Separator, preparation method for separator, secondary battery and electric device

Abstract

A separator and a preparation method therefor, a secondary battery and an electric device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the positive electrode sheet and the negative electrode sheet are arranged on two sides of the separator; and the separator comprises a base film, the compression ratio of the base film, which has been maintained at a pressure of 5 MPa and a temperature of 25°C for 1 min, is measured to be 40-85%, and the porosity of the base film is 67-90%.

Description

Diaphragm, diaphragm preparation method, secondary battery and power device Cross-reference to related applications

[0001] This application claims priority to Chinese Patent Application No. 202411483955.7, filed on October 23, 2024, entitled “Separator, Method for Preparing Separator, Secondary Battery and Electrical Device”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery technology, and in particular to separators, methods for preparing separators, secondary batteries, and electrical devices. Background Technology

[0003] With the development of battery technology, high-energy-density batteries, such as silicon-based batteries, metal-based batteries, and negative electrode-free batteries, will experience huge volume expansion and contraction during cycling. This can cause problems such as the collapse of the internal battery structure, cracking of wound electrode sheets, shear stress in stacked structures, or poor electrolyte wetting.

[0004] The existing solution is to suppress the expansion of the negative electrode by adding a skeleton structure to the negative electrode material. However, this solution is complicated to operate, costly, and the reliability of the enhanced skeleton decreases after multiple cycles. Moreover, it has a negligible effect on alleviating expansion in high-expansion systems and has no significant effect on improving long-term lifespan. Summary of the Invention

[0005] To alleviate the volume expansion problem during the cycling process of secondary batteries, this application provides a separator, a method for preparing the separator, a secondary battery, and an electrical device.

[0006] This application provides a secondary battery, including a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode and the negative electrode are disposed on both sides of the separator. The separator includes a base film. The base film has a compression ratio of 40%-85% after being tested at 25°C and 5MPa for 1 minute, and the porosity of the base film is 67%-90%.

[0007] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application, it can be implemented according to the contents of the specification. In order to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0008] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0009] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of this application.

[0010] Figure 2 is an exploded view of a secondary battery according to an embodiment of this application shown in Figure 1.

[0011] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0012] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0013] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0014] Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of this application.

[0015] The accompanying drawings are not drawn to scale.

[0016] Explanation of reference numerals in the attached figures:

[0017] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0018] The embodiments of this application will be described in further detail below with reference to the accompanying drawings and examples. The detailed description of the following embodiments and the accompanying drawings are used to illustrate the principles of this application by way of example, but should not be used to limit the scope of this application, that is, this application is not limited to the described embodiments.

[0019] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the secondary battery, power-consuming device, separator, and method for preparing the same, according to this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0020] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0021] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0022] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0023] 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.

[0024] Unless otherwise specified, the terms "comprising" and "including" as used in this application are open-ended. For example, "comprising" and "including" may mean that other components not listed may also be included or contained.

[0025] Unless otherwise specified, the term "or" is inclusive in this application. For example, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0026] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged after being discharged to activate the active materials and continue to be used.

[0027] Typically, a secondary battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. During charging and discharging, active ions (such as lithium ions) repeatedly insert and extract between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing active ions to pass through. The electrolyte, also located between the positive and negative electrodes, mainly conducts the active ions.

[0028] [Septum]

[0029] As analyzed in the background section, conventional methods use a framework structure in negative electrode materials to suppress negative electrode expansion, but the improvement effect does not meet expectations. To address the problems caused by negative electrode expansion, this application proposes an alternative approach to improve the separator of the secondary battery.

[0030] The first embodiment of this application provides a diaphragm, comprising a base membrane. The base membrane, when tested at 25°C and 5 MPa for 1 minute, exhibits a compression ratio of 40%-85% and a porosity of 67%-90%. The compression ratio can be measured at different temperatures (25°C or 95°C) and different holding times (1 minute or 5 minutes). For the compression ratio defined in this invention, it refers to the change in thickness before and after pressurization at room temperature (25°C) after applying a pressure of 5 MPa and holding it for 1 minute. The porosity testing method is as described in the Examples section.

