the above values. This increases the proportion of the first organic film in the composite separator, reduces the risk of metal dendrites piercing the first organic film and subsequently the entire separator, reduces the probability of short circuits between the positive and negative electrodes, and improves the cycle life and storage life of the battery.

[0067] According to some embodiments of this application, the second organic membrane comprises a second polyolefin resin.

[0068] As an example, the second polyolefin resin may include one or both of polyethylene resin or polypropylene resin.

[0069] According to some embodiments of this application, when the separator includes a first organic film and a second organic film, the first organic film is disposed close to the positive electrode and the second organic film is disposed close to the negative electrode. Therefore, when the secondary battery is a sodium-ion battery without a negative electrode, the peak pore size of the second pores on the second organic film is relatively large. When a metal layer is deposited on the negative electrode current collector, some metal can be deposited within the second pores. This mitigates the volume expansion of the secondary battery through the second organic film and reduces the risk of metal dendrites penetrating the separator through the first organic film.

[0070] According to some embodiments of this application, after the first charge and discharge, a metal layer is deposited in situ on the negative electrode current collector of the secondary battery, and the metal layer includes a metal element corresponding to the active metal ions. That is to say, the secondary battery can be a negative electrode-less secondary battery, or a secondary battery in which the metal corresponding to the active metal ions is used as the negative electrode current collector.

[0071] In this application, "the metal layer includes a metal element corresponding to the active metal ion" means that when the active metal ion is a sodium ion, the metal layer is a sodium metal layer; when the active metal ion is a lithium ion, the metal layer is a lithium metal layer.

[0072] According to some embodiments of this application, the active metal ions may include lithium ions or sodium ions. That is, the secondary battery may include one of the following: a negative electrode-free lithium secondary battery, a lithium metal battery, a negative electrode-free sodium secondary battery, or a sodium metal battery.

[0073] According to some embodiments of this application, the secondary battery may also be a lithium-ion secondary battery or a sodium-ion secondary battery.

[0074] In this application, a negative electrode-free secondary battery refers to a secondary battery in which no negative electrode active material layer is provided on the negative electrode sheet, and a metal layer is formed on the negative electrode current collector during the first charge and discharge of the secondary battery.

[0075] According to some embodiments of this application, when the secondary battery is a negative electrode-less secondary battery, the negative electrode current collector may include copper foil or aluminum foil.

[0076] According to some embodiments of this application, the negative electrode sheet may further include a conductive layer located on at least one surface of the negative electrode current collector. For example, when the secondary battery is a sodium-ion secondary battery without a negative electrode, the negative electrode sheet includes a negative electrode current collector and a conductive layer located on one side of the negative electrode current collector. This improves the ionic conductivity of the negative electrode sheet.

[0077] According to some embodiments of this application, the thickness of the negative electrode current collector is 3μm-20μm, for example, it can be 3μm, 5μm, 10μm, 15μm or 20μm, or it can be any range of the above values.

[0078] According to some embodiments of this application, the thickness of the conductive layer is 1μm-20μm, for example, it can be 1μm, 5μm, 10μm, 15μm or 20μm, or it can be any range of the above values.

[0079] According to some embodiments of this application, the conductive layer may include a conductive agent and a binder, wherein the conductive agent accounts for 1%-50% of the total mass of the conductive layer, and the binder accounts for 50%-99% of the total mass.

[0080] As an example, the mass percentage of the conductive agent can be 1%, 10%, 20%, 30%, 40%, or 50%, or a range of any of the above values.

[0081] As an example, the mass percentage of the adhesive can be 50%, 60%, 70%, 80%, 90%, or 99%, or a range of any of the above values.

[0082] Therefore, while increasing the ionic conductivity of the negative electrode, the risk of conductive layer shedding is reduced.

[0083] According to some embodiments of this application, the coating weight of the positive electrode film can be 150 mg / 1540.25 mm. 2 -450mg / 1540.25mm 2 For example, it could be 150mg / 1540.25mm 2 200mg / 1540.25mm 2 250mg / 1540.25mm 2 300mg / 1540.25mm 2 350mg / 1540.25mm 2 400mg / 1540.25mm 2 Or 450mg / 1540.25mm 2 The range can be any of the values ​​mentioned above. This increases the energy density of the secondary battery.

