Secondary battery and manufacturing method therefor, electrode sheet, and electric device

Abstract

A secondary battery and a manufacturing method therefor, an electrode sheet, and an electric device (6). The secondary battery comprises an electrode sheet; the electrode sheet comprises a current collector and an electrode active layer disposed on at least one surface of the current collector; and the electrode active layer comprises an electrode active material and a polymer binder. The polymer binder comprises a core and a shell covering at least part of the surface of the core; at least one of the shell and the core comprises a polymer; both the shell and the core have hole structures; and the number of holes in the shell is greater than that of holes in the core.

Description

Secondary battery, preparation method thereof, electrode sheet and electric device Cross-reference to related applications

[0001] This application claims priority to Chinese Patent Application No. 202411226086.X, filed on September 3, 2024, entitled "Secondary battery, preparation method thereof, electrode sheet and electric device", which is incorporated by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the field of batteries, in particular to a secondary battery, a preparation method thereof, an electrode sheet and an electric device. BACKGROUND

[0003] In recent years, with the increasingly wide application of secondary batteries, secondary batteries are widely used in energy storage power supply systems such as water, fire, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc. The great development of secondary batteries has made the consumer market demand higher for their fast charging performance. Therefore, how to improve the fast charging performance of secondary batteries is a technical problem to be solved at present. SUMMARY

[0004] The present application provides a secondary battery, a preparation method thereof, an electrode sheet and an electric device, aiming to improve the fast charging performance of the secondary battery.

[0005] In a first aspect, the present application provides a secondary battery, which comprises an electrode sheet, the electrode sheet comprising a current collector and an electrode active layer arranged on at least one surface of the current collector, the electrode active layer comprising an electrode active material and a polymer binder.

[0006] The polymer binder comprises a core and a shell covering at least part of the surface of the core, at least one of the shell and the core comprises a polymer, both the shell and the core have a pore structure, and the number of pores in the shell is greater than the number of pores in the core.

[0007] In the above-mentioned secondary battery, the number of pores in the shell of the polymer binder is greater than the number of pores in the core, which can improve the transfer rate of active ions between the electrode active material and the electrolyte, reduce the transmission resistance of active ions, and increase the electrolyte storage capacity, improve the average liquid absorption rate of the electrode sheet, and improve the interface of the electrode sheet. Therefore, during the large current charging and discharging cycle of the secondary battery, active ions can quickly insert into or extract from the electrode active material, thereby improving the fast charging performance of the secondary battery.

[0008] In some embodiments, the average pore size of the inner core is 0.5 nm to 3 nm. With the average pore size of the inner core in the above range, the active ions can migrate rapidly in the polymer binder, which is conducive to further improving the fast-charging performance of the secondary battery. Further, the average pore size of the inner core is 0.5 nm to 1.5 nm.

[0009] In some embodiments, the average pore size of the outer shell is 0.5 nm to 3 nm. With the average pore size of the outer shell in the above range, the active ions can migrate rapidly in the polymer binder, which is conducive to further improving the fast-charging performance of the secondary battery. Further, the average pore size of the outer shell is 0.5 nm to 1.5 nm.

[0010] In some embodiments, the outer shell and the inner core both comprise polymers, and the polymers in the inner core are different from the polymers in the outer shell.

[0011] In some embodiments, the polymers in the inner core comprise one or more of polybutadiene, styrene-butadiene copolymer, ethylene-vinyl acetate copolymer, polybutyl acrylate, and graft copolymer of polyvinyl alcohol and isooctyl acrylate.

[0012] In some embodiments, the polymers in the outer shell comprise one or more of a polymer containing an acrylate structural unit and a polymer containing an olefin structural unit.

[0013] In some embodiments, the polymer containing an acrylate structural unit comprises one or more of styrene-methyl methacrylate copolymer, polymethyl methacrylate, styrene-methyl methacrylate copolymer, and methyl methacrylate-isobornyl methacrylate copolymer.

[0014] In some embodiments, the polymer containing an olefin structural unit comprises one or more of polystyrene and styrene-methyl methacrylate copolymer.

[0015] By designing the polymers in the inner core to be different from the polymers in the outer shell, or selecting the polymers in the inner core and / or the polymers in the outer shell to be the above-mentioned types of materials, the adhesion between the polymer binder and the electrode active material can be improved, and the fast-charging performance of the secondary battery can be further improved.

[0016] In some embodiments, the volume average particle size Dv50 of the polymer binder is 100 nm to 500 nm.

[0017] In some embodiments, the average liquid absorption rate of the electrode sheet is 1 mL / s to 4.5 mL / s. With such a design, the active ions have a suitable migration path, which is more conducive to improving the fast-charging performance of the secondary battery.

[0018] In some embodiments, the electrode tab is a negative electrode tab, the current collector is a negative electrode current collector, the electrode active layer is a negative electrode active layer, and the electrode active material is a negative electrode active material. When the electrode tab is a negative electrode tab, the introduction of the polymer binder into the negative electrode tab can reduce the transmission resistance of active ions and the risk of lithium precipitation in the negative electrode tab.

[0019] In some embodiments, the negative electrode active material includes a carbon-based material, and the carbon-based material includes graphite.

[0020] In some embodiments, the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer stacked in sequence along a direction away from the surface of the negative electrode current collector, the first negative electrode active layer and the second negative electrode active layer both contain the polymer binder, and the mass percentage content of the polymer binder in the first negative electrode active layer is greater than or equal to the mass percentage content of the polymer binder in the second negative electrode active layer. In this way, the second negative electrode active layer can retain certain pores and active ion transmission channels, which is conducive to improving the infiltration rate of the electrolyte when the electrolyte infiltrates from the first negative electrode active layer to the second negative electrode active layer, thereby improving the transmission rate of active ions and further improving the fast-charging performance of the secondary battery.

[0021] In some embodiments, the mass percentage content of the polymer binder in the first negative electrode active layer is 0.8% to 1.8%. In this way, the adhesion between the polymer binder and the negative electrode active material in the first negative electrode active layer can be improved, and the fast-charging performance of the secondary battery can be further improved.

[0022] In some embodiments, the mass percentage content of the polymer binder in the second negative electrode active layer is 0.6% to 1.8%. In this way, the adhesion between the polymer binder and the negative electrode active material in the second negative electrode active layer can be improved, and the fast-charging performance of the secondary battery can be further improved.

[0023] In some embodiments, the first negative electrode active layer and the second negative electrode active layer both contain the negative electrode active material, and the mass percentage content of the negative electrode active material in the first negative electrode active layer is less than or equal to the mass percentage content of the negative electrode active material in the second negative electrode active layer.

[0024] In some embodiments, the mass percentage content of the negative electrode active material in the first negative electrode active layer is 96% to 97%.

[0025] In some embodiments, the mass percentage content of the negative electrode active material in the second negative electrode active layer is 97% to 98%.

[0026] In some embodiments, the electrode tab is a positive electrode tab, the current collector is a positive current collector, the electrode active layer is a positive electrode active layer, and the electrode active material is a positive electrode active material.

[0027] In a second aspect of the present application, a method for preparing a secondary battery is provided, comprising a step of preparing an electrode tab; the step of preparing the electrode tab comprises:

[0028] emulsion polymerization of a first solution comprising a first pore-forming template agent and a first monomer to form an emulsion comprising an inner core;

[0029] mixing a second solution comprising a second pore-forming template agent and a second monomer with the emulsion comprising an inner core to form an outer shell on at least part of the surface of the inner core through emulsion polymerization, to obtain a polymer binder intermediate; the mass fraction of the second pore-forming template agent in the second solution is higher than the mass fraction of the first pore-forming template agent in the first solution;

[0030] removing the first pore-forming template agent and the second pore-forming template agent in the polymer binder intermediate to form a pore structure in the inner core and the outer shell, and the number of pores in the outer shell is greater than the number of pores in the inner core, to obtain a polymer binder;

[0031] coating an electrode slurry comprising an electrode active material and the polymer binder on at least one surface of the current collector, and performing a drying process to form an electrode active layer, to obtain the electrode tab.