[0031] The compressibility of the base membrane in this application provides compressible space when the battery expands in volume. A higher compressibility provides more compressible space, thus more effectively mitigating battery expansion. However, the compression of the base membrane causes the electrolyte impregnated within it to be squeezed out, leading to a decrease in the membrane's electrolyte retention capacity. This application further controls the porosity and compressibility of the base membrane within the aforementioned range, enabling the base membrane to have sufficient electrolyte impregnation while maximizing compressible space. Therefore, when the base membrane has the defined compressibility and porosity, it can provide more expansion space and improve the membrane's electrolyte retention capacity, thereby improving the battery's cycle life and capacity retention.

[0032] In some embodiments, the base film has a compression ratio of 40%-85%, optionally 45%-70%, and more preferably 52%-70%, thereby further providing expansion space.

[0033] In some embodiments, the porosity of the base membrane is 67%-90%, optionally 70-85%, and more preferably 70-80%, thereby further enhancing the liquid retention capacity of the diaphragm.

[0034] In some embodiments, the thickness of the base film is 8-50 μm, optionally 12-30 μm, and more preferably 15-25 μm, thereby further providing expansion space.

[0035] This application does not specify a particular pore size for the base membrane; reference can be made to the pore size of base membranes used in conventional separators. In some embodiments, the pore size of the base membrane is 20-80 nm. This pore size is suitable for wetting conventional electrolyte solvents, additives, and other molecules of conventional size, thereby further improving the liquid retention capacity of the separator.

[0036] In some embodiments, the base film comprises a high molecular weight polyolefin with a weight average molecular weight ≥700,000 Daltons, optionally from 800,000 to 3,000,000 Daltons, and more preferably from 900,000 to 2,000,000 Daltons, and a low molecular weight polyolefin with a weight average molecular weight ≤600,000 Daltons, optionally from 300,000 to 600,000 Daltons, and more preferably from 400,000 to 600,000 Daltons. The combination of high and low molecular weight polyolefins makes the processability of the base film of the present invention more optimized.

[0037] In some embodiments, the polyolefins in the high molecular weight polyolefin and the low molecular weight polyolefin may each independently include one or both of polypropylene and polyethylene. Using the polypropylene and the polyethylene enables excellent processability of the resulting base film.

[0038] In some embodiments, the amount of high molecular weight polyolefin used is 0.1% to 3% by weight, based on the total weight of the base film. The strength of the base film can be further improved by using a limited amount of high molecular weight polyolefin.

[0039] In addition to the base membrane described above, in some embodiments, the diaphragm also includes a heat-resistant layer and / or an adhesive layer. The heat-resistant layer and adhesive layer can be disposed in various ways; for example, a heat-resistant layer can be disposed on one or both sides of the base membrane, with the adhesive layer disposed on the side of the heat-resistant layer furthest from the base membrane; or a heat-resistant layer can be disposed on one side of the base membrane and an adhesive layer on the other side.

[0040] The heat-resistant material in the heat-resistant layer can be ceramic material or nanocellulose. The adhesive used in the heat-resistant layer and the adhesive layer can refer to the adhesive used in conventional diaphragms, and will not be described in detail in this application.

[0041] [Membrane Preparation Method]

[0042] A second embodiment of the present invention provides a method for preparing a diaphragm, the diaphragm comprising a base membrane, the preparation method comprising a base membrane preparation process comprising: mixing the base membrane with a polymer and a low flash point solvent oil to obtain a mixture, and forming the obtained mixture into a membrane sheet, wherein the flash point of the low flash point solvent oil is ≤80°C; performing biaxial stretching of the membrane sheet in the longitudinal and transverse directions to obtain a base membrane precursor; and removing the low flash point solvent oil from the base membrane precursor to obtain the base membrane.

[0043] In the preparation of the above mixture, a low flash point solvent oil with a flash point ≤80℃ is used. Its rapid evaporation rate results in cross-pore formation, leading to higher porosity and a richer pore structure in the base film. This allows for the production of a base film with high porosity and appropriate compressibility, such as a porosity of 67%-90% and a compressibility of 40%-85%. Utilizing this porosity and compressibility provides expansion space and enhances the separator's liquid retention capacity, thereby improving the battery's cycle life and capacity retention.

[0044] In some embodiments, the polymer resin may be a polyolefin-based resin. Specific examples of polyolefin-based resins may include any one of polyethylene, polypropylene, polybutene, and polypentene, or mixtures of two or more thereof, such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene.