[0084] According to some embodiments of this application, the compaction density of the positive electrode film is 1.6 g / cm³. 2 -2.2g / cm 2 For example, it could be 1.6 g / cm³. 2 1.8g / cm 22.0g / cm 2 Or 2.2g / cm 2 The range can be any of the values ​​mentioned above. This increases the energy density of the secondary battery.

[0085] According to some embodiments of this application, the positive electrode active material may include Na. q Fe y P m O n and Na x-a A a V w-b M b (PO4) 2-2c (DO4) 2c F z-d Q d One or two of the following, wherein 3.9≤q≤4.2, 2.8≤y≤3.1, 3.8≤m<4, 14.5≤n≤15.5, element A represents an alkali metal element that replaces element Na, element M represents a metal element that replaces element V, element D represents a dopant element that replaces element P, element Q represents a dopant element that replaces element F, 3.5≤x≤4.5, 0≤a≤0.15x, 0.8≤w≤1.1, 0≤b≤0.3w, 0≤c≤0.15, 0.8≤z≤1.1, 0≤d≤0.2z.

[0086] According to some embodiments of this application, element A includes one or both of K and Li, element M includes one or more of Fe, Cr, Al, Sc, Ga, In, Ti, Zr, Mn, Zn, Ni, Cu, and Co, element D includes one or both of Si and S, and element Q includes one or both of Cl and O.

[0087] 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.).

[0088] In some embodiments, when the secondary battery is a lithium-ion secondary battery, a negative electrode-less lithium secondary battery, or a lithium metal battery, the positive electrode active material can be a positive electrode active material known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.8 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0089] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0090] 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.

[0091] [Electrolytes]

[0092] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements.

[0093] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0094] In some embodiments, when the secondary battery is a lithium-ion secondary battery, a negative electrode-free lithium secondary battery, or a lithium metal secondary battery, the electrolyte salt may include at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0095] In some embodiments of this application, when the secondary battery is a sodium-ion secondary battery, a sodium secondary battery without a negative electrode, or a sodium metal secondary battery, the electrolyte sodium salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium tetrafluoroborate, sodium dioxalate borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, or sodium bis(trifluoromethanesulfonyl)imide.

[0096] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0097] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0098] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery 5 as an example.

[0099] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 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 cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can 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 battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0100] In some implementations, the batteries can be assembled into battery modules, and the number of batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0101] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple batteries 5 can be fixed in place using fasteners.

[0102] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of batteries 5 are received.

[0103] 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.

[0104] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 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.

[0105] A second aspect of this application provides a method for preparing a secondary battery, comprising: preparing a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material providing active metal ions; preparing a negative electrode sheet, the negative electrode sheet including a negative current collector; preparing a separator membrane, the separator membrane including a first organic membrane having a plurality of first pores, the peak pore size of the plurality of first pores being D1, wherein D1 satisfies: D1≤30nm; and assembling the positive electrode sheet, the negative electrode sheet, and the separator membrane into a secondary battery. The prepared secondary battery exhibits excellent cycle life and storage life.

[0106] Taking a sodium-ion secondary battery without a negative electrode as an example, the steps of this method are explained in detail:

[0107] S10: Preparation of positive electrode sheet

[0108] According to some embodiments of this application, the positive electrode sheet can be prepared by dispersing the components used to prepare 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.

[0109] S20: Preparation of negative electrode sheet

[0110] According to some embodiments of this application, the negative electrode sheet can be prepared by dispersing a conductive agent and a binder in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing, and other processes.

[0111] S30: Preparation of the separator membrane

[0112] According to some embodiments of this application, a method for preparing the first organic membrane includes: melting and extruding a first polyolefin resin with an alkane mixture, extracting the alkane mixture, and performing a first stretching to form the first organic membrane. The alkane mixture comprises alkanes with 16-31 carbon atoms, and the stretching force during the first stretching is 10 MPa-50 MPa. This reduces the stretching force during stretching, thereby reducing the peak pore size and average pore size of the first organic membrane, reducing the risk of metal dendrites piercing the separator, and improving the cycle life and storage life of the secondary battery.