[0032] In the secondary battery prepared by the above method, the number of pores in the outer shell of the polymer binder is greater than the number of pores in the inner core, which can improve the transfer rate of active ions between the electrode active material and the electrolyte, reduce the transmission resistance of active ions, and increase the electrolyte storage capacity, improve the average liquid absorption rate of the electrode tab, and improve the interface of the electrode tab. Therefore, during the large-current charge and discharge cycle of the secondary battery, active ions can quickly insert into or de-insert from the electrode active material, thereby improving the fast-charging performance of the secondary battery. Moreover, the above method is relatively simple and suitable for large-scale production.

[0033] In some embodiments, the current collector is a negative current collector, the electrode active material is a negative electrode active material, the electrode slurry is a negative electrode slurry, the electrode active layer is a negative electrode active layer, and the electrode tab is a negative electrode tab.

[0034] In some embodiments, the step of coating an electrode slurry comprising an electrode active material and the polymer binder on at least one surface of the current collector, and performing a drying process to form an electrode active layer, to obtain the electrode tab, comprises:

[0035] coating a first negative electrode slurry containing the negative electrode active material and the polymer binder on at least one surface of the negative electrode current collector to prepare a first negative electrode active layer, to obtain a negative electrode sheet intermediate;

[0036] coating a second negative electrode slurry containing the negative electrode active material on a surface of the negative electrode sheet intermediate away from the negative electrode current collector, and performing a drying process to form a second negative electrode active layer, to obtain the electrode sheet, the electrode sheet being the negative electrode sheet; the mass percentage content of the polymer binder in the first negative electrode active layer is greater than or equal to the mass percentage content of the polymer binder in the second negative electrode active layer. In this way, the second negative electrode active layer can retain certain pores and active ion transmission channels, which is conducive to improving the infiltration rate of the electrolyte from the first negative electrode active layer to the second negative electrode active layer, thereby improving the transmission rate of the active ions and further improving the fast-charging performance of the secondary battery.

[0037] In some embodiments, the second negative electrode slurry further contains the polymer binder.

[0038] In some embodiments, the electrode sheet is a positive electrode sheet, the current collector is a positive electrode current collector, the electrode active layer is a positive electrode active layer, and the electrode active material is a positive electrode active material.

[0039] In some embodiments, the first monomer includes one or more of an olefin compound, an acrylate compound, and an enol compound.

[0040] In some embodiments, the second monomer includes one or more of an olefin compound and an acrylate compound.

[0041] In some embodiments, the first pore-forming template agent and the second pore-forming template agent each independently include an inorganic salt, and the inorganic salt includes one or more of ammonium phosphate, ammonium sulfate, and ammonium chloride.

[0042] In some embodiments, the first solution further contains one or more of a first initiator, a first emulsifier, and a first molecular weight regulator.

[0043] In some embodiments, the second solution further contains one or more of a second initiator, a second emulsifier, and a second molecular weight regulator.

[0044] In some embodiments, the first initiator and the second initiator each independently include one or more of a persulfate salt and an azo compound.

[0045] In some embodiments, the first emulsifier and the second emulsifier each independently include one or more of an anionic emulsifier, a non-ionic emulsifier, and an amphoteric emulsifier.

[0046] In some embodiments, the first molecular weight regulator and the second molecular weight regulator each independently comprises one or more of a mercaptan compound, a halogenated compound, and a nitroxide radical compound.

[0047] In a third aspect of the present application, an electrode tab is provided, which comprises a current collector and an electrode active layer disposed on at least one surface of the current collector, the electrode active layer comprising an electrode active material and a polymer binder;

[0048] The polymer binder comprises a core and a shell covering at least part of the surface of the core, both the shell and the core having a pore structure, the number of pores in the shell being greater than the number of pores in the core.

[0049] The electrode tab described above has the same advantages as the electrode tab in the secondary battery of the present application.

[0050] In a fourth aspect of the present application, a power-using device is provided, which comprises at least one of the secondary battery of the first aspect of the present application, the secondary battery prepared by the preparation method of the second aspect of the present application, and the electrode tab of the third aspect of the present application.

[0051] The power-using device of the present application comprises at least one of the secondary battery described above, the secondary battery prepared by the preparation method described above, and the electrode tab described above, and thus has at least the same advantages as the secondary battery, the secondary battery prepared by the preparation method, or the electrode tab.

[0052] The details of one or more embodiments of the application are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the application will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF DRAWINGS

[0053] To better describe and illustrate the embodiments or examples provided by the present application, one or more drawings can be referred to. Additional details or examples used to describe the drawings should not be considered as limiting the scope of the disclosed application, the presently described embodiments or examples, and any one of the best modes of these applications presently understood. Moreover, the same reference numbers are used throughout the drawings to represent the same components. In the drawings:

[0054] FIG. 1 is a schematic view of a battery cell according to an embodiment of the present application.

[0055] FIG. 2 is an exploded view of the battery cell shown in FIG. 1 according to an embodiment of the present application.

[0056] FIG. 3 is a schematic view of a battery module according to an embodiment of the present application.

[0057] FIG. 4 is a schematic view of a battery pack according to an embodiment of the present application.

[0058] FIG. 5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 4.

[0059] FIG. 6 is a schematic view of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0060] Explanation of Reference Numerals:

[0061] 1: battery pack; 2: upper case; 3: lower case; 4: battery module; 5: battery cell; 51: case; 52: electrode assembly; 53: cover plate; 6: electric device. DETAILED DESCRIPTION

[0062] Hereinafter, some embodiments of a secondary battery and a method of manufacturing the same, an electrode sheet, and an electric device according to the present application are described in detail with appropriate reference to the accompanying drawings. However, there can be cases where unnecessary detailed description is omitted. For example, there can be cases where detailed description of matters well known, repeated description of substantially the same structure, is omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0063] The "ranges" disclosed in the present application can be defined in the form of a lower limit and an upper limit, a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The range defined in this way can be inclusive or exclusive of the end values, either end value can be independently included or excluded, and can be arbitrarily combined, i.e., 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 particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. In addition, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is also listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" represents a shorthand manner of describing each and every numerical value that is contained within the range, wherein "a" and "b" are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shorthand manner of describing those numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to listing the parameter as, for example, integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when it is stated that a parameter is an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0064] In the present application, "a plurality of", "a plurality of kinds", and the like, if not specifically limited, refer to more than two or equal to two in number. For example, "one or more" means one or more than two.

[0065] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0066] In the present application, the phrase "embodiments" means that the specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment or embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment to other embodiments. Those skilled in the art understand explicitly and implicitly that the embodiments described herein can be combined with other embodiments. The phrase "embodiments" in the present application has a similar understanding.

[0067] Those skilled in the art can understand that in the method of each embodiment or embodiment, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The detailed execution order of each step should be determined by its function and possible internal logic. If not specifically stated, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method comprises steps (a) and (b), which means that the method can comprise sequentially performed steps (a) and (b), or sequentially performed steps (b) and (a). For example, the method can also comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0068] In the present application, the open technical features or technical solutions described by the words "contain", "include", "comprise" and the like, if not otherwise stated, do not exclude additional members from the listed members, which can be considered as providing both a closed feature or solution composed of the listed members, and an open feature or solution including additional members in addition to the listed members. For example, A includes a1, a2 and a3, if not otherwise stated, it can also include other members, or it can not include additional members, which can be considered as providing the feature or solution that "A is composed of a1, a2 and a3", and also providing the feature or solution that "A includes a1, a2 and a3, and also includes other members". In the present application, if not otherwise stated, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0069] In the present application, "optionally", "optional" or "optional" means optional, i.e. selected from either of the two parallel schemes "with" or "without". If there are multiple "options" in a technical solution, each "option" is independent if there is no special description, no contradiction or mutual restriction.

[0070] The fast charging performance of the secondary battery is related to the charge-discharge current. In the case of large current charge-discharge, the electrode sheet is limited by the lithium intercalation speed, which may cause lithium precipitation on the electrode sheet, thereby greatly limiting the fast charging capacity of the secondary battery.

[0071] Based on this, one embodiment of the present application provides a secondary battery, which comprises an electrode sheet, the electrode sheet comprising a current collector and an electrode active layer arranged on at least one surface of the current collector, the electrode active layer comprising an electrode active material and a polymer binder;

[0072] The polymer binder comprises a core and a shell covering at least part of the surface of the core, at least one of the shell and the core comprises a polymer, both the shell and the core have a pore structure, and the number of pores in the shell is greater than the number of pores in the core.