[0045] In some embodiments, the mixture prepared in the foregoing steps is extruded using an extruder to form a film. There are no particular limitations on the extruder, and it can be any extruder conventionally used in the art. Non-limiting examples of extruders include those fitted with a T-die or a tubular die. The extrusion process can be carried out at currently used extrusion temperatures, but is preferably carried out at temperatures 10°C to 100°C above the melting point of the polymer resin. When the extrusion temperature is excessively above the above-defined range, film formation becomes difficult due to thermal degradation of the polymer resin, and the mechanical properties of the finished base film are undesirably reduced. Extruded films can be obtained through an extrusion process.

[0046] In some implementations, the extruded film is introduced into an alignment process. The alignment process can be carried out using a conventionally used alignment machine. Suitable alignment machines include, but are not limited to, sequential biaxial alignment machines. The mechanical strength of the porous substrate can be increased by the alignment of the extruded film. Alignment can be performed sequentially or simultaneously in the machine direction (MD, longitudinal) and transverse direction (TD).

[0047] In some embodiments, the flash point of the low flash point solvent oil is 65°C-80°C.

[0048] The low flash point solvent oil is a material dispersed in a polymer, imparting heterogeneity to a base film prepared by extrusion, orientation, or the like, and subsequently removed from the base film. Therefore, the portion of the base film containing the solvent oil exists in a porous form after evaporation. The low flash point solvent oil is preferably a material present in a liquid state during extrusion. In some embodiments, the low flash point solvent oil includes one or more of white oil (≤80°C), heptyl acetate (flash point 67.8°C), n-decane (flash point 46.11°C), isodecane (flash point 46°C), n-undecane (flash point 60°C), n-dodecane (flash point 71°C), cyclononane (flash point 42.78°C), and cyclodecane (flash point 65°C). The aforementioned low flash point solvent oils are widely available, chemically stable, and easily removed. In this invention, the flash point is a closed-cup flash point.

[0049] In some embodiments, the transverse stretching ratio is 1-3.5. The transverse stretching ratio is significantly reduced compared to the stretching ratio of conventional processes, and combined with the use of low flash point solvent oil, the porosity and compressibility of the resulting base film are further optimized; moreover, the strength of the membrane is improved.

[0050] In some embodiments, the longitudinal stretching ratio is 1-3.5. The longitudinal stretching ratio is also significantly reduced compared to the stretching ratio of conventional processes. Combined with the use of low flash point solvent oil, the porosity and compressibility of the resulting base film are further optimized, and the strength of the membrane is improved.

[0051] In some embodiments, the total weight percentage of the polymer used for the base film in the mixture is 15-35% by weight. This ensures that the polymer used for the base film is plasticized and mixed evenly during the mixing process, and also improves the quality of the film.

[0052] In some embodiments, the polymer used for the base membrane comprises a high molecular weight polyolefin with a weight average molecular weight ≥700,000 Daltons and a low molecular weight polyolefin with a weight average molecular weight ≤600,000 Daltons. The high molecular weight ensures that the membrane strength meets the application performance requirements, while the low molecular weight polyolefin improves processability.

[0053] In some embodiments, the weight-average molecular weight of the high molecular weight polyolefin is between 800,000 and 3,000,000 Daltons.

[0054] In some embodiments, the weight-average molecular weight of the low molecular weight polyolefin is 300,000 to 600,000 Daltons.

[0055] In some embodiments, the amount of high molecular weight polyolefin used is 0.1% to 3% by weight, based on the total weight of the polymer used in the base film.

[0056] When preparing the mixture, the heating temperature can be selected based on the melting point and other characteristics of the polymer used for the base film. In some embodiments, the temperature for preparing the mixture is 160°C to 250°C, preferably 170°C to 190°C. This selected temperature improves plasticizing efficiency while avoiding excessively high temperatures that could cause polymer thermal aging.

[0057] Because low flash point solvent oils evaporate rapidly, in order to minimize their loss during mixture preparation and control raw material costs, in some embodiments, the mixture is prepared under an absolute pressure of 1.0 atm to 3.0 atm. By preparing the mixture under pressure, the evaporation of the low flash point solvent oil can be controlled as much as possible. Therefore, low boiling point solvent oil can be added in moderation, thus achieving the goal of fully utilizing the low flash point solvent oil for pore formation.

[0058] Since the low flash point solvent oil used in this application has the characteristic of rapid evaporation, it gradually evaporates as pores are formed during the biaxial stretching process. The residual low flash point solvent oil can be removed by heating. For example, in some embodiments, the low flash point solvent oil is removed by heating the base film precursor to 110°C to 120°C.