[0113] According to some embodiments of this application, the alkane mixture comprises alkanes with 16-31 carbon atoms; specifically, the alkane mixture is a mixture of saturated cycloalkanes with 16-31 carbon atoms and chain alkanes with 16-31 carbon atoms. According to some embodiments of this application, the alkane mixture is white oil.

[0114] According to some embodiments of this application, the method for preparing the first organic membrane includes: mixing and stirring polyethylene resin and white oil to form a mixture, melting and plasticizing the mixture, extruding it through a die to form a cast sheet, extracting the white oil, and performing a first stretching to form the first organic membrane.

[0115] According to some embodiments of this application, the tensile force during the first stretching can be between 10 MPa and 50 MPa, for example, it can be 10 MPa, 20 MPa, 30 MPa, 40 MPa, or 50 MPa, or any range of the above values. Therefore, reducing the tensile force during the first stretching reduces the peak pore size and average pore size of the first organic film, reduces the risk of metal dendrites piercing the separator, and improves the cycle life and storage life of the secondary battery.

[0116] According to some embodiments of this application, when extracting the white oil, the solvent may include dichloromethane.

[0117] According to some embodiments of this application, based on the total mass of the first polyolefin resin and the alkane mixture, the mass percentage of the first polyolefin resin can be 95%-99%, and the mass percentage of the alkane mixture can be 1%-5%. Thus, during the preparation of the first organic membrane, the first polyolefin resin and the alkane mixture form a homogeneous insoluble body after mixing. After extraction of the alkane mixture, first pores are formed on the first organic membrane. By increasing the mass percentage of the alkane mixture, the number of homogeneous insoluble bodies in the system is increased. After extraction, more first pores can be formed on the first organic membrane. Furthermore, by reducing the tensile force during the first stretching, the peak pore size and average pore size of the formed first pores are reduced, decreasing the risk of metal dendrites piercing the separator and improving the cycle life and storage life of the secondary battery.

[0118] As an example, the mass percentage of the first polyolefin resin can be 95%, 96%, 97%, 98%, or 99%, or a range of any of the above values.

[0119] As an example, the mass percentage of the alkane mixture can be 1%, 2%, 3%, 4%, or 5%, or a range of any of the above values.

[0120] According to some embodiments of this application, the method may further include melting and extruding a second polyolefin resin with the alkane mixture, extracting the alkane mixture, and performing a second stretching to form a second organic film, wherein the tensile force during the second stretching is 10 MPa-60 MPa; forming the second organic film on at least one surface of the first organic film, wherein the second organic film includes a plurality of second pores, the peak pore size of the plurality of second pores being D3, and satisfying D1≤D3.

[0121] According to some embodiments of this application, the method may further include mixing polyethylene resin with white oil, melting and plasticizing it, extruding it through a die to form a cast sheet, extracting the white oil, and performing a second stretching to form a second organic film, wherein the tensile force during the second stretching is 10 MPa-60 MPa; forming the second organic film on at least one surface of the first organic film, wherein the second organic film includes a plurality of second pores, the peak pore size of the plurality of second pores being D3, and satisfying D1≤D3. On the one hand, the first organic film reduces the risk of metal dendrites piercing the first organic film through its smaller peak pore size; on the other hand, the first organic film and the second organic film reduce the probability of metal dendrites penetrating the separator through a layering effect, thereby improving the cycle life and storage life of the secondary battery.

[0122] As an example, the tensile force during the second stretching can be 10MPa, 20MPa, 30MPa, 40MPa, 50MPa or 60MPa, or a range of any of the above values.

[0123] According to some embodiments of this application, based on the total mass of the second polyolefin resin and the alkane mixture, the mass percentage of the second polyolefin resin is 97.5%-99%, and the mass percentage of the alkane mixture is 1%-2.5%.

[0124] As an example, the mass percentage of the second polyolefin resin can be 97.5%, 98%, 98.5%, or 99%, or a range of any of the above values.

[0125] As an example, the mass percentage of the alkane mixture can be 1%, 1.5%, 2%, or 2.5%, or a range of any of the above values.

[0126] A third aspect of this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for 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.

[0127] As for the aforementioned electrical equipment, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0128] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this device, a battery pack or battery module can be used.

[0129] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can be powered by a battery.