[0073] In the above-mentioned secondary battery, the number of pores in the shell of the polymer binder is greater than the number of pores in the core, which can improve the transfer rate of active ions between the electrode active material and the electrolyte, reduce the transmission resistance of active ions, and increase the electrolyte storage capacity, improve the average liquid absorption rate of the electrode sheet, and improve the interface of the electrode sheet. Therefore, during the large current charge-discharge cycle of the secondary battery, active ions can quickly intercalate or deintercalate from the electrode active material, thereby improving the fast charging performance of the secondary battery.

[0074] As a non-limiting example, the current collector has two opposite surfaces in the thickness direction of itself, and the electrode active layer is arranged on any one or both of the two opposite surfaces of the negative electrode current collector.

[0075] In some embodiments, the average pore size of the core is 0.5 nm to 3 nm. When the average pore size of the core is within the above range, the active ions can quickly migrate in the polymer binder, which is beneficial to further improve the fast charging performance of the secondary battery. Specifically, the average pore size of the core includes but is not limited to 0.5 nm, 0.8 nm, 1 nm, 1.2 nm, 1.5 nm, 1.8 nm, 2 nm, 2.3 nm, 2.5 nm, 2.8 nm, 3 nm, or a range between any two of the foregoing. Further, the average pore size of the core is 0.5 nm to 1.5 nm.

[0076] In some embodiments, the average pore size of the shell is 0.5 nm to 3 nm. With the average pore size of the shell in the above range, the active ions can migrate rapidly in the polymer binder, which is beneficial to further improve the fast charging performance of the secondary battery. Specifically, the average pore size of the shell includes, but is not limited to, 0.5 nm, 0.8 nm, 1 nm, 1.2 nm, 1.5 nm, 1.8 nm, 2 nm, 2.3 nm, 2.5 nm, 2.8 nm, 3 nm, or a range between any two of the foregoing. Further, the average pore size of the shell is 0.5 nm to 1.5 nm.

[0077] In some embodiments, both the shell and the core include a polymer, and the polymer in the core is different from the polymer in the shell.

[0078] In some embodiments, the polymer in the core includes one or more of polybutadiene, styrene-butadiene copolymer, ethylene-vinyl acetate copolymer, polybutyl acrylate, and graft copolymer of polyvinyl alcohol and isooctyl acrylate.

[0079] In some embodiments, the polymer in the shell includes one or more of a polymer containing an acrylate structural unit and a polymer containing an olefin structural unit.

[0080] By designing the polymer in the core to be different from the polymer in the shell, or by selecting the polymer in the core and / or the polymer in the shell to be the above-mentioned types of materials, the adhesion between the polymer binder and the electrode active material can be improved, and the fast charging performance of the secondary battery can be further improved.

[0081] In some embodiments, the polymer containing an acrylate structural unit includes one or more of styrene-methyl methacrylate copolymer, polymethyl methacrylate, styrene-methyl methacrylate copolymer, and methyl methacrylate-isobornyl methacrylate copolymer. The lone pair electrons in the acrylate structural unit of the above-mentioned polymer can provide a channel for the transport of active ions, and the polymer is more likely to react with the hydroxyl and carboxyl groups of the electrode active material to form adhesive forces. Therefore, the above-mentioned polymer containing an acrylate structural unit can improve the adhesion between the polymer binder and the electrode active material, and further improve the fast charging performance of the secondary battery.

[0082] In some embodiments, the polymer containing an olefin structural unit includes one or more of polystyrene and styrene-methyl methacrylate copolymer.

[0083] In some embodiments, the volume average particle size Dv50 of the polymer binder is in the range from 100 nm to 500 nm. Specifically, the volume average particle size Dv50 of the polymer binder includes, but is not limited to, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, or a range between any two of the aforementioned values.

[0084] In some embodiments, the average liquid absorption rate of the electrode tab is in the range from 1 mL / s to 4.5 mL / s. With such a design, the active ions have a suitable migration path, which is more conducive to improving the fast-charging performance of the secondary battery. Specifically, the average liquid absorption rate of the electrode tab includes, but is not limited to, 1 mL / s, 1.5 mL / s, 2 mL / s, 2.5 mL / s, 3 mL / s, 3.5 mL / s, 4 mL / s, 4.5 mL / s.

[0085] In some embodiments, the electrode tab is a negative electrode tab, the current collector is a negative current collector, the electrode active layer is a negative electrode active layer, and the electrode active material is a negative electrode active material. When the electrode tab is a negative electrode tab, the introduction of the above-mentioned polymer binder into the negative electrode tab can reduce the transmission resistance of the active ions and the risk of lithium precipitation of the negative electrode tab.

[0086] In some embodiments, the negative electrode active material includes a carbon-based material, and the carbon-based material includes graphite. As a non-limiting example, the graphite can include one or more of artificial graphite and natural graphite.

[0087] In some embodiments, the negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer that are sequentially stacked along a direction away from the surface of the negative current collector, and the first negative electrode active layer and the second negative electrode active layer each contain a polymer binder, and the mass percentage content of the polymer binder in the first negative electrode active layer is greater than or equal to the mass percentage content of the polymer binder in the second negative electrode active layer. With such a design, the second negative electrode active layer can retain certain pores and active ion transmission channels, which is conducive to improving the infiltration rate of the electrolyte when the electrolyte infiltrates from the first negative electrode active layer to the second negative electrode active layer, thereby improving the transmission rate of the active ions and further improving the fast-charging performance of the secondary battery.

[0088] In some of the embodiments, the mass percentage content of the polymer binder in the first negative electrode active layer is in the range from 0.8% to 1.8%. In this way, the adhesion between the polymer binder and the negative electrode active material in the first negative electrode active layer can be improved, and the fast-charging performance of the secondary battery can be further improved. Specifically, the above-mentioned mass percentage content includes, but is not limited to, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or a range between any two of the aforementioned values.

[0089] In some embodiments, the second negative active layer includes a polymer binder in a mass percentage of 0.6% to 1.8%. In this way, the adhesion between the polymer binder and the negative active material in the second negative active layer can be improved, and the fast charging performance of the secondary battery can be further improved. Specifically, the mass percentage includes, but is not limited to, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, or a range between any two of the foregoing.

[0090] In some embodiments, the first negative active layer and the second negative active layer both include a negative active material, and the mass percentage of the negative active material in the first negative active layer is less than or equal to the mass percentage of the negative active material in the second negative active layer. In this way, the second negative active layer can retain certain pores and active ion transmission channels, which is conducive to improving the infiltration rate of the electrolyte from the first negative active layer to the second negative active layer, thereby improving the transmission rate of the active ions and further improving the fast charging performance of the secondary battery.

[0091] In some embodiments, the mass percentage of the negative active material in the first negative active layer is 96% to 97%. Specifically, the mass percentage includes, but is not limited to, 96%, 96.2%, 96.4%, 96.6%, 96.8%, 97%, or a range between any two of the foregoing.

[0092] In some embodiments, the mass percentage of the negative active material in the second negative active layer is 97% to 98%. Specifically, the mass percentage includes, but is not limited to, 97%, 97.2%, 97.4%, 97.6%, 97.8%, 98%, or a range between any two of the foregoing.

[0093] In some embodiments, the negative current collector can be a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be obtained by forming a metal material on a polymer material base layer. In the negative current collector, non-limiting examples of the metal material can include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, and the like. In the negative current collector, non-limiting examples of the polymer material base layer can include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

[0094] In some embodiments, the negative active layer further optionally includes other binders. The other binders can include one or more 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).

[0095] In some embodiments, the negative active layer further optionally includes other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)), and the like.

[0096] In some embodiments, the negative active layer further optionally includes a conductive agent. The conductive agent can include one or more of super-p carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0097] In some embodiments, the first negative active layer and the second negative active layer each independently further include other binders. The other binders can include one or more 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).

[0098] In some embodiments, the first negative active layer and the second negative active layer each independently further include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)), and the like.

[0099] In some embodiments, the first negative active layer and the second negative active layer each independently further include a conductive agent. The conductive agent can include one or more of super-p carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0100] In some embodiments, the electrode tab is a positive electrode tab, the current collector is a positive current collector, the electrode active layer is a positive active layer, and the electrode active material is a positive active material. The introduction of the above-mentioned polymer binder in the positive electrode tab can reduce the transmission resistance of active ions and improve the interface of the positive electrode tab.