[0059] In some embodiments, to improve the removal rate of low flash point solvent oils, the solvent removal process is carried out under an absolute pressure of 0.5 atm to 0.9 atm. Removing the solvent under these pressure conditions accelerates the solvent removal process.

[0060] [Rechargeable Battery]

[0061] The third embodiment of this application provides a secondary battery, which includes a positive electrode, a negative electrode, an electrolyte, and a separator. The positive electrode and the negative electrode are disposed on both sides of the separator. The separator includes a base film, which is the base film provided in any embodiment of the first embodiment described above. For example, the base film has a compression ratio of 40%-85% and a porosity of 67%-90%.

[0062] Although the mechanism is not yet clear, the applicant has unexpectedly discovered that the base membrane used in the secondary battery of this application has a limited compressibility and porosity, providing expansion space and improving the liquid retention capacity of the separator, thereby improving the cycle life and capacity retention of the battery.

[0063] [Positive electrode plate]

[0064] A positive electrode typically includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0065] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0066] 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.).

[0067] In some embodiments, the positive electrode active material may be a known battery positive electrode active material. As an example, when the secondary battery is a lithium-ion secondary battery, 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) 0.85 Co 0.15Al 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.

[0068] When the secondary battery is a sodium-ion secondary battery, as an example, the positive electrode active material of the sodium-ion secondary battery may include at least one of the following materials: sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds. However, this application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries may also be used.

[0069] As an optional technical solution in this application, the transition metal in the sodium transition metal oxide can be at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. For example, the sodium transition metal oxide is Na. x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0 < x ≤ 1.

[0070] As an optional technical solution in this application, the polyanionic compound can be a compound containing sodium ions, transition metal ions, or a tetrahedral (YO4) structure. n- A class of compounds with anionic units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents (YO4). n- The price state.

[0071] Polyanionic compounds can also contain sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds containing anionic units and halide anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si, and n represents (YO4). n- The valence state; the halogen can be at least one of F, Cl and Br.

[0072] Polyanionic compounds can also be sodium-containing tetrahedral (YO4) compounds. n- Anionic unit, polyhedral unit (ZO) y ) m+ And a class of compounds with optional halide anions. Y can be at least one of P, S, and Si, and n represents (YO4). n-The valence state; Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; m represents (ZO) y ) m+ The valence state; the halogen can be at least one of F, Cl and Br.

[0073] Polyanionic compounds include, for example, NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2 (P2O7), NaM'PO4F (M' is one or more of V, Fe, Mn and Ni), and Na3(VO4) y )2(PO4)2F 3-2y At least one of (0≤y≤1).

[0074] Prussian blue compounds can be a class of compounds containing sodium ions, transition metal ions, and cyanide ions (CN-). The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Examples of Prussian blue compounds include Na. a Me b Me' c (CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0 < a ≤ 2, 0 < b < 1, 0 < c < 1.

[0075] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0076] 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.

[0077] 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.

[0078] [Negative electrode plate]

[0079] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer comprising a negative electrode active material. Alternatively, the negative electrode sheet can be a negative current collector, wherein no negative electrode active material is disposed or only a conductive layer is disposed. The negative electrode material contains a certain content of silicon, 0.1% to 100%. It is one of low-silicon, medium-silicon, and high-silicon negative electrode materials.

[0080] 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.

[0081] 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.).

[0082] In some embodiments, when the negative electrode has a negative electrode active material, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0083] In some embodiments, the negative electrode film layer may optionally include a binder. As an example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0084] In some embodiments, the negative electrode film may optionally include a conductive agent. As an example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0085] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0086] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0087] [Electrolytes]

[0088] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0089] In some embodiments, the electrolyte is liquid and includes an electrolyte salt and a solvent.

[0090] In some embodiments, the electrolyte salt may be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0091] In some embodiments, the solvent may be selected from ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl ethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), diisopropyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), n-Propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), and propyl propionate (n-Propyl) Propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), tetramethylene sulfone (SF), methyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE) are one or more of these compounds.

[0092] In some embodiments, the electrolyte may optionally include additives. As examples, 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.

[0093] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0094] In some implementations, the secondary battery includes a single secondary battery cell, or a battery module and a battery pack.

[0095] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0096] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0097] This application does not impose any particular limitation on the shape of the secondary battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square-structured secondary battery cell 5 as an example.

[0098] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0099] In some implementations, the secondary battery cells can be assembled into a battery module. The number of secondary battery cells contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0100] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple secondary battery cells 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 secondary battery cells 5 can be fixed in place using fasteners.