[0130] To make the technical problems, technical solutions, and beneficial effects solved by the embodiments of this application clearer, the following will provide a more detailed description in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0131] Example 1

[0132] 1. Preparation of positive electrode sheet

[0133] Sodium iron pyrophosphate (the positive electrode active material), carbon nanotubes (the conductive agent), and metahexafluorophosphate (the binder) were thoroughly mixed in N-methylpyrrolidone (NMP) at a mass ratio of 95:2:3 to form a uniform positive electrode slurry. This slurry was then coated onto the surface of an aluminum foil current collector. After drying, cold pressing, and die-cutting, a positive electrode sheet with a thickness of 200 μm was obtained. The coating weight of the positive electrode film was 350 mg / 1540 mm². 2 .

[0134] 2. Preparation of negative electrode sheet

[0135] 5g of sodium carboxymethyl cellulose (CMC-Na) was weighed and dissolved in 1000mL of water. Then, 5g of conductive carbon black was added, and the mixture was ultrasonically dispersed to prepare a slurry. The slurry was coated onto the surface of the negative electrode current collector copper foil, and after drying, slitting, and cutting, the negative electrode sheet was obtained.

[0136] 3. Preparation of electrolyte

[0137] In an argon-filled glove box with a water content of <1ppm, diethylene glycol dimethyl ether and tetrahydrofuran were mixed at a mass ratio of 1:3, and sodium hexafluorophosphate (NaPF6) with a concentration of 1.0mol / L was added. After stirring evenly, an electrolyte was obtained.

[0138] 4. Preparation of the separating membrane

[0139] Polyethylene resin and white oil are mixed and stirred to form a mixture with a polyethylene resin content of 99% and a white oil content of 1%. The mixture is melted and plasticized, and extruded through a die to form a casting. The white oil is extracted with dichloromethane and finally longitudinally stretched with a tensile force of 50 MPa to form a first organic film. The parameters of the first organic film are shown in Table 1.

[0140] 5. Preparation of secondary batteries

[0141] The positive electrode, separator, and copper foil obtained in the above steps are stacked in sequence, so that the separator is between the positive electrode and the copper foil and can isolate the positive electrode and the copper foil. Then, the stacked components are wound to obtain the battery cell. The electrode assembly is placed in the housing, dried, and then injected with electrolyte. After formation, settling and other processes, a sodium metal battery without a negative electrode is obtained.

[0142] The preparation methods of the secondary batteries in Examples 1-4 and Examples 6-9 are the same as those in Example 1, and the differences are detailed in Table 1.

[0143] Example 5

[0144] The preparation method of the secondary battery is the same as in the example, except that:

[0145] The positive electrode active material on the positive electrode sheet is lithium iron phosphate;

[0146] Preparation of electrolyte

[0147] In an argon-filled glove box with a water content of <1ppm, diethylene glycol dimethyl ether and tetrahydrofuran were mixed at a mass ratio of 1:3, and lithium hexafluorophosphate (LiPF6) with a concentration of 1.0mol / L was added. After stirring evenly, an electrolyte was obtained.

[0148] Example 10

[0149] The preparation method of the secondary battery is the same as in the embodiment, except that the preparation method of the separator includes:

[0150] Polyethylene resin and white oil are mixed and stirred to form a mixture with a polyethylene resin content of 96% and a white oil content of 4%. The mixture is melted and plasticized, and extruded through a die to form a casting sheet. The white oil is extracted with dichloromethane, and finally longitudinally stretched with a tensile force of 10 MPa to form a first organic film.

[0151] Polyethylene resin and white oil are mixed and stirred to form a mixture with a polyethylene resin content of 99.6% and a white oil content of 0.4%. The mixture is melted and plasticized, and extruded through a die to form a cast sheet. The white oil is extracted with dichloromethane, and finally longitudinally stretched with a tensile force of 60 MPa to form a second organic film.

[0152] The first organic membrane and the second organic membrane are pressed together at 80°C to form a separation membrane.

[0153] The preparation methods of the secondary batteries in Examples 11 and 12 are the same as those in Example 10, and the differences are detailed in Table 1.