[0101] In some embodiments, the positive electrode tab includes a positive active material, and the positive active material includes one or more of lithium-containing phosphate with olivine structure, lithium transition metal oxide, and a modified compound of each thereof.

[0102] Non-limiting examples of lithium-containing phosphates of olivine structure can include, but are not limited to, one or more of lithium iron phosphate, a composite of lithium iron phosphate and carbon, lithium manganese phosphate, a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon. Examples of lithium transition metal oxides can include, but are not limited to, one or more of lithium cobalt oxide (such as LiCo02), lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof, etc. Non-limiting examples of lithium cobalt oxide can include LiCo02; non-limiting examples of lithium nickel oxide can include LiNi02; non-limiting examples of lithium manganese oxide can include LiMn02, LiMn204, etc.; non-limiting examples of lithium nickel cobalt manganese oxide can include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(also can be referred to simply as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2(also can be referred to simply as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2(also can be referred to simply as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(also can be referred to simply as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2(also can be referred to simply as NCM 811 ), etc. Non-limiting examples of lithium nickel cobalt aluminum oxide can include LiNi 0.8 Co 0.15 Al 0.05 O2.

[0103] It can be understood that the secondary battery will be accompanied by lithium (Li) deintercalation and consumption during charging and discharging, and the content of Li in the positive electrode sheet is different when the secondary battery is discharged to different states. In the enumeration of the positive electrode active material in this application, the content of Li is the initial state of the material unless otherwise stated. When the positive electrode active material is applied to the positive electrode sheet in the secondary battery system, the content of Li in the positive electrode active material contained in the sheet will usually change after charging and discharging cycles. Among them, the content of Li can be quantified by molar content, but is not limited thereto. As for "the content of Li is the initial state of the material", the initial state of the material refers to the state before being put into the positive electrode slurry. It can be understood that the new material obtained by properly modifying the listed positive electrode active material is also within the scope of the positive electrode active material, and the foregoing proper modification refers to acceptable modification methods for the positive electrode active material, and non-limiting examples include coating modification.

[0104] In the enumeration of the positive electrode active material in this application, the content of oxygen (O) is only the theoretical state value, and the release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual content of O will fluctuate. Among them, the content of O can be quantified by molar content, but is not limited thereto.

[0105] In some embodiments, the positive current collector can use a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be obtained by forming a metal material on a polymer material base material. In the positive current collector, non-limiting examples of the metal material can include one or more of aluminum, an aluminum alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, and a silver alloy. In the positive current collector, non-limiting examples of the polymer material base material can include one or more of a polypropylene (PP), a polyethylene terephthalate (PET), a polybutylene terephthalate (PBT), a polystyrene (PS), and a polyethylene (PE) base material.

[0106] In some embodiments, the positive electrode active layer can also optionally include a conductive agent. As a non-limiting example, the conductive agent can include one or more of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0107] In some embodiments, the positive electrode sheet can be prepared by dispersing the components described above for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the polymer binder, and any other components, in a solvent to form a positive electrode slurry; coating the positive electrode slurry on at least one side surface of the positive electrode current collector; and drying, cold-pressing, or the like, to obtain the positive electrode sheet. The solvent can be selected from, but is not limited to, any of the aforementioned embodiments, such as N-methylpyrrolidone (NMP). The positive electrode slurry can be coated on a single surface of the positive electrode current collector, or on both surfaces of the positive electrode current collector. The positive electrode slurry can be coated on a single surface of the positive electrode current collector, or on both surfaces of the positive electrode current collector. The solid content of the positive electrode slurry can be 40 wt% to 80 wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000 millipascal-seconds (mPa-s) to 25000 mPa-s.

[0108] In some embodiments, the secondary battery further includes an electrolyte. The electrolyte has a function of conducting ions between the positive electrode sheet and the negative electrode sheet. The type of the electrolyte is not particularly limited in the present application, and can be selected as needed. For example, the electrolyte can be liquid.

[0109] In some embodiments, the electrolyte uses an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0110] In some embodiments, the electrolyte salt can include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluoro-oxalato-borate (LiDFOB), lithium bis-oxalato-borate (LiBOB), lithium difluoro-bis-oxalato-phosphate (LiDFOP), and lithium tetrafluoro-oxalato-phosphate (LiTFOP).

[0111] In some embodiments, the solvent can include one or more of ethylene carbonate (EC, ), propylene carbonate (PC, ), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate , fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, methyl ethyl sulfone, and diethyl sulfone.

[0112] In some embodiments, the electrolyte can optionally further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the secondary battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0113] In some embodiments, the additive in the electrolyte can include, but is not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0114] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0115] In some embodiments, the material of the separator can include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0116] In some embodiments, the thickness of the separator is 6 micrometers (pm) to 40 pm, and can be optionally 12 pm to 20 pm.

[0117] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to make an electrode assembly through a winding process or a stacking process.

[0118] In some embodiments, the secondary battery is a lithium-ion secondary battery.

[0119] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0120] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and further, non-limiting examples of the plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, etc.

[0121] The secondary battery includes at least one battery cell. The secondary battery can include one or more battery cells.

[0122] In the present application, a "battery cell" refers to a basic unit capable of converting chemical energy and electrical energy to each other, and further, generally includes at least a positive electrode sheet, a negative electrode sheet, and an electrolyte. During the charging and discharging of a secondary battery, active ions are intercalated and deintercalated between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct the active ions between the positive electrode sheet and the negative electrode sheet.

[0123] The shape of the battery cell is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, FIG. 1 is a battery cell 5 of a square structure as an example.

[0124] In some embodiments, referring to FIG. 2, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by a person skilled in the art according to actual needs.

[0125] The secondary battery can be a battery module 4 or a battery pack 1.

[0126] The battery module includes at least one battery cell. The number of battery cells contained in the battery module can be one or more, which can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0127] FIG. 3 is a battery module 4 as an example. Referring to FIG. 3, in the battery module 4, a plurality of battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. Further, the plurality of battery cells 5 can be fixed by fasteners.

[0128] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0129] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, which can be selected by a person skilled in the art according to the application and capacity of the battery pack.

[0130] FIGS. 4 and 5 are a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 is capable of being disposed on the lower case 3 and forms an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0131] Another embodiment of the present application provides a preparation method of a secondary battery, comprising a step of preparing an electrode tab; the step of preparing an electrode tab comprises:

[0132] emulsion polymerization of a first solution comprising a first pore-forming template agent and a first monomer to form an emulsion comprising an inner core;

[0133] mixing a second solution comprising a second pore-forming template agent and a second monomer with the emulsion comprising the inner core, and emulsion polymerization to form an outer shell on at least part of the surface of the inner core to obtain a polymer binder intermediate; the mass fraction of the second pore-forming template agent in the second solution is higher than the mass fraction of the first pore-forming template agent in the first solution;

[0134] removing the first pore-forming template agent and the second pore-forming template agent in the polymer binder intermediate to form a pore structure in the inner core and the outer shell, and the number of pores in the outer shell is greater than the number of pores in the inner core, to obtain a polymer binder;

[0135] coating an electrode slurry comprising an electrode active material and a polymer binder on at least one surface of the current collector, and drying to form an electrode active layer to obtain an electrode tab.

[0136] In the secondary battery prepared by the above preparation method, the number of pores in the outer shell of the polymer binder is greater than the number of pores in the inner core, which can improve the transfer rate of active ions between the electrode active material and the electrolyte, reduce the transmission resistance of active ions, and increase the electrolyte storage capacity, improve the average liquid absorption rate of the electrode tab, and improve the interface of the electrode tab. Therefore, during the large-current charge and discharge cycle of the secondary battery, active ions can quickly insert into or separate from the electrode active material, thereby improving the fast-charging performance of the secondary battery. Moreover, the above preparation method is relatively simple and suitable for large-scale production.

[0137] In some embodiments, the current collector is a negative electrode current collector, the electrode active material is a negative electrode active material, the electrode slurry is a negative electrode slurry, the electrode active layer is a negative electrode active layer, and the electrode tab is a negative electrode tab.