[0101] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple secondary battery cells 5 are received.

[0102] 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.

[0103] 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.

[0104] In addition, this application also provides an electrical device, which includes the secondary battery provided in this application. The secondary battery 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.

[0105] As the electrical device, a single secondary battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0106] 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 the secondary battery for this device, a battery pack or battery module can be used.

[0107] [Example]

[0108] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0109] Porosity testing: Testing equipment: electronic balance; micrometer; ruler; weighing method used (density: 0.95 g / cm³). 3 Porosity = 1 - membrane mass / (membrane volume * membrane density), reference standard: ISA-MD-SBP-GEN-9999.

[0110] Compression ratio test: Under room temperature (25℃) conditions, the pressure was applied to 5MPa and held for 1min. The thickness change before and after the pressure was applied was tested.

[0111] Aperture: Obtained by measuring mercury porosimetry using a gas method.

[0112] Thickness: The thickness was obtained by testing multiple areas of the diaphragm using a 1 / 2000-degree thickness gauge.

[0113] Weight-average molecular weight: obtained by high-temperature gel permeation chromatography.

[0114] Sample preparation & testing: Accurately weigh a certain amount of sample using a balance and place it into a 20mL sample preparation bottle. Add 10mL of mobile phase (1,2,4-trichlorobenzene). Weigh the sample preparation bottle after adding the solvent. Calculate the sample concentration c using the density of the mobile phase. Place the sample preparation bottle into the sample preparation system and shake to dissolve at 140℃ for 3 hours. After dissolution, transfer the dissolved sample to a 2mL test bottle using a pipette filter to begin testing. The molecular weight of polyethylene can then be obtained.

[0115] Equipment: Chromatographic column: Agilent PL gel MIXED-BLS (300mm×7.5mm, 10μm)×2; Guard column: Agilent PL gel Guard (50mm×7.5mm, 5μm).

[0116] The following provides examples of membrane preparation and corresponding battery examples.

[0117] Example 1

[0118] First, a weight-average (Mw) molecular weight of 8.0 × 10⁻⁶ was selected. 5 The first polyethylene with a molecular weight of 3.0 × 10⁻⁶ Mw 5 The second polyethylene was mixed and then mixed with commercially available white oil with a flash point of 75°C to form a mixture at a mixing pressure of 1.2 atm absolute pressure. The mixture was heated and melted in an extruder, then cooled and cast to obtain a cast film with a thickness of 25 μm. The cast film was then stretched longitudinally and transversely, with a longitudinal stretching ratio of 2.3 times at a longitudinal stretching temperature of 110°C and a transverse stretching ratio of 2.3 times at a transverse stretching temperature of 115°C. After stretching, the white oil was removed by heating to 115°C and reducing the pressure (0.8 atm absolute pressure) to form pores, followed by heat setting at 133°C to obtain a porous polyethylene-based film with a thickness of 5.2 μm.

[0119] Table 1 below summarizes the process data from the membrane preparation examples and comparative examples of this invention. The gas-phase separation involving solvent removal methods in the table below is achieved using an oven. In the embodiments of this invention, the base membrane thickness can be reduced by increasing the stretching ratio.

[0120] Table 2 below shows the base membrane data obtained in the above-mentioned membrane preparation examples and comparative examples.

[0121] (1) Preparation of positive electrode sheet

[0122] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black are dissolved in solvent N-methylpyrrolidone (NMP) at a mass ratio of 97:1:2. After thorough mixing, a positive electrode slurry is obtained. The positive electrode slurry is then uniformly coated onto the positive electrode current collector, and after drying and cold pressing, a positive electrode film is formed and slit to obtain the positive electrode sheet.

[0123] (2) Preparation of negative electrode sheet

[0124] The active material artificial graphite, Si material, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) are dissolved in deionized water at a mass ratio of 93:2:2:2:2:1 to prepare a negative electrode slurry. The negative electrode slurry is then uniformly coated onto a copper foil current collector and dried to obtain a negative electrode film. After cold pressing and slitting, a negative electrode sheet is obtained.

[0125] (3) Preparation of electrolyte

[0126] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), 1 mol / L LiPF6 was dissolved in an organic solvent (ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:2:1), and stirred until homogeneous to obtain the corresponding electrolyte.

[0127] (4) The diaphragm is a diaphragm prepared from the base membrane in Example 1, wherein the diaphragm includes a conventional coating.