[0154] Table 1

[0155] Performance testing

[0156] 1. Cyclic performance test

[0157] Each battery cell is charged at a rate of 0.33C to a voltage of 3.65V at room temperature, and then discharged at a rate of 0.33C to a voltage of 1.5V. The reversible capacity is measured as C0. This charging and discharging process is repeated until the discharge capacity C of a certain cycle is reached. n The total number of cycles until C0 ≤ 80% is recorded as X-Cycle. Where C... n It is the reversible capacity at the nth cycle.

[0158] 2. Storage lifespan test

[0159] Each battery cell was charged at a rate of 0.33C to a voltage of 3.65V at room temperature, and then discharged at a rate of 0.33C to a voltage of 1.5V. The reversible capacity was measured to be C. 0， The battery cells were then stored at 25°C and charged at a rate of 0.33C every 30 days until the voltage reached 3.65V, and then discharged at a rate of 0.33C until the voltage reached 1.5V. The reversible capacity was measured to be C. n， Then, the storage life degradation rate of the battery cell is obtained by dividing Cn / C0. The cells are continuously stored and repeatedly charged and discharged until the discharge capacity C of a certain cycle is reached. n Until / C0 ≤ 80%. Record the number of days D the battery is stored. n .

[0160] The test results of the secondary batteries in Examples 1-12, Comparative Example 1, and Comparative Example 2 are shown in Table 2.

[0161] Table 2

[0162] Conclusion: As can be seen from the comparison between Examples 1-12 and Comparative Examples 1 and 2, the secondary battery using the separator proposed in this application has excellent cycle life and storage life. This indicates that by using the first organic membrane as the base membrane and reducing the peak pore size of the first pore, the probability of metal dendrites entering the first pore and passing through the first organic membrane, thus causing a short circuit between the positive and negative electrodes, can be reduced. Under the condition that there are no other dense coatings (such as ceramic layers) on the first organic membrane, the risk of short circuit between the positive and negative electrodes can also be reduced.

[0163] As can be seen from Examples 1-4, by adjusting the content of polyethylene resin and white oil in the first organic membrane and the tensile force during the first stretching, the peak pore size, average pore size and tortuosity of the first organic membrane can be adjusted, thereby improving the cycle life and storage life of the secondary battery.

[0164] As can be seen from Examples 6-9, when the separator only includes the first organic membrane, by adjusting the thickness of the first organic membrane, the probability of short circuit between the positive and negative electrodes can be reduced, thereby improving the cycle life and storage life of the secondary battery.

[0165] As can be seen from Examples 10-12, when the separator includes both a first organic membrane and a second organic membrane, the probability of short circuit between the positive and negative electrodes can be further reduced, thereby improving the cycle life and storage life of the secondary battery.

[0166] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

A type of secondary battery, wherein, include: A positive electrode, comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer comprising a positive active material providing active metal ions; a negative electrode, comprising a negative current collector; and a separator, located between the positive electrode and the negative electrode, the separator comprising a first organic membrane having a plurality of first pores, the peak pore size of the plurality of first pores being D1, wherein D1 satisfies: D1≤30nm. According to claim 1, the secondary battery, wherein, The tortuosity of the first organic membrane is τ1, and τ1 satisfies: τ1≥4. According to claim 2, the secondary battery, wherein, 4≤τ1≤4.

8. The secondary battery according to any one of claims 1-3, wherein, The average pore size of the plurality of first pores is D2, wherein D2 satisfies: 11nm≤D2≤22nm. The secondary battery according to any one of claims 1-4, wherein, The first organic membrane comprises a first polyolefin resin. The secondary battery according to any one of claims 1-5, wherein, The thermal shrinkage rate of the first organic film is 3%-5%. According to claim 6, the secondary battery, wherein, The thickness of the first organic film is a, and it satisfies 7μm≤a≤22μm. The secondary battery according to any one of claims 1-7, wherein, The isolation membrane further includes a second organic membrane located on at least one surface of the first organic membrane. The second organic membrane includes a plurality of second pores, the peak pore size of the plurality of second pores being D3, and satisfying D1≤D3. The secondary battery according to claim 8, wherein, The ratio of the thickness of the first organic membrane to the total thickness of the separator membrane is greater than or equal to 0.