[0138] In some embodiments, the step of coating an electrode slurry comprising an electrode active material and a polymer binder on at least one surface of the current collector, and drying to form an electrode active layer to obtain an electrode tab comprises:

[0139] coating a first negative electrode slurry containing a negative electrode active material and a polymer binder on at least one surface of the negative electrode current collector to prepare a first negative electrode active layer, to obtain a negative electrode intermediate;

[0140] coating a second negative electrode slurry containing a negative electrode active material on the surface of the negative electrode intermediate away from the negative electrode current collector, and performing a drying process to form a second negative electrode active layer, to obtain an electrode sheet, the electrode sheet being a negative electrode sheet; the mass percentage content of the polymer binder in the first negative electrode active layer is greater than or equal to the mass percentage content of the polymer binder in the second negative electrode active layer.

[0141] In the above embodiments, the second negative electrode active layer can retain certain pores and active ion transmission channels, which is beneficial to improving the infiltration rate of the electrolyte when the electrolyte infiltrates from the first negative electrode active layer to the second negative electrode active layer, thereby improving the transmission rate of the active ions and further improving the fast-charging performance of the secondary battery.

[0142] In addition, during the drying process, the solvent in the layer where the second negative electrode active layer is located is first volatilized, and the solvent in the layer where the first negative electrode active layer is located rapidly rises, and in the rising process, the solvent carries part of the polymer binder, causing the polymer binder to float up. Therefore, the mass percentage content of the polymer binder in the first negative electrode slurry with respect to the solid material in the slurry is less than the mass percentage content of the polymer binder in the second negative electrode slurry with respect to the solid material in the slurry. The mass of the solid material in the slurry is the mass of the slurry after the solvent is removed. Since the polymer binder will float up, the second negative electrode slurry can contain or not contain the polymer binder. The floating amount of the polymer binder = (the content test value of the polymer binder in the first negative electrode active layer - the content design value of the polymer binder in the first negative electrode active layer) / the content design value of the polymer binder in the second negative electrode active layer x 100%.

[0143] The possible reasons for the floating of the polymer binder are: (1) surface tension effect; (2) concentration gradient effect; (3) capillary effect; (4) thermodynamic movement of solid particles; and (5) density difference between the layers where the first negative electrode active layer and the second negative electrode active layer are located caused by particle sedimentation in the first negative electrode slurry and the second negative electrode slurry.

[0144] In some embodiments, the second negative electrode slurry further contains a polymer binder.

[0145] In some embodiments, the electrode sheet is a positive electrode sheet, the current collector is a positive electrode current collector, the electrode active layer is a positive electrode active layer, and the electrode active material is a positive electrode active material.

[0146] In some embodiments, the first monomer includes one or more of an olefinic compound, an acrylate compound, and an enolic compound. Optionally, the olefinic compound includes one or more of styrene, butadiene, ethylene, and vinyl acetate. Optionally, the acrylate compound includes one or more of butyl acrylate and isooctyl acrylate. Optionally, the enolic compound includes one or more of vinyl alcohol and propylene alcohol.

[0147] In some embodiments, the second monomer includes one or more of an olefinic compound and an acrylate compound. Optionally, the olefinic compound includes one or more of styrene and phenylpropylene. Optionally, the acrylate compound includes one or more of methyl methacrylate and isobornyl methacrylate.

[0148] In some embodiments, the first pore-forming templating agent and the second pore-forming templating agent each independently include an inorganic salt including one or more of ammonium phosphate, ammonium sulfate, and ammonium chloride.

[0149] In some embodiments, the first solution further includes one or more of a first initiator, a first emulsifier, and a first molecular weight regulator.

[0150] In some embodiments, the second solution further includes one or more of a second initiator, a second emulsifier, and a second molecular weight regulator.

[0151] In some embodiments, the first initiator and the second initiator each independently include one or more of a persulfate salt and an azo compound.

[0152] In some embodiments, the first emulsifier and the second emulsifier each independently include one or more of an anionic emulsifier, a non-ionic emulsifier, and an amphoteric emulsifier. Optionally, the anionic emulsifier includes one or more of sodium dodecyl sulfate and sodium dodecylbenzenesulfonate. Optionally, the non-ionic emulsifier includes a polyoxyethylene alkyl ether. Optionally, the amphoteric emulsifier includes a betaine-type emulsifier.

[0153] In some embodiments, the first molecular weight regulator and the second molecular weight regulator each independently include one or more of a mercaptan compound, a halogenated compound, and a nitroxyl compound. Optionally, the mercaptan compound includes dodecyl mercaptan. Optionally, the halogenated compound includes one or more of carbon tetrachloride and tetrabromoethane. Optionally, the nitroxyl compound includes 2,2,6,6-tetramethylpiperidine-1-oxyl radical.

[0154] In some embodiments, the step of removing the first pore-forming template agent and the second pore-forming template agent from the polymer binder intermediate comprises mixing an acid or a base with the polymer binder intermediate to remove the first pore-forming template agent and the second pore-forming template agent through a neutralization reaction.

[0155] By adjusting the process parameters in the above preparation method, the secondary battery provided in an embodiment of the present application can be obtained.

[0156] An electrode tab provided in another embodiment of the present application comprises a current collector and an electrode active layer disposed on at least one surface of the current collector, wherein the electrode active layer comprises an electrode active material and a polymer binder.

[0157] The polymer binder comprises a core and a shell covering at least part of the surface of the core, at least one of the shell and the core comprises a polymer, and the shell and the core both have a pore structure, and the number of pores in the shell is greater than the number of pores in the core.

[0158] In some embodiments, the electrode tab is the electrode tab in the secondary battery described above.

[0159] A power-using device provided in another embodiment of the present application comprises at least one of the secondary battery described above, the secondary battery prepared by the preparation method described above, and the electrode tab described above.

[0160] The secondary battery can be used as a power source of a power-using device or as an energy storage unit of a power-using device. The power-using device can include a mobile device, an electric vehicle, an electric train, a ship or a satellite, an energy storage system, etc., but is not limited thereto. The mobile device can be a mobile phone, a notebook computer, etc., for example, and the electric vehicle can be a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but is not limited thereto.

[0161] The secondary battery can be selected according to the use requirements of the power-using device.

[0162] FIG. 6 is a power-using device 6 as an example. The power-using device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the secondary battery for the power-using device, a battery pack or a battery module can be used.

[0163] Another example of the device can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and a secondary battery can be used as a power source.

[0164] Hereinafter, the examples of the present application will be described. The examples described below are illustrative and are given for the purpose of illustration and explanation only and are not to be understood as limiting the present application. In the examples, the techniques or conditions not mentioned are performed according to the techniques or conditions described in the literature in the field or according to the product manual. The reagents or instruments used are not mentioned by the manufacturer, and are all conventional products that can be obtained commercially.

[0165] Test method related to the examples:

[0166] (1) Test of average particle size Dv50 of the polymer binder:

[0167] The dispersion medium used in the test is water, the refractive index of the dispersant is 1.33, and the test instrument is Malvern 3000 from the United States. The parameters of the test instrument are as follows: refractive index is 1.42, absorbance is 0.005, and obscuration (scattering intensity) is 8% to 12%.

[0168] Specifically, the polymer binder sample is taken with a disposable pipette. After the sample is poured into the sample tower, it is circulated with the solution to the test light path system. Under the irradiation of the laser beam, the particle size distribution characteristics of the particles can be obtained by receiving and measuring the energy distribution of the scattered light (obscuration: 8% to 12%). According to the test data, a particle size volume distribution graph is drawn. From the distribution graph, it can be seen that the particle diameter of 50% of the total volume is greater than a certain Dv50 value, and the particle diameter of the other 50% of the total volume is less than the Dv50 value. Therefore, the Dv50 value is the volume average particle size of the particles.

[0169] (2) Test of mass percentage content of the polymer binder:

[0170] The mass percentage content of the polymer binder in the first negative active layer and the second negative active layer is tested by STA449F3 differential scanning calorimetry, and the specific test procedure is as follows: take the double-sided tape and bond it on the surface of the negative electrode sheet to be tested, then roll it three times in the same direction with a roller, tear off the double-sided tape, and the second negative active layer powder is adhered to the tape; the first negative active layer powder is scraped off the negative electrode sheet with a spatula, and attention should be paid not to scrape the negative current collector. The first negative active layer powder and the second negative active layer powder are used to test the content of the polymer binder in the first negative active layer and the second negative active layer according to the following steps, and the test steps are as follows: weigh about 50 milligrams (mg) of sample in an Al2O3 crucible and shake it flat; parameter setting: nitrogen atmosphere, flow rate of purge gas 60 milliliters per minute (mL / min), flow rate of protective gas 20 mL / min; temperature rising program: 10 degrees Celsius per minute (℃ / min), 35℃-600℃, ensure complete dispersion of the sample, then draw the thermogravimetric and DTG-temperature change graph, heat flow-temperature change graph, and calculate the mass percentage content of the polymer binder in the first negative active layer and the second negative active layer.