[0128] (5) Preparation of lithium-ion batteries

[0129] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in the battery casing, dried, and then injected with electrolyte. After formation and settling processes, a lithium-ion battery cell 1 is obtained.

[0130] The separators of Example 1 were replaced sequentially with the separators of Examples 2 to 5 and Comparative Examples 1 to 2 to form lithium-ion battery cells 2 to 7.

[0131] Capacity retention test method at 25℃: The pouch battery is hung on a charge / discharge test device at 25±2℃ and subjected to 1C charge / discharge cycles. The charging time, charging current, and charging voltage, as well as the discharging time, discharging current, and discharging voltage are monitored during the process. The capacity retention rate is calculated as: discharge capacity after 200 cycles / initial discharge capacity.

[0132] Test method for cycle life at 25℃: Hang the soft pack on a charge and discharge test device at 25±2℃ and perform charge and discharge cycles at a 1C rate. Monitor the charging time, charging current (2.3A), and charging voltage during the process, as well as the discharging time, discharging current (2.3A), and discharging voltage. The cycle life is the number of cycles in which the capacity decays to 80%.

[0133] The results are shown in Table 3 below.

[0134] As shown in the table above, the battery obtained by the separator of the present invention is significantly better than the comparative example in terms of capacity retention and cycle life, thus fully demonstrating the advantages of the present invention. From the capacity retention and cycle life of battery cells 1 to 4 above, it can be seen that as the compression ratio and porosity gradually increase, the capacity retention and cycle life of the obtained battery cells are correspondingly improved. The compression ratio of battery cell 5 is lower than that of battery cell 4 but higher than that of battery cells 1-3, and the porosity is lower than that of battery cells 3-4 but higher than that of battery cells 1-2. Overall, the capacity retention and cycle life of the battery cell are slightly lower than that of battery cell 4, but better than those of battery cells 1-3. This illustrates the combined effect of the base film compression ratio and porosity in the technical solution of the present invention.

[0135] Compression behavior test:

[0136] This application investigated the compression behavior of the high compression ratio diaphragm designed under this scheme at different temperatures (25℃ or 95℃) and different holding times (1 min or 5 min) (as shown in the table below). In the table, CCS refers to the coating containing ceramic material (1 micrometer on each side); PCS refers to the coating containing polymer binder (coating density 0.9 g / m²). 2 Compression ratio (32 layers) refers to the compression ratio measured by stacking 32 layers of membrane; 7-micron PE base film (weight average molecular weight 600,000 Daltons) serves as the control group, and the high compression ratio base film comes from Example 3.

[0137] Table 4

[0138] The compression data above shows that the high compression ratio membrane has a compression capacity 30 times that of a conventional membrane; and even after full compression, it can still retain approximately 38% porosity (about 5% higher than the porosity of a conventional membrane) for liquid retention. The high compression ratio base membrane in this invention exhibits a significantly better compression ratio than the 7-micron PE base membrane used in the control group.

[0139] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A secondary battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are disposed on opposite sides of the separator, wherein the separator comprises a base film, the base film having a compression ratio of 40%-85% obtained by holding at 25°C and 5MPa for 1 min, and the porosity of the base film being 67%-90%.

2. The secondary battery according to claim 1, wherein the compression ratio of the base film is 45%-70%.

3. The secondary battery according to claim 1, wherein the compression ratio of the base film is 52%-70%.

4. The secondary battery according to any one of claims 1 to 3, wherein the porosity of the base film is 70%-85%.

5. The secondary battery according to any one of claims 1 to 4, wherein the base film has a compression ratio of 65%-70% and a porosity of 71%-85%.

6. The secondary battery according to any one of claims 1 to 5, wherein the thickness of the base film is 8 μm to 50 μm.

7. The secondary battery according to any one of claims 1 to 5, wherein the thickness of the base film is 12 μm to 30 μm.

8. The secondary battery according to any one of claims 1 to 7, wherein the pore size of the base film is 20 nm to 80 nm.

9. The secondary battery according to any one of claims 1 to 8, wherein the base film comprises a high molecular weight polyolefin with a weight average molecular weight ≥ 700,000 Daltons and a low molecular weight polyolefin with a weight average molecular weight ≤ 600,000 Daltons.

10. The secondary battery according to claim 9, wherein the high molecular weight polyolefin has a weight-average molecular weight of 800,000 to 3,000,000 Daltons.