4. The secondary battery according to claim 8 or 9, wherein, The second organic membrane comprises a second polyolefin resin. The secondary battery according to any one of claims 8-10, wherein, The following conditions must be met: the first polyolefin resin includes polyethylene resin; the second polyolefin resin includes one or both of polyethylene resin and polypropylene resin. The secondary battery according to any one of claims 8-11, wherein, The first organic film is disposed close to the positive electrode, and the second organic film is disposed close to the negative electrode. The secondary battery according to any one of claims 1-12, wherein, After the first charge and discharge, the secondary battery deposits a metal layer in situ on the negative electrode current collector, and the metal layer includes metal elements corresponding to the active metal ions. The secondary battery according to any one of claims 1-13, wherein, The negative electrode current collector includes copper foil or aluminum foil. The secondary battery according to any one of claims 1-14, wherein, The negative electrode sheet further includes a conductive layer, which is located on at least one surface of the negative electrode current collector. The secondary battery according to claim 15, wherein, The following conditions must be met: the thickness of the negative electrode current collector is 3μm-20μm; the thickness of the conductive layer is 1μm-20μm. The secondary battery according to claim 15 or 16, wherein, The conductive layer comprises a conductive agent and a binder. Based on the total mass of the conductive layer, the conductive agent accounts for 1%-50% of the mass, and the binder accounts for 50%-99% of the mass. The secondary battery according to any one of claims 1-17, wherein, One or more of the following conditions must be met: the coating weight of the positive electrode film is 150 mg / 1540.25 mm. 2 -450mg / 1540.25mm 2 The compaction density of the positive electrode film is 1.6 g / cm³. 2 -2.2g / cm 2 The positive electrode active material includes Na. q Fe y P m O n and Na x-a A a V w-b M b (PO4) 2-2c (DO4) 2c F z-d Q d One or two of the following, wherein 3.9≤q≤4.2, 2.8≤y≤3.1, 3.8≤m<4, 14.5≤n≤15.5, element A represents an alkali metal element that replaces element Na, element M represents a metal element that replaces element V, element D represents a dopant element that replaces element P, element Q represents a dopant element that replaces element F, 3.5≤x≤4.5, 0≤a≤0.15x, 0.8≤w≤1.1, 0≤b≤0.3w, 0≤c≤0.15, 0.8≤z≤1.1, 0≤d≤0.2z. The secondary battery according to claim 18, wherein, The element A includes one or both of K and Li, the element M includes one or more of Fe, Cr, Al, Sc, Ga, In, Ti, Zr, Mn, Zn, Ni, Cu, and Co, the element D includes one or both of Si and S, and the element Q includes one or both of Cl and O. A method for preparing a secondary battery, wherein, include: A positive electrode is prepared, comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer comprising a positive active material providing active metal ions; a negative electrode is prepared, comprising a negative current collector; a separator is prepared, comprising a first organic membrane having a plurality of first pores, the peak pore size of the plurality of first pores being D1, wherein D1 satisfies: D1≤30nm; the positive electrode, the negative electrode, and the separator are assembled into a secondary battery. The method according to claim 20, wherein, The method for preparing the separator membrane includes: melting and extruding a first polyolefin resin and an alkane mixture, extracting the alkane mixture, and performing a first stretching to form the first organic membrane, wherein the alkane mixture comprises alkanes with 16-31 carbon atoms, and the tensile force during the first stretching is 10 MPa-50 MPa. The method according to claim 21, wherein, The method further includes: mixing and melting the second polyolefin resin with the alkane mixture and extruding it; extracting the alkane mixture; and performing a second stretching to form a second organic membrane, wherein the tensile force during the second stretching is 10 MPa-60 MPa; forming the second organic membrane on at least one surface of the first organic membrane; wherein the second organic membrane includes a plurality of second pores, the peak pore size of the plurality of second pores is D3, and D1≤D3. The method according to claim 22, wherein, One or two of the following conditions must be met: based on the total mass of the first polyolefin resin and the alkane mixture, the mass percentage of the first polyolefin resin is 95%-99%, and the mass percentage of the alkane mixture is 1%-5%; based on the total mass of the second polyolefin resin and the alkane mixture, the mass percentage of the second polyolefin resin is 97.5%-99%, and the mass percentage of the alkane mixture is 1%-2.5%. An electrical appliance, wherein, The secondary battery includes the secondary battery according to any one of claims 1-19 or the secondary battery prepared by the method according to any one of claims 20-23.

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