[0171] (3) Average liquid absorption rate test of electrode sheet:

[0172] 1) Dry the electrode sheet under the condition of 80℃ / 4 hours (h); 2) cut the electrode sheet to an appropriate size, test the thickness of the electrode sheet, and fix it on the sample stage; 3) select a capillary tube with an inner diameter d = 200 μm, polish it to the port with 5000 mesh sandpaper; 4) turn on the microscope, adjust the lens magnification to 20-30 times, adjust the height of the sample stage and the angle between the lens and the sample stage to make the picture clear; 5) the capillary tube absorbs the above-mentioned electrolyte, controls the electrolyte height h = 3 millimeters (mm), clamps the capillary tube on the bracket, further adjusts the picture to be clear, and lowers the capillary tube to make it contact with the electrode sheet; the liquid surface of the capillary tube drops at the same time, and the stopwatch is used to time; 6) when the liquid surface drops to the bottom, read the liquid absorption time t, and record the data; 7) calculate the average liquid absorption rate v of the electrode sheet according to the formula v = π × (d / 2)^2 × h × ρ / t, where ρ is the density of the electrolyte.

[0173] (4) Lithium precipitation test:

[0174] At 25℃, the battery is charged at a rate of 1C to a voltage of 3.65V, then charged at a constant voltage until the current is less than or equal to 0.05C, and then discharged at a rate of 1C to a voltage of 2.8V. This cycle is repeated for 500 times, then the battery is charged at a rate of 1C to a voltage of 3.65V, and then charged at a constant voltage until the current is less than or equal to 0.05C to reach the full charge state. The battery in the full charge state is disassembled in a low humidity room, and the surface of the electrode sheet is observed to see if lithium is precipitated.

[0175] (5) Fast charging performance test:

[0176] The battery was first charged and discharged at a current of 1C (i.e. the current value of fully discharging the theoretical capacity within 1h), specifically including: at 35℃, the battery was charged at a current of 1C to a voltage of 3.65V, then charged at a constant voltage to a current of ≤0.05C, rested for 5min, then discharged at a current of 0.33C to a voltage of 2.8V, and the actual capacity was recorded as C0. Then the battery was sequentially charged at a current of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, 3.0C0, 3.5C0, 4C0, 4.5C0, 5C0 to the full battery charging cutoff voltage of 3.65V or the negative electrode cutoff potential of 0V (whichever is reached first), and after each charging was completed, it was discharged at a current of 1C0 to the full battery discharge cutoff voltage of 2.8V, and the negative electrode potential corresponding to 10%, 20%, 30%, …, 80% SOC (State of Charge, when “SOC=0”, it indicates that the battery is completely discharged, and when “SOC=100%”, it indicates that the battery is completely charged) was recorded, and the charging rate-negative electrode potential curve at different SOC states was drawn, and the charging rate corresponding to the negative electrode potential of 0V at different SOC states was obtained by linear fitting, which was the charging window at the SOC state, and was recorded as C10% SOC, C20% SOC, C30% SOC, C40% SOC, C50% SOC, C60% SOC, C70% SOC, C80% SOC, respectively. The charging time T from 10% SOC to 80% SOC of the battery was calculated according to the formula (60 / C20% SOC+60 / C30% SOC+60 / C40% SOC+60 / C50% SOC+60 / C60% SOC+60 / C70% SOC+60 / C80% SOC) x 10%, and the unit was min. The shorter the time, the better the fast charging performance of the battery.

[0177] Example 1

[0178] (1) Preparation of positive electrode sheet

[0179] The positive electrode active material lithium iron phosphate (LiFePO4), the conductive agent carbon black (Super P), and the binder polyvinylidene fluoride (PVDF) were mixed uniformly in a solvent N-methyl pyrrolidone (NMP) at a mass ratio of 96.2:2.7:1.1 to obtain a positive electrode slurry; the positive electrode slurry was coated on the double-sided surface of a positive electrode current collector aluminum foil, and through processes such as drying, cold pressing, slitting, and cutting, a positive electrode sheet was obtained.

[0180] (2) Preparation of negative electrode sheet

[0181] (2.1) Preparation of the polymer binder

[0182] (2.1.1) A polymerization kettle is vacuumed, 250 grams (g) of deionized water is added, then sodium dodecyl sulfate (first emulsifier), ammonium persulfate (first initiator) and ammonium phosphate (first pore-forming template agent) are added, stirring is started and the temperature is raised to 70°C, then a mixed solution of 1,3-butadiene (first monomer), styrene (first monomer) and dodecyl mercaptan (first molecular weight regulator) is added dropwise, the dropwise time is 3h, then the temperature is raised to 90°C for 0.5h, emulsion polymerization is carried out to form an emulsion containing an inner core; wherein the mass ratio of 1,3-butadiene (first monomer), styrene (first monomer), ammonium phosphate (first pore-forming template agent), sodium dodecyl sulfate (first emulsifier), ammonium persulfate (first initiator) and dodecyl mercaptan (first molecular weight regulator) is 10:90:1.25:3.5:0.6:1, and the mass of styrene (first monomer) is 90g;

[0183] In step (2.1.1), the mass fraction of ammonium phosphate (first pore-forming template agent) in the first solution formed by 1,3-butadiene (first monomer), styrene (first monomer), ammonium phosphate (first pore-forming template agent), sodium dodecyl sulfate (first emulsifier), ammonium persulfate (first initiator) and dodecyl mercaptan (first molecular weight regulator) is denoted as W1;

[0184] (2.1.2) To the entire emulsion containing the inner core obtained in step (2.1.1), 170g of deionized water, ammonium phosphate (second pore-forming template agent), sodium dodecylbenzenesulfonate (second emulsifier), ammonium persulfate (second initiator) are added, stirring is started and the temperature is raised to 70°C, then a mixed solution of 1,3-butadiene (second monomer), styrene (second monomer), methacrylate (second monomer) and dodecyl mercaptan (second molecular weight regulator) is added dropwise at a constant speed, the dropwise time is 3h, then the temperature is raised to 90°C for emulsion polymerization reaction, the reaction time is 3h, an outer shell is formed on part of the surface of the inner core to obtain a polymer binder intermediate; wherein the mass ratio of 1,3-butadiene (second monomer), styrene (second monomer), methacrylate (second monomer), ammonium phosphate (second pore-forming template agent), sodium dodecylbenzenesulfonate (second emulsifier), ammonium persulfate (second initiator) and dodecyl mercaptan (second molecular weight regulator) is 90:90:5:1.25:3.5:0.6:1, and the mass of 1,3-butadiene (second monomer) is 90g;

[0185] In step (2.1.2), the mass fraction of ammonium phosphate (pore-forming template agent) in the second solution formed by 1,3-butadiene (second monomer), styrene (second monomer), methacrylate (second monomer), ammonium phosphate (second pore-forming template agent), sodium dodecyl benzene sulfonate (second emulsifier), ammonium persulfate (second initiator), and dodecyl mercaptan (second molecular weight regulator) is denoted as W2, W2>W1;

[0186] (2.1.3) The polymer binder intermediate is cooled, and after cooling, the pH value is adjusted to 6-8 with ammonia water (alkali liquor), and the first pore-forming template agent and the second pore-forming template agent in the inner core and the shell are removed by neutralization reaction, so that the inner core and the shell both form a pore structure, and the number of pores in the shell is more than the number of pores in the inner core, to obtain a polymer binder. The average pore diameter of the inner core is 1 nm, and the average pore diameter of the shell is 1 nm.