11. The secondary battery according to claim 9 or 10, wherein the low molecular weight polyolefin has a weight-average molecular weight of 300,000 to 600,000 Daltons.

12. The secondary battery according to any one of claims 9-11, wherein the weight content of the high molecular weight polyolefin is 0.1% to 3% based on the total weight of the base film.

13. The secondary battery according to any one of claims 9-12, wherein the separator comprises a heat-resistant layer disposed on at least one surface of the base membrane, wherein the heat-resistant layer comprises an adhesive resin containing inorganic particles, and the inorganic particles comprise oxides, hydroxides, or both, wherein the oxides and the hydroxides comprise at least one element selected from the group consisting of aluminum, magnesium, silicon, zirconium, calcium, strontium, barium, antimony, tin, zinc, and rare earth elements.

14. The secondary battery according to any one of claims 9-13, wherein the separator comprises an adhesive layer disposed on at least one surface of the base membrane.

15. The secondary battery according to any one of claims 1 to 14, wherein the negative electrode comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material.

16. The secondary battery according to claim 15, wherein the silicon-based material comprises one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, or silicon alloy.

17. An electrical appliance, wherein, The secondary battery includes any one of claims 1-16.

18. A diaphragm, wherein, The diaphragm includes a base membrane, wherein the base membrane has a compression rate of 40%-85% and a porosity of 67%-90% when tested at 25°C and 5MPa for 1 minute.

19. The diaphragm according to claim 18, wherein the compression ratio of the base membrane is 45%-70%.

20. The diaphragm according to any one of claims 18-19, wherein the compression ratio of the base membrane is 52%-70%.

21. The diaphragm according to any one of claims 18-20, wherein the porosity of the base membrane is 70%-85%.

22. A method for preparing a diaphragm, the diaphragm comprising a base membrane, the preparation method comprising a base membrane preparation process, the preparation process comprising: A base film is prepared by mixing a polymer and a low flash point solvent oil to obtain a mixture, and the resulting mixture is then made into a film sheet, wherein the flash point of the low flash point solvent oil is ≤80℃; The membrane is bidirectionally stretched in both the longitudinal and transverse directions to obtain the base membrane precursor; The low flash point solvent oil in the base film precursor is removed to obtain the base film.

23. The preparation method according to claim 22, wherein the flash point of the low flash point solvent oil is 65℃-80℃.

24. The preparation method according to claim 22, wherein the low flash point solvent oil comprises one or more of white oil, n-decane, isodecane, n-undecane, n-dodecane, cyclononane, and cyclodecane.

25. The preparation method according to claim 24, wherein the low flash point solvent oil is white oil.

26. The preparation method according to any one of claims 22-25, wherein the total weight of the polymer used for the base film accounts for 15-35% by weight in the mixture.

27. The preparation method according to any one of claims 22 to 26, wherein the preparation of the mixture is carried out under an absolute pressure of 1.0 atm to 3.0 atm.

28. The preparation method according to any one of claims 22-27, wherein the polymer for the base film comprises a high molecular weight polyolefin with a weight average molecular weight ≥700,000 Daltons and a low molecular weight polyolefin with a weight average molecular weight ≤600,000 Daltons.

29. The preparation method according to claim 28, wherein the high molecular weight polyolefin has a weight-average molecular weight of 800,000 to 3,000,000 Daltons.

30. The preparation method according to claim 28 or 29, wherein the low molecular weight polyolefin has a weight-average molecular weight of 300,000 to 600,000 Daltons.

31. The preparation method according to any one of claims 22-30, wherein, Based on the total weight of the polymer used in the base film, the amount of the high molecular weight polyolefin is 0.1% to 3% by weight.

32. The preparation method according to any one of claims 22-31, wherein the temperature for preparing the mixture is 160°C to 250°C.

33. The preparation method according to any one of claims 22-32, wherein the temperature for preparing the mixture is 170°C to 220°C.

34. The preparation method according to any one of claims 22 to 33, wherein the transverse stretching ratio is 1-3.

5.

35. The preparation method according to any one of claims 22 to 34, wherein the longitudinal stretching ratio is 1-3.

5.

36. The preparation method according to any one of claims 22 to 35, wherein the solvent removal process is carried out under an absolute pressure of 0.5 atm to 0.9 atm.

37. The preparation method according to any one of claims 22 to 36, wherein the low flash point solvent oil is removed by heating the base film precursor to 110°C to 120°C.

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