[0187] (2.2) Preparation of the negative electrode active layer

[0188] (2.2.1) The negative electrode active material graphite, carboxymethyl cellulose sodium (CMC-Na), conductive agent carbon black, and the above polymer binder are weighed according to the mass ratio of 96.3:1.0:0.4:2.3, then the negative electrode active material graphite, carboxymethyl cellulose sodium (CMC-Na), and conductive agent carbon black are mixed, stirred uniformly, and then deionized water is added for stirring and kneading to obtain graphite agglomerates with good wettability; carboxymethyl cellulose sodium (CMC-Na) is added to the graphite agglomerates, stirred, and then dispersed after the addition of deionized water, and then the above polymer binder is added, stirred, to obtain a first negative electrode slurry;

[0189] (2.2.2) The negative electrode active material graphite, carboxymethyl cellulose sodium (CMC-Na), conductive agent carbon black, and the above polymer binder are weighed according to the mass ratio of 97.8:1.0:0.7:0.5, then the negative electrode active material graphite, carboxymethyl cellulose sodium (CMC-Na), and conductive agent carbon black are mixed, stirred uniformly, and then deionized water is added for stirring and kneading to obtain graphite agglomerates with good wettability; carboxymethyl cellulose sodium (CMC-Na) is added to the graphite agglomerates, stirred, and then dispersed after the addition of deionized water, and then the above polymer binder is added, stirred, to obtain a second negative electrode slurry;

[0190] (2.2.3) The first negative electrode slurry is coated on the double-sided surfaces of the negative electrode current collector copper foil, and after drying, a first negative electrode active layer is formed; the second negative electrode slurry is coated on the surface of the first negative electrode active layer away from the copper foil, and after drying, a second negative electrode active layer is formed, to obtain a negative electrode sheet.

[0191] (3) Separation film

[0192] A 12-μm-thick polypropylene separation film is selected.

[0193] (4) Preparation of electrolyte

[0194] Ethylene carbonate (EC) and methyl ethyl carbonate (EMC) were mixed in a mass ratio of 30:70 to obtain an organic solvent, 3 wt% of LiFSI based on the whole electrolyte was dissolved in the mixed solvent, and LiPF6 which was sufficiently dried was dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0195] (5) Preparation of battery

[0196] The positive electrode sheet, the separator, and the negative electrode sheet were stacked in order, with the separator between the positive and negative electrode sheets to play a separating role, and then wound into an electrode assembly, which was then loaded into an aluminum plastic film, and then baked at 80°C to remove water, and then the above electrolyte was injected, sealed, and subjected to processes such as standing, hot and cold pressing, formation, and capacity distribution to obtain a lithium ion secondary battery.

[0197] The lithium ion secondary batteries of Comparative Examples 1-2 were prepared basically the same as in Example 1, except that the mass of the first pore-forming template agent in step (2.1.1) and the mass of the second pore-forming template agent in step (2.1.2) were changed, as shown in Table 1.

[0198] Table 1

[0199] The lithium ion secondary batteries of Examples 2-4 were prepared basically the same as in Example 1, except that the dropping time of the mixed solution in step (2.1.1) was changed, as shown in Table 2.

[0200] Table 2

[0201] The lithium ion secondary batteries of Examples 5-7 were prepared basically the same as in Example 1, except that the dropping time of the mixed solution in step (2.1.2) was changed, as shown in Table 3.

[0202] Table 3

[0203] The lithium ion secondary batteries of Examples 8-11 were prepared basically the same as in Example 1, except that the type of the second monomer in step (2.1.2) was changed, or the type of the first monomer in step (2.1.1) and the type of the second monomer in step (2.1.2) were changed, as shown in Table 4.

[0204] In Example 8, the 1,3-butadiene, styrene, and methacrylate in step (2.1.2) were replaced by styrene.

[0205] In Example 9, the 1,3-butadiene, styrene, methacrylate in step (2.1.2) is replaced by methyl methacrylate;

[0206] In Example 10, the 1,3-butadiene, styrene in step (2.1.1) is replaced by butyl acrylate, the 1,3-butadiene, styrene, methacrylate in step (2.1.2) is replaced by styrene and methyl methacrylate, the mass ratio of styrene (second monomer), methyl methacrylate (second monomer), ammonium phosphate (second pore-forming template), sodium dodecyl benzene sulfonate (second emulsifier), ammonium persulfate (second initiator) and dodecyl mercaptan (second molecular weight regulator) in step (2.1.2) is 92.5:92.5:1.25:3.5:0.6:1, and the mass of styrene (second monomer) is 92.5 g.

[0207] In Example 11, the 1,3-butadiene in step (2.1.1) is replaced by ethylene, the styrene in step (2.1.1) is replaced by ethyl acetate; the 1,3-butadiene, styrene, methacrylate in step (2.1.2) is replaced by methyl methacrylate and isobornyl methacrylate, the mass ratio of methyl methacrylate (second monomer), isobornyl methacrylate (second monomer), ammonium phosphate (second pore-forming template), sodium dodecyl benzene sulfonate (second emulsifier), ammonium persulfate (second initiator) and dodecyl mercaptan (second molecular weight regulator) in step (2.1.2) is 92.5:92.5:1.25:3.5:0.6:1, and the mass of methyl methacrylate (second monomer) is 92.5 g.

[0208] Table 4

[0209] The lithium ion secondary batteries of Examples 12-13 are prepared according to the method of Example 1, except that the mass of the first emulsifier in step (2.1.1) and the mass of the second emulsifier in step (2.1.2) are changed, as described in Table 5.

[0210] Table 5

[0211] The lithium ion secondary batteries of Examples 14-15 are prepared according to the method of Example 1, except that the mass ratio of graphite, sodium carboxymethyl cellulose, carbon black and polymer binder in step (2.2.1) and step (2.2.2) are changed, as described in Table 6.

[0212] Table 6

[0213] The lithium ion secondary battery of Example 16-17 was prepared substantially the same as Example 1, except that the types of the first pore-forming template and the second pore-forming template in step (2.1.1) and step (2.1.2) were changed, as described in Table 7.

[0214] Table 7

[0215] The lithium ion secondary battery of Example 18 was prepared substantially the same as Example 1, except that step (2.2.3) was different, in which the first negative electrode slurry was coated on the double-sided surface of the negative electrode current collector copper foil, and after drying, the first negative electrode active layer was formed to obtain the negative electrode sheet, and the thickness of the first negative electrode active layer in this example was the same as the total thickness of the first negative electrode active layer and the second negative electrode active layer in Example 1, as described in Table 8.

[0216] Table 8

[0217] The lithium ion secondary battery of Example 19 was prepared substantially the same as Example 1, except that step (1) and step (2.2) were different.

[0218] In the preparation of the positive electrode sheet in step (1) of this example, the positive electrode active material lithium iron phosphate (LiFeP04), the conductive agent carbon black (Super P), and the polymer binder prepared in step (2.1) of Example 1 were mixed uniformly in a solvent water in a mass ratio of 96.2:2.7:1.1 to obtain a positive electrode slurry; the positive electrode slurry was coated on the double-sided surface of the positive electrode current collector aluminum foil, and through processes such as drying, cold pressing, slitting, and cutting, the positive electrode sheet was obtained.

[0219] In the preparation of the negative electrode active layer in step (2.2) of this example, the negative electrode active material graphite, the thickening agent sodium carboxymethyl cellulose, the binder styrene-butadiene rubber, and the conductive agent acetylene black were mixed in a mass ratio of 97:1:1:1, deionized water was added, and a negative electrode slurry was obtained under the action of a vacuum stirrer; the negative electrode slurry was uniformly coated on the double-sided surface of the copper foil; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 h, and then cold-pressed and cut to obtain the negative electrode sheet.

[0220] The lithium ion secondary battery of Comparative Examples 3-4 was prepared substantially the same as Example 19, except that the mass of the first pore-forming template in step (2.1.1) and the mass of the second pore-forming template in step (2.1.2) were changed, as described in Table 9.

[0221] Table 9

[0222] As can be seen from Tables 1 to 8, compared with Comparative Examples 1 to 2, the polymer binder with the shell and the core both having the pore structure and the number of pores in the shell being more than that in the core introduced into the negative electrode sheet of the battery of Examples 1 to 18 can effectively improve the fast-charging performance of the battery and reduce the risk of lithium precipitation of the negative electrode sheet.

[0223] As can be seen from Table 9, compared with Comparative Examples 3 to 4, the polymer binder with the shell and the core both having the pore structure and the number of pores in the shell being more than that in the core introduced into the positive electrode sheet of the battery of Example 19 can effectively improve the fast-charging performance of the battery and improve the interface of the positive electrode sheet.

[0224] The above description of the various embodiments tends to emphasize differences between the various embodiments, and the same or similar parts can be referred to each other, and for the sake of brevity, will not be described herein.

[0225] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the gist of the present application, various modifications that can be thought of by those skilled in the art, other modes constructed by combining part of the configuration elements of the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery comprising an electrode tab comprising a current collector and an electrode active layer provided on at least one surface of the current collector, the electrode active layer containing an electrode active material and a polymer binder; the polymer binder comprising a core and a shell covering at least part of the surface of the core, at least one of the shell and the core comprising a polymer, both the shell and the core having a pore structure, the number of pores in the shell being greater than the number of pores in the core.

2. The secondary battery according to claim 1, wherein The secondary battery has at least one of the following characteristics (1) and (2): (1) The average pore diameter of the core is 0.5 nm to 3 nm; (2) The average pore diameter of the shell is 0.5 nm to 3 nm.

3. The secondary battery according to claim 1 or 2, wherein The secondary battery has at least one of the following characteristics (1) and (2): (1) The average pore diameter of the core is 0.5 nm to 1.5 nm; (2) The average pore diameter of the shell is 0.5 nm to 1.5 nm.

4. The secondary battery according to any one of claims 1 to 3, wherein Both the shell and the core comprise a polymer, the polymer in the core being different from the polymer in the shell.

5. The secondary battery according to any one of claims 1 to 4, wherein The secondary battery has one or more of the following characteristics (1) and (2): (1) The polymer in the core comprises one or more of polybutadiene, styrene-butadiene copolymer, ethylene-vinyl acetate copolymer, polybutyl acrylate, and graft copolymer of polyvinyl alcohol and isooctyl acrylate; (2) The polymer in the shell comprises one or more of a polymer containing an acrylate structural unit and a polymer containing an olefin structural unit.

6. The secondary battery according to claim 5, wherein The secondary battery has one or more of the following characteristics (1) and (2): (1) The polymer containing an acrylate structural unit comprises one or more of styrene-methyl methacrylate copolymer, polymethyl methacrylate, styrene-methyl methacrylate copolymer, and methyl methacrylate-isobornyl methacrylate copolymer; (2) The polymer containing an olefin structural unit comprises one or more of polystyrene and styrene-methyl methacrylate copolymer.

7. The secondary battery according to any one of claims 1 to 6, wherein The volume average particle size Dv50 of the polymer binder is 100 nm to 500 nm.

8. The secondary battery according to any one of claims 1 to 7, wherein The average liquid absorption rate of the electrode tab is 1 mL / s to 4.5 mL / s.

9. The secondary battery according to any one of claims 1 to 8, wherein The electrode tab is a negative electrode tab, the current collector is a negative electrode current collector, the electrode active layer is a negative electrode active layer, and the electrode active material is a negative electrode active material.

10. The secondary battery according to claim 9, wherein The negative electrode active material comprises a carbon-based material, and the carbon-based material comprises graphite.

11. The secondary battery according to claim 9 or 10, wherein The negative electrode active layer comprises a first negative electrode active layer and a second negative electrode active layer stacked in order along the direction away from the surface of the negative electrode current collector, both the first negative electrode active layer and the second negative electrode active layer contain the polymer binder, and the mass percentage content of the polymer binder in the first negative electrode active layer is greater than or equal to the mass percentage content of the polymer binder in the second negative electrode active layer.

12. The secondary battery according to claim 11, wherein The secondary battery has at least one of the following characteristics (1) and (2): (1) the mass percentage content of the polymer binder in the first negative electrode active layer is 0.8% to 1.8%; (2) the mass percentage content of the polymer binder in the second negative electrode active layer is 0.6% to 1.8%.

13. The secondary battery according to claim 11 or 12, wherein The first negative electrode active layer and the second negative electrode active layer both contain the negative electrode active material, and the mass percentage content of the negative electrode active material in the first negative electrode active layer is less than or equal to the mass percentage content of the negative electrode active material in the second negative electrode active layer.

14. The secondary battery according to claim 13, wherein The secondary battery has at least one of the following features (1) and (2): (1) the mass percentage content of the negative electrode active material in the first negative electrode active layer is 96% to 97%; (2) the mass percentage content of the negative electrode active material in the second negative electrode active layer is 97% to 98%.

15. The secondary battery according to any one of claims 1 to 8, wherein The electrode tab is a positive electrode tab, the current collector is a positive current collector, the electrode active layer is a positive electrode active layer, and the electrode active material is a positive electrode active material.

16. A method of manufacturing a secondary battery, comprising the step of manufacturing an electrode sheet. The step of preparing the electrode tab includes: emulsion polymerization of a first solution containing a first pore-forming template agent and a first monomer to form an emulsion containing a core; mixing a second solution containing a second pore-forming template agent and a second monomer with the emulsion containing the core, and emulsion polymerization to form a shell on at least part of the surface of the core, to obtain a polymer binder intermediate; the mass fraction of the second pore-forming template agent in the second solution is higher than the mass fraction of the first pore-forming template agent in the first solution; removing the first pore-forming template agent and the second pore-forming template agent in the polymer binder intermediate, so that the core and the shell both form a pore structure, and the number of pores in the shell is greater than the number of pores in the core, to obtain a polymer binder; coating an electrode slurry containing an electrode active material and the polymer binder on at least one surface of the current collector, and drying to form an electrode active layer, to obtain the electrode tab.

17. The method of making according to claim 16, wherein, The current collector is a negative current collector, the electrode active material is a negative electrode active material, the electrode slurry is a negative electrode slurry, the electrode active layer is a negative electrode active layer, and the electrode tab is a negative electrode tab.

18. The method of making according to claim 17, wherein, The step of preparing the electrode tab includes: coating a first negative electrode slurry containing the negative electrode active material and the polymer binder on at least one surface of the negative current collector to prepare a first negative electrode active layer, to obtain a negative electrode tab intermediate; coating a second negative electrode slurry containing the negative electrode active material on the surface of the negative electrode tab intermediate away from the negative current collector, and drying to form a second negative electrode active layer, to obtain the electrode tab, which is the negative electrode tab; the mass percentage content of the polymer binder in the first negative electrode active layer is greater than or equal to the mass percentage content of the polymer binder in the second negative electrode active layer.

19. The method of making according to claim 18, wherein, The second negative electrode slurry also contains the polymer binder.

20. The method of making according to claim 16, wherein, The electrode tab is a positive electrode tab, the current collector is a positive current collector, the electrode active layer is a positive electrode active layer, and the electrode active material is a positive electrode active material.

21. The production process according to any one of claims 16 to 20, wherein, The preparation method satisfies at least one of conditions (1) to (5) as follows: (1) the first monomer includes one or more of an olefin compound, an acrylate compound, and an enol compound; (2) the second monomer includes one or more of an olefin compound and an acrylate compound; (3) the first pore-forming template agent and the second pore-forming template agent each independently include an inorganic salt, the inorganic salt including one or more of ammonium phosphate, ammonium sulfate, and ammonium chloride; (4) the first solution further contains one or more of a first initiator, a first emulsifier, and a first molecular weight regulator; (5) the second solution further contains one or more of a second initiator, a second emulsifier, and a second molecular weight regulator.

22. The method of making according to claim 21, wherein, The preparation method satisfies at least one of conditions (1) to (3) as follows: (1) the first initiator and the second initiator each independently include one or more of a persulfate salt and an azo compound; (2) the first emulsifier and the second emulsifier each independently include one or more of an anionic emulsifier, a nonionic emulsifier, and an amphoteric emulsifier; (3) the first molecular weight regulator and the second molecular weight regulator each independently include one or more of a mercaptan compound, a halogenated compound, and a nitroxyl compound.

23. An electrode tab, the electrode tab including a current collector and an electrode active layer disposed on at least one surface of the current collector, the electrode active layer containing an electrode active material and a polymer binder; The polymer binder includes a core and a shell covering at least part of the surface of the core, both the shell and the core having a pore structure, the number of pores in the shell being greater than the number of pores in the core.

24. An electrical device, including at least one of the secondary battery of any one of claims 1 to 15, the secondary battery prepared by the preparation method of any one of claims 16 to 22, and the electrode tab of claim 23.